IGS
International GPS Services
for Geodynamics

1997 Technical Reports
October, 1998

IGS Central Bureau
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California U.S.A.
Edited by
Ivan Mueller
Ruth Neilan
Ken Gowey

This publication was prepared by the Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National Aeronautics and Space
Administration.
Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not constitute or imply its
endorsement by the United States Government, the National Aeronautics and Space
Administration, or the Jet Propulsion Laboratory, California Institute of Technology.

ii

Abstract
Applications of the Global Positioning System (GPS) to Earth Science are numerous.
The International GPS Service (IGS), a federation of government agencies and universities,
plays an increasingly critical role in support of GPS-related research activities.
Contributions from the IGS Governing Board and Central Bureau, analysis and data
centers, station operators, and others constitute the 1997 Annual Report. This report has
a companion publication, the 1997 Technical Reports. Hard copies of each volume can beobtained by contacting the IGS Central Bureau at the Jet Propulsion Laboratory.

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iv

Contents

Contents
Introduction ................................................................................................... ix

1

Executive Reports
The Development of the IGS in 1997 -The Governing Board’s Perspective .......................3
Gerhard Beutler
Activities of the IGS Central Bureau in 1997 .......................................................................7
Ruth Neilan
Recommendations and Action Items - IGS Governing Board Retreat.................................9
Ivan Mueller

2Analysis Center Reports
Analysis Coordinator Report..............................................................................................23
J. Kouba and Y. Mireault

Analysis Centers
Code Analysis Center of the IGS .......................................................................................73
M. Rothacher, T.A. Springer, et al.
The ESA/ESOC IGS Analysis Centre .................................................................................89
T. J. Martin Mur, J. M. Dow, et al
GFZ Analysis Center of the IGS - Annual Report 1997 ......................................................99
Gerd Gendt, Galina Dick, et al.
JPL IGS Analysis Center Report, 1997............................................................................109
D. C. Jefferson, Y. E. Bar-Sever, et al.
NRCan IGS Analysis Centre Report, 1997 .......................................................................117
P. Tétreault, C. Huot, et al.
Scripps Orbit and Permanent Array Center
1997 Analysis Center Report ...........................................................................................123
Peng Fang, Matthijs van Domselaar, et al.

Associate Analysis Centers
GNAACsThe Newcastle Global Network
Associate Analysis Center Annual Report ......................................................................139
Ra’ed Kawar, Geoffrey Blewitt, et al.
MIT T2 Associate Analysis Center Report .......................................................................149
Thomas A. Herring
GNAAC Coordinate Comparisonsat JPL for GPS Weeks 813-964........................................................................................155
M B Heflin, D C Jefferson, et alv

IGS 1997 Technical Reports

RNAACsThe EUREF RNAAC: 1997 Annual Report.........................................................................159
Carine Bruyninx, et al
Report of the Regional Network Associate Analysis Center
for Far-East Asia ..............................................................................................................165
K. Murakami
The PGC Regional Network Associate
Analysis Centre: 1997 Annual Report..............................................................................167
H. Dragert, J. Henton, et al
Annual Report 1997 of the RNAAC SIRGAS .....................................................................173
Wolfgang Seemüller and Hermann Drewes
Rapid Service and Prediction
U.S. Naval Observatory Center
for Rapid Service and Predictions...................................................................................177
J.R. Ray, J.R. Rohde, et al

IERS Contributions
IERS References
Contribution of the Central Bureau of IERS......................................................................185
C. Boucher, D. Gambis,, et al3Data Center Reports
Global Centers
CDDIS Global Data Center Report ....................................................................................197
Carey E. Noll
Scripps Orbit and Permanent Array Center
1997 Global Data Center Report ......................................................................................211
Jeff Dean, Yehuda Bock, et al
Regional/Operational Centers
BKG Regional IGS Data Center Report ............................................................................219
Heinz Habrich

4

Network and Station Reports
Global, Regional, and Local Networks
AUSLIG 1997 IGS Annual Report
Networks & Stations.........................................................................................................227
M. R. Hendy
The GPS Receiver Network of ESOC: Malindi, Maspalomas,
Kourou, Kiruna, Perth and Villafranca .............................................................................231
C.Garcia-Martinez, T.Martin-Mur, et alviContents

GPS Global Network Operations at
JPL and the UNAVCO Facility ..........................................................................................239
S. Fisher, D. Stowers, et al
Individual Stations
Status of the IGS Stations Provided by
the Norwegian Mapping Authority in 1997 .......................................................................245
Svein Rekkedal and Hans-Peter Plag
UPAD Status Report for 1997 ...........................................................................................247
Alessandro Caporali
Report on IGS Global Station Jozefoslaw (JOZE)............................................................249
Janusz Sledzinski
The BOR1 IGS Station ......................................................................................................251
M. Lehmann, L. Jaworski
Peculiarity of POL2, SELE GPS Stations Operations .....................................................253
Alexandr Matix, Yuri Trapeznikov

5

Pilot Projects/Committees
GLONASS IGEX-98 Campaign
IGS 1997 Annual Report...................................................................................................257
Pascal Willis
IGS/BIPM Time Transfer Pilot Project..............................................................................259
J. RayIGS Combination of Tropospheric Estimates -The Pilot Experiment........................................................................................................265
Gerd Gendt
1997 IGS Activities in the Area of the Ionosphere ..........................................................271
J. Feltens
IGS Infrastructure Committee Report...............................................................................273
Yehuda BockContinuous GPS Positioning of Tide Gauges:
Some Preliminary Considerations ..................................................................................287
Michael Bevis

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IGS 1997 Technical Reports

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viii

Introduction

Introduction
The U.S. Global Positioning System (GPS) constellation of satellites plays amajor role in regional and global studies of Earth. In the face of continued growth anddiversification of GPS applications, the worldwide scientific community has made aneffort to promote international standards for GPS data acquisition and analysis, and to
deploy and operate a common, comprehensive global tracking system.
As part of this effort, the International GPS Service for Geodynamics (IGS) wasestablished by the International Association of Geodesy (IAG) in 1993 and began formal
operation in January 1994. The IGS, with a multinational membership of organizations
and agencies, provides GPS orbits, tracking data, and other data products in support ofgeodetic and geophysical research. In particular, since January 1994, the IGS has madeavailable to its user community the IGS official orbit, based on contributions from theseven current IGS Analysis Centers. The IGS also supports a variety of governmental
and commercial activities and develops international GPS data standards andspecifications.
Highly accurate and reliable data and data products supplied by the IGS meet thedemands of a wide range of applications and experimentation. They can be accessed on
the Internet through the Information System maintained by the IGS Central Bureau,
which is sponsored by the National Aeronautics and Space Administration (NASA) andmanaged for NASA by the Jet Propulsion Laboratory (JPL) of the California Institute ofTechnology. In 1996, the IGS became a member of the Federation of Astronomical andGeophysical Data Analysis Services (FAGS).
The Central Bureau Information System can be accessed using the World Wide
Web (WWW) or via anonymous File Transfer Protocol (FTP) as follows:••

WWW — http://igscb.jpl.nasa.gov
FTP — igscb.jpl.nasa.gov (or 128.149.70.171)

Use the directory /igscb. See README.TXT for online help, and TREE.TXT and
IGSCB.DIR for directory and file information.

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xExecutive Reports

2

The Development of the IGS in 1997 The Governing Board’s Perspective

The Development of the IGS in 1997 The Governing Board’s Perspective
Gerhard Beutler
Chair, IGS Governing Board
Astronomical Institute
University of Berne
Berne, Switzerland

1

Overview

It was stated many times that the IGS, as a very active service in support of Earth
Sciences and Astronomy, is in continuous development. Many changes have occurred
recently.
One of these recent changes concerns the format of the IGS annual reports. The
first IGS annual report was written in 1994 (although IGS activities started as early as1992), and the annual reports for 1995 and 1996 followed. The 1994 Annual Reportcontains 330 pages, the 1995 Volume 282 pages (using a more dense format than the 1994
report), and the 1996 report contains 446 pages (using essentially the same format as in1995). On one hand, this increase in size was viewed as a positive development whichhighlighted and documented many new activities (e.g., the work performed in the context
of the densification of the ITRF). On the other hand, it became increasingly difficult for
non-IGS-experts to get a concise overview of IGS activities.
This was why the IGS Governing Board, at its 1998 Darmstadt Business Meetingon Sunday, February 8 1998, decided to produce the IGS Annual Report for 1997 in twocorresponding volumes. Volume 1 would contain the top level information (CB report,
IGS Analysis Center Coordinator report, report about current projects, etc.), and Volume
2 would contain the technical reports (analysis center reports, the station reports, etc.).
As a matter of fact, Volume 1 has been completed and should be distributed insummer 1998. Volume 2 is being finalized in September 1998. Volume 1 was edited by
the Central Bureau much the way the 1996 Annual Report was done, and Volume 2 will
be published to closely resemble "camera ready manuscripts.” Volume 1 contains about
TBD pages and will be broadly distributed (approximately 1500 copies) inside andoutside the IGS. Volume 2 is primarily designed for internal IGS use and will be
distributed to the IGS participants, associates, and libraries. Both reports will be madeavailable in electronic form.
The Table of Contents of Volume 2 indicates the high documentary value ofVolume 2: Here, the IGS Analysis Center Coordinator and all IGS Analysis Centers andAssociate Analysis Centers summarize their latest and greatest improvements andchanges, the IERS (Rapid Service and Predictions and Central Bureau) provide technicalfeedback and comments to the IGS. This again underlines the excellent relationship3

IGS 1997 Technical Reports

between the two services in support of science. In subsequent sections, the Data centers
discuss the issues of data handling, and an overview is provided concerning the state of
the IGS network.
The development of the IGS in size and quality since 1992 is remarkable. Crucial
to this development are its pilot projects, working groups, and committees. The final
section of Volume 2 gives an overview of these IGS components that are currently active.
The topic is also addressed in the next subsection of this report.
Volume 2 contains another first; an executive chapter. Initially, it was importantthat the topic of this new section does not restate that which is contained in Volume 1 ofthe 1997 Annual Report. I was therefore initially reticent to write a new contribution forVolume 2. However, after some reflection, I found that the Governing Board clearlystates that the Technical Reports volume of the 1997 Annual Report was different incontent but of equal importance as Volume 1.
So that I do not repeat that which is in Volume 1, the development of the IGS asan IAG- and a FAGS-Service, the essential IGS Events in 1997, and a few remarks
concerning the IGS Retreat 1997 may be found in my report in the first part of the 1997
IGS Annual Report.
Let me address here two topics which kept the Governing Board (and others)quite busy in 1997 and 1998. (As opposed to the first part of the annual report, I am
thus also addressing events which took place in 1998 – a practice which was always
followed in the IGS Analysis Center Reports). The first topic addresses the future IGS
policy regarding pilot projects and working groups, the second the IGS retreat -- which
will then be handled in detail in the second contribution to this introductory chapter.
2

IGS Policy for the Establishment of IGS Projects and Working Groups

As one may conclude from Section 5 (Pilot Projects/Committees) of this report
there are quite a few pilot projects or working groups active within the IGS. These
working groups were set up in the past by the Governing Board on a more or lessspontaneous "ad hoc" basis, where the goals and responsibilities were not always clearly
defined. When the Governing Board saw the smooth development of the ambitious"IGS/BIPM Pilot Project to Study Accurate Time and Frequency Comparisons" it
became obvious that the IGS needed well-defined rules for how to set up pilot projects
and working groups.
This issue was addressed at the Business Meeting of the IGS Governing in
Darmstadt because of the request to create an IGS ionosphere working group. It was
decided that John Dow and the IGS Chairman should•

draft a general "charter" for setting up Working Groups or Pilot Projects within
the IGS, and to circulate this draft within the Governing Board, and•develop, in close cooperation with the “ionosphere club,” the charter for theionosphere working group and circulate this draft within the “ionosphere group.”
4

The Development of the IGS in 1997 The Governing Board’s Perspective

Such general rules and a draft charter for the ionosphere working group wereactually set up and presented to the Governing Board at its ninth meeting in Boston,
Mass, at the end of May 1998. The outcome may be found in IGS Mail Message No.
1916: The rules were accepted by the Board and will be applied whenever new IGS
working groups or pilot projects are created. Moreover, the ionosphere working group
was created with Dr. Joachim Feltens from ESA as chairman.
It is the explicit wish of the Board that existing IGS Working Groups, Pilot
Projects, etc., should follow the same rules in the future. This process will be invokedsoon and will eventually lead to a clearer and better structure of the IGS. The acceptedrules are stated in a special document which will have the status of a "by-law" of the IGS.
The document will be referred to in the new Terms of Reference (to be adapted by theend of 1998). Some of the essential points of the document are:•

An IGS Working Group deals with a particular topic related to the IGS
components. An IGS Pilot Project aims at the development of one or moreparticular IGS product(s) using data from the IGS network.

•

Working groups and projects are operating autonomously under the leadership ofthe chairperson.

•

The IGS Governing Board regularly organizes special meetings, where IGS
projects and working groups are reviewed. Such meetings may be special sessions
at IGS Workshops.

•

IGS Working Groups and IGS Pilot Projects are set up by the IGS Governing
Board at one of its regular meetings. At such a "constitutional meeting" the IGS
Governing Board
•
•

•

3

approves the draft Working Group Charterappoints the chair of the Working Group or Project for two years.

Proposals to terminate the work, to essentially change the Charter, to (re-)appoint
chairpersons are made at these meetings. These proposals are presented to the
IGS Governing Board at its next regular Meeting.
The IGS Retreat in December 1998

At the seventh IGS Governing Board Meeting in Rio de Janeiro it was decided toorganize an "IGS Retreat" in December 1997 with the IGS Governing Board Members
and a very limited group of IGS Associates with the goal to come up with a plan for the5

IGS 1997 Technical Reports

future development of the IGS which then should be discussed by the entire IGS
community and the Board (IGS Mail Message No. 1683).
The retreat actually took place in Napa Valley, December 12-14, 1997.
Recommendations and action items were presented at the Business Meeting of the IGS
Governing Board in Darmstadt. The report was prepared by Ivan I. Mueller, who wasalso the program chair of the retreat. The recommendations and action items will be
discussed in the next section of this introductory chapter by the same author. The report
could only be discussed at the business meeting, decisions on this matter were taken at
the 9th IGS Governing Board Meeting on 28 May, 1998 in Boston. Many of the
proposed action items were already properly addressed at the 1998 IGS Analysis Center
Workshop, others require adaptations in the Terms of Reference, a work which isunderway right now.
The Governing Board considers the "recommendations and action items" of the
IGS Governing Board retreat in Napa Valley, December 12-14, 1997 (as prepared by Ivan
I. Mueller) as an extremely useful document defining the development of the IGS at least
till the end of the millennium..
4

Acknowledgments

We should keep in mind that the IGS is based on a voluntary collaboration of alarge number of scientific and survey institutions. It is also worth pointing out that thecontributing organizations are not funded by the IGS, but have to raise funds for their
IGS-related activities. Thus, an organization like the IGS only works properly if allcontributing institutions are dedicated to the IGS mission and its performance, and if thebenefit from IGS activities justifies the investments.
The other pillar of the IGS success is the personal engagement of many individuals
who devote their time to the IGS. Prior to my involvement with the IGS, I was not awareof the large number of enthusiasts willing to cooperate on a voluntary basis for the benefit
for the scientific community. I am convinced that most IGS associates share thesefeelings. On behalf of the IGS Governing Board, I would like to cordially thank all
institutions and individuals devoting time and funds to the success of the IGS.
Many of us contributing to the 1997 IGS Annual Report found it difficult tosubmit manuscripts on time. The fact that delays stayed within "reasonable limits" is due
to Prof. Ivan Mueller. His help in the editorial process allowed the Central Bureau toproduce the Annual Reports in a timely manner -- which is of greatest importance forsuch a document. In this context I also would like to congratulate Dr. John Dow from
ESA for his very efficient production and distribution of the Proceedings of the 1998 IGS
Workshop in Darmstadt.

6

Activities of the IGS Central Bureau in 1997

Activities of the IGS Central Bureau in 1997
Ruth E. Neilan
Director, IGS Central Bureau
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California

1

Overview

The end of 1997 marks a ‘rite of passage’ for the IGS, the first four year period
devoted to nurturing this fledgling scientific service based on the Global Positioning
System (GPS). The past four years have resulted in solidifying this now well known
international activity and reinforcing its importance for scientific and research applications.
During this time period, the IGS has become the fundamental supporting infrastructure fornumerous geodetic, geophysical and geodynamic applications that depend on the utilizationof GPS technology. The IGS also advocates standards and specifications for achieving
excellence in precision use aspects of GPS from network operations through GPS analysis
and applications, so that users worldwide can make use of the wealth of data and products
afforded by the IGS.
It is quite clear that the strength of the IGS is directly due to the many participating
individuals and their sponsoring agencies as noted by Gerhard Beutler above. The
achievement of the IGS is something that each can lay claim to and it is the recognition thatthrough mutual cooperation much greater benefit is realized by all.
2

Network Status and Update

The IGS network consists of precision, geodetic dual-frequency GPS stations thatobserve the GPS satellites on a continuous, 24-hour basis. These globally distributed
stations are funded, implemented and operated by one of the IGS participating. At the endof 1997, nearly 200 stations were listed as part of the IGS network, an increase of nearly
70 stations registering with the IGS in 1997. Currently, the data files from each station
span a 24-hour period, although the IGS is planing sub-daily data retrievals in the future,
on an hourly or four to six hour basis. A Network Workshop is planned in November
1998 to address the current and future operations of the network, and many new
requirements that affect these operations. This is in response to increasing demands on theinfrastructure.
3

IGS / CB Activities in 1997

The IGS Terms of Reference, the relations by which the IGS is governed (i.e., bylaws) were established at the beginning of the service in 1994. These terms state that the“Central Bureau of the International GPS Service is responsible for the overall coordinationand management of the Service”. In order to fulfill this role, the CB has been be actively
engaged in the many activities of the IGS. Given the current scope of IGS activities, theon-going fundamental processes and the new projects and directions of related GPS
applications, the personnel of the CB must have a number of different talents to collectively
perform the necessary tasks in order to coordinate with the various components of theservice.

7

IGS 1997 Technical Reports

The CB is in the process of reorganizing the office based on the recommendationsof the Napa Retreat in December 1997, see Ivan Mueller, this volume. One of the mostnoticeable results will be a nearly full-time Director and a full-time position of a Deputy
Director. These staffing allocations are appropriate given the necessity of the CB to assume
more of the daily coordination of the IGS, especially with regard to the robust performanceof the ~200 station network, and the need to assume the role of the Executive arm of theGoverning Board. In the first year or two of IGS operations the contributing agencies
were all working to achieve their objectives as part of the IGS, in the spirit of the IGS
mission statement. During this period, it took time to develop and solidify the working
relationships internal to the IGS. Today, we are increasingly aware that additional effort iswarranted in two areas: sustaining the fundamental IGS and providing interface to users,
both internal and external
The Central Bureau has been actively working to completely upgrade the Central
Bureau Information System (CBIS) which was made active in June of 1998. This web site
and FTP server contain all of the fundamental information of the IGS. All IGS products areheld here, as well as at the Global Data Centers (GDC). Most external users access the
CBIS, while internal IGS users generally access the GDCs of choice. The CBIS willcontinue to evolve so that information is easily accessible and web based tutorials on theuse of IGS products will be developed.
One of the other duties of the Central Bureau is to organize workshops andmeetings, much effort was devoted to the first joint workshop between the PermanentService for Mean Sea Level (PSMSL) and the IGS, the Workshop on Methods for
Monitoring Sea Level and Altimeter Calibration, a joint IGS and Permanent Service for
Mean Sea Level (PSMSL) workshop. This resulted in proceedings that are very valuablefor these applications . Other workshops include supporting the 1997 Analysis Center
workshop convened by Mike Watkins and Yehuda Bock. All meetings of the Governing
Board are arranged by the CB, this year in held in Pasadena, Rio de Janeiro, San Francisco
and the IGS Retreat in Napa Valley.
The Central Bureau managed six IGS exhibits at various international locations thisyear in order to promote information and use of data and products from the IGS. These
exhibits include a computer slide show, back-drop of information, publications for pick-up
or order and people stationed at the booth to answer questions.
The Central Bureau has devoted a great deal of time and resources to publications,which include 1996 IGS Annual Report, IGS Directory 1997, and IGS Resource Packets,
updated quarterly. The IGS brochure was completely redesigned and rewritten:
IGS Brochure Monitoring Global change with Satellite Tracking, this brochure is alsoavailable in Spanish and future revisions will be available in other languages also.
The direction of the Central Bureau in the future will be to shift the focus somewhat frompublications, exhibits, and meeting organization, and move to coordinate the Service in amore active fashion, as recommended by the Governing Board. To achieve this objective,
the current reorganization of the CB will result in closer working relations with the various
IGS components, including the Analysis Center Coordinator, the IGS working groups andcommittees. .

8

Recommendations and Action Items - IGS Governing Board Retreat, 1997

Recommendations and Action Items IGS Governing Board Retreat
Napa Valley, December 12-14, 1997
Ivan I Mueller
1

Overview

One of the conclusions reached at the Retreat was that the IGS Terms ofReference (January 1996 version), with some “fine tuning”, still reflects the current needsof the IGS. For this reason, and also to provide a framework for the Retreat’s
Recommendations (Rs) and Action Items (As), relevant portions of the terms are
reproduced below, in bold letters between dotted lines, with the Rs and As inserted at theappropriate locations.
In order to keep the Retreat as conducive to open discussion as possible, formal
Minutes were not kept. A “short hand”/informal record, suitable to jag the memories ofthe participants, is available from the Central Bureau.
The Recommendations/Action Items and the explanatory text as presented below
are based on the final summary discussion of the Retreat Coordinators on December 14,
1998, and on correspondence and conversations held after the Retreat.
2

International GPS Service for Geodynamics - Terms of Reference

The term “Geodynamics” within the name IGS, at its inception, was meant toindicate that the primary users of the service are scientists involved in geodynamics,
specifically using GPS for determining and/or monitoring positions on the surface of the
Earth with the highest accuracy. Since other types of users (especially from theatmospheric and oceanic science communities) are appearing on the horizon, thesuggestion was made to eliminate the term “Geodynamics” from the title of IGS.
•

R1: The name of the Service be the “International GPS Service”.

• A1: Governing Board (GB) needs to consider R1 and vote.
--------------------------------------------------------------------------------------The primary objective of the IGS is to provide a service to support, through GPS
data products, geodetic and geophysical research activities. Cognizant of theimmense growth in GPS applications the secondary objective of the IGS is tosupport a broad spectrum of operational activities performed by governmental or
selected commercial organizations. The Service also develops the necessary

9

IGS 1997 Technical Reports

standards/specifications
conventions.

andencourages

international

adherencetoits

IGS collects, archives and distributes GPS observation data sets of sufficientaccuracy to satisfy the objectives of a wide range of applications andexperimentation. These data sets are used by the IGS to generate the following
data products:
•
•
•
•
•
•

high accuracy GPS satellite ephemerides
earth rotation parameters
coordinates and velocities of the IGS tracking stations
GPS satellite and tracking station clock information
ionospheric information
tropospheric information.

The accuracies of these products are sufficient to support current scientificobjectives including:• realization of global accessibility to and the improvement oftheInternational Terrestrial Reference Frame (ITRF)• monitoring deformations of the solid earth• monitoring earth rotation• monitoring variations in the liquid earth (sea level, ice- sheets, etc.)• scientific satellite orbit determinations• ionosphere monitoring• climatological research, eventually weather prediction.
--------------------------------------------------------------------------------------In the past, the IGS combined products used primarily have been those related to
the IGS Reference Frames, both terrestrial and inertial, recommended for GPS users.
These are the station coordinates with their variations in time (defining the terrestrial
frame) and the orbits of GPS satellites (defining the inertial frame), and the transformation
parameters relating the two (the earth-rotation parameters). There have been somequestions as to the internal consistencies of the above products.
Due to user requirements for using the GPS signals in various efficient modes
and/or leading to more accurate results, it appears necessary for IGS to produce
combined, timely, and consistent additional products. Specifically, this includes GPS
clock corrections (possibly an IGS time scale), tropospheric zenith biases and global
and/or regional ionosphere models. These, together with the reference frames (all basedon the IERS Conventions, 1996), constitute the IGS Reference System, assuringconsistency for all GPS users of positioning in all modes.
Although non-positioning GPS user requirements are not clear at this time, itappears that there is (or will be in the near future) an increasing demand for rapid (real-10

Recommendations and Action Items - IGS Governing Board Retreat, 1997time) and more accurate GPS orbits, as well as the inclusion of other non-GPS satellites in
the IGS framework (primarily the GLONASS and LEO satellites).
•

R2: IGS is to produce combined, internally consistent, global products based on GPS
observations as follows (several of these to a fair extent are already accomplished):a) station coordinates and velocities (incl. IGS SINEX products)b) orbital parametersc) earth rotation parametersd) GPS clock correctionse) IGS time scalef) tropospheric zenith delaysg) ionosphere models•

A2.1: The Analysis Center Workshop in Darmstadt should address the issues a) - d)and f) and g) and make recommendations.

•A2.2: The recently established IGS-BIPM Pilot Project should address issues as
already decided by the GB.

•

R3: IGS should continue producing accurate orbits based on rapid and/or high ratedata, investigate new requirements (e.g., for real time meteorology forecasting atwenty-station network providing 30s data down loaded every 6-12 hours issuggested. For LEO see A4.2 below) and suggest and implement improvements inavailability (IGR) and precision (IGP).

•

A3: The Analysis Center Workshop in Darmstadt should address this issue and makerecommendations.

•

R4: IGS should support the tracking of GLONASS and LEO satellites.

•

A4.1: The GB should support tracking of GLONASS satellites by actively promoting
within IGS the International GLONASS Experiment (IGEX), currently scheduled
Sep.-Dec.,1998, pending on the discussion on GLONASS at the GB business meetingin Darmstadt.
A4.2: The LEO Working Group should continue its work (in collaboration withvarious groups involved in the use of LEOs for atmospheric science). Specificrecommendations are to be made on the appropriate number of tracking stations andsampling rate (1-5s?) and on the feasibility of IGS processing of occultation and/or
other flight data.

•--------------------------------------------------------------------------------------The IGS accomplishes its mission through the following components:
11

IGS 1997 Technical Reports

networks of tracking stations• data centers• Analysis and Associate Analysis Centers• Analysis Coordinator• Central Bureau• Governing Board

NETWORKS OF TRACKING STATIONSIGS Stations provide continuous tracking using high accuracy receivers andhave data transmission facilities allowing for a rapid (at least daily) datatransmission to the data centers (see below). The stations have to meetrequirements which are specified in a separate document. The tracking data of
IGS stations are regularly and continuously analyzed by at least one IGS Analysis
Center or IGS Associate Analysis Center....
IGS Stations which are analyzed by at least three IGS Analysis Centers forthe purpose of orbit generation, where at least one of the Analysis Centers lies ona different continent than the station considered, are in addition called IGS Global
Stations.
All IGS stations are qualified as reference stations for regional GPS
analyses. The ensemble of the IGS stations forms the IGS network (polyhedron).
---------------------------------------------------------------------The IGS global network needs an overall enhancement. The IGS Infrastructure
Committee is involved considering issues related to the existing network e.g.,
instrumentation, monumentation, reporting, performance, data communication and flow,
quality control, archiving, site and RINEX standards. Plans for a coordinated systematiceffort to expand/densify the network to the proposed (about 200 stations) Polyhedronare still lacking. On the other hand, the regional densification efforts are progressing, andlimits are to be set up as to the inclusion of the regional stations into the IGS Polyhedron(being pro-active at the same time). Use of the network for climatology would also
require the installation of high stability accurate barometers.
•

R5: The global IGS Network should be enhanced in the overall sense.

•

A5.1: The IGS Infrastructure Committee is to continue its work and report to the GB
at its next regular meeting in Boston.
A5.2: The GB should consider appointing a Network Manager/ Coordinator, within or
outside the CB, to coordinate a systematic effort to complete the IGS Polyhedron.

•

12

Recommendations and Action Items - IGS Governing Board Retreat, 1997

The responsibility would include the formulation of network standards and checking
performance.
• A5.3: The CB/GB should make a systematic and concerted effort to request stations
to install high stability/accuracy barometers (the alternative of using routinely
produced atmospheric pressure grids should be explored, although their availability innear real time might be a challenge).
• A5.4: The GB should consider organizing an IGS Network Workshop to have an open
discussion on network/station issues and to develop a direct interaction between the
GB and the stations, upon which rest all IGS activities.
--------------------------------------------------------------------------------------DATA CENTERS
The data centers required fall into three categories: Operational, Regional, andGlobal Data Centers.
The Global Data Centers are the main interfaces to the Analysis Centers and theoutside user community. Their primary tasks include the following:•receive/retrieve, archive and provide on line access to tracking data
received from the Operational/Regional Data Centers• provide on-line access to ancillary information, such as site
information, occupation histories, etc.,• receive/retrieve, archive and provide on-line access to IGS products
received from the Analysis Centers• backup and secure IGS data and products.
--------------------------------------------------------------------It was noted that, with the exception of CDDIS (which is doing an admirable job),
not all Global Data Centers are regularly producing their Access Reports. In view of theimportance of keeping track of the users of IGS products, it is recommended that suchreports be published on a regular basis.
•

R6: It is recommended that all Global Data Centers publish Access Reports on amonthly basis.

•

A6: The CB is to contact the relevant Global Data Centers and encourage them tocomply with R6.
---------------------------------------------------------------------------------------

13

IGS 1997 Technical Reports

ANALYSIS CENTERS
The analysis centers fall into two categories: Analysis Centers and Associate
Analysis Centers.
The Analysis Centers receive and process tracking data from one or more data
centers for the purpose of producing IGS products. The Analysis Centers arecommitted to produce daily products, without interruption, and at a specified time
lag to meet IGS requirements. The products are delivered to the Global Data
Centers and to the IERS (as per bilateral agreements), and to other bodies, using
designated standards.
The Analysis Centers provide as a minimum, ephemeris information and earthrotation parameters on a weekly basis, as well as other products, such as
coordinates, on a quarterly basis. The Analysis Centers forward their products to
the Global Data Centers.
Associate Analysis Centers are organizations that produce unique products, e.g.,
ionospheric information or Fiducial Station coordinates and velocities within acertain geographic region. Organizations with the desire of becoming Analysis
Centers may also be designated as Associate Analysis Centers by the Governing
Board until they are ready for full scale operation.
--------------------------------------------------------------------------------------•

R7: Depending on the outcome of the Analysis Center Workshop in Darmstadt theabove descriptions of the Analysis and Associate Analysis Centers should be
reviewed. The GB decisions in San Francisco/Napa Valley re. the
GNAACs/RNAACs, may also have an effect.

•

A7: The AC Coordinator together with the Chair of the Densification Project
recommend the necessary changes to the Terms of Reference as per R7, if necessary.
----------------------------------------------------------------------------------ANALYSIS COORDINATOR
The Analysis Centers are assisted by the Analysis Coordinator.
The responsibility of the Analysis Coordinator is to monitor the Analysis Centers
activities to ensure that the IGS objectives are carried out. Specific expectations
include quality control, performance evaluation, and continued development ofappropriate analysis standards. The Analysis Coordinator is also responsible forthe appropriate combination of the Analysis Centers products into a single set of14

Recommendations and Action Items - IGS Governing Board Retreat, 1997

products. As a minimum a single IGS ephemeris for each GPS satellite is to be
produced. In addition, IERS will produce ITRF station coordinates/velocities andearth rotation parameters to be used with the IGS orbits.
The Analysis Coordinator is to fully interact with the Central Bureau and the
IERS. Generally the responsibilities for the Analysis Coordinator shall rotate
between the Analysis Centers with appointments and terms specified by theGoverning Board.
--------------------------------------------------------------------------------------In view of R2 above, the present Analysis Coordinator’s role will be significantly
expanded and it is unlikely that a single person (or organization) will be able to handle theresponsibilities related to all the different combined global products now contemplated.
There is also a question of coordinating the regional densification projects (connected to
the Polyhedron) in some central way.One of the responsibilities here would also be theeducation of users on how to use IGS products.
•

R8: It is recommended that Working Groups be appointed for Tropospheric
Products, for Ionospheric Products, for ITRF Densification and possibly others(pending on the recommendations of the Analysis Center Workshop in Darmstadt).
The Analysis Center Coordinator should be an ex-officio member of all Working
Groups. The alternative of appointing individual “Coordinators” for each application(instead of the Working Groups) may also be considered.

•

A8.1: Based on the recommendations of the Darmstadt Analysis Workshop, the GB
should appoint new Working Groups or Coordinators as per R8 and clarify theirrelationship/interaction (reporting requirements, etc.) with the CB and the GB.
A8.2: The concept of Working Groups or additional Coordinators, together with their
responsibilities and reporting/interaction requirements should be incorporated in theTerms of Reference.

•--------------------------------------------------------------------------------------CENTRAL BUREAUThe Central Bureau (CB) is responsible for the general management of the IGS
consistent with the directives and policies set by the Governing Board. The
primary functions of the CB are to facilitate communications, coordinate IGS
activities, establish and promote compliance to IGS network standards, monitor
network operations and quality assurance of data, maintain documentation, andorganize reports, meetings and workshops, and insure the compatibility of IGS
and IERS by continuous interfacing with the IERS. To accomplish these tasks the
CB fully interacts with the independent Analysis Coordinator described above.

15

IGS 1997 Technical Reports

Although the Chairperson of the Governing Board is the official representative of
the IGS at external organizations, the CB, consonant with the directives
established by the Governing Board, is responsible for the day-to-day liaison withsuch organizations....
The CB coordinates and publishes all documents required for the satisfactory
planning and operation of the Service, including standards/specifications
regarding the performance, functionality and configuration requirements of allelements of the Service including user interface functions.
The CB operates the communication center for the IGS. It maintains a hierarchyof documents and reports, both hard copy and electronic, including network
information, standards, newsletters, electronic bulletin board, directories,
summaries of IGS performance and products, and an Annual Report.
In summary, the Central Bureau performs primarily a long term coordination andcommunication role to ensure that IGS participants contribute to the Service in aconsistent and continuous manner and adhere to IGS standards.
--------------------------------------------------------------------------------------The Central Bureau has performed well, especially in the areas of coordinating the
network and communication. However, due in part to the rapid expansion of IGS overthe past several years, other CB tasks described in the Terms of Reference either had tobe farmed out to persons (usually volunteers) outside the CB, contracted to otherorganizations (e.g., UNAVCO), or neglected.
In addition to the rapid expansion of IGS, the other major difficulty faced by the
CB is in trying to fulfill its responsibilities which are primarily structural andorganizational in nature. Although it is difficult to assess the situation from the outside, it
seems evident that because no single person has full time responsibility within the CB,
every one is “spread too thin” and fragmented. The Director of the CB has at least threejobs and it appears that only one person reports to her (the liaison to UNAVCO). The
UNAVCO contract to help with the network involves one staff position spread out over
six persons. Others working for the CB, instead of reporting to the Director, in fact
report to one of JPL’s Group Supervisors, who in turn reports to certain Section/Division
heads, and is not directly in charge of the Director of the CB. It appears that such astructure (although maybe efficient for other purposes), combined with the fragmentationof individual responsibilities, lead to difficulties in meeting JPL’s original commitment to
IGS and in some cases even to conflicts of interests within JPL.
•

R9: It is recommended that the tasks of the CB as described in the Terms ofReference be reviewed and the future tasks of the CB clearly defined, with the “leftover” responsibilities appropriately assigned to organizations or individuals outside
the CB, which will closely interact with the CB.
16

Recommendations and Action Items - IGS Governing Board Retreat, 1997•

R10: It is recommended that the host organization of the CB review and streamline
the CB organization, with fragmentation reduced to a minimum and lines of reporting
and responsibilities clearly defined.

•

R11: It is also recommended that at least two persons should be given full time
responsibility within the CB. One of these should be the Director, the other may be
the Network Coordinator (see A5.2 above).

•

R12: It is recommended that, provided that the recommendation for the additional
Coordinators are adopted (see R8 above), their interaction with the CB be clearly
defined.

•A9: The Director of the CB should discuss R9-11 with the appropriate officials of
the host organization and present a plan to eliminate the above difficulties to the GB
and the progress at its next regular meeting in Boston.

•

A10: A10: The GB should appoint a sub-committee to work with the Infrastructure
Committee and the Director of the CB to accomplish R9 and R12.

•

A11: The Central Bureau section of the Terms of Reference will have to be modified
after the fact.
--------------------------------------------------------------------------------------GOVERNING BOARD
The Governing Board (GB) consists of fifteen members. They are distributed asfollows:
Elected by IGS Associates (see below):
Analysis Centers' representatives 3
Data centers' representative
Networks' representatives
2

1

Elected by the Governing Board upon recommendations from the Central Bureau,
for the next term:
Representatives of Analysis,
Data Centers or Networks
2
Members at large
2
Appointed members:
Director of the Central Bureau 1
Representative of the IERS
1
IGS representative to the IERS
1
17

IGS 1997 Technical Reports

IAG/FAGS representative
President of IAG Sect. II
or Com.VIII (CSTG
Total

1
1
15

The appointed members are considered ex officio and are not subject toinstitutional restrictions. The other ten persons must be members of different
organizations and are nominated for each position by the IGS components they
represent as listed above (six persons) , or by the Central Bureau (four persons)for a staggered four year term renewable once. The GB membership should beproperly balanced with regard to supporting organizations as well as to geography.
The election for each position is by the number of nominations received from therelevant IGS component, i.e., from the networks (for this purpose organizations
operating two or more Global Stations are considered a network), from theAnalysis Centers and from the Data Centers. In case of a tie, the election is by themembers of the Governing Board and the IGS Associate Members (see below) by asimple majority of votes received. The election will be conducted by a nominating
committee of three members, the chair of which will be appointed by the Chair of
the IGS Governing Board...
The IAG / FAGS representative is appointed by the IAG Bureau (or by FAGS) fora maximum of two four-year terms...
The secretariat of the GB is provided by the Central Bureau...
--------------------------------------------------------------------------------------The experiences of the past several years indicate that the nomination procedurefor both groups of elected GB members (i.e., those nominated by the IGS Associates andthose by the CB), may be improved to assure wider participation in the nomination
process. In addition, it has been suggested to include all (or most) Coordinators in the
deliberations of the GB. The appointed representation of IAG and FAGS on the GB
needs clarification as well.
•

A12: The GB should appoint a sub-committee to review the current
nomination/appointment procedures for GB membership and to recommend
improvements by the end of 1998.

---------------------------------------------------------------------------------------

18

Recommendations and Action Items - IGS Governing Board Retreat, 1997

Additional Recommendations/Action Items:•

A13: Periodic performance review requirement for each IGS component beincorporated in the Terms of Reference. The GB is to set up procedures for such
regular reviews (how often and how?) and for the follow up of the recommendations(whether positive or negative).

•

R13: The GB should consider forming an Advisory Committee for Commercializationof IGS products. The Committee should include representatives of organizations
experienced in such ventures, e.g., WMO, UCAR/NCAR, IRIS, ESA (its business
arm).

•

R14: The GB should consider forming a committee, with external participation, withthe task to prepare the IGS Long Range and Strategic Plan. Reporting should be at the
IAG General Assembly in 1999.
(January 31, 1998).

19

IGS 1997 Technical Reports

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20Analysis CentersAnalysis Center Coordinator1997 Analysis Coordinator Report

1997 Analysis Coordinator Report
J. Kouba and Y. Mireault
Geodetic Survey Division
Geomatics Canada, Natural Resources Canada
Ottawa, Canada
1

Introduction

This report is a complement to the 1997 Analysis Coordinator Report found in Volume Iof the 1997 IGS Annual Report (Kouba, 1998). Changes, enhancements and new
products implemented during 1997 as well as the combination and evaluation procedures
and statistics of orbits, clocks and Earth Orientation Parameters (EOP) are reviewed. In
1997, three IGS combinations were routinely performed: the IGS Prediction (IGP/daily),
Rapid (IGR/daily) and Final (IGS/weekly) combinations. The United States Naval
Observatory (USNO), an Associate Analysis Centre (AAC) joined the Rapid IGS
production group in April 1997 (Wk 902) but does not contribute to the Prediction or the
Final IGS products.
2

Changes, Enhancements and New Products in 1997

In 1997, all IGS products continued to be oriented to ITRF94 which had been introduced,
along with many other changes, on June 30, 1996 (Wk 860) (Kouba and Mireault, 1997).
This year’s changes and enhancements were mostly related to the IGS LOD/UT andclock correction combinations. The IGS Rapid solution deadline, to make IGS Rapid(IGR) orbits available to the Analysis Centres (AC) participating in the combined
Prediction products, was advanced by two hours, from 24 to 22 hr UTC. The changes
and enhancements are summarized in Table 1.
2.1

IGS Orbit Prediction Combination (IGP)

As mentioned in last year’s Annual Report (Kouba and Mireault, 1997), the IGS orbit
Prediction combination, a 2-day orbit prediction (i.e. 24 to 48h) was officially introducedon March 2, 1997 (GPS Wk 895). This year, more comprehensive statistics regarding
AC and IGS orbit/clock prediction (IGP) performance have been incorporated (seesection 5). The accuracy code found in the IGP sp3 file is based on the comparison with
IGR, which is reported daily with a 2-day delay, as part of the IGR combinations.
Predicted satellite orbits, with missing comparison information with respect to IGR, areassigned an accuracy code of 15 (i.e. precision of ~33 m) in the sp3 file for that day.
Comparison RMS between IGR and IGP below 10 m are left with the accuracy codes
estimated by the combination program. Accuracy codes for satellites with comparison
RMS above 10 m are adjusted according to the RMS.

23

IGS 1997 Technical Reports

Table 1. Summary of changes and enhancements in 1997.
Wk/DayProducts

Changes

894/0
895/0

Final
Rapid

IGS LOD combination/integration officially implemented
using only the IERS Bulletin A UT1 values that included 24h
VLBI data.

896/0
898/0

Final
Rapid

IGS LOD combination/integration using observed IERS
Bulletin A UT1 values.

902/0

Rapid
only

IGS Rapid submission deadline shortened by two hours (from24 to 22 hr UTC).

907/0
908/3

Final
Rapid

IGS LOD combination/integration using up to five days prior
to the most current observed IERS Bulletin A UT1 values.

917/0
919/0

Final
Rapid

New EOP comparison table implemented.

929/0
931/0

Final
Rapid

New IGS LOD weighting scheme based on the AC LOD RMS
bias calibration (1/RMS2) with respect to IERS Bulletin A.

933/0

Rapid
only

Use of EMR UT-derived LOD values in the IGS Rapid LOD
combination/integration.

935/0
936/1

Final
Rapid

Non-SA based AC clock alignment and weighting abandoned
for a more reliable method using one reference AC, allbroadcast satellites and the absolute deviation of AC aligned
clocks with respect to the unweighted mean for AC clock
weighting.

938/0
940/1

Final
Rapid

New alignment correction to AC clocks based on the
difference in the radial component between AC and IGS
combined orbits.

2.2

IGS LOD/UT Combination

IGS LOD/UT combination was briefly described in the 1996 IGS Annual Report (Kouba
and Mireault, 1997) although officially implemented in 1997 only. The IGS Rapid andFinal LOD/UT combinations were respectively implemented on Wks 895 and 894.
To summarize, IGS combined LODs are now based on a weighted average of AC LOD
solutions that are first calibrated/aligned with respect to IERS Bulletin A over a 21 day
period ending five days prior to the last observed (i.e. non-predicted) IERS Bulletin A
UT1 value (also called the anchor point). The combined IGS LOD values are thenintegrated into IGS UT starting from the anchor point and up to the day required. The
integration period is generally 6-10 days for the Rapid combinations and 1-4 days for the
Final combinations. Figure 1 presents a schematic view of the LOD combination andintegration procedure while section 4 describes in more detail the combination method
used.

24

1997 Analysis Coordinator Report

Last Bulletin A observed UT1

5 daysncal
AC LOD Bias Calibrationnday
AC LOD Combination /Integration period

Start-21days

Start(Anchor)Day required

Figure 1. LOD combination and integration procedure.
Initially, only the IERS Bulletin A UT1 values containing 24h VLBI data were used for
the LOD calibration and integration. The strategy was modified a few weeks later to also
include all observed, i.e. non-predicted values. After extensive testing, it was shown thatusing IERS Bulletin A UT1 values ending five days prior to the last observed UT1 value
would give more optimal and reliable results (Mireault et al., 1997).
At first, as in the IGS Polar Motion (PM) combinations, AC orbit weights were used for
AC LOD weights. Other weighting strategies based on AC LOD calibration standard
deviation (1/σ2) and calibration RMS (1/RMS2) were tested. The 1/RMS2 weighting
scheme produced the best results overall (when compared to IERS Bulletin A) and wastherefore implemented. Another improvement (as already noted by (Ray, 1996)) was
realized when EMR UT-derived LOD values were used instead of its estimated LOD.
Tests were conclusive and the new strategy was immediately implemented (strategy IGRC-3* in Table 2). LOD strategy changes that occurred in 1997 are listed in Table 1 while
Table 2 contains a sample of the results from some of the LOD tests performed.

25

IGS 1997 Technical Reports

Table 2. Comparison of strategies of IGS LOD/UT combination and IERS Bulletin A
Product tested: IGS Rapid (IGR)
Period covered: Wk 921/day 0 to Wk 937/day 6

Strategy used

UT Comparison wrt IERS Bulletin A
RMS (µsec)

RMS Improvement(%)

IERS Bulletin A(originally)

377

---

IGR-A-1
IGR-B-1
IGR-C-1
IGR-C-2
IGR-C-3
IGR-C-3*

162
158
137
127
121
96

57
58
64
66
68
75

Description of IGR LOD combination/integration strategies used:
IERS Bulletin A (originally):UT values taken directly from IERS Bulletin
A (at the time of the combination);(A): IGR LOD combination with AC LOD bias calibration based on IERS
Bulletin A UT1 values that included 24h VLBI data;(B): IGR LOD combination with AC LOD bias calibration based on all
observed (i.e. non predicted) IERS Bulletin A UT1;
(C): IGR LOD combination with AC LOD bias calibration based on IERS
Bulletin A UT1 values ending five days prior to its last observed UT1value;(1): AC LOD weights from AC orbit combinations;(2): AC LOD weights based on the standard deviation of AC LOD bias
calibration wrt IERS Bulletion A (1/σ 2);(3): AC LOD weights based on the RMS of AC LOD bias calibration
with respect to IERS Bulletion A (1/RMS 2);(*): Use of EMR UT-derived LOD and use of AC LOD for all others.

