Precise Orbit Determination for FORMOSAT-3/COSMIC and Gravity Application

The orbits of FORMOSAT-3/COSMIC (FC) are determined using the GPS data at a 5-s sampling rate. Both reduced dynamic and kinematic solutions are employed. GPS data from only one antenna is used. The average RMS orbital differences between the reduced dynamic and the kinematic solutions for FM1 to FM6 is 6 cm in each coordinate component. The RMS orbital difference at the overlapped arc is regarded as the internal orbital accuracy. Typical RMS overlapping differences from the reduced and kinematic solutions are about 5 cm. While the reduced dynamic orbit is smooth and free from outliers in most cases, the kinematic solution yields large orbit errors when the satellite attitude values are large or missing. Expected orbital improvement can be achieved by combining GPS data from the two POD antennae. As an external accuracy assessment, we numerically integrate FC orbits using the initial state vectors from the GPS solutions and modeled perturbing forces acting on FC satellites. The RMS differences between GPS-determined and numerically integrated orbits are about 10 cm. For gravity application, we model the centers of mass of the six FC satellites and their time variations by considering the consumption of fuel and motion of solar panels. We will also present experimental determinations of gravity harmonic coefficients using kinematic orbits of FC in different scenarios. The result of a combination solution of gravity harmonic coefficients using FC and GRACE data will also be presented.