SPACECRAFT DOPPLER TRACKING WITH A VLBI ANTENNA

Accurate measurements of the Doppler shift of the Voyager 2 spacecraft (26 A. U.) have been performed in the period 23/7/88-12/8/88 with the 32m VLBI antenna at Medicina (BO). The device which allows the use of VLBI instrumentation for precision Doppler tracking is basically an open-loop receiver and is called digital tone extractor (DTE). Simultaneous measurements were performed at NASA's Deep Space Network (DSN) stations of the Madrid complex, using a different technique (closed loop). In the most interesting tracking configuration, the signal was transmitted in S-band (2.1 GHz) from the 70 m DSN station at Canberra (Australia), retransmitted back coherently from the spacecraft in X-band (8.4 GHz-19 W) and received 7h 6min later at Medicina and Madrid. The main purpose of the experiment was to test the capabilities of this new system and to characterize its performances. This taks has been successfully accomplished with the acquisition of high-quality data, which are presently under analysis. We plan also to compare and correlate the data with the ones acquired at Madrid stations, in order to separate out different noise contributions to the measured signal. The use of Medicina antenna as a listen-only station is planned for the gravitational wave experiment of the Ulysses spacecraft (ESA), in 1992. © 1990 Società Italiana di Fisica.