**IGLOS: The International GLONASS Service**

Werner Gurtner

Astronomical Institute

University of Berne

Sidlerstrasse 5

CH-3012 Bern

**Summary**

The International GLONASS Service as a Pilot Project of the
International GPS Service IGS is a follow-on project of the successful
first global GLONASS campaign IGEX-98. Its major purpose is the full
introduction of the GLONASS satellite system into the operation of IGS
by a step-wise approach. This paper reviews some of the requirements
needed and gives recommendations especially with respect to the data
organization and data flow.

**1. The International GLONASS Experiment IGEX-98**

Both the navigation and scientific communities have an interest in
exploiting GLONASS, primarily as an augmentation to GPS. Thus, four
organizations jointly decided to organize a first global GLONASS
tracking campaign IGEX-98:

-   the International GPS Service (IGS)

-   the Institute of Navigation (ION) through its GLONASS-GPS
    Interoperability Working Group

-   the International Association of Geodesy\'s Commission VIII
    (International Coordination of Space Techniques for Geodesy and
    Geodynamics, CSTG)

-   the International Earth Rotation Service (IERS).

through an eight persons Steering Committee under the chairmanship of
Pascal Willis (IGN, France).

The objectives of the experiment were:

-   Collection of a globally-distributed GLONASS data set over a long
    time period, primarily using dual-frequency GLONASS receivers
    collocated with GPS stations

-   Precise orbit determination at the meter level or better

-   Receiver evaluation

-   Software development

-   Reference frame comparisons between the GLONASS PZ-90 system, the
    GPS WGS 84 system, and the International Terrestrial Reference Frame
    (ITRF)

-   Time and time transfer applications

-   Stimulus for other scientific applications

In addition the International Laser Ranging Service ILRS was asked to
track nine GLONASS satellites to allow for an independent possibility to
assess the quality of the generated orbits and to study the level of
agreement between the two different satellite observation techniques.

The International GLONASS Experiment (IGEX-98) was conducted from
October 19, 1998 to April 19, 1999.

Using the successful model of the IGS, an organizational structure was
put in place that included data centers to store and distribute data and
products, and analysis centers to compute precise satellite orbits and
other products. A Data Center Coordinator, an Analysis Center
Coordinator and a Network Coordinator oversaw the operation. The
information flow between all interested parties was ensured by the
e-mail exploder IGEXMAIL at IGN, France, similar to the well-known
IGSMAIL.

Guidelines were issued for receiver installation, data communications
and data formats. A data flow scheme was designed so that tracking data
would be forwarded to local, regional and global data centers. An
observation sampling rate of 30 seconds was selected and RINEX was
designated as the standard data format. This required some small
modifications to the RINEX standard. The tracking protocol called for
continuous (24 hours/day, 7 days/week) tracking, and data transmission
once per day from each station. Conventions were also established for
station, receiver and antenna identification, and station documentation.

Care was taken to avoid undesired interference with the routine IGS
operation by keeping the data and information flow as well as the
processing and the distribution of the results separate from the IGS
channels.

More than 60 stations took part in the campaign. Dual-frequency
GPS/GLONASS receivers by Ashtech, Javad, 3SNavigation, ESA/ISN and a
couple of single-frequency receivers submitted data to the IGEX data
centers.

A major accomplishment of IGEX-98 was the production of precise orbits
for all of the operational GLONASS satellites for every day of the
campaign. Eleven organizations generated orbits using a variety of
software. Six of these Analysis Centers were able to generate orbits on
a more or less routine basis, one of which based its computations on SLR
observations alone. Comparisons and tests confirmed an orbit accuracy of
the order of one decimeter.

After the end of the campaign in April 1999 all the contributors to the
IGEX-98 campaign were asked to continue with their activities on a best
effort basis until decisions were taken about the future of the use of
GLONASS for high-precision geodetic applications. Not all of the
original contributors could continue. Currently (July 2000 about 30
stations still submit mixed data, two third of which are dual-frequency
receivers on both systems.

GPS Week 1067

(June 18 through June 24, 2000; Days 00170 through 00176)

Mon. No.

