  A relativistic delay model for space very long baseline interferometry
(hereafter SVLBI) observation of sources at infinite distance is derived. In
SVLBI, where one station is on a spacecraft, the orbiting station's maximum
speed in an elliptical Earth orbit is much bigger than the ground VLBI (here
after GVLBI), leading to a higher delay rate . The delay models inside the VLBI
correlators are usually expressed as fifth-order polynomials in time that good
for a limited time interval, which are evaluated by the correlator firmware and
track the interferometer delays over a limited time interval. The higher SVLBI
delay rate requires more accurate polynomial fitting and evalution, more
frequent model updates.