2.3

New EOP Comparison Table

In order to have a closer look at the AC EOP performance, a new EOP comparison table
was introduced in the IGS/IGR summary reports (August, 1997). The new table shows
the difference between IGS/IGR combined values and the AC individual contributions
for the Earth Rotation Parameters (ERP: Xpole, Ypole, Xrt, Yrt) and Length Of Day(LOD). Table 3 shows an example of the new table (Table 4.wwww.d) for the IGS Rapid
combination of GPS Wk 933 day 2. It includes a header describing the EOP involved,
the units and some short explanatory comments on flags used. The table is divided into 3
sections: (1) AC names and flags used; (2) ERP (i.e. Xpole, Ypole, Xrt, Yrt) and LOD
comparisons of IGR versus each AC (IGR-AC) and (3) LOD bias estimation summary
Table 3. Example of the new EOP comparison table (Table 4)26

1997 Analysis Coordinator Report

Table 4: Earth Orientation Parameters daily summary.
Daily Centre ERP, ERP Rates and LOD differences with respect to
IGR combined values.
Xpole,Xrt: x pole and x pole rate (10**-5")
Ypole,Yrt: y pole and y pole rate (10**-5"/day)
LOD: Length Of Day (µsec)
AC LOD BIAS: 21-day mean LOD bias with respect to Bulletin A (us)AC LOD BIAS RMS: RMS of AC LOD BIAS (us)FLAG: "u" (used), "x" (excluded), "-" (no submission)for Xpole, Ypole, Xrt, Yrt and LOD
A star ("*") beside the AC name indicates that AC LOD values were
derived from AC UT1-UTC values.

Table 4.0933.2

GPS week: 0933

Day: 2

MJD: 50777.5
AC LOD
AC LOD
CENT FLAG | Xpole Ypole
Xrt
Yrt
LOD |
BIAS BIAS RMS-----------------------------------------------------------------------coduuuuu |
-30
44
-6
48
-27 |
-14
55
emr* uuuuu |
25
-48
102
-102
12 |
4
29
esa
uuuuu |
-5
16
-90
-41
-9 |
3
61
gfz
uuuuu |
-1
-5
11
13
-9 |
-1
32
jpl
uuuuu |
59
-49
27
71
1 |
7
32
ngs
uuuuu |
-81
-87
-148
-75
-15 |
28
56
sio
uuuuu |
-54
76
-18
68
-29 |
-14
185
usn
uuuuu |
34
-29
21
-70
24 |
12
48

with respect to IERS Bulletin A. For the Final combinations, one week of daily EOP
Tables (Table 7.wwww.d) are generated preceded by a weekly summary Table (Table
6.wwww.d) compiling statistics for the whole week.
2.4

Clock Combinations

Almost from the beginning of the IGS, the AC clock weighting strategy has been basedon the absolute deviation of AC initial clock alignment with respect to the broadcast
satellites without Selective Availability (SA) (3 satellites at the beginning). Now, since
only one non-SA satellite remains (PRN15) this clock weighting strategy has become
unreliable.
Given the good performance of AC clock corrections during 1997, it was decided tochange from the clock combination weighting strategy based on non-SA satellites, to thefollowing one: a chosen reference AC is aligned to the broadcast GPS clocks using all
satellites, then all other ACs are aligned to that reference AC. The clock weights arecomputed from the absolute deviation of the AC aligned clock corrections with respect tothe unweighted mean. This strategy was used whenever the alignment using non-SA
satellites gave inappropriate weights and unrealistic clock RMS. It was also used early in1994 (Kouba et al., 1995) before the non-SA strategy weighting scheme was introduced.

27

IGS 1997 Technical Reports

A strength of this method is that it does not really matter which AC is chosen forreference as long as it has a very high number of relatively good clock corrections and noclock resets. Its biggest advantage is that AC solutions showing poor performance are
greatly down-weighted resulting in more precise and consistent IGS combined clock
corrections without having to explicitly exclude any AC solutions, thus making theprocedure easier and faster to execute. This strategy was reinstated in the Rapid andFinal clock combinations starting Wk 936/day 1 and Wk 935/day 0 respectively.
To further improve the consistency of IGS orbits and clocks, a new alignment correction
to AC clocks, based on the difference in the radial component between AC and IGS
combined orbits, was implemented. A first attempt was made almost 3 years ago but noimprovement was noticed at that time due to the then higher AC clock RMS (~1.0 ns thencompared to ~0.3 ns now). Considering the above two changes, tests using precise point
positioning (Springer et al., 1998) have shown a factor of two improvement over using
the former non-SA satellite weighting method. With the latest implementations in the
IGS satellite clock combination, typical repeatability of 10 mm horizontal and 14 mm
vertical was found for stationary site coordinates (Springer et al., 1998). This strategy
was implemented on Wk 938/day 0 and Wk 940/day 1 (late 1997 and early 1998) for
both the Final and Rapid combinations respectively.
3

Orbit and Clock Evaluations

The Long Arc (LA) orbit evaluation was described in more detail in the IGS 1994 Annual
Report (Kouba et al., 1995) and, therefore will not be described here. Note though, that
LA evaluation is only performed for the Final orbit products that are generated on a
weekly cycle. LA RMS are presented in Figure 29.
Between January 1996 and March 1998, the IGS combined orbits/clocks as well as all
AC solutions which contain both the orbit and clock corrections data, have been further
evaluated by an independent single point positioning program (navigation mode)developed at NRCan (GPSPACE or GPS Positioning from Active Control System (ACS)Clocks and Ephemerides). This was done to verify clock solution precision andorbit/clock consistency for both the Rapid and Final orbit/clock products. Pseudorange
data from three stations (BRUS, USUD and WILL) were used daily and theircorresponding position RMS (with respect to ITRF94) are summarized in the Rapid/Finalsummary reports. Tables 4a and 4b summarize the point positioning results obtained
from both the Rapid and Final orbit/clock products for 1997. Note that the IGR
orbits/clocks are included in the Final IGS summary reports for performance comparison,
i.e. they are not used in the IGS Final combinations. For most ACs, both Rapid and Final
3D Navigation RMS ranged from ~30 cm to ~65 cm overall (depending on the station
processed) with the height component being the least precise.
Starting March 1998, the daily precise navigation statistics found in the IGS reports are
now based on receiver phase data using JPL's GISPY-OASIS II point positioning
capability recently installed at NRCan. Section 6 gives more details.

28

1997 Analysis Coordinator Report

Table 4a. 1997 IGS Rapid Combination Point Positioning RMS using GPSPACE(Pseudorange data - navigation mode) for ACs providing orbit/clock solutions
BRUS
ACs
emr
esa
gfz
igr
jpl
usn

Lat

Lon

47
88

USUD

Ht

3D

Lat

Lon

36

93

63

37

55

126

94

61

52

37

101

69

48

35

91

48

35

48

38

WILL

Ht

3D

Lat

Lon

Ht

3D

29

79

53

27

18

47

33

52

138

92

37

30

70

49

53

32

87

62

31

20

54

38

63

38

29

81

54

27

18

48

33

92

63

37

27

78

52

27

17

46

32

98

67

41

31

89

59

29

19

50

35

Table 4b. 1997 IGS Final Combination Point Positioning RMS using GPSPACE(Pseudorange data - navigation mode) for ACs providing orbit/clock solutions
BRUS
ACs
cod
emr
esa
gfz
igs
jpl

USUD

WILL

Lat

Lon

Ht

3D

Lat

Lon

Ht

3D

Lat

Lon

Ht

3D

49

36

97

66

37

29

79

53

26

17

46

32

50

36

99

67

37

29

79

53

27

18

48

33

57

39

100

70

43

33

89

60

33

23

57

40

51

39

96

67

39

31

82

55

29

21

53

37

48

36

96

65

37

29

80

53

27

17

48

33

49

36

98

66

36

28

79

53

27

16

46

32

Period covered: GPS Wks 886-937 (Dec.29/1996-Dec.27/1997)
Units: centimeters (cm)
RMS ≥ 999 cm were excluded from the RMS computations
4

IGS Orbit, Clock and EOP Combinations by Weighted Average:
Method Description

Table 5 summarizes the Prediction, Rapid and Final combination procedures for the orbit,
clock and EOP products at the end of 1997. Changes that occurred during the year are
listed in Table 1. A more detailed description including the formulas involved in theorbit/clock combinations can be found in the IGS 1994 Annual Report (Kouba et al.,1995). Two different procedures for the EOP combination are used, one for Xpole,
Ypole, Xrt and Yrt (group I) and another for LOD/UT (group II).

29

IGS 1997 Technical Reports

Table 5. Orbit, clock and EOP combination/evaluation procedures at the end of 1997
1. Long Arc Ephemerides Evaluation for each AC:
Final Combination: seven daily satellite ephemerides are used as pseudo-observations in anorbit adjustment program and RMS residuals are examined;
Prediction and Rapid Combinations: none
2. Transformation to Common Reference:(a) Orbit
Prediction, Rapid and Final Combinations: performed directly in the ITRF94 reference
frame without EOP alignment;(b) Clock
Rapid and Final Combinations: are aligned with respect to broadcast GPS clock
corrections. First, a reference AC is aligned to the broadcast GPS clocks using all
satellites and then each AC is aligned to the reference AC.
Prediction Combination: none
3. Orbit and Clock Combinations:
Prediction, Rapid and Final Combinations: AC orbit weights are computed from absolutedeviations with respect to unweighted mean orbits; satellite ephemerides are then combined
as weighted averages of AC solutions.
Rapid and Final Combinations: AC clock weights are computed from the absolute deviationof the AC aligned clock corrections with respect to the unweighted mean;
Satellite clock corrections are then combined as weighted averages of AC solutions.
Prediction Combination: none (CODE’s predicted clock corrections are used for IGP
predicted clock corrections).
4. EOP Combination:
Rapid and Final Combinations: PM x and y and PM rates are combined as weighted averages
from available AC EOP values using orbit weights.
Rapid and Final Combinations: AC LOD alignment is based on comparisons with the IERS
Bulletin A during the three week period ending five days before the last observed Bulletin A
daily value. AC LODs are then combined as weighted averages according to the AC LOD
alignment RMS. IGS LODs are then integrated into IGS UTs.
Prediction Combination: PM x and y, PM rates, LODs and UTs are taken directly from themost current IERS Bulletin A.
5. Long Arc Ephemerides Evaluation for the IGS Combined Orbits:
Final Combination: seven daily satellite ephemerides are used as pseudo-observations in anorbit adjustment program and RMS residuals are examined.
Prediction and Rapid Combinations: none.
6. Independent Point Positioning Evaluation (navigation mode):
Rapid and Final Combinations: all AC solutions which contain orbits and clocks (including
IGS/IGR combinations) are evaluated using the three IGS stations: BRUS, USUD and WILL.
Prediction Combination: none.

30

1997 Analysis Coordinator Report

Group I: ERP (Xpole, Ypole, Xrt, Yrt)
The parameters in the first group are combined as a straightforward weighted mean of
AC (Cent) available values using AC orbit weights (eqn. 1).
Ncent

ERP Comb = ∑ ERPCent ⋅ WCent
Orb

(1)

Cent

Orb
where: WCentis the AC orbit weight (Kouba et al., 1995) and

ERP Comb is the combined IGS value of one of the parameters of group I.

Group II: LOD/UTInitially, each AC LOD series is calibrated with respect to the IERS Bulletin A. This isdone by estimating a 21-day calibration (ncal) bias between IERS Bulletin A and each
AC individually (eqn. 2). AC LOD values from day “Start” (or anchor point) and up to
the day required (nday total) are then properly corrected (eqn. 3) before combining them
to form IGS combined LOD values (eqn. 4). AC LOD weights are based on the bias
calibration RMS from (eqn. 2) and given explicitly in (eqn. 5 and 6). Finally, IGS
combined LODs from (eqn. 4) are integrated into IGS UTs (eqn. 7) from the anchor point
up to the day required. All the equations are given below. Figure 1 illustrates the generalprocedure.
ncal

Bias Cent =∑ (LOD

Bull.A− LODCent )

ical

(2)

ncal

LOD′Cent iday = LODCent iday + Bias Cent
Ncent

LODcomb iday = ∑ WLODCent ⋅ LOD′Cent iday

iday = 1,nday(3)

iday = 1,nday

(4)

Cent

where:
WLOD Cent =1
RMS 2BiasCent

(5)

2

ncal

RMS Bias Cent =∑ (LOD

Bull.A− LODCent )

ical

(6)

ncal

31

IGS 1997 Technical Reports

Finally,iday

UT combiday = UTBull.AStart − ∑ LOD Combi

iday = 1,nday(7)iwhere:“Start”
is 5 days prior to the IERS Bulletin A last observed UT1,
LOD Bull.A is IERS Bulletin A UT1-derived LOD,
LODCent is the AC LOD or AC UT-derived LOD (in the case of EMR),
Bias Cent
LOD′Cent

is the AC LOD calibration bias,
is the corrected AC LOD or AC UT-derived LOD (e.g. EMR),LODComb is the estimated IGS combined LOD,
“ncal”
is the number of calibration days (maximum 21),“nday”
is the number of combination/integration days,
UTBull.AStart is the IERS Bulletin A UT1 starting or anchor value, and,UT Comb
4

is the estimated IGS combined UT.

IGS Prediction, Rapid and Final Combination Results in 1997

In this section, results for the fourth year of IGS service, i.e. December 29, 1996 to
December 27, 1997 (Wks 886-937), are presented.
Tables 6 and 7 show the Prediction and Rapid product statistics of the translation, therotation, and the scale parameters from the daily Helmert transformations with respect tothe IGS Rapid (IGR) orbits. Similarly, Table 8 shows the Final product statistics of thesame parameters but this time with respect to the IGS Final orbits. A complete series ineach table would have 364 days except for EMP (Table 6) which started on Wk 887 / day
5 (352 days), ESP (Table 6) which started on Wk 899 / day 3 (270 days) and USN (Table
7) which started its Rapid contribution on Wk 902 / day 3 (249 days). Note also thatrotations (RX, RY and RZ) greater than 5 mas in Table 7 were excluded from the ACmeans and standard deviations for more meaningful AC overall statistics.
Figures 2-9 (Prediction products) and Figures 10-17 (Rapid products) display, for eachAC, the daily translations, rotations and scales of the X, Y and Z satellite coordinateswith respect to the IGS Rapid orbits (IGR). Broadcast results (Figure 2) are included forcomparison only and do not contribute to the IGS orbit and clock combinations except for
the AC Rapid clock alignment. In Figure 2, each translation and rotation series are offset
by 1.0 m and 20 mas respectively for visibility.

32

1997 Analysis Coordinator Report

Table 6. IGS Prediction Combination - GPS Wks 886-937 (performed directly in the
ITRF94 reference frame); means (µ) and standard deviations (σ) of the daily
Helmert Transformation Parameters
Center

DX

DY
(m)

DZ

RX

RY
(mas)

RZ

SCL
(ppb)

DAYS

brd

µ
σ

0.01
0.12

0.03
0.13

0.02
0.28

0.52
2.69

-0.50
4.44

4.36
6.92

-3.1
2.8

364

cop

µ
σ

0.00
0.02

0.02
0.02

-0.03
0.06

-0.52
0.97

0.11
1.30

0.15
3.32

-0.2
0.5

364

emp

µ
σ

0.00
0.02

0.02
0.02

-0.01
0.04

0.35
1.16

0.08
1.75

-0.04
7.09

-0.1
0.7

346(1)

esp

µ
σ

0.00
0.02

0.02
0.02

-0.01
0.04

0.67
0.98

0.21
1.59

0.66
5.52

0.2
0.5

256(2)

gfp

µ
σ

0.00
0.01

0.02
0.01

0.01
0.03

0.42
1.78

0.25
1.63

-0.45
2.47

-0.3
0.6

342

jpp

µ
σ

0.00
0.01

0.02
0.01

0.00
0.04

0.07
2.10

-0.44
2.65

-0.13
2.66

0.2
0.7

363

sip

µ
σ

0.00
0.03

0.02
0.03

0.00
0.07

-0.44
1.26

0.05
1.26

-2.21
8.20

-0.1
1.2

337

igp

µ
σ

0.00
0.01

0.02
0.01

0.00
0.03

0.02
0.92

0.00
1.13

-0.25
2.57

-0.1
0.4

364

(1): emp submissions started only on Wk 887 day 5 (Maximum 352 days)(2): esp submissions started only on Wk 899 day 3 (Maximum 270 days)

In Figures 3-9, each translation and rotation series are offset by 0.2 m and 10 mas
respectively. Finally, in Figures 10-17, the translation series are offset by 0.1 m and eachrotation/pole difference series are offset by 2 mas respectively.
Figures 18-25 show the Final results for the same daily transformation parameters but
this time with respect to the Final IGS orbits. IGR results (Figure 25) are included forcomparison only and do not contribute to the IGS orbit and clock combinations. Again
for visibility, the same offsets, i.e. 0.1 m for the translation series and 10 mas for therotation/pole difference series were used. Figures 10-25 (middle plots) display, in
addition to the rotations in X, Y and Z, the PM differences with respect to IGR/IGS. This
was added to monitor AC orbit/EOP consistency and performance. PM differences in y/x
should correspond to orbital X/Y rotations respectively. A perfect correlation would betranslated as a 1.0 value in Figure 26, which shows the correlation coefficient of each AC
X/Y rotations versus AC PM differences in y/x for the same period as shown in Figures
10-25.

33

IGS 1997 Technical Reports

Table 7. IGS Rapid Combination - GPS Wks 886-937 (performed directly in the ITRF94
reference frame); means (µ) and standard deviations (σ) of the daily Helmert
Transformation Parameters
Center

DX

DY
(m)

DZ

RX

RY
(mas)

RZ

SCL
(ppb)

DAYS

cod

µ
σ

0.00
0.01

0.02
0.01

-0.01
0.01

-0.59
0.35

0.11
0.29

0.09
0.29

-0.2
0.2

363

emr

µ
σ

0.00
0.01

-0.01
0.02

0.00
0.01

0.36
0.47

-0.02
0.39

0.12
0.44

-0.1
0.2

325

esa

µ
σ

0.00
0.01

0.01
0.02

0.00
0.01

0.03
0.35

-0.20
0.38

0.19
0.38

0.2
0.2

334

gfz

µ
σ

0.00
0.01

-0.03
0.01

0.01
0.01

0.23
0.44

0.10
0.35

-0.33
0.44

-0.1
0.2

322

jpl

µ
σ

0.01
0.01

0.02
0.02

0.00
0.01

-0.06
0.35

-0.14
0.28

0.06
0.31

0.1
0.2

283

ngs

µ
σ

0.00
0.02

-0.04
0.02

-0.01
0.02

0.22
0.58

0.14
0.50

-0.06
0.47

-0.2
0.3

315

sio

µ
σ

0.00
0.01

0.02
0.02

0.00
0.02

-0.34
0.67

0.02
0.40

-0.44
1.90

0.2
0.5

311

usn

µ
σ

0.00
0.02

-0.03
0.04

0.00
0.02

0.34
0.48

0.00
0.43

0.22
0.37

0.2
0.2

219(1)(1): usn submissions started on Wk 902 day 3 (Maximum 249 days)
Note also that Rotations (RX,RY,RZ) greater than 5.0 mas were notincluded in AC means and standard deviations for more meaningful annual
statistics. Number of outliers: esa:2; gfz:1; jpl:3; sio:22; usn:4.

The AC Rapid correlations show a slightly more consistent orbit/EOP series than the
Final. However, in almost all cases, the consistency improved between the first andsecond half of 1997.
Figure 27 shows the orbit coordinate RMS of all AC Prediction submissions with respect
to the IGS Rapid (IGR) combinations. Two types of RMS are displayed: the combination
RMS median (i.e. the median of all AC satellite combination RMS) and the weighted
combination RMS (WRMS). Similarly, Figure 28 shows orbit coordinate RMS of all AC
Rapid submissions with respect to the IGR combinations. Finally, Figure 29 shows the
AC Final submission orbit RMS results where the combination RMS median was
replaced by the 7-day Long Arc RMS. Figures 30-31 show the AC Prediction and Rapid
clock RMS respectively with respect to IGR and Figure 32 displays AC Final clock RMS
with respect to the IGS Final clocks.

34

1997 Analysis Coordinator Report

Table 8. IGS Final Combination - GPS Wks 886-937 (performed directly in the ITRF94
reference frame); means (µ) and standard deviations (σ) of the daily Helmert
Transformation Parameters
Center

DX

DY
(m)

DZ

RX

RY
(mas)

RZ

SCL
(ppb)

DAYS

cod

µ
σ

0.00
0.00

0.01
0.01

-0.01
0.01

-0.27
0.18

0.11
0.13

0.22
0.22

-0.2
0.1

364

emr

µ
σ

0.00
0.01

-0.01
0.01

-0.01
0.01

0.38
0.21

-0.07
0.17

0.43
0.38

-0.1
0.2

357

esa

µ
σ

0.00
0.01

0.00
0.01

0.00
0.01

0.05
0.23

-0.13
0.20

0.15
0.21

0.1
0.1

364

gfz

µ
σ

0.00
0.00

0.00
0.01

0.01
0.01

0.11
0.16

0.06
0.13

-0.21
0.20

-0.1
0.1

364

jpl

µ
σ

0.00
0.01

0.01
0.01

0.00
0.01

-0.04
0.16

-0.12
0.10

0.02
0.12

0.3
0.1

357

ngs

µ
σ

0.00
0.01

-0.05
0.01

0.00
0.02

0.20
0.45

0.25
0.30

-0.13
0.36

-0.1
0.2

364

sio

µ
σ

0.00
0.01

0.01
0.01

0.00
0.01

-0.01
0.31

0.17
0.66

-0.36
0.30

0.2
0.4

336

igr

µ
σ

0.00
0.01

-0.01
0.01

0.00
0.01

0.10
0.23

0.10
0.21

-0.03
0.25

0.0
0.1

364

The predicted broadcast clocks (IGP) are provided by COD (COP). BRD and IGRclocks/orbits are always used for comparison purposes only. Note that COD (Rapid) andNGS (Rapid and Final) clocks correspond to broadcast clocks as provided by the satellite
navigation message (therefore not included in the clock combination) and that SIO(Rapid and Final) does not provide any clock corrections. As mentioned in previous
Annual Reports, erroneous satellite orbit and clock solutions are excluded from the
combination if they bias the IGS combined solution but are always included in the RMS
computation.
All exclusions are reported in the IGS weekly/daily combination reports. IGS predicted
clocks (IGP) are at the same precision level as the broadcast clocks but with more
outliers. IGS Rapid and Final clock results have reached the 0.2-0.3 ns precision levelwith the Final combination results being slightly more consistent. The predicted orbit
precision (IGP) has reached the ~50 cm RMS median level which is considerably better
than the ~200 cm for the broadcast orbits. As expected, predicted orbits have occasional
outliers caused by unpredictable satellite events. Rapid and Final orbit position precision
are below the 10 cm and at, or below, the 5 cm level respectively. Again, the Rapid
results are somewhat noisier than the Final results due mostly to the very short delivery
time which causes occasional lack of tracking data (submission deadline of 21 hours).

35

IGS 1997 Technical Reports

41998 and Future Improvement

By the time this report is published, at least two new major enhancements will havealready been implemented. Starting March 1, 1998 (Wk 947), the new ITRF96 reference
system will be used for all IGS products. Also in March 1998, (Wks 948 and 950 for IGS
Final and IGS Rapid respectively), a new precise point navigation program utilizing
phase instead of pseudorange data, will have been implemented. A brief summary was
already given in the Volume I of this year’s Annual Report (Kouba, 1998) and a morethorough analysis will be presented in next year’s Annual Report. Figure 33 shows anexample of IGR and IGS performance using this point positioning approach. When
fixing IGS orbits/clocks, navigation positioning precision of only a few cm is now
possible anywhere in the world without the need for any base station data. Current IGS
sp3 clock sampling of 15 minutes and SA limit navigation to 15 min intervals only. For
higher sampling, more frequent IGS clocks are needed to allow precise interpolation of
the SA clock effects.
5

Summary

1997 was again a very busy year for both the ACs and the IGS AC coordinator. ACs
have again improved the reliability and precision of their products or at least maintained
the quality level already reached in the past, which in itself is quite an accomplishment.
By the end of 1997, the best AC orbit solutions were at or below the 5 cm level for the
Final solutions and between 5-10 cm for the Rapid solutions. Prediction orbit precision, a
new product made available in 1997, has a RMS median of ~50 cm or lower compared to~200 cm for the broadcast orbit. Better AC clock corrections along with improved clockcombination strategies resulted in combined clock correction precision reaching an
unprecedented 0.2–0.3 ns level (and as precise as 0.1 ns on some occasions!) for both the
Rapid and Final solutions. The IGS LOD combination and integration into IGS UT werealso introduced in 1997. Several combination strategies were tested and implemented
throughout the year resulting each time in a more precise and consistent IGS LOD/UT
series when compared to IERS Bulletin A. A new EOP comparison table to monitor AC
EOP performance was also added to the IGR/IGS summary reports. Finally, AC Rapid
combination submission deadline was advanced by 2 hours, i.e. from 24 to 22 hours
UTC, allowing ACs to use IGR in their orbit predictions.
6

References

Kouba J., 1998, 1997 Analysis Coordinator Report, in International GPS Service for
Geodynamics, 1997 Annual Report, Volume I, Jet Propulsion Laboratory, Pasadena, in
preparation.
Kouba J. and Y. Mireault, 1997, 1996 Analysis Coordinator Report, in International GPS
Service for Geodynamics, 1996 Annual Report, Jet Propulsion Laboratory, Pasadena,
California, pp. 55-100.

36

1997 Analysis Coordinator Report

Kouba J., Mireault Y. and F. Lahaye, 1995, Appendix to the Analysis Coordinator
Report, in International GPS Service for Geodynamics, 1994 Annual Report, Jet
Propulsion Laboratory, Pasadena, California, pp. 70-94.
Mireault Y., Kouba J. and J. R. Ray, 1997, Use of GPS for Near Real Time Estimates of
UT1, EOS Trans. AGU, 78(17), Spring Meeting Suppl., S109, 1997.
Ray J.R., 1996, Measurements of Length of Day using the Global Positioning System,
Journal of Geophysical Research, Vol. 102, No. B9, pp. 20141-20149.
Springer T.A. , Zumberge J.F. and J. Kouba, 1998, The IGS Analysis Products and theConsistency of the Combined Solutions, 1998 AC Workshop Proceeding (in preparation),
Darmstadt, Germany, Feb. 9-11.

37

IGS 1997 Technical Reports

BRD vs IGR
3

2

1

0

X Y Z Translations (m)
-1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

BRD vs IGR
60

40

20

0

X Y Z Rotations (mas)
-20
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

BRD vs IGR
6
3
0
-3
-6
-9 (ppb)
Scale
-12
-15
886

890

894

898

902

906

910

914

918

Scale

Figure 2. BRD 1997: Daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations are each offset by
20 mas)38

1997 Analysis Coordinator Report

COP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

COP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

COP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 3. COP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)

39

IGS 1997 Technical Reports

EMP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

EMP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

EMP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 4. EMP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)40

1997 Analysis Coordinator Report

ESP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

ESP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

ESP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 5. ESP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)

41

IGS 1997 Technical Reports

GFP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

GFP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

GFP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 6. GFP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)42

1997 Analysis Coordinator Report

JPP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

JPP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

JPP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 7. JPP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)

43

IGS 1997 Technical Reports

SIP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

SIP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

SIP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 8. SIP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)

44

1997 Analysis Coordinator Report

IGP vs IGR
0.6

0.4

0.2

0.0

X Y Z Translations (m)
-0.2
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

922

926

930

934

922

926

930

934

Z Trans

IGP vs IGR
40

30

20
10

0

X Y Z Rotations (mas)
-10
886

890

894

898

902

906

X Rot

910

914

Y Rot

918

Z Rot

IGP vs IGR
3
2
1
0
-1

Scale (ppb)
-2
-3
886

890

894

898

902

906

910

914

918

Scale

Figure 9. IGP 1997: Prediction daily seven-parameter Helmert (X, Y and Z
Translations are each offset by 1 metre; X, Y and Z Rotations areeach offset by 10 mas)

45

IGS 1997 Technical Reports

COD vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

COD vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

COD vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 10. COD 1997: Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

46

1997 Analysis Coordinator Report

EMR vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

EMR vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

EMR vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 11. EMR 1997: Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

47

IGS 1997 Technical Reports

ESA vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

ESA vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

ESA vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 12. ESA 1997: Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)48

1997 Analysis Coordinator Report

GFZ vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

GFZ vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

GFZ vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 13. GFZ 1997: Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

49

IGS 1997 Technical Reports

JPL vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

JPL vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

JPL vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 14. JPL 1997:

Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)50

1997 Analysis Coordinator Report

NGS vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

NGS vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

NGS vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 15. NGS 1997: Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

51

IGS 1997 Technical Reports

SIO vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

SIO vs IGR
12
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

SIO vs IGR
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 16. SIO 1997:

Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)52

1997 Analysis Coordinator Report

USN vs IGR
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

USN vs IGR
10
8
6
4
2
0

X Y -2Z Rotations and PM diff. (mas)
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

USN vs IGR
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 17. USN 1997: Rapid daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)53

IGS 1997 Technical Reports

COD vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

COD vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

COD vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 18. COD 1997: Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)54

1997 Analysis Coordinator Report

EMR vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

EMR vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

EMR vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 19. EMR 1997: Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)55

IGS 1997 Technical Reports

ESA vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

ESA vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

ESA vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 20. ESA 1997: Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

56

1997 Analysis Coordinator Report

GFZ vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

GFZ vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

GFZ vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 21. GFZ 1997: Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)57

IGS 1997 Technical Reports

JPL vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

JPL vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

JPL vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 22. JPL 1997:

Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

58

1997 Analysis Coordinator Report

NGS vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

918

Y Trans

922

926

930

934

926

930

934

930

934

Z Trans

NGS vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

NGS vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 23. NGS 1997: Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

59

IGS 1997 Technical Reports

SIO vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

SIO vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

SIO vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 24. SIO 1997:

Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)60

1997 Analysis Coordinator Report

IGR vs IGS
0.3

0.2

0.1

0.0

X Y Z Translations (m)
-0.1
886

890

894

898

902

906

X Trans

910

914

Y Trans

918

922

926

930

934

926

930

934

930

934

Z Trans

IGR vs IGS
10
8
6
4
2
0
-2
886

890

894

898

X Rot

902

906

Ypole

910

914

Y Rot

918

Xpole

922

Z Rot

IGR vs IGS
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
886

890

894

898

902

906

910

914

918

922

926

Scale

Figure 25. IGR 1997: Final daily seven-parameter Helmert transformations (X, Y and Z
Translations are each offset by 0.1 metre; X, Y, Z Rotations and Y and X
pole differences are each offset by 2 mas)

61

IGS 1997 Technical Reports

Correlation between
Rapid X Rotations and Rapid Ypole Differences
1.0

0.8

0.6

0.4

0.2

0.0

cod

emr

esa

gfz

1st half of 1997

jpl

ngs

2nd half of 1997

sio

usn

1997

Correlation between
Rapid Y Rotations and Rapid Xpole Differences
1.0

0.8

0.6

0.4

0.2

0.0

cod

emr

esa

gfz

1st half of 1997

jpl

ngs

2nd half of 1997

sio

usn

1997

Correlation between
Final X Rotations and Final Ypole Differences
1.0

0.8

0.6

0.4

0.2

0.0

cod

emr

esa

gfz

1st half of 1997

igr

jpl

2nd half of 1997

ngs

sio

1997

Correlation between
Final Y Rotations and Final Xpole Differences
1.0

0.8

0.6

0.4

0.2

0.0

cod

emr

esa

gfz

1st half of 1997

igr

jpl

2nd half of 1997

ngs

sio

1997

Figure 26. 1997 correlation coefficients between AC X/Y rotations and AC PM differencesin y/x with respect to IGR/IGS.

62

1997 Analysis Coordinator Report

BRD vs IGR

COP vs IGR

800

400

700

350

600

300

500

250

400

200

300

150

200

100

100

50

Orbit Position RMS (cm)
0
886

890

894

898

Orbit Position RMS (cm)
902

906

910

914

Comb. RMS Median

918

922

926

930

0
886

934

890

894

898

Comb. WRMS

902

EMP vs IGR

350

350

300

300

250

250

200

200

150

150

100

100

50

918

922

926

930

934

930

934

930

934

930

934

Comb. WRMS

50

Orbit Position RMS (cm)
890

894

898

Orbit Position RMS (cm)
902

906

910

914

Comb. RMS Median

918

922

926

930

0
886

934

890

894

898

Comb. WRMS

902

906

910

914

Comb. RMS Median

GFP vs IGR

918

922

926

Comb. WRMS

JPP vs IGR

400

400

350

350

300

300

250

250

200

200

150

150

100

100

50

50

Orbit Position RMS (cm)
890

894

898

Orbit Position RMS (cm)
902

906

910

914

Comb. RMS Median

918

922

926

930

0
886

934

890

894

898

Comb. WRMS

902

906

910

914

Comb. RMS Median

SIP vs IGR

918

922

926

Comb. WRMS

IGP vs IGR

400

400

350

350

300

300

250

250

200

200

150

150

100

100

50

50

Orbit Position RMS (cm)
0
886

914

ESP vs IGR
400

0
886

910

Comb. RMS Median

400

0
886

906

890

894

898

Orbit Position RMS (cm)
902

906

910

914

Comb. RMS Median

918

922

926

930

0
886

934

Comb. WRMS

890

894

898

902

906

910

914

Comb. RMS MedianFigure 27. 1997 Prediction Daily orbit position RMS

63

918

922

926

Comb. WRMS

IGS 1997 Technical Reports

COD vs IGR

EMR vs IGR

40

40

35

35

30

30

25

25

20

20

15

15

10

10

5

5

Orbit Position RMS (cm)
0
886

890

894

898

Orbit Position RMS (cm)
902

906

910

Comb. WRMS

914

918

922

926

930

0
886

934

890

894

898

Comb. RMS Median902

ESA vs IGR

35

35

30

30

25

25

20

20

15

15

10

10

5

918

922

926

930

934

930

934

930

934

930

934

Comb. RMS Median5

Orbit Position RMS (cm)
890

894

898

Orbit Position RMS (cm)
902

906

910

Comb. WRMS

914

918

922

926

930

0
886

934

890

894

898

Comb. RMS Median

902

906

910

Comb. WRMS

JPL vs IGR

914

918

922

926

Comb. RMS MedianNGS vs IGR

40

40

35

35

30

30

25

25

20

20

15

15

10

10

5

5

Orbit Position RMS (cm)
890

894

898

Orbit Position RMS (cm)
902

906

910

Comb. WRMS

914

918

922

926

930

0
886

934

890

894

898

Comb. RMS Median

902

906

910

Comb. WRMS

SIO vs IGR

914

918

922

926

Comb. RMS Median

USN vs IGR

40

40

35

35

30

30

25

25

20

20

15

15

10

10

5

5

Orbit Position RMS (cm)
0
886

914

GFZ vs IGR
40

0
886

910

Comb. WRMS

40

0
886

906

890

894

898

Orbit Position RMS (cm)
902

906

910

Comb. WRMS

914

918

922

926

930

0
886

934

Comb. RMS Median

890

894

898

902

906

910

Comb. WRMS

Figure 28. 1997 Rapid Daily orbit position RMS

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1997 Analysis Coordinator Report

COD vs IGS

EMR vs IGS

30

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JPL vs IGS

914

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NGS vs IGS

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Figure 29. 1997 Final Daily orbit position RMS

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IGS 1997 Technical Reports

BRD vs IGR
200

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Figure 30. 1997 Prediction Daily clock RMS

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1997 Analysis Coordinator Report

COD vs IGR

EMR vs IGR
3

120
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20

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Figure 31. 1997 Rapid Daily clock RMS

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COD vs IGS

EMR vs IGS

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NGS vs IGS

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Figure 32. 1997 Final Daily clock RMS

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Rapid IGS (IGR)
30

25
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3D RMS (cm)
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25
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5

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954

WILL

Figure 33. Precise Point Positioning (Navigation) using phase data and JPL’s GIPSY-OASIS II
software. The top graphic shows the 3D-RMS for the Rapid IGS (IGR) while thebottom one shows the 3D-RMS for the Final IGS.

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70Analysis Centers

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72

1997 Annual Report - Code Analysis Center of the IGS

1997 Annual Report - Code Analysis Center of the IGS
M. Rothacher, T.A. Springer, S. Schaer, G. Beutler, D. Ineichen
Astronomical Institute, University of Berne

U. Wild, A. Wiget
Federal Office of Topography, Wabern

C. Boucher, S. Botton
Institut Géographique National, Paris

H. Seeger
Bundesamt für Kartographie und Geodäsie, Frankfurt

1

Introduction

CODE, the Center for Orbit Determination in Europe, is a joint venture of thefollowing institutions:••••

the Federal Office of Topography (L+T), Wabern, Switzerland,
the Federal Agency of Cartography and Geodesy (BKG), Frankfurt, Germany,
the Institut Géographique National (IGN), Paris, France, and
the Astronomical Institute of the University of Berne (AIUB), Berne,
Switzerland.

CODE is located at the AIUB. All solutions and results are produced with thelatest version of the Bernese GPS Software [Rothacher and Mervart, 1996].
This report covers the time period from May 1997 to June 1998. It focuses on the
major changes in the routine processing during this period and shows the newdevelopments and products generated at CODE. The processing strategies used till April
1997 are described in the annual reports of previous years [Rothacher et al., 1995, 1996a,
and 1997a].
Figure 1 shows the number of global IGS stations and the number of doubledifference phase observations processed at CODE for each day in the time interval fromJanuary 1997 to June 1998. The number of stations increased from about 80 to 100. An
upper limit of 100 for the number of sites to be processed has been set in May 1998. If
there is data available from more than 100 sites, the sites with long data gaps are removed
first and then sites are selected according to their importance and data quality. The
number of observations shows a jump in October 1997 (day 278), where the elevation
cut-off angle for the global data processing was changed from 20 to 10 degrees (see next
sections for more details).

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IGS 1997 Technical Reports

Figure 1. Statistics of the global 1-day solutions computed at CODE
Due to this processing change and due to the increase in the number of sites withtime the number of observations has roughly doubled from January 1997 to June 1998.
The significant decrease in the number of stations and observations in February 1998 wascaused by a computer problem at one of the operational centers and clearly shows thatbackup components are needed for such cases.
2

Changes in the Routine Processing

The major changes implemented in the CODE routine analysis since May 1997
are listed in Table 1. Modifications prior to this date have already been reported in theannual report of last year [Rothacher et al., 1996a].
3

Product Quality and Results

3.1

Change of Elevation Cut-Off Angle

The most significant changes in the last year are related to lowering the elevation
cut-off angle from 20 to 10 degrees. Since April 1997, CODE has tested several
processing strategies using the data of the permanent European network. The cluster ofabout 40 stations has been processed over many months using eight slightly different
processing schemes. The eight solutions differ in the elevation cut-off angle, thetropospheric modeling, and the observation weighting model. More details about thesepro-cessing strategies may be found in [Rothacher et al., 1997b; Springer et al., 1997].

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1997 Annual Report - Code Analysis Center of the IGS

Table 1. Modification of processing scheme at the CODE Analysis Center fromMay 1997 to June 1998.
Date
23-Sep-97

Doy/Year
266/9705-Oct-97

278/9705-Oct-97

278/97

19-Oct-97

292/9726-Jan-98

026/9801-Mar-98

066/9801-Mar-98

066/9801-Apr-98

088/9806-Jun-98

155/9811-Jun-98

160/9817-Jun-98

168/98Description of Change at CODE
Generation of IONEX files containing daily global andEuropean ionosphere maps.
Major changes of global solutions: elevation cut-off angle set
to 10 degrees (previously 20 degrees), Niell dry mapping
function for troposphere delays, elevation-dependent
weighting of the observations.
Solid Earth tides according to the IERS Conventions 1996.
Polar tides were also included.
Troposphere gradient parameters estimated in global 1-day
solution for test purposes.
Maximum degree of spherical harmonics for ionospheremodels increased from 8 to 12. A global solution with station-specific ionosphere models using smoothed code observations was set up.
Switch from ITRF94 to ITRF96. Reference frame definedby 37 sites (of the set of 42 selected by the IGS).
Ocean loading model according to [Scherneck, 1991] implemented (ocean tide maps from [Le Provost et al., 1994];
see also [McCarthy, 1996]).
2-hour (instead of 24-hour) time resolution for global ionosphere maps. Use of a solar-geomagnetic reference frame forionospheric modeling.
In addition to GPS, GLONASS orbit predictions are now
routinely produced for the SLR community.
1-day and 2-day ionosphere map predictions made available
on anonymous ftp at CODE.
First set of global IONEX files sent to CDDIS.

The results of the different European processing strategies clearly indicate thatlowering the elevation cut-off angle significantly improves the internal consistency of thestation coordinate estimates. This is mainly caused by the better decorrelation of thestation heights and the tropospheric zenith path delay parameters (see e.g. [Rothacher
and Beutler, 1997]). It was also found that it is important to account for the increased
noise of the low-elevation data by using an elevation-dependent weighting of theobservations. Furthermore, a well per-forming tropospheric mapping function, e.g., theNiell mapping function [Niell, 1993], has to be used.
Based on these European results it was decided to decrease the elevation cut-off
angle also for all global solutions. There the cut-off had always been set to 20 degrees

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IGS 1997 Technical Reports

since the beginning of the IGS in June 1992. A small test series, based on 5 days, wasgenerated using similar strategies as for our European network to verify if the same
improvements may be seen in the global solutions. The results of the different tests are
summarized in Table 2.
Table 2. Results based on global solutions from 5 days using different processing
strategies.
Elev.
Cut-Off
(deg)
20
15
15
10
10
5
5
5
5

Mapp.
Func.

Elev.
Weight.

Saast.
Saast.
Niell
Saast.
Niell
Saast.
Niell
Saast.
Niell

No
No
Yes
Yes
Yes
No
No
Yes
Yes

Coord.
Repeat.
(mm)
3.10
3.10
3.06
2.95
2.92
3.52
3.28
3.00
2.96

RMS
(mm)
--10.3
6.5
10.5
5.4
29.3
14.4
14.9
9.5

Relative to 20° solution
Scale
#Obs.
#Param.
(ppb)
Inc.(%)
Inc.(%)-------0.7
12
30
-0.1
12
30
-0.3
23
63
0.9
23
63
0.1
26
75
0.5
26
75
0.0
26
75
0.9
26
75

Several interesting results can be observed here. First of all, the results clearly deteriorate
if the cut-off angle is decreased without proper weighting of the measurements. The
results also reveal that the Saastamoinen mapping function [Saastamoinen, 1973] is notadequate for low-elevation data. Secondly, the amount of data gained is strikingly large:
almost a 25% increase can be seen when lowering the elevation cut-off from 20° to 10°.
Unfortunately, the amount of ambiguity parameters also increases significantly (60-70%),
which reflects the higher noise level of the low-elevation data which makes the data
cleaning more difficult. Nevertheless, the degree of freedom of the solutions increases
significantly (21% when going from 20° to 10°). Finally, significant terrestrial scalechanges are observed which are depending on the elevation cut-off angle, the weighting of
the observations, and the tropospheric mapping function. These scale changes are
surprisingly large considering the fact that 9 stations were held fixed to their ITRF94
positions in these tests. As a result of these tests, the solution using a cut-off of 10°, withelevation-dependent weights w = sin 2 e , and using Niell's (dry) mapping function waschosen for our global IGS solutions. The 5° solution was rejected mainly due to the factthat the IGS set of antenna phase center variations (IGS_01) is only valid down to 10°[Rothacher et al., 1996b; Rothacher, 1996]. Additional information on the change of theelevation cut-off angle may be found in IGSMAIL #1705 and IGSREPORT #4247. Notethat the change of approximately 1 ppb in the scale observed in these tests is still visible
in our current official solutions. In a free network solution, the scale change is even more76

1997 Annual Report - Code Analysis Center of the IGS

pronounced (2-3 ppb), indicating once more that the scale defined by GPS is not very
reliable and heavily depends on the processing strategy and modeling.
3.2

Change of Terrestrial Reference FrameOn March 1, 1998 (GPS week 0947), the IGS changed its realization of theterrestrial reference frame by switching from ITRF94 to ITRF96. At the same time, theset of the 13 “fixed” reference stations was change. A completely new and much larger setof reference stations was selected because the original set of 13 stations was no longer
adequate to accurately realize the terrestrial reference frame for the IGS products. From
this newly selected set of 48 reference stations CODE selected 36 stations. One morestation (REYK) was added to this list. The ITRF96 positions of these 37 stations areconstrained to 1 mm in our official solutions.
Although ITRF94 and ITRF96 nominally have the same orientation and origin,some small effects of the reference frame change were observed in the IGS products,
namely, a small rotation around the Earth's Z-axis of about 0.3-0.4 mas and a small change
in the Y-coordinate of the pole of about 0.2 mas. More information about these changes
may be found in IGSMAIL #1829 and #1838 and IGSREPORT #4698. The change to
ITRF96 and the use of the much larger set of reference stations significantly improved theCODE products, in particular the earth rotation parameters ERPs). The precision of the
ERP estimates are now below the 0.1-mas level (see IGSMAIL #1853 for details).
3.3

The European Network Solution

Besides being one of seven IGS Analysis Centers, CODE also plays an essential
role in the maintenance and densification of the European Reference Frame (EUREF).
Within the framework of EUREF, CODE participates as one of currently ten Associate
Analysis Centers (AACs) and is responsible for the combination of the individual AAC
results into an official combined EUREF solution. Each of the EUREF AACs processes acertain subset of available permanent GPS sites in Europe. The main goal of processing
the European network, apart from participating as an EUREF Analysis Center, is to
study new processing techniques. Eight types of solutions, each using slightly different
processing options, are currently generated each day. Two additional solutions are set up
to compute regional ionosphere maps and to monitor the ionospheric activity over
Europe.Table 3 shows the internal consistency of the eight different solutions and gives ashort description of the basic differences between the solutions. A significant
improvement may be seen, too, when tropospheric gradient parameters are estimated, but
only for those sites which actually track satellites at low elevations. This is not evident
fromBecause the repeatabilities listed there are dominated by a few “bad” stations,which provide little or no low elevation data. All stations with low elevation data show ahighly significant improvement (up to a factor of 2), if gradients are estimated. The
improvement is mainly in the horizontal component but also the height repeatability is

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IGS 1997 Technical Reports

slightly better. For more details we refer to [Rothacher et al., 1997b; Springer et al.,1997].
Table 3. Overall repeatability of the daily European solutions at CODE basedon days 060-157 of 1998. Repeatabilities are given in millimeters.
Sol.
ID.

Amb.
Fixed

Ele. Mapp.
Cut. Func.

Elev.
Wgt.

