Satellite Orbit and Ephemeris Determination using Inter Satellite Links

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Autonomous Onboard ProcessingInter Satellite LinksI SPg 0Specific Impulsenominal gravity force (9.81 m/s²)we can estimate the required burn time bymttStartBurnBurn− tmI=BurnStartSP⋅⋅ gTISP⋅g00T⋅ m= eStart⎛ ∆v⎞⎜ ⎟−ISPg⎝ ⋅ 0 ⎠⎛⋅⎜1−e⎜⎝5≈ 7.63⋅10s ≈ 212 h⎛ ∆v⎞⎜ ⎟−ISPg⎝ ⋅ 0 ⎠⎞⎟⎟⎠Eq. 7.3-2For station keeping of a GEO satellite, Hughes therefore recommends two 5 hour propulsivephases per day. To provide nominal availability of GNSS 2, the satellites will have to beavailable during these propulsive phases. Although there are further developments like the HS702 thruster providing 165 mN thrust, there are still 2 x 30 minutes thrust phases per dayrequired for station keeping. Thus, the broadcast message has to be adapted to account for thefrequent or nearly permanent presence of propulsive forces. There are several possibilities todo that.7.3.1 Continued Service during ManoeuvresIf a broadcast message format similar to the GLONASS navigation message is used, wherethe satellite position is derived from numerically integrating a simple force model, andpresumed the thrust phase is sufficiently short, a special navigation message extender couldbe send. Such a message could look like the followingENGINE_START_TIMEENGINE_CUT_OFF_TIMEAVARAGE_THRUST_XAVARAGE_THRUST_YAVARAGE_THRUST _ZTable 7-3 Thrust Phase Navigation Message ExtensionThe user receiver would than simply add the thrust forces during the time span covered by thenavigation message extender. A geometric ephemeris format based on Keplerian elements,Page 160R. Wolf
Inter Satellite LinksAutonomous Onboard Processinglike the one used by GPS, is much less suited for augmentation. The propulsive forces wouldhave to be modelled as generic “orbit perturbations” which is likely to require more modelparameters than in the example above. Although limited to short thrust periods, this method issuited for nearly arbitrary (high) thrust levels.If thrust levels are low (low thrust chemical or ion propulsion), the thrust can be considered asan additional force in the orbit prediction process. The normal broadcast message is then fitover an interval containing a thrust phase, as would be over a normal free flight phase.The error introduced by this depends strongly on the acceleration by the propulsion system. Ifthe error introduced remains small, this solution would be favourable, because there is noadditional navigation message. In the following, results concerning this method will beshown.To evaluate the errors introduced by orbit manoeuvres, three different orbit types have beenconsidered: GEO, IGSO and LEO. Thrust and velocity increment have been altered tosimulate typical manoeuvres. The following table shows the parameters used in thesimulation.Chemical PropulsionIon PropulsionSpecific Impulse 315 s 2568 sThrust 4 x 10 N 2 x 18 mNBurn Time for a 50 m/sManoeuvreBurn Time for a 1 m/sManoeuvre11.36 min Not Considered(212 h ~ 9 days)13.7 s 4.2 hTable 7-4 Simulation ParametersThe chemical propulsion case is represented by 4 x 10 Newton thrusters using storablepropellant like MMH / NO 4 . The ion propulsion consist of two Hughes XIPS thrusters fromthe HS 601 HP.Two manoeuvres have been performed for all three satellite types, covering the followingcases:• The 50 m/s manoeuvre represents the case, where orbit manoeuvres are conductedinfrequently, with a high velocity increment. This only makes sense using high thrustpropulsion, thus the ion propulsion has not been considered for this case.• The 1 m/s manoeuvre represents the case, where orbit manoeuvres are conductedfrequently, but with a low velocity increment. In this case, the ion propulsion has beenconsidered, although the thrust phase is not impulsive, but more like a permanent actingforce.R. Wolf Page 161
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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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Page 126 and 127: Simulations and ResultsInter Satell
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Satellite Orbit and Ephemeris Determination using Inter Satellite Links

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