Integration of Radio Tracking and Feature-based Optical Measurements for Geophysical Investigations

Accurate estimation of geophysical parameters, including total mass, moment of inertia and rotational state of planetary bodies is essential for understanding their degree of differentiation, constraining their internal structure, and gaining insights into their evolutionary path. To improve the accuracy of these key estimates, we have developed an integrated framework that combines Earth-based radio tracking data with navigation measurements based on the observation of relevant surface features on the body’s surface.

Two-way Doppler and range measurements provide robust constraints on the spacecraft motion along the line of sight and are traditionally used for gravity and geophysical investigations. Surface imagery of the central body offers complementary information, supporting the estimation of the target body’s spin vector and deviations from uniform rotational state, such as longitudinal librations.

The proposed approach leverages the tracking of relevant surface features to jointly reconstruct the spacecraft trajectory and estimate geophysical parameters of the target body. Features tracked across partially overlapping images acquired sequentially during closely spaced orbital passes improve the internal consistency of the trajectory reconstruction, whereas features observed across different mission phases contribute to the refinement of the body’s rotational state. To address challenges arising from variable illumination conditions and resolution discrepancies in planetary images, hybrid strategies are adopted for feature tracking, combining conventional computer vision with Artificial Intelligence-based feature detection and matching.

The framework is validated using data from the MESSENGER spacecraft during its science orbital phase around Mercury. The novel approach improves estimation accuracies with respect to single-instrument solutions and provides a flexible, effective tool for maximizing the scientific return of deep-space missions.