Repeatability
N
E
U

Remarks

EG_
EQB
NMF
NMW
EQ_
ET_
NM5
NMG

No
Yes
Yes
Yes
Yes
Yes
Yes
Yes

15
15
15
15
10
10
5
5

No
No
No
Yes
Yes
Yes
Yes
Yes

2.1
1.9
1.9
1.7
1.8
1.7
1.8
1.7

Ambig. free
Ambig. fixed
Niell mapping
Elev.dep.weighting
Cut-off angle 10°
Global trop. intro.
Cut-off 5°
Tropo. gradients

3.4

Saast.
Saast.
Niell
Niell
Niell
Niell
Niell
Niell

2.6
1.9
1.9
1.7
1.7
1.7
1.8
1.7

5.7
5.6
5.8
5.5
4.9
4.5
4.8
4.8

The CODE Solar Radiation Pressure Model

The largest error source in GPS orbit modeling is the impact of solar radiationpressure. Over the last few years many improvements have been made in modeling theorbits of GPS satellites within the IGS. However, most improvements were achieved byincreasing the number of estimated orbit and/or solar radiation pressure (RPR)
parameters. This increase in the number of estimated satellite parameters weakens thesolutions of all estimated parameters. Due to correlations, the additional parameters cancause biases in other estimated quantities like, e.g., the length of day.
A new radiation pressure model was derived by fitting 5-day arcs through all
CODE final orbits since 1996. By analyzing the resulting time series of RPR parameters,a model for each of the five estimated parameters was computed. The quality of themodel was tested by performing a 7-day fit using this new model and estimating only two
RPR parameters: a scale term and the y-bias. Using the ROCK4/42 models the RMS ofthis 7-day fit was around 75 cm whereas with the new CODE model an RMS of only 6
cm resulted, an improvement by almost an order of magnitude. The new model moreover
allows a reduction of the number of orbit parameters that have to be estimated. The
CODE model was presented at the 1997 AGU Fall meeting and at the IGS 1998
Workshop in Darmstadt. More information may be found in [Springer et al., 1998a,
1998b] and in IGSMAIL #1842.

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3.5

Earth Rotation Parameters

In April 1994, CODE started to estimate nutation rate corrections in longitude andobliquity relative to the IAU 1980 theory of nutation. The series of nutation rateestimates covers by now a time interval of more than 4 years. A detailed analysis of thisseries by [Rothacher et al., 1998] has shown, that GPS may contribute to the highfrequency part of the nutation spectrum, i.e., for periods below about 20 days. The
nutation amplitudes estimated in this range of periods are comparable to the best VLBI
results.
The series of 2-hourly ERP estimates, started in January 1995, is another uniqueproduct of CODE. The uninterrupted series of 3.5 years of sub-daily ERPs may be used
to estimate diurnal and semi-diurnal ocean tide amplitudes. The GPS results derived fromthis series are of equal quality as the best ocean tide models obtained from altimeter data,from VLBI, and from SLR. A thorough discussion of the high-frequency ERP results from
CODE may be found in [Rothacher, 1998].
3.6Time and Frequency Transfer with GPS

In 1991 a common project of the Swiss Federal Office of Metrology (OFMET)
and CODE/AIUB was initiated to develop time transfer terminals based on geodetic GPS
receivers. The goal is the comparison of time offsets with sub-nanosecond accuracy andfrequencies with an accuracy of 10 −15 over one day for two or more (GPS-external) clocks.
The OFMET is amongst others responsible for time and frequency maintenance anddissemination in Switzerland. Within this field of activities, time and frequency transfer
over a wide range of distances using many different methods (among other TV methods,
GPS common view techniques, etc.) are of primary interest.
The software used for this project is the Bernese GPS Software. It was originallya pure “double-difference” software package. For the time transfer project it wasessential to modify the software to allow for zero-difference (undifferenced) and singledifference processing. A first step was made in September 1995, enabling zero-difference
processing using code observations. In January 1997, the capability to process
undifferenced phase data was built into the software.
It was clear from the start of the project, that optimum use should be made of the
GPS code and phase measurements and that only geodetic GPS equipment should be
used. The emphasis was put on the comparison of external clocks, as opposed toreceiver-internal clocks. Calibration of delays in cables, temperature-dependent delays,etc., were and are of vital interest in the context of the joint OFMET/AIUB project (seeFigure 2). Let us mention at this point that the control of these delays is absolutely
mandatory for GPS-based time transfer. The corresponding requirements are much lessstringent for frequency transfer.
Today two prototype Geodetic Time Transfer terminals (GeTT terminals) areavailable and a third will be ready in the near future. The terminals contain modified

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IGS 1997 Technical Reports

Ashtech Z-12 receivers. More information about the time transfer project and the GeTT
terminals may be found in [Schildknecht et al., 1990; Overney et al., 1998].
CODE will participate, in collaboration with the OFMET, in the IGS/BIPM timetransfer project. After two experiments on European baselines in 1997 (OFMET-NPL,
PTB-NPL), the GeTT terminals will be deployed on a transatlantic baseline during thesecond half of 1998. This will in fact be the first comparison of the GeTT method with
the independent two-way satellite technique (TWSTFT) on an intercontinental baseline.

Figure 2. Temperature dependence of the GPS receiver delays
for P1 code and L1 phase measurements during one
day.
3.7

Troposphere Gradients

As mentioned in a previous section, tropospheric gradients have been estimated in
the European solutions of CODE since April 1997 [Rothacher et al., 1997b]. In October
1997, a test solution with the estimation of daily troposphere gradients was activated in
the global CODE processing. Figure 3 shows, as an example, the troposphere gradientparameters (excess path delay at 10 degrees elevation angle) of the site Onsala for about
500 days.

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1997 Annual Report - Code Analysis Center of the IGS

Figure 3. Troposphere gradients for Onsala at 10 degrees elevation.
We recognize a seasonal signature and an offset in both components, especially inthe north. Most sites located on the northern hemisphere exhibit, on the average,
significantly larger delays towards the south than the north and vice versa for sites on thesouthern hemisphere. The same characteristics have also been reported by the VLBI
community.
3.8

Ionosphere
At present the following ionosphere products are generated on a routine basis:•••

2-hourly global ionosphere maps (GIMs) are produced using double-difference
phase or phase-smoothed code observations. The phase-derived TEC maps
proved their usefulness for ambiguity resolution (AR) on long baselines. Rapid
global maps are available with a delay of about 12 hours, the final ones after 3 to 4
days.
Regional (European) maps are produced as well and are also used to support AR.
On the average 90% of the initial carrier phase ambiguities can be resolved reliably− without making use of code measurements.
Daily sets of differential code biases (DCBs) for all GPS satellites (and allcontributing receivers) are estimated at CODE since October 1997. The day-today scattering of the satellite-specific DCBs is better than 0.1 ns.

81

IGS 1997 Technical Reports•

Since June 1998, 1-day and 2-day predicted GIMs are regularly derived. The
prediction procedure performed is described in [Schaer et al., 1998b].

In order to improve the ionosphere estimation the following changes were made in
1997/1998: The maximum degree of the spherical harmonic (SH) expansion was increased
from 8 to 12 to be able to resolve smaller TEC structures like, e.g., the equatorial
anomaly. The temporal resolution was increased from 24 hours to 2 hours and slight
relative constraints between consecutive sets of SH coefficients were introduced (to getreasonable TEC results for regions where no stations are located). Moreover, we recently
refer the SH expansion to a solar-geomagnetic reference frame (instead of a solargeographic one).
Starting with June 1, 1998, our final GIMs are delivered weekly to CDDIS incompressed IONEX form [Schaer et al., 1998a] fulfilling the standards as stated in[Feltens and Schaer, 1998]. The CODE IONEX files also contain RMS maps and a set of
DCB values for the satellites. Figure 4 shows 12 TEC snapshots of the global TEC for
June 1, 1998, referring to times 01:00, 03:00, …, 23:00 UT. Bright areas indicate low
TEC, dark ones high TEC. The dotted line corresponds to the geomagnetic equator.
The long-time series of global TEC parameters available at CODE covers 3.5 years
by now and includes up to 1788 SH coefficients per day. The zero-degree coefficient
representing the mean TEC on a global scale characterizes the ionospheric activity pretty
well. The evolution of this particular TEC parameter during a period of low solar activity
is shown in Figure 5. An automatically updated figure showing the complete time series
and a one-year prediction of the Earth's mean TEC can be found on the WWW page
http://www.cx.unibe.ch/aiub/ ionosphere.html.

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1997 Annual Report - Code Analysis Center of the IGS

Figure 4. CODE 2-hour GIMs for day 152, 1998

Figure 5. Evolution of the Earth's mean TEC computed by CODE since January 1, 1995
4

GLONASS

Since December 1997, a single-frequency receiver (Ashtech GG24) is running
permanently at the Zimmerwald observatory. A daily single-point-positioning solution iscomputed and the time difference between GLONASS time and GPS time is monitored.
In Figure 6 we see the systematic difference of about 2 µsec between the time systems(after subtracting the leap seconds of UTC (Moscow)). Also, the difference varies withinseveral ten nanoseconds.

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IGS 1997 Technical Reports

The fit of 3-day arcs through the orbits broadcast by the GLONASS satellites
indicate that the precision of the GLONASS orbits is in general 2-3 meters, a quality
similar to that of the GPS broadcast orbits. As in the case of GPS, improved GLONASS
orbits have to become available in order to make the GLONASS measurements useful forgeodetic and geodynamic applications. Presently, the Bernese GPS Software is modified
to enable the processing of dual-frequency GLONASS carrier phase data including
ambiguity resolution.

Figure 6. Time difference between the GLONASS and GPS time system.
8

Outlook

For the next year, we plan to realize a combined GPS/GLONASS solution,
starting with the activities related to the International GLONASS Experiment (IGEX). It
would also be beneficial (especially for tropospheric gradient parameters, that will soon
be implemented into the official 3-day solutions) to include data down to 5 degrees
elevation, but a new set of antenna phase center calibrations going down to 5 degrees will
be needed beforehand. The increase in the amount of data due to the addition of
GLONASS and low-elevation data will allow a refined modeling of the troposphereparameters. CODE will continue its special ERP series (sub-daily ERPs and nutation). In
view of the present quality of the GPS ERPs and the increasing length of the series, thestudy of new phenomena in earth rotation (e.g. high-frequency atmospheric normal
modes) may become possible. Because sub-daily site displacements are strongly
correlated with sub-daily ERPs, a detailed study of ocean loading, atmospheric loading,and short-term variations in site coordinates in general, will be another important field ofinterest at CODE.

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1997 Annual Report - Code Analysis Center of the IGS

9REFERENCES
Feltens, J., and S. Schaer (1998), IGS Products for the Ionosphere, in Proceedingsof the IGS Analysis Center Workshop in Darmstadt, Germany, February 9-11,
1998.
Le Provost, C., M. L. Genco, F. Lyard, P. Vincent, and P. Canceil (1994),
Spectroscopy of the world ocean tides from a finite element hydrodynamic
model, Journal of Geophysical Research, 99, 24777-24797.
McCarthy, D.D. (1996), IERS Conventions (1996), IERS Technical Note 21,
Observatoire de Paris, Paris, July 1996.
Niell, A. E. (1993), Global Mapping Functions for the Atmosphere Delay atRadio Wavelengths, Journal of Geophysical Research, 101 (B2), 3227-3246.
Overney, F., L. Prost, G. Dudle, T. Schildknecht, G. Beutler, J. A. Davis, J. M.
Furlong, and P. Hetzel (1998), GPS Time Transfer Using Geodetic Receivers(GeTT): Results on European Baselines, in Proceedings of the 12th European
Frequency and Time Forum EFTF 98, Warsaw, Poland, March 10-12 1998.
Rothacher, M. (1996), Mean Antenna Phase Offsets and Elevation-Dependent
Phase Center Corrections, submitted by e-mail to all Analysis Centers (July, 1).
Rothacher, M. (1998), Recent Contributions of GPS to Earth Rotation andReference Frames, Habilitation (2nd Ph.D.), University Press, University ofBerne.
Rothacher, M., and G. Beutler (1997), The Role of GPS in the Study of Global
Change, accepted by Physics and Chemistry of the Earth, February 1998.
Rothacher, M., G. Beutler, E. Brockmann, L. Mervart, R. Weber, U. Wild, A.
Wiget, H. Seeger, S. Botton, and C. Boucher (1995), Annual Report 1994 of the
CODE Processing Center of the IGS, in IGS 1994 Annual Report, edited by J.F.
Zumberge et al., pp. 139-162, Central Bureau, JPL, Pasadena, Ca.
Rothacher, M., G. Beutler, E. Brockmann, L. Mervart, S. Schaer, T. A. Springer,
U. Wild, A. Wiget, H. Seeger, and C. Boucher (1996a), Annual Report 1995 of
the CODE Processing Center of the IGS, in IGS 1995 Annual Report, edited by
J.F. Zumberge et al., pp. 151-174, IGS Central Bureau, Jet Propulsion
Laboratory, Pasadena, California U.S.A., September, 1996.

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IGS 1997 Technical Reports

Rothacher, M., W. Gurtner, S. Schaer, R. Weber, and H. O. Hase (1996b),
Azimuth- and Elevation-Dependent Phase Center Corrections for Geodetic GPS
Antennas Estimated from GPS Calibration Campaigns, in IAG Symposium
No.115, edited by W. Torge, pp. 335-339, Springer-Verlag.
Rothacher, M., T. A. Springer, S. Schaer, G. Beutler, E. Brockmann, U. Wild, A.
Wiget, C. Boucher, S. Botton, and H. Seeger (1997a), Annual Report 1996 of the
CODE Processing Center of the IGS, in IGS 1996 Annual Report, edited by J.F.
Zumberge et al., pp. 201-219, IGS Central Bureau, Jet Propulsion Laboratory,
Pasadena, California U.S.A., November 1997.
Rothacher, M., T. A. Springer, S. Schaer, and G. Beutler (1997b), Processing
Strategies for Regional GPS Networks, in Proceedings of the IAG General
Assembly in Rio, September, 1997, Springer, (in press).
Rothacher, M., G. Beutler, T.A. Herring, and R. Weber (1998), Estimation of
Nutation Using the Global Positioning System, JGR, submitted in March 1998.
Rothacher, M., and L. Mervart (1996), The Bernese GPS Software Version 4.0,
Astronomical Institute, University of Berne, September 1996.
Saastamoinen, I.I. (1973), Contribution to the theory of atmospheric refraction,
Bulletin Géodésique, 107, 13-34.
Schaer, S., W. Gurtner, and J. Feltens (1998a), IONEX: The IONosphere Map
EXchange Format Version 1, February 25, 1998, in Proceedings of the IGS
Analysis Center Workshop in Darmstadt, Germany, February 9-11, 1998.
Schaer, S., G. Beutler, and M. Rothacher (1998b), Mapping and Predicting the
Ionosphere, in Proceedings of the IGS Analysis Center Workshop in Darmstadt,
Germany, February 9-11, 1998.
Scherneck, H.-G. (1991), A parametrized solid earth tide model and ocean tideloading effects for global geodetic baseline measurements, Geophys. J. Int., 106,677-694.
Schildknecht, T., G. Beutler, W. Gurtner, and M. Rothacher (1990), Towards
Subnanosecond GPS Time Transfer using Geodetic Processing Techniques, inProceedings of the Fourth European Frequency and Time Forum EFTF 90, pp.
335-346, Neuchatel, Switzerland.
Springer, T. A., W. Gurtner, M. Rothacher, and S. Schaer (1997), EUREF
Activities at the CODE Analysis Center, in Proceedings of the 4th International86

1997 Annual Report - Code Analysis Center of the IGS

Seminar on GPS in Central Europe, Vol. 4(27), pp. 165-176, Warsaw
University of Technology, Warsaw, Poland, May 7-9, 1997.
Springer, T. A., G. Beutler, and M. Rothacher (1998a), Improving the Orbit
Estimates of the GPS Satellites, Journal of Geodesy, submitted January 1998.
Springer, T.A., G. Beutler, and M. Rothacher (1998b), A new Solar Radiation
Pressure Model for the GPS Satellites, in Proceedings of the IGS Analysis
Center Workshop, edited by J.Dow et al., ESA, Darmstadt, Germany, February
1998.

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The ESA/ESOC IGS Analysis Centre

The ESA/ESOC IGS Analysis Centre
T. J. Martin Mur, J. M. Dow, C. Garcia Martinez, J. Feltens and P. Bernedo
ESA/European Space Operations Centre
Darmstadt, Germany

1

Introduction

In 1997 we have continued our contribution to the IGS that was initiated in 1992.
We have incremented the number of products to accomplish the last IGS requirements
and recommendations.
This year has been characterized by the following:•

The production of predicted orbits has been developed and initiated. We
contribute regularly to the IGS predicted orbit since April 1997.

•

The quality of our products has been improved by the estimation of empiricalaccelerations and the implementation of new tropospheric and ocean loading
models.

•Further developments have been carried out in the Ionospheric Monitoring facility
in preparation to the regular production of combinations and the creation of a new
IGS product in 1998.

•

In the field of satellites carrying GPS receivers, the TOPEX/Poseidon analysis
activities, performed more than two years ago, have been resumed with theAutomated Rendezvous Pre-development (ARP). The analysis of the first Flight
Demonstration took place in 1997. Several of our routine IGS products have beenused for the data analysis.

2

ESOC GPS Web Pages
Additional information on GPS related activities at ESOC can be found at our web

pages:
html://nng.esoc.esa.de

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IGS 1997 Technical Reports

3

ESOC IGS Analysis

An updated description of models and parameters used in our routine processing
is located at:
http://igscb.jpl.nasa.gov/igscb/center/analysis/esa.acn
4Computer Resources and Software Description

The software has been integrated in the last years under the GPS Tracking andData Analysis Facility (GPS-TDAF). It is a UNIX based environment for which main
tools have been developed in FORTRAN for the numerical computations and in tcl/tk forthe scripts that control the automatic processing and for the graphical user interfaces.
The GPS TDAF is made up of three main tools:•••

The Remote Station monitor and control. It retrieves, preprocesses anddistributes the data of the ESA GPS receivers Network.
The External Sites monitor and control. It retrieves the RINEX data that areused for the processing of our IGS products.
The IGS processing monitor and control. It allows the operations related to of
the final, rapid and predicted products.

A special emphasis has been put on the system automation, and it has proven to
be robust for periods of several days with a very sparse remote operator control. The
main problems arise from the computer loading at ESOC, the huge amount of involved
data and the sequential nature of the products, that depend on the success of theprocessing of the previous days.
5

Data Archiving

A 7 Gigabytes disk is accessible to the workstations through the ESOC network.
There is space for the computation of two or three weeks and then products and data arearchived in a robot tape system. Old data can be put on line for reprocessing.
6

ESOC IGS Analysis Centre Major Routine Analysis Changes in 1997

31/03/97
09/03/97
02/04/97

Estimation of sine and cosine radial empirical acceleration instead of impulses
every 12 hours
Saastamoinen tropospheric model replaced the Willman one
ESA predicted orbits available

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The ESA/ESOC IGS Analysis Centre

21/04/97
22/07/97
30/11/97
7

Deadline of rapid orbits reduced from 23:00 to 21:00 UTC
Hatanaka compression implemented for the analysis and for the data
distribution of the ESA receivers
Ocean loading implemented based on the Scherneck model

Tracking Network

The following panel is part of the GPS Tracking and Data Analysis Facility andshows for a given day the stations that are considered in the processing.

8

Predicted Orbits

Earth fixed positions taken from the rapid IGS solution are used as basic
observable. The number of days for the fit is variable, currently set to four. If the IGR
rapid orbit is not accessible our corresponding rapid solution plus eop's are used instead.
Measurements of the last day have a weight three times the one of the initial days.
Per satellite are estimated ROCK4T scale factor, ybias and sine and cosine one
cycle per revolution empirical accelerations in the three orbital directions. The initial statevector is taken from the corresponding rapid solution. It is also estimated.
Earth orientation parameters are taken from the rapid solution for the fit intervaland from the IERS rapid service for the prediction. xp, yp and ut1 in the prediction
interval are corrected for the offset with respect to the IGS rapid eop's.
Satellites are deweighted if

91

IGS 1997 Technical Reports• they have been deweighted in the rapid solution• are missing any of the days of the rapid combination• are in eclipse period• the orbit determination fit of the 4 igr orbits is poor.
Satellites with a extremely bad fit in the 4 day igr interval are replaced by the
propagation of our last rapid solution. It has been proven that a propagation of a smaller
arc is better.
9

Rapid and Final Orbits

Rapid and final orbits are mainly differentiated by the amount of data available.
Rapid orbits are initiated at 14:00 UTC after collection of all the available data. The dataarc is 12+24 hours and the processing time about 4 hours. Final orbits are initiated with adelay of several days for data collection. The data arc is 12+24+12 hours and theprocessing time about 12 hours in a SUN Sparc 20.
The orbit modelling is common. Per satellite we estimate the following parameters:
•
•

•

•
10

Position and velocity at epoch.
Scale for ROCK4T, y-bias, sine and cosine radial component one cycle per
revolution empirical accelerations. These parameters replaced the delta-v
impulses every 12 hours for all the satellites in March 1997.
For eclipsing satellites the observations are excluded half an hour before andafter the eclipse. Delta-v are estimated in the three orbital components at
eclipse exit time.
Any delta-v due to spacecraft manoeuvres.

Tropospheric Estimates

Tropospheric zenith path delays are produced in our routine analysis. They are
estimated along with orbits, eop's and station coordinates. We use the Saastamoinen
model since March 1997. The model consists of two hourly step functions with apriori
values taken from the previous day.
In the following example the results for three stations, USUD, TSKB and TAEJ
from the same region are presented. The three curves show the same seasonal variation.
Taejon and Tsukuba are located close to the sea level while Usuda is at about 1500 meters
height. That explains the difference of about 400 mm between the curves. The
geographical proximity between Usuda and Tsukuba makes both profiles to look very
similar in spite of the height difference.

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The ESA/ESOC IGS Analysis Centre

11

Ionosphere Processing

At the beginning of 1998 the routine processing of ionosphere TEC maps andsatellite/receiver differential code biases (DCBs) in final and in rapid mode has been
started at ESOC. The first day that was processed in final mode was the 28 December
1997, and processing in rapid mode started for 19 March 1998. 24 hours of so called TEC
observables, derived from carrier phase leveled to code data, are fitted to 2-d single layer
TEC models, as well as to Chapman profile models to resolve the ionosphere's electron
content 3-dimensionally. The processing sequence is as follows:
1) A nighttime TEC data fit is made to obtain reference values for the DCBs. The
nighttime TEC itself is absorbed in this fit with a low degree and order sphericalharmonic. In the other fits 2) - 4) these DCBs are then introduced as constraints.
2) A Single layer Gauss-Type Exponential (GE) function is fitted to the TEC
data. The results of this estimate are intended for the ESOC-internal interpretationof results and comparison with the other fits.
3) A Chapman profile model is fitted to the TEC data, where the layer ofmaximum electron density N0 and its height h0 are estimated as functions ofgeomagnetic latitude and local time. h0 is restricted to achieve values within apredefined height range only, currently 400 km =< h0 =< 450 km.
4) A Chapman profile model is fitted to the TEC data, where now h0 is fitted as a
global constant.

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IGS 1997 Technical Reports

Since the beginning of June 1998 ESOC is contributing to the IGS Ionosphere
Working Group's pilot phase. Fit no 3) in final mode is currently delivered to the IGS
Ionosphere pilot project. The figure below shows a TEC map from a fit of type 3).

Latitude

0 hours

12 hours

24 hours

Local Time

12

Manoeuvres Estimation

For the estimation, the manoeuvres must be previously announced in the
NANU's. If at least two receivers track the manoeuvring satellite we try to detect the time
when the thrusters are fired using two alternative ways:•

Studying the residuals of the phase triple differences for combinations that include
the satellite. The time is determined by looking for a step in the triple differences
caused by the new range rates produced by the change in velocity. The
preliminary value of along track delta-v is estimated by the energy change. Seeplot below.

•

Using clock bias free ionospheric free carrier phase time differences. We havedeveloped an algorithm to detect delta-v changes based on the comparison of theobservations to a propagated orbit. We obtain directly the estimate of delta-v.

Once the manoeuvre time is known and with a initial estimate of delta-v, our orbit
determination program BAHN estimates the orbit and impulse in three directions. Radialand cross track components are normally negligible compared to the along track

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The ESA/ESOC IGS Analysis Centre

component. Observations one hour before and after the manoeuvre are not considered
because of difficulties with the fit.
We have estimated all manoeuvres since the beginning of IGS. Only in a very few
cases did the lack of visibility not allow us to detect the firing time. The current network
densification facilitates the detection.

13

Station Coordinates

The stability of the station coordinates has been improved -specially the verticalcomponent- with the implementation of the Saastamoinen tropospheric model in March
1997 and also with the consideration of ocean loading based on the Scherneck model. Thatcan be clearly seen below on the height estimation of Maspalomas, a station very affectedby the ocean loading effect. Several tidal effects, specially the monthly one, have been
substantially reduced.

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IGS 1997 Technical Reports

Oceanloading

Saastamoinen model

14

Products
Our routine products are the following:• Final orbits esawwwwd.eph, being wwww the gps week and d the day of theweek (0-6), distributed via CDDIS. 11 days delay.
• Rapid orbits esawwwwd.eph, being wwww the gps week and d the day of theweek (0-6), distributed via EMR. 21 hours delay since April 1997Predicted
orbits espwwwwd.eph, distributed via EMR.
• daily rapid eop (pole, LODR) solutions in IERS format: esawwwwd.erp.
• weekly final eop (pole, LODR) solutions in IERS format: esawwww7.erp.
• weekly summaries: esawwww7.sum.
• weekly free network station coordinate solution in the SINEX format:
esawwww7.snx• daily tropospheric files containing Zenith Path Delay estimations
esawwwwd.tro

15

References
J. M. Dow, T. J. Martin Mur, and M. M. Romay Merino, ESA's Precise Orbit
Determination Facility, ESA Bulletin no. 78, May 1994, pp. 4O-50.
H. Fliegel, T. Gallini, and E. Swift, Global Positioning System radiation force
model for geodetic applications, J. Geophys. Res. 97(B1), pp. 559-568, January
1992.

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The ESA/ESOC IGS Analysis Centre

D. D. McCarthy (ed.), IERS Standards (1992), IERS Technical Note 13,
Observatoire de Paris, July 1992.
D. D. McCarthy (ed.), IERS Conventions (1996), IERS Technical Note 21,
Observatoire de Paris, July 1996.
J. T. Wu, S.C. Wu, G.A. Hajj, W.I. Bertiger, and S.M. Lichten, Effects of antennaorientation on GPS carrier phase, Manuscripta Geodaetica (1993) 18, pp. 91-98.
Sinex Working Group. SINEX - Solution (Software/technique) INdependent
EXchange Format. Version 1.00 (April 01, 1996).
T.J.M. Mur, J.M. Dow, C.G. Martinez, J. Feltens. The ESA/ESOC IGS Analysis
Centre. 1996 IGS Annual Report.

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GFZ Analysis Center of the IGS - Annual Report 1997

GFZ Analysis Center of the IGS - Annual Report 1997
Gerd Gendt, Galina Dick, Wolfgang Söhne
GeoForschungsZentrum, Div. 1
D-14473 Potsdam, Telegrafenberg A17

1

Introduction

In the last year, a further improvement of accuracy and quality of the products
has been gained. The rapid and final orbits reached the 6 cm and 4 cm level, respectively,
and the polar motion crossed the 0.1 mas “boundary.”
The software and the technology were changed to get the various products
compatible to the final orbits. The clock solution was aligned to the GPS time frame.
The zenith path delay (ZPD) products from all the ACs are combined at GFZ to
an official IGS product (see Gendt, this volume).
2

Routine IGS Processing - Overview

The technology for the generation of rapid and final products was alreadydescribed in the last annual report. During 1997, changes were made to be more effective
and to get compatible final products (see Table 1). An overview of both GFZ products,
and all daily and weekly activities, is given in Table 2.

Fig. 1. Global distribution of stations used in the IGS analysis of GFZ(s - sites added for rapid analysis)

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IGS 1997 Technical Reports

Table 1. Modification in software and technology
Week
902
903
921
929
939
944
945
947
949
951
958

Date
1997-04-20
1997-04-27
1997-08-31
1997-10-26
1998-01-04
1998-02-08
1998-02-15
1998-03-01
1998-03-15
1998-03-28
1998-05-17

Description
Use of an optimized set of ~30 sites for rapid analysis
ERP from 3-day arcs (middle day), UT is solved for
Calibration of clock products (alignment to GPS time frame)
Introduction of Niell mapping function
Compatible clocks (use of 3-day orbits for clocks)
5-min sampling rate, 15 degree elevation cutoff angle
Introduction of tropospheric gradients
Introduction of ITRF-96 with 47 core sites
SINEX using 3-day orbits (compatible to SP3 orbits)
Error for PRN8 corrected (IIR to IIA)
PRN13: antenna phase center offset changed from 1.6746 m to
1.0229 m

Since March 1998, the raw RINEX data are handled in the Hatanaka format. All
input files not submitted in this format are converted into this format just after ftp prior
to analysis.
Starting with GPS week 944, we switched from 20 degree elevation cutoff to 15
degrees. At the same time, the sampling rate was reduced from 6 to 5 minutes, so that we
could get exact satellite clock estimates for all 15-minute epochs in the sp3 product.
It should be mentioned that for PRN8 (started in November 1997), a wrong blocknumber (IIR instead of IIA) was erroneously introduced. After correction in March 1998,
our overall orbit quality improved by 1 cm (Figure 3). This demonstrates that at the levelof 5 cm, such an incorrect parameter influences the orbit solutions for all other satellites.
2.1

Rapid Analysis

A set of 20-30 well-distributed sites is sufficient to get high-quality satellite
orbits. Starting with GPS week 902, an optimized set of ~30 sites was selected, resulting
in reduced computation time for the total rapid analysis (about 2-3 hours). To have along history, we tack on a selected set of sites in our final analysis. However,occasionally these sites do not come in a timely manner, so for the rapid analysis, some
additional sites were selected.
2.2

Final Analysis

The data cleaning part was changed to use the precise point positioning (PPP)
technique based on the satellite clocks from the rapid analysis. This cleaning is more
effective and more flexible than the formerly-used double difference procedure, which is
now used only for the rapid analysis.

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GFZ Analysis Center of the IGS - Annual Report 1997

The strategy for deriving the final products was changed to get products that arecompatible to each other. The old products for ERP and SINEX were based on daily
orbits, whereas the sp3 product was formed using 3-day arcs.
The new technology is the following:
Table 2. Overview of IGS routine analysis and generated product (D denotes actual day)
1-day orbits
n Rapid analysis for D-1(start 9:00 UT)• Optimized set of ~30 sites
gfzwwwwd.sp3p• DD-Clean• Analysis and post-fit-cleaning (iteratively)

gfzwwwwd.erp

n Predictions• IGR products for D-4 to D-2 and
GFZ rapid products for D-1 used

gfpwwwwd.erp

including sat-clocks(12:00 UT)n Final analysis• PPP-cleaning using rapid clocks
Final Solution, weekly
n 3-day orbits by combining NEQ of 1-day orbits• Output of NEQ implicitly containing
the 3-day orbits (not fixed !)n ERP are taken from the middle day of the
3-day orbit solutions

gfzwwwwd.sp3

gfzwwwwd.erp

n Weekly SINEX is computed by stacking the above NEQ
for 7 days (data of each day are used in three NEQs)

gfzwwwwd.snx

n To get compatible products clocks and (real-valued)
ambiguities are computed fixing the above sp3 orbits

sat-clocks

n ZPD solutions with a higher sampling rate are effectively
computed by fixing the above ambiguities

gfzwwww.tro

All products are based on fixing the station coordinates of the core sites (since
March 1998 47 sites of ITRF96, before that 13 sites of ITRF94). The sp3 orbits arecomputed using 3-day arcs (by stacking NEQs of 1-day arcs). Simultaneously with theorbit product, a 3-day NEQ is formed containing only the station coordinates and ERPs,
but also implicitly including the 3-day orbits (Helmert blocking; orbits are not fixed!).

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IGS 1997 Technical Reports

For each 3-day arc daily ERPs are computed with continuation constraints for the day
boundaries (i.e., polar motion and trend as well as UT and LOD form a continuous
polygon). The solution for the middle day of each arc is the official product. Each week,
a UT series is compiled by concatenation of the daily values and starting at Bulletin A
value.
Because the sp3 orbits are computed by stacking NEQ there are no
simultaneously solved clocks. Therefore an additional step was introduced at which allclocks and (real valued) ambiguities are determined by fixing the given orbits. These
products are now consistent to the best orbits. Such clocks can be used in the wellknown precise point positioning. For the derivation of the ZPD product with hourly
sampling rate (in the routine analysis 4 hours are used), the derived ambiguities are
introduced and fixed. This way the new analysis can be done within minutes. I n
addition, the ZPD values are adjusted using the adjacent days (3 days in total) to smooththe solution at the day boundaries.
3

Improvement of the Clock Solution

Before GPS week 921, the GFZ clock solutions (rapid as well as final) showedsignificant offsets and drifts compared to the IGS combined solutions. The differences in
the offset reached values up to 1 microsecond and the trend difference was normally inthe range of 80-90 nanoseconds per day. T o overcome the problem of singularity within
the estimation process, the satellite and station clocks are reduced to a specified reference
clock. Usually as a reference, one of the station clocks is taken which is known to bevery stable. At GFZ, the station clock of Algonquin is used as reference most of thetime. Before GPS week 921, the satellite clocks taken directly from the adjustment weredelivered as the final results. However, the connection to a specified reference clock led
to differences compared to GPS time because of the behavior of the reference clock. It isonly stable with the exception of a small rest, and therefore includes a time-varying offset
and drift. Beginning with GPS week 921, the GFZ clock solution was corrected for theseeffects. In a first step, possible resets or jumps of the reference clock which appear
sometimes are detected and corrected by comparing the reference clock with other stablestation clocks. Missing epochs of the reference clock, which normally led to gaps in the
final clock solution, can now be bridged over. The offset correction is then performed by
calculating an average over all satellites. The drift correction is estimated by the trend over
all mean values.
Figure 2 shows the improvement of the final clocks of GFZ for both offset andtrend as a result of the changes. Before week 921 the drift was nearly constant in therange of 90 nanoseconds per day (reflecting the clock characteristic of Algonquin) whereas
after the software improvement it was usually a few nanoseconds.

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GFZ Analysis Center of the IGS - Annual Report 1997

Fig. 2. Offset (l, [ns]) and trend (s, [ns/d]) of GFZ satellite clock results as comparedto the GPS time frame
It should be mentioned that there are some inconsistencies between the ACs
concerning the antenna phase center offset for PRN13 which results in higher clock
differences. When GFZ switched from 1.6764 m to 1.0229 m, the value used by mostACs, the level of our clock differences improved to 0.2 ns (Figure 3).

Fig. 3. Daily median of GFZ final (l) and rapid (s) orbits compared to the official IGS
Product
4

High Rate Satellite Clocks

Based on the results of the IGS final analysis which is usually done with sampled
data, the computation of high-rate satellite clocks (i.e., with the original data rate) can be
carried out with a small additional amount of computation time.

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IGS 1997 Technical Reports

After the IGS final processing, precise GPS satellite orbits are available which canbe introduced as fixed parameters. On the other hand, all ambiguities within the sampled
data are estimated within the IGS adjustment, and all bad data and outliers are identified.
With this information, the raw 30-sec RINEX data files are reduced to those parts forwhich the ambiguities are valid. New data before or after are neglected. As a result ofsuch pre-processing, new ambiguities may not be found within the high rate data.
Beyond this, the GPS data between the low rate epochs are separately inspected for bad
records. A selection of about 20 stations seems to be sufficient for a good global
coverage. Because this number of stations in general is smaller than the number ofstations used in the IGS final analysis, the starting time of the ambiguities is not identicalwith the starting time of the original scene. Therefore, the ambiguities have to be shiftedto the correct new epochs to avoid the problem that new ambiguities are automaticallyfound by the program.
At this moment the precise high-rate clocks are not estimated routinely but aperiodical and automatic computation can be installed easily. More details and results canbe found in (Söhne, 1998).
5

Products

5.1

OrbitsThe quality of the orbit solutions was 5-6 cm for the final and 6-10 cm for the
rapid variant. The introduction of the ITRF96 with the 47 core sites to be held fixed(week 947), stabilized the realization of the reference frame and led to an improvement forthe orbits, especially for the rapid solution. The orbits are now on the 4 cm level for the
finals and on many days on the 6-7 cm level for the rapids (Figure 3).
5.2

Earth Rotation Parameters

The accuracy for the ERP determination was already on a high level (see Table 3)
and could even be improved during the past year. By using 3-day arcs for thedetermination of ERP (starting in week 904), the LOD solution improved significantly(Figure 4). A further improvement was reached in March 1998 by the introduction of theenlarged set of core sites which stabilized the reference frame realization during the daily
analysis. The largest effect can be seen for the rapid products (Table 3, Figure 4), whichhave now an accuracy of 0.15 mas, and corresponds to the former accuracy of the final
product. The improvement in the rates is not as dramatic. Since week 903, GFZ also
submits UT results which are aligned to Bulletin A on the first day of each week. The
accuracy within the week is at 0.07 ms.

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GFZ Analysis Center of the IGS - Annual Report 1997

Table 3. Accuracy of ERP determination for Rapid and Final Products

xp
yp
LOD

Rapid*Final
ITRF94 ITRF96
ITRF94
ITRF96
±0.29
±0.16
±0.11
±0.07
±0.29
±0.14
±0.15
±0.09
±0.36
±0.31
±0.26
±0.16
* outliers removed

Fig. 4. Differences of GFZ final (l) and rapid (s) ERP solutions to the IGS final results.
5.3

Global Reference Frame and Plate Kinematics

To continue our investigations of recent global tectonics and for the determinationof a global reference frame, one more year of IGS data was added to our analysis. The
solution for the reference frame was computed according to the description in previous
reports (Gendt et al., 1997). Site velocities were adjusted for all sites which wereanalyzed for more than one year (65 sites). The velocity in height was loosely
constrained. The orientation of the system was defined by applying no-net-rotation
constraints both for the site coordinates and the site velocities, as a reference the ITRF96
reference frame including velocities was used.
The quality of the determined reference frame is on the sub-cm level. Helmert
transformation to ITRF96 gives residuals in north, east and up of 1.9 mm, 7.1 mm, 5.6

105

IGS 1997 Technical Reports

mm, respectively (for the 47 core sites). Larger discrepancies in the east component wereobserved for SANT, AREQ and KWJ1. For the dense parts of the network in Europe
and North America (28 sites), the residuals are 1.2 mm, 2.8 mm, 3.2 mm. The global
residuals to ITRF96 for the site velocities are in the range of 2-3 mm/y for all threecomponents (2 mm/y in Europe and North America). Again some sites were excluded due
to large discrepancies in the velocities, e.g., SANT, AREQ, MALI, OHIG.
Figure 5 presents the station velocities from our global solution over 5 years. The
corresponding values from ITRF96, so far as available, are shown for comparison. A
considerable improvement can be noted for the Australian sites (e.g., YAR1, HOB2,
TIDB), as well as for the sites of East Asia and Japan (TAIW, USUD, TSKB), whichshowed rather large discrepancies in the past. Also a number of relatively new sites like
GUAM, KERG, KWJ1, CAS1 and DAV1 are in better agreement now, so that in generala better quality both of ITRF and GFZ solution can be stated. The most remarkableregion with large discrepancies remains South America, where further investigations andchecks of the solution quality are obviously needed.

Fig. 5. Site velocities from GFZ solution from 5 year IGS data together with ITRF96
valuesFigure 6 demonstrates a significant improvement of the GFZ coordinate solution
achieved after the introducing of ITRF96 at week 947. Here, differences in 3 components
between the GFZ coordinate solutions and the combined JPL GNAAC solutions are
given.

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GFZ Analysis Center of the IGS - Annual Report 1997

Fig. 6. Differences between GFZ coordinate solution and the combined solution of JPL
GNAAC in north (l) , east (n) and height (s).

5.4

Troposphere

The quality of the zenith path delay solution is on the level of 3-4 mm (Moredetails see Gendt, 1998).

6

References

Gendt, G., G. Dick and W. Söhne (1997): GFZ Analysis Center of IGS - Annual Report
1996, International GPS Service for Geodynamics, 1996 Annual Report, ed. by J.F.
Zumberge, D. E. Fulton and R. E. Neilan, JPL, California, pp. 169-181
Gendt, G. (1998): IGS Combination of Tropospheric Estimates - Experience from Pilot
Experiment, in Proceedings of the IGS Analysis Center Workshop, Darmstadt,
February 9-11, 1998, in press
Söhne, W. (1998): Precise high-rate satellite clocks at GFZ, in Proceedings IGS Analysis
Center Workshop, Darmstadt, February 9-11, 1998, in press

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IGS 1997 Technical Reports

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108

JPL IGS Analysis Center Report, 1997

JPL IGS Analysis Center Report, 1997
D. C. Jefferson, Y. E. Bar-Sever, M. B. Heflin, M. M. Watkins,
F. H. Webb, and J. F. Zumberge
Jet Propulsion Laboratory/California Institute of Technology
Pasadena, California 91109 USA

1

Summary

JPL activities as an IGS Analysis Center continued throughout 199. Regulardeliveries of rapid (1-day), precise, and high-rate (30-sec) GPS orbits and clocks, Earthorientation parameters, and free-network ground station coordinates (now in SINEX 1.0)
were maintained. A new product was made available in 1997 - daily troposphereestimates in the IGS exchange format. The estimation of tropospheric gradients has
further improved the accuracy of our solutions. In early 1998, a larger subset of the
newly-augmented group of 47 IGS fiducial stations was put in use, and all fixed-network
solutions were made to align with ITRF96. Shortly thereafter, only free-network
products rotationally aligned with ITRF96 are submitted.
2

Evolution in 1997

Material relating to JPL (Jet Propulsion Laboratory) participation as an IGS
analysis center, beginning in 1992, can be found in [1] and references therein. [2]describes JPL activities as a GNAAC (Global Network Associate Analysis Center).
Table 1 indicates the evolution of our activities during 1997. A major event was
the estimation of tropospheric gradients (see section 5 of this document). Also, JPL
station coordinate solutions now conform to the SINEX 1.0 format. We thank Remi
Ferland of NRCan for providing the SINEX conversion utilities and assisting in theirimplementation at JPL.
Table 1: Analysis evolution, 1997 through early 1998
ActionProduce troposphere files in IGS Exchange format
Produce station coordinate files in with SINEX 1.0 format
Produce free-network transformation files routinely
Do not process satellites deleted from rapid-service product
Estimate tropospheric gradients
Use TurboRogue MAD2 (in place of MADR) as a fiducial site
Correct mismodeling of SRP for PRN03
Use NRC1 (instead of ALGO) as default reference clock

109

Date
Jan 26
Jan 26
Apr 15
Apr 23
Aug 24
Nov 9
Nov 9
Dec 14

IGS 1997 Technical Reports

Use 32-hour nominal orbit interval, map final orbits for 30 hours
Use ITRF96 coordinates and velocities for 22+ subset of 47 IGS fiducials
Use free-network estimates in troposphere products
Lower threshold of global site RMS isolation from 2000 to 1800 km
Use USNO (instead of NRC1) as default reference clock
Increase a priori sigma for pole rates from 0.18 to 0.50 mas/day
Use all free-network solutions for products and AC summary report statistics
3

Feb 1, 1998
Mar 1
Mar 15
Mar 18Apr 26May 6
Jun 28

Product Summary

Tables 2 and 3 summarize the regular products that result from JPL IGS AC
activities. New products are the daily, site-specific troposphere estimates in IGS
Exchange format. These are described in Section 5 of this report. Table 4 indicatesaddresses of World Wide Web pages with related information.
Table 2: Regular products from the JPL IGS Analysis Center, at
ftp://sideshow.jpl.nasa.gov/pub/jpligsac
Example File

Contents

0937/jpl0937.sum.Z

narrative summary for GPS week 0937

0937/jpl0937[0-6].sp3.Z

precise orbits for days 0-6 (Sun through Sat) of GPS week 937

0937/jpl0937[0-6].yaw.Z

yaw-rate data for eclipsing satellites, days 0-6, GPS week 937

0937/jpl09377.erp.Z

fixed-network Earth orientation parameters for GPS week 937(free-network beginning week 964)

0937/jpl09377.snx.Z

free-network station coordinates for GPS week 937(7-parameter transformation to ITRF beginning wk 947)
(3-parameter rotation to ITRF beginning wk 964)
fixed-network troposphere solutions, days 0-6, for GPS week 937(free-network beginning week 949)

0937/jpl0937[0-6].tro.Z

hirate/JPL0937[0-6].sp3.Z

high-rate (30-s) precise orbits and clocks, days 0-6, GPS week 937

1997.eng.Z

engineering data for 1997, sites in global solution

1997_p.eng.Z

engineering data for 1997, point-positioned sites

ytd.eng

year-to-date engineering data, sites in global solution

ytd_p.eng

year-to-date engineering data, point-positioned sites

110JPL IGS Analysis Center Report, 1997

Table 3: Other products at ftp://sideshow.jpl.nasa.gov/pub/gipsy_products
Example File
VeryRapidService/*

Contents
hourly Earth orientation, orbits, and clock data foruse in GIPSY
quick-look precise orbits for days 0-6 (Sun throughSat) of GPS week 937

RapidService/orbits/jpl0937[0-6].sp3.Z

RapidService/orbits/jpl0937[0-6]_pred.sp3.Z quick-look 3-day predicted orbit for days 0-6,
GPS week 937
RapidService/orbits/1997-12-21.*

daily quick-look and predicted files for use in GIPSY

1997/clocks/1997-12-21.*

1997 daily free- and fixed-network clocks
and yaw-rates for use in GIPSY

1997/orbits/1997-12-21.*

1997 daily free- and fixed-network precise orbits,
polar motion, shadow-events data for use in GIPSY

hrclocks/1997-12-21.*high-rate clocks (in TDP format) for use in GIPSYIERSB/*

IERS Bulletin-B information

Table 4: Addresses of relevant web pages______________________________________________________________________________
Address
Contents
http://sideshow.jpl.nasa.gov/mbh/series.html
graphical time-series of site coordinates
http://sideshow.jpl.nasa.gov/mbh/global/table.html table of site coordinates and velocities
http://milhouse.jpl.nasa.gov/eng/jpl_hp2.html
summaries and plots of station and satellite
performance______________________________________________________________________________

4

Site Selection

Due to the continuous growth of the global network and the impracticality (withcurrent computer resources) of simultaneously analyzing data from all stations, analgorithm for selecting a well-distributed subset of sites along with required sites such as
the IGS fiducials was implemented in late 1994 (see [1]). This scheme chooses N groundstations on the basis of isolation. That is, the Nth site is chosen so as to maximize its

111

IGS 1997 Technical Reports

distance from the nearest of the N-1 already chosen sites. The RMS isolation z (further
described in [3]) is used to assess the distribution after all sites have been selected.
The site selection process has evolved since its first implementation, and currently
37 stations are selected as follows:
•
•

•
•

•

•

5

Choose a reference clock station (usually NRC1 as of week 936; USNO as ofweek 955 ).
Use 24-hour rapid-service processing results to make a separate list of stations
with highly stable clocks. Although these are usually sites with H-masers, ingeneral, these are any stations for which there are at least 250 5-minute clock
solutions (out of a maximum of 288) that are smooth at the 4-cm level ontimescales of 5 minutes.
Based on isolation, choose the next 8 most isolated sites from the list of stableclock sites. These will aid in post-processed high-rate clock production.
Add any sites not yet selected that are fiducial sites and use pseudorange
observations (i.e., TurboRogue fiducials). Note that as of GPS week 947, thelist of fiducials to choose from is a predetermined, well-distributed 22 station
subset of the 47 sites newly designated as IGS fiducials.
Again based on isolation, choose a number of well-distributed stations using
pseudorange (typically TurboRogues), accounting for other fiducials anddesired isolated stations not using pseudorange.
Choose the remaining most isolated stations to complete the 37 total. Ensurethat any of these that are of the 47-site IGS fiducial set will be constrained
during the fixed-network portion of the processing.

