This paper is to present a new method to quantify the GPS orbital prediction errors based on the symmetry characteristics in the errors of the orbit from a forward integration versus those from a backward integration starting with the same initial condition (IC). As a reflection of the symmetry level, the correlation of >99% is observed between the orbital errors from forward and backward integrations. In the context that extended ephemeris (EE) at the age of multiple days is used in GNSS navigation, the proposed method is an ideal candidate to provide realistic User Range Errors (URE) for the EE as an quality indicator. The method also has the potential to be used for providing the URA for broadcast ephemeris in future GNSS.
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