Satellite Orbit and Ephemeris Determination using Inter Satellite Links

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Autonomous Onboard ProcessingInter Satellite LinksThe case where the orbit manoeuvre is performed using the apogee kick motor, has not beenconsidered, because these high thrust engines (> 400 N) are not accurate enough to performsmall orbit corrections. Frequently, the apogee-kick is performed using a solid rocket motor,which can't be re-ignited anyway.The following simulation have been performed considering the orbit manoeuvre in theprediction of the precise ephemeris. The broadcast message, although not intended forpropulsive flight phases, has been fit over an interval which contains at least the beginning ofthe manoeuvre. This is the worst case, because the acceleration changes not smooth, but witha step. The following table represents the simulation results for the three types of orbitmanoeuvres.Manoeuvre Component GEO IGSO LEO50 m/sRadial 1.3 m 72 m 0.4 mAlong Track 1.4 km 1.2 km 1.5 mChemicalPropulsionCross Track 97 m 891 m 49 mURE 221 m 243 m 41 m1 m/sChemicalPropulsionRadial 0 1.4 m 0.2 mAlong Track 51 m 41 m 0.8 mCross Track 11 m 29 m 14.5 mURE 8 m 8 m 12.2 m1 m/sRadial 0 m 0.05 m 0.08 mIon PropulsionAlong Track 0.35 m 0.42 m 0.29 mCross Track 0.35 m 0.31 m 0.17 mURE 0.07 m 0.1 m 0.29 m1 m/sRadial 0.03 m 0.06 m 0.08 mIon Propulsion Along Track 0.10 m 0.14 m 0.29 m15 MinutesCross Track 0.13 m 0.26 m 0.17 mUpdate Rate URE 0.05 m 0.07 m 0.29 mTable 7-5 Ephemeris Error during an Orbit ManoeuvreThe fit error over an interval containing a 50 m/s manoeuvre is intolerable high. In case of aplanned orbit correction which requires a high velocity increment, the satellite has to beswitched to unhealthy.A short, but frequently performed orbit correction using chemical propulsion produces alsointolerable high fit errors. Due to the fact, that the manoeuvre last only about 14 seconds, thePage 162R. Wolf
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
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Satellite Orbit and EphemerisDeterm
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Inter Satellite LinksAbstractGlobal
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Inter Satellite LinksTable of Conte
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Inter Satellite Links6.2.2 IGSO WAL
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Inter Satellite LinksFigure 6-38 Tr
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Inter Satellite LinksList of Tables
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Inter Satellite LinksJ2Earths Oblat
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Inter Satellite LinksIntroduction1
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Inter Satellite LinksIntroductionGE
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Inter Satellite LinksState Estimati
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State EstimationInter Satellite Lin
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Orbit ComputationInter Satellite Li
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Satellite Orbit and Ephemeris Determination using Inter Satellite Links

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