Troposphere Products and Strategy Update

Beginning with GPS week 890 (January 26, 1997), JPL began to submit acontribution to the troposphere estimate combination compiled by Gerd Gendt at GFZ.
These files contain our daily estimates of the total (wet + dry) zenith tropospheric delay
at each site used in the fixed-network global solution. Initially, troposphere parameters
were estimated using the Lanyi troposphere mapping function, a satellite elevation cutoffof 15 degrees, and a random walk model with 1.02 cm/sqrt(hr) process noise. The formatof the troposphere products was designed by Yoaz Bar-Sever (JPL) and Gerd Gendt, andthe JPL solution may be obtained as listed in Table 2.
Starting with GPS week 920 (August 24, 1997), tropospheric gradients wereadded to the list of parameters estimated for each ground station. In implementing thisstrategy, the following modifications were made to the estimation process:•

The Niell troposphere mapping function has replaced the Lanyi mapping
function.

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JPL IGS Analysis Center Report, 1997

6•

Troposphere horizontal delay gradients are estimated at all the stations. The two
gradient parameters are modeled as random walk with a sigma of 0.03
cm/sqrt(hour) and are estimated every five minutes.

•

The random walk sigma on the estimated zenith wet delay has been reduced from1.02 cm/sqrt(hour) to 0.30 cm/sqrt(hour).

•The carrier phase post-fit residual rejection criterion has been reduced from 5 cm
to 2.5 cm.

•

Beginning week 949, troposphere products are representative of the free-network
estimates (fixed-network prior to this).

•

The elevation angle cutoff has not changed; it remains at 15 degrees. Details of
this estimation strategy are presented in [4].
New in 1998

Beginning with GPS week 947 (March 1, 1998), JPL adopted IGS procedures set
forth at the IGS Analysis Center Workshop in Darmstadt, Germany, February 9-11,
1998 regarding the use of an augmented set of fiducial ground stations and theirrespective ITRF96 positions. Monument coordinates and velocities are taken fromftp://lareg.ensg.ign.fr/incoming/ITRF96_IGS_RS47.SNX(nowavailable
atftp://igscb.jpl.nasa.gov/igscb/station/coord/ITRF96_IGS_RS47.SNX.Z), and antennaheights from ftp://igscb/igscb/station/general/igs.snx. (Antenna reference point to L1 andL2 phase centers are from ftp://igscb/igscb/station/general/igs_01.pcv.) As only 37
stations are used in the daily analyses, a subset of the 47 sites included in these files isselected as described in section 4 of this document. Moreover, as of this date, SINEX
files submitted from JPL contain free-network station coordinates rigorously transformed
into ITRF96 via a 7-parameter transformation.
It was also decided at the same AC workshop to eventually discontinue the use of
the non-minimal fiducial constraints described above. Therefore, as of GPS week 964(June 28, 1998), all orbits, clocks, EOP, station coordinates, and relevant statistics
reported in our weekly AC summary reports are representative of free-network solutions
that are minimally aligned with ITRF96. Only 3 rotations are applied so that geocenterand scale changes can continue to be observed. Also, JPL SINEX files now containweekly Earth orientation parameters which are consistent with the corresponding sp3formatted orbits and clocks.
Additionally, a slight change has been made to our automation process. As
described in [5], we primarily determine “processing readiness” by periodicallycalculating the RMS isolation, a measure of the global distribution of available ground113

IGS 1997 Technical Reports

sites. As more stations have become a part of the IGS network, in mid-March 1998, welower the threshold of this value from 2000 km to 1800 km. While this may mean alonger delay in some instances, the four-day maximum wait time is still enforced.
A new set of products for GIPSY users is also now available, namely, hourly GPS
ephemeris and clock solutions in the same format as analagous products found in Table 3.
These solutions, based on a set of 18 global stations from which hourly data are received,
have an accuracy of approximately 1 to 2 meters, and are provided on a “caveat emptor”
basis.
7

Results

Figure 1 indicates the further improvement in orbit quality since 1995. As in thepast, our metric for orbit quality is the day-to-day consistency of the solutions, i.e., thedegree to which estimates from adjacent days agree near the midnight boundaries.
Contributing factors are the continuing expansion of the global network, the use of global
phase ambiguity resolution, and the estimation of tropospheric gradients.

30

20

15

10

median weekly orbit repeatability (cm)

25

5

0
1995

1996

1997

1998

year

Figure 1: JPL orbit repeatability (3drms) since 1995. Each data point indicatesthe median over all satellites and days for a particular GPS week. (The daily
number for a given satellite indicates the degree to which the precise orbit agrees
with those of adjacent days near the midnight boundary.) Weeks during which AS
was off are marked with an ‘X’.

114

JPL IGS Analysis Center Report, 1997

8

Acknowledgment

This work was performed at the Jet Propulsion Laboratory, California Institute ofTechnology, under contract with the National Aeronautics and Space Administration.
9

References[1] J. F. Zumberge, M. B. Heflin, D. C. Jefferson, M. M. Watkins, and F. H. Webb, Jet
Propulsion Laboratory IGS Analysis Center 1994 Annual Report, in IGS 1994 Annual
Report, edited by J. Zumberge, R. Liu and R. E. Neilan, IGS Central Bureau, Jet
Propulsion Laboratory, Pasadena CA, 1995, JPL Publication 95-18[2] M. B. Heflin, D. C. Jefferson, M. M. Watkins, F. H. Webb, and J. F. Zumberge,
Comparison of Coordinates, Geocenter, and Scale from All Centers: GNAAC Activities
at JPL for weeks 813-897, in IGS 1996 Annual Report, edited by J. F. Zumberge, D. E.
Fulton, R. E. Neilan, IGS Central Bureau, Jet Propulsion Laboratory, Pasadena, CA, 1997[3] J. F. Zumberge, M. B. Heflin, D. C. Jefferson, M. M. Watkins, F. H. Webb, Precise
point positioning for the efficient and robust analysis of GPS data from large networks,
1997, J. Geophys. Res., Vol. 102, No. B3, p. 5005
[4] Bar-Sever, Y.E., P.M. Kroger and J.A. Borjesson, Estimating Horizontal Gradients of
Tropospheric Path Delay with a Single GPS Receiver, 1998, J. Geophys. Res., Vol. 103,
No. B3, pp. 5019-5035.
[5] D. C. Jefferson, M. B. Heflin, M. M. Watkins, F. H. Webb, J. F. Zumberge, and Y. E.
Bar-Sever, Jet Propulsion Laboratory IGS Analysis Center Report, 1996, in IGS 1996
Annual Report, edited by J. F. Zumberge, D. E. Fulton, R. E. Neilan, IGS Central Bureau,
Jet Propulsion Laboratory, Pasadena, CA, 1997

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116

NRCan IGS Analysis Centre Report, 1997

NRCan IGS Analysis Centre Report, 1997
P. Tétreault, C. Huot, R. Ferland, Y. Mireault, P. Héroux, D. Hutchison and J. Kouba
Geodetic Survey Division
Geomatics Canada, Natural Resources Canada
Ottawa, Canada

1

Summary

During 1997, NRCan initiated submission of daily predicted orbits to IGS andparticipated in the IGS developmental work on the estimation of tropospheric andionospheric delays. The existing NRCan processing strategy was maintained during 1997
(Tétreault et al., 1996, 1997).
2

Orbit Prediction Strategy

On January 10, 1997 (GPS Week 887), NRCan started contributing daily
predicted orbits to the IGS. A 2-day prediction is obtained from 4 IGR rapid orbit
solutions by estimating 6 Keplerian elements and 9 radiation pressure parameters using
the Bernese software version 3.5. The IGR x and y Pole position series are used alongwith the Bulletin A UT1 series to provide the necessary Earth Orientation Parameters.
The use of the Bulletin A UT1 series, initiated on GPS Week 934, has improved the z
rotation of the NRCan predicted orbits as can be seen in Figure 1. Extrapolations of the
EMR and IGR UT1 series were use prior to Bulletin A up to and including GPS Week
916 and 933 respectively. NRCan 2-day predicted orbits are currently about 50-cm
median RMS with respect to IGR orbits for non-eclipsing satellites and about 100-cm for
eclipsing satellites. Further evaluation of NRCan predicted orbits can be found in [Kouba
and Mireault, 1998].

117

IGS 1997 Technical Reports

emp vs igr
20

X Y Z Rotations (mas)

15
10
5
0
-5
-10
-15
-20
888

892

896

900

904

908

X Rot

912

916

Y Rot

920

924

928

932

936

940

Z Rot

Figure 1. Seven –parameter Helmert transformation between IGR and NRCan
Predicted orbits (emp) for GPS Weeks 888 to 943
3

Tropospheric and Ionospheric Modeling

March 1997, NRCan started contributing to the IGS pilot project for theestimation of tropospheric zenith delay. NRCan estimates tropospheric delays at a 7.5
minute interval using a random walk stochastic model with a 1 cm/ hr sigma. Lastepoch estimates from the previous day are weighted and used as apriori values for thecurrent day estimation. For all stations included in NRCan final daily solutions, Total
Zenith Delay (TZD) estimates at 2 hour intervals are submitted to IGS on a daily basis(Gendt, 1998).
NRCan, in support of the Canadian Active Control System (CACS) and in
preparation for the IGS Ionospheric Pilot Project, has developed a grid model ofionospheric delays. The NRCan regional ionospheric model is based on stations covering
the Canadian territory. It is computed using carrier phase smoothed pseudo-range
observations with an elevation cut-off angle of 15 degrees and an elevation dependent
weighting model. A spherical single layer shell at 350 km elevation and a cosine of thezenith angle mapping function are used. The model is based on 24-hour averages
computed in a sun-fixed geographical reference frame. Table 1 presents precise point
positioning monthly averages of daily RMS that were achieved using four different
processing strategies.
In

118

NRCan IGS Analysis Centre Report, 1997

Table 1. Point positioning RMS using NRCan ionospheric modelingMonthly Averages of Daily RMS
Station NRC1, January 1998
(from position estimates at 15 min. intervals, andusing phase smoothed pseudo-range observations)

Latitude
1.3
1.0
0.8
0.5

RMS (m)Longitude Height
0.7
3.6
0.7
1.7
0.4
1.1
0.4
1.1

Processing Strategy
L1
L1 + SLM
L1 + SLM + CAL
L3

L1 = L1 Frequency
SLM = Single Layer Ionospheric Model
CAL = Satellite Differential L1/L2 Code Bias
L3 = Ionospheric Free Combination

4

Final Product Analysis

A 2.5-year spectral analysis of the NRCan station position residual series led tothe discovery that the use of erroneous ocean loading coefficients were causing a 13.7-day
signal. This was corrected on February 23, 1997 and a new spectral analysis for GPS
Weeks 894 to 938 confirmed that the 13.7-day signal had been removed.
The consistency of NRCan products was assessed by computing the differences
between NRCan and ITRF/IGS products for station coordinates, orbits and EOP’s. The
estimated 7 parameter transformations are listed in Table 2. UTI-UTC was not included
due to its long-term drift, evident in the weekly averaged means and standard deviations.
The differences between the IGS and NRCan polar motion and UT1-UTC series areshown in Figures 2 (a) and (b) respectively. The 0.3mas bias seen in the y pole series
starting on day 327 was caused by mistakenly over-constraining the weekly station
coordinates and EOP combinations for GPS Weeks 933 to 938 inclusively. Weeks 933 to
938 were not used to compute the transformations presented in Table 2.

119

IGS 1997 Technical Reports

Table 2. Weekly Averaged Differences Between NRCan and ITRF/IGS Products(NRCan – ITRF/IGS)(Weeks 886 to 932)
Solution
Coordinatesa
sigmaTranslation (cm)Rotation (mas)Scale(ppb)
T1
T2
T3
R1
R2
R3
Sc
0.0
0.4 0.2
0.092
0.017
0.002 -0.47
0.1
0.2 0.1
0.044
0.025
0.016 0.33

Orbits b
sigma

0.0
0.8

-1.5
0.9

-0.4
0.6

EOPc
sigma

0.352
0.145

-0.072
0.129

0.316
0.142

-0.116
0.154

0.335
0.285

-0.08
0.09

(a) Weekly averaged transformations between the combined NRCan weekly SINEX
coordinate solutions and ITRF coordinates for the 13 IGS fiducial stations.
(b) Weekly averaged transformations between NRCan and IGS daily orbits.
(c) Weekly averaged differences between NRCan and IGS daily polar motion.

1

Polar Motion (mas)x-pole
0.5

0

-0.5

-1

y-pole
-1.5
0

50

100

150

200

Days of 1997

(a)

120

250

300

350

400

NRCan IGS Analysis Centre Report, 1997

12000

UT1-UTC (micro-sec)

11500

11000
10500
10000

9500

9000
8500
0

50

100

150

200

250

300

350

400

Days of 1997(b)
Figure 2. Difference between IGS Final EOP’s and NRCan Daily EOP solutions
As in previous years, a multi-year solution of NRCan EOPs, station coordinatesand velocities was computed (IERS, 1998). Specifically, the 1997 combination was
performed using daily solutions from 1994 to 1997 inclusively. In order to mitigate thesmall misalignment present in NRCan daily solution, the daily variance-covariancematrices were unconstrained in order to remove the implicit effect of the geocentre prior
to the combination. The annual solution was constrained at ITRF96, epoch 1998.0 using
stations: ALGO, AREQ, DAV1, DRAO, FAIR, FORT, GOLD, GUAM, HART, IRKT,KERG, KIT3, KOKB, LHAS, MADR, SANT, TIDB, TROM, TSKB, WTZR, YAR1
and YELL.
5

References
Gendt, G., (1998), IGS Combination of Tropospheric Estimates – Experiencefrom Pilot Project, 1998 Analysis Center Workshop Proceedings (edited by J.
Dow, J. Kouba and T. Springer) February 9-11, Darmstadt, Germany (in print)
IERS, (1998), Annual Report for 1997, Central Bureau of IERS, Observatoire deParis, Paris, France, 1998
Kouba, J. and Y. Mireault,(1998), “1997 Analysis Coordinator Report”, 1997
IGS Annual Report, IGS Central Bureau, Pasadena, CA (this volume)121

IGS 1997 Technical Reports

Tétreault, P., C. Huot, R. Ferland, J. Kouba and J. Popelar,(1997), “NRCan
Analysis Centre Annual Report for 1996”, 1996 IGS Annual Report, IGS Central
Bureau, Pasadena, CA, November 1997, pp. 221-232
Tétreault, P., R. Ferland, J. Kouba and J. Popelar,(1996), “NRCan (EMR)
Analysis Centre 1995 Annual Report to the IGS”, 1995 IGS Annual Report, IGS
Central Bureau, Pasadena, CA,
September 1996,pp.
175-186122

Scripps Orbit and Permanent Array Center, 1997 Analysis Center Report

Scripps Orbit and Permanent Array Center
1997 Analysis Center Report
Peng Fang, Matthijs van Domselaar, and Yehuda Bock
Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California San Diego
La Jolla, California, 92037 USA
http://lox.ucsd.edu

1

Introduction

The Scripps Orbit and Permanent Array Center (SOPAC) at the Scripps Institutionof Oceanography has been providing precise satellite ephemerides and Earth orientation
parameters since 1991. The development of the Permanent GPS Geodetic Array (PGGA)
in southern California at that time served as a catalyst for the generation of precise GPS
orbits, and continues to be a catalyst with the development of the Southern California
Integrated GPS Network (SCIGN).
This report will focus on SOPAC’s analysis procedures and reprocessing of early
IGS products. The 1997 annual report of the SOPAC Global Data Center is in a separate
document.
2

Products Submitted

IGS data and products are available at SOPAC’s GARNER archive, which isaccessible through anonymous ftp (ftp://ftp.lox.ucsd.edu) or via our homepage(http://lox.ucsd.edu). To retrieve SIO products via ftp, change directory to /pub/products,
and select the appropriate GPS week. The following SOPAC analysis products arecontributed to the IGS:
Type of Product
Final ProductsFile Format
SIOwwwwn.SP3
SIOwwww7.ERP
SIOwwww7.SUM
SIOwwww7.SNX
SIRwwwwn.SP3
Rapid Products
SIRwwwwn.ERP
Prediction Products SIPwwwwn.SP3
SIPwwwwn.SP3

Description
Daily ephemeris files
Weekly EOP (pole, UT1-UTC, lod)
Weekly processing summary
Weekly SINEX files
Daily rapid orbit solutions
Daily rapid EOP solutions
24 hour orbit predictions
48 hour orbit predictions

The final products are generally available within 4 days of the end of the GPS
week; the rapid and prediction products are available within 18 hours of the end of the UTC
day. The daily processing volume at SOPAC is about 280 stations per day.
3
Reprocessing of IGS Products (1992-1995)
During 1997 a major effort was made to reprocess GPS data from as early as June
1992. Improvements and changes in analysis software and processing algorithms makes
the coordinate time series, precise ephemerides, and EOP inconsistent over longer periodsof time and results derived by them may be biased. Therefore, we reanalyzed IGS data

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between June 1992 and June 1995. The results are a significant improvement in our site
position time series, orbit ephemerides and EOP. Also, problems encountered during thisreanalysis were recorded so that future reprocessing campaigns will be as smooth as
possible and therefore, less time consuming.
Since changes in the software are continuously made, re-analysis of earlier data
after major improvements is necessary to acquire the best products possible. The next
reprocessing which will commence in 1998 will also include data obtained before June
1992.
4

Site Selection

Various new sites were added to the IGS network in 1997. As a result, the amountof sites contributing to the daily solution increased slightly to approximately 60 per day atthe end of 1997 (Figure 2). The stations used by SOPAC in the global processing areshown in Figure 1.

Figure 1. IGS stations processed by SOPAC
To ensure fast turn around of the rapid solution, the number of stations in thissolution was limited to 47 (Figure 2), but new sites of better quality and latency wereincorporated in the rapid analysis schemes to ensure higher reliability. The days for whichdata of less than 40 sites were available for the rapid analysis were mostly correlated withweekends. The latency of the data during this time is apparently worse than during thework week. The availability of SIO’s rapid solution has improved (Figure 2) and isgenerally available within 18 hours after the end of the day.

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Scripps Orbit and Permanent Array Center, 1997 Analysis Center Report

5

Analysis Procedure

SIO continues to use a multi-day processing scheme in distributed mode for its
rapid and predicted solutions. The primary reason for doing distributed processing is toincrease processing efficiency by dividing a large network into smaller subnetworks. For
SIO’s rapid and predicted solutions, the global network is divided into two subnetworks
with 5 stations in common to provide sufficient overlap when the two solutions are
combined at a later stage. For these 5 common stations, there are a few near-by stations
chosen as backup in the event that any of these stations’ data are not available. In order tomaintain a uniform processing speed, the maximum number of stations of each subnetwork

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IGS 1997 Technical Reports

is limited to 26 which are chosen to best suit our processing hardware configuration
without sacrificing product quality.

Figure 3. Processing timeline for SIO rapid and predicted solutions
Since the total number of IGS global stations is growing steadily, our processing
procedure uses a prioritized station selection scheme to ensure the use of the stations thatare most evenly distributed and of the highest data quality. In this scheme, good
geographical location bears highest weight. This fully automated procedure and its
execution timeline are illustrated in Figure 3. During processing, tight constraints are
imposed on a priori values of ITRF96 coordinates and the UT1 values from IERS Series 7
obtained from USNO weekly (later biweekly) submissions. The main adjusted parameters
are station coordinates, orbital parameters, pole-x, pole-y positions, and LOD. In addition,
tropospheric delay parameters are estimated at one-hour intervals for each station. The time
for orbital initial conditions (IC) is set to the midday of day 0 for both day-1 and day 0
solutions. The solutions are performed with the GAMIT software package incorporating aweighted least square approach. These two sets of 24 hour solutions on individual
subnetworks are then combined with the GLOBK software incorporating a Kalman filter
approach. The resulting orbits and EOP become SIO rapid products. Once the rapid orbits
and EOP are generated, they are used for orbit extrapolation (through integration) further
into 24 and 48 hour predicted orbits. Then the rapid orbits and EOP together with 48-hour
predicted orbits are submitted to the IGS for its combination solutions. The 24-hour
predicted orbits are sent to the SIO data/product server to replace the previous day’s 48hour predicted orbits. This particular arrangement has been proven very useful since manynear real-time users who themselves have some data collection delays may take advantage
of 24 hour predictions as they are of higher quality than 48 hour predictions. The 24-hour
predicted orbits are also used as the next day’s a priori orbits.

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Scripps Orbit and Permanent Array Center, 1997 Analysis Center Report

SIO’s regular daily solution is carried out in distributed mode. Compared to therapid solutions, the regular daily solutions are single-day solutions. The maximum numberof stations in a subnetwork is set to 42 in order to include as many global stations as
possible. The latency of the daily solutions is between 4 to 6 days. The main reason for this
delay is to wait for more data and to take advantage of better a priori UTC values. The
rapid orbits serve as a priori orbits for the regular daily solutions. The selection of stationsin the global network is mainly dictated by geographical distribution. Regions with higher
concentrations of stations are grouped into regional networks, such as Europe (designatedas EURA), California (PGGA, DGGA, BARD), and U.S. (CORS). The CORS array isprocessed primarily to obtain tropospheric delay estimates at frequent time intervals (every
30 minutes).
SIO’s final weekly solution is generated after 7 regular daily solutions arecompleted. This solution, employing a Kalman filter approach with GLOBK, uses a set ofcovariance matrices from unconstrained daily solutions of both global and regional
subnetworks, tightly constraining a set of core IGS stations defined in ITRF96, to estimate
station positions, orbits, and EOP. It should be noted that the unconstrained solutions areproduced in the same GAMIT runs as the constrained solutions.
Detailed description on the modeling of parameters and other strategic settings are
given in the IGS data processing center questionnaire (see appendix), also available at IGS
Central Bureau homepage (http://igscb.jpl.nasa.gov).
Regional networks are processed with orbital parameters tightly constrained after
corresponding global solutions becomes available. The relationship among the abovementioned processes is depicted in Figure 4. (The real-time sliding window is tested but
not yet operational).
Since January 15, 1996, all SIO solutions adapted the Neill mapping function[Niell, 1996] to replace the CFA 2.2 mapping function. At the same time, the elevation
angle cutoff was lowered from 15° to 7°.

Figure 4. The relationship of various processing elements and theirproducts.

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IGS 1997 Technical Reports

6

Contact Information
To learn more about SIO’s analysis please contact:
Mailing Address:
Peng Fang or Matthijs van Domselaar
Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
9500 Gilman Drive
La Jolla, CA 92093-0225
Phone: (619) 534-2445
FAX: (619) 534-9873
E-mail: pfang@ucsd.edu, matthijs@lox.ucsd.edu
WWW: http://lox.ucsd.edu

7

Acknowledgements

Funding for analysis is provided by the U.S. National Science Foundation, theSouthern California Earthquake Center (SCEC), the William M. Keck Foundation, and
SIO. We thank all our colleagues at SOPAC, IGS, and SCIGN for their support, and Bob
King, Tom Herring, and Simon McClusky for GAMIT/GLOBK assistance.
8

References
Ash, M. E., Determination of Earth satellite orbits, Tech. Note 1972-5, Lincoln
Laboratory, MIT, 19 April 1972.
Bar-Sever, Y. E., A new module for GPS yaw attitude, in Proc. IGS Workshop:
Special Topics and New Directions, edit. G. Gendt and G. Dick, pp. 128-140,
GeoForschungsZentrum, Potsdam, 1996.
Beutler, G. (1990). Numerische Integration gewoehnlicher Differentialgleichungssysteme: Prinzipien und Algorithmen. Mitteilungen der SatellitenBeobachtungsstation Zimmerwald, No. 23, Druckerei der Universitat Bern,
1990.
Beutler, G., E. Brockmann, W. Gurtner, U. Hugentobler, L. Mervart, and M.
Rothacher, Extended Orbit Modeling Techniques at the CODE Processing
Center of the International GPS Service for Geodynamics (IGS): Theory andInitial Results, Manuscripta Geodaetica, 19, 367-386, 1994.
Dong, D., and Y. Bock, Global Positioning System network analysis with phaseambiguity resolution applied to crustal deformation studies in California,
Journal of Geophysical Research, 94, 3949-3966, 1989.
Feigl, K. L., and 14 others, Space geodetic measurement of crustal deformation incentral and southern California, 1984-1992, Journal of Geophysical Research,
98, 21,667–21,712, 1993.

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Scripps Orbit and Permanent Array Center, 1997 Analysis Center Report

Herring, T. A., GLOBK: Global Kalman filter VLBI and GPS analysis program
Version 4.17, Internal Memorandum, Massachusetts Institute of Technology,
Cambridge, 1998.
Herring, T. A., and D. Dong, Measurement of diurnal and semi-diurnal rotationalvariation and tidal parameters of the Earth, Journal of Geophysical Research,
99, 18,051, 1994.
King, R. W., and Y. Bock, Documentation of the GAMIT GPS Analysis Software
version 9.72, Mass. Inst. of Technol., Cambridge, 1998.
McCarthy, D. D. (ed.) (1992). IERS Standards (1992). IERS Technical Note 13,
Observatoire de Paris, July 1992.
McCarthy, D. D. (ed.) (1996). IERS Conventions (1996). IERS Technical Note
21, Observatoire de Paris, July 1996.
Niell, A. E., Global mapping functions for the atmospheric delay, J. Geophys.
Res., 101, 3227-3246, 1996.
Schaffrin, B., and Y. Bock, A unified scheme for processing GPS phaseobservations, Bulletin Geodesique, 62, 142–160, 1988.
Springer, T. A., G. Beutler, and M. Rothacher, A new solar radiation pressure
model for the GPS satellites, IGS Analysis Center Workshop, Darmstadt, 9-11
February 1998.
Wu, J. T., S. C. Wu, G. A. Hajj, W. I. Bertiger, S. M. Lichten, Effects ofantenna orientation on GPS carrier phase. Manuscripta Geodaetica 18, 1993,
91-98, 1993.

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IGS 1997 Technical Reports

APPENDIX=============================================================================

INTERNATIONAL GPS SERVICE FOR GEODYNAMICS
SOPAC Processing Strategy Summary=============================================================================
Analysis Center| Scripps Orbit and Permanent Array Center (SOPAC),| Institute of Geophysics and Planetary Physics (IGPP),| Scripps Institution of Oceanography (SIO),| University of California, San Diego (UCSD)|| 9500 Gilman Dr., San Diego, CA 92093-0225, USA| Phone: ++ 1 619 534 0229
| Fax:++ 1 619 534 9873----------------------------------------------------------------------------Contact Person(s) | Yehuda Bocke-mail: ybock@ucsd.edu|
phone : ++ 1 619 534 5292
| Peng Fange-mail: pfang@ucsd.edu|
phone : ++ 1 619 534 2445
| Matthijs van Domselaar
e-mail: mvandomselaar@ucsd.edu|
phone : ++ 1 619 534 2031----------------------------------------------------------------------------Software Used
| GAMIT v. 9.72, GLOBK v. 4.17,
developed at MIT/SIO----------------------------------------------------------------------------Final Products| siowwwwn.sp3
GPS ephemeris files in 7 daily
generated for|files at 15 min intervals in SP3 format,
GPS week 'wwww'|
including accuracy codes computed fromday of week 'n'|overlapping analysis wrt. previous day.
(n=0,1,...,6)| siowwww7.erp
ERP (pole, UT1-UTC) weekly solution| siowwww7.sum
Summary of weekly solution combining both|
IGS global and regional solutions.
| siowwww7.snx
Weekly coordinates in SINEX format| siowwwwn.tro
Daily files of 1-h troposphere delay|
estimates in SINEX format (based|on 1-day solutions).
Rapid Products| sirwwwwn.sp3
Daily orbits for current-1 day. ~16 hour|
delay.
| sirwwwwn.erp
Daily EOP for current-1 day. ~16 hour|
delay.
Predictions| sipwwwwn.sp3
24 hour predicted orbits. Partially|real-time. This file will replaces 48 hour|
predicted orbits of previous day upon|generation.
| sipwwwwn.sp3
48-hour real-time predicted orbits|----------------------------------------------------------------------------Preparation Date | July 13, 1998----------------------------------------------------------------------------Effective Date for| May 31, 1998
Data Analysis|=============================================================================

130

Scripps Orbit and Permanent Array Center, 1997 Analysis Center Report----------------------------------------------------------------------------|MEASUREMENT MODELS||---------------------------------------------------------------------------|| Observable| Doubly differenced, ionosphere-free combination of L1 ||| and L2 carrier phases. Pseudoranges are used only||
| to obtain receiver clock offsets and in ambiguity||| resolution.
||---------------------------------------------------------------------------|| Data weighting| Sigma on doubly difference LC phase: 64 mm||| Sampling rate: 2 minutes||| Elevation angle cutoff : 7 degrees||---------------------------------------------------------------------------|| Data Editing| Cycle slip fixing is performed at one-way level using ||| a combination of one-way and double-difference obser- ||| vations. If a cycle slip cannot be fixed reliably,||| the data observation is flagged and an additional||| bias parameter estimated implicitly in the solution.
||---------------------------------------------------------------------------|| RHC phase| Phase polarization effects applied (Wu et al, 1993)|| rotation corr.
|||---------------------------------------------------------------------------|| Ground antenna| Elevation-dependent phase center corrections are|| phase center| applied according to the model IGS_01. The corrections || calibrations| are given relative to the Dorne Margolin T antenna.
||---------------------------------------------------------------------------|| Troposphere| A priori zenith delay: nominal constant;||| sub-daily corrections estimated as described below|||--------------------------------------------------------||| Met data input: none|||--------------------------------------------------------||| Mapping functions: (Niell, 1996)||---------------------------------------------------------------------------|| Ionosphere| Not modeled (ionosphere eliminated by forming the||| ionosphere-free linear combination of L1 and L2).
||---------------------------------------------------------------------------|| Plate motions| ITRF96 velocities (see position constraints in||| estimated parameters and reference frame below)||---------------------------------------------------------------------------|| Tidal| Solid earth tidal displacement:|| displacements|
constant Love number tides|||
frequency dependent radial tide (K1)|||--------------------------------------------------------||| Pole tide: not applied|||--------------------------------------------------------||| Ocean loading: not applied||---------------------------------------------------------------------------|| Atmospheric| Not applied|| loading|||---------------------------------------------------------------------------|| Earth orientation| IERS Bulletin A plus diurnal and semidiurnal variations||| in x,y, and UT1 models (EOP) using VLBI based model ||| of Herring and Dong (1994); daily corrections to||| UT1 rate, pole position and rate estimated as||| described below|

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IGS 1997 Technical Reports|---------------------------------------------------------------------------|| Satellite center | Block I
x,y,z: 0.2100, 0.0000, 0.8540 m|| of mass|--------------------------------------------------------|| correction| Block II/IIA x,y,z: 0.2794, 0.0000, 0.9519 m|||--------------------------------------------------------||| Block IIR
x,y,z: -0.0031,-0.0012, 0.0000 m||---------------------------------------------------------------------------|| Satellite phase | Not applied|| center calibrat ||----------------------------------------------------------------------------| Relativity| Relativistic corrections applied|| corrections||----------------------------------------------------------------------------| GPS attitude| Yaw computed using model of Bar-Sever (1996), using|| model| nominal rates or estimates supplied by JPL|--------------------------------------------------------------------------------------------------------------------------------------------------------|ORBIT MODELS||---------------------------------------------------------------------------|| Geopotential| GEM T3 degree and order 8|||--------------------------------------------------------||| GM = 398600.4415 km**3/sec**2|||--------------------------------------------------------||| AE =
6378.1363 km||---------------------------------------------------------------------------|| Third-body| Sun and Moon as point masses|||--------------------------------------------------------||| Ephemeris: CfA PEP NBODY 740|||--------------------------------------------------------||| GMsun = 132712440000 km**3/sec**2|||--------------------------------------------------------||| GMmoon =4902.7989 km**3/sec**2||---------------------------------------------------------------------------|| Solar radiation | A priori: nominal block-dependent constant direct|| pressure|
acceleration; corrections to direct, y-axis, |||and B-axis constant and once-per-rev terms|||
estimated (see below) (Beutler et al., 1994; |||Springer et al. 1998)|||--------------------------------------------------------||| Earth shadow model: umbra and penumbra|||--------------------------------------------------------||| Earth's albedo: not applied|||--------------------------------------------------------||| Satellite attitude model not applied||---------------------------------------------------------------------------|| Tidal forces| Solid earth tides: frequency independent Love’s||| number K2= 0.300|||--------------------------------------------------------||| Ocean tides: UT CSR model (IERS 1996)||---------------------------------------------------------------------------|| Relativity| Applied (IERS 1996, Chapter 11, Eqn.1)||---------------------------------------------------------------------------|

132

Scripps Orbit and Permanent Array Center, 1997 Analysis Center Report| Numerical| Adams-Moulton fixed-step, 11-pt predictor-corrector|| Integration| with Nordsieck variable-step starting procedure||| (see Ash, 1972 and references therein)|||--------------------------------------------------------||| Integration step-size: 75 s; tabular interval: 900 s|||--------------------------------------------------------||| Arc length: 24 hours|--------------------------------------------------------------------------------------------------------------------------------------------------------|ESTIMATED PARAMETERS (A PRIORI VALUES & SIGMAS)||---------------------------------------------------------------------------|| Adjustment| Batch weighted least squares||---------------------------------------------------------------------------|| Station| Up to 50 IGS stations for rapid solutions|| coordinates| Up to 80 IGS stations for regular daily solutions||---------------------------------------------------------------------------|| Satellite clocks | Time fixed from Broadcast ephemeris; phase variations || bias| estimated for editing only, eliminated in solution||| by double differencing||---------------------------------------------------------------------------|| Receiver clock| Time estimated from pseudoranges phase variations|| bias| estimated for editing only, eliminated in solution||| by double differencing||---------------------------------------------------------------------------|| Orbital| 6 Keplerian elements plus 9 radiation-pressure terms: || parameters| constant and sin/cos once-per-rev terms for a direct, ||| y-axis, and b-axis acceleration; all but direct and||| y-axis constant term constrained to 1% of zero||---------------------------------------------------------------------------|| Troposphere| Knots of a linear spline in zenith delay estimated||| once per hour for each station constrained by a||
| random-walk process to 2 cm/sqrt(hr); one N-S and one ||| E-W gradient parameter per day for each station,||| constrained to 3 cm at 10 deg elevation angle||---------------------------------------------------------------------------|| Ionospheric| Not estimated (first-order effect eliminated by linear || correction| combination of L1 and L2 phase)||---------------------------------------------------------------------------|| Ambiguity| Resolution attempted for baselines < 500 km using||| phase with an 8 ppm ionospheric constraint and pseudo- ||| range (Dong & Bock, 1989; Feigl et al., 1993)||---------------------------------------------------------------------------|| Earth Orient.
| Pole X/Y and their rates, and UT1 rate estimated once || Parameters (EOP) | per day.
||---------------------------------------------------------------------------|| GPS attitude| Not estimated|| model|||---------------------------------------------------------------------------|

133

IGS 1997 Technical Reports----------------------------------------------------------------------------|REFERENCE FRAMES||---------------------------------------------------------------------------|| Inertial| Geocentric; mean equator and equinox of 2000 Jan 1||| at 12:00 (J2000.0)||---------------------------------------------------------------------------|| Terrestrial| ITRF96, with 37 stations constrained 2-3 mm in||| horizontal and 10-14 mm in vertical coordinates )||---------------------------------------------------------------------------|| Interconnection | Precession: IAU 1976|||--------------------------------------------------------||| Nutation: IAU 1980|-----------------------------------------------------------------------------

134

$VVRFLDWH$QDO\VLV&HQWHUV*OREDO1HWZRUN$VVRFLDWH$QDO\VLV&HQWHUVThe Newcastle Global Network Associate Analysis Center Annual Report

The Newcastle Global Network
Associate Analysis Center Annual Report
Ra’ed Kawar, Geoffrey Blewitt and Philip Davies*
Department of Geomatics
University of Newcastle upon Tyne
NE1 7RU, UK* Currently at the Ordnance Survey, Southampton, UK

1

Introduction

The Newcastle Global Network Associate Analysis Center (GNAAC) continued
its activities during the year 1997. It produced the weekly combined coordinate solution
for the global and the regional networks, based on ACs and RNAACs data respectively.
In spite of some delays in submitting few reports; the NCL GNAAC services are fully
restored now.
In this report a summary of our results and analysis are described for the period of
GPS week 887 through GPS week 938. During this period, we have kept the same
software and algorithms, as year 1996. Therefore, the details of our methods andalgorithms are described in the 1996 annual report.
2

New Strategy

In October 1997, because of the introduction of the IGS formal station log, wehave stopped using the input SINEXes for station information; and started using thelogistics file, updated daily, from the IGS Central Bureau. This method has saved most of
the discrepancies caused by the incorrect dome number or antenna height in the input
SINEXes. However, as a result of implementing this strategy, some of the stations which
do not have an official IGS log file are not analyzed. The regional stations were the mostaffected. We have contacted our colleagues who are running these networks and now
most of the RNAACs have official stations logs for all their stations. Figure 1 shows atime series of the number of stations analyzed by NCL P-network which clearly
demonstrate the drop of the number of GPS stations after October 1997.

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IGS 1997 Technical Reports

Introduction of loghist usage

160
140
120
100

aus
eur
gsi
NCL
pgc
sir

80
60

N_STATIONS

40
20
0
880

890

900

910

920

930

940

GPS WEEK

Figure 1: Time series of the number of stations analyzed by the NCL P-net.
3

GNET RESULTS

Figure 2 shows a time series of the number of input GPS stations by the ACs andthe number of stations analyzed by the Newcastle GNAAC. It also shows that in 1997,
we continued analyzing data from 6 Analysis Centers (COD, EMR, ESA, GFZ, JPL,NGS & SIO). It was noted that the number of stations being analyzed by the NCL
GNAAC has steadily increased toward the end of the year.

90

80

70
cod
emr
esa
gfz
jpl
ngs
sio
NCL

60

N_STATIONS

50

40

30

20
880

890

900

910

920

930

940

GPS week

Figure 2: Time series of the number of stations analyzed by the NCL G-net.

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The Newcastle Global Network Associate Analysis Center Annual Report

As mentioned in the 1996 report, the G-solution is estimated as block of normal
equations composed of deconstrained network in terms of coordinates without any
reference frame. This loose solution is later transferred and scaled to the CORE 13stations of ITRF94 using 7 parameters Helmert transformations. Figures 3 through 6
show the transformation and scaling parameters for X, Y, Z & scale for the ACs and NCL
GNAAC to ITRF94. These figures demonstrate the relatively smooth and low-value
parameters calculated for the Newcastle GNAAC transformation and scale compared to
the ACs values.

4.0E-2

3.0E-2
cod
emr
esa
gfz
jpl
sio
ngs
NCL

2.0E-2

1.0E-20.0E0
Tx (m)-1.0E-2-2.0E-2-3.0E-2-4.0E-2
880

890

900

910

920

930

940

GPS week

Figure 3: Time series of Tx transformation parameters for the ACs and NCL GNAAC to
ITRF.

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IGS 1997 Technical Reports

1.0E-1

cod
emr
esa
gfz
jpl
ngs
sio
NCL

-1.5E-8-1.0E-1
TY (m)-2.0E-1-3.0E-1

880

890

900

910

920

930

940

GPS week

Figure 4 Time series of Ty transformation parameters for the ACs and NCL GNAAC to
ITRF.

0.3
cod
emr
esa
gfz
NCL
jpl
ngs
sio

0.2

0.1
TZ (m)
0.0

-0.1
880

890

900

910

920

930

940

GPS Week

Figure 5: Time series of Tz transformation parameters for the ACs and NCL GNAAC to
ITRF.

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The Newcastle Global Network Associate Analysis Center Annual Report

cod
emr
esa
gfz
NCL
jpl
ngs
sio

1.0E-88.0E-96.0E-94.0E-9Scale

2.0E-90.0E0-2.0E-9-4.0E-9
880

890

900

910

920

930

940

GPS WEEK

Figure 6: Time series of scaling parameters for the ACs and NCL GNAAC to ITRF.
4

RNAAC Results

In year 1997 we have also continued analyzing the RNAACs data to complete thefull polyhedron. These analysis are based on the A-SINEXes input and the weekly input
R-SINEXes from the RNAACs. In order to process an R-SINEX, the responsible
RNAAC should submit at least three global stations in each weekly solution. However,this was not the case for most of the RNAACs and only few were analyzed on regularbasis, see Figure 1.
As mentioned in the 1996 report that we use the “weight-space formulae for
efficiency” to attach the R-Network to the polyhedron. This acts in turn as Helmert
transformation for 3D rotation, 3D translation and scale. Figure 7 shows a time series for
the Root Mean Square error of R-network transformation when attached to the global
network.

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IGS 1997 Technical Reports

40

aus
gsi
eur
pgc
sir

30

20

RMS (mm)

10

0

880

890

900

910

920

930

940

GPS WEEK

Figure 7: Time series of the RMS transformation of the RNAACs to NCL G-NET.
5

ITRF Realisation

As part of the Newcastle GNAAC contribution we submit our weekly G-SINEX
and P-SINEX; which contain coordinate solution for Global and regional stations. The
solution also shows better repeatability and smoother time series. Some examples ofTime series of stations analyzed by NCL GNAAC are presented in figures 8 & 9.

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The Newcastle Global Network Associate Analysis Center Annual Report

ALGO
0.05
0.04
0.03
0.02
0.01
0.00

East (m)

-0.01
-0.02
-0.03
-0.04
-0.05
880

890

900

910

920

930

940

920

930

940

920

930

940

GPS week

ALGO
0.05
0.04
0.03
0.02
0.01
0.00

North (m)

-0.01
-0.02
-0.03
-0.04
-0.05
880

890

900

910
GPS week

ALGO
0.05
0.04
0.03
0.02
0.01
0.00

Up (m)

-0.01
-0.02
-0.03
-0.04
-0.05
880

890

900

910
GPS week

Figure 8 : ALGO Time series for East, North and Up components

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IGS 1997 Technical Reports

YAR1
0.05
0.04
0.03
0.02
0.01
0.00

East (m)

-0.01
-0.02
-0.03
-0.04
-0.05
880

890

900

910

920

930

940

920

930

940

920

930

940

GPS week

YAR1
0.05
0.04
0.03
0.02
0.01
0.00

North (m)

-0.01
-0.02
-0.03
-0.04
-0.05
880

890

900

910
GPS week

YAR1
0.05
0.04
0.03
0.02
0.01
0.00

Up (m)

-0.01
-0.02
-0.03
-0.04
-0.05
880

890

900

910
GPS week

Figure 9 : YAR1 Time series for East, North and Up components

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The Newcastle Global Network Associate Analysis Center Annual Report

6

Conclusions

Based on the analysis and diagrams presented in this report; it became obviousthat Newcastle GNAAC contributes a smooth and low rms error margin coordinate
solution for the IGS and IERS, each week. This solution is also used for tectonic andgeophysical studies, to understand the plate kinematics and boundary conditions.
7

References

International GPS Services for Geodynamics, 1996 Annual Report, edited by J.F.
Zumberge, D.E. Fulton and R. E. Neilan. JPL Publication, Jet Propulsion Labratory,
Pasadena, California, 1997.

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148

MIT T2 Associate Analysis Center Report

MIT T2 Associate Analysis Center ReportThomas A. Herring
Department of Earth, Atmospheric, and Planetary Sciences,
Massachusetts Institute of Technology, Cambridge, MA 02139

1

Abstract

We discuss the analysis of the 1997 combined solutions generated from the
SINEX files submitted by the IGS analysis centers. We highlight the changes to theanalysis procedures reported in previous annual reports. Analysis of our combined
solutions shows mean fits to the (up to) 49 ITRF96 reference sites of 4.0 mm. For the
G-SINEX combinations the median RMS repeatability in north, east, and height are 2.1,
2.6 and 6.7 mm, respectively for 124 sites. For the P-SINEX combinations, the RMS
repeatabilities are 2.2, 3.0, and 7.6 mm, respectively for 181 sites.
2

Analysis Procedure Changes

As reported previously [Herring, 1996,1997], two analyses are performed eachweek. One of these uses the IGS Analysis Center (AC) weekly A-SINEX files togenerate a combined G-SINEX file and the other uses the T1 Associate Analysis Center(AAC) R-SINEX files combined with the G-SINEX file to generate a weekly P-SINEX
files. In 1997, the G-SINEX files contain 124 sites that were used more than 10 times
during the year and 82 sites that were used every week. The corresponding values for the
P-SINEX files are 181 and 114 sites, respectively. The G- and P-SINEX analyses are
performed 3 and 7 weeks delays.
The basic procedures we use have not been changed and are documented in theweekly summary files submitted with the combined SINEX files. The three changes ofnote that have been are associated with (a) deconstraining SINEX files, (b) translation andscale estimation, and (c) generation of a weekly residual file for the G-SINEX
combination.
2.1

Deconstraining AC SINEX files

The procedure we use to deconstrain SINEX files is to invert the covariancematrices for the estimates and apriori constrains, and to subtract the apriori constraints
from the estimates. New weak constraints (±5 meters for most centers) are applied to thestation coordinates and the system re-inverted. For some analysis centers, thisprocedures leads to numerical stability problems which results in the deconstrained
covariance matrices being non-positive definite. To diagnose and correct this problem, we
now compute the eigenvalues of the deconstrained covariance matrix and, if negative

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IGS 1997 Technical Reports

eigenvalues are found, scale the diagonal by increments of 1 part-per-million until all the
eigenvalues are positive. Throughout 1997, the COD and NGS analysis centers generated
negative eigenvalues when their SINEX files were deconstrained to ±5 m. (After April
1998, COD SINEX files generate all positive eigenvalues).
Our analysis of the numerical stability problems indicates that it probably arises
from large negative correlations in the deconstrained covariance matrices. The negative
correlations arise from the implicitly determined center-of-mass origin for the GPS
coordinate system. Sites that are located on opposite sides of the Earth have negatively
correlated Cartesian coordinates so that if one coordinate increases, the other must
decrease in order to keep the center of the coordinate system in the same location. These
negative correlations (which unlike positive correlations that simply must always be lessthan unity) generally can not exceed specific negative values if the whole complete
covariance matrix is to be positive definite.
2.2

Translation and Scale Estimation

We modified slightly the method used to estimate translations and scale of the
SINEX files from individual analysis centers. As in the past, during variance rescaling andthe combination solution we do not explicitly estimate translation and scale parameters.
However, now when we compare each center individually to ITRF94 (during 1997 and
now ITRF96) and to the combined solution, we explicitly estimate translations and scaleparameters. The advantage of this approach is that generates more realistic standard
deviations for the estimates of the translations and scale. The estimates of the center ofmass position and the coordinates of the sites did not seem to be greatly effected by thischange. The standard deviations of the translation and scale parameters were effected,
and for most centers increased by generally at least a factor of three and became mostconsistent with the week-to-week scatter of the values. For some centers the standard
deviations decreased when change was made.
At the end of 1997, we started using ITRF96 as the basic coordinate system inwhich our analysis is performed. We increased also from the 13-IGS core sites (whichwere reduced to less than 10 at the time of change) to 49-IGS reference sites defined in the
ITRF96 coordinate system for variance rescaling and translation estimation. In our
analyses presented here we used this new system to evaluate our 1997 combined SINEX
files.
2.3

G-SINEX Residual file

Near the end of 1997, we introduced a new file in our submission that contains
residuals by station for each of the seven IGS analysis centers. The files are named
mitwwwwg.res where wwww is the GPS week number. For the coordinate components(north, east and up) of each site the root-mean-square (RMS) scatter of the residuals foreach center to the combined solution coordinate estimate, the square root of c2 per degreeof freedom of these residuals, and the individual center residuals and standard deviations

150

MIT T2 Associate Analysis Center Report

are given. At the bottom of the file the statistics for each center are given based on all thesites analyzed by the center.

