IL NUOVO CIMENTO VOL. 13 C, N. 1 Gennaio-Febbraio 1990 Spacecraft Doppler Tracking with a VLBI Antenna(*)(**). G. COMORETTO Osservatorio Astrofisico di Arcetri - Firenze L. IESS Istituto di Fisica dello Spazio Interplanetario, CNR - Frascati (Roma) B. BERTOTTI Dipartimento di Fisica Nucleare e Teorica, Universit~ di Pavia - Pavia J. P. BRENKLE and T. HORTON Jet Propulsion Laboratory - Pasadena, USA G. GRUEFF Dipartimento di Fisica, Universita di Bologna - Bologna (ricevuto il 31 Dicembre 1988) Summary. -- 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 32 m 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 (2.1GHz) from the 70m 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 character- ize its performances. This task has been successfully accomplished with the (*) Paper presented at the 4th Cosmic Physics National Conference, Capri, September 11-14, 1988. (**) To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee. 169