Operational Management of Collision Risks for LEO Satellites at CNES

Because of the ever-increasing number of orbital debris, the possibility of a satellite collision with space debris or another satellite is becoming more likely. This phenomenon particularly concerns the LEO altitude regime, the most frequently used region in space, where the environment has reached a point in which collisions will become the dominant debris generating mechanism in the future. Today, CNES performs the station keeping of 16 spacecraft (14 LEO and 2 GEO) in its Operations Sub-directorate. In this context, CNES has placed an effort to evolve into a fully operational state of its collision risk monitoring activities for LEO satellites. These operational activities are gathered at OCC (Orbit Computation Center). This paper presents the current OCC contingency operational procedure for collision risks management. This procedure is divided in five main stages: automated screening, manual risk assessment, dangerous conjunctions fine assessment, conjunction mitigation and formal end of conjunction mitigation. Collision risks automated screening involves the daily monitoring of close conjunctions of the 14 LEO spacecraft with all the USSTRATCOM catalogued objects. At this stage, fully automated computations are performed from the latest Two-Line Elements (TLE) data available using an in-house estimate of the orbital position uncertainty. Only close conjunctions which exceed a collision maximum probability threshold are short-listed for the next stage. The manual risk assessment stage identifies dangerous conjunctions with an empirical approach that considers several criteria, such as collision probability, trending miss distance and conjunction geometry. The dangerous conjunctions fine analysis is a much more complex procedure involving new orbit data and 3D visualization. When a high risk is corroborated, the risk mitigation stage is triggered. An avoidance maneuver is then calculated taking into account satellite mission and platform constraints, as well as new high-risk conjunction events in the post maneuver trajectory. This maneuver is eventually executed if the risk remains high while getting closer to the conjunction date. Finally, the formal end of conjunction mitigation capitalizes the case experience. The CNES methodology is described with comprehensive justifications. A real case example is introduced to illustrate the whole operational procedure.