Report on First Inflight Data of BepiColombo's Mercury Orbiter Radio Science Experiment

BepiColombo's Mercury orbiter radio science experiment (MORE) was conceived to enable extremely accurate radio tracking measurements of the Mercury Planetary Orbiter to precisely determine the gravity field and the rotational state of Mercury, and to test theories of gravitation (e.g., Einstein's theory of general relativity). The design accuracy of the radio tracking data was 0.004 mm/s (at 1000 s integration time) for the range-rate measurements and 20 cm for the range (at a few seconds of the integration time). These accuracies are attained due to a combination of simultaneous two-way microwave links at X (7.2-8.4 GHz) and Ka-band (32-34 GHz) to calibrate the dispersive plasma noise component. In this letter, we present the first analysis of the range and range-rate data collected by ESA's deep-space antenna (DSA) during the initial cruise phase of BepiColombo. The novel 24 Mcps pseudonoise (PN) modulation of the Ka-band carrier, enabled by MORE's Ka-band transponder, built by Thales Alenia Space Italy, Rome, Italy, provided two-way range measurements to the centimeter-level accuracy, with an integration time of 4.2 s at 0.29 astronomical units. In tracking passes with favorable weather conditions, the range-rate measurements attained an average accuracy of 0.01 mm/s at 60 s integration time. Data from May 20-24, 2019 were combined in a multi-pass analysis to test the link stability on a longer timescale. The results confirm the noise level observed with the single-pass analysis and provide a preliminary indication that the MORE PN ranging system at 24 Mcps is compatible with the realization of an absolute measurement, where the need to introduce the range biases in the orbital fit is much more limited than in the past. We show that in the initial cruise test the BepiColombo radio link provided the range measurements of unprecedented accuracy for a planetary mission, and that, in general, all target accuracies for radio-metric measurements were exceeded.