Site Name Name Receiver Agency 170 171 172 173 174 175 176 Days

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-- \-\-\-- \-\-\-\-\-\-\-\-\-\-\-\-\-\-\--
\-\-\-\-\-\-\-- \-\-- \-\-- \-\-- \-\-- \-\-- \-\-- \-\-- \-\-\--

Borowiec BORG 3S Navigation PAS X X X X X X X 7

Brisbane SUNM JPS Legacy GGD DNR X X X X - X X 6

Davis DAVR Ashtech Z18 AUSLIG X X X X X X X 7

Delft DLFT JPS LEGACY GGD DUT X X X X X X X 7

Delft VSLD 3S Navigation R1 VSL X X X X X X X 7

Graz GRAB Ashtech Z18 ISR X X X X X X X 7

Greenbelt GODZ Ashtech Z18 NASA - X X X X X X 6

Herstmonceux HERP 3S Navigation RGO X X X X X X X 7

Kiruna KR0G Ashtech Z18 NLS X X X X X X X 7

Maartsbo MR6G Ashtech GG24C NLS X X X X X X X 7

Mattersburg MTBG JPS LEGACY TUVienna X X X X X X X 7

Mcmurdo CRAR JPS Legacy USGS X X X X X X X 7

Mitaka MTKA Ashtech Z18 ENRI X X X X X X X 7

Mt. Stromlo STR2 JPS LEGACY AUSLIG X X X X X X X 7

Neustrelitz NTZ1 3S Navigation DLR X X X X X X X 7

Ondrejov GOPE Ashtech Z18 GOP X X X X X X X 7

Onsala OS0G Ashtech Z18 OSO X X X X X X - 6

Pretoria CSIR 3S Navigation CSIR X X X X X X X 7

Reykjavik REYZ Ashtech Z18 BKG X X X X X X X 7

Sydney (Lindfiel LINR 3S Navigation NML - X X X X X X 6

Teddington NPLC 3S Navigation R1 NPL X X - X X X X 6

Thule THU2 Ashtech Z18 KMS X X X X X X X 7

Visby VS0G Ashtech GG24C NLS X X X X X X - 6

Washington, DC USNX 3S Navigation USNO X X X X X X X 7

Wettzell WTZZ Ashtech Z18 BKG X X X X X X X 7

Wroclaw WROC Ashtech Z-XIIV POLAND - X X X X X X 6

Yarragadee YARR Ashtech Z18 AUSLIG X X X X X X X 7

Zimmerwald ZIMJ JPS LEGACY GGD AIUB - - X X X X X 5

Zimmerwald ZIMZ Ashtech Z18 AIUB X X X X X X X 7

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-- \-\-\-- \-\-\-\-\-\-\-\-\-\-\-\-\-\-\--
\-\-\-\-\-\-\-- \-\-- \-\-- \-\-- \-\-- \-\-- \-\-- \-\-- \-\-\--

Totals: 29 stations 25 28 28 29 28 29 27 194

Table 1: Data holding at CDDIS

A workshop was held on September 13-14, 1999 in Nashville, Tennessee
(U.S.A.) for participants to discuss their experiences and present the
results of their work.

Topics of discussion were:

-   Receiver clock differences between GLONASS and GPS data

-   Phase center variations of the antennas

-   Solar radiation pressure modeling

-   Orbit accuracies, verification procedures

-   Influence of GLONASS observations on the accuracy of coordinates and
    Earth rotation parameters

-   Reference frame comparisons (ITRF - PZ-90) and the stability of the
    frames

-   Systematic differences between SLR and microwave orbits

-   Time transfer using GLONASS satellites and precise orbits

-   Integrity monitoring of the satellites

The workshop decided in resolutions to

-   continue global, internationally coordinated GLONASS tracking and
    orbit determination in the time interval 1999-2003

-   ask CSTG and IGS to continue to collaborate in an International
    GLONASS \"Pilot Service\" and to organize it as an IGS Pilot Project

-   form a new Project Committee to prepare a charter for the \"Pilot
    Service\", prepare and send out a new Call For Participation, and to
    announce the creation of the \"Pilot Service\"

**2. The International GLONASS Service Pilot Project (IGLOS-PP)**

At the December 1999 IGS Governing Board meeting the proposal was voted
on and given provisional approval.

The International GLONASS Pilot Service will have the following goals
and objectives:

-   Establish and maintain a global GLONASS tracking network by
    collocation of dual-frequency, combined GPS/GLONASS receivers with
    GPS receivers at IGS sites, upgrading of existing IGS GPS receivers
    to dual-frequency, combined GPS/GLONASS receivers or introduce such
    receivers at new sites

-   Apply International GPS Service (IGS) network operations standards

-   Produce precise (10-cm level) orbits, satellite clock estimates, and
    station coordinates incorporating SLR observations

-   Monitor and assess GLONASS system performance

-   Investigate the use of GLONASS to improve Earth Orientation
    Parameters

-   Improve atmospheric products of the IGS

-   Finally: Fully integrate GLONASS into IGS products, operations and
    programs.

