Validation of SGP4 and IS-GPS-200D Against GPS Precision Ephemerides

Many applications use the NORAD SGP4 orbital model for predicting satellite ephemerides and often these applications require knowledge of the errors associated with those predictions. Unfortunately, the SGP4 orbital data, in the form of two-line element (TLE) sets, does not provide any kind of accuracy information. Some approaches have been published which purport to estimate these errors by performing consistency or abutment checks, but they do not validate their assumptions or provide any validation by comparison to high-accuracy ephemerides. This paper will assess the suitability of these approaches by comparing SGP4 ephemerides to precision ephemerides available for the GPS constellation.