3.0

Analysis of Combined Solutions

Our analysis of 1997 combined SINEX files examines the internal consistency ofthese combinations and their agreement with ITRF96. (At the time the combinations
were made, the ITRF94 system was used). Figure 1 we show the RMS agreement
between the 49 ITRF96 reference sites (list of sites given in weekly summary files) for
each weekly combination in 1997 and the number of sites used in the realization. ThisRMS is computed from the combination of the north, east, and height differences after atranslation, rotation, and scale are removed from the weekly combination. In computing
the RMS, the height is down-weighted by a factor of 3, i.e., we construct a weight matrix
with the heights given one-tenth the weight of the horizontal components. This weightmatrix is used in the computing the RMS. In Figure 2, we show the values of thetranslations and scale factors estimated for each weekly G-SINEX file. As we have seenin the past, the Y- and Z-components of the translation appear to have significant annual
signatures. The procedures used to determine the transformation between coordinate
systems are discussed in Dong et al.. [1997].

Figure 1: RMS fit of the weekly combinations to the (up to) 49 ITRF96 reference sites.
The mean RMS fit is 4.0 mm with a median of 43 stations form the reference site list
used.

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IGS 1997 Technical Reports

Figure 2: Estimated translation and scale factors to bring the weekly G-SINEX files intoalignment with the ITRF96 reference sites. The error bars on the scale are inflated due toour down weighting the heights in the coordinate system realization.
In Figure 3, we show the histograms for the repeatabilities of the sites in the G- and Pcombinations. Although the RMS scatters are small, they are typically three-times larger
than the standard deviations of the estimates. The time series of the position estimatesalso show systematic variations as previously reported.

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MIT T2 Associate Analysis Center Report

Figure 3: Histogram of the repeatabilities from the G- and P-SINEX combinations. For
the height, six and eight sites are of the range for the G- and P-SINEX files. The medianvalues are 2.0, 2.6, and 6.7 for the north, east and height in the G-SINEX combinations
and 2.2, 3.0, and 7.6 mm in the P-SINEX combinations. The scatters are typically 3times larger than the standard deviations for indicate.
4

References
Dong D., T. A. Herring, and R. W. King, Estimating Regional Deformation from a
Combination of Space and Terrestrial Geodetic Data, J. Geodesy, 72, 200–214,
1997
Herring, T. A., MIT Global Network Associate Analysis Center Report,
International GPS Service for Geodynamics (IGS) 1995 Annual Report, pp. 203207, 1996.
Herring, T. A., MIT T2 Associate Analysis Center, International GPS Service for
Geodynamics (IGS)1996
Annual Report,
pp.
255-267,
1997.

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IGS 1997 Technical Reports

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154

GNAAC Coordinate Comparisons at JPL for GPS Weeks 813-964

GNAAC Coordinate Comparisonsat JPL for GPS Weeks 813-964
M B Heflin, D C Jefferson, M M Watkins, F H Webb, and J F Zumberge
Jet Propulsion Laboratory, California Institute of Technology
Pasadena, CA 91109 USA

Global Network Associate Analysis Center (GNAAC) activities began at JPL
starting with GPS week 813. Constraint removal was implemented on week 821 and afully rigorous combination was computed starting with week 837. Sinex 1.0 format was
implemented on week 890. A total of 152 weekly comparison reports have been
produced to date.
Many improvements are either completed or in progress. Standard antennaheights are now provided by the IGS central bureau and incorporated into the weekly
coordinate solutions. Each center will implement a weak or minimal constraint method
for all products. Daily eop estimates and their full covariance information will be
included with the coordinate solutions each week. These changes were implemented at
JPL starting with GPS week 964. All parameters including orbits, clocks, tropospheres,coordinates, and eop are estimated daily with weak constraints of no more than 10 m onany coordinate. Orbits, coordinates, and eop are rotated into alignment with ITRF96.
The geocenter and scale are left at their estimated values.
Solutions submitted from COD, EMR, ESA, GFZ, JPL, NGS, and SIO areobtained from the CDDIS each week. If necessary, a-priori constraints are removed to
the level of about 10 m. Each pair of solutions is compared after application of internal
constraints by estimating a 7-parameter Helmert transformation to minimize the leastsquares coordinate residuals. All common sites are used. The errors from each solution
are scaled to make CHI^2/DOF roughly equal to one for all pairs and four sigma outliers
are removed. The transformation parameters for each pair are given in the report alongwith the WRMS of residuals.
A free-network combination of solutions from all centers is also computed. Eachsolution is scaled and edited according to the results of pair-wise comparisons. Then all
free-network solutions are rigorously combined using their full covariance matrices. The
free-network combination is submitted to the CDDIS along with the summary report.
Sites common to all solutions are used to compare each solution with the combination.
The comparison is carried out by application of internal constraints and estimation of a 7parameter Helmert transformation. The WRMS residuals are tabulated in the report.
Results for weeks 837-964 are summarized in Tables 1 and 2. Table 1 indicatesthe mean WRMS for weekly comparisons of each center with the combination rounded to
the nearest mm. The full strength of all common sites is used for the pairwise

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IGS 1997 Technical Reports

comparisons and the transformation parameters are well determined for each pair. The
mean geocenter and scale offsets are given for each center relative to JPL in Table 2.
Table 1. Mean WRMS for GPS weeks 837-964.

Center
COD
EMR
ESA
GFZ
JPL
NGS
SIO

North
mm
2
5
4
3
2
11
3

East
mm
3
9
7
7
2
15
4

Vertical
mm
8
12
23
11
7
14
8

Table 2. Mean geocenter and scale offsets with respect to JPL for GPS
weeks 837-964.

Center
COD
EMR
ESA
GFZ
NGS
SIO

TX
cm
0.8
0.4
0.3
-0.4
0.4
-0.1

TY
cm
-0.3
-11.5
1.7
-6.0
-19.2
-0.2

TZ
cm
0.3
7.9
2.7
2.4
9.4
6.4

Scale
ppb
0.5
0.0
1.8
0.3
-1.7
-0.4

Mean geocenter offsets range from the mm level to more than 10 cm. Mean scaledifferences are less than 1 part per billion for all but two centers. Overall, weekly
comparisons show agreement in horizontal coordinates at the mm level, agreement invertical coordinates at the 1 cm level, agreement of geocenter estimates at the 1-10 cm
level, and agreement of scale estimates at the level of a few parts per billion.
This work was performed at the Jet Propulsion Laboratory, California Institute ofTechnology, under contract with the National Aeronautics and Space Administration.

156

Regional Network Associate
Analysis Centers

The EUREF RNAAC: 1997 Annual Report

The EUREF RNAAC: 1997 Annual Report
Carine Bruyninx
Royal Observatory of Belgium
B-1180 Brussels, Belgium

Daniel Ineichen, Tim Springer
Astronomical Institute, University of Bern
CH-3012 Bern, Switzerland

1

Introduction

The IAG Subcommission for the European Reference Frame (EUREF) continued
in 1997 to monitor and process the European GPS tracking network. Ten European
analysis centers process each a subnetwork and the individual subnetwork solutions arethen combined into the official weekly EUREF solution. Coordinate time series for all theEuropean stations are made available at the EUREF Permanent Network Web site :http://www.oma.be//KSB-ORB/EUREF/eurefhome.html

2

The Proposed Network
Not included in map :THU1 and KELY
in Greenland

NYAL

VARD

TROM KIR0
KIRU

REYK

SODA

VIL0

HOFN

TRON

OSLO

STAV

VAAS
MAR6
VIS0

ONSA

METS

JOEN
SVTL
MDVO

RIGA

ZWEN

LAMA

HERS

POTS
WSRT
BOR1 BOGO
DELF
KOSG
BRUS
JOZE
GOPE WROC
DENT WARE
WTZR
DOUR
MOPI
OBER
HFLK
PENC
PFAN
VENE GRAZ
ZIMM
UPAD

TOUL

GRAS

ZECK

MEDI
SOFI

MADR

VILL

EBRE

MATE
CAGL

ANKR

NOTO

SFER

NICO

MAS1

Figure 1. Network of EUREF Tracking Stations, as of January 1 1998.

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IGS 1997 Technical Reports

The EUREF network, which consisted of 56 stations (January '97), was extended
in 1997 with 9 new stations : Höfn (Iceland), Oberpfaffenhofen. (Germany), Nicosia(Cyprus), Pfänder (Austria), Sodankylä (Finland), Sofia (Bulgaria), Toulouse (France),
Westerbork (Netherlands), Zelenchukskaya (Russia). The present network is shown in
Figure 1.
3

The EUREF Local Analysis

Since mid 1996, weekly SINEX contributions from 10 European Local Analysis
Centers are combined into one official weekly EUREF solution.
Since the summer of 1997, the EUREF local analysis centers follow all theanalysis recommendations set up at the "EUREF Analysis Workshop" held in Brussels(April 1997) :• common elevation cut-off angle of 15°• use precise satellite orbits (IGS or CODE), all centers perform a fixed-orbit
processing• use consistent orbits and Earth Rotation Parameters• adopt the IGS tables modeling the constant and elevation dependent antennaphase eccentricities• adopt similar strategies for the modeling of the troposphere ; estimation of atropospheric zenith path delay/2 hours
Also in the summer of 1997, a proposal for redistribution of the subnetworks wasapproved by the EUREF analysis centers and it has been effective since September 1997.
Presently 18% of the EUREF stations have their data routinely analyzed by 2 EUREF
analysis centers, 79% by 3 centers and 3% by 4 analysis centers.
Figure 2 shows, as an example, the RMS values derived from the Helmert
residuals of the ASI/CGS analysis center with respect to the weekly combined EUREF
solution. The results for the other EUREF Local Analysis Centers are similar.

Fig. 2. RMS values derived from the Helmert residuals for ASI/CGS and EUR(combined solution) in 1997.

160

The EUREF RNAAC: 1997 Annual Report

4

The EUREF Regional Analysis

4.1

The EUREF Combined Solution

In the combination scheme of the weekly SINEX solutions, delivered by the 10
local analysis centers, first all the applied constraints are removed and the covariancematrices are rescaled. In the final step new constraints are applied to tie the solution to
the terrestrial reference frame, currently ITRF96.
Two different solutions are generated each week :• a free network solution : well suited for the detection of problems within the
combined solution and as a consistency check of the various subnetwork
solutions ;• a solution tightly constrained to the ITRF96 : the official weekly EUREF
solution.
In addition, a combination of the most recent solution with the previous six
weekly solutions is generated. This gives an insight into the behaviour of the sites over a
period of almost two months and provides a measure of the inner consistency of the
network. The RMS of this combination is below 2 mm horizontally and below 5 mm inheight.
Since March 1, 1998 (GPS week 0947), the official EUREF combined solution istied to the ITRF96. Before this time it was tied to the ITRF94. The following sites wereselected for the realization of the new reference frame : BOR1, GRAZ, KOSG, MATE,
ONSA, POTS, REYK, VILL, WTZR, ZIMM and ZWEN. These stations are tightly
constrained (0.1 mm) to their ITRF96 coordinates.
The SINEX files of the combined EUREF solution is submitted every week to the
CDDIS, BKG, ROB and CODE. More details about the combination scheme can befound in (Bruyninx et al, 1997).
4.2

Submission to the IERS (EUREF97)

As is the case for the ITRF96 realization, EUREF will contribute to therealization of the ITRF97 with a combined solution for the European permanent GPS
network (EUREF97).
One of the major problems with regional solutions, like the EUREF solution, isthat they are generated using the IGS (or CODE) orbits without orbit improvement. As aconsequence, the reference frame changes of the orbits show up in the coordinate
estimates of the regional solutions. Especially changes from ITRF92 to ITRF93 andsubsequently from ITRF93 to ITRF94 led to a significant change in the reference frame,
mainly caused by the ITRF93 being differently defined. These reference frame differences
had to be removed before combining the weekly EUREF solutions.

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IGS 1997 Technical Reports

The EUREF 97 contribution was done as follows :• First, the reference frame change due to the transition from ITRF93 to
ITRF94 (GPS week 860) was eliminated by estimating the values of a 7parameter Helmert transformation between the two reference frames. No
significant translations or rotations were found between ITRF94 and ITRF96(GPS week 0947).
• Solutions for problematic sites had to be removed for some time periods, e.g.
an 'old' receiver (MADR), snow on the antenna ( SODA), and changes of theantenna/radome set-up (MOPI). More problems existed in the original EUREF
solutions, but they were noticed and handled during the generation of theweekly EUREF solutions.
• If significant coordinate jumps were detected, then a new coordinate solution
was set up, e.g. for the sites OSLO, STAV, TRON and VARD, due to equipmentchanges.
• The reference frame was defined by constraining the coordinates (1 mm) andthe velocities (1 mm/y) of the following sites to their ITRF96 coordinates andvelocities: BOR1, GRAZ, KOSG, MATE, ONSA, POTS, REYK, THU1, VILL,WTZR, ZIMM and ZWEN.
• All weekly EUREF solutions since GPS week 0834 to 0957 were used, andthe coordinates and velocities of all stations were estimated.
The EUREF97 solution was compared to the ITRF96 and EUREF96
solutions by performing a 7-parameter Helmert transformation. The resulting RMS errors
in position and velocity are given in Table 2. The agreement is excellent and is very
consistent with the internal precision of the weekly solutions (1 mm and 5 mm for thehorizontal and vertical component, respectively).
In Figure 3, the estimated EUREF97 velocities are plotted relative to the motionof the Eurasian plate as defined by the NUVEL-1A model. Plotting the relative velocities
allows to detect easily sites with unexpected motions. The velocities for the sites STAV,TRON and OSLO differ significantly from their expected Eurasian motion. In these cases,
the GPS velocity estimates are not reliable due to a change in GPS equipment, causing theshort observation history to be split into two independent pieces. The site REYK showsa large (expected) relative velocity because it is located on the North-American plate.
Many sites in the Mediterranean area show some movement, e.g. ANKR, NICO, MATE,
and MEDI. This may be explained by the fact that the Mediterranean area is a
tectonically active area where the African and Arabian plate are colliding with theEurasian plate.

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The EUREF RNAAC: 1997 Annual Report

Table 2. Position and velocity RMS derived from the comparison of ITRF96,
EUREF96 and EUREF97.
#
STA

EUREF96 - ITRF96
EUREF97 - ITRF96
EUREF97 - EUREF96
5

52
53
52

POS

RMS

[mm]

N

E

U

0.8
1.3
1.1

0.8
1.7
1.3

4.5
4.2
3.1

VEL.RMS
TO
T
2.7
2.8
2.1

[mm/y]
4.5
4.7
4.1

OutlookA temptative list with about 20 future EUREF stations can be found at
http: //www.oma.be//KSB-ORB/EUREF/planned.html.

In order to assess the velocity of the European plate with respect to neighbouring
tectonic plates, EUREF will try to include permanent stations from outside Europe(North-Africa,

Fig. 3. : Map of the EUREF97 velocities relative to the motion of the Eurasianplate.

163

IGS 1997 Technical Reports

Middle-East) into its routine network processing. These stations will beconsidered as "Associated EUREF stations".
In the course of 1998, a supplementary local analysis center, located at IGN
France, will be contributing to EUREF including the new permanent stations planned in
France.
6

Acknowledgments

Without the labour and the commitment of the responsible agencies, theirrepresentatives at the observation sites, the data centers and the analysis centers, the
EUREF network would not be the success that is it today. The authors would like toacknowledge especially the responsibles of the EUREF analysis centers who havecontributed to this report : J. Dousa (Geodetic Observatory Pecny, Czech Republic), W.
Ehrnsperger (Bayerische Kommission für die Internationale Erdmessung, Germany), C.
Ferraro, M. Fermi, A. Nardi, C. Sciarretta and F. Vespe, (ASI Space Geodesy Center,
Italy), M. Figurski and J. Rogowski (Warsaw University of Technology, Poland).
7

References
Bruyninx C. et al (1997), "The EUREF Associate Analysis Center : 1996 Annual
Report", in International GPS Service for Geodynamics, 1996 Annual Report,
Jet Propulsion Laboratory, Pasadena, California

164

Report of the Regional Network Associate Analysis Center for Far-East Asia

Report of the Regional Network Associate Analysis Center
for Far-East Asia.
K. Murakami
Geodetic Observation Center.
Geographical Survey Institute , Japan

1

Introduction

In early 1996, a pilot project aimed at analyzing the performance of Regional
Network Associate Analysis Center (RNAACs) was initiated to prove the feasibility of
the concept of distributed data processing in its operational services. In response to theInternational GPS Service for Geodynamics (IGS)'s call for participation,Geographical
Survey Institute of Japan (GSI) started to process 7 sites from its dense network withother IGS global sites.
2

Outline of the Processing

7 sites are selected for the pilot project and have been processed with
International GPS Service (IGS) global tracking station. (Figure 1a,1b). Daily coordinate
solutions are generated using GAMIT version 9.45 and they are combined with GLOBK
version 4.0 to generate weekly constraint solutions.
•
•
•
•
•
•

3

Characterizing features of the performed solutions are
Final IGS orbits and Earth orientation parameters are applied.
Measurement elevation angle cut off 20 degrees, sampling rate 2 minutes for
single-day adjustments.
Tropospheric zenith delays are estimated every 3 hours.
Orbit relaxation strategy is used.
Station coordinates estimated in the International Terrestrial Reference Frame(ITRF), applying a priori sigma of _}10 .

Summary

Estimated parameters are obtained as Software/Solutions Independent
Exchangeable (SINEX) format and submitted to Crustal Dynamics Data Information
System (CDDIS). Densification of ITRF is important in Eastern Asia Region to
construct a rigorous reference frame.

165

IGS 1997 Technical Reports

Figure 1. International GPS Service (IGS) global tracking station.

166

The PGC Regional Network Associate Analysis Centre: 1997 Annual Report

The PGC Regional Network Associate
Analysis Centre: 1997 Annual Report
H. Dragert, J. Henton, Y. Lu, M. Schmidt
Geological Survey of Canada
Pacific Geoscience Centre
Sidney, British Columbia, Canada

1

Introduction

The Western Canada Deformation Array (WCDA) is a regional network ofcontinuous GPS tracking stations in southwestern British Columbia established by the
Geological Survey of Canada (GSC) beginning in 1991. This network currently consists of
8 sites (see Table 1) whose data, along with data from Whitehorse and Neah Bay, areanalyzed at the Pacific Geoscience Centre (PGC) located in Sidney, British Columbia, 17
km north of Victoria. The purpose of the WCDA is to monitor crustal deformation along
the northern Cascadia margin and thereby provide a better understanding of crustal
dynamic processes and earthquake hazard in this region.
TABLE 1
Location:

Site ID:

Domes No.:

Latitude:Longitude: Start:

Whitehorse, Y.T.

WHIT

40136M001

60.7508-135.2203

Jun-96

Williams Lk, B.C. WILL

40134M001

52.2369-122.1678

Oct-93

Holberg, B.C.

HOLB

40130M001

50.6403-128.1350

Jul-92

Whistler, B.C.

WSLR

40141M001

50.1265-122.9212

Sep-96

Penticton, B.C.

DRAO

40105M001

49.3225-119.6250

Feb-91

Nanoose, B.C.

NANO

40138M001

49.2948-124.0865

May-95

Ucluelet, B.C.

UCLU

40140M001

48.9256-125.5413

May-94Sidney, B.C.

PGC140129M002

48.6486-123.4511

Dec-89Victoria, B.C.

ALBH

40129M003

48.3897-123.4875

May-92Neah Bay, WA

NEAH

40139M001

48.2978-124.6249

Jul-95

Table 1: GPS sites included in the regional network analyses carried out at PGC . All
sites except NEAH are operated by the Geological Survey of Canada; NEAH is operated
by the University of Washington as part of the Pacific Northwest Geodetic Array(PANGA). All GSC sites use Rogue SNR-8000 receivers and Dorne-Margolin chokering
antennas; NEAH currently uses an Ashtech Z12 receiver with an Ashtech DorneMargolin chokering antenna.

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IGS 1997 Technical Reports

The GSC also operates two additional stations in central Canada (see Table 2).
Data for all Canadian sites (except PGC1 which is a site for testing receivers) are available
via anonymous ftp from our server sikanni.pgc.nrcan.gc.ca. Alternatively, the files can beaccessed through the WCDA home page at:
http://www.pgc.nrcan.gc.ca/geodyn/wcda.htm.
The daily data files are also forwarded to the Geodetic Survey of Canada, Ottawa,and to NASA’s CDDIS located at Goddard.
TABLE 2
Location:
Flin Flon, Man.

Site
ID:
FLIN

Domes No.: Latitude: Longitude Start::Start:40135M001 54.7257-101.9780 May-96

Lac Dubonnet, Man. DUBO 40137M001 50.2588
Man. Man.

-95.8662

Oct-96

Table 2: Continuous GPS sites in central Canada operated by the Geological Survey ofCanada under a cooperative program with NASA to study glacial rebound. The Rogue
SNR-8000 receivers at these sites have been provided by NASA and are supported by
UNAVCO. Support for site operations is also provided in part by Manitoba Hydro.

2

Data Analysis

Daily station solutions are obtained using an L3 double-differencing strategy with
IGS final orbits held fixed and a fixed reference station (DRAO). All other stations areloosely constrained (sigmas = 10 m). The variance-covariance matrix from this differential
solution is scaled by a factor of 9 to facilitate a less rigid network integration of theseregional results with global results and augmented to a full matrix by including thereference station position as a loosely (sigmas = 10 m) constrained observation. (To avoid
a potential non-positive definite condition of the matrix arising from numerical precisionof the computing platform, the nominal XX, YY, and ZZ variances for DRAO were set at
100 + 1E-08.) Daily station solution files are combined into a weekly solution using the
program "stacomb" from the Geodetic Survey Division, NRCan, Ottawa. The input files
are the 7 daily station-solution Sinex files generated at PGC as well as a reduced versionof the commensurate EMR weekly Sinex file which provides the coordinates and thecovariances of sites common to both analyses as a priori constraints. The common sites
are DRAO, ALBH, WILL, and WHIT. Currently, the a priori covariances introduced
from the reduced version of the EMR weekly solution has been limited to the diagonalterms only; i.e. the a priori correlation between sites has been removed.
Other parameters used to characterize the WCDA regional network solution aresummarized in the list below (for details see Dragert et al., 1995, and Chen, 1998):

168

The PGC Regional Network Associate Analysis Centre: 1997 Annual Report

Solution Identifier:
Software Used:
Reference Frame:
Reference Epoch:
GM:
Gravity Model:
Tidal Corrections:
Ocean Loading:

Tropospheric Model:
Solution Type:

Solution Basis:

3

UT date and time stamp of combined weekly file
CGPS22 V1.0 (UNIX ) (Kouba and Popelar, 1990)
Defined by IGS orbits (nominally ITRF94 during
1997)
Current date
398600.4415 km3/s2
GEMT3(8,8) + C21 + S21
Solid earth tide and pole tide corrections
Pagiatakis, global_model
Note: A new table of ocean loading corrections was
generated and applied beginning with GPS Week
892. The Schwiderski 1 x 1 degree global modal wasused to calculate the M2, S2, K1, O1, N2, & P1
constituents; beginning with Week 894, nineconstituents were used by adding K2, Q1, and Mf.
Stochastic coloured noise with a correlation time of
10 hr
L3 double-differenced phase (ionosphere-free linear
combination of L1 and L2) with orbits held fixed &ambiguities not held fixed
120 sec intervals using all available satellites (max.
8) above 15o elevation

Results

The precision of the daily network solutions is gauged by the L3 double-difference
phase range residuals averaged over all satellites and all stations. In general, the average ofthese residuals vary from 6 to 9 mm. Formal errors for the daily solutions of network
station locations relative to the reference station (DRAO) are less than 1, 2 , and 4 mm for
relative latitude, longitude, and height respectively. More realistic estimates of errors are
given by the day-to-day variations of relative positions which have sigmas that are double
the values listed above.
Detailed analysis of temporal variations in the relative positions of network sites
have resolved secular motions at the level of a few millimetres per year and also revealed
significant transient signals of non-tectonic origin. For sites on southern Vancouver Island,
velocity estimates based on linear trends in daily solutions of relative positions are in
general agreement with current (pseudo) three-dimensional elastic models of a locked
thrust fault. The margin-parallel motion of the GPS site on northern Vancouver Island(HOLB) is more consistent with shear-strain expected from Pacific Plate/North America
Plate interaction across northwest trending strike-slip faults as opposed to elastic
shortening across a locked convergent margin.

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IGS 1997 Technical Reports

Figure 1: Horizontal motions of WCDA network sites relative to DRAO estimated fromlinear trends in the daily solutions. Length of observations at each site vary from over 4
yr to less than 2 yr.

The linear trends obtained from regressions on daily GPS solutions of relative site
positions which form the basis of the crustal velocity estimates are made more uncertain
by instrumental effects and by transient signals of non-tectonic origin. For example,
changes in the position of the antenna phase centres as large as 3 mm in the horizontal and
19 mm in the vertical have been observed to accompany changes in the physical mountingof the antennas. Annual and semi-annual signals with amplitudes of several millimetres
have been observed predominantly in the relative vertical components for almost all
network sites. Non-stationary spectral peaks with amplitudes as large as 7 mm also
appear in the relative vertical components in the period range of 10 to 20 days. The exact
causes of these non-linear temporal variations are not known. However, the amplitudes ofsome of these signals appear to vary with elevation cut-off used in the analyses,
indicating a near-field multipathing effect as a possible factor in these phase-centre
changes.
Large sudden offsets in the phase centres of antennas are the most detrimental tounbiased estimates of linear trends in a station’s time series. Table 3 provides a chronology
for all WCDA sites of when physical changes were made in the set-up of the antennas
which resulted in an apparently significant (2 to 3 mm in the horizontal; 5 to 20 mm in
the vertical) change of relative position. The magnitudes of these changes are strongly

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The PGC Regional Network Associate Analysis Centre: 1997 Annual Report

dependent on the nature of the GPS data analysis (L3 solution; double differencing;elevation cut-off; etc.); consequently, no absolute calibration of these steps can beprovided. If data from WCDA sites are being used to estimate absolute or relative crustal
motions, Table 3 can be used to flag times when discontinuities may have occurred.
Table 3Date

Site

Description of Changes

94:041DRAO

New antenna; New antenna mount;

94:124HOLB

New antenna;

94:173WILL

Added acrylic dome & mounting ring;

95:011

ALBH

New antenna mount;

95:103DRAO

New antenna mount;

95:158ALBH

New antenna;

95:189WILL

New antenna mount; Added RF screening skirt;

95:202

ALBH

Added RF screening skirt;

95:223

UCLU

Added RF screening skirt;

96:010DRAO

Added RF screening skirt;

96:046

NANO

Added RF screening skirt;

96:089HOLB

New antenna mount;

97:033
NEAH
New antenna; New Ashtech cone dome;
Table 3: Listing of changes in the antenna set-up at WCDA sites which affected
the mean phase-centre positions. Dates give year followed by the Julian day.

4

References
Chen, X., Continuous GPS monitoring of crustal deformation with the Western
Canada Deformation Array: 1992-1995. M.Sc.E, thesis, Dept. of Geodesy
and Geomatics Engineering Technical Report No. 195, University of New
Brunswick, Fredericton, N.B., Canada, 158 pp, 1998.
Dragert, H., X. Chen, and J. Kouba, GPS monitoring of crustal strain in southwest
British Columbia with the Western Canada Deformation Array,
Geomatica, Vol. 49, No. 3, 301-313, 1995.
Kouba, J., and J. Popelar, GPS satellite monitoring and data analysis, Proceedingsof the 2nd International Symposium on Precise Positioning with GPS,
Ottawa, Canada, 656-662, 1990.

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172

Annual Report 199 of the RNAAC SIRGAS

Annual Report 1997 of the RNAAC SIRGAS
Wolfgang Seemüller and Hermann Drewes
Deutsches Geodätisches Forschungsinstitut
München, Germany

1

Introductions

The IGS Regional Network Associate Analysis Center for the South American
Reference System (RNAAC SIRGAS) is processing, on a weekly basis, all available dataof permanently observing GPS stations in the mainland of South America and thesurrounding areas. The resulting station coordinates solutions are routinely forwarded as
SINEX files to the IGS Global Data Centers, and then combined within the polyhedron
solutions of the Global Network Associate Analysis Centers (GNAAC). In early 1998,
there are 22 stations included in these solutions.
2

Station Network

The RNAAC SIRGAS configuration has been extended in 1997 by including four
new stations in Brazil (Cuiaba, Imperatriz, Manaus, Vicosa), and one in Venezuela(Maracaibo). Instead of the two global IGS stations Bermuda and Richmond, the newstation Barbados was included as a reference station. The complete actual network isshown in Figure 1.

Figure 1: Network processed by the IGS RNAAC SIRGAS

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IGS 1997 Technical Reports

3

Processing and Results

The processing is continuously done using the automated Bernese Software
Version 4.0 (Rothacher and Mervart 1996). The results are controlled by daily andweekly time series comparisons (see also Seemueller and Drewes 1998). A typicalexample is shown for the new station Vicosa in figure 2. We clearly see the broad
variations of the diurnal solutions (only weakly constrained in the IGS orbits reference
frame) and the smooth behaviour in the GNAAC‘s weekly global polyhedron solutions(constrained by IGS stations in the ITRF). The deviations between both solutions,
RNAAC and GNAAC, are typically in the sub-centimeter level for horizontal
components and one to two centimeters in the vertical.

Figure 2: Variations of coordinate solutions for station Vicosa (Brazil)
4

References
Rothacher, M. and L. Mervart (eds.): Bernese GPS Software Version 4.0.
Astronomical Institute, University of Berne, 1996.
Seemueller, W. and H. Drewes: The IGS Regional Network Associate Analysis
Center for South America at DGFI. Proc. IGS Analysis Center Workshop,
Darmstadt, 1998

174

Rapid Service and Prediction

U.S. Naval Observatory Center for Rapid Service and Predictions

U.S. Naval Observatory Center
for Rapid Service and Predictions
J.R. Ray, J.R. Rohde, P. Kammeyer, and B.J. Luzum
Earth Orientation Dept., U.S. Naval Observatory
Washington, DC 20392 USA

1Introduction

The mission of the U.S. Naval Observatory (USNO) includes determining thepositions and motions of the Earth, Sun, Moon, planets, stars and other celestial objects,
providing precise time, measuring the Earth's rotation, and maintaining the master clock
for the U.S. The Earth Orientation (EO) Department contributes to this mission bycollecting suitable observations and performing data analyses to determine and predict the
time-varying orientation of the terrestrial reference frame within the celestial reference
frame. The key parameters determined and disseminated are polar motion coordinates,
universal time (UT1), precession, and nutation. The user community includes the U.S.
Department of Defense, other U.S. government agencies, scientific researchers, and thegeneral public. The primary applications are for high-accuracy navigation and positioning
with an emphasis on real-time uses.
In order to accomplish these objectives, USNO collaborates closely with a large
number of other groups and organizations, and relies on a combination of results from avariety of techniques. Very long baseline interferometry (VLBI) is essential in order tomaintain accurate knowledge of UT1, the celestial pole, and the celestial reference frame,
which is realized by the positions of about 600 extragalactic radio sources. Increasingly,
however, GPS is being used to satisfy important aspects of the USNO mission. This
report summarizes the current status of USNO participation with the IGS and GPS,
together with some recent results.
2

IERS Sub-Bureau for Rapid Service and Predictions

The IERS Sub-bureau for Rapid Service and Predictions of Earth orientation
parameters (EOPs) is hosted by the EO Dept. at USNO. EOP results contributed bymany analysis centers derived from observations by VLBI, satellite laser ranging (SLR) to
LAGEOS, lunar laser ranging (LLR), or GPS are combined into a homogeneous daily timeseries which is updated and distributed twice each week as IERS Bulletin A. Combined
EOP values for the recent past are published together with predictions extending a year
into the future.
In recent years, the Bulletin A polar motion results have been dominated by theprecise, daily determinations of the IGS combined Final products, with the Rapid series
being used for the most recent measurements. The Rapid determinations are quite
important for Bulletin A by providing timely (delivered within 22 hours after each UTC

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IGS Technical Reports

midnight), high-quality results which are most significant for the polar motion predictions
needed by real-time users. The accuracy of these series during 1997 is estimated to beabout 0.1 mas (per component) for the Finals and 0.2-0.3 mas for the Rapids.
Implementation on 01 March 1998 of the much more robust and improved terrestrialreference frame realization proposed by Kouba et al. (1998), where the coordinates andvelocities of 47 sites are constrained to their ITRF96 values, will surely produce
significantly more stable polar motion results. Early indications are that the accuracy of
the Rapid polar motion values has been improved to about 0.1 mas (Ray, 1998a). This,in turn, should improve the quality and reliability of near-term polar motion predictions.
In addition to polar motion, IERS Bulletin A has become increasingly reliant on
IGS estimates of length of day (LOD). The IGS started producing an official LOD
product on 02 March 1997 using a weighted combination of LOD results submitted byeach IGS Analysis Center (AC). To calibrate for LOD biases, each series is comparedwith the most recent 21 days of non-predicted UT1 values from Bulletin A (Kouba and
Mireault, 1997; Ray, 1996). Shortly after its advent, the IGS combined LOD results
were introduced into the Bulletin A combination to extend the UT1 value of the mostrecent VLBI determination forward by integration. A few months later an independent
set of GPS-based estimates of univeral time, derived at USNO and described below, werealso included in Bulletin A. About two weeks of the most recent estimates are used, after
calibration in offset and rate compared to overlapping UT1 results from VLBI. In April
1998, an analogous universal time series from the EMR AC (Natural Resources Canada)
was added in a similar manner. The EMR analysis strategy differs from the other IGS
groups in applying a priori orbit constraints that allow both universal time and LOD tobe estimated simultaneously. These three series together have proven very successful inextending UT1 results forward from the latest VLBI determinations, which can have alatency of up to about a week. As a consequence, the last non-predicted UT1 value in
Bulletin A is now generally more accurate than 100 :s, usually considerably more so.
Errors in predicted EOP values are a significant source of systematic error in the
IGS Predicted orbits, although they rarely dominate the overall error budget. MartRn
Mur et al. (1998) have stressed the need for improved EOP predictions for use incomputing the IGS Predicted orbits. Partly to address this concern, refinements alreadyunder development were implemented in Bulletin A on 03 March 1998 (Ray and Luzum,1998). The improvement is most significant for the shortest prediction intervals (53% for
1 day) with dimishing effect over longer spans. Research is continuing into further
improvements, which are likely to be implemented later in 1998.
For real-time users, given two updates of Bulletin A each week, the longest
prediction interval is 7 days (for Tuesday updates compared with the previous Thursday
issue which normally contains most recent data from 2 days earlier). This means that
real-time users can experience polar motion prediction errors up to ~2.4 mas (in an RMS
sense per component) and UT1 errors as large as ~1 ms (15 mas). Predictions of UT1
variation are more problematic because the geophysical excitation is about an order ofmagnitude larger than for polar motion. To reduce these prediction errors significantly

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U.S. Naval Observatory Center for Rapid Service and Predictions

will require more frequent Bulletin A updates, preferably done daily shortly after the IGS
Rapid products are released. Such a process is expected to be implemented during 1998
and will reduce the longest prediction interval from 7 days to ~58 hours, under normal
circumstances. In that case, the maximum errors for real-time users should be less than
~0.7 mas (RMS per component) for polar motion and less than ~0.3 ms for UT1.
As part of its contribution to the IGS, USNO prepares regular reports and plotsof the performance of each IGS Analysis Center compared with Bulletin A, which areavailable at http://maia.usno.navy.mil/bulletin-a.html. Additional analysis reports areprepared occasionally to assess changes in IGS performance (e.g., Ray, 1998a) or toevaluate the geophysical implications (e.g., Eubanks et al., 1998).
3

IGS Rapid Service Associate Analysis Center

Given the significance of the IGS results and our increasing reliance upon them, it
is natural for USNO also to contribute actively as a data analysis center. Beginning 23
April 1997 USNO officially became a contributor to the IGS Rapid products. Originally
we intented to eventually begin submitting Final and other products to become a full IGS
AC. Since that time, however, other GPS-related activities have developed, particularly
the IGS/BIPM timing project (Ray, 1998b), which are more closely tied to the USNO
mission and therefore take higher priority. Given limited resources, USNO would, for thetime being, prefer not to assume the additional responsibilities of a full AC and planinstead to maintain the status of a Rapid Service Associate Analysis Center (RSAAC)
contributing to the IGS Rapid and (eventually) Prediction products. We hope to begin
producing and contributing predicted GPS orbits by late 1998.
Basic features of the USNO Rapid analysis strategy are summarized in Table 1.
The software used is GIPSY/OASIS II, developed and maintained by JPL.
Table 1. USNO Analysis Strategy.
SoftwareObservables
Arc length
NetworkElevation cutoff
Sat. parameters

Attitude

GIPSY/OASIS II (vers. 4.8)
carrier phase & pseudo-range at 7.5 min.
3 + 24 hr.
30 global sites (usually)
15 deg.
initial position & velocity,rad. pressure scale & Y-bias as constants,
X,Z rad. pressure scales as stochastic
yaw rates for eclipsing sats.

An emerging interest at USNO is improved GPS orbits for a variety of real-time
applications, including precise time transfer. This requires the highest quality Rapid andPredicted orbits and EOPs. These interests mesh naturally with full participation in the
IGS/BIPM timing project. In support of this effort, USNO deployed a TurboRogue
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IGS Technical Reports

SNR12 receiver in Washington, DC connected to the USNO Master Clock (MC) as its
frequency reference. Data were released to the IGS starting 01 May 1997. A second
receiver was deployed at Falcon (recently renamed Schriever) Air Force Base in Colorado
Springs on 25 March 1998. This is the site of the USNO Alternate Master Clock (AMC)
and the operations center for GPS. The MC and AMC are kept closely synchronized byhourly two-way satellite time transfer (TWSTT) observations. These IGS sites cantherefore serve as important comparison sites in the IGS/BIPM project.
A "timing solution" strategy is currently under development for the IGS/BIPM
timing project. The approach is to adopt the IGS combined orbits (either Rapid or Final,
as appropriate) without adjustment and to determine all the station and satellite clocks ina fully consistent way, relative to a reference station clock (normally, USNO). It isessential to use a global network of stations, preferably equipped with highly stablefrequency standards, and to resolve as many phase cycle ambiguities as possible.
Computational constraints limit the size of the tracking network to about 30 stations. In
order to densify the clock network it will be necessary to supplement such an analysis
with an adaptation of the precise point positioning technique (Zumberge et al., 1997).
4

GPS Determinations of Universal Time

Elsewhere Kammeyer (1998) has described his method to determine UT1-likevariations from an analysis of GPS orbit planes as estimated for IGS Rapid submissions.
Briefly, the Earth-fixed GPS ephemerides determined operationally at USNO arecompared to orbit planes propagated using a modeled radiation pressure acceleration
normal to each orbit plane. For each satellite, the modeled acceleration is expressed
relative to the projection of the Sun direction on the orbit plane and depends only on theangle from the orbital angular momentum to the Sun. The models being used wereobtained empirically from observed experience when this angle was greater than 90N
during 1994-1995. For each satellite, there is a unique axial rotation angle which brings
the observed Earth-fixed positions into alignment with the propagated orbit plane. Since
the propagated orbit plane of each satellite is different from its osculating orbit plane in an
inertial frame, offsets are added to the rotation angles to form single-satellite estimates ofGPS-based universal time. The median of these values for the 13 satellites modeled gives
the UT estimate reported to IERS Bulletin A.
Kammeyer's results show quite encouraging short-term performance. The
relatively slow drifts allow sliding segments of these data to be included in the Bulletin A
combination after calibration only for an offset and a rate. The residual scatter is about 75:s over 3-week intervals. The procedure is described above and has proven very
successful in extending more accurate but less timely VLBI measurements of UT1 to nearreal-time. Over longer spans these GPS-based determinations drift systematically, up to
about 600 :s in six months.
Several improvements can be made and are being pursued. The long-term drifts
reflect deficiencies in empirical models for the orbit plane motions. With the longer seriesof orbits now available, better models should be feasible. In particular, the present

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U.S. Naval Observatory Center for Rapid Service and Predictions

models are based on data collected before the current yaw bias was implemented. Models
can be constructed for all satellites, not just the 13 currently being used. The procedure
could be applied to the more precise and reliable IGS combined orbits rather using the
USNO orbits.
5

References
Eubanks, T.M., J.R. Ray, and B.J. Luzum, The atmospheric excitation of rapid polar
motions, Eos Trans. American Geophysical Union, 79(17), S55, 1998.
Kammeyer, P., Determination of a UT1-like quantity by comparing GPS positions topropagated orbit planes, in preparation 1998.
Kouba, J., and Y. Mireault, Analysis Coordinator report, in International GPS Service
for Geodynamics, 1996 Annual Report, edited by J.F. Zumberge, D.E. Fulton, andR.E. Neilan, Jet Propulsion Laboratory, Pasadena, California, pp. 55-100, 1997.
Kouba, J., J. Ray, and M.M. Watkins, IGS reference frame realization, in 1998 IGS
Analysis Center Workshop Proceedings, European Space Operations Centre,
Darmstadt, Germany, in press 1998.
MartRn Mur, T.J., T. Springer, and Y. Bar-Sever, Orbit predictions and rapid products,
in 1998 IGS Analysis Center Workshop Proceedings, European Space Operations
Centre, Darmstadt, Germany, in press 1998.
Ray, J., GPS measurement of length-of-day: Comparisons with VLBI and consequencesfor UT1, in 1996 IGS Analysis Center Workshop Proceedings, edited by R.E. Neilan,
P.A. Van Scoy, and J.F. Zumberge, Jet Propulsion Laboratory, Pasadena, California,
pp. 43-60, 1996.
Ray, J., EOP improvements due to the ITRF96 change, IGS Electronic Mail #1853,
http://igscb.jpl.nasa.gov/igscb/mail/igsmail/igsmess.1853, 02 Apr. 1998a.
Ray, J, The IGS/BIPM time transer project, in 1998 IGS Analysis Center Workshop
Proceedings, European Space Operations Centre, Darmstadt, Germany, in press
1998b.
Ray, J., and B. Luzum, Improved polar motion predictions, IGS Electronic Mail #1816,
http://igscb.jpl.nasa.gov/igscb/mail/igsmail/igsmess.1816, 27 Feb. 1998.
Zumberge, J.F., M.B. Heflin, D.C. Jefferson, M.M. Watkins, and F.H. Webb, Precise
point positioning for the efficient and robust analysis of GPS data from large
networks, J. Geophys. Res., 102(B3), 5005-5017, 1997.
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IERS Contributions

IERS References - Contribution of the Central Bureau of IERS

IERS References
Contribution of the Central Bureau of IERS
C. Boucher, Z. Altamimi, P. Sillard
Institut Géographique National

D. Gambis, E. Eisop
Paris Observatory

1

The International Terrestrial Reference Frame

The main ITRF event in 1997 of interest to IGS is the generation of a new
solution, namely the ITRF96, which is used as reference by IGS starting on March 1,
1998.
1.1

ITRF96The ITRF96 solution represents a new generation of realization of theInternational Terrestrial Reference System (ITRS). It is achieved by combining
simultaneously positions and velocities using full variance- covariance information
provided, in SINEX format, by the IERS analysis centers. Moreover, a rigorous
weighting scheme is used, based on the analysis and estimation of the variancecomponents using Helmert method.
A brief description of the ITRF96 solution could be found in the 1997 IGS
Annual Report, Volume 1. For more details about the results and analysis of the ITRF96,
see (Boucher et al, 1988). Moreover, in Volume 1, we also provided some information
about the selection of 47 ITRF96 reference stations to be used by IGS. For more details,
see also (Altamimi, 1998). All the ITRF96 related files are available via the Web at:
http://lareg.ensg.ign.fr/ITRF/ITRF96.html
Here, we will provide more information on ITRF96 of interest to IGS.
The ITRF96 global combination is achieved using the following properties:••

17 selected space geodetic solutions provided by the IERS analysis centersand 70 SINEX files, containing positions and covariances computed fromlocal ties.
The reference frame definition (origin, scale, orientation and time evolution) of
the combination is achieved in such a way that ITRF96 is in the same system
as the ITRF94.
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IGS 1997 Technical Reports

•
•

1.2.

Velocities are constrained to be the same for all points within each site.
Matrix Scaling Factors have been rigorously estimated during this combined
adjustment, which was then iterated.

