**Prospective Development of the International VLBI Service for Geodesy
and Astrometry over the next few years (**Status: July, 2002)

Wolfgang Schlüter[^1], Nancy Vandenberg[^2],

for the IVS Directing Board

1.  **General Remarks**

The International VLBI Service for Geodesy and Astrometry (IVS) is a
Service of the International Association of Geodesy (IAG), International
Astronomical Union (IAU) and of the Federation of Astronomical and
Geophysical Data Analysis Services (FAGS). The charter and the basis for
international collaboration is given by the Terms of Reference (ToR)
accepted by IAG and IAU and by the proposals provided by individual
agencies in response to the call for participation.

IVS is an international collaboration of organizations that operate or
support Very Long Baseline Interferometry (VLBI) components. The goals
are

-   to provide a service to support geodetic, geophysical and
    astrometric research and operational activities,

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-   to promote research and development activities in all aspects of the
    geodetic and astrometric VLBI technique,

-   to interact with the community of users of VLBI products and to
    integrate VLBI into a global Earth observing system.

The IVS inauguration date was March 1, 1999. As IVS has no funds of its
own, but is tasked by IAG and IAU for the provision of timely, highly
accurate products (Earth Orientation Parameters (EOPs), Terrestrial
Reference Frame (TRF), Celestial Reference Frame (CRF), etc.), IVS is
dependent on the support of individual agencies. On the other hand, as
IVS is dependent on the support of its collaborating institutions, the
final responsibility for success is reflected back to these
institutions.

**2. Activities, results and improvements since March 1999**

In order to maintain the strong requirement for consistency, which is
the basis for realizing and maintaining global reference frames such as
the CRF and TRF, IVS initially employed and accepted existing
infrastructure, observing programs, and related data handling. During
its first two years of existence, the efforts of IVS were concentrated
on the installation of new components and adoption of new IVS tasks.
Coordination of activities within the service took effort, resources and
time to mature.

All the activities of the first years are documented in the Annual
Reports of the IVS for the years 1999, 2000 and 2001 \[1\], \[2\],
\[3\]. The first General Meeting was held in Kötzting/Germany in
February 2000, the second General Meeting was held in Tsukuba/Japan in
February 2002 and several technical meetings concerning analysis and
technology aspects were conducted. Proceedings of the General Meetings
are available \[4\],\[5\].

Under the leadership of the IVS Technology Coordinator the VLBI Standard
Interface (VSI) was developed and the experts internationally agreed
upon the specifications for the hardware interface. Now the work for the
software specifications is proceeding with similar promise of success.
The VSI will allow the combination of different recording techniques
used in VLBI e.g. the Mk4/Mk5 developed in the USA, the S2 developed in
Canada or the K4 developed in Japan. Currently Mk5 is under development.
Hard disks will replace tapes, which will save maintenance cost on the
tape drives and reduce shipping costs. Mk5 will also enable development
of e-VLBI -- near real time VLBI by sending data via internet and
support remote control of observing stations.

Emphasis was placed on data analysis, coordinated by the Analysis
Coordinator. Today six analysis centers provide a timely, reliable,
continuous solution for the entire set of five Earth Orientation
Parameters (EOPs) - two pole motion coordinates, UT1, two celestial pole
coordinates. The IVS Analysis Coordinator makes a combined solution --

the official IVS product - as timely input for the IERS and its
combination with the GPS, SLR/LLR and DORIS solutions. It turns out that
the IVS combined solution gains 20% in accuracy over the single VLBI
solutions. To improve the products of the IVS, more analysis centers
were motivated to contribute and are invited to participate in Pilot
Projects led by the Analysis Coordinator. Their solutions were compared,
discussed and improved as all analysis centers reach for high standards.

Improving the quality of the observations and the network is a major
goal of the Network Coordinator. Investigations and statistics are
employed to find weak points. More improvements could be achieved, some
needed improvements are difficult to realize because of the large amount
of funds required for maintenance and upgrades. On the international
level some additional stations support or will soon support IVS, such as
the new station TIGO, which is in operation in Concepcion/Chile since
April 2002. More stations will become involved which are equipped with
data acquisition, recording, or transmission systems developed by Canada
(S2) and Japan (K4) or which will be equipped with Mk4/Mk5 through
collaboration of supporting institutes e.g.Hobart/Tasmania.

