ANALYSIS OF RADIOMETRIC TRACKING DATA OF THE JUNO SPACECRAFT X-BAND SIGNAL RECEIVED BY THE SARDINIA DEEP SPACE ANTENNA

The Sardinia Deep Space Antenna (SDSA) is a facility in the south of Sardinia, Italy, managed by the Italian Space Agency (ASI). It shares the basic functionalities with the Sardinia Radio Telescope (SRT), which is a key infrastructure in the network of Italian large antennas. With a primary mirror of 64-m diameter and 1116 electromechanical actuators to control its active surface, this all-Italian station employs one of the largest and most advanced radio-telescopes in Europe for Radio Astronomy and Space Research. Since its technical commissioning in 2018, SDSA has been employed in several tracking activities, but its radiometric tracking performance for deep-space missions has never been fully and systematically evaluated or compared to the one of similar deep space antennas. In the context of a collaboration between the Italian Space Agency (ASI) and the University of Bologna (UniBO) aimed at the characterization of the performance of SDSA, an acquisition campaign of radiometric tracking data has been carried out between December 2020 and June 2021. In this timeframe, SDSA has been systematically tracking the NASA spacecraft Juno at X-band in a three-way configuration, where the uplink signal was provided by Robledo’s NASA Deep Space Network DSS 55 near Madrid. Thanks to an IFMS backend borrowed from the European Space Agency (ESA) and installed in an appropriately shielded environment, SDSA was able to record both closed-loop and open-loop raw data for further processing and comparison. The same radiometric data acquired simultaneously in two and three-way mode at the NASA stations was also retrieved, providing the possibility of identifying noise sources common to both signal paths, and therefore isolate and highlight the station contributions to the uncertainties in the tracking of the spacecraft Juno. The main objective of this acquisition campaign was to provide nominal values for the antenna performance, as well as to compare different operational settings of the ground complex subsystems and identify the best tracking configuration to be used in the future. To this end, Caltech/JPL’s orbit determination toolkit MONTE has been used to generate the Doppler residuals based on the acquired tracking data. Such residuals have then been thoroughly investigated to provide stability and performance metrics and evaluate the antenna noise contributions. In this paper, the results of the tracking activities of the Juno spacecraft with SDSA are presented. The effects on the tracking performance of different antenna sub-systems settings are also discussed, showing the contribution of the active surface control and the pointing strategy to the stability of the Doppler residuals. Additionally, an error budget for Doppler tracking with SDSA is presented and discussed. This model was developed using radiometric tracking data with different mission geometries and transmission paths, and makes it possible to isolate the contribution to the Doppler tracking uncertainties from the antenna hardware. Finally, the fluctuations in the Doppler residuals are analysed to develop a general model of the station frequency stability as a function of time, and an assessment of the differential tracking accuracy between SDSA and the NASA receiving antennas employed in the Robledo-Madrid complex during the same tracking arcs is provided.