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mOrbit determination strategies suitable for on-board processing in real time are investigated with JPL's GIPSY-OASIS ii software. The study includes the effects of Selective Availability (SA), increased drag during solar-active conditions for a Low Earth orbiter. and various other time dependant error effects: It is shown that the use of a higher order gravity field (and thus a better dynamic model) can effectively smooth the effects introduced by SA. However. the storage requirements. and computer resources required in integrating the orbit with a large number terms can be prohibitive .

It is possible to reduce the size of die field required with the use of a software package known as the Satellite Linear Orbit Perturbation analysis tool (SLOP). The terms that have the largest magnitude effect can be determined and a specialized field consisting of only those terms can be used. The possibility of using selected gravity terms from a large (50x50) truncation (rather than a smaller truncation such as 20x20) shows promise. Further improvement to the OD accuracy results when these selected terms are tuned for the specific mission. This is analogous to tuning the gravity field with data from the first few revs of the actual orbit. Even with the tuning and subsequent creation of a tailored gravity field. the orbit error approaches a few meters in magnitude when SA is active. However. this is substantial improvement over the 30 to 50 meters that would result from SA in a point-positioning mode.