Monitoring and Mitigation of Close Proximities in Low Earth Orbit

The German Space Operations Center (GSOC) is currently building up an operational proximity monitoring and mitigation system. Proximity events are detected based on the “Two-line Elements” (TLEs) from the US Strategic Command (USSTRATCOM) and precise orbit information from locally operated missions. Despite evident deficiencies in the quality and timeliness of the available orbit information, TLEs are currently the only source of orbit information for the numerous space objects. While an overly trust in the quality of the orbital data might result in an underestimation of the true collision risk, a pessimistic accuracy assessment would result in frequent proximity warnings. The TLE uncertainty needs to be carefully assessed to avoid such implications. Even after a realistic error analysis, the orbit information of a possible jeopardising object has to be refined for a proper planning and implementation of collision avoidance manoeuvres. For this purpose, the use of FGAN radar tracking is currently planned, for which an accuracy assessment is to be considered. In this paper, the proximity statistics and TLE accuracy analysis as well as the FGAN tracking campaign are discussed, together with their application to the collision risk management of satellites in a Low-Earth-Orbit (LEO). The cumulative frequency of predicted proximities is first estimated for the selected GSOC missions based on a one-year simulation. The TLE accuracy is then discussed by comparing ephemerides derived from TLEs with those derived from precise orbit determination of locally controlled satellites. As a special case, the recent collision between a Cosmos and an Iridium satellite is also analysed. Complementary to these assessments, the orbit prediction accuracy using FGAN tracking is discussed from campaign results. The paper concludes with a discussion of the operational application to the active proximity monitoring and mitigation strategies.