Modeling orbit dynamics of FORMOSAT-3/COSMIC satellites for recovery of temporal gravity variations

The precise GPS high-low tracking data from the joint Taiwan-USA mission FORMOSAT-3/COSMIC (COSMIC) can be used for gravity recovery. The current orbital accuracy of COSMIC kinematic orbit is 2 cm and is better than 1 cm for 60-s normal points. We model the perturbing forces acting on the COSMIC spacecraft based on standard models of orbit dynamics. The major tool for the numerical work of force modeling is NASA Goddard's GEODYNII software. Considering that COSMIC spacecrafts are not equipped with accelerometers, the accelerations due to atmospheric drag, solar radiation pressure, and other minor surface forces are modeled by estimating relevant parameters over one orbital period from COSMIC's kinematic and reduced dynamic orbits. We carry out experimental solutions of time-varying geopotential coefficients using one month of COSMIC kinematic orbits (August 2006). With the nongravity origin forces properly modeled by GEODYN II, residual orbital perturbations (difference between kinematic and reference orbits) are assumed to be linear functions of time-varying geopotential co-efficients and are used as observations to estimate the latter. Both COSMIC and combined COSMIC and GRACE gravity solutions are computed. The COSMIC solution shows some well-known temporal gravity signatures but contains artifacts. The combined COSMIC and GRACE solution enhances some local temporal gravity signatures in the GRACE solution.