Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Inter Satellite LinksAutonomous Onboard Processingsatellite should be switch to unhealthy. The error introduced by ion propulsion is very low.This is due to the fact that the ephemeris message can easier be fit to a slowly varying forcethan to a fast changing. The resulting URE is acceptable, when the period of validity isdecreased to 15 minutes.7.3.2 Frequently Updated Ephemeris CorrectionsThe simulation performed in the preceding section are independent whether the orbitdetermination is performed onboard or not. Besides the fitting error of the broadcast model,there is another error contributing to the prediction of a power trajectory: the uncertaintyintroduced by the engine with respect to• thrust level• thrust direction• exact time when nominal thrust level is reached• engine cut off behaviour.The significance of these error sources increase with thrust level. During orbit prediction of apowered flight, the Kalman filter process is adapted by increasing process noise.A major advantage of onboard processing is now that - presumed that measurements areavailable – orbit corrections can be computed at a high update rate.7.4 Application Example: Autonomous Onboard Integrity MonitoringIn the following example, an autonomous onboard processing scenario is demonstrated usingthe Galileo Option 1, consisting of 33 MEO satellites. It is assumed, that the broadcastephemeris for the next 24 hours have already been determined (onboard or on ground) in postprocessing, and uploaded. The broadcast ephemeris model used is the 15 parameter extendedGLONASS type.Each satellite has an onboard processor, processing ground and intersatellite links. Theonboard processor uses a Kalman filter with the following state vector.⎛ ∆x⎞⎜ ⎟⎜ ∆y⎟⎜ ∆z⎟⎜ ⎟⎜∆T⎟x = ⎜ ⎟⎜∆x⎟⎜ ∆y⎟⎜ ⎟⎜ ∆z⎟⎝∆T⎠And the following simplified transition matrixEq. 7.4-1R. Wolf Page 163