Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Simulations and ResultsInter Satellite Linksarcs. For example, if the broadcast model is fit over a complete MEO orbit, the GPS modelyields an errors smaller than the 15 DOF force model integration.The estimation of Keplerian elements bears some additional problems: if orbits withexcentricity of inclination near zero have to be repesented, additional contrains have to beintroduced to make the least squares estimation process converge.6.3.2 Orbit Determination and Propagation ErrorTo obtain useful broadcast ephemeris, the fit interval has to reside in the future. Therefore, thedetermined orbit has to be propagated from the last known position using a sophisticated forcemodel. The orbit determination process yields a position, which is accurate only to certaindegree. If propagated, it will slowly diverge from the true orbit. In the following example theorbit of a satellite from the Galileo Option 1 constellation has been determined using groundlinks only. The satellite position and velocity estimated by the real time Kalman filter yields arelatively noisy estimate of the satellite state vector with a 1 σ accuracy of around• 35 cm in the radial component• 1 meter in the along track component• 80 cm in the cross track componentNote, that the real error needs not to be as high as that. The accuracy is taken from thecovariance matrix of the filter and represents the internal confidence of the estimation. If thisraw estimate is propagated without further smoothing, the orbit errors increase with timerelatively fast, as depicted in the following figure.Page 138R. Wolf