Analysis of conjunctions between two satellites with no communication
Creators: name:  R. LUCKEN 
name:   A. PETIT 
name:   S. REDEL 
name:   F. DELEFLIE 
name:   E. RAMIREZ 
name:   J. SALGADO 
name:   M. RAMIREZ 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: SPACE DEBRIS MODELING
REENTRY

description: The space traffic management is becoming more and more complex with the growing number of maneuvering satellites in particular in low Earth Orbit (LEO) due to the race for large constellations of communication satellites. One of the major challenges for CNES [1] and the EU SST [2] in particular is the analysis of the collision risk when a conjunction that involves one of these satellites is scheduled. Preliminary analysis showed that about 20% of the close approach conjunctions with active satellites involve a secondary object with some maneuver capability, while this figure was rather below 10% a couple of years ago. This evolution is partially due to the deployment of more than 800 Starlink satellites over year 2020. In some cases, satellite operators can share their ephemerides and coordinate their actions to minimize the collision risk. However, in most cases, the behavior of the secondary object is unknown. The purpose of this work is to identify patterns of life of the secondary object to reduce the risk of collision. A conjunction is mainly characterized by a time of closest approach (TCA), a miss distance, a relative velocity, and uncertainty in position at TCA which allows to compute a probability of collision. Recent studies discussed the realism of uncertainties and proposed solutions to reassess the collision risk [3, 4, 5]. These works impact the decision making process that precedes a collision avoidance maneuver. However, if a maneuver is performed by the secondary satellite between the prediction date and the TCA, the risk assessment can turn out to be wrong. More importantly, some dangerous events can also be missed or detected just a few hours before TCA. In general, the whole process of orbit fitting and trajectory forecast becomes inaccurate when the secondary objects performs regular frequent station-keeping maneuvers. To tackle this issue, secondary objects with maneuver capability are treated separately from the passive objects. First, a list of such objects is maintained on a daily basis using satellite databases. Secondly, the two-line elements (TLE) data of these objects are analyzed to determine how often station keeping maneuvers are realized. This step is based on filtered differences on the orbital elements [6] and yields a first time interval where each maneuver occurred, as well as an estimate of the maneuver Delta-V. Then, an estimate of the thrust capability and thrust direction of the secondary satellite is determined using an iterative method based on forward and backward orbit propagation of the special perturbation (SP) orbital data from the US Air Force. The uncertainty on the object position comes mainly from the uncertainty related to the time and duration of the maneuvers, and the Gaussian assumption is not valid anymore. Hence, we define an exclusion volume at each date whose bounds are defined by the extreme maneuver scenarios that would be possible for the secondary satellite. In order to avoid a collision, the primary satellite should keep out of this exclusion volume, at any epoch. In this paper, we describe each of these methods in details and present simulation results for a limited set of primary objects in LEO. [1] M Moury and L Newman. Middle man concept for in-orbit collision risk mitigation: Caesar and cara examples. In SpaceOps 2014 Conference, page 1637, 2014. [2] P Faucher, R Peldszus, and A Gravier. Operational space surveillance and tracking in europe. Journal of Space Safety Engineering, 7(3):420425, 2020. [3] M D Hejduk and L C Johnson. Approaches to evaluating probability of collision uncertainty. NASA Technical Reports (Presentation), 2016. [4] S Laurens, P Seimandi, J Couetdic, and JC Dolado. Covariance matrix uncertainty analysis and correction. In 68th International Astronautical Congress, 2017. [5] A B Poore, J M Aristoff, Joshua T Horwood, R Armellin, William T Cerven, Y Cheng, C M Cox, R S Erwin, J H Frisbee, M D Hejduk, et al. Covariance and uncertainty realism in space surveillance and tracking. Technical report, Numerica Corporation Fort Collins United States, 2016. [6] T. Kelecy and K. Hamada. Satellite maneuver detection using tow-line element (tle) data. In AMOS, 2007.