Catalogue-Based Screening for In-Orbit Proximity and Threat Detection

The growing number of Earth orbiting objects poses a serious threat to the safe operation of satellites and the long-term sustainability of space activities. To prevent it, rendezvous and proximity operations (RPO) capabilities are attracting increasing interest, since they enable various tasks ranging from debris removal to in-orbit inspection, repair, and refuelling of satellites. In this context, this paper focuses on the detection and monitoring of proximity operations and space-to-space threats. Specifically, it investigates methods to detect close and long-lasting encounters between two space objects, namely situations where they maintain a relatively small distance and exhibit a low relative speed. Catalogue screening techniques typically used in collision risk assessment scenarios, such as the Orbit Path filter and the Apogee-Perigee filter, are not deemed suitable for this application. Hence, this paper proposes an original methodology based on the screening of the Two-Line Elements catalogues. A first filtering step is required to eliminate all space objects that cannot experience proximity or pose a threat depending on the type of close approach to be detected. A second filtering step is then applied to select a subset of pairs that are characterized by low distance and low relative speed, by adopting thresholding criteria on the differential orbital parameters and on the actual position and velocity of the satellites along their orbits (Synchronisation filter). Finally, additional filters may be used according to the type of RPO event under analysis. At the end of this screening process, the minimum distance, the minimum speed and the Proximity Time Fraction (PTF), defined as the percentage of time instants in which the relative distance and the relative speed are kept below a certain threshold, are evaluated for each candidate encounter. The most interesting close approaches, resulting from such analysis, are further investigated focusing on the time evolution of an ad-hoc defined Proximity Index (PI) and of the differential orbital parameters, as well as on the relative motion with particular regard to the PTF. The proposed methodology is tested by using real datasets and considering both the close approaches between a debris and a satellite and those between two satellites.