Satellite Orbit and Ephemeris Determination using Inter Satellite Links

More documents

Recommendations

Info

ReferencesInter Satellite Linksintegrity result is then transmitted to an uplink station where it is uploaded to a spacecraftwhere it can be broadcast to the user. System latency is at minimum four seconds.Using ISLs for integrity monitoring demands a high technological effort. There is the issue ofthe access method, for example: a Time Division Multiple Access (TDMA) method like forthe GPS Block IIR cross links will not be appropriate because of the time to alarmrequirement, which raises the demand for either Code Division Multiple Access (CDMA) orFrequency Division Multiple Access (FDMA). Pure CDMA on one single frequency is notfeasible due to the near-far effect, which simply means a spacecraft can not receive on thesame frequency it is transmitting. A pure FDMA approach however raises the question of howmany frequencies will be needed? One per spacecraft? Frequencies are one of the very rareresources in satellite navigation, thus it is unlikely that 30 frequencies will be allocated toISLs for Galileo. As a viable option appears the combination CDMA and FDMA. Forexample, assuming a number of six frequencies and allowing each satellite to send on threeand to receive on the remaining three frequencies. For each satellite, the combination of sendand receive frequencies is different. Using this approach we would have⎛6⎞6!⎜ ⎟ = =⎝3⎠(6 − 3)!3! ⋅72036= 20possible combinations with only 6 frequencies needed, meaning that 20 bi-directional ISLscan be established simultaneously with any combination providing one matching frequencypair.Even the question of antennae placement is solvable. There is no need to mount 30 antennaeon a single S/C bus. Phased array antennae, which use electronic beam steering to manipulatethe reception direction appear to be the right technology. Besides solving the antennaeplacement problem they additionally provide SDMA (Space Division Multiple Access).8.2 Recommendations for GalileoWhile the system design phase for the next generation of satellite navigation systems GNSS 2is already in progress, the results obtained in this Ph.D. thesis lead to severalrecommendations for future satellite navigation systems. In the frame of Galileo ISL's havebeen studied and evaluated with respect to their capability for orbit determination andintegrity monitoring. The technological effort has been found very high for orbitdetermination, but worth further investigation with respect to integrity monitoring. Withintegrity being the major design driver, ISL is still an option for Galileo today.Although the use of intersatellite links places a high requirement on the space segment, i.e.the satellites with respect to complexity, the gain could be worth the effort. The ISL providesnot only ranging capability, but also offers a communication channel between the satelliteswhich can be exploited to exchange status information as well as broadcast messages whichare dedicated to the user. For example, GPS Block IIR spacecraft are capable to use theirPage 190R. Wolf
Inter Satellite LinksReferences"cross-links" to disseminate the broadcast ephemeris of the entire constellation. Thisovercomes the problem that ephemeris upload can only be done by the master control station,which has rare contact (only twice a day) to each SV. This removes the necessity for the usercommunity to use orbit parameters computed already 24 hours ago, thus improving accuracy.GPS Block IIR has also the capability for autonomous navigation, i.e. observing andimproving broadcast ephemeris parameters unaided from ground. The underlying TDMAprocess with a period of 37 seconds, however, does not support integrity with respect to thetime-to-alarm requirement.The effort of building complex space vehicles may be balanced by the reduction of (numberof) ground stations. Even if all monitoring and orbit determination is done on ground, theadditional orbit information obtained from the intersatellite ranging can bridge gaps in groundnetwork coverage.Moreover, the benign geometry especially for higher orbit altitudes like MEO or GEO/IGSOsatellites, allows very rapid estimation of the orbits using shorter intervals of observation.This leads to increased availability after manoeuvres, and also allows an increased rate ofbroadcast ephemeris update rate. The communication capability of ISLs can also be used tokeep error due to ageing of broadcast ephemeris low. Keeping the accuracy goal of Galileo inmind: this is an option to achieve it!Combination of onboard autonomous processing and intersatellite links, although not feasiblein an optimal filter, is the most interesting option for autonomous integrity monitoring of thefuture Galileo system. And last but not least: The two ephemeris models developed in theframe of this work are a perfect match for the need of the Galileo system in terms offlexibility and accuracy.8.3 AchievementsSoftware ConAn (Constellation Analyser)In the frame of this Ph.D. thesis, the theory of orbit determination and orbit computation hasbeen reviewed and a new approach for precise orbit and ephemeris determination using intersatellite links has been developed. To investigate the achievable accuracy, the elaboratedmodels have been coded in a complex software package allowing system level performanceanalysis as well as detailed evaluation of orbit computation and orbit estimation algorithms. Itincludes several gravity models, precise planetary ephemeris (JPL DE200, see [STA-90]) andorbit estimation in real time using Kalman filtering as well as conventional batch processingof measurements. The simulations, which are a cornerstone of this Ph.D. thesis have all beenconducted using ConAn, as well as the comparison and visualisation of results.Development of two new ephemeris modelsThe broadcast ephemeris model of both today's existing satellite navigation systems, GPS andGLONASS have been investigated. It has been shown that superior performance of the GPSmodel is mainly due to the number of parameters, or simply spoken, due to the degrees offreedom provided by the model. Especially for short periods of validity, i.e. much shorter thanR. Wolf Page 191
Page 1 and 2:
Satellite Orbit and EphemerisDeterm
Page 3 and 4:
Inter Satellite LinksAbstractGlobal
Page 5 and 6:
Inter Satellite LinksTable of Conte
Page 7 and 8:
Inter Satellite Links6.2.2 IGSO WAL
Page 9 and 10:
Inter Satellite LinksList of Figure
Page 11 and 12:
Inter Satellite LinksFigure 6-38 Tr
Page 13 and 14:
Inter Satellite LinksList of Tables
Page 15 and 16:
Inter Satellite LinksJ2Earths Oblat
Page 17 and 18:
Inter Satellite LinksIntroduction1
Page 19 and 20:
Inter Satellite LinksIntroductionGE
Page 21 and 22:
Inter Satellite LinksISL Observatio
Page 23 and 24:
Inter Satellite LinksState Estimati
Page 26 and 27:
State EstimationInter Satellite Lin
Page 28 and 29:
Page 30 and 31:
Page 32 and 33:
Page 34 and 35:
Page 36 and 37:
Orbit ComputationInter Satellite Li
Page 38 and 39:
Page 40 and 41:
Page 42 and 43:
Page 44 and 45:
Page 46 and 47:
Page 48 and 49:
Page 50 and 51:
Page 52 and 53:
Page 54 and 55:
Page 56 and 57:
Page 58 and 59:
Page 60 and 61:
Page 62 and 63:
Page 64 and 65:
Page 66 and 67:
Page 68 and 69:
Page 70 and 71:
Page 72 and 73:
Page 74 and 75:
Page 76 and 77:
Page 78 and 79:
Page 80 and 81:
Page 82 and 83:
Page 84 and 85:
Page 86 and 87:
Software DescriptionInter Satellite
Page 88 and 89:
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Simulations and ResultsInter Satell
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Page 130 and 131:
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141:
Page 142 and 143:
Page 144 and 145:
Page 146 and 147:
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
Page 156 and 157: Simulations and ResultsInter Satell
Page 168 and 169: Autonomous Onboard ProcessingInter
Page 208 and 209: ReferencesInter Satellite Linkshalf
Page 210: ReferencesInter Satellite LinksSan
show all

Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Create successful ePaper yourself

Delete template?

Save as template?