First results from cruise tests of the Mercury Orbiter Radio science Experiment (MORE) of ESA’s BepiColombo mission

The ESA-JAXA mission BepiColombo, launched on 20 October 2018 and currently in cruise, will arrive at Mercury in 2025 to investigate its surface, interior structure and magnetosphere. The MORE radio science experiment onboard the Mercury Planetary Orbiter (MPO) aims at determining the gravity field, rotational state and librations of the planet with unprecedented accuracy, using precise tracking of the spacecraft during its orbital phase around Mercury. Doppler and range measurements collected during the cruise phase will also be used to test the theory of general relativity. The MORE experiment exploits two-way microwave links simultaneously at X (7.2 GHz) and Ka-band (32.4 GHz) from ESA and NASA tracking stations to measure accurately BepiColombo’s line-of-sight velocity and the round-trip light-time of the signal. The calibration of the dispersive plasma noise component through the combination of the X/X, X/Ka and Ka/Ka links and the use of water vapor radiometers to correct for the path delay due to Earth’s troposphere will result in an accuracy of ~3 µm/sec (at 1000-s integration time) on the Doppler and centimeter-level range accuracies. We report on the first analysis of range and Doppler data collected by ESA and NASA stations at X and Ka-band during the initial cruise phase of BepiColombo. The comparison of the observed noise with the predictions shows results exceeding the expectations. In particular, the 24 Mcps pseudo-noise modulation of the Ka-band carrier, enabled by MORE’s KaT transponder built by Thales Alenia Space Italia, provided two-way range measurements accurate to ~1 cm with just 4 s integration time, at a distance of 0.3 AU. In good passes, the range rate has shown an accuracy of 100 µm/s at 1 s integration time, in line with the expected end-to-end performance.