Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Simulations and ResultsInter Satellite Links6 SIMULATION AND RESULTSSeveral simulations have been performed to assess the achievable orbit determinationaccuracy, varying in• Types of orbits and constellation• Ground network• Observation types, i.e. ground based only or ground and intersatellite links.This chapter gives an overview of the analysed scenarios, as well as the results.6.1 Constellations, Ground Networks and Simulation Scenarios6.1.1 ConstellationsMost of the constellation are so called Walker constellations, characterised by three numbersT/P/Fwhere T Total number of satellitesP Number of orbit planesF Factor of pattern unit (PU = 360°/T), to obtain phase difference betweensatellites on adjacent orbit planesThe following equations hold two obtain the orbital parameters for each satellite.Satellite spacing:Orbit plane spacing:Phase difference between adjacent planes:360°⋅ PT360°P360°⋅ FTThe phase difference has to be interpreted the following way: Assuming a phase difference of30 ° and a satellite on one orbit plane is passing his ascending node (i.e. mean anomaly = 0°),the next satellite on the right hand adjacent plane is already ahead in mean anomaly by 30°.Theses Walker constellations are a good starting point for constellation analysis, because theyprovide reasonable earth coverage with direct computable satellite orbit parameters. Note thatthese constellations are reasonable, but not optimal for satellite navigation systems. Walkerconstellation have one inherent draw back, due to their symmetry. An optimised constellation,like the today's GPS constellation has more or less evolved from a 24/6/1 Walkerconstellation, but the satellites and orbit planes are not evenly spaced anymore.Page 92R. Wolf