Accelerometer Measurements for Orbit and Gravity Recovery: Challenges and Benefits for the BepiColombo Mission

TL;DR

This study evaluates how accelerometer measurement modeling affects orbit and gravity field recovery for the BepiColombo mission, highlighting strategies to improve parameter estimation accuracy amid observational challenges.

Contribution

It introduces a simulation-based approach to optimize accelerometer bias estimation and gravity field recovery, addressing challenges in parameter co-estimation during planetary orbit determination.

Findings

Postponing accelerometer bias estimation improves gravity field accuracy.

One year of Doppler data enables gravity field recovery up to degree and order 40.

Bias estimation errors increase when the beta-Earth angle drops below 45°.

Abstract

The European Space Agency's BepiColombo mission continues its pioneering voyage to Mercury, the innermost planet of the Solar System. Among the advanced instruments onboard the Mercury Planetary Orbiter (MPO) is the Italian Spring Accelerometer (ISA), whose scientific objectives are closely linked to the Mercury Orbiter Radio-Science Experiment (MORE). Together, these instruments aim to provide high-precision data on the spacecraft's orbit, as well as Mercury's gravity field and internal structure. This simulation study investigates how the modeling and parametrization of accelerometer measurements influence orbit and gravity field recovery and explores strategies to overcome the challenges associated with the co-estimation of all parameters. We assess the integrated retrieval accuracy of the spacecraft orbit, gravity field, and accelerometer parameters under different noise levels and varying observation geometries during the mission. The MPO orbit is propagated, and Doppler and accelerometer measurements are simulated based on available noise models. Our results indicate that postponing the estimation of accelerometer biases until a preliminary gravity field is established helps prevent gravity field mismodelings from being absorbed into the accelerometer parameters. The daily estimation of bias was found to be essential. Using one year of Doppler tracking data and applying Kaula regularization, we demonstrate the potential to recover Mercury's gravity field up to degree and order 40. Low-degree coefficients improve under more optimistic noise conditions. Errors in cross-track bias estimation rise sharply when the beta-Earth angle falls below 45{\deg}, corresponding to the degraded Doppler observability. Orbit determination achieved centimetre-level radial accuracy and metre-level along- and cross-track accuracy, with increased cross-track errors during low-observability periods.