Satellite Orbit and Ephemeris Determination using Inter Satellite Links

Simulations and ResultsInter Satellite Links0,060,05URE [m]0,040,030,020,010,002450997,0 2450997,2 2450997,4 2450997,6 2450997,8Julian Date [Days]Figure 6-49 URE with 12 hours smoothing using ISL's6.3.3 Ephemeris Accuracy of ScenariosIn the following section, the achievable accuracy of broadcast ephemeris has beeninvestigated for the different scenarios. To get comparable results, the 15 degree of freedomGLONASS type ephemeris model has been used for all constellations. However, the fitinterval has been adopted to the different orbit types to obtain a “useful” accuracy in terms ofURE. The following table indicates the fit intervals chosen for the different orbit classes:Orbit ClassLEO 1250 kmMEOGEO / IGSOFit Interval15 Minutes2 hours4 hoursInput to the simulations had been the reference orbits and estimated orbits from section 6.2.One has to keep in mind that the propagation of the raw estimate bears some danger: theposition error will not be the same for all “starting position”, because the real error is notconstant but shows a noise / random walk behaviour within the 3 sigma margin (see section6.2). Therefore some of the raw estimates had to be replaced by an artificial introduced smalloffset to obtain the results. However, the numbers derived are representative.Page 148R. Wolf