IGS Contribution to ITRF96

6 global IGS/GPS solutions were included in the ITRF96 adjustment. Moreover,two European GPS solutions, computed by CODE, were also included as part of EUREF
contribution to the ITRF. These GPS solutions contain about 194 stations. Table 1 lists
these solutions together with solutions coming from the other IERS techniques. Figure 1
shows the coverage of the 4 IERS technique sites implied in the ITRF96.
To have an idea about the quality of the individual solutions used to generate the
ITRF96, Table 1 lists the global 3D Weighted RMS in position as well as in velocity. In
position, the best level reached in 3D precision is greater than 1 cm for VLBI and GPS,around 1 cm for SLR, and 3 cm for DORIS. In velocity, the best 3D precision is about 2
mm/year for SLR and VLBI, 4 mm/year for GPS, and 8 mm/year for DORIS.
Conversely, the largest discrepencies (excepting 2 outliers) between the origins andscales of the individual solutions are respectively 4 cm and 4 x 10-9.
Table 1. ITRF96: Used data and global residuals per solution.
------------------------------- --------------------------------------------------------------------Solution

Numberof points

Data Spanyy-yy

Position RMS
mm

Epoch
yy:doyVelocity RMS
mm/y----------------------------------------------------------------------------------------------------VLBI
SSC(GSFC) 97 R 01
SSC(GIUB)
97 R 01
SSC(NOAA)95 R 01
SSC(JPL)
97 R 01

120
43
111
8

79-97
84-96
79-94
91-96

5.80
13.60
14.70
20.70

93:001
93:001
93:001
93:001

1.90
.50
1.90

SLR
SSC(CSR)
SSC(GSFC)

96 L 01
97 L 01

89
38

76-96
80-96

11.10
10.90

93:001
86:182

3.80
1.70

GPS
SSC(EMR)
SSC(GFZ)
SSC(CODE)SSC(EUR)
SSC(EUR)
SSC(MIT)
SSC(NCL)
SSC(JPL)

97 P 01
97 P 02
97 P 02
97 P 04
97 P 03
97 P 01
97 P 01
97 P 02

36
66
100
39
58
132
114
113

95-97
93-96
93-97
95-96
96-97
94-97
95-97
91-96

10.00
16.80
7.10
2.40
2.90
8.50
5.40
9.40

96:001
94:365
95:076
96:09096:33997:151
96:001
96:001

3.50
3.30
1.90
.30
.30
9.20
6.30
3.80

DORIS
SSC(GRGS) 97 D 01
SSC(CSR) 96 D 01
SSC(IGN)
97 D 04

48
54
62

93-96
93-96
90-97

26.90
26.10
28.30

93:001
93:001
95:100

8.00
10.60
12.80

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IERS References - Contribution of the Central Bureau of IERS

VLBI

SLR

GPS

DORIS

Figure 1. ITRF96 Sites
2.

Earth Orientation

2-1

Contribution of GPS to Polar Motion and LOD

GPS EOP solutions (polar motion and LOD) have a significant contribution in themulti-technique combined solutions derived by the IERS. Since 1994, a combined
solution of the various GPS series is performed and is used in our current analyses. Table
2 shows the mean differences and the unbiased RMS agreements of the various series
GPS which contributed to this combined solution, and of different solutions derived fromother techniques to (IERS) C04. These statistics reflect the accuracy reached by thedifferent techniques. Note the significant improvement (shown in brackets) in theconsistency between the various solutions due to the adoption of the ITRF96 in the GPSdata analysis after March 1998. The relative weighting of GPS in the IERS C04 solution
is now about 70 %.

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IGS 1997 Technical Reports

Table 2 - biases and unbiased RMS of the differences of various solutions to (IERS) C04---------------------------------------------------------------------------------------------------Differences to
X- biasRMS
Y-biasRMS
lod-bias
RMS(IERS) C04
mas
mas
mas
mas
.1 ms
.1
ms
---------------------------------------------------------------------------------------------------GPS solutions
CODE 98 P 01
.33
.16 (11)-.10.19 (11).15.31 (16)
EMR 96 P 03
.12
.21 (20).40.22 (17).09.45 (34)
ESOC 96 P 01
.13
.22 (14).21.27 (14)-.10.37 (28)
GFZ 96 P 02
.24
.13 (09).14.18 (14)-.06.37 (24)
JPL 96 P 03
.12
.14 (10).03.17 (10).11.67 (34)NOAA 96 P 01 .54.49 (26).22.54 (26).06.57 (39)
SIO 96 P 01.20
.19 (16).04.19 (16).00.61 (37)
IGS 96 P 02
.32
.18 (17).17.25 (13).00.31 (24)
IERS 97 P 01.00
.08 (06)-.01.11 (05)-.02.28 (20)
Other individual series
USNO 97 R 08
CGS 98 L 01
CSR 95 L 01
DUT 98 L 01
GZ 98 L 02
IAA 98 L 01

.04
.38
-.34
.64
.53
.06

.20 (18).43.21 (21).38 (47).25.14 (16)

.02
.03
-.04
.50
.55
-.11

.15 (12).40 (11).23 (12).39 (41).28.15 (13)-.13.39 (28).04.14 (14)

Combined series
USNO 98 C 01 -.07
.10 (09)-.10.12 (07)
SPACE 98 C 01 -.05
.19 (11)-.02.14 (11)----------------------------------------------------------------------------------------------------Figures 2a, 2b and 2c show for x-pole, y-pole and LOD, the plots of the differences of theseven individual GPS series, and other series with the combined IERS C04.

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IERS References - Contribution of the Central Bureau of IERS

Figure 2a and 2b - x and y-pole coordinates in 1997/1998. Daily differencesof individual GPS series with IERS C04.
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IGS 1997 Technical Reports

Figure 2c - LOD in 1997/1998. Daily differences of individual GPS series with IERS
C04.
2-2

Use of GPS LOD Estimates for Near-real Time Universal Time
Determination

Due to the difficulty of determining the long-term behaviour of the non rotating
system realized through the orbit orientation, Universal Time UT1 cannot be accurately
derived from satellite techniques but only from inertial methods like VLBI. On the other
hand, these techniques can determine the length of day variations (lod), derivative ofUniversal Time together with the orbital parameters; of course their spectrum show
similar systematic errors than those of Universal Time directly estimated. Various studies(Gambis et al 1993, Gambis 1996) have shown that the high-frequency signal contained in
the lod estimates on time scales limited to a couple of months derived fron SLR and GPS
can be used to densify the series obtained by the VLBI technique and also for near-realtime earth orientation monitoring.
The published near-real Universal Time was so far extrapolated from VLBI
determinations. The present rapid availability of the GPS LOD solutions allows tosignificantly improve it when VLBI UT1 determination is available and consequently it
gives a much better UT prediction. The following table represents the results obtained bythis procedure over one year of operational analysis (1997.5 to 1998.5). The C04 solution

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IERS References - Contribution of the Central Bureau of IERS

is now permanently computed. On tuesdays, 5 GPS LOD estimates are available
whereas no VLBI UT1 solution is. Table 3 shows the accuracy reached using these GPS
LOD estimates compared to regular predictions from the last available VLBI value. This
corresponds to a near-real time service. The improvement is significantly of one order ofmagnitude for a 5-day time lag.
Table 3 - RMS errors (in microseconde) of the Universal Time solution based on GPS andcompared to the current prediction based on an auto-regressive process.
------------------------------------------UNIT: 1 ms
5 days------------------------------------------Pure Prediction 1000
GPS estimates100------------------------------------------3

References
Altamimi, Z., 1998, IGS reference stations classification based on ITRF96 residual
analysis, in J.M. Dow, J. Kouba and T. Springer (eds), Proceedings of the
IGS 1998 Analysis Center Workshop, Darmstadt, February 9-11, 1998.
Boucher, C., Altamimi, Z., and Sillard, P., 1998, Results and Analysis of the
ITRF96}, IERS Technical Note 24, Observatoire de Paris, Paris.
Boucher C, Z. Altamimi, D. Gambis, E. Eisop and M. Feissel, Contribution of theCentral Bureau of IERS, 1995 IGS Annual report. JPL Publication, 27..
Gambis D., Essaifi N., Eisop E. and M. Feissel, 1993, Universal time derived from
VLBI, SLR and GPS, IERS technical note 16, Dickey and Feissel (eds),
ppIV15-20.
Gambis D. , 1996, Multi-technique EOP combinations,proceedings of the 1996
IGS Analysis Center workshop, Silver spring.MD, edited by P. Van Scoyand R.E. Neilan, Pasadena, CA, JPL, JPL Publication 96-23.
Gray, J.E. and Allan, D.W. 1974: A method for estimating the frequency stabilityof an individual oscillator, Proc 8th Ann. Symp. on Frequency Control},2439, 277--287.

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192

Data Centers

Global Centers

CDDIS Global Data Center Report

CDDIS Global Data Center Report
Carey E. Noll
Terrestrial Information Systems Branch,
NASA Goddard Space Flight Center, Code 922,
Greenbelt, MD 20771

1

Introduction

The Crustal Dynamics Data Information System (CDDIS) has supported theInternational GPS Service for Geodynamics (IGS) as a global data center since 1992. The
CDDIS activities within the IGS during 1997 are summarized below; this report also
includes any changes or enhancements made to the CDDIS during the past year. General
CDDIS background and system information can be found in the CDDIS data centersummary included in the IGS 1994 Annual Report (Noll, 1995) as well as the subsequent
updates (Noll, 1996 and Noll, 1997).
2

System Description

The CDDIS archive of IGS data and products are accessible worldwide by way of
a password-protected user account. New users can contact the CDDIS staff to obtain therequired username and password, as well as general instructions on the host computer,directory structure, and data availability.
2.1

Computer Architecture

During 1997, the CDDIS was operational on a dedicated Digital Equipment
Corporation (DEC) VAX 4000 Model 200 running the VMS operating system. The
CDDIS is located at NASA’s Goddard Space Flight Center (GSFC) and is accessible tousers 24 hours per day, seven days per week. The CDDIS is available to users globally
through electronic networks using TCP/IP (Transmission Control Protocol/Internet
Protocol) and DECnet (VAX/VMS networking protocol) and through dial-in service.
At this time, two magnetic disk drives, totaling 6.4 Gbytes in volume, are devoted
to the storage of the daily GPS tracking data. A dual-drive, rewriteable optical disk
system provides additional on-line disk storage for GPS data as well as the long-termarchive medium for GPS data on the CDDIS. With the current nearly 120 station
network, only three days of GPS tracking data can be stored on a single side of one ofthese platters. The older data continues to be stored on these optical disks and can easily
be requested for mounting and downloading remotely by the user. Alternatively, if therequest for older data is relatively small, data are downloaded to magnetic disk, providing

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IGS 1997 Technical Reports

temporary on-line access. A 4.3 Gbyte magnetic disk drive is devoted to the on-line
storage of IGS products, special requests, and supporting information.
3

Archive Content

As a global data center for the IGS, the CDDIS is responsible for archiving andproviding access to both GPS data from the global IGS network as well as the products
derived from the analyses of these data.
3.1

GPS Tracking Data

The GPS user community has access to the on-line and near-line archive of GPS
data available through the global archives of the IGS. Operational and regional data
centers provide the interface to the network of GPS receivers for the IGS global data
centers. For the CDDIS, the following operational or regional data centers make data
available to the CDDIS from selected receivers on a daily basis:
•
•
•
•
•
•
•
•
•
•
•

Australian Survey and Land Information Group (AUSLIG) in Belconnen,
Australia
European Space Agency (ESA) in Darmstadt, Germany
GeoforschungsZentrum (GFZ) in Potsdam, Germany
Geographical Survey Institute (GSI) in Tsukuba, JapanNOAA’s Geosciences Laboratory (GL/NOAA) Operational Data Center(GODC) in Rockville, Maryland
Korean Astronomy Observatory in Taejeon, Korea
Jet Propulsion Laboratory (JPL) in Pasadena, California
National Geography Institute in Suwon-shi, Korea
National Imagery and Mapping Agency (NIMA), formerly Defense Mapping
Agency (DMA), in St. Louis, Missouri
Natural Resources of Canada (NRCan) in Ottawa, Canada
University NAVSTAR Consortium (UNAVCO) in Boulder, Colorado

In addition, the CDDIS accesses the other two IGS global data centers, Scripps
Institution of Oceanography (SIO) in La Jolla California and the Institut Géographique
National (IGN) in Paris France, to retrieve (or receive) data holdings not routinely
transmitted to the CDDIS by a regional data center. Table 1 lists the data sources andtheir respective sites that were transferred daily to the CDDIS in 1997. Nearly 42K
station days from 146 distinct GPS receivers were archived at the CDDIS during 1997; acomplete list of all archived sites can be found on the web site(http://cddisa.gsfc.nasa.gov/reports/gpsdata/cddis_summary.1997).

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Table 1: Sources of GPS data transferred to the CDDIS in 1997

SourceAUSLIG CAS1 COCONOAA/GL AOML BARBUSNA USNO
NRCan ALBH ALGO
SCH2 STJOESA KIRU KOURGFZ KIT3 KSTU
GSI TAIW TSKBIGN ANKR BOR1HERS HOFN(KSTU) LHAS

Sites
DAV1 HOB2 MAC1
BRMU FORT HNPTWES2 WUHNCHUR DRAO DUBO
WHIT YELLMALI MAS1 PERTLPGS OBER POTS

KELY

RCM6 SOL1

No.
Sites
5
12

FLIN

NRC1

12

VILL
ZWEN

PRDS

6
6
2
32
(39)

BRUS EBRE GRAS GRAZ HARK HART
IRKT JOZE KERG (KIRU) (KIT3) KOSG
(LPGS) (MAS1 MATE MDVO METS NTUS)
NYAL OHIG ONSA PAMA (POTS) REYK TAHI TROMWETT WSRT WTZR ZECK ZIMM (ZWEN)
JPL AOA1 AREQ ASC1 AUCK AZU1 BOGT BRAZ CARR60
CASA CAT1 CHAT CICE CIT1 CRO1 CSN1 DGAR
EISLFAIR GALA GODE GOL2 GOLD GUAM HARVHRAO IISC
JPLM KOKB KRAK KWJ1 LBCH MAD2
MADR MCM4 MDO1 MKEA MOIN NLIB OAT2 PIE1
POL2 PPYN QUIN SANT SELE SEY1 SHAO SNI1
SPK1 THU1 TID2 TIDB UCLP USC1 USUD WHC1
WHI1 WLSN XIAN YAR1
KAO TAEJ
1
NGI SUWN
1NIMA BAHR
1
SIO MAG0 MONP PETR PIN1 PVEP SIO3
VNDP YAKA9YAKZUNAVCO POL2
1
Totals:
146 sites from 13 data centers

Note: Sites in () indicate backup delivery route
Sites in italics indicate sites new to the CDDIS in 1997
Once they arrive at the CDDIS, these data are quality-checked, summarized, andarchived to public disk areas in daily subdirectories; the summary and inventory
information are also loaded into an on-line data base. Typically, the archiving routines on
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IGS 1997 Technical Reports

the CDDIS are executed several times a day for each source in order to coincide with theirautomated delivery processes and to ensure timely arrival in the CDDIS public disk areas.
In general, the procedures for archiving the GPS tracking data are fully automated,
requiring occasional monitoring only, for replacement data sets or re-execution because of
system or network problems.
The CDDIS GPS tracking archive consists of observation, navigation, andmeteorological data, all in compressed (UNIX compression) RINEX format.
Furthermore, summaries of the observation files are generated by the UNAVCO qualitychecking (QC) program and are used for data inventory and quality reporting purposes.
During 1997, the CDDIS archived data on a daily basis from an average of 115 stations;
toward the end of the year, this number increased to nearly 125 stations. Under thecurrent 125 station network configuration, about 100 days worth of GPS data are
available on-line to users at one time. Each site produces approximately 0.8 Mbytes ofdata per day; thus, one day’s worth of GPS tracking data, including the summary andmeteorological data files, totals nearly 100 Mbytes. For 1997, the CDDIS GPS dataarchive totaled over 35 Gbytes in volume; this figure represents data from nearly 42K
observation days. Of the 125 or more sites archived each day at the CDDIS, not all are of“global” interest; some, such as those in Southern California, are regionally oriented. The
CDDIS receives data from these sites as part of its NASA archiving responsibilities.
During 1997, tests were conducted to incorporate a “compact RINEX” into the
IGS data flow. This software, developed by Hatanaka Yuki (GSI) and Werner Gurtner(AIUB), when used with UNIX compression, reduces the size of the RINEX data byapproximately a factor of eight (as compared to approximately 2.5 with using UNIX
compression alone). Tests were performed at various data center levels within the IGS,
with the intent to use files in this format as the in exchange between data centers andanalysis centers. The CDDIS continues, however, to archive and make data available inthe compressed RINEX format for the greater user community.
The majority of the data delivered to and archived in the CDDIS during 1997 wasavailable to the user community within 24 hours after the observation day. As shown in
Figure 1, over forty percent of the data from all sites delivered to the CDDIS wereavailable within six hours of the end of the observation day; over fifty percent wereavailable within eight hours. These statistics were derived from the results of the dailyarchive report utilities developed by the IGS Central Bureau and executed several times
each day on the CDDIS.
The CDDIS staff often receives requests from users for the daily broadcast
ephemeris file (denoted BRDCddd0.yyN_Z). To reduce the amount of time spent onthese requests by the CDDIS staff, a new disk area has been established(GPS3:[GPSDATA.BRDC.yyyy]) to store the historic BRDC files.

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CDDIS Global Data Center Report

1 hour delay
2 hour delay
4%
8%
3 hour delay
9%
9 or more hour
delay
46%

4 hour delay
10%

6 hour delay
11%
8 hour delay
12%

Figure 1: Median delay in GPS data delivery (all sites) to the CDDIS in
1997

IGS
SIO
NRCan
NOAA
JPL
GFZ
ESA

SourceCODE

JPL (comparison)

MIT (comparison)
NCL (comparison)
MIT (polyhedron)
NCL (polyhedron)

AUS
EUR
GIA
GSI
PGC
SIR

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

Median Time Delay (Days)

Figure 2: Median delay in GPS product delivery to the CDDIS (by source) in 1997

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IGS 1997 Technical Reports

3.2

IGS Products

The seven IGS data analysis centers (ACs) retrieve the GPS tracking data daily
from the global data centers to produce daily orbit products and weekly Earth rotation
parameters (ERPs) and station position solutions; the nine IGS associate analysis centers(AACs) also retrieve IGS data and products to produce station position solutions. The
CDDIS archives the products generated by both types of IGS analysis centers. Thesefiles are delivered to the CDDIS by the IGS analysis centers to individual user accounts,
copied to a central disk archive, and made available in ASCII format (generally
uncompressed) on the CDDIS by automated routines that execute several times per day.
The Analysis Coordinator for the IGS, located at NRCan, then accesses the CDDIS (orone of the other global analysis centers) on a regular basis to retrieve these products andderive the combined IGS orbits, clock corrections, and Earth rotation parameters as wellas to generate reports on data quality and statistics on product comparisons. Users
interested in obtaining precision orbits for use in general surveys and regional experiments
can also download the IGS products. The CDDIS currently provides on-line access to all
IGS products generated since the start of the IGS Test Campaign in June 1992. As of
1996, access to the on-line archive of CDDIS products can also be performed through theWorld Wide Web (WWW) as well as through ftp.
During 1996, Regional Network Associate Analysis Centers (RNAACs) began thegeneration and submission of station position solutions for regional networks in Software
INdependent EXchange (SINEX) format. The three Global Network AACs (GNAACs)
continued their comparison of these files during 1997 and submitted the resulting SINEX
files to the CDDIS. The GNAACs accessed the SINEX files from the IGS ACs andRNAACs and produced comparison and combined, polyhedron station position
solutions.
The derived products from the IGS ACs are typically delivered to the CDDIS
within seven days of the end of the observation week; delivery times for AAC products
vary, but average 25 days for regional solutions. Figure 4 presents the median delay
during 1997, in days and by source, of AC and AAC products delivered to the CDDIS.
The statistics were computed based upon the arrival date of the solution summary file forthe week. The time delay of the IGS products and the combined SINEX solutions are
dependent upon the timeliness of the individual IGS analysis centers; on average, thecombined orbit is generated within one to two days of receipt of data from all analysis
centers and is typically available to the user community within ten days.
The rapid orbit and ERP products generated by the IGS Analysis Coordinator
were also made available to the IGS global data centers starting in June 1996. These
products are produced daily, within 24 hours UTC; automated procedures at the CDDIS
download these files from NRCan in a timely fashion. Starting in early 1997, the IGS
Analysis Center Coordinator began generating predicted orbit, clock, and Earth rotation
parameter combinations based upon the individual ACs’ predicted solutions. These
solutions, designated IGP, are available within 0.5 hours of the beginning of theobservation day.

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3.3

Meteorological Data

The CDDIS currently receives meteorological data from approximately twenty
sites. In 1997, additional IGS sites began providing meteorological data from collocatedsensors; these stations are: Albert Head, Ottawa, Priddis, St. John’s, and Yellowknife
Canada, Colorado Springs CO, McDonald TX, USNO Washington D.C., Zimmerwald
Switzerland, and Zwenigorod Russia. The meteorological data provided are dry
temperature, relative humidity, and barometric pressure at thirty minute samplingintervals. These data are stored on CDDIS with the daily GPS observation and navigation
data files in parallel subdirectories.
3.4

Supporting Information

Daily status files of GPS data holdings, reflecting timeliness of the data delivered
as well as statistics on number of data points, cycle slips, and multipath continue to begenerated by the CDDIS. By accessing these files, the user community can receive aquick look at a day’s data availability and quality by downloading a single file.
Furthermore, monthly summaries of the data quality for the IGS sites are also generated.
Both the daily and monthly status files are available through the WWW at URL
http://cddisa.gsfc.nasa.gov/gpsstatus/. The daily status files are also archived in the daily
GPS data directories.
Ancillary information to aid in the use of GPS data and products are also
accessible through the CDDIS. Weekly and yearly summaries of IGS tracking dataarchived at the CDDIS are generated on a routine basis and distributed to the IGS user
community through IGS Report mailings. These summaries are now accessible throughthe WWW at URL http://cddisa.gsfc.nasa.gov/gpsdata/gpsdata_list.html. The CDDIS
also maintains an archive of and indices to IGS Mail, Report, and Network messages.
4

System Usage

Figures 3 through 5 summarize the monthly usage of the CDDIS for the deposit
and retrieval of GPS data during 1997. These figures were produced daily by automatedroutines that peruse the log files created by each network access of the CDDIS. Figure 3
illustrates the amount of data retrieved by the user community during 1997. Over one
million files were transferred in 1997, totaling approximately 360 Gbytes in volume.
Averaging these figures, users transferred 90K files per month, totaling nearly 30 Gbytes
in size. The chart in Figure 4 details the total number of host accesses per month with thenumber of distinct (i.e., unique) hosts per month shown as an overlay. Here, a hostaccess is defined as an initiation of an ftp session; this session may transfer a single file,or many files. Figure 5 illustrates the profile of users accessing the system during 1997;
these figures represent the number of distinct hosts in a particular country ororganization. Nearly two-thirds of the users of GPS data available from the CDDIS comefrom U.S. government agencies, universities, or corporations.
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IGS 1997 Technical Reports

120,000
Misc.
Product
Data

Number of Files

100,000

80,00060,00040,00020,000

0
Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Month

Figure 3: Number of GPS related files transferred to/from the CDDIS in 1997
30,000350
Total Number of Hosts

300
25,000250
20,000200
15,000150

10,000100

Number of Distinct Hosts

Total Number of Host Accesses

Number of Distinct Hosts

5,000
50

0

0
Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Month

Figure 4: Number of hosts accessing GPS data and products on the CDDIS in 1997
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CDDIS Global Data Center Report

Unknown

113

U.S. Network

55

U.S. Military

17

U.S. Government

98

U.S. Education

109

U.S. Commercial

54

Turkey1

The Netherlands

10

Taiwan39

Switzerland

13

Sweden

35

Spain

7

South Korea

7

South Africa

2

Host Category

Russia17

Poland6Peru

6

Norway29

Non-Profit

3

New Zealand

11

Nambia4

Mexico

4

Japan

13

Italy

14

India

3

Hungary2

Greece5Germany55

G. Britain

46

France11

Finland2

Denmark

9

Czech Republic

5

Canada33

Bulgaria

1

Brazil23

Belgium

2

Austria

10

Australia

22

Argentina

2
0

20

40

60

80

Number of Distinct Hosts

Figure 5: Distribution of IGS users of the CDDIS in 1997

205

100

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IGS 1997 Technical Reports

Figures 3, 4, and 5 present statistics for routine access of the on-line CDDIS GPS
data archives. However, a significant amount of staff time is expended on fielding
inquiries about the IGS and the CDDIS data archives as well as identifying and making
data available from the off-line archives. Table 2 summarizes the type and amount ofspecial requests directed to the CDDIS staff during 1997. To satisfy requests for off-line
data, the CDDIS staff must copy data from the optical disk archive to an on-line magnetic
disk area, or for larger requests, mount the optical disks in a scheduled fashion,
coordinating with the user as data are downloaded.
Table 2: Summary of special requests for GPS data and information in 1997
Type of Request
General IGS/CDDIS
information
Off-line GPS data
Amount of off-line data
requested
Volume of off-line data
requested

Totals~215 requests (phone, fax, email)~130 requests (phone, fax, email)~40,650 station days†~35 Gbytes

Notes: †In this context, a station day is defined as one day’s
worth of GPS data (observation and navigation file in
RINEX format)
5

Publications

The CDDIS staff attended several conferences during 1997 and presented papers
on or conducted demos of their activities within the IGS, including:••“Flow of GPS Data and Products for the IGS” (Carey E. Noll) was presented at
the Workshop on Methods for Monitoring Sea Level in March 1997
“GIS and GPS Applications at the National Aeronautics and Space
Administration” (Lola Olsen and Carey E. Noll) was presented at the Georesearch
GPS/GIS ‘97 conference in May 1997

Hypertext versions of this and other publications can be accessed through the
CDDIS on-line documentation page on the WWW at URL
http://cddisa.gsfc.nasa.gov/documents.html.

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6

Future Plans

6.1

Computer System Enhancements

Procurement of a replacement hardware platform for the CDDIS VAX system
was undertaken in early 1997. This system will be a DEC AlphaServer 4000 running theUNIX operating system; the system will have over 120 Gbytes of on-line magnetic disk
storage. A significant amount of the CDDIS staff time was spent during 1997 developing
data processing and archiving routines for this new system. The staff hopes to have all
GPS data activities transferred to the UNIX platform by mid-1998; the host name for thiscomputer is cddisa.gsfc.nasa.gov. The CDDIS anonymous ftp and WWW sites, however,will be operational on the UNIX platform in early 1998 (in fact, all URLs in thisdocument reflect the web site on the new CDDIS computer).
An area of ongoing concern to the CDDIS staff is the ability to respond to special
requests for older, off-line GPS data. Currently, this is a time-consuming activity for thestaff since all older data are stored on optical disks in VAX VMS file format and the
CDDIS VAX system is equipped with only two optical disk drives. The future CDDIS
AlphaServer system under UNIX will not be equipped with these magneto-optical drives;
therefore, a new medium for long-term storage of the historic GPS archive must be
identified. The CDDIS staff has decided to utilize CD-ROMs for this archive. A CD
recordable system and 600 platter jukebox were purchased during 1997. The CDrecordable system consists of a Macintosh computer and a CD-ROM tower with thecapability of recording up to five copies of a CD. The existing GPS archive on magnetooptical disks (in VAX/VMS format) will be migrated to CD-ROM during 1998. The data
will most likely be written to CD-ROM by GPS week.
6.2

Changes in the Data Archive

The CDDIS data and product archive directories will be consolidated in mid-1998
once the system is operational on the new UNIX computer. This change will simplify
data access for the user community since all data will be under one directory path.
Tests are underway in mid-1998 to provide hourly data to the IGS user
community. During the tests, hourly data will be transmitted to CDDIS from JPL for
several NASA sites. The hourly data will be archived to a public disk area on CDDIS in atimely fashion and retained there for three days. After three days, the hourly data will bedeleted; the daily file, transmitted through normal channels with typically a one to twohour delay, will have been received and archived already and thus the hourly data are oflittle use.
In early 1998, a Call for Participation in the International GLONASS EXperiment(IGEX-98) was issued. IGEX-98 is sponsored by several organizations, including the
IGS, and requests participation by stations, data centers, and analysis centers. The
CDDIS responded to this call and hopes to make GLONASS data available to the IGS

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user community. The CDDIS plans to establish on-line directories for these data and toincorporate GLONASS data in normal data processing procedures.
6.3

Changes in the Product Archive

Starting in early 1998, the IGS Analysis Center Coordinator began generating
predicted orbit, clock, and Earth rotation parameter combinations based upon theindividual ACs’ predicted solutions. These solutions, designated IGP, will be available
within 0.5 hours of the beginning of the observation day. The IGS global data centers,
including the CDDIS, will make these products available as soon as possible each day toensure the timely utility to the user community.
Also early in 1998, the IGS Analysis Center Coordinator began generating
accumulated IGR and IGS ERP files on a daily and weekly basis; these data are used witheither the final or the rapid orbits. These files will be produced at the same time as the
IGS rapid and final products are generated and downloaded by the IGS Global Data
Centers. The files are designated IGS95P02.ERP (to be used with the IGS final orbits)
and IGS96P02.ERP (to be used with IGS rapid orbits).
The CDDIS began generating “short-SINEX” files, designated with an .SSC
extension in early 1998. These files contain the site information from the SINEX file but
no matrices. The files are stored in the weekly IGS product subdirectories.
Since January 1997, the IGS has conducted a pilot experiment on the combinationof troposphere estimates. Using a sampling rate of two hours, the zenith path delay(ZPD) estimates generated by the IGS analysis centers were combined by GFZ to form
weekly ZPD files for approximately 100 IGS sites. These troposphere products will beavailable at all IGS Global Data Centers, including the CDDIS starting in early 1998.
As of June 1, 1998, several IGS Analysis Centers will be supplying daily, global
ionosphere maps of total electron content (TEC) in the form of IONEX (an official
format for the exchange of ionosphere maps) files. These products will also be available
from the IGS Global Data Centers. At the CDDIS, the IONEX files will be located in
subdirectories of the main product area, rather than under the weekly subdirectory
structure, since the files are produced daily.
7

Contact Information

To obtain more information about the CDDIS or a username and password toaccess the IGS archive of data and products, contact:
Ms. Carey E. Noll
Manager, CDDIS
Code 922
NASA/GSFC
Greenbelt, MD 20771

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Phone:FAX:
E-mail:WWW:8(301) 286-9283(301) 286-0213
noll@cddis.gsfc.nasa.gov or CDDIS::NOLL
http://cddisa.gsfc.nasa.gov/cddis_welcome.html

Acknowledgments

The author would once again like to thank members of the CDDIS staff, Dr.
Maurice Dube and Ms. Ruth Kennard (Raytheon-STX). Their continued, consistently
outstanding support of the CDDIS has helped to make this system a success in the user
community.
9

References
Noll, C. E. “CDDIS Global Data Center Report” in IGS 1994 Annual Report.
September 1995.
Noll, C. E. “CDDIS Global Data Center Report” in IGS 1995 Annual Report.
September 1996.
Noll, C. E. “CDDIS Global Data Center Report” in IGS 1996 Annual Report.
November 1997.

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Scripps Orbit and Permanent Array Center – 1997 Global Data Center Report

Scripps Orbit and Permanent Array Center
1997 Global Data Center Report
Jeff Dean, Chris Roelle and Yehuda Bock
Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California San Diego
La Jolla, California, 92037 USA
http://lox.ucsd.edu

1

Introduction

The Scripps Orbit and Permanent Array Center at the Scripps Institution of
Oceanography (SIO) has served as an IGS Global Data Center since 1994. SIO iscommitted to collecting, archiving, and publishing high-quality continuous GPS data in atimely manner to support the global GPS research community. This report describes SIO’s
archiving operations, data access procedures and user statistics. The annual report of the
SOPAC Analysis Center is in a separate document.
2

Archive Content

SIO’s IGS archiving responsibilities include collection, storage, and distribution of
IGS tracking data in RINEX format and IGS products. SIO also archives RINEX files
from a variety of worldwide regional GPS networks. In particular, SIO is the data archivefor the Southern California Integrated GPS Network (SCIGN) and a continuous GPS dataarchive for the University NAVSTAR Consortium (UNAVCO). Presently SIO collects andarchives data from over 350 permanent GPS stations.
SIO collects and archives GPS products from all IGS analysis centers, including
combined, rapid and predicted orbits, and Earth orientation parameters (EOP). In addition
to products and tracking data, SIO archives tropospheric estimates, meteorological andnavigation files in RINEX format, SINEX solutions, raw receiver data for some arrays,and site log files. SIO also archives and maintains data for users of the GAMIT/GLOBK
software [King and Bock, 1998; Herring, 1998].
3

Computer Facilities

SIO’s archiving facilities are made up of a distributed network of Sun Ultra Sparc
workstations and one SGI data server. Data are collected, quality checked and stored on
250GB of primary on-line magnetic disk storage. Older data are stored on an 8-drive
magneto-optical removable cartridge library, providing over 500GB of on-line secondary
storage. With over 750GB of data storage, SIO can maintain all of its data holdings online. Figure 1 depicts SIO’s archiving computer network.
4

Archiving Operations

Data collection and archiving are entirely automated. At set intervals all
predetermined data archives are probed for new or re-submitted data. Any data that areneeded are collected via ftp to a holding area and quality checked (QC’d). Upon successful
QC, the data are catalogued, archived, mirrored, and immediately made available for

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IGS 1997 Technical Reports

anonymous ftp. All of the data collection software is written in PERL with command line
and Web-based interfaces. The cataloging is currently flat file based, but is being migrated
to an Object Relational Database Management System (ORDBMS), Oracle 8.

5

Tracking Data

On average, data were collected from 250 sites in 1997. Table 1 shows thebreakdown of sites by array and the number of sites collected from each. The table reflects
the current network breakdown (circa July 1998).
Table 1. GPS arrays archived at SIO
Array Code
IGS
SCIGN
ARGN
BARD
CORS

Array NameIGS Global Network
Southern California Integrated GPS Network
Australian Regional GPS Network
Bay Area Regional Deformation Array, California
U.S. Continuous Operating Reference Stations

212

Number
110
53
10
24
91

Scripps Orbit and Permanent Array Center - 1997 Global Data Center Report

EUREF
INEGI
PANGA
SOI
WCDA
MISC

European Reference Network
Instituto Nacional de Estadística, Geografía eInformática, Mexico
Pacific Northwest Geodetic Array
Survey of Israel
Western Canada Deformation Array
Miscellaneous sites

45
1

Total

9
3
7
14
367

The following GPS data archives were accessed to complete our RINEX file data
holdings. Some are publicly available, others are password protected.
Table 2. GPS Archives used to complete SIO’s RINEX file data holdings.
SourceAgenzia Spaziale Italiana
Australian Surveying and Land Information Group
Crustal Dynamics Data Information System
Institut Geographique National
Bundsumt fur Kartographic und Geodasie
Geosciences Operational Data Center
Central Washington University
Canadian Active Control System
Forecast Systems Laboratory
Australian Academy of Science, Institute of Space
Research – Dept of Satellite Geodesy
University of Washington
Jet Propulsion Laboratory
Survey of Israel
Rensselaer Polytechnic InstituteRoyal Observatory of Belgium
Regional GPS Data Acquisition/Analysis Center on
Northern Eurasia
United States Geological Survey

213

Address
geodaf.mt.asi.it
ftp.auslig.gov.au
cddisa.gsfc.nasa.gov
mozart.ensg.ign.fr
igs.ifag.de
gracie.grdl.noaa.gov
panga.cwu.edu
macs.geod.emr.ca
spruce.fsl.noaa.gov
flubiw01.tu-graz.ac.at
ftp.geophys.washington.edu
bodhi.jpl.nasa.gov
ftp1.netvision.net.il
inusa.geo.rpi.edu
ftpserver.oma.be
ria.ipmnet.r
dixie.gps.caltech.edu

IGS 1997 Technical Reports

214

Scripps Orbit and Permanent Array Center - 1997 Global Data Center Report

6

IGS Products

IGS products are collected from the IGS analysis and associate analysis centers
including SIO’s own rapid and predicted ephemeris. Weekly products have a latency of 4days. SIO’s predicted and rapid ephemerides are available within 18 hours from the end of
the previous observation (UTC) day in UTC. The IGS combined, rapid, and predicted
orbits are available with a 20-22 hour latency from the end of previous observation day.
7

User Activity

The number of files extracted from our archive via ftp in 1997 totaled over 580,000(~348GB), averaging approximately 48,000 (~30GB) transfers per month. An average of
300 different hosts accessed SIO each month. Figure 2 depicts the file transfers per monthfrom SIO’s archive in 1997, and a breakdown of U.S. domain users from March ‘97 to
March ‘98.
The increase in on-line storage capacity has made it possible for SIO to put all dataon-line, eliminating the need for data requests. User activity has increased over 100%since the end of 1997, and we are projecting an average of over 100,000 file transfers permonth in 1998.
8

Data Policy

SIO maintains an open data policy without any restrictions. All data are madeavailable as soon as they are collected. The open data policy eliminates the need for formal
request procedures, thus reducing the time it takes to gain access to data. SIO encourages
this policy wherever possible.
9

Archive Access

SIO’s archive is accessible via ftp (ftp://lox.ucsd.edu/pub/DATATYPE). Users can
ftp to the archive, navigate the data tree for data they are interested in, and collect any data
they wish. SIO also provides a data interface on the WWW (http://lox.ucsd.edu). The
Web page design is dynamic, but there will always be links to data availability.
10

Future Developments

SIO has increased the data archive capacity by at least 100% during 1998, allowing foron-line storage of 1 TB of data. SIO is implementing a high-availability computer network,
which will greatly minimize system and archive downtime and improve disaster recovery
situations.
The plans to migrate from flat files to an object relational database (Oracle 8) are wellunderway and should be implemented by late 1998.
SIO is also involved with the Seamless GPS Archive effort with many other data
centers organized by the University NAVSTAR Consortium (UNAVCO); prototype
applications and seamless data access will hopefully be available by the end of 1998.
11

User Information
To learn more about SIO’s archiving facilities please contact:
Jeff Dean or Chris Roelle
Institute of Geophysics and Planetary Physics

215

IGS 1997 Technical Reports

Scripps Institution of Oceanography
8602 La Jolla Shores Drive
La Jolla, CA 92037
Phone: (619) 534-7692
FAX: (619) 534-9873
E-mail: jdean@ucsd.edu, cr@ucsd.edu
WWW: http://lox.ucsd.edu
12

Acknowledgments
Funding for archiving is provided by the U.S. National Science Foundation, theSouthern California Earthquake Center (SCEC), the William M. Keck Foundation,
NASA, U.S. Geological Survey, and SIO. We thank all arrays (Table 1) archiving
centers (Table 2) for allowing access to their data, and our colleagues at SOPAC,
IGS, SCIGN, UNAVCO, and U.S. CORS for their support.

13

References
Bock. Y., J. Behr, P. Fang, J. Dean, and R. Leigh, Scripps Orbit and Permanent
Array Center (SOPAC) and Southern California Permanent GPS Geodetic
Array (PGGA), The Global Positioning System for the Geosciences, pp. 5561, National Academy Press, Washington, D.C., 1997.
Fang, P. and Y. Bock, Scripps Orbit and Permanent Array Center 1995 Report to
IGS, International GPS Service for Geodynamics 1995 Annual Report, J. F.
Zumberge, M. P. Urban, R. Liu, and R. E. Neilan, eds., IGS Central Bureau,
Jet Propulsion Laboratory, Pasadena, pp. 103-124, 1996.
Herring, T. A., GLOBK: Global Kalman filter VLBI and GPS analysis program
Version 4.17, Internal Memorandum, Massachusetts Institute of Technology,
Cambridge, 1998.
King, R. W., and Y. Bock, Documentation of the GAMIT GPS Analysis Software
version 9.72, Massachusetts Institute of Technology, Cambridge, 1998.

216

Regional/Operational Centers

BKG Regional IGS Data Center Report

BKG Regional IGS Data Center Report
Heinz HabrichFederal Agency for Cartography and Geodesy
D-60598 Frankfurt at Main, Germany

1

Introduction

The Federal Agency for Cartography and Geodesy (BKG) operates the Regional
IGS Data Center for Europe since the beginning of the IGS Test Campaign in June 21,
1992. GPS tracking data from permanent GPS sites in Europe are obtained fromOperational Data Centers (ODC’s), Local Data Centers (LDC’s) or directly from thestations. Also tracking data from stations outside of Europe are transferred to BKG, if
these stations are operated by an European institution. The received data are uploaded to
the Global Data Center (GDC) at the Institut Géographique National (IGN) in Paris andthe Center for Orbit Determination in Europe (CODE) in Berne, and are also madeavailable to other users and archived.
The IGS products as computed by the IGS Analysis Centers are downloaded
from the GDC to BKG in order to provide these information to European users. The IGS
tracking data and products together with the series of ITRF-solutions, which is alsoavailable at BKG, allow the users to get comprehensive information for various GPS
applications using the Internet capability.
2

Computer Architecture

The Regional IGS Data Center at BKG operates on an HP-9000/J210 workstation
running the HP-UX operating system. This workstation is connected to the Internet witha maximum transfer rate of 128 kbit/s and two harddiscs (each of 4 Gbyte capacity) store
the on-line data. Data are archived on CD-ROM discs. If users connect to the computer
using the anonymous ftp account, the directory structure given in Table 1 is available.
3

GPS Tracking Data

All institutions sending GPS data to BKG store these files in the "indata"
directory. Only this directory has a "write permission" for the anonymous ftp user. The
GPS tracking data received in the "indata" directory are uploaded to IGN and CODE, andavailable in the "outdata", "gpsdata" and "IGS" directory.
The outdata" directory stores the observation, summary and meteorological filesof all stations for a period of one month. Daily GPS navigation files are concatenated to a
file with the station abbreviation "IFAG" (e.g. IFAG1750.97N.Z). Because the number offiles in this directory may grow up to about 8,000 it is recommended to use this directory
not for manual access to the data but for automated procedures.

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IGS 1997 Technical Reports

Table1. Directory Structure on igs.ifag.de/indata/outdata/gpsdata/ssssddd0.yyD.Z/ssssddd0.yyO.Z/ssssddd0.yyS.Z/ssssddd0.yyM.Z/IFAGddd0.yyN.Z
/ssss

/IGS

/NAV
/yyyy

/ORBITS/station
/wwww

/COOR
/IGSMAIL/ITRFyy/IGSMESS.nnnn/IGSMESS.INDEX
ssss = Station Abr.
yy = year in 2 digiti = day of week/ssssddd0.yyD.Z/ssssddd0.yyS.Z/ssssddd0.yyM.Z/IFAGddd0.yyN.Z/ddd/ssssddd0.yyD.Z/ssssddd0.yyS.Z/ssssddd0.yyM.Z/IFAGddd0.yyN.Z/ssssyymm.log/CODwwwwi.EPH/CODwwww7.ERP/IGRwwwwi.SP3/IGRwwwwi.ERP/IGSwwwwi.SP3/IGSwwww7.ERP/xxx

ddd = Day of year
wwww = GPS weekxxx = ITRF-solution

yyyy = year in 4 digit
mm = monthnnnn = Message no.

The "gpsdata" directory shows station separated subdirectories and can be used
for manual data access. The GPS tracking data are available in these subdirectories for aperiod of 2 months. For the same period the navigation files are on-line in the "NAV"
subdirectory.
The "IGS" directory includes the GPS data in daily subdirectories which are
created with a delay of 5 days after the end of observation and the data are on-line for aperiod of 6 months.
The GPS tracking data are archived on CD-ROM and old data will be made on-line
available on special request. In this case a complete CD-ROM is mounted on the
anonymous ftp account point or a subset of files is copied to the daily subdirectories fora limited time period.

220

BKG Regional IGS Data Center Report

Table 2. Contact Information
Contact Address
Heinz Habrich
Bundesamt fuer Kartographie und Geodaesie
Richard-Strauss-Allee 11
D-60598 Frankfurt am Main
Germany

Tel: +49-69-6333267
Fax: +49-69-6333425
E-Mail: habrich@ifag.de

Data Center Accesshostname: igs.ifag.de (141.74.240.26)
account: anonymous ftp
URL: http://www.ifag.de
4

IGS Products

IGS precise orbits and earth rotation parameters are on-line available at BKG
starting with GPS week 729 corresponding to the official start of the IGS at the beginningof 1994. Now the IGS rapid orbits, the IGS final orbits and the CODE orbits are stored in
subdirectories for every GPS week. IGS final orbits are also archived on CD-ROM and
CD-ROM copies can be delivered on special request.
5

User Activity

In 1997 the tracking data of 59 permanent GPS sites were collected and archived
on daily basis at the BKG Data Center. Approximately 50 distinct users (institutions)contact the Data Center on regular basis with about 400 connections every day.
6

Future Activity

In order to start with tests for an hourly upload of RINEX files a new
subdirectory "/IGS/nrt" was already created. It is planned to store the hourly files for aperiod of 3 days. BKG acts also as a data center for the IGEX-98 GLONASS experiment.
For this purpose the directory "/IGEX" was created. GLONASS tracking data will be
stored in this directory. BKG will participate in the IGEX campaign as an Analysis
Center and make the products available in the "/IGEX" directory.

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IGS 1997 Technical Reports

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222

Networks and Stations

Global, Regional, andLocal Networks

AUSLIG 1997 IGS Annual Report - Networks & Stations

AUSLIG 1997 IGS Annual Report
Networks & Stations
M. R. Hendy, R. Twilley, and J. Manning
Australian Surveying & Land Information Group
Canberra, Australia

1

Australian Regional GPS Ground Station (ARGN)The ARGN is a network of 15 GPS permanent ground stations extending from
Antarctica to Cocos Island.
It consists of:• Eleven AUSLIG sites• One DSTO site, Salisbury South Australia• ESOC site at Perth (IGS site)• Tidbinbilla DSN/JPL site (IGS site)• Yaragadee JPL (IGS site)
Of the eleven AUSLIG sites, five currently provide data to the IGS:
•
•
•
•
•

Casey
Davis
Maquarie Island,
Cocos Island
HobartAUSLIG has also installed backup receivers at Tidbinbilla and Yaragadee sites.
Data from all sites is processed by the AUSLIG Space Geodesy Analysis Centre as an
IGS regional associate analysis centre.
2.

