Abstract

NGA's New Software Improves the Accuracy of the GPS Ephemerides
Track: Defense and Intelligence
Authors: Clifton Minter

The National Geospatial-Intelligence Agency (NGA) has been the authoritative standard for GPS precise orbits and clocks. The precise ephemeris process serves several purposes:
• The station network, that provides data for the process, also defines the World Geodetic System 84 (WGS 84) reference frame.
• The NGA precise ephemeris serves as a quality check for the GPS broadcast ephemeris.
• The NGA precise ephemeris supports high-precision positioning and survey activities.

On 26 February 2012, the GPS Division at NGA officially replaced its orbit determination system. The previous package, the Orbit Mensuration and Navigation Improvement System (OMNIS), has been in service since an initial version was deployed in August 1987. The new package is the Estimation and Prediction of Orbits and Clocks to High Accuracy (EPOCHA), which significantly changes the NGA GPS ephemeris from a computational and operational standpoint. Results indicate that EPOCHA provides a 10-20% overall accuracy improvement.