### 3. Scientific rationale

For research and applications in the fields of geodesy, geokinematics,
geodynamics and astronomy, as well as in space research and space
application programs, highly reliable reference frames are a strong
requirement. Both the CRF and the TRF are demanded, and the EOPs that
are the connecting elements between the systems must be derived from
observations. In order to make full use of all the results obtained in
related fields and to compare results from the past with results to be
obtained in the future, the consistency of the reference frames is not
only of great importance, it is the strongest requirement. Descriptions
of "global change" as, for example, the result of mass transport or
circulation in the atmosphere, oceans and lithosphere, or as the
interaction of the masses of the planets, presume that the reference
systems must be realized at the state of the art. Existing space
missions such as CHAMP, and upcoming missions such as GRACE and GOCE,
will provide results that describe variations in the gravity field with
extremely high time resolution. The high precision of new altimeter
missions will result in very precise information on profiles of the
Earth. Not only is the consistency of the reference frames over decades
of time a demand but also the ability to describe geometric and
geophysical phenomena in one common reference frame will be necessary to
generate comparable results. The accuracy of information will be on the
level of 10^-8^ to 10^-9^, which at least requires a uniform reference
frame precise to 10^-9^.

A reference frame at the level of 10^-9^ cannot be achieved by only one
technique. The limitation comes from observing errors, modeling errors
and unknown systematic effects. A demand by IAG is the combination of
results provided by various techniques so as to benefit from the
different sensitivities. While satellite techniques must include the
orbit integration, and employ a gravity field, VLBI directly connects
the inertial reference frame, EOPs and TRF, thus eliminating one major
error source. It has to be emphasized that VLBI play the fundamental
role in maintaining global reference frames in particular the CRF.

Combination means that all techniques provide independent comparable
time series, which can be combined using SINEX or, preferably, on the
observation level. Combination does not mean to derive the required
products by one single technique (e.g. GPS) and, if required, along with
a few selected determinations of corrections from a superior technique
(VLBI). This will not help to eliminate and understand systematic errors
and will not support the science driven by the best time series
available.

IAG has promoted the establishment of the services in order to obtain
such products as time series, which finally are combinable by IERS. In
the sense of IAG, the services (IVS, ILRS and IGS) must generate timely
products so that a joint automatic procedure can be established to
generate the final results. Investigations are required to understand
discrepancies and to improve analysis procedures. Such a bootstrap
technique, successfully used in the past, will help obtain a better
understanding of the "changing Earth" and improve models for describing
the phenomena. There is no doubt that time series obtained during the
past 100 years, at the time generated on the level of state of the art,
have been, and still are, of fundamental interest for research and for
proving the models in related fields. As such the services are the basis
for research and will drive most of the research by themselves. In its
new structure IAG acknowledges the services as the basis for all its
research activities by giving the services a high rank, on the same
level as the Commissions. IGGOS (Integrated Global Geodetic Observing
System), a candidate IAG project, points out the strong requirement for
the services and the demand for the reference frame on the level of
10^-9^.

IVS is recognized as a service of IAU, as the VLBI technique is the only
technique that uniquely realizes the inertial reference frame, the EOPs,
and the TRF, which are urgently required for astronomical research and
for all missions in space.

It has to be emphasized that IVS is working hard towards developing the
procedures to provide their products in the required form. Further steps
in organization and development are needed.

