Spacecraft navigation using the VLBA

In 2008, the VLBA observed the Phoenix spacecraft for 8 sessi on , each of about three hours, over a period of four months up to the Mars landing. Phase-referen cing VLBI techniques were used between the Phoenix 8.4-GHz telemetry signal, the continuu m emission from nearby quasars, and the telemetry signal from the two Mars orbiters. The astrome tric results of these observations were compared with the accurate trajectory of Phoenix determine d after landing in order to assess the VLBA positional accuracy and its feasibility for future NASspacecraft tracking. The nominal celestial position of Phoenix was obtained to about 0.15 mas accuracy, but achieved an accuracy of 0.015 mas (15 meters at Mars) with respect to the Mars cente r of mass within two days of landing using in-beam observations of Phoenix and the two Ma rs orbiters.