Subject: [IGSMAIL-2946]: Joint working group on VLBI mapping of GPS phase centers
From: Brian Corey <[email-masked]>
To: [email-masked], [email-masked], [email-masked]
Cc: [email-masked]
Date: Sat, 22 Jul 2000 16:20:25 EDT

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IGS Electronic Mail      22 Jul 13:21:44 PDT 2000      Message Number 2946
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Author:   Brian Corey

               Announcement of joint working group on
            VLBI mapping of GPS satellite phase centers

A working group comprised of representatives from the IVS, IGS, and ILRS
communities has been established to investigate the feasibility of
using VLBI to aid in determining the locations of the phase centers of
the GPS transmitters with respect to the satellite centers of mass.

The initial meeting of the WG was held in Koetzting, Germany during the
IVS General Meeting on 23 February 2000.  The minutes from that meeting
are appended to this announcement.  Included in the minutes is a list of
the members of the WG, information on the background and organization of
the WG, and a brief summary of some of the initial technical discussions.

A working group web page, which is accessible via a link on the IVS home
page, has been set up at http://ivscc.gsfc.nasa.gov/WG/wg1.  The page
includes a link to a paper presented by Tim Springer at the IVS General
Meeting, in which he discusses very lucidly the problems with the GPS
phase center positions and the effect of the uncertainty in those
positions on the GPS-derived terrestrial scale.

We invite participation in the WG discussions by individuals who wish to
contribute to the investigation of the various technical issues, which
include, but are not limited to, mapping of spatially coherent sources
and differential satellite-quasar VLBI astrometry.  An email distribution
list has been set up by the IVS to facilitate WG discussions.  Anyone
wishing to contribute to the discussions should contact Brian Corey
([email-masked]) to have their names added to the list.  An
email archive of all messages to the list is maintained on the WG web
site.

Brian Corey
WG chair

MIT Haystack Observatory
Westford, MA 01886 U.S.A.
[email-masked]

 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

     Minutes of the initial meeting of a joint WG (IVS-IGS-ILRS)
       for the study on the determination of the phase centers
               of the GPS transmitters employing VLBI

                      held in Koetzting/Germany
                      February 23, 2000 / 19:30

Participants
============

For IVS:        Brian Corey (IVS-nominated Rep.)
                Ed Himwich (IVS-nominated Rep.)
                Axel Nothnagel (IVS-Analysis coordinator)
                Wolfgang Schlueter (IVS-chair)
                Nancy Vandenberg (IVS-CC)

For IGS:        Tom Herring (IGS-nominated Rep.)
                Tim Springer (IGS-nominated Rep.)
                
For ILRS:       Graham Appleby (ILRS-nominated Rep.)
                Richard Biancale (ILRS-nominated Rep.)

Objectives of the WG
====================

At the IGS Analysis Center Workshop held at Scripps Institution of
Oceanography in 1999 the stability of the GPS global reference frame
was discussed. It became apparent that there is a large uncertainty
about the location of the phase center of the GPS transmitter.  The
possibility of "mapping" the GPS-phase center using VLBI was discussed,
and it was suggested that IVS could help to find a solution.

IVS considered this at its 2nd Directing Board Meeting in Birmingham,
even though there was only an informal request.  The Board proposed to
establish a Working Group on this topic and to invite members of the
other services.

The objectives of this WG were defined as follows:

"The WG could be set up to study the feasibility, equipment, time
required, and if it could be done with accuracy sufficient to make it
worthwile."

It was clearly pointed out that VLBI plays the leading role, but as
orbit determination and scaling problems are involved, the SLR technique
will contribute too.  The nominations of the WG members have been made
by the three services through their respective chairpersons.

Regulations of Working Groups
=============================

The WG operates under the leadership of the IVS. The IVS regulations for
WG are similar to those for IGS and are in accordance with ILRS
regulations.  In particular, the WG will have
* a finite life span,
* a membership list (see participants, all nominated representatives).
* The IVS Directing Board has named Brian Corey as chair of the WG.
* The WG operate autonomously and will report to the boards of the three
   services as necessary.

Discussions
===========

The present situation was reviewed with regard to apparent phase center
offsets of the satellite transmitting antennas, and the relationships
among the phase center offsets, receiving antenna phase patterns, and
terrestrial scale errors were discussed.  Scale changes of ~10**-8 and
satellite antenna offsets of order 1 meter in the radial direction are
observed when these quantities are estimated from GPS data.  The GPS
analyses assume the satellites are point sources, whereas the satellite
antennas are phased arrays consisting of two concentric rings of helical
antennas. There is also a 5 cm bias between the orbits determined by IGS
and by laser ranging to the GPS satellites.

Useful information that VLBI observations of the GPS satellites could
potentially provide includes estimates of the phase center location
relative to the satellite center of mass.  One possible observing
scenario that was discussed was to use the VLBA, perhaps in conjunction
with the EVN, to do differential observations between each GPS satellite
and neighboring extragalactic sources.  Mapping the individual elements
of the satellite phased arrays is desirable; unlike the case with
extragalactic sources, however, the satellite signal is spatially
coherent at the source, so the standard radio astronomical methods for
inverting interometric visibilities to generate a map of the source do
not apply.

The initial goal of the WG is to investigate the feasibility of mapping
the transmission wave fronts and to determine the accuracy with which
phase center offsets could be estimated from VLBI observations (in
conjunction with information provided by the IGS or ILRS).  If the
expected accuracy is deemed worthy (e.g., error in phase center under
1 meter), then a proposal to conduct observations, correlate the VLBI
data, and analyze the results would be submitted to a suitable funding
agency (e.g., U.S. National Science Foundation) under the guidance of
the WG.

Next steps
==========

The next step is to gather detailed information on the satellite
specifications (dimensions, power, etc.).

The WG will set up an e-mail list via the IVS-CC.  Experts in radio
imaging of coherent sources will be invited to participate in WG
discussions.