4\. Review of products and observing programs

At the 4^th^ IVS Directing Board meeting held in September 2000 in
Paris, the requirement for reviewing the products and the related
observing programs was discussed with the view that IVS must meet its
service requirements and improve its products. Because such a review
requires overall expertise, a broad discussion and acceptance within the
entire community, a Working Group (WG2) for Product Specification and
Observing Programs was established at the 5^th^ Directing Board Meeting
in February 2001. (The Minutes of all meetings are published and made
available on the IVS web site.) The assignment of WG2 was to

-   review the usefulness and appropriateness of the current definition
    of IVS products and suggest modifications,

-   recommend guidelines for accuracy, timeliness, and redundancy of
    products,

-   review the quality and appropriateness of existing observing
    programs with respect to the desired products,

-   suggest a realistic set of observing programs which should result in
    achieving the desired products, taking into account existing agency
    programs,

-   set goals for improvements in IVS products and suggest how these may
    possibly be achieved in the future,

-   present a written report to the IVS Directing Board .

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-   

To establish a broad basis for discussion and to secure acceptance in
the community, the members were chosen from among experts in the field
of geodetic/astrometric VLBI. Led by Professor Harald Schuh from
Technical University of Vienna as chair, the following experts are the
members of the Working Group:

-   Patrick Charlot, Observatoire Bordeaux/France

-   Hayo Hase , Bundesamt für Kartographie und Geodäsie,
    Concepcion/Chile,

-   Ed Himwich, NVI Inc/Goddard Space Flight Center, Greenbelt/USA,

-   Kerry Kingham, US Naval Observatory, Washington D.C. /USA,

-   Calvin Klatt, Geodetic Survey Division of Natural Resources Canada;
    Ottawa/Canda,

-   Chopo Ma, Goddard Space Flight Center, Greenbelt/USA,

-   Zinovy Malkin, Institut of Applied Astronomy, St. Petersburg/Russia,

-   Arthur Niell, MIT Haystack Observatory, Westford-Haystack/USA,

-   Axel Nothnagel, Geodatisches Institut, Uni-versität Bonn /Germany,

-   Wolfgang Schlüter, Bundesamt für Karto-graphie und Geodäsie,
    Wettzell/Germany,

-   Kazuhiro Takashima, Geographical Survey Institute, Tsukuba/Japan,

-   Nancy Vandenberg , NVI Inc. Goddard Space Flight Center,
    Greenbelt/USA

A report of the WG2 was presented in November 2001. The IVS Directing
Board reviewed the final version and accepted it for publication ,
available at <http://ivscc.gsfc.nasa.gov/WG/wg2> or in the Annual Report
2001 \[6\]. The report is the basis for continuous improvements and for
related research within IVS over the next few years. The results of the
report will help IVS meet the objectives and future requirements set up
by the IAG and IAU for research in the geosciences and astronomy. The
following sections are based on the WG2 report..

5\. Summary of current products and prospective improvements in the next
few years

IVS is required to deliver products according to its ToR. Some products
are uniquely provided by VLBI: UT1, CRF, and celestial pole; other
products are available from more than one technique: Polar Motion, EOP,
TRF, and certain geodynamical and physical parameters. The IVS products
can be defined in terms of their accuracy, reliability, frequency of
observing sessions, temporal resolution of the estimated parameters,
time delay from observing to final product, and frequency of solutions.
The current situation with IVS products is described in detailed tables
in the WG2 report. In summary, the products and their current accuracies
are:

EOP UT1 24 hr sessions UT1: 5 s

1 hr sessions UT1: 15 s

EOP pole 24-hr sessions Xp:200as

Yp:100as

TRF single solution position: 5 to 20 mm

global solutions positions: 1 to 4 mm

vel..: 0.1 to 1 mm/yr

CRF source positions 0.25 mas

Celestial pole (precession/nutation) 0.1 \... 0.4 mas

As of late 2001, IVS products are generated from \~3 days/week observing
with 6-station networks. The time delay ranges from 10 days to 6 months,
with an overall average value of 60 days. Over the next four years, the
goals of IVS with respect to its products are the following (specific
goals for each product are listed in the WG2 report tables):

-   improve the accuracies of all EOP and TRF products by a factor of 2
    to 4 and improve the sky distribution of the CRF,

-   decrease the average time delay from 60 to 30 days, and designate 2
    days per week as rapid turnaround sessions with a maximum delay of
    3-4 days, starting 2002,

-   increase the frequency of observing sessions from 3 to \~7 days per
    week,

-   deliver all products on a regular, timely schedule.

