Demonstration of Adaptive Extended Kalman Filter for Low-Earth-Orbit Formation Estimation Using CDGPS

Carrier-phase Di®erential GPS is an ideal sensor for formation °ying missions in low Earth orbit since it provides a direct measure of the relative positions and velocities of the vehicles in the °eet. This paper presents results for a relative navigationlter that achieves centimeter-level precision using measurements from a customized GPS receiver. This work uses a precise, robust extended Kalmanlter that is based on rela- tively simple measurement models and a linear Kep- lerian propagation model. To increase the robustness of thelter in the face of environment uncertainty, the ¯lter is adapted using MMLE algorithms. The adap- tivelter is used to accurately identify the process noise and sensor noise covariance for the system. De- spite the simplicity of thelter, hardware-in-the-loop simulations performed on the Formation Flying Test- bed at the Goddard Space Flight Center demonstrate that thislter can achieve ¼2 cm relative position ac- curacy and <0.5 mm/s relative velocity accuracy for a range of Low Earth Orbit formations.