<h2>AAS  96-145</h2><h2>A KALMAN FILTER FOR ORBIT DETERMINATION WITH APPLICATIONS TO GPS, STELLAR, AND X-RAY NAVIGATION</h2><h4>P. J. Melvin, Naval Research Laboratory</h4><h2> Abstract </h2>An algorithm is presented for the computation of the gravity gradient tensor from tesseral harmonics in Earth fixed inertial coordinates.  This array occurs in the dynamics matrix governing small orbit errors and is integrated into the state transition matrix used for covariance propagation.  This modification to an orbit determination Kalman filter from GPS pseudoranges is then tested against the TOPEX precise orbit.  Optical interferometers are reaching a precision level that they could be used for satellite navigation using stellar aberration alone and a covariance analysis is performed for geosynchronous navigation.  A simulation is also carried out for the X-ray detector on the ARGOS spacecraft.<br><br>