On May 25, 2000 the Call for Participation was distributed through
IGSMail and other electronic mailing systems with a deadline for answers
set to be July 7, 2000. The CfP is available at the IGS web site.

**3. Open Issues**

Below I try to discuss some of the open issues to be solved for a
successful start of the Pilot Project:

**3.1 Receiver Performance**

The Call for Participation explicitely asks for dual-frequency /
dual-system receivers, but it allows for receivers of less performance
for special purposes like time transfer (e.g., 3S Navigation receivers).

How shall the stations and the data of such receivers of reduced
performance be handled? Should the log and data files be kept in
separate directories? Are such stations be accepted as \"IGS stations\"?
We propose to keep such stations separate from the others in different
directories or even in special-purpose data centers.

**3.2 Antenna Phase Centers**

A special concern is the behavior of the mixed GPS/GLONASS antennas with
respect to phase center variations. Currently no .pcv files exist for
these antennas. The determination of the phase centers and their
elevation-dependent variations need to be done and/or officially
accepted for the dual-system antennas in use.

**3.3 Receiver and Antenna Naming**

IGEX-98 and the follow-on period used a naming file for receivers and
antennas defined before the beginning of the campaign in the fall of
1998. In the meantime IGS redesigned and redefined the naming
conventions. IGLOS will have to switch to the new official IGS Table.

**3.4 Station Log Files**

IGEX-98 and the follow-on period used a log file template which was
based on the IGS template but modified to account for the dual-system
(GPS/GLONASS) nature of the project.

IGS will have to generate a new, generalized template including
GLONASS-related items or, better still, to modify the template for
general GNSS tracking stations.

**3.5 RINEX Observation Files**

We propose to include the mixed GPS/GLONASS observation files of
dual-frequency, dual-system stations into the official IGS data
directories for observation data at the data centers with the start of
the IGLOS Pilot Project. Those Analysis Centers or other customers not
(yet) processing GLONASS data will just have to ignore the mixed files.
As soon as such groups have modified their procedures to deal with mixed
files (either by processing or filtering the GLONASS observations) they
can start to include those files at their discretion.

**3.6 RINEX Navigation Message Files for GLONASS**

The inclusion of the GLONASS navigation message files into the data
center directories is not problematic as they differ from the GPS
navigation message files by their filename extension. Most data centers
have special directories for the navigation message files. There we
recommend to define parallel directories for the GLONASS files.

**3.7 Precise Orbit Files**

For the IGEX-98 campaign a new version of the SP3 format was developed
to have the possibility to store both the GPS and GLONASS satellite
positions into one SP3 file. Modifications concerned the satellite
identifier (\"Gnn\" for GPS, \"Rnn\" for GLONASS, \"nn\" being the PRN
or slot number, respectively), the time system (GPS or UTC), the file
type (\"G\"=GPS, \"R\"=GLONASS, \"M\"=mixed), and a modified format
version identifier (changed from \"a\" to \"b\").

We propose IGS to accept this new \"b\"-version for mixed files and to
allow, after a suitable delay, the same version to be used for pure GPS
orbit files, too.

We propose Analysis Centers who process both GPS and GLONAS data to
generate, at least for a limited time, both GPS-only and mixed
GPS/GLONASS precise orbit files and to distinguish the mixed files from
the pure GPS files through the three-digit center code by replacing the
third character by a \"x\".

**3.8 SINEX Files**

The SINEX format definition uses the IERS indicators to distinguish the
individual stations/receivers/antennas among the various space
techniques (microwave, SLR, VLBI) . We propose the Analysis community
and IERS to check if additional indicators should be defined to show
GLONASS-only or mixed GPS/GLONASS stations, or if a distinction should
be done by other means or not at all.

**3.9 IGLOSMAIL vs. IGSMAIL**

Analysis Centers and other customers should be made aware of IGLOSMAIL,
which is the successor of the discontinued IGEXMAIL.

Messages related to mixed GPS/GLONASS stations (announcement of new
stations, changes, problems, etc) might only be distributed through
IGLOSMAIL. However, we propose to discontinue IGLOSMAIL as soon as the
full integration of the mixed stations into the IGS network will be
realized.