One of the ambitious projects of ISRO is to demonstrate an experimental technology for advanced satellitecommunication systems, which will be a high capacity multi-beam Ku/Ka band spacecraft. India plans to develop itsheaviest communication satellite to provide advanced telecom services. The 4500 kg satellite, GSAT-11 will belaunched by the end of 2012 on board the Geo-Synchronous Satellite Launch Vehicle (GSLV) or Ariane-5 and carryabout 36 transponders in the Ku-band and Ka-band frequencies. The spacecraft is planned to be placed at 74 deg eastlongitude in geostationary orbit. It is further being planned the spacecraft contains new technology such as usage ofelectrical propulsion system (EPS) for north/south station keeping as an augmentation to chemical propulsion toincrease the life of spacecraft. Orbit determination (OD) point of view, the usage of electric propulsion system has agreat influence on orbit determination results, because it is very hard to predict or confirm exact performance of theelectric propulsion, and its uncertainty degrades the orbit determination accuracy. This paper describes how theelectric propulsion acceleration is modeled in ISRO's GEO missions' operational orbit determination system and toestimate the performance of thrusters in addition to state and other model parameters such as solar radiation pressurecoefficient and measurement biases. GSAT-11 electrical propulsion system employs four stationary plasma thrusters(SPT), out of which two facing north and other two facing south. ISRO's GEO missions' operational orbitdetermination software was updated to estimate electric propulsion. Tracking data from network of tracking stationsconfigured for the mission was simulated for various test cases of SPT values. Orbit determinations were carried outwith and without thruster estimation. The main computation process of the orbit determination system is exercisedfor the purpose of trajectory generation and estimation. Cowell's method is used for trajectory generation throughnumerical integration. Weighted least squares technique and iterative differential correction process is used to obtainthe refined state and SPT force parameters. This paper describes main computations involved in orbit determinationprocess namely trajectory generation, observation modeling and estimation. To demonstrate raggedness of orbitdetermination process, very non-nominal initial thrust parameters were considered as initial parameters to ODprocess. It is observed that the updated orbit determination system for GSAT-11 estimates precise thrust parametersalong with precise orbit solutions. The maximum difference between nominal and estimated thrust parameters isfound to be within one percent.
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