IVS/IGS/ILRS Joint Working Group on GPS Phase Center Mapping
      Report to the 4th Meeting of the IVS Directing Board
                     September 17, 2000
  ============================================================


Motivation for WG
-----------------

The immediate impetus for the formation of the IVS/IGS/ILRS joint
working group on GPS phase center mapping was a series of discussions
at the IGS Analysis Center Workshop at the Scripps Institution of
Oceanography in 1999 regarding the uncertainty in the location of the
phase centers of the GPS satellite transmitters.  The phase center
location is a critical issue as it bears directly on the scale of the
GPS terrestrial reference frame.  Estimates of the phase center
offsets from the satellite centers of mass based solely on GPS data
have yielded surprising results, including biases as large as 1 meter
between satellites.  It was suggested at the workshop that VLBI
observations of the satellites could provide independent information on
the phase center locations, including perhaps the relative locations
for the individual radiating elements in the phased array transmitters.
An informal request was made to the IVS to lead an investigation of the
feasibility of such observations.


Establishment and organization of WG
------------------------------------

At the second IVS Directing Board meeting in July 1999, the board
decided to establish a working group on this subject and to invite the
IVS, IGS, and ILRS chairs to nominate representatives to serve on it.
The charge to the working group was to study the feasibility of the
proposed VLBI observations, the equipment and time required, and the
expected accuracy of the phase center location estimates.  Following the
nomination of members from the three services, the WG was officially
established under the rules of the IVS at the third IVS Directing Board
meeting on 20 Feburary 2000.

The initial meeting of the WG was held on 23 February 2000 in Koetzting,
Germany, during the first IVS General Meeting.  The members of the WG,
all of whom were present at the meeting, are: Graham Appleby (ILRS rep.),
Richard Biancale (ILRS rep.), Brian Corey (IVS rep. and WG chair),
Tom Herring (IGS rep.), Ed Himwich (IVS rep.), Axel Nothnagel (IVS ex
officio), Wolfgang Schlueter (IVS ex officio and IVS chair), Tim Springer
(IGS rep.), and Nancy Vandenberg (IVS ex officio).  The purpose of the
meeting was primarily organizational, but some technical issues regarding
the phase center estimates from GPS observations and the proposed VLBI
measurements were also discussed.


Initial WG activities
---------------------

In March, a WG web site was set up at http://ivscc.gsfc.nasa.gov/WG/wg1.
Background information on the phase center problem, general information
on the WG, and a WG email archive are available on the web site.

Following official notification of the IGS and ILRS chairs by the IVS
chair that the WG had been established, a general email announcement was
distributed to the IGS, ILRS, and IVS communities to inform them of the
existence and purpose of the WG.  The announcement included an
invitation for interested individuals with expertise in relevant fields
to participate in the WG studies.  Messages were also sent to selected
experts in the fields of VLBI imaging and astrometry to solicit their
advice.

The initial WG discussions have concentrated on identifying the key
technical issues to be addressed.  The task of measuring the phase
center offsets can conceptually be broken down into two subtasks:
(1) measure the mean phase center location of the full, 12-element
phased array on each satellite, and (2) measure the relative offsets
of the 12 individual radiating elements in each array.  Differential
GPS/quasar VLBI astrometry, combined with GPS or SLR estimates of the
location of the satellite center of mass, should allow the mean phase
center offset to be estimated.  The questions to be answered are what
the potential accuracy is and what the technical roadblocks are to
observing the satellites with a VLBI network, correlating the VLBI
data, and analyzing the results.  The second task of imaging the phased
array wavefront is more problematical; the feasibility of measuring the
relative offsets of the array elements, even in principle, is currently
a matter of discussion.