  We present a demonstration of near real-time spacecraft astrometry with the
VLBA. We detect the X-band downlink signal from Mars Reconnaissance Orbiter and
Odyssey with the VLBA and transmit the data over the internet for correlation
at the VLBA correlator in near real-time. Quasars near Mars in the plane of the
sky are used as position references. In the demonstration we were able to
obtain initial position measurements within about 15 minutes of the start of
the observation. The measured positions differ from the projected ephemerides
by a few milliarcseconds, and the repeatability of the measurement is better
than 0.3 milliarcseconds as determined from measurements from multiple scans.
We demonstrate that robust and repeatable offsets are obtained even when
removing half of the antennas. These observations demonstrate the feasibility
of astrometry with the VLBA with a low latency and sub-milliarcsecond
repeatability.
