This paper presents a multi-sensor navigation approach to allow a formation of spacecraft to autonomously navigate in the proximity of a Near Earth Asteroid. Multiple measurements collected by on-board cameras, attitude sensors and LIDAR are used to estimate the state of each spacecraft with respect to the asteroid. Inter-spacecraft position measurements are then combined with spacecraft-to-asteroid position measurements to improve accuracy. The paper analyses the use of different filtering techniques to estimate the state of a 4-spacecraft formation with respect to the asteroid. Different combinations of measurements are constructed to evaluate the improvement in navigation performance offered by the data fusion of the measurements gathered by the four spacecraft. Moreover the robustness of the navigation system is tested against the occurrence of failures. Results show that the navigation performance is significantly improved by adding the inter-spacecraft position measurements. Finally, an asteroid orbit determination method is proposed that combines asteroid's line of sight measurements from multiple spacecraft and Sun Doppler shift sensor with spacecraft-to-ground tracking data. Different approach configurations are evaluated for a 2-spacecraft formation and it is shown that the integrated use of spacecraft-to-asteroid and ground-to-spacecraft measurements provides an effective way to improve the ephemerides of the asteroid.
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