Cnes Operational Feedbacks in Collision Avoidance for Leo Satellites
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CNES OPERATIONAL FEEDBACKS IN COLLISION AVOIDANCE FOR LEO SATELLITES Grégory BEAUMET CNES DCT/OP/MO, 18 av. Edouard Belin, 31401 Toulouse cede 9, France Phone: %33-5-61-27-43-91 Email: gregory..eaumet0cnes.fr ABSTRACT Because of the ever-increasing number of orbital debris, the possibility of a satellite collision with a space debris or another satellite is becoming more and more likely. This phenomenon particularly concerns the LEO altitude regime, the most fre uently used region in space, where the environment has reached a point in which collisions will become the dominant debris generating mechanism in the future. Therefore CNES performs collision risk monitoring for the 15 LEO satellites it controls. Most of the operational Collision Avoidance activities are gathered at OCC (Orbit Computation Center) in its Operations Sub-directorate. First this paper briefly presents the current CNES Collision Avoidance Process which includes OCC contingency operational procedure for collision risk management, procedure which is divided in five stages, each successive stage representing an escalation in the contingency from collision risk screening to dangerous con,unctions fine analysis and risk mitigation. The different criteria, actors, types of avoidance manoeuvres will be presented with the information flow settled between different actors. The analysis of known collisions will be presented in order to e-periment the efficiency of the process. Then this paper tries to give feedbacks of two years of operational Collision Avoidance assessment for 15 LEO satellites in order to underline the preference actions which should be taken to improve global efficiency of the process. Therefore, key figures for the operational activities are given. number of alerts, number of risks, number of avoidance manoeuvres, etc... The evolution of the operational process within the past two-year e-perience is presented, especially the need for more and more automation. In order to give an idea of the variety of situations which need to be handled in terms of Collision Avoidance, first a real typical case e-ample is introduced. Then the case of a close approach between two controlled satellites is discussed and the differences are illustrated with real cases encountered. Finally the case of a close approach between an operational satellite 0ASON 1 with a non-operational one TOPE1 which was (and still is) on a close orbit for mission purpose is presented with its operational mitigation of close encountered close approaches. 1. INTRODUCTION There are eight recorded collisions and four of which with catalogued space debris. In 2ecember 1331, the Russian satellite COSMOS 1334 and a COSMOS 328 debris collided. Five years later, on 0uly 24 th 1338, the French satellite CERISE was hit by a debris. On 0anuary 17 th 2005 there was a collision between THOR BURNER and a debris from Long March. And recently, on February 10 th 2003, an operational satellite IRI2IUM 33 and a non operational one COSMOS 2251 collided in orbit. One difficulty of the collision risk management is the limited number of tracked ob,ects. only 13,000 out of 100,000 potentially dangerous debris. Another difficulty is the unavailability of the accuracy of the publicly available data needed to properly monitor collision risks. CNES uses different data sources. • Space-Trac3 database. the main Two Lines Elements (TLE) database. 2ue to its breadth and update fre uency, it has been adopted as the primary source for the screening phase. Nevertheless, its accuracy and orbit propagation models are not fully available. Therefore, comple- estimations are re uired to estimate and improve consistency. • 4RA7ES database. the French Air Force Space Surveillance System (built by ONERA). • Specific or.ital data . radar measurements from French 2efence facilities, FGAN radar and CNES Guyana Launch Center. These sources produce very accurate measurements but they are only used to improve the orbit determination of dangerous ob,ects due to their limited availability. 