Satellite Orbit and Ephemeris Determination using Inter Satellite Links

IntroductionInter Satellite Linksmaximum possible separation angle between two ground referenced observations. This leadsto a significant larger uncertainty in the off-radial components of the orbit, than in the radialcomponent. In the following figure the distances between satellites and earth, as well as theearth’s diameter are approximately drawn to scale.GEOGEOFigure 1-2 ISL Tracking Geometry for a GEO SatelliteAn ISL to another GEO satellite results in a much better observability of the tangential orbiterrors. As a result, the decorrelation of the clock error and the radial orbit error is enhancedand shortened. Another benefit from ISL’s is the improvement in satellite tracking capabilityfor satellites at low earth orbit (LEO). Usually a large scale ground network is required toprovide reasonable coverage of the complete LEO satellite orbit. The ground network of theDORIS system, for instance, consist of 51 ground beacons distributed over the entire world. Iffor example a GEO would be used to establish an ISL, the LEO satellite would remain inview to that satellite for more than one third of its orbit. The next figure indicates the trackinggeometry for a LEO / GEO inter satellite link.Page 2R. Wolf