Autonomous Orbit Determination for CubeSat Adapted to Proximity Operations at an Asteroid

Deep space exploration has turned into multi-spacecraft operations where the navigation of auxiliary probes is a new challenge. We have developed, in a previous work, an autonomous on-board orbit determination concept for nanosatellites cruising in deep space and we adapt the concept here to proximity operations at a small solar system body. We consider a science case inspired by the HERA mission that will carry two CubeSats, considering here a CubeSat performing a fly-by of an asteroid at low altitude and low velocity. The aim is that the CubeSat determines its own orbit in full autonomy. The method uses an optical sensor and a Kalman filter. The sensor measures the absolute directions of selected bodies in front of background stars, the observed bodies may or may not be resolved. The measurements feed an Unscented Kalman Filter (UKF) well suited for strongly non-linear models. The performances in this work show an orbit reconstruction with a 3 a accuracy below 1 km and even 500 m after 1.5 day at processing observations. The algorithm can be further improved if we combine optical measurements with radio link measurements performed between the CubeSat and its mothercraft. The assumptions on the optical and radio link performances are realistic at CubeSat scale. The accuracy is improved down to 15 meters at 3 a. It uses moderate processing resources and shows great potential for improvements. Some instabilities are still reported that need to be investigated further. Nevertheless, this level of performance in the HERA context would allow to perform multiple fly-bys in strong autonomy and even make it possible to let the CubeSat determine by itself the mass of the moonlet Dimorphos, thus directly contributing to the scientific results.