Simulation Study of A Low-Low Satellite-to-Satellite Tracking Mission

The Gravity Recovery and Climate Experiment (GRACE) is a dedicated spaceborne mission to map the Earth’s gravity field with unprecedented accuracy. The GRACE mission is planned to launch in 2001, for a lifetime of approximately 5 years. It consists of two satellites, co-orbiting in nearly polar orbit, at approximately 300-500 km altitude, separated by 100-500 km along track. Primary measurements are the range change between the two satellites, which represents the gravity perturbation differences between the two locations. These range changes are measured by a high accuracy microwave raging system. To detect the non-gravitational perturbations, which also affect the range change, three axis accelerometers are used. In this study, full numerical simulations were performed to evaluate the gravity recovery accuracy, and to determine sensitivity of gravity estimation to mission and design parameters. The measurement and dynamic model equations were derived and presented along with the description of the simulation procedure. The error models on two major instruments, the inter-satellite ranging system and accelerometer, were described and their effect on the estimation accuracy were discussed. Series of extensive simulations were performed to analyze the impact of the various simulation parameters, which included the orbit parameters, measurement types, and so on. Comprehensive error models made it possible to perform realistic analyses. To the extent that the error sources assumed in the simulations represent those actually encountered during the mission, the simulation results will predict the performance of the GRACE mission. The GRACE mission is expected to improve the current knowledge of the Earth gravity field by order of magnitude. The geoid error level is expected to be less than 1cm to spherical harmonic degree 70.