State-of-the-art radio tracking data performance of BepiColombo MORE during cruise

Abstract

The BepiColombo mission, a collaborative venture between ESA and JAXA, started its journey on October 20, 2018, from the Kourou spaceport in French Guiana, with the goal of reaching Mercury by late 2025. During its 7-year cruise phase, BepiColombo encounters eleven superior solar conjunctions, an alignment where the Sun is positioned between the spacecraft and Earth. The Mercury Orbiter Radio Science Experiment (MORE) utilized the first six conjunctions to test gravitational theories by measuring the relativistic time delay and frequency shift of photons as they pass near the Sun. This process involved collecting range and Doppler data from the ESTRACK DSA-3 antenna in Malargue (Argentina) and NASA’s DSS 25 antenna in Goldstone (California). Over 15 radio tracking passes were scheduled for each of the six campaigns (March 2021, February 2022, July 2022, February 2023, July 2023, and December 2023). A comprehensive calibration process, addressing various noise sources, was essential for conducting the general relativity tests. Central to the superior conjunction experiments are the onboard Deep Space Transponder (DST) and the Ka-band transponder (KaT). MORE's KaT establishes a two-way coherent radio link in the Ka-band, enabling both uplink and downlink communications and incorporating a novel 24 Mcps pseudo-noise (PN) modulation. The DST supports two-way coherent X/X and X/Ka Doppler and PN ranging at 3 Mcps. Multi-frequency radio links are crucial for calibrating dispersive noise, primarily from solar corona plasma. This technique has been previously employed in Cassini's cruise radio science experiments, providing plasma-free observations up to a minimum impact parameter of 7 solar radii. Furthermore, the Tropospheric Delay Calibration System (TDCS) at DSA-3 significantly reduces the impact of tropospheric water vapor. The KaT is equipped with a self-calibration loop for measuring the transponder group delay, while the DST relies on pre-flight ground test values. DSA-3's group delay calibration system conducts pre-tracking pass measurements for each link (X/X, X/Ka, Ka/Ka), with continuous in-pass measurements available exclusively for the Ka/Ka link, the primary data source for the radio science experiments. This work presents the performance of calibrated range and Doppler data from DSA-3 across five solar conjunction experiments. We report the spectral properties of the calibrated residuals, highlighting the improvements achieved through calibration procedures. Notably, these campaigns marked the first use of a plasma-cancellation scheme for calibrating range data, proving effective up to an impact parameter of about 4 solar radii. The final accuracy of the radiometric data was  about 0.02 mm/s  at 60-second integration time and ~4.5 cm at 2 s interval, respectively for range rate and range measurements.