It is certainly feasible to achieve these challenging goals for IVS
products, if the proposed observing programs are carried out and if
required improvements are realized.

### 6. Evolving observing programs 

To meet its product goals IVS has designed an observing program which
was coordinated with the international community beginning with the 2002
observing year. The observing program includes the following sessions:

-   EOP: Two rapid turnaround sessions each week, initially with 6
    stations, increasing to 8 when station and recording media resources
    are available in later years. These networks will be designed to
    have comparable Xp and Yp results. One-baseline 1-hr INTENSIVE
    sessions four times per week, with at least one parallel session.

-   TRF: Monthly TRF sessions with 8 stations including a core network
    of 4 to 5 stations and using all other stations three to four times
    per year. The number of stations may be increased if the correlator
    can support the increase data load.

-   CRF: Bi-monthly RDV sessions using the VLBA and 10 geodetic
    stations, plus quarterly astrometric sessions to observe mostly
    southern sky sources.

-   Monthly R&D sessions to investigate instrumental effects, research
    the network offset problem, and study ways for technique and product
    improvement.

-   Annual, or semi-annual if resources are available, 14-day continuous
    sessions to demonstrate the best results that VLBI can offer, aiming
    for the highest sustained accuracy.

Although certain sessions have primary goals, such as CRF, all sessions
are scheduled so that they contribute to all geodetic and astrometric
products. Sessions in the observing program that are recorded and
correlated using S2 or K4 technology will have the same accuracy and
timeliness goals as those using Mk4 or Mk5.

To support the IVS Coordinating Center in the coordination of the
observing program with the international community and to realize the
new program an Observing Program Committee has been established.

The observing programs and product delivery can only be accomplished by
making some changes and improvements in IVS observing program resources
(station days, correlator time, and magnetic media), by improving and
strengthening analysis procedures, and by pursuing a vigorous technology
development program.

### 7. Requirements to IVS components

The WG2 report contains many recommendations for different aspects of
IVS, its products, and its programs. What follows is a summary of the
most important ideas.

[Program resources]{.underline}: The number of required station
observing days increases by about 30% over the next two years and by
2005 the top dozen geodetic stations will need to be observing up to 4
days per week. Increased station reliability and unattended operations
can improve temporal coverage by VLBI and also allow substantial savings
in operating costs. Higher data rate sessions can yield more accurate
results, and therefore all geodetic stations must be upgraded to Mk4 or
Mk5 capability as soon as possible. More stations need to be equipped
with S2 and K4 systems so that global geodetic networks can be designed
using these systems. The present level of support at the three Mk4
correlators must be sustained to meet the IVS product goals, and support
is needed from the S2 and K4 correlators. The efficiency of the
correlators needs to approach a processing factor of unity, i.e. one day
processing for one day observing. All correlators must commit to
handling the IVS data with priority processing for meeting timely
product delivery requirements. Additional recording media capacity,
equivalent to \~100 Mk4 tapes, will be needed to support the higher data
rate observing that is necessary for increased accuracy. Alternatively,
additional media capacity can be realized by using rapid shipping modes
to shorten the tape cycle time.

[Analysis]{.underline}: More analysis centers and those using different
software packages should participate in the analysis that is required
for robust IVS products. The increased amount of VLBI data to be
produced under the new observing program will require analysis centers
to handle a larger load. Partially automated analysis procedures will
help improve the timeliness of product delivery. New IVS products such
as EOP rates, a combined TRF solution, tropospheric parameters and
geodynamical parameters should be developed because they can contribute
to scientific investigations.

[Technology upgrades and improvements]{.underline}: The Mk5 system under
development should be deployed as soon as feasible because not only will
it enable higher data rate recording but it will also improve station
and correlator reliability and efficiency. In addition, new methods for
data transmission, including electronic media, should be strongly
pursued because higher data rates, automated observing and processing
methods will lead to increased accuracy, reliability, timeliness, and
efficient use of resources.