GPS Data Center
The configuration of the GPS receivers and antennae at the sites are:

SITE
Casey
Cocos
IslandDavis

GPS RxTurboRogue/ Ashtech Z12
TurboRogue

GPS
Vers.
3.2.33.1
1F60
2.8.33.2

TurboRogue

3.2.33.1

ANTENNA

MONUMENT

DOME
Hemi Sph

Dorne
Margolin T
Dorne
Margolin T
Dorne

227

Concrete pedestal on Yes
rock
Concrete pillar
Yes
Steel rods in rockYes

IGS 1997 Technical Reports

SITE

GPS RxHobart/ Ashtech Z12
TurboRogue

GPS
Vers.
1F60
3.2.33.1

MacquarieIslandTurboRogue/ Ashtech Z123.2.33.1
1F60

ANTENNA

MONUMENT

DOME
Hemi Sph

Margolin T
Dorne
Margolin T
Dorne
Margolin T

Concrete pillar

No

Concrete pillar

Yes

A new acrylic dome was placed on the Casey monument during December.
Improvements to the GPS data system at AUSLIG have resulted in complete
redundancy in the system for conversion of receiver data files to RINEX. A RAID
system is now in place on the AUSLIG ftp server.
3

Station Reports

3.1

CAS1

During 1997 the data logging system was partially upgraded, and an Ashtech Z12
GPS receiver was installed as backup to the TurboRogue and to support DGPS during theAntarctic summer. The receiver operates on the external rubidium oscillator.
3.2

DAV1

During 1997 the data logging system was completely upgraded, and an Ashtech
Z12 GPS receiver was installed as backup to the TurboRogue and to support DGPS
during the Antarctic summer. The receiver operates on the internal quartz oscillator.
3.3

MAC1

During 1997 the data logging system was completely upgraded, and an Ashtech
Z12 GPS receiver was installed as backup to the TurboRogue and to support DGPS
during the Antarctic summer. The receiver operates on the internal quartz oscillator.
3.4

HOB2
No changes were made to this site during 1997.

3.5

COC1

No changes were made to this site during 1997. The receiver operated on theinternal quartz oscillator.

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AUSLIG 1997 IGS Annual Report - Networks & Stations

4.

Plans for 1998

The network will be developed to meet full IGS specifications and all sites madeavailable to IGS. A new IGS site will be established during 1998 colocated with SLR,
Absolute gravity and DORIS at Mt. Stromlo near Canberra.
5.

Contract Details
Hendy@auslig.gov.au

BobTwilley@auslig.gov.au

229

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230

The GPS Receiver Network of ESOC: Malindi, Maspalomas, Kourou, Kiruna, Perth and Villafranca

The GPS Receiver Network of ESOC: Malindi, Maspalomas,
Kourou, Kiruna, Perth and Villafranca
C.Garcia-Martinez (1), J.M. Dow,
T.Martin-Mur, J.Feltens (2), P.Bernedo (1).
(1) GMV at ESOC ; (2) mbp at ESOC.

1

Introduction

ESOC is currently involved in the establishment of a network of high precision
geodetic receivers on ESA ground sites. So far, six installations have been carried out at
the sites of Malindi, Maspalomas, Kourou, Kiruna, Perth and Villafranca. The
establishment of this network is one of the objectives of the ESA GPS-TDAF (Tracking
and Data Analysis Facility). Figure 1 shows the geographical distribution of the receivers.

FIGURE 1

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IGS 1997 Technical Reports

2

Location of the Receivers.

The ESOC receivers are being installed at the ESA ground stations. In this way
they can take advantage of the facilities that the stations provide. They are integrated inracks in rooms with temperature and humidity control, connected to the frequency
standards of the stations and to the permanent communication links between the stations
and the control centre at ESOC. They provide, along with the rest of the GPS-TDAF,
also several services. Examples are the monitoring of the behaviour of the timing system,the 1PPS output and the ionosphere monitoring over the station.
2.1

MalindiThe receiver is located at the base camp of the San Marco Scout launching site,which is a complex of facilities situated near the equator in Formosa bay near Malindi,
Kenya. The station is on the coast about 115 km north of Mombasa.
2.2

Maspalomas

The GPS receiver is installed at the Maspalomas ground station, that is propertyof the spanish institute INTA. It is located in the southern part of the Gran Canaria
Island, municipal district of San Bartolome de Tirajana, Spain. The site is approximately
1750 metres from the coast.
2.3

KourouThe GPS receiver is installed at the ESA Kourou Diane station that is locatedabout 27 km from the town of Kourou, in French Guyana.
2.4

KirunaThe GPS receiver is installed in the ESA Kiruna ground station, that is at
Salmijarvi, 38 km east of Kiruna in northern Sweden.
2.5

Perth

The receiver is located at the ESA Perth station, that is approximately 20km northof the city of Perth on the western coast of Australia. The station is situated on the Perth
International Telecommunications Centre Complex, which is operated by Telstra
Corporation Limited.

232

The GPS Receiver Network of ESOC: Malindi, Maspalomas, Kourou, Kiruna, Perth and Villafranca

2.6

Villafranca

The receiver is situated in the Villafranca (VILSPA) ground station, located in
Villafranca del Castillo, 30 km west of Madrid, Spain.
3

History and Evolution

The development of the network started at the beginning of 1992 when two
MiniRogues SNR-8C, the most advanced receiver then, were ordered from AOA. After aperiod of testing in ESOC, the first installation was completed in the week before thestart of the IGS campaign at Maspalomas. Data was available from 22/06/92. The antenna
was mounted on a monument belonging to the Spanish IGN, that participated in several
geodetic campaigns with the marker name MPA1. For IGS the selected marker name was
MASP.
ESOC constructed another monument and on 11/04/94 installed a new GPS
system with a TurboRogue SNR-8100. Both systems were operated in parallel for several
weeks until the decommission of the old receiver. The marker name of the new monument
is MAS1 and the IERS DOMES Number 31303M002 was assigned to it.
In the last months of 1995 the TurboRogue SNR-8100 experienced a degradationin the quality and quantity of the data that made necessary the replacement of the unit.
Two new TurboRogues SNR 12 had been ordered and in April 96, shortly after thedelivery and testing in ESOC, one of the new units was installed in Maspalomas.
The second of the MiniRogues was installed on late July 1992 at Kourou. Initiallythe data were downloaded directly from the receiver to ESOC using Telebit modems.
Unfortunately the quality of the public telephone lines between Europe and French
Guyana were very irregular. The data was obtained for a period of 10 days in August, andsporadically thereafter. Attempts made from Pasadena to dial up the Kourou modem
were also unsuccessful. The low transfer rates and the irregular quality of the telephonelines made very problematic the completion of the file transfers using XMODEM. A new
solution had to be implemented. It was based on the permanent links between the stationand the control centre ESOC shared by several ESA projects. The regular operation of thereceiver started on 18/10/92 when the connection to the new data link was completed.
During the period when communications were not possible, a permanent concretemonument was constructed for the antenna there (see IGS mail No. 144). The antenna
was moved by about -3.0,-1.1,1.1 m in longitude, latitude and height respectively from its
previous position. The software of the MiniRogue was upgraded to version 7.8 on
06/10/94. The receiver was operated permanently without hardware problems for morethan five years. In October 1997 the MiniRogue was replaced by an eight channels
TurboRogue with the corresponding Dorne Margolin T antenna.
A set of five receivers model TurboRogue SNR-8100, was ordered at the end of
1992. After the testing period in ESOC, the first receiver was despatched to Kiruna andinstalled on July 1993. The receiver was placed in a building several metres away from themain building of the station. From here the distance to the monument is shorter. The

233

IGS 1997 Technical Reports

monument is on top of a slope surrounded by trees. The antenna was replaced in May
95.
The second TurboRogue SNR8100 installation was performed on 13/08/93 at
Perth. Unfortunately, a few days after the beginning of the operation, the receiver wasdamaged during a lightning storm on 03/09/93. A new receiver was immediately delivered.
The earthling of the antenna has been improved to try to avoid the same problem
happening again. The original receiver and antenna were repaired and reinstalled on
27/04/94.
Villafranca was set up on 12/11/94. At this site the cabling from the monument to
the racks of the main building, where the receiver is integrated, is about 150 metres long.
This is 50 m longer than the standard set-up of the receiver. This made necessary the
installation of an additional line amplifier close to the antenna. With this modification thesignal level has nominal values. At the beginning of 1998 this configuration presented
several signal level problems and the receiver was moved to a portable station avoiding thenecessity of the large cable.
The last installation has been Malindi. A MiniRogue SNR-8C was deployed at thestation and started the data collection at the end of 1995. The data retrieval was initially
via an analogue line that at the beginning of 1996 was replaced by a 64 Bit/s digital circuit.
This facilities depend on other ESA projects and will be discontinued. A test with dial up
modems using the recently improved PSTN at Malindi has been carried out successfullyin May 96. The receiver is connected to a external 5 MHz quartz reference. In October
1997 a 12 channels TurboRogue replaced the old MiniRogue.
4

Monumentation

Figure 2 shows the monument specially developed for the GPS-TDAF. It isbasically a reinforced concrete cylinder of 50 cm diameter that is situated over afoundation. On top of the cylinder there is an embedded horizontal metal plate. The
marker is the centre of this plate, on the upper surface.
Three iron bolts are used to fix the antenna mounting in a horizontal position. The
antenna is screwed to the mounting.

234

The GPS Receiver Network of ESOC: Malindi, Maspalomas, Kourou, Kiruna, Perth and Villafranca

Figure 25

Equipment

The physical configuration of all the equipment involved in the remote stations
part of the GPS TDAF is summarized in Figure 3.
The remote stations are continuously tracking the GPS satellites. The antenna isconnected to the receiver normally with a standard 300 ft RG-214 coaxial cable. Only
Villafranca has a cable 450 ft long, as remarked in the last section.
The timing system of the stations are used as 5 MHz reference frequency. They
are cesiums manufactures by OSCILLOQUARTZ with long term drift controlled bytiming GPS system.
All ESA stations are equipped with TurboRogues. In 1997 the last MiniRogues in
Kourou and Malindi have been upgraded to TurboRogues.
One of the serial ports of the receivers is connected to a device that provides forcommunications and optionally for data storage. This device is a PC that runs a script of acommunications package. Shortly after 00:00 UTC the PC downloads the data from thereceiver with the XMODEM protocol, waits the remainder of the day for the call fromthe control centre ESOC and allows the remote control of the computer.

235

IGS 1997 Technical Reports

FIGURE 3

There are two main reason for the necessity of the intermediate device. First it
buffers data. Several months can be stored on the disk. In addition it allows the datatransfer to ESOC using a wide range of protocols. The XMODEM protocol, the only one
supported by the receivers, is not suitable for the packet-switched networks that are
sometimes involved in the communications with the control centre. It also provides flow
control with the DCE (Data Communication Equipment).
The communication with the receiver is performed using the same line that is used
for data downloading. The commands are sent to the PC that stores them and immediatelychanges the active comm port to the one connected to the receiver, sends them, waits theanswer and stores it. The active port is swap again to the one connected to thecommunication device and the answer of the receiver is echoed. Several attempts havebeen done with a secondary line (PAD or modem) connected to the free port of thereceiver for interaction with it in terminal mode, but the system has been shown to be
more reliable without this secondary link.
For the communications with ESOC the permanent links between ESOC and thestations are used whenever possible. They are very reliable and do not introduce
additional costs due to the small amounts of data involved.
At ESOC there is one workstation with two serial ports. One is attached to a
Telebit modem and the other to an internal LAN of ESOC that gives access to the ESA
ground station via X.25/PAD. This workstation retrieves, decompresses, reformats,

236

The GPS Receiver Network of ESOC: Malindi, Maspalomas, Kourou, Kiruna, Perth and Villafranca

validates, archives, recompresses and distributes every day the data automatically. The
nominal time when all the processes are finished is 02:00 UTC.
The data are available to the IGS community in RINEX format via the official data
centres.
At Malindi the receiver is a TurboRogue SNR 12 RM. The antenna is Dorne
Margolin T with a height of 0.1347 m and is located at the centre of the station. Data havebeen retrieved by means of a permanent digital circuit and since the end of 1996 we aremaking use of the very improved PSTN at Malindi station for data downloading using
OCTOCOM dial-up modems.
In Maspalomas the receiver is a TurboRogue SNR 12 RM. The antenna, Dorne
Margolin T, is mounted over a monument located several metres east of the Main
Equipment Room. The antenna height is 0.033 m. The data retrieval is performed with a
Telebit T2500 modem. A PAD (Packet Assembler-Disassembler) that runs over a 64
Bit/s line has been used in the past.
Kourou is equipped with a TurboRogue SNR-8100. The antenna is Dorne
Margolin T with a height of 0.045 m and is located about 25 m from the MCR (MainControl Room) building.
Kiruna has a TurboRogue SNR-8100 and a Dorne Margolin T antenna with aheight of 0.062 m. The communications are performed using a PAD that runs over a
permanent circuit between ESOC and Kiruna Station.
The TurboRogue of Perth is connected to a Dorne Margolin T antenna which hasa height of 0.0595 m. The communications are carried out by means of a PAD that issituated in a different building of the station. To overcome this problem, two local
modems had to be used. They provide for communications between PC and PAD.
Villafranca has also a TurboRogue with a Dorne Margolin T antenna. The antennaheight is in this case 0.0437 m.
6

Plans for the future

There are currently two ESA sites that offer possibilities for future installations.
They are Odenwald (Germany) and Redu (Belgium). They are really more interesting forother projects than for IGS. The baseline of the plans for the future, concerning IGS, ismore than new installations, the improvements of the current ones with the last hardware
and software available and provide for an even more robust communications tending to
the real time data availability at ESOC.
We are working on a Real Time Infrastructure project that will replace thesoftware that currently is running in the remote stations PC's to a more versatile one thatwill provide for continuous data downloading to the control centre at ESOC and will
enhance the data analysis capabilities.
In 1998 communications between ESOC and the Ground Stations will be reviewed
and the GPS TDAF will have to look for new connectivity solutions for several stations.

237

IGS 1997 Technical Reports

7

References•

GPS TDAF Stations Configuration Manual. Version 1.1, December 1995, ESOC.

•The GPS receiver network of ESOC: Maspalomas, Kourou, Kiruna, Perth andVillafranca. C.Garcia-Martinez, J.M. Dow, T.Martin-Mur, J.Feltens, M.A.
Bayona-Perez. 1994 IGS Annual Report.

•

The GPS receiver network of ESOC: Maspalomas, Kourou, Kiruna, Perth andVillafranca. C.Garcia-Martinez, J.M. Dow, T.Martin-Mur, J.Feltens, M.A.
Bayona-Perez. 1995 IGS Annual Report.

•

The GPS receiver network of ESOC: Maspalomas, Kourou, Kiruna, Perth andVillafranca. C.Garcia-Martinez, J.M. Dow, T.Martin-Mur, J.Feltens, M.A.
Bayona-Perez. 1996 IGS Annual Report.

238

GPS Global Network Operations at JPL and the UNAVCO Facility

GPS Global Network Operations at
JPL and the UNAVCO Facility*S. Fisher, R. Khachikyan , U. Lindqwister, M Marcin, A. Moore, D. Stowers
Jet Propulsion Laboratory, Pasadena, CA, USA* Also at the UNAVCO Facility

L. Estey, C. Meertens, D. Mencin, J. Normandeau, B. Perin, V. Richardson
University NAVSTAR Consortium Facility, Boulder, CO, USA

1

Transfer of Station Engineering and Maintenance Activities to UNAVCO

A proposal to collaboratively operate the NASA GPS Global Network (GGN)
was developed jointly by JPL and the UNAVCO Facility and was submitted to NASA in
April 1997. Arrangements were formalized in June, upon notification by NASA that the
combined effort would be funded, and part of the responsibility for operating the GGN
was transferred from JPL to the UNAVCO Facility during the last half of the year.
Under this new arrangement, JPL is providing overall management of the GGN effort andis providing most of the data management system development and operations. The
UNAVCO Facility is handling most of the station-level engineering and maintenance,
including performing the new station installations and establishing data flow; enhancing
the capabilities of existing GPS stations and data return; sharing the network monitoring
responsibilities; supporting station-level troubleshooting and repairs; maintaining station
configuration information, including the IGS logs; retrieving data from some of the more
problematic stations; and supporting improvements to data retrieval, verification,
management and reporting systems.
2

GPS Station Installations and MaintenanceWorking in close cooperation with various international partners, JPL and the
UNAVCO Facility have supported the installation of eight new GPS tracking stations in
1997 on behalf of NASA, including two new stations in Russia, three in Kyrgyzstan, one
in Colombia, one in South Africa, and one in Turkey. Five of these already regularly
provide data to the IGS, as will the others once they are capable of reliably returning data.
Significant planning and preparations were also accomplished for several new stations to
be installed during 1998 at locations including Brazil, China, Cote D’Ivoire, Morocco, and
the Philippines.
Of the approximately 60 stations comprising the NASA Global Network(excluding the dense array stations in California and other locations), fifteen required thereplacement of faulty receivers or antennas during 1997; four required the replacement ofon-site computers; two required replacement of faulty telecommunications equipment;

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IGS 1997 Technical Reports

two required establishment of new telecommunications services; and four required
engineering staff visits for on-site troubleshooting, repair or re-configuration.
3

Improvements to Data Retrieval, Verification and Management Systems

Improvements made to data retrieval, verification and management systems havefocused on enhancing overall data return and timelines of data deliveries to users, and on
improving the quality and reliability of GPS data and associated descriptive information.
JPL’s GNRT (Generalized Near-Real Time) software, which provides rapidturnaround, data-driven, queued processing of any type of data, was tested in aconfiguration which produces daily RINEX files for NASA’s Global Network stations.
This set of Perl scripts is able to complete the RINEX production within minutes ofreceipt of raw GPS data and it is envisioned that this scheme may provide the IGS with a"Rapid Service" RINEX function in the future.
The JPL GNEX software, a PC/Linux based software package, which performs
unattended data retrievals at remote GPS stations, was improved to include a prototype
module for controlling Trimble receivers, based on the UNAVCO LAPDOGS (Local
Automated Process for Downloading of Global Sites) software tool, which is currently
being used at a number of installations. Capability was also added to support a broader
set of commonly available modem types.
The UNAVCO software tool TEQC (Translate/Edit/Quality Check) was modified
to include translation capability for ConanBinary and TurboBinary data types. For
TurboBinary, this includes translation capability for normal-rate data, high-rate data (upto 50 Hz sampling rate), and the mixed "30-1" format contained LC observables. The raw
data can contain any mixture of these types, and TEQC allows the user to easily extract
the desired portion, e.g. extraction of just normal-rate (30 second sampling) from the "301" format. This code was also adapted for TurboBinary from the Allen Osborne
Associates Benchmark receiver, to store the C/A-derived phase value for the RINEX L1
observable. The ConanBinary translator currently handles ConanBinary from theTurboRogue/TurboStar series and Benchmark receivers, though not for the original Roguereceiver. The TEQC software is used by several IGS participants, including UNAVCO,
JPL and the CDDIS (Crustal Dynamics Data Information System) Global Data Center,
for GPS data file conversion and validation/verification. UNIX and DOS versions areavailable
on-lineat
the
UNAVCO
Facility
website(http://www.unavco.ucar.edu/software/).
Scripts to automatically compare descriptive information from the Central Bureau
Information System (CBIS) and the CDDIS station log summary files and RINEX files
were developed and are run daily to identify any discrepancies, with results posted at the
CBIS. This information has been used extensively by JPL and the UNAVCO Facility toverify the GGN station log and RINEX header metadata and correct inconsistencies.

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GPS Global Network Operations at JPL and the UNAVCO Facility

4

Plans for 1998

Up to seven new permanently operating GPS stations will be installed at global
locations in 1998 in support of NASA activities and all will provide data to the IGS.
Funding has been secured to improve data flow at several existing GPS stations by
upgrading some with new PC/LINUX download systems running GNEX and improving
Internet connectivity at another subset. At least four new automated meteorological data
recording packages, funded by the US National Science Foundation, will be placed at
GGN stations during 1998, with possibilities for additional units becoming available fromother sources later in the year.
GNEX software functionality will be expanded to support Ashtech receivers anda new module to automatically establish dial-up PPP (Point-to-Point Protocol)
connections will be added. Other modifications are also planned, including integrating
data from automated meteorological recording instruments into the data flow, andrewriting sections of the code to improve operability and maintainability of the software.
The lightweight GNRT system for producing RINEX products is self-contained
and portable among UNIX systems, making it appropriate for use as a backup to the JPL
data retrieval and management operations in the event of a large-scale failure. The GNRT
system will be installed at the UNAVCO Facility during 1998, and operational
procedures will be developed and regularly exercised to assure adequate and maintainableback-up coverage. The Pacific Geosciences Center also hosts a minimal additional backup system and a GNRT installation is planned there, as well.
A 24-station near-real time subnetwork of the GGN, which has provided hourly
data since late 1996, is the springboard for the development of a subnetwork of high-rate,
low-latency sites for ground support to CHAMP, GRACE, and other Low-Earth
Orbiters (LEOs) being pursued cooperatively by several agencies. Utilizing existing GPS
tracking stations is envisioned, with communications and receiver systems upgrades
where appropriate. Highly capable, yet economical LINUX computers at the stations
would host the "Smart Sites" software, currently being developed based on the GNEX
package, allowing on-the-fly data verification/validation, active notification of error
conditions, and other sophisticated new modes of reliable, automated site operations.
GNRT processing would provide the required rapid turnaround of the LEO ground GPS
network data to users. Detailed GPS ground network segment requirements are currently
emerging and NASA and other funding is being pursued to support the LEO efforts.
The value of hourly data to applications such as rapid orbit prediction andatmospheric monitoring is clear, and JPL/UNAVCO will strive to increase the coverage ofthe hourly subnetwork and make these products broadly available. In addition to the
GGN sites with sufficient communications capability for hourly data retrievals, several
other partners such as SK, BKG, GFZ, and OSO, also make hourly data available fromsome stations and it is hoped that the IGS will endorse and promote the acceptance ofhourly data in the future.

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242

Individual Stations

Status of the IGS Stations Provided by the Norwegian Mapping Authority in 1997

Status of the IGS Stations Provided by
the Norwegian Mapping Authority in 1997
Svein Rekkedal and Hans-Peter Plag
Norwegian Mapping Authority, Hønefoss, Norway

1

Introduction

In 1997, the Norwegian Mapping Authority (NMA) provided data from twocontinuously operating stations in Norway, namely Ny-Ålesund and Tromsø, to IGS. In
addition, NMA gathered and distributed data from the Scandinavian stations Onsala,Metsahovi and Thule.
2

Operation and Changes at the Sites

During the year, we experienced considerable problems with old Rogue SNR-8
receivers at Ny-Ålesund and Tromsø. However, with extensive support from JPL, wewere able to get them operational again.
For several reasons, we could not guarantee the stability of our antenna pillars inTromsø and Ny-Ålesund. Therefore, new antenna pillars were built at both sites. On
these new pillars, a Rogue SNR-800 was installed in Tromsø, and a Rogue RM-12 at NyÅlesund. The old Rogue SNR-8 receivers and their antennas will be kept working on theirold antenna pillars as long as possible for investigation of imaging.
At Høfn, Iceland, a new station has been established with NMA equipment.
However, due to environmental problems, it was not possible to get our equipment
operational in 1997 and to download data reliably.
We decided to operate our antennas without radomes. In order to avoid problems
with snow, the antennas are mounted on steel towers of 3 to 5 m height. The plates at the
top have minimal sizes thus reducing multi path.
3

Data

As a result of the problems with the Rogue SNR-8 receivers the stations did notprovide data reliably over the complete year. In particularly, Tromsø did not produce
data for several months.
The download software has been modified. It now handles data from different
receivers, and data are download every hour. At Ny-Ålesund and Tromsø, the computersused for down-loading are parts of NMA-LAN and connected to Internet. At Høfn, thecomputer is connected to a ISDN line.
Within a JPL project related to low orbiting satellites, JPL collects the data directfrom the stations operated by NMA every hour. NMA collects data from the stations

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IGS 1997 Technical Reports

once a day, concatenates the hourly files to one daily file, RINEXes the data and FTPs
these files to IGS.
4

OutlookIn 1998, NMA will incorporate a quality check of all data as a part of the
RINEXing program.
This quality check will aim at increasing the data coverage and the reliability andconsistency of the data. Furthermore, it is planned to create a data archive for our IGS, aswell as other continuously operating stations, where authorized users will get access tocurrent and past RINEX files.

246

UPAD Status Report for 1997

UPAD Status Report for 1997
Alessandro Caporali
Dipartimento di Geologia Paleontologia e Geofisica
Universita’ degli Studi di Padova
Via Giotto 1
I-35137 Padova, Italy

1

Introduction

The GPS station UPAD of the University of Padova has been operating since
1994 as a permanent installation in support of the International GPS Service for
Geodynamics (IGS), and of the European initiatives EUREF and CERGOP. In 1997, ourUniversity joined UNAVCO. The UPAD station serves the scientific and tutorial needsof the Department of Geology, Palaeontology and Geophysics for the application of GPS
data to Earth Sciences, and of the Interdepartmental Center for Space Activities (CISAS)
for the application of GPS techniques to Space Engineering, Space Communication andNavigation. The station is located in downtown Padova, on the roof of the University
Main Building, near a Geodetic Dome formerly used for astrolabe observations.

Figure 1: daily statistics of the UPAD data computed with QC.
except clock drift (msec/hour)247

All unitlessIGS 1997 Technical Reports

Figure 2: Network of permanent stations processed at UPAD. Units shown are
Molasse(1), External units (2), Penninic- oceanic (3), Austroalpine (4) and Southalpine
and Dinaric (5).
2Instrumentation

As in the previous years, the station operated throughout 1997 with the
TRIMBLE 4000SSE receiver and geodetic antenna with groundplane,. In September, newequipment was tested off-line. This included a TRIMBLE 4000 Ssi, chocke ring antenna
and the control software URS, under OS2. The tests occasionally implied a period of
down-time for the 4000 SSE receiver. Before the new system went to regular operation,
an increase of edited data, as reported by the QC program (Figure 1), was noticed. Thiswas probably due to a transmitter nearby, but the final computation of the baselines
seems unaffected. The receiver clock also showed a decrease in drift this summer, causedby temperature gradients in the room housing the receiver. Multipath was at nominal
values and dropped sensibly after Dec. 21 with the new system, indicating a higher
immunity of the chocke ring antenna. A new modem was installed to support the local
BBS. The local PC is configured as a FTP server to provide both by phone andInternet/FTP access.
3

Analysis Software

Bernese 4.0 was installed on a dedicated PC Pentium 266MHz in April.
Systematic, semiautomatic analysis started in July 1997, with data from 11 stations being
processed with analysis strategies recommended for regional networks (Figure 2).

248

Report on IGS Global Station Jozefoslaw (JOZE)

Report on IGS Global Station Jozefoslaw (JOZE)
Janusz Sledzinski
Institute of Geodesy and Geodetic Astronomy
Warsaw University of Technology
Warsaw, Poland

The IGS permanent GPS station Jozefoslaw (JOZE) is located at the
Astrogeodetic Observatory of the Institute of Geodesy and Geodetic Astronomy of the
Warsaw University of Technology, 14 km southwards from the Warsaw city center. The
Observatory was established in 1959 and at present, the following permanent services aremaintained:•

GPS permanent service has been maintained since August 1993. Earlier, thestation participated in the IGS Epoch'92 Campaign. As a basic GPS equipment
the Trimble 4000SSE receiver serial No. 3249A02090 and antenna Trimble
Geodetic L1/L2 No. 3247A66429 are used. Three rubidium frequency standards
are available at the station; one of them is used as an external standard for IGS
service. On January 1, 1995 the second GPS receiver, a TurboRogue SNR8000,
serial No. 339 with the antenna type Dorne Margolin T No. 442, was installed at
the station. The permanent GPS IGS service is maintained by both receivers(Trimble 4000SSE and TurboRogue SNR8000). The Trimble 4000SSE serves as
the main receiver and the observations collected by this receiver are transmitted tothe international data centers. The observations from Jozefoslaw are used forboth IGS service and for maintenance of the EUREF system. The observations of
the TurboRogue SNR receiver are available upon request for all interested centers
for scientific research. Additionally, in some periods of 1996 and 1997, othertypes of GPS receivers were temporary installed at the station Jozefoslaw. They
were: Ashtech ZXII-3, Leica SR9500 and Zeiss RM24. The observations were
performed to study some instrumental effects, multipath and atmospheric(ionosphere and troposphere) influences.

•

The station JOZE takes part in the works of the IGS Ionosphere Working Group.

•

Gravimetric permanent tidal observations are carried out using LaCoste &Romberg, mod. G gravity meter. This service has been maintained sinceNovember 1993. The Observatory is incorporated to the international network oftidal observatories of the International Center for Earth Tides (ICET) of theFederation of Astronomical and Geophysical Data Analysis Services (FAGS) as
station No. 0909. The Observatory Jozefoslaw is one of the fundamental pointsof the Polish national gravimetric network; many absolute gravity determinations

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IGS 1997 Technical Reports

have been performed by Polish and international observing groups. The Polishabsolute gravity meter is installed at the station. A meridional gravimetricbaseline, 26 km long, was established at the Observatory in 1976; periodic
observations are made four times a year. The observations are used jointly withclassical astrometric determinations for monitoring the changes of the vertical.
•

Astrometric latitude observations have been carried out since 1959 in theinternational cooperation with BIH and IPMS and now the observations are usedby Shanghai Observatory (international coordinator of the optical astrometry) and
GOSTSTANDARD, Moscow. These observations are still used ascomplementary ones for the analyses of the time variations of the plumb line.

•

Meteorologic service maintained at the station can be supported by nearby
permanent meteo service of the Warsaw airport (Warszawa-Okecie). The station
Jozefoslaw is located in a distance of a few kilometer from the Warsaw airport.

•

In some periods the observations of atmospheric electricity are made at theObservatory by the team of the Polish Academy of Sciences.

The monumentation of the reference point for IGS GPS observations was madeaccording to the IGS standards. The network of control points is available. Due to thegeological situation the pillar could not be monumented on the bedrock. Station
Jozefoslaw is the reference point of several international GPS networks, e.g. EUREF(European Reference Frame), EXTENDED SAGET (Satellite Geodetic Traverses),
CEGRN (Central Europe GPS Reference Network realized in the frame of the project CEICERGOP (Central European Initiative Central Europe Regional Geodynamics Project)
and BSL (Baltic Sea Level Project). The eccentricity of the EUREF point with respect tothat of other campaigns is X = 0.079 m, Y = 0.030 m, Z = 0.108m. In the 1960ties,
1970ties and 1980ties the Observatory also participated in other astrometric as well assatellite Doppler and GPS campaigns.
The Institute's Processing Center acts as IGS Regional Network Associate
Analysis Center, EUREF Local Analysis Center and as CEI CERGOP Processing Center.
The routine permanent GPS data processing and transmission are made for IGS and
EUREF; also other GPS campaigns organized in Central Europe for geodynamic studiesof the Teisseyre-Tornquist Contact Zone, the Carpathians Belt and Subalpine Regions areprocessed in the Center.

250

The BOR1 IGS Station

The BOR1 IGS Station
M. Lehmann, L. Jaworski
Space Research Centre , Polish Academy of Sciences,
Borowiec & Warsaw, Poland

Stability of the Borowiec Local Network
The geodetic network at Astrogeodynamical Observatory in Borowiec consists of
few points which are using for different applications.
Most important from them are:• 12205M002 Borowiec – SLR 7811 marker (BORL),• 12205S002 BOR1 – permanent IGS marker with ROGUE SNR–8000 receiver,• 12205S001 BORO – EUREF marker ( 0216 BOROWIEC) used in GPS
campaigns in Poland,
Vectors between these three points were measured with GPS and classic
techniques for comparison and stability analysis.
Below, in Table 1, we present values for height differences and distances for 6
years period from selected campaigns.
Mean values of differences of geodetic and cartesian coordinates for vector
BOR1–BORO are presented in Table 2.
Table: 1 Results obtained from selected campaigns
BOR1- BORL
BOR1 - BORO
BORO - BORL
s
s
s
∆h
∆h
∆h
7.535
134.4977.532
134.4975.811
98.6047.536
134.496
5.819
98.6031.723
77.3187.533
134.4995.811
98.6077.533
134.4997.533
134.500

observations
periodcampaign
14 daysepoch’92
2 days LASER1’93
4 days LASER3’94
classic
3 days Jaworski ‘97
19 days Jaworski ‘98

Table 2. The eccentricity values in geodetic and cartesian components forvector BOR1 -BORO∆B [° ′ ″]+0° 00' 03.03712"∆X [m]-38.382∆L [° ′ ″]- 0° 00' 05.06454"∆Y [m]-112.239251∆h [m.]
7.533∆Z [m.]
63.397

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252

Peculiarity of POL2, SELE GPS Stations Operation

Peculiarity of POL2, SELE GPS Stations Operations
Alexandr Matix, Yuri Trapeznikov
Institute for High Temperatures - Association, Russian Academy of Sciences,
CIS, 720049, Kyrgyzstan, Bishkek, OIVTRAN

The POL2 station operates in Kyrgyzstan (TIEN SHAN) from the 10th of May 1995
and SELE - from the 08 of May 1997. TurboRogue SNR 8000 receivers have been installed atboth stations. Short Haul Modems are used for data downloading.
Some of SVs (or each of SVs) disappearing during TurboRogue operation without
any reason is the one usual problem of the stations and of course, pauses in data flow.
Sometimes TurboRogue hangs up for no reason whatever (owing to TR firmware bugs,
probably), so it doesn't allow to download the data and simply doesn't react on 'bang'command. The TurboRogue dispower/power process can only fix this problem. So, thenecessity in frequent visits to the station is obvious.
The TurboRogue doesn't operate properly sometimes, when the computer used
for downloading process is restarted. We can also get the same result after sending any
disorder message to TurboRogue. In this case the reply would be as follows:"User unknown" or "Data not found"and the data would stay not downloaded.
TurboRogue operates better, when FreeWave modems are used, because thecomputer connects to TurboRogue only close to the downloading process and after thedata is downloaded, this communication line becomes disconnected.
To avoid such troubles we decided to undertake the below solution. One
commutator was installed between Short Haul Modem and TurboRogue to increase theTurboRogue operation reliability of POL2 and SELE. The commutator is controlled by
two special sequences of 3 bytes. One sequence provides connection of the Short Haul
Modem and TurboRogue and another - disconnection of them. Before the start of data
downloading computer sends one sequence of bytes to commutator and the commutatorswitches SHM to TR. After the downloading, computer sends to commutator anothersequence of bytes and the commutator switches SHM from TR. So, all the time between
downloading process, the TurboRogue is not connected to computer serial port and SHM
too. This solution decreases the quantity of pauses in data flow.
If the TurboRogue is hung up, the third and forth control sequences of bytes areused. The third sequence disconnects the power supply from TurboRogue and the forthconnects the power supply to TR. The tracking is stopped at first by the 'terminate'command, then the power supply becomes disconnected from TurboRogue by the third
sequence of bytes, then the power supply becomes connected to TurboRogue by the253

IGS 1997 Technical Reports

forth sequence of bytes after a short delay. The TurboRogue starts automatically andwrites data on the Flash Card.
All control sequences are sent automatically by XTalk program according to theTurboRogue messages within the downloading process.
For more information, please email matix@gdirc.ru.

254

PilotProjects/Committees

GLONASS IGEX-98 Campaign

GLONASS IGEX-98 Campaign
IGS 1997 Annual Report
Pascal Willis
Institut Geographique National, ENSG/LAREG,
6-8, Avenue B. Pascal, 77455 Marne-la-Vallee, France
Email: pascal.willis@ensg.ign.fr
Web site: http://lareg.ensg.ign.fr/IGEXThe GLONASS system is a Russian navigation satellite system, very similar in its
principle to the GPS system. Even if the GLONASS constellation of satellite is not yet
operational, this system may have some geodetic applications in geodesy and geophysics
that need to be studied, specially combined use of GLONASS/GPS receivers.
At the 1997 IAG Scientific Assembly in Rio de Janeiro in Brazil, it was decided
that the CSTG Subcommission for Precise Microwave Satellite System È should organize
an international campaign of observation for the GLONASS satellites. This action was
done jointly with the IGS as the IGS Directing Board also decided at its Rio de Janeiro
meeting to organize such a campaign.
Several scientific goals were foreseen: precise GLONASS orbits using GLONASS
data from a worldwide international tracking network, terrestrial reference frame issues,possible use of GLONASS and GPS data.
A Steering Committee was appointed in Rio de Janeiro to organize such a
campaign, called IGEX-98 (International GLONASS Experiment): G. Beutler, W.
Gurtner, G. Hein, R. Neilan, J. Slater, Pascal Willis (chair). This campaign was supported
by several associations or services: the International Association of Geodesy (IAG), theInternational GPS Service (IGS), the International Earth Rotation Service (IERS) and the
ION (US Institute of Navigation).
In order to organize such a campaign, the Steering Committee met at the AGU Fall
Meeting in December 1997 and decided to send out in early 1998 an International Call for
Participation for stations (combined GPS and GLONASS measurements), Data Centers,
Data Analysis Groups for GLONASS and SLR data. A Web site has been created(http://lareg.ensg.ign.fr/IGEX) in order to give information to submit the proposal ofparticipation.
Several institutions have answered positively to the International for Participation(the list is accessible at this Web site). The Steering Committee met in Marne-la-Vallee,
France on June 29, in order to validate and analyse these answers. On June 26, 47
proposals were received, leading to a possible network of 30 to 50 GLONASS/GPS
receivers collocated at existing IERS sites (mainly IGS sites).
An IGEX mail facility (equivalent to the IGS mail) has been started in order toregularly exchange information between the different participants. The campaign isplanned for 3 months starting from September 20, 1998 to December 20, 1998.
More information can be found on the Web at the following address:
http://lareg.ensg.ign.fr

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258

IGS Combination of Troposheric Estimates - The Pilot Experiment

IGS/BIPM Time Transfer Pilot Project
J.R. Ray
Earth Orientation Dept., U.S. Naval Observatory
Washington, DC 20392 USA

1Introduction

The “IGS/BIPM Pilot Project to Study Accurate Time and Frequency
Comparisons using GPS Phase and Code Measurements” was authorized in December
1997 jointly by the International GPS Service for Geodynamics (IGS) and the Bureau
International des Poids et Mesures (BIPM). A Call for Participation was issued shortly
afterwards with responses received from about 35 groups. The respondents have formed
a working group co-chaired by C. Thomas, BIPM, and J. Ray, U.S. Naval Observatory(USNO).
A number of groups have been working for several years to develop the capabilityof using geodetic GPS techniques for accurate time and frequency transfer. A variety ofconvincing demonstrations has already been performed showing the potential for
determining clock differences at the level of a few hundred picoseconds. The current stateof maturity of both the global tracking network and data analysis techniques now allows
practical applications to be considered. The central goal of this Pilot Project is toinvestigate and develop operational strategies to exploit GPS measurements for improved
availability of accurate time and frequency comparisons worldwide. This will become
especially significant for maintaining the international UTC timescale as a new generationof frequency standards emerges with accuracies of 10-15 or better.
The Project is expected to run until January 2000. By that time, those aspects
which are suitable for integration into the operational activities and official products of
the IGS or BIPM should be underway. To the extent that some functions may not besuitable for the existing structure of the IGS, a new coordinator for this might beappropriate. The progress of the Project and other related information is maintained at
the Web site http://maia.usno.navy.mil/gpst.mil.
2

Areas of Participation

2.1

Deployment of GPS Receivers

In addition to the GPS receivers already installed as part of the IGS global tracking
network, other receivers at laboratories having accurate time standards are sought. Theseshould be high-quality geodetic receivers capable of recording and rapidly transmitting
dual-frequency pseudorange and carrier phase observations. The station configurationand data distribution should conform to IGS standards and appropriate documentation

259

IGS 1997 Technical Reports

must be filed with the IGS Central Bureau. A log file should be completed and sent to the
IGS Central Bureau for each IGS station. For this Project, due consideration should begiven to electronic stability, environmental control, and other factors which might affectthe timing results. Upgrading of existing tracking stations for better timing performance isalso encouraged. Deployment of dual-frequency GLONASS receivers, especiallycollocated at IGS sites, would provide an additional data source of interest.
2.2

GPS Data Analysis

Strategies for analyzing GPS phase and pseudorange observations must be
developed, consistent with other IGS products, to allow the routine, accuratecharacterization of time standards at a large number of independent GPS receiver sites andonboard the GPS satellites. This work will be done in close cooperation with the IGS
Analysis Center Coordinator. It is expected that regular reports will be issued byparticipating analysis centers, analogous to those distributed by the IGS for otheractivities, and filed in the IGS Electronic Reports series.
The precise relationship between the analysis activities that are needed for thisPilot Project and those required for the official products of the IGS is not entirely clear at
this point. Certainly, the Project should build and rely upon the existing IGS structure.
There may, however, exist a need for clock analysis and related products beyond thecharter of the IGS. Also, some changes in the current analysis procedures of the IGS may
be advantageous for enhanced timing performance. For these reasons it is essential that
the Analysis Coordinator be actively involved.
2.3

Analysis of Instrumental Delays

In order to relate clock estimates derived from GPS data analysis to external timing
standards it is necessary to understand the instrumental electronic delays introduced by
the associated hardware. Studies are sought to characterize the short-term and long-term
sensitivities to environmental changes and to develop suitable calibration methods.
Differences for the L1 and L2 frequencies must be considered. Studies of both GPS
ground sites as well as the GPS satellites are sought.
2.4

Time Transfer Comparisons

Simultaneous, independent time and frequency comparison data are needed tocompare with the GPS-derived estimates. Collaborations are sought with groups
performing time transfer experiments using a variety of techniques. Close cooperation is
expected with the Consultative Committee for Time and Frequency (CCTF) of the
ComitJ International des Poids et Mesures (CIPM).

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IGS Combination of Troposheric Estimates - The Pilot Experiment

3

Objectives

To accomplish the overall goal of improved global accessibility to accurate timeand frequency using GPS, several specific objectives can be set.
3.1

Accurate and Consistent Satellite Clocks

Satellite clock estimates are among the “core” products of the IGS (Kouba et al.,1998). The IGS combined solutions for satellite clocks are distributed together with the
IGS combined orbits in the sp3 product files. It is essential that the clock information be
as accurate as possible and also that it be fully consistent with the other IGS products.
Kouba et al. (1998) describe the importance of global consistency to ensure that the point
positioning technique (Zumberge et al., 1997) can be applied without degradation.
A type of point positioning likely to become increasingly important is for tracking
low Earth-orbiting satellites equipped with onboard GPS receivers. For this application
the 15-minute tabulation interval of the sp3 orbit files is not adequate because the SA
corruption of the broadcast clocks does not allow accurate interpolation over intervals
longer than about 30 s (Zumberge et al., 1998a). For this and other applications, the IGS
ACs have been asked to provide satellite clock products with 30-s sampling rates and the
IGS will probably begin producing a corresponding combined product (Springer et al.,1998). Methods for efficiently computing high-rate satellite clocks have been presented
by Zumberge et al. (1998b) and Soehne et al. (1998). A new exchange format will beneeded to permit easy distribution of the new high-rate results.
3.2

Accurate and Consistent Station Clocks

Presently, the IGS does not produce clock information for the GPS groundstations although doing so is mentioned in the IGS Terms of Reference. There is a clear
interest in the user community for this information. Apart from time transfer uses, it
could be used to characterize and monitor the performance of station frequency standards.
Clock solutions from stations equipped with very stable frequency standards (especially
H-masers) are needed to apply the method of Zumberge et al. (1998a) to estimate highrate satellite clocks. For this purpose, station clock determinations at intervals of about 5
minutes can be accurately interpolated to the 30-s intervals needed to solve for thesatellite clocks provided that the ground stations are referenced to stable clocks.
For time transfer applications, such as envisioned for this Pilot Project, accurate
analysis results for the station clocks are mandatory. As with high-rate satellite clocks, a
suitable exchange format must be developed. Regular summary reports to describe theanalysis results characterizing satellite and station clocks will be encouraged. Theseshould be publicly distributed in the IGS Electronic Reports series. Some IGS ACs,particularly JPL and EMR, already include valuable clock information in the weekly
analysis summary reports that accompany their Final product submissions.

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IGS 1997 Technical Reports

From geodetic analyses of the GPS data, the effective “clock” of each station isdetermined for the ionosphere-corrected L3 phase center of the antenna displaced by theelectronic delay to the point in the receiver where the time tags are assigned to the phasemeasurements. These clock determinations are relative measurements in the sense thatusually a single station is chosen as a time reference and not adjusted. From theviewpoint of geodetic applications, the precise reference point of the analysis clocks isirrelevant. As a result, manufacturers of geodetic receivers have generally not taken care
to provide easy or accurate access to the time reference points. However, for timing
applications, such as time transfer comparisons with other techniques, the precise
location of the clock reference and accurate access to it are essential. Consequently, theinvestigation of instrumental path delays and access points is critical to the success of the
Pilot Project.
Even if one imagines a shift in the timing paradigm so that the GPS receivers are
eventually regarded as a part of the outer ``electronics package'' of stable frequency
standards, it is nonetheless vital to establish accurate access to the clock reference points.
The effects of environmental influences will be even more important in that case and must
be minimized. Doing so will require new approaches for isolating GPS receiver
equipment, such as efforts by Overney et al. (1997).
3.3

Accurate and Stable Reference Timescale

Ultimately, it is necessary that all clock information, for satellites and stations, bereferenced to a common, consistent timescale. Individual sets of results from different
ACs generally refer to different reference clocks. Thus, in the IGS combination process,the AC submissions must be realigned. This is currently done by choosing one
submission as a reference solution, realigning its satellite clock estimates to GPS time
based on the broadcast clocks for all the satellites (using only daily offset and rate terms),
and then realigning all the other AC submissions to the reference solution (Springer et al.,1998). Corrections are applied to each solution set to account for radial orbit differencescompared to the IGS combined orbits. The IGS combined satellite clock estimates arethen formed from the weighted average of the realigned, corrected submissions.
It has been suggested that the clock realignment and combination process would be
improved if a common set of “fiducial” station clocks were used in all analyses andincluded in the IGS submissions (Springer et al., 1998). Naturally, only stations
equipped with very stable frequency standards (preferably geometrically well distributed)
should be considered as candidate fiducials. Recommendations for this station set will
likely be made during 1998.
Likewise, it is questionable whether GPS time is an appropriate choice for theunderlying IGS timescale. The ideal choice should be accurate, accessible, and stable over
all relevant time intervals (namely, 30 s and longer). GPS time is readily accessible but
not with an accuracy comparable to other IGS products due to SA effects. Nor is GPS
time particularly stable. The clocks of the GPS constellation are monitored from USNO

262

IGS Combination of Troposheric Estimates - The Pilot Experimentand this information is provided to GPS operations with the goal of maintaining GPS timewithin 28 ns (RMS) of UTC(USNO), allowing for accumulated leap second differences.
In practice, the two timescales have been kept within about 6.5 ns (modulo 1 s) over thelast two years (for 24-hour averages). However, the GPS time steering algorithm has a``bang-bang'' character resulting in a saw-tooth variation with a typical cycle of about 25
days. This is equivalent to a frequency error greater than 10-14 over days to weeks, whichchanges periodically in an abrupt, nearly step-like fashion.
Almost certainly, an internal ensemble of the frequency standards used in the IGS
network can be formed which would possess better stability than GPS time (Young et al.,1996). There are currently about 27 IGS stations using H-masers, and about 40 with
Cesium or Rubidium standards. Addition of new IGS sites located at primary timing
laboratories would only improve this situation. A purely internal IGS timescale would
probably not be stable against long-term drifts so some linkage to external laboratory
timescales is required. Indeed, tracability to UTC(BIPM) is most desirable. In principle,
this could be accomplished using the instrumental calibration data mentioned above,
especially for the fiducial clock sites. It will be technically difficult, however, to achieve
comparable accuracies for the calibration measurements to the few hundred picosecond
level possible for the data analysis clocks. This will be one of the greatest challenges forthis Pilot Project.
An alternative approach to provide external linkage that can be readily
implemented uses monitor data for the GPS constellation that are collected and comparedat the timing labs. USNO collects such data using pseudorange timing observations andmakes the results publicly available. Using the observed offsets of GPS time relative toUTC(USNO), the corresponding IGS clock estimates can be related to UTC(USNO).
Because of the effects of SA such comparisons would only be useful to remove long-term
differences. This is probably sufficient, at least for an initial realization. Other timing
laboratories would be encouraged to provide similar monitor data for a more robust tie toUTC(BIPM). A potential problem with this approach is possible biases between theeffective clocks transmitted by the satellites as measured from the pseudorange and carrier
phase observables.
Apart from the issues discussed above concerning calibration and externalreferencing for an IGS timescale, there are other practical questions that must be resolved.
In particular, it may be difficult to form and maintain a timescale within the IGS productdelivery schedule. This is likely to be especially true for the Rapid products even thoughthat is probably also where the greatest user interest lies. Fundamentally, this does notseem overwhelming although it will require entirely new and highly automated IGS
processes. Other practical concerns are minimizing discontinuities at day boundaries,
dealing with clock discontinuities and drop-outs in the ensembling process, and finding anappropriate robust ensembling algorithm. These subjects, together with those mentioned
above, should be studied during this Pilot Project.

