29th Annual P7.ec.ise Time and Time Interval (PTTI) Meeting Performance of the Kalman Filter of the Global Positioning System Operational Control Segment

We characterize the errors in the broadcast estimates of the Global Positioning System’s satellite ephemeris and clock states made by the Operational Control Segment’s (OCS) Kalman filter over the period when the system ephemeris Q’s were changed. This change was called the Ephemeris Enhancement Endeavor (EEE). We report on the accuracy, stability, and periodic variations of the GPS Kalman filter’s estimates of satellite clocks and ephemerides, particularly how they changed with EEE. Root-sum-squared (RSS) ephemeris error variations decreased by a factor of 1.7. The amplitude of the 2 cycldd component decreased by a factor of 2.2. While the 2 cycle/d periodic effects dropped from 0.65 to 0.49 of the RSS ephemeris error, they still seem to be a major component. Radial ephemeris errors appear to be negatively biased by about 1 m. This bias is unchanged by EEE. Error deviations are generally no more than SV clock stabilities- the Kalman filter Q’s are generally good. Clock estimak errors are worse than ephemeris errors- there may still be some room for improvement. Broadcast ephemeris errors add diurnal variations to the GPS time offset fiom the U. S. Naval Observatory Master Clock (USNO MC). GPS- USNO MC variatians for averaging times near 1 d are consistent with White PM, averaging down to 1.5 11s at 1 d with 48 pohtdd. GPS- USNO MC appears to have periodic variations with periodicity spread approximately over periods from 16 to 24 d and a total amplitude of up to 4.5 ns