Accelerometer Measurements for Orbit and Gravity Recovery

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), one of the two spacecraft that comprise the BepiColombo mission, is the Italian Spring Accelerometer. The instrument’s primary scientific goals are closely linked to the Mercury Orbiter Radio-Science Experiment. Together, these instruments aim to provide valuable data on the spacecraft’s orbit, as well as Mercury’s gravity field and internal structure. This simulation study examines how accelerometer measurements affect orbit and gravity field recovery, and it explores strategies to overcome the challenges associated with the co-estimation of all parameters. In it, we evaluate the accuracy of the integrated retrieval of spacecraft orbit, gravity field, and accelerometer parameters under different noise levels and varying observation geometries during the mission. To achieve these goals, we propagate the orbit of MPO and simulate Doppler observations and accelerometer measurements based on the available noise models. We consider two scenarios by including either a realistic or a more conservative accelerometer noise level. 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 carry out orbit determination and gravity field recovery under varying assumptions regarding the accelerometer noise and the observation geometry. Improved recovery of low-degree coefficients was observed under realistic noise assumptions. Moreover, the strong influence of observation geometry in shaping the error behavior was observed. namely, errors in cross-track bias estimation increased sharply when the (Formula presented) -Earth angle dropped below (Formula presented), corresponding to the degraded Doppler observability periods. Orbit determination achieved decimeters-level accuracy in the radial direction and metre-level accuracy in the along- and cross-track directions, with a substantial rise in cross-track orbit errors observed during periods of reduced Doppler observability. This study also demonstrates the applicability of the planetary extension of the Bernese GNSS Software as an independent tool for orbit determination and gravity recovery for the BepiColombo mission.