Subject: [IGEXMail-0290] message from IGEX-98 Steering Committee (plans for the future)
From: Pascal Willis <[email-masked]>
Date: 14 Apr 1999 18:57:16

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IGEX Electronic Mail    14-Apr-1999 18:57:16                   Message No 0290
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Author: Pascal Willis <[email-masked]>
Subject: message from IGEX-98 Steering Committee (plans for the future)

Dear contributors to the IGEX-98 experiment, dear colleagues,

The official end of the extended IGEX-98 campaign is approaching rapidly.
IGEX-98 was a success beyond expectation, and there is a growing and
important scientific user community, in particular in the field time and
frequency transfer, urging us to continue with the IGEX-like activities.

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THIS WHY THE IGEX-98 STEERING COMMITTEE REQUESTS YOU TO CONTINUE WITH YOUR
IGEX-98 ACTIVITIES ON A BEST EFFORT BASIS AT LEAST UNTIL THE 1999 IGEX
WORKSHOP ON SEPTEMBER 13-14 IN NASHVILLE U.S.A., WHERE A CONCRETE PROPOSAL
FOR A MORE PERMANENT CONTINUATION OF THE IGEX-98 WILL BE PRESENTED BY THE
IGEX STEERING COMMITTEE.
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Before commenting this request, please allow us to express our sincere
thanks to all individuals and all institutions, who/which made the IGEX-98
a great success.

As a matter of fact, the IGEX-98 campaign has exceeded its initial goals
and also its original duration: It is now possible to obtain precise
GLONASS orbits from four different analysis groups working with three
different software packages using data from the global IGEX-98 GPS/GLONASS
tracking network.

Several people and institutions already have expressed their willingness
to continue their IGEX-98 commitments (for data acquisition, data
archiving and data analysis).

Other groups have expressed their vital interest in a continuation of the
IGEX activities, in particular in the uninterrupted availability of
precise GLONASS orbits referring to the IGS realization of the ITRF. As a
matter of fact, the IGEX steering committee recently received a letter
from Dr. Quinn, Head of Bureau International des Poids et Mesures,
expressing the timing community's greatest interest in continuing to
receive precise GLONASS orbits (letter attached to this e-mail).

There are thus strong arguments to continue the IGEX-98 activities after
April 17, 1999 on a more permanent, currently a "best effort" basis. We
request you in particular to

  - continue GLONASS/GPS data acquisition for all
    dual-frequency GPS/GLONASS instruments
  - continue SLR tracking of GLONASS satellites (on a reduced scale
    to be established at the next ILRS Governing Board Meeting)
  - continue archiving GPS/GLONASS data
  - continue GLONASS precise orbit computations
  - complete the scientific investigations you want to present at the
    IGEX-Workshop in September

Regarding single-frequency data collection we propose to apply the
following policy:

  - The current number and distribution of dual-frequency receivers allows
    for a precise orbit computation without the inclusion of single-frequency
    data. The single-frequency data set collected during the official IGEX-98
    campaign is probably sufficient for all special investigations one might
    have in mind.

  - Proposal: Single frequency data is considered useful for validation purposes
    and for special studies. If there are, however, budgetary or other reasons
    for a station to discontinue single-frequency operation, this would be
    acceptable and of no harm at the present stage of the IGEX-experiment.

As stated above, the IGEX-98 steering committee considers it appropriate
to move from the campaign-type to a more permanent organization. It will
take some time, however, to achieve such a goal. Also, such an
organization should involve more than just the IGEX steering committee. We
believe that it would be most appropriate that a proposal for a more
permanent IGEX-continuation should be presented by the IGEX steering
committee at the IGEX-Workshop of September in Nashville, where the
proposal can be discussed and accepted as a resolution. Such a proposal
must be based on your input. We therefore propose the following procedure:

 - The IGEX steering committee sends out end of April a questionnaire
   concerning the continuation of the IGEX-98.

 - IGEX Steering Committee informs the IGS Governing Board on the June 7,
   1999 IGS Governing Board Meeting

 - Proposal for a future IGEX organization prepared by the IGEX steering
   committee in June/July

 - Proposal finalized at the IGEX Steering Committee meeting in
   July (at the IUGG meeting in Birmingham).

 - Proposal distributed through IGEXMail and IGSMail in August.

 - Reviewed proposal should be discussed, modified, and adopted as a
   resolution at the IGEX Workshop in Nashville

 - IGEX steering committee presents the proposal to the next IGS
   Governing Board Meeting for approval.

We would like to thank you very much in advance for your collaboration.

On behalf of the IGEX-98 steering committee

                                               Pascal Willis, Chair
                                               IGEX-98

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Attachment: Letter by Dr. T.J. Quinn, Director of BIPM
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Dear Colleagues,

The International GLONASS Experiment (IGEX-98), which has been in progress
for about six months, has been well received by the international time
metrology community, as the results of this experiment contribute greatly
to the research on the best use of the GLONASS constellation for
international time transfer.

The international time scales - International Atomic Time (TAI) and
Coordinated Universal Time (UTC) - are based on data from some 220 atomic
clocks located in about 50 time laboratories around the world. Today, the
sole method of comparing these clocks to provide data for TAI is the GPS
C/A-code common-view technique. The performance of this technique barely
covers present needs, and will be not sufficient in the near future to
compare the new generation of clocks. This is why the time metrology
community is engaged in a number of studies of new time transfer
techniques, among which is the use of GLONASS or combined GPS and GLONASS
systems.

The first tests of the GLONASS precise code time transfer show a
significant improvement in performance. Also the availability of GLONASS
precise code on two frequencies allows measurements of the ionosphere. In
order to take full advantage of GLONASS capabilities for intercontinental
time transfer, however, the use of post-processed precise ephemerides is
necessary, as it is for GPS.  An uncertainty of about 2 ns or better is
expected for intercontinental GLONASS time transfer when using precise
ephemerides, and only about 10 ns when using broadcast ephemerides.

The large participation and excellent results of the IGEX-98 campaign
raise the question of transforming this experiment into a permanent
service. The Bureau International des Poids et Mesures (BIPM) strongly
supports the creation of such a permanent service.  The BIPM is already
routinely using GPS precise ephemerides produced by IGS. Permanently
available GLONASS precise ephemerides would greatly contribute to the
diversification of time transfer methods used for the construction of TAI.

I congratulate you on the excellent manner in which the IGEX-98 experiment
has been conducted and wish you every success in the future.

Yours sincerely

T. J. QUINN

To IGEX-98 Steering Committee:

Pascal Willis,      - IGN, Chair
Gerhard Beutler     - AIUB
Werner Gurtner      - AIUB
Ruth Neilan         - JPL
Guenther Hein       - UbM
Jim Slater          - NIMA

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 Pascal Willis                                                      
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