The Tracking System of the Mercury Orbiter Radioscience Experiment

Precise microwave tracking of interplanetary spacecraft has been a crucial tool in solar system exploration. Not only range and range rate measurements are the main observable quantities in spacecraft orbit determination and navigation, but they have been widely used to refine the dynamical model of the solar system and to probe planetary interiors. Thanks to the use of Ka-band and multifrequency radio links, a significant improvement in microwave tracking systems has been demonstrated by the radio science experiments of the Cassini mission to Saturn. The Cassini radio system has been used to carry out the most accurate test of General Relativity to date. Further developments in the radio instrumentation have been recently started for the Mercury Orbiter Radioscience Experiment (MORE), selected for the ESA mission to Mercury, BepiColombo. MORE addresses the mission's scientific goals in geodesy, geophysics and fundamental physics. In addition, MORE will carry out a navigation experiment, aiming to a precise assessment of the orbit determination accuracies attainable with the use of the novel instrumentation. The key instrument is a Ka/Ka band digital transponder enabling a high phase coherence (to a level of 10^-15 over 1000 s integration time) between uplink and downlink carriers and supporting an accurate ranging system designed for an end-to-end accuracy of 20 cm. The onboard instrumentation must be complemented by a ground system, capable of simultaneous transmission and reception of multiple frequencies at X and Ka-band. Ground support for the MORE experiment is currently foreseen from the DSN station DSS25 (Goldstone, California). DSS 25 was specifically upgraded in support of the Cassini Radio Science experiment and, at present, is the only deep space antenna with Ka-band uplink and multifrequency capabilities. On the ESA side, the upgrade to Ka-band of the 35-m Cebreros antenna in Spain is also being considered with the goal of increasing the science return of the experiment and to provide Europe with state-of-the-art tracking systems. In this paper we review the experimental configuration of MORE and outline a plan for the engineering development of the ground segment which is currently under consideration in ESA in support of the BepiColombo radio science experiment.