Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Simulations and ResultsInter Satellite Links6.3 Accuracy of Broadcast Ephemeris (User Ephemeris)The ephemeris of a navigation satellite, which is broadcast to the user is derived in severalsteps:t0 – t1t1 - t2Observation and Processing:From observation, the error with respect to a reference trajectory isdetermined. This is done either by real time estimation (Kalman filtering) orin batch process. The result is a time series of satellite positions, as well assome estimated physical model parameters.Propagation and Adjustment:The satellite trajectory is propagated ahead in time, from t1 up to t2 using thebest estimate of the satellites state vector as well as the best available forcemodel. Due to limitations in the accuracy of determining the state vector, aswell as the model parameters, the position of the satellite will diverge fromthe true position with time.A simple orbit propagation model will be adjusted to this propagatedtrajectory. These are the broadcast or user ephemeris.Therefore, quality of the broadcast ephemeris is driven by multiple factors:• Model fitting error: even is the model is fitted on the (in reality unknown) true trajectory,it will have an error due to its simplicity.• Orbit determination error: even if the satellite trajectory is propagated using a perfectforce model, the initial position and velocity will not be perfect, due to limitations in theorbit determination process.• Orbit propagation error: even if the initial state (position / velocity) of the satellite wouldhave been known perfectly, the imperfection of the force model will cause the propagatedtrajectory to diverge slowly from the true one.6.3.1 Model Fitting ErrorIn the following simulations, several candidates for broadcast ephemeris have been evaluatedby fitting them over a specified orbit arc. The orbit class has been varied from about 1250 kmorbit altitude (LEO) up to 36000 km (GEO). The error has been derived by comparing theposition derived from the broadcast model with that derived from a high order force modelintegration.Four different broadcast ephemeris models have been evaluated. All four models aredescribed in chapter 4.• The GLONASS model using 9 degrees of freedom• GLONASS type force model integration model using 12 degrees of freedomPage 136R. Wolf