2. CURRENT CNES COLLISION AVOIDANCE PROCESS 2.1 OCC cont ngency operat onal procedure for coll s on r s) management The CNES Collision Avoidance (CA) process is operational since 0uly 2007 on all LEO satellites controlled by CNES. In order to be capable of handling a collision risk whenever it occurs, on-call teams in each speciality are in place. Two on-call teams of flight dynamics engineers are available 24/7. one in charge of CA analyses (CA team) and one in charge of the station-keeping of the LEO satellites controlled at CNES. Obviously, when an avoidance manoeuvre is considered, the CA procedure also relies on all the other on-call teams (vehicle, payload, ground segment, station network ?). The CNES procedure is divided in 5 stages as presented in Fig. 1, each stage being an escalation in the contingency. Fig. 1. Five stages procedure (PoC D Probability of Collision) The automated screening stage consists of a fully automated process which is performed daily at OCC. Collision risk predictions are made within an horizon of 7 days. The orbits of the CNES satellites are given by ephemeris provided by the dedicated control centers. The secondary ob,ect information comes from the public NORA2 catalogue (Space-Track database) and is eventually completed with data from the GRAAES catalogue. The estimation of the orbital position uncertainty is performed using in-house tools as described in B1C. The manual risk assessment stage consists in the manual analysis of the risks detected by the previous stage. This stage cannot be completely automated since each con,unction characteristic (collision probability, geometry and date, orbit uncertainties, miss distance on each a-is, trending miss distance and ob,ect size) is important and has to be analysed and taken into account to evaluate the risk. The aim of the dangerous con,unctions fine assessment stage is to determine the con,unctions that need to be mitigated. This is also a manual stage and it involves re uesting radar data in order to improve the knowledge of the ob,ect orbit. At this stage, three kinds of risks can lead to con,unction mitigation. - if the secondary ob,ect orbit is confirmed by radar measurements and the operational collision probability e-ceeds 10 -3, - if radar measurements are not available and the operational collision probability e-ceeds 10 -2, - if the risk is confirmed by NASA (when the con,unction concerns one of the CNES satellites involved in the A-Train). 2uring the con,unction mitigation stage Collision Avoidance team keeps on analysing the risk so that any change on the collision risk can be detected. At the same time, possible avoidance manoeuvres are computed in cooperation with the station-keeping team taking into account satellite mission, platform and operational constraints, as well as potentially new high-risk con,unction events in the post manoeuvre tra,ectory. This stage is coordinated by the on-call mission representative through the e-ceptional Operational Coordination Group (OCG) which decides if an avoidance manoeuvre has to be performed. The final stage is the formal end of the con,unction mitigation. At this stage, the CA team tries to analyse the lessons learned in order to improve the process. Post Collision Avoidance reports summarizing the risk management are produced and presentations are performed in annual e-ploitation reviews. 2.2 Actors and nformat on flow settled ,etween d fferent actors The actors mentioned in the previous section are summarized in the Fig. 2 with the information flow settled between each of them. CNES OCC S GRAVES c TLEs r CONTROL e e CONTROLCENTER n M A i Alert CENTER n i s CONTROL CENTER NORAD g t s TLEs i e g s a s t m One team per mission: i e o n Radar n t DV - Mission coordination measurements analysis - Station-keeping - Vehicle NASA - Ground segment Fig. 2. CNES Collision Avoidance information flow 3. REAL CASE E.AMPLES This section describes real cases handled by CNES CA team since the beginning of its operational activities. The ob,ective is to illustrate the variety of situations encountered, firstly describing a typical case and then presenting two special cases for which the involvement of several CNES entities was re uired. 3.1 Typ cal close approach On 0uly 1E th 2007 the automatic screening provided the following information concerning a new collision risk B1C between SPOT5 satellite and a debris of the Chinese FENGFUN satellite. • time of closest approach (TCA). 0uly 22 nd 2007 at 05.43G • collision ma-imum probability e-ceeding 10 -4G • con,unction geometry. o miss distance D 125 mG o in-track separation D 30 mG o radial separation D 50 mG o cross-track separation D 70 m. 1 13 0.8 0.6 8 0.4 0.2 3 0 -0.2 -2 -0.4 I differencesI (km) I differences (km) -0.6 -7 -0.8 -1 -12 24/1 13/2 5/3 25/3 14/4 4/5 24/5 13/6 3/7 0 1 2 3 4 5 6 7 8 910 Date Number of extrapolation days 0.2 0.3 0.15 0.2 0.1 0.1 0.05 0 0 -0.1 -0.05 -0.2 -0.1 R differences (km) -0.3 Rdifferences (km) -0.15 -0.4 -0.2 -0.5 24/1 13/2 5/3 25/3 14/4 4/5 24/5 13/6 3/7 0 1 2 3 4 5 6 7 8 910 Date Number of extrapolation days 0.2 0.4 0.15 0.2 0.1 0 0.05 -0.2 0 -0.4 -0.05 -0.6 -0.1 -0.15 -0.8 C differences (km) -0.2 -1 Cdifferences (km) -0.25 -1.2 -0.3 -1.4 24/1 13/2 5/3 25/3 14/4 4/5 24/5 13/6 3/7 0 1 2 3 4 5 6 7 8 910 Date Number of extrapolation days Fig.