Precise Orbit Determination for NASA's Earth Observing System Using GPS

An appfication of a precision orbit determination technique for NASA's Earth Observing System (EOS) using the Global Positioning System (GPS) is described. This technique allows the geometric information from measurements of GPS carrier phase and P-code pseudo-range to be exploited while minimizing requirements for precision dynamical modeling. Briefly, the method combines geometric and dynamic information to determine the spacecraft trajectory; the weight on the dynamic information is controlled by adjusting fictitious spacecraft accelerations in three dimensions which are treated as firstorder exponentially time-correlated stochastic processes. By varying the time correlation and uncertainty of the stochastic accelerations, the technique can range from purely geometric (for zero time correlation, infinite uncertainty) to purely dynamic (for infinite time correlation, zero uncertainty). Performance estimates for this technique as applied to the orbit geometry planned for the EOS platforms indicate that decimeter accuracies for EOS orbit position may be obtainable. The sensitivity of the predicted orbit uneertainties to model errors for station locations, non-gravitational platform accelerations, and Earth gravity is also presented.