**8. Summary**

IVS has the capacity to meet the requirements set up by IAG and IAU in
the realization of the reference frames and related products. In
general, precise time series of the products, with sufficient accuracy
(bias free), density, and timeliness must be generated. IVS as a
Technique Center of IERS must guarantee the realization of precise
celestial and terrestrial reference frames that are consistent over
decades.

To meet this guarantee, improvements are required in the availability
and reliability of the network stations. Automation for unattended
observing will help to overcome the weekend gaps. More capacity is
required in data transmission media, which will be solved by the
development of a modern disk based recording system (Mk5) and by the
ability to transfer data via the Internet (e-VLBI). These new systems
will reduce the time delay and dramatically reduce expenses currently
needed for tapes and tape drives. The global network configuration has
to be improved, especially in the southern hemisphere, and more
observing time is required. Encouraging additional related institutions
and including the S2 and K4 technologies will also improve the
situation. High priority has to be placed on rapid turnaround sessions
at the correlator. To avoid backlogs the throughput at the correlators
has to be improved. More analysis centers with different software are
required to improve the analysis and to increase the robustness of the
products.

IVS's primary duty is to provide the best possible results through
optimized and efficient coordination of all the resources available. The
new product specifications and the new related observing programs should
give the basis for cooperation and for contributions by collaborating
institutions. Nevertheless, the current IVS situation is highly
dependent on only a few institutions and requires the strong, continued
support and contributions of those key players.

**9. Resolution**

At the 2^nd^ IVS General Meeting held in Tsukuba/Japan in February 2002
the following resolution has been released:

# Tsukuba, February 6, 2002

## The International VLBI Service for Geodesy and Astrometry

recognizing

1)  that geodetic and astrometric VLBI is fundamental for the
    establishment and maintenance of the ICRF and contributes
    extensively to the generation of the ITRF, and

2)  that geodetic VLBI plays an essential role in geodesy and astrometry
    due to its uniqueness in observing the complete set of Earth
    orientation parameters (EOPs) which describes the transformation
    between the ICRF and ITRF stable over a time span longer than a few
    days, and

3)  that providing the reference frames and EOPs consistent over decades
    on the highest accuracy level will be a challenging role for IVS,

noting

the final report of the IVS Working Group 2 for Product Specifications
and Observing Programs,

recommends

that all IVS components should concentrate their effort and resources to
accomplish the following objectives:

-   signifcant improvements of the accuracy of VLBI products

-   shorter time delay from observation to availability of results

-   continuous temporal coverage by VLBI sessions.

### 10. References

\[1\] Vandenberg, N.R. (editor): Annual Report 1999,
NASA/TP-1999-209243, Greenbelt, MD, August 1999

\[2\] Vandenberg, N. R., Baver K.D.(editors): Annual Report 2000,
NASA/TP-2001-209979, Greenbelt, MD; February 2001

\[3\] Vandenberg, N. R., Baver K.D.(editors): Annual Report 2001,
NASA/TP-2002-210001, Greenbelt, MD; February 2002

\[4\] Vandenberg, N. R., Baver K.D.(editors): 2000 General Meeting
Proceedings, NASA/CP-2000-209893, Greenbelt, MD; June 2000

\[5\] Vandenberg, N. R., Baver K.D.(editors): 2002 General Meeting
Proceedings, NASA/CP-2002-210002, Greenbelt, MD; June 2002

\[6\] Schuh, H. et al.; IVS Working Group 2 for Product Specification
and Observing programs, Final Report, Annual Report 2001, page 13 -- 45,
NASA/TP-2002-210001, Greenbelt, MD; February 2002

[^1]: ^1^ Wolfgang Schlüter (Chair), Bundesamt fuer Kartographie und
    Geodäsie, Fundamentalstation Wettzell, D-93444 Koetzting, E-Mail:
    schlueter@wettzell.ifag.de

[^2]: ^2^ Nancy Vandenberg (Director IVS Coordinating Center), NVI,
    Inc./GSFC, Code 920.1, Greenbelt MD,20771, USA, E-Mail:
    nrv@gemini.gsfc.nasa.gov