<h2>AAS 99-143</h2>
<h2>Mitigating the Effects of Selective Availability                                                                                                 </h2>
<h4>K. Gold, G. Born, S. Mitchell, B. Frazier                                                                                                                                           </h4>
University of Colorado, Bouler, CO                                                                                                                                        
<h2> Abstract </h2>
Orbit determination strategies suitable for on-board processing in real time are investigated with JPL's GIPSY-OASIS II software. The effects of SA, and other time dependent errors are included. The use of a higher order gravity field can effectively smooth effects introduced by SA, however the required computational resources are prohibitive in a flight-computer environment. A smaller gravity field is selected with terms that have the largest effect on the particular orbit, and this field is further turned. Resulting orbits are accurate to several meters, which is substantial improvement over the 30 to 50 meters from SA in a point-positioning mode.