A new approach for close approach analysis of ISRO's operational satellites Close Approach Prediction Software: CLAPS

摘要

During the lifetime of an operational spacecraft, a situation may occur when it faces a close approach with other orbiting space objects. The mitigation strategy for minimizing threat from orbiting space objects is to first carry out proximity analysis for operational spacecrafts (primary) with all other catalogued orbiting space objects (secondary). In case of a possible close approach, to plan an evasive collision avoidance maneuver. The ever increasing number of space objects around the Earth demands this kind of analysis on daily basis by satellite operators. Presently, ISRO is operating close to 50 satellites in LEO and GEO/GSO orbits and this number is increasing each year at a rapid rate. At present the NORAD TLE catalogue consists of around 18000 unclassified space objects. This includes operational satellites and debris (defunct satellites, launch vehicle upper stages and tracked space junk created by past collisions) objects. The large number of object pairs (primary & secondary) require enormous amount of computational load to do such kind of analysis on daily basis. Clearly, an efficient and user friendly tool to predict the future close approaches and plan appropriate evasive collision avoidance maneuver for all operational satellites is necessary for spacecraft control centres. In this paper, methodology is designed and developed for carrying out close approach analysis. In the design, efficient approach is adopted to reduce the computation time. Each object pair goes through screening process using pre-filters like perigee-apogee test and smart sieves. These filters are based on basic flight dynamics rules. Only those pairs which are passed by all filters are subjected to relative distance function method for finer assessment. In this method cubic splines are used as approximating functions to determine minimum distance in closed form. For the candidate pairs which violate the specified minimum Inter-Satellite-Distance (ISD) limit, collision probability is computed and appropriate collision avoidance maneuver is suggested, if required. For operational satellites, latest available orbit determination results at control centre are used and orbit propagation is done with high fidelity ephemeris model. SGP4 model is used for orbit propagation of catalogued objects. Using this design, CLAPS software is developed to predict the future close approaches for multiple operational spacecrafts with complete TLE catalogue in single run. CLAPS s/w is tested for various close encounter cases and results are validated with STK's AdvCAT. The close approach time and minimum distance are found to match up to millisecond and millimetre level respectively. Few real close approach scenario results are discussed and compared with SOPA results. At present, CLAPS is operational at Master Control Facility (MCF), Hassan for routine monitoring of close approaches. It was also used during MICROSAT-R mission at ISTRAC.