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4

References
Kouba, J., J. Ray, and M.M. Watkins, IGS reference frame realization, in 1998
IGS Analysis Center Workshop Proceedings, European Space Operations
Centre, Darmstadt, Germany, in press 1998.
Overney, F., Th. Schildknecht, and G. Beutler, GPS time transfer using geodetic
receivers: Middle-term stability and temperature dependence of the signal
delays, Proc. 11th European Frequency and Time Forum, 504-508, 1997.
Soehne, W., et al., Precise high-rate satellite clocks at GFZ, in 1998 IGS Analysis
Center Workshop Proceedings, European Space Operations Centre, Darmstadt,
Germany, in press 1998.
Springer, T.A., J.F. Zumberge, and J. Kouba, The IGS analysis products and theconsistency of the combined solutions, in 1998 IGS Analysis Center Workshop
Proceedings, European Space Operations Centre, Darmstadt, Germany, in press
1998.
Young, L.E., D.C. Jefferson, S.M. Lichten, R.L. Tjoelker, and L. Maleki,
Formation of a GPS-linked global ensemble of Hydrogen masers, andcomparison to JPL's linear ion trap, Proc. 50th Annual Institute of Electronics
Engineers (IEEE) International Frequency Control Symposium, 1159-1162,
05-07 June 1996.
Zumberge, J.F., M.B. Heflin, D.C. Jefferson, M.M. Watkins, and F.H. Webb,
Precise point positioning for the efficient and robust analysis of GPS data fromlarge networks, J. Geophys. Res., 102(B3), 5005-5017, 1997.
Zumberge, J.F., M.M. Watkins, and F.H. Webb, Characteristics and applicationsof precise GPS clock solutions every 30 seconds, Navigation, in press 1998a.
Zumberge, J.F., et al., Efficient estimation of precise high-rate GPS clocks, in 1998
IGS Analysis Center Workshop Proceedings, European Space Operations
Centre, Darmstadt, Germany, in press 1998b.

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IGS Combination of Troposheric Estimates - The Pilot Experiment

IGS Combination of Tropospheric Estimates -The Pilot Experiment
Gerd Gendt
GeoForschungsZentrum Potsdam, Div. 1
D-14473 Potsdam, Telegrafenberg A17

1

Introduction

The existing global and regional networks of permanent GPS receivers installed forgeodetic and navigational applications can be used with marginal additional cost for
determination of atmospheric water vapor with high temporal and spatial resolution. In
different countries projects are under way in which the impact of GPS derived water
vapor on the improvement of weather forecast are studied. Within the IGS a network ofmore than 100 globally distributed sites is analyzed on a daily basis. The zenith pathdelay (ZPD) values obtained should be converted into precipitable water vapor (PWV)
and should be made available to the scientific community.
This IGS product could meet the demands for climatological studies. Here a time
resolution of 2 hours (this is what IGS will provide) is sufficient, because long-term
characteristics are of interest only, and a time delay of a few weeks for product delivery is
acceptable.
Since the beginning of the Pilot Experiment six of the global IGS Analysis Centers(ACs) have regularly contributed (Table 1). Different mapping functions and elevation
cutoff angles of 10, 15 or 20 degrees are implemented. The number of sites per AC varies
from 30 to 85, and their were ∼100 sites in total. The product from the combination is aweekly file for each site containing the ZPD estimates and precipitable water vapor ifconversion is possible. Additionally, a combination report will summarize some statistics
on the differences to the IGS Mean (bias, standard deviation), for the global mean of eachAC and separately for all sites.
Table 1. Contributing Analysis Centers with some relevant parameters
ZPD[min]

Cutoff[deg]No.Sites

CODE

120

85

Saastmoinen; Niell (week 926)

EMR
ESA
GFZ
JPL
NGS

60
120
60
5
120

20, 10(wk
926)
15
20
20
15
15

30
50
55
37
55

Lanyi
Saastmoinen
Saastmoinen; Niell (week 929)
Lanyi; Niell (week 920)
Niell

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Mapping Function

IGS 1997 Technical Reports

2Comparisons, Results

For comparisons the differences between individual AC estimates and the IGS canbe used. More than 60 sites are used by at least three ACs, so that sufficient statisticalinformation about the quality of the tropospheric estimates can be gained. For most sites
and ACs the stddev is ±6 mm ZPD (which corresponds to ±1 mm PWV) and it
approaches in many cases the ±3 mm level. The magnitude of the stddev is of course
highly correlated with the magnitude in the repeatability of the estimated station
coordinates. Looking into the geographical distribution of the magnitude for the stddev thelargest stddevs can be found in the equatorial region. The bias for most sites is below ±3
mm. Even for sites with a larger bias its repeatability is very high.
The global mean stddev (mean over all sites) of the best ACs is at the 4 mm level(Figure1). Only a small global bias at the 1 mm ZPD level can be stated. However,significant effects of ±1-2mm from AC to AC exist. ACs having changed their parameters(cmp. Table 1) show larger jumps correlated with those changes.

Fig. 1. Difference between AC ZPD and IGS Combined ZPD. Mean values (meanover all sites) per week and Analysis Center
The biases for fiducial (or other well determined) sites are very small, and therepeatability is at the 2 mm ZPD level. However, larger systematic effects can be foundfor some sites too, especially in the equatorial region. Here systematic effects of about ±6
mm exist with single peak to peak differences in the weekly biases of 20 mm. The bias
differences could be reduced by taking into account the well-known correlation between
the station height and the ZPD estimates. This works rather well for some sites, but not
for all. However, such a procedure will not be recommended because any corrections to266

IGS Combination of Troposheric Estimates - The Pilot Experiment

the estimates are dangerous. It is better to reduce the scattering in the determined station
heights. One step in this direction will be the enlarged set of 52 fiducials, which will be
constrained to a certain extent by all the ACs. The introduction of a smaller elevation
cutoff angle may also help to reduce the bias.
3

Conversion into Precipitable Water Vapor

For the conversion meteorological surface measurements are needed. At the
moment 19 sites report regularly their met data to the global data centers. Ten furthersites have announced the installation of met packages, but the data are not yet available.
The met data must be of high precision (1 mbar corresponds to 0.35 mm in PWV) andreliability (continuous time series). For some sites too many missing days or larger gaps
must be stated. In those cases no meaningful series of PWV could be produced.
Unfortunately, only 10 to 15 reliable sites with met sensors exist at the moment (a small
percentage of all analyzed sites).
The GPS derived PWV estimates can be compared with WVR measurements to
get a measure for the absolute accuracy. Only at POTS measurements of a collocated
WVR were available. A WVR-1100 of Radiometrics Corporation is operated byMeteorological Observatory Potsdam of the German Weather Service, and is located400m apart from the GPS receiver. The agreement of the GPS results (both CODE and
GFZ) with the WVR is at the 1 mm level (Figure 2). The stddev of the difference
approaches ±0.5 mm, the bias has a level of ±1 mm and shows some long-periodic
behavior for both GPS results. The difference between the two GPS solutions is smaller
than their differences to the WVR measurements.

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IGS 1997 Technical Reports

Fig. 2. Comparison of PWV estimates from GPS and WVR at POTS. WVR data
are from Meteorological Observatory Potsdam, 400 apart from GPS receiver.
4

Summary

During the one year experiment all components involved in the combination have
performed well and timely. The ZPD estimates have a high quality for all the weeks. The
consistency is at the 4 to 5 mm level both for the bias and for the stddev.
The importance of the IGS contribution to climate research will not only dependon the quality of the ZPD estimates but also on the number of sites which could beequipped with met packages. The number of instruments available now is not sufficient.
To get a better insight into the behavior of the bias more collocated WVR should
be made available, either at existing IGS sites or at non-IGS sites which then should beanalyzed by all IGS ACs for some test periods.
The pilot phase for the IGS Combined Tropospheric Product is finished and the
combined zenith path delay (ZPD) estimates are an official product now. The conversion
into precipitable water vapor will be postponed until a sufficient number of surface met
packages is available. At the moment it is to the customer to convert the ZPD by relying
both on the existing RINEX met files as well as on interpolation within global or regional
meteorological fields. The product will be archived at the global Data Centers.

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IGS Combination of Troposheric Estimates - The Pilot Experiment5

References
Gendt, G., 1996, Comparison of IGS Tropospheric Estimations, in Proceedings
Analysis Center Workshop Silver Spring 1996, Eds. R E Neilan, P A Van Scoy,
J F Zumberge, Pasadena, CA U.S.A., pp. 151-163
Gendt, G., 1997, IGS Combination of Tropospheric Estimates (Status Report), in
Proceedings Analysis Center Workshop Pasadena 1997, in press
Gendt, G., 1998, IGS Combination of Tropospheric Estimates - Experience from
Pilot Experiment, in Proceedings Analysis Center Workshop Darmstadt
1998, in press

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270

1997 IGS Activities in the Area of the Ionosphere1997 IGS Activities in the Area of the Ionosphere
J. Feltens
EDS at Flight Dynamics Division
ESA/European Space Operations Centre
Darmstadt, Germany

A subsession was dedicated to the ionosphere at the IGS workshop in Potsdam in
May 1995 and again at the IGS workshop held in Silver Spring in March 1996. In
preparation for the latter, an intercomparison of ionosphere TEC maps and satellite
differential code biases was initiated. Five Analysis Centers (ACs) contributed to thisintercomparison, for which TEC data over a time span of five weeks had been used. The
intent of this intercomparison was to get an idea of the accuracies that can be achieved.
Additionally it could be identified which ACs were interested in contributing to an IGS
ionosphere product.
However in 1996, most of the ACs were still in a development stage with theirmodeling and software, and at this time they were not in a position to produce ionosphere
products for the IGS on routine basis. During the year 1997, the ionosphere activities at
the ACs were thus concentrated in the background rather than in the foreground, andsome of the ACs modeling capabilities were significantly improved during that time.
These improvements include the successful prediction of ionosphere model parameters aswell as first attempts to model the ionosphere's electron content 3-dimensionally.
Another important thing that was achieved in 1997 was the development of theso-called IONosphere Map EXchange Format (IONEX) (Schaer et al., 1997), defining forthe first time which and how ionosphere products should be exchanged. The IONEX
became in the meantime the mandatory ionosphere format within the IGS.
At the Darmstadt IGS workshop in February 1998 it was then decided to start acoordinated routine processing and a combination of IGS ionosphere products. Based ona Position Paper (Feltens and Schaer, 1998) and a Terms of Reference (Schaer andFeltens, 1998) an IGS Ionosphere Working Group has been established at 28 May 1998,
which started immediately with its pilot phase in June 1998.
The importance of an IGS engagement within the area of ionosphere science isobvious, and representatives of the ionosphere community have indicated a great interestin a continuous series of global IGS ionosphere models. Since mid 1996 we areapproaching the next solar maximum. Therefore good and rapidly available information
about the ionosphere's actual state becomes increasingly important. Users of satellite
navigation systems need accurate corrections to remove signal degradation caused by theionosphere, information on the ionosphere's behavior is of great importance for radio
signal propagation applications, and scientists will benefit from up-to-date and long-term
ionosphere information.

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References
Feltens, J., and S. Schaer (1998): 'IGS Products for the Ionosphere', in Proceedingsof the 1998 IGS Analysis Centers Workshop, ESOC, Darmstadt, Germany,
February 9-11, 1998.
Schaer, S., W. Gurtner, and J. Feltens (1997): 'IONEX: The IONosphere Map
EXchange Format Version 1', February 25, 1998, in Proceedings of the 1998 IGS
Analysis Centers Workshop, ESOC, Darmstadt, Germany, February 9-11, 1998.
Schaer, S., and J. Feltens (1998): 'Terms of Reference for the IGS Ionosphere
Working Group', distributed via IGSmail, Message Number 1918, sent on 8 June
1998.

272

IGS Infrastructure Committee Report

IGS Infrastructure Committee Report
Yehuda Bock
Scripps Institution of Oceanography
La Jolla, CA 92093-0225
E-mail: ybock@ucsd.edu

Committee Members: Yehuda Bock (Chair), Werner Gurtner, Ulf Lindqwister, Chuck
Meertens, Ruth Neilan, Carey Noll, Steve Fisher
1

Introduction

The IGS Infrastructure Committee was established by the IGS Governing Board atits meeting in Rio de Janeiro in September, 1997. The primary motivations for theformation of this committee was to assist the IGS Central Bureau in:
1. eliminating existing discrepancies that impact the accuracy, reliability, andtimeliness of IGS products (e.g., incorrect or ambiguous antenna heights,
incorrect or ambiguous RINEX data file headers),
2. identifying weaknesses in station infrastructure and recommending actions(e.g., replacing aging and obsolete equipment, evaluation of monumentationand long-term site stability, evaluation of data flow), and3. re-evaluating and promoting adherence to IGS standards within the IGS
organization (e.g., site standards, station reporting, data quality and reliability,
data archiving and dissemination).
The committee identified several actions that could improve the IGS infrastructure:
1. Development of guidelines for IGS tracking stations and operational centers
2. Promotion of improved communications between operation centers
3. Recommendation of courses of action to the Governing Board and Central
Bureau4. Preparation of an automated site log preparation scheme to solve the persistentproblem of contradicting or missing information about sites
5. Preparation of a network “road map” to chart data collection and data flow
6. Preparation of a station performance table
2

Progress

The committee in consultation with the IGS Central Bureau prepared two
documents (initially drafted by Werner Gurtner) which were adopted as IGS policy by theGoverning Board at its meeting in Boston in May, 1998:
1. Joining the IGS
2. Guidelines for IGS Stations and Operational Centers
The first document describes the procedures to be followed when establishing a
new IGS station. The second document gives guidelines for operating IGS tracking
stations and Operational Centers.
The documents can be found in Appendices A and B. The latest version of these
documents is being maintained by the IGS Central Bureau (//http://igscb.jpl.nasa.gov/)
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IGS 1997 Technical Reports

Committee members have been active in planning for the IGS Network Workshop
planned for November, 1998 in Annapolis, Maryland. An organizational meeting of the
network planning committee was held at Scripps Institution of Oceanography in August,1998. At the meeting the following actions were adopted in preparation of the November
workshop and to meet the objectives of the infrastructure committee:
1. Preparation of a network “road map” to chart data collection and data flow (ledby the Central Bureau).
2. Preparation of an automated site log preparation scheme to solve the persistentproblem of contradicting or missing information about sites (led by Scripps
Orbit and Permanent Array Center).

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IGS Infrastructure Committee Report

APPENDIX A: Joining the IGS
Version 1.1 ((reformatted for IGS Report, August 31, 1998))(prepared by IGS Infrastructure Committee and Central Bureau)***************************************************
A.1

Introduction: Procedures for Becoming an IGS Station

This document describes the procedures to be followed when establishing a new
IGS station. It is applicable to stations of global and regional interest, and includes detailed
steps to be taken by the responsible agency, in consultation with the IGS Central Bureau(CB). Also described in this document are maintenance of the public station log files andprocedures related to data flow, archiving, and analysis.
Note: The IGS recommends that these procedures also be applied to stations oflocal interest, as well. However, the IGS does not take any responsibility forcompleteness, correctness, nor for data processing and data archiving for“Local” stations.
For more detailed descriptions of station requirements with respect tomonumentation, receiver, antenna, data handling, documentation, and data formats pleaserefer to “Guidelines for IGS Stations and Operational Centers.”
All electronic messages, unless otherwise specified, should be sent to the CB at
igscb@igscb.jpl.nasa.gov.
A.2

Initial Steps

Contact the CB concerning the intent to install the station(s). Include a statement ofdesire for the station to be considered as part of the IGS network. Clearly identify theschedule for implementation, the responsible installation agency, the responsible
operational agency once the site is established, and the funding agency (including prospects
for long-term funding support).
A proposed four character station identifier should also be included for confirmationby the CB. The CB will assist in the designation of the identifier to prevent duplication.
The CB will announce the new station on its schedule of future or proposed
stations.
A.3

Site Installation

IGS standards need to be followed when installing the station. (see “Guidelines for
IGS Stations and Operational Centers”).
Once the station is installed and operational, a communication should be addressedto the CB indicating the data availability.
A.4

Station Log

If the new station is part of an existing network, the responsible Operational Centerhas to update the center description form (download /igscb/center/oper/'center'.ocn from
igscb.jpl.nasa.gov, modify, and send an electronic notification to the CB).
If the station is part of a new network, the new Operational Center has to create acenter description form (download /igscb/center/oper/BLNKFORM.OCN from
igscb.jpl.nasa.gov, modify, and send electronic notification to the CB).

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IGS 1997 Technical Reports

A
stationlog
mustbe
prepared(download/igscb/station/general/BLNKFORM.LOG from igscb.jpl.nasa.gov); many examples areavailable in /igscb/station/log.
Forward the station log to the IERS with a request for a DOMES number; this is thenumbering system that is used by the International Earth Rotation Service (IERS) to keep
track of all space geodetic stations. Instructions on obtaining a DOMES number is available
fromtheLaboratoiredeRechercheen
Géodésie(LAREG)(http://lareg.ensg.ign.fr/ITRF/domesreq.html). The IERS will assign a DOMES number
for the station and any other monument or reference marker located at the same site
location.
Forward completed station log (updated with DOMES number) to the CB to be
included into the Central Bureau Information System (CBIS). E-mail files to the CB.
Mandatory items include: Station name, 4-character site code, DOMES number,approximate coordinates (X/Y/Z and/or lat/lon/elev), antenna height and description,
receiver and antenna types, antenna diagram (see files rcvr_ant.gra, antenna.gra available at
ftp://igscb.jpl.nasa.gov/igscb/station/general for correct names), eccentricity elements (ifany) to nearby geodetic markers at the same site location, contact addresses.
Important Note: Station log files with missing or incorrectly enteredinformation will not be accepted by the CB.
When all the information is available on the CBIS an announcement should be
prepared by the implementing agency for distribution through IGSMail. (It is not necessary
to distribute the log file itself through IGSMail).
A.5

Station Log Updates

Whenever there is an update or change to the information contained in the stationlog file, the current log file should be downloaded from the CBIS (/igscb/station/log) andmodified by adding the new information and the modification date. This file should be sent
back to the CB and an announcement of the modification should be made through an
IGSMail message. This procedure ensures that modifications of the station log done by the
CB will be preserved.
Important Note: Any changes at the site must be relayed promptly (preferably
in advance) to the CB and posted via IGS Mail. In particular, these include but
are not limited to changes in monument, GPS equipment, and antenna height.
A.6

Data Format

It is the responsibility of the Operational Agency to provide GPS data to the IGS in
Receiver Independent Exchange (RINEX) format. The RINEX files must be preparedfollowing the guidelines for IGS stations and Operational Centers (“Guidelines for IGS
Stations and Operational Centers”, “rinex2.txt”).
To ensure compatibility, send an extract of a sample daily RINEX file (header, firstand last few epochs of data records, ASCII) per e-mail to the CB.
A.7

Data Flow and Archive
The long-term archiving of the data falls within the responsibility of the Operational

Center.

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IGS Infrastructure Committee Report

If the station is part of a local, regional or special network (e.g. SCIGN, EUREF,
CIGNET, JPL FLINN) the daily data files have to be sent to the respective data centersfrom where they will be forwarded, if appropriate, to other IGS Data Centers.
The operational agency needs to inform the CB to which IGS Data Center it isplanning to send its data.
The data from regional networks that will not be processed by more than one IGS
Analysis Center can be kept in the Regional Data Center and not forwarded to an IGS
Global Data Center.
The data of sites of local networks not to be processed by an IGS Analysis Center
should be kept in their respective Local Data Center, or forwarded to a Regional Data
Center.
Important Notes:
Daily data may not be accepted by the IGS Data Centers if station log files are
missing or incomplete, if the RINEX data files are not correctly formatted or
incomplete, or if the station log files and RINEX file headers are inconsistent.
Individual data files may not be accepted by the IGS Data Centers if the files
contain less than 10 percent of the nominal amount of data.
A.8

Data Analysis“Global sites” are those sites of global interest that are processed by at least three
Analysis Centers located on more than one continent. That is, data from these sites
contribute significantly to the production of IGS global products (e.g., satellite orbits,
EOP). New sites which contribute significantly to regional densification of the IGS
polyhedron are classified as “Regional sites.” Otherwise, a site is classified as a “Local
site.” Analysis of Local sites will be performed by a local analysis center and will not be theresponsibility of the IGS.
For densification purposes the results (especially coordinates and velocities) may be
forwarded as SINEX files to Associate Analysis Centers to be included into combined
solutions (SINEX: see ftp://igscb.jpl.nasa.gov/igscb/data/format/sinex.txt).
Important Note: IGS Analysis and Associate Analysis Centers may not processdata and submit results to the IGS of those sites not meeting all of the requirements
outlined in this document.

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APPENDIX B: Guidelines for IGS Stations and Operational Centers
Version 1.2 (reformatted for IGS Report, August 31, 1998)(prepared by IGS Infrastructure Committee and Central Bureau)***************************************************
B.1

Introduction

This document gives guidelines for operating IGS tracking stations and Operational
Centers. For potential or new IGS participants, please refer to the document “Joining the
IGS” (Appendix A).
B.1.1

Organization of the IGS Data Flow

En route to Analysis Centers and other users, the tracking data collected bypermanent GPS receivers flow through the following components of the IGS network:•

Tracking Stations (TS): They set up and operate the permanent GPS
tracking receivers and antennae on suitable geodetic markers.

•

Operational Centers (OC): They perform data validation, conversion of raw
data to Receiver Independent Exchange Format (RINEX), datacompression, and data upload to a data center through the Internet. For
some sites the OC is identical with the institution responsible for therespective site (i.e., the OC is identical with the TS).

•

Local Data Center (LDC): They collect the data of all stations of a local
network and distribute them to the (local) users. One or more of thesestations are part of the IGS network. For many of the local networks the
LDC will be identical with their Operational Center.
The LDC will forward the data (of a selection) of the local sites to the
Regional Data Center.
If there is not a LDC for particular data, data will flow directly from the OC
to the designated Regional Data Center.
Note: These guidelines do not pertain to local networks which are notpart of the IGS network, nor to any actions of Local Data Centersrelated to non-IGS stations.

••Regional Data Centers (RDC): They collect all the data of a certain region(e.g., Europe) or special network (e.g., JPL) and make it available to theusers, especially to those of the respective region. They forward the data of
the sites of global interest to one of the three Global Data Centers.

Global Data Center (GDC): They collect the data of global interest from theRegional Data Centers. They equalize their data holdings among themselves andmake the data available through anonymous ftp to the users, especially to the
IGS Analysis Centers.

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B.1.2.

Requirements for Permanent Stations

Because the user of the network will be mainly interested in the data and thenecessary auxiliary information about the tracking sites, we do not make a clear distinction
between the activities and responsibilities of the Tracking Stations and the Operational
Centers.
Very strict rules are inconsistent with the nature of the IGS. However for a station
to be included in the IGS Network, the following guidelines will be used to judge themerits of a candidate station. Please consult also the check list of how to become an IGS
station.
Instrument
The GPS receivers should•

track both codes and phases on both frequencies under non-AS- (antispoofing) as well as AS-conditions•

track at least 8 satellites, simultaneously•

track at least with 30 seconds sampling rate. If the sampling rate is faster,
the data should be decimated to 30 seconds prior to upload to the Regional
Data Center•synchronize the actual instant of observation with true GPS time within +- 1
millisecond of the full second•

be protected from power failures wherever feasible. If the data aredownloaded in real time to an external PC without being stored in thereceiver for a certain time, the same protection should include the PC as well

Antenna•

The antenna should include a Dorne-Margolin element and a choke ring•

The antenna should be oriented in the manner proposed by the
manufacturer.

•

The antenna should be protected against a heavy snow load, othermeteorological factors, and vandalism by use of an antenna cover(“radome”). The effect of the radome on the antenna phase center should bedetermined. Various radome designs are available from the IGS Central
Bureau.

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Marker
The marker should fulfill standard requirements for a first order geodetic monumentwith respect to stability, durability, long-term maintenance, documentation, and access.
The marker description should be fully documented in the IGS site log file (seeDocumentation below).
Obstruction should be minimal above 15 degrees elevation, but visibility to lower
elevations is encouraged whenever possible.
Signal reception quality has to be verified, especially with respect to interference of
external signal sources like radars, and with respect to multipath.
The antenna height corresponds to the vertical distance of the agreed-upon physicalreference point (see antenna diagrams) on the antenna above the marker.
Local ties to other markers on the sites should be determined in the ITRF coordinate
system to guarantee 1-mm precision in all three dimensions. Offsets are given in delta-X,
delta-Y, delta-Z, X,Y,Z being the geocentric Cartesian coordinates (ITRF).
Documentation
A standard IGS site description file (log file) should be filled out and sent to the
IGS Central Bureau at e-mail:igscb@igscb.jpl.nasa.gov
Blank forms are available through anonymous ftp at the IGS Central Bureau
Information System (CBIS):
ftp://igscb.jpl.nasa.gov/igscb/station/general/blank.log
Consistency in both format and content is strongly encouraged. Additional site
descriptions (photos, maps, etc) should be sent to the Central Bureau.
B.1.3

Operational Centers
Responsibilities

The Operational Centers control the sites of a particular (local) network from theoperational point of view.
They form a link between the sites and the Data Center. The Data Center then makes
available the data to the Analysis Centers, other Data Centers, and individual users.
The Operational Centers are responsible for•the download of the raw data from the receivers of the local network•

the archiving of the raw data•

the reformatting of the data into the agreed-upon exchange format (RINEX)•

the quality check of the data on a station by station basis

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the generation of status messages (abnormal conditions)•the alert/engagement of on-site personnel (abnormal conditions)•the upload of the data to the Data Center at agreed-upon times

Within the International GPS Service for Geodynamics there are many independent
tracking sites (e.g. Zimmerwald, Herstmonceux) that are not part of a local or special
network. As such they are not connected to an actual Operational Center. In this case theorganization operating the site also performs the tasks outlined above.
Data Download From the Stations
The downloading from the receiver to the Operational Center's computer system
can either be done directly or indirectly through a small on-site computer, e.g. a PC:+-----------++------------+| receiver | -------- | on-site PC | -----++-----------++------------+||+--------------------++----- | Operational Center ||+--------------------++-----------+|| receiver | -----------------------------++-----------+The PC could download the data from the receiver continually, using e.g. somemanufacturer-provided download software.
The communication between the Operational Center and the stations can be achieved
through any one of several means, including dialup modem, Internet, special-purpose data
links, Inmarsat, etc. Station communication configuration information should be included
in documentation provided to the CB.
Data Archive
As the exchange data format does not conserve all information found in the raw data
it should not be used for the primary data archiving. The Operational Center is responsible
for the long-term data archiving unless this task has been delegated to the Tracking
Stations. The original raw data files or compressed (e.g. zipped) raw data files are usuallyarchived.
The IGS Data Centers archive the RINEX files for the general benefit of the IGS.
Data Format
The data are to be prepared in daily (24 hours) RINEX files, both for observations
and broadcast navigation messages. This is the current standard. For emerging applications(e.g., atmospheric sensing, Low-Earth-Orbiters -- LEO’s) more frequent data collection
and preparation may be considered by the IGS.
The daily observation files contain the observations collected between 00:00:00 and
23:59:59 GPS time. The sampling rate (observation interval) must be the adopted standard,

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currently 30 sec. In case of a higher original observation rate a decimation of the data to theadopted standard is mandatory.
The header information, especially the station name, receiver and antenna
information, and antenna height, must be up-to-date and has to strictly follow the agreedupon conventions (see blank log forms for the stations). Consistency in both format andcontent is strongly encouraged.
The navigation message file contains all messages with TOC/TOE (time of clock,
time of ephemeris) between 00:00 and 23:59 GPS time of the respective day.
It is recommended to generate a combined daily RINEX navigation file containing
non-redundantly all navigation messages collected by all sites of a local network. The
filename (part "ssss", see below) should then contain a 4-character code of the Operational
Center.
The RINEX navigation files are prepared in a compressed form using the standard
UNIX compress program. The RINEX observation files are additionally compressed using
the Hatanaka compression scheme. Compress and decompress programs for otherplatforms (PC/DOS, VAX and Alpha VMS) are available at the IGS CBIS:
ftp://igscb.jpl.nasa.gov/igscb/software/compress (for the UNIX Compression)
ftp://igscb.jpl.nasa.gov/igscb/software/rnxcmp (for the Hatanaka Compression)
File TypeASCII File
Compressed Files
UNIX
UNIX
VMS
Observation
ssssdddf.yyO
ssssdddf.yyD.Z ssssdddf.yyD_Z
Navigation
ssssdddf.yyN
ssssdddf.yyN.Z ssssdddf.yyN_Z
Meteorologicalssssdddf.yyM
ssssdddf.yyM.Z ssssdddf.yyM_Z
ssss: 4-character station code
ddd : day of the year of the first record
f : file sequence number within the day
0: containing all data of the day
yy : yearDOS
ssssdddf.yyE
ssssdddf.yyN
ssssdddf.yyM

The extension yyD (or yyE in DOS) indicates Hatanaka-compressed files. Moreinformation about the Hatanaka compression can be found in
ftp://igscb.jpl.nasa.gov/igscb/software/rnxcmp/docs/README
Files sent to another host must be named on the target system in accordance withthe target operating system:
Example: Put a file from a UNIX to a VMS system:
binaryput zimm1230.94D.Z ZIMM1230.94D_Z
Data Validation
Data should be checked before being sent to a Data Center. A minimum verification
should consist of a check of•••

the total number of observations
the total number of observed satellites
the date of the first observation record in the file282

IGS Infrastructure Committee Report•

the station name, receiver/antenna types, antenna height

The use of a true quality check program is highly recommended, e.g. UNAVCO's
QC program (available through the IGS Central Bureau Information System, directory/igscb/software/qc).
Files which do not meet the minimum verification should not be sent to a Data
Center.
Data Handling
It is highly recommended that all steps in the data handling -- download from thereceiver, reformatting, data validations, generation of statistics, data archiving andtransmissions -- be fully automated. Data failing to pass the validation step should be kept
back for manual treatment and then posted.
Data Upload
The data must be sent daily to a designated IGS Data Center, via a Local Data
Center if appropriate.
The IGS Data Center to be used is assigned by the Central Bureau. Alternate andback-up routing will also be assigned in case of emergency and should be fully tested.
Timeliness of the Data Transfer
In order to allow for rapid analysis of the data and to avoid the times of heavy
network traffic on the international data lines the data should arrive at the Regional Data
Center not later than 2 hours after midnight Universal Time, to be forwarded to one of the
Global Data Centers not later than 03:00 UT.
Documentation
The IGS Central Bureau Information System makes available a blank form for an
Operational Center description file (blankform.ocn in /igscb/center/oper). This form should
be filled out by the Operational Center or by the agency operating an independent
permanent GPS station and sent to the Central Bureau.
B.1.4 Local Data Centers
GeneralDepending on the policy of the respective agencies, the tracking data and theauxiliary station information of the local network can be made available through computer
networks or bulletin boards etc. This task would be the responsibility of a Local Data
Center.
Stations not part of a local network send their data directly to the designated IGS
Data Center.

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Data Access
It is recommended to allow access to (all or part of) the data through Internet's
anonymous ftp, currently being the most effective and easy to use access method,especially for automated data download. Another access procedure on Internet is throughWorld Wide Web (WWW) servers with easy to use browsers such as Mosaic or Lynx.
Available Information
It is recommended to make available at least the following information:•

Station information (site log files, requested)•

Data Center Information (blank forms and examples can be found in the
IGS CBIS (ftp://igscb.jpl.nasa.gov/igscb/center/data).

•

Other network information: Data flow, Operational Center description,reference and access to other networks (EUREF, IGS, ...)•

Daily tracking data (RINEX files), including results of quality checks•

Data holding information (i.e. a machine-readable summary of the available
tracking data).
Examples can be foundin directory
ftp://igscb.jpl.nasa.gov/igscb/data/holding. A program to generate such adata holding file can be obtained through the IGS Central Bureau.

•

Status information about the network and GPS in general and crossreferences to other information systems

Sites for which there is incomplete, incorrect, or contradictory documentation willbe identified automatically at the Central Bureau. A notification will be made to users by the
CBIS. Users of data may use this information as they see fit. However, the expectation isfor these discrepancies to be eliminated.
Data Organization
It is recommended to organize at least the tracking data in a hierarchical directory
structure. From the user's point of view it is usually preferable to combine all the data of
one day (or one week) into one directory than to have station-dependent directories.
Examples of directory organizations can be found in the IGS Data Center description files(ftp://igscb.jpl.nasa.gov/igscb/center/data).
All the other information can be made available either through ASCII files in a
directory tree or through more advanced means like data bases of hypertext documents or,preferably, through both.
The Local Data Center description file and the daily updated data holding file should
be made available to the IGS Central Bureau Information System.

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Data Transfer to the Regional Data Center
The tracking data of stations that are also part of an upper level network (EUREF,
IGS) have to be forwarded through Internet to the respective Regional Data Center byanonymous ftp, or other reliable electronic means. It is recommended that a binary (image)"put" be used.
B . 1 . 5 Data Analysis•

Although the IGS Analysis Centers are free regarding which sites to process
they should not report results (such as coordinates and velocities) for sites
for which there are no correctly filled out log files available at the IGS
CBIS. In principle the corresponding RINEX data files should also be
available at least one IGS Regional Data Center. Data available only from
Local Data Centers should be used by special arrangement only.

•

The Analysis Centers are requested to use the primary site information(DOMES numbers, receiver and antenna names, antenna heights, antennaphase center information) provided in master files available at the CBIS.

•The Analysis Centers and Associate Analysis Centers are requested toensure that their SINEX files are consistent with the site information
provided in master files available at the CBIS.

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Continuous GPS Positioning of Tide Gauges: Some Preliminary Considerations

Continuous GPS Positioning of Tide Gauges:
Some Preliminary Considerations
Michael Bevis
School of Ocean and Earth Science and Technology
University of Hawaii
Honolulu, Hawaii 96822
USA

1

Introduction

The IGS is working with the oceanographic community to begin implementationof the long-considered step of adding continuous GPS (CGPS) stations to tens of tidegauges around the world so as to reference sea level at this set of points to the ITRF. A
technical committee formed under the auspices of the IGS and the PSMSL wasestablished at the 1997 JPL Workshop in order to write a set of standards for groups
constructing these CGPS stations. During its discussions the committee has come torealize that the oceanographic community is pursuing several different agendas, and thatthese efforts have distinct accuracy requirements and are impacted in different ways by
various geotechnical considerations and local environmental factors. There are two main
agendas:(i)

The ‘centimeter’ agenda. The idea is to establish the absolute vertical
position of the tide gauge and its benchmarks (and ultimately the local sea
level history) with an accuracy of 2 - 3 cm, for the purpose of calibrating
and/or validating modern satellite altimeter measurements (e.g. TOPEX).
The oceanographers engaged in this activity typically place little
importance on providing tectonic corrections for historical sea level dataobtained at their calibration stations.

(ii)

The ‘millimeter’ agenda. Tide gauge measurements reflect vertical motionsof both the land (to which the gauge is attached) and the sea surface. The
intra-annual, annual and even decadal fluctuations recorded by a tide gauge
are usually totally dominated by absolute sea level changes. But when
averaged over much longer periods of time (~100 years) the rates ofvertical motion of the land and the sea surface are often similar inmagnitude (~ 1 mm/yr). In order to obtain an absolute sea level history
from a long historical record of relative sea level change, it is necessary toestimate the rate of vertical motion of the land or structure supporting thetide gauge. The goal is to use CGPS to measure the vertical velocities oftide gauge stations with an accuracy of better than 1 mm/year within ten
years, and with considerably better velocity accuracy over longer periods
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IGS 1997 Technical Reportsof time. This more ambitious agenda was the main focus of the well-known
Carter reports.
The second agenda is much more challenging and will lead to quite severe
requirements for the selection of ‘suitable’ tide gauge sites, the CGPS installations anddata analysis. Tide gauges that can be instrumented with CGPS for the purposes of thecentimeter agenda may often be unsuitable for pursuing the millimeter agenda. As suchthe technical committee is now working on separate sets of recommendations for these
distinct ‘end case’ applications. A key realization is that these recommendations need tobe considered as tide gauges are being selected, and not just during the implementation
stage.
2

The Impact of Geodetic Requirements on Site Selection

Some requirements are common to any positioning agenda and would apply to
any tide gauge. For example, one should always ‘tie’, by precise leveling, the GPS
antenna to the system of tide gauge benchmarks (TGBMs) as well as to a reference pointon the tide gauge itself. This is especially important when one is working at a tide gauge
with a long history and this history is a major focus of the effort. This is because overmany decades the tide gauge is likely to have been modified, moved and even replaced on
several and perhaps many occasions. It is the TGBMs and the associated program ofprecise leveling measurements that provide the temporal continuity of the vertical datumat such stations. In this sense one could argue that when the focus is on a very long sea
level time series, the system of TGBMs is even more important than the specific tidegauge that is operating today. Nevertheless, the need to tie the GPS antenna to the
TGBMs as well as to the tide gauge is clear no matter what application we are pursuingor where we are working. Not all requirements are this straightforward, however.
The technical requirements for geodetic positioning of tide gauges need to beconsidered during the selection of gauges for GPS augmentation, and not justsubsequently. Only a small fraction of all available tide gauges will be retrofitted with a
CGPS station. It would be short-sighted to base the selection of these tide gauges purely
on oceanographic criteria, without regard to the relative geodetic suitability of thecandidate tide gauges. The ease with which a given tide gauge can be positioned
geodetically depends both on the positioning accuracy required and the local
environmental conditions. Consider the standard issue of sky view, for example. It is
unlikely that one could ever obtain subcentimeter vertical accuracy by making GPS
measurements at a tide gauge located at the base of a 200 meter cliff. This raises thequestion of how far the CGPS station can be removed from the tide gauge. In many cases
the existence of walls or buildings besides the tide gauge will require the GPS antenna to
be moved at least several meters from the gauge itself. This will rarely matter as long as
the GPS antenna mount is connected to the same structure as the gauge, making significant
relative motion of the tide gauge and GPS antenna impossible. But could it ever be
acceptable to install a CGPS station hundreds of meters or even kilometers away from the288

Continuous GPS Positioning of Tide Gauges: Some Preliminary Considerations

tide gauge in order to optimize conditions for the GPS measurements (e.g. an improved
sky view) or for other purposes such as security of the GPS equipment?
There are a number of factors that must be addressed before one can answer thisquestion. What positioning agenda are we pursuing? How often would the GPS antenna
be referenced to the tide gauge using precise leveling? What is the nature of the groundbeneath and between the tide gauge and the GPS antenna? I do not have space to discuss
here all the relevant issues in a comprehensive manner, but the following points may
illustrate the nature of the problems and trade-offs involved.
Some tide gauges are attached to seawalls or piers that were constructed on
sediments or, worse still, engineering fill. The well-known tide gauge at Valparaiso (Chile)is an example of the latter situation. These structures typically subside as the underlying
material compacts. Oceanographers have long been aware that tide gauges may be subject
to purely local vertical motions of this kind, and this is one of the reasons why they havereferenced their tide gauges to TGBMs. Some oceanographers have concluded that a
CGPS station serves a similar role to a TGBM, and so there is no particular need to place
the CGPS station very close to the tide gauge. However, a CGPS station is being
positioned in a global reference frame on a daily basis. If a somewhat remote CGPS
station is moving by an unknown amount relative to its tide gauge, then the absolute
position of the tide gauge is being determined only as frequently as the levelling tie isbeing made. This may be just once a year. This represents a massive dilution of thepositioning power of CGPS.
The negative impact of losing the temporal continuity of CGPS measurements isfurther illustrated by the following example. Some tide gauges are tied to large standing
structures such as piers that may be undergoing thermoelastic displacements withamplitudes of millimeters and sometimes even centimeters. Suppose one such tide gauge
is being tied by precise leveling to a CGPS station built on a rock outcrop 1 km away. If
the leveling survey happens roughly annually and always takes place around the sametime of year, even large annual thermoelastic motions of the gauge might never beobserved at all!
If a tide gauge and a non-colocated CGPS station are not constructed on a coherent
outcrop of solid rock, the possibility of subsurface displacements changing the relative
level of the tide gauge and the GPS antenna is present even when near-surface‘engineering’ instability of a pier or a harbor wall can be ruled out. An analysis of all
releveling data obtained in the USA prior to 1980 showed that most of the largest verticaldisplacements occurred in and near cities built on sediments such as those underlying thecoastal planes of the Atlantic and Gulf Coasts. Most of these signals manifest differential
subsidence associated with water withdrawal. Many tide gauges are based in this kind ofsetting.
One also needs to keep in mind that in many parts of the world precise levelingsurveys do not take place at reasonable intervals (except on paper). In this case there will
be no strong constraints on possible relative motions of the tide gauge and the GPS
antenna.

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If one is pursuing the centimeter agenda it will usually be acceptable to offset the
GPS station from the tide gauge by distances as great as 1 km, provided that frequent
first-order leveling ties are performed. In some settings the leveling ties should be
performed several times during the first year to ensure that annual cycles of deformation
affecting any large structures supporting the gauge (or the GPS antenna) are not going
undetected or being misunderstood due to aliasing. Additionally these surveys should be
performed throughout the lifetime of the observation program to ensure that slower (butnot necessarily steady) secular movements will be adequately characterized. The larger
the separation of the GPS station and the tide gauge, the more difficult and expensive theleveling program will be, and the more likely it will not be executed properly throughout
the lifetime of the program. The inconvenience of frequent leveling should becontemplated very carefully before one abandons the idea of putting the GPS station very
close to the tide gauge.
It will rarely be acceptable to separate the tide gauge and the GPS antenna by
more than a few meters when one is pursuing the millimeter agenda. The only exception
to this would be when both the tide gauge and the GPS station were being installed in asingle massive outcrop of competent and locally rigid rock. Under these circumstances
one could separate the tide gauge and the GPS antenna by a few tens of meters.
Suppose the oceanographic agenda in a given region could be satisfied by adding a
CGPS station either to tide gauge A or tide gauge B. Suppose gauge A is built on a solidrock outcrop in an area with minimal human impact, and it will be possible to build amonument that couples the GPS antenna to the rock substrate just a few meters from thetide gauge. Suppose that tide gauge B is located on a harbor wall at the base of a large
building. The nearest place with a good sky view is over a kilometer away and this place,the tide gauge, and the road between them overlie a sedimentary basin, which includes aheavily pumped aquifer. It would be unreasonable to select B over A even if the locationof B made it slightly preferable for purely oceanographic reasons.
The selection of tide gauges for GPS augmentation must involve some sort ofcompromise between oceanographic and geodetic requirements. The higher the positioning
accuracy required, the more weight must be placed on the environmental and geodetic
suitability of the site. We must be very careful when we determine whether we are
pursuing the centimeter or the millimeter agenda at a given site. If the geodesists hear one
or two oceanographers say that their primary interest is the centimeter agenda then it isnaturally tempting to settle on that goal, since it makes it much more likely that the
proposed site can be considered suitable, and because the standards for installation will be
more lax and therefore easier to achieve. However, in many cases, stations built to thisstandard will not be suitable for pursuing the millimeter agenda. So if the oceanographerschange their minds about the precision levels they seek to attain, then much of early workon CGPS positioning of tide gauges will subsequently be viewed as substandard. This
prospect is particularly worrying given the history of upwards creeping accuracy
requirements in almost every application of GPS geodesy.

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3

The Writing of Standards for CGPS Installations at Tide Gauges

The above discussion should make clear that the writing of standards for CGPS
installations at tide gauges is more involved than most members of our technicalcommittee first realized. Nevertheless we are working on a document which is based bothon multidisciplinary discussions, and on experience gained during past installations.
The University of Hawaii is one of several groups working in this area. Our effort
is a collaboration between the Pacific GPS Facility led by the author, and by the UH Sea
Level Center led by Mark Merrifield, another member of the technical committee. Our
first project, begun last year, was the installation of a CGPS station at the Honolulu Tide
Gauge. This dataset is freely available on a daily basis. We are presently engaged inselecting another six tide gauges for GPS augmentation. Early site selection studies
indicate that almost every station will be a special case! We will be documenting eachproject as it occurs, and will make this information available on the web. We hope that
other groups will be doing this too. Hopefully we will be able to refer to these websites in
the IGS/PSMSL standards document, so that specific cases can be made to serve as
illustrations of the general requirements associated with CGPS positioning of tide gauges.
4

Data Processing and the Role of the IGS

By the beginning of the year 2000 we expect that 15 - 25 tide gauges around theworld will have been retrofitted with CGPS stations. These data will be made available tothe IGS via one or more data centers. The IGS needs to consider now what mechanisms
need to be established to ensure that these data are processed in an optimum manner. We
must keep in mind that, initially at least, most of these stations will not be reporting
within a few days of real-time. Thus the processing streams associated with IGS orbit
production are not relevant to this agenda. Additionally, because many tide gauges arebuilt on relatively unstable structures, these new CGPS stations may not be of muchinterest to geodetic groups already engaged in ITRF densification efforts if their primary
motivation is crustal motion research. Who is going to be responsible for this newprocessing effort? And who is going to pay for it?
5

Getting Involved

Anyone wishing to join the IGS/PSMSL technical committee on CGPS
positioning of tide gauges, and/or participate in our email discussions, should send an
email message to bevis@soest.hawaii.edu.

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