<h2>AAS 99-186</h2>
<h2>Orbit Determination for Medium-Altitude Eccentric Orbits Using GPS                                                                               </h2>
<h4>J.G. Neelon, Jr.*, P.J. Cefola*, R.J. Proulx*, J.E. Draim**                                                                                                                         </h4>
*The Charles Stark Draper Lab, Cambridge, MA, ** Ellipso, Inc., Washington, DC                                                                                            
<h2> Abstract </h2>
This work examines GPS Receiver CA navigation solutions as a measurement source in the orbit determination process for higher altitude elliptical orbits. Some concerns are the visibility between such orbits and the GPS constellation, and the increase of ionospheric delay experienced when using a nadir facing GPS antenna. The current work extends an earlier investigation, which demonstrated the use of actual navigation solutions as a measurement source for sever LEO orbits (TOPEX, TAOS, and EUVE). A program was constructed to simulate the GPS navigation solutions. The simulation includes realistic error models and visibility considerations. The simulation uses the Message Passing Interface (MPI) parallel-processing paradigm. Simulated and actual navigation solution errors are compared for the TOPEX case. Simulated navigation solutions for he Ellipso Borealis, Ellipso Concordia, and Molniya orbits were processed with the Goddard Trajectory Determination System (GTDS) orbit determination program. A variety of tests were performed including those that combine realistic measurement errors with perturbation force mis-modeling.