  In order to establish the position of the center of mass of the Earth in the
International Celestial Reference Frame, observations of the Global Positioning
Satellite (GPS) constellation using the IVS network are important. With a good
frame-tie between the coordinates of the IVS telescopes and nearby GPS
receivers, plus a common local oscillator reference signal, it should be
possible to observe and record simultaneously signals from the astrometric
calibration sources and the GPS satellites. The standard IVS solution would
give the atmospheric delay and clock offsets to use in analysis of the GPS
data. Correlation of the GPS signals would then give accurate orbital
parameters of the satellites {\bf in the ICRF reference frame}, i.e. relative
to the positions of the astrometric sources. This is particularly needed to
determine motion of the center of mass of the earth along the rotation axis.
