IGS Combination of Precise GPS Satellite Ephemerides and Clocks

Abstract

Since January 1, 1994, the Geodetic Survey Division (GSD) of Natural Resources Canada (formerly Energy Mines and Resources, EMR) has been combining and comparing the GPS satellite ephemerides and clock corrections produced by the seven Analysis Centres contributing to the International GPS Service for Geodynamics (IGS). The IGS ephemeris/clock combination is produced weekly and provides information on the internal and external consistency between results of the IGS Analysis Centres. 7-day orbital fits characterize the internal consistency whereas comparisons between properly re-aligned daily satellite orbit solutions provide the external validation; the statistics are summarized in weekly reports which, along with the combined orbits and the corresponding Earth Orientation Parameters (EOP), are available electronically from the IGS Global Data Centres. Improved weighting, editing and handling of multiple reference clock resets produced significantly improved clock combinations. Steady improvements of results by all Analysis Centres and other improvements of combination techniques have resulted in accuracies approaching 10 cm for IGS combined orbits and 1 ns for combined satellite clocks under Anti-Spoofing (AS).