Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Autonomous Onboard ProcessingInter Satellite Links7 AUTONOMOUS ONBOARD PROCESSING7.1 Why Onboard Processing?In a typical conventional orbit estimation process, ranging signals are transmitted by thesatellite whereas measurements are taken by the ground stations. There are two exceptions,the DORIS and the PRARE system: both systems are performing measurements onboard.• PRARE uses two way the range and range rate measurements in the X-band withphase coherent ground transponders. It has four channels; therefore it is limited to foursimultaneous measurements. Moreover, due to the fact that the transponders are phasecoherent and X band frequencies require directive antennae, the ground transponderscan serve only one satellite at a time. Although PRARE is used for orbit heightsbetween 500 and 2000 km, it is principally not limited to a special orbit class. Duringthe AUNAP project (1996) PRARE has been evaluated as an option for anautonomous navigation processor onboard an IGSO satellite.• DORIS receives codeless carrier signals on two frequencies (S-band and UHF) fromso called ground beacons and performs Doppler 1 measurements. Because range ratemeasurements are independent of the clock offset, one “Master Beacon” transmits akind of ranging code, which is needed to perform at least coarse synchronisation of theonboard clocks. Doppler measurements allow precise orbit estimation if the satellitedynamics are high, therefore it is more or less restricted to LEO orbits.Nevertheless, even in those systems the measurements are downloaded and transmitted viadata link to a central facility for further processing. In navigation applications like GPS andGLONASS, the central processing facility performs then orbit determination, orbit predictionand broadcast ephemeris adjustment.But given the fact that measurements are available, onboard processing has some advantages.The data latency can be reduced to a minimum. Therefore it is best suited for applicationswere fast reaction is desired. In navigation applications, the following four parameters are ameasure for the performance of a system:• Accuracy• Availability• Continuity of Service• IntegrityThe first two parameters are driven by the system’s design. Accuracy is mainly driven by twofactors, the radio frequency link (signal-in-space) and the broadcast ephemeris, and can beenhanced e.g. by1 Although DORIS performs no ranging but Doppler measurements, the two frequencies are needed to correctfor ionospheric effects, which are in fact an issue due to 10 seconds integration time. Because of this longintegration time, it would be more appropriate to speak of phase rate instead of Doppler measurements.Page 152R. Wolf