  The planetary ephemeris is an essential tool for interplanetary spacecraft
navigation, studies of solar system dynamics (including, for example,
barycenter corrections for pulsar timing ephemeredes), the prediction of
occultations, and tests of general relativity. We are carrying out a series of
astrometric VLBI observations of the Cassini spacecraft currently in orbit
around Saturn, using the Very Long Baseline Array (VLBA). These observations
provide positions for the center of mass of Saturn in the International
Celestial Reference Frame (ICRF) with accuracies ~0.3 milli-arcsecond (1.5
nrad), or about 2 km at the average distance of Saturn. This paper reports
results from eight observing epochs between 2006 October and 2009 April. These
data are combined with two VLBA observations by other investigators in 2004 and
a Cassini-based gravitational deflection measurement by Fomalont et al. in 2009
to constrain a new ephemeris (DE 422). The DE 422 post-fit residuals for Saturn
with respect to the VLBA data are generally 0.2 mas, but additional
observations are needed to improve the positions of all of our phase reference
sources to this level. Over time we expect to be able to improve the accuracy
of all three coordinates in the Saturn ephemeris (latitude, longitude, and
range) by a factor of at least three. This will represent a significant
improvement not just in the Saturn ephemeris but also in the link between the
inner and outer solar system ephemeredes and in the link to the inertial ICRF.
