Approaching Mercury – Preparation for Geodesy Studies by the BepiColombo Laser Altimeter (BELA)

As BepiColombo gets closer to its target Mercury, the BepiColombo Laser Altimeter (BELA) and the Mercury Orbiter Radio-Science Experiment (MORE) teams prepare for operation, scientific measurements, and analysis. For that purpose, both teams simulate the future measurements: altimetric range measurements by BELA and orbit determination product from MORE radio science measurements. These simulated measurements provide a test bed for the different analysis algorithms and applications.

We report on three different approaches to derive Mercury’s Love number h₂, that were developed and are being optimized within the BELA team: co-registration [1, 2]; cross-over analysis [2, 3]; global grid approach [4, 5]. All of these approaches were already validated in applications to existing laser altimetry data from the Moon and Mercury. In this study we are challenging these approaches by utilizing simulated BELA data and estimate their individual performance in terms of estimation biases and uncertainty assessment. For the simulation of BELA data, we performed an iterative approach. We started with an idealized BELA range measurement simulation, assuming error-free measurements without false detections up to 1400 km. A tidal signal with a defined h₂ value was incorporated into the simulated measurements (dataset v1). Next, we added a simple performance model that includes false detections but keeps range measurements perfect (v2). The third version employed a full BELA performance model [6], which also accounts for slope and roughness at the observation site (v3). For all simulations, the spacecraft’s orbit, attitude, and Mercury’s rotation were assumed to be perfectly known. This assumption changed in the next iteration, where we utilized spacecraft orbits reconstructed by the MORE team from simulated radio-science data (v4). Because the accuracy of h₂ and other geodetic parameters depends on orbit reconstruction quality, careful modelling of the orbit is mandatory.

The MORE simulation consists of an orbit-determination process: first, synthetic radiometric observables are generated by numerically integrating the spacecraft trajectory; second, a least-squares estimation compares these synthetic observables with predictions to reconstruct the trajectory.

In a blind test, all three h₂ inversion approaches received different versions of the simulated observations and were tasked with retrieving the assumed h₂. All tidal models successfully derived the unknown Love number, though differences in estimation accuracy were observed, reflecting each method’s strengths, weaknesses, and the specific parameters used.

 

 

References

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