Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Inter Satellite LinksIntroduction1 INTRODUCTIONThe conventional way of precise orbit determination is to take pseudorange, Doppler or anglemeasurements of a satellite with respect to a fixed point on the ground, and apply differentialcorrections to a (more or less accurate) predicted reference orbit.A radio signal travelling from one satellite to another can also be used to derive the distancebetween these two space crafts. Although the distance is not measured between a satellite anda known point– like a ground station – but between two satellites, these measurements can beused to derive the satellite’s state vectors, i.e. their position and velocity at a given time.Although these measurements can not be used solely, i.e. with out any ground reference, theyprovide additional information. The following picture shows two satellites, which areconducting inter satellite measurements. At the same time, ranging stations on the ground takemeasurements from both satellites.Figure 1-1Principle of Inter Satellite MeasurementsThe ISL (inter satellite links) provides an observation with a geometry completely differentfrom those of the ground referenced links, as can be seen from the figure. This is an aadvantage especially for satellites at higher orbits. From a satellite in geostationary orbit, theearth is seen under a small angle of approximately 17°, which implies also the limit for theR. Wolf Page 1