Satellite Orbit and Ephemeris Determination using Inter Satellite Links

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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
Inter Satellite LinksAutonomous Onboard Processing• providing two frequencies to allow ionospheric corrections• increasing chipping rate on the ranging signal• increasing update rates of broadcast ephemeris• using accurate broadcast models / short fit intervals• using accurate clocksAvailability, especially with respect to visibility of enough S/V to perform navigation, isdriven by constellation design, and can be enhanced by• putting enough S/V into service (actives, as well as spares and replenishment)• choosing benign orbits with respect to visibilityThe last two parameters are a bit more critical. They are mainly driven by reliability of thespace vehicles and environmental influences degrading the signal-in-space like RFinterference, atmospheric effects or jamming. Keeping these parameters high is of utmostinterest for civil aviation.System inherent continuity and integrity of the two existing navigation systems GPS andGLONASS does not meet the requirements of civil aviation and can therefore be not used as asole means of navigation. To overcome system limitations with respect to integrity,augmentation systems like WAAS, EGNOS and MSAS are under development. Their mainoutput are corrections for• ionospheric effects• satellite clock• satellite ephemerisemitted by geostationary Inmarsat space crafts. A central processing facility has torecomputed satellite orbits to provide orbit and clock corrections at a high update rate. Thishas to be done for up to 51 satellites. Fast corrections which are applied directly to the rangemeasurement are provided at an update interval smaller than 6 seconds to meet time-to-alarmrequirements for CAT I. So called “long term” corrections provide vector corrections forposition and velocity which are updated approximately every 6 minutes. Both, fast and longterm corrections have to be applied additionally to the broadcast ephemeris transmitted by theGPS and GLONASS space crafts.Summarising the measures taken to enhance integrity we find1. ephemeris correction2. at a high update rate3. with minimum data latency4. and corrections to the ionospheric effectsFuture satellite navigation systems like Galileo will provide at least dual or maybe even triplefrequency links. Even the existing GPS system is going to be enhanced and the nextgeneration of replenishment satellites (starting with Block II F) will provide a civil availableranging code on two frequencies. What’s left, is the integrity of Satellite orbit and clock.R. Wolf Page 153
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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 LinksList of Figure
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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 LinksISL Observatio
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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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Software DescriptionInter Satellite
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Simulations and ResultsInter Satell
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Satellite Orbit and Ephemeris Determination using Inter Satellite Links

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