GPS Accuracy Versus Number of NIMA Stations

The current GPS ground system uses five worldwide Air Force monitor stations to collect ranging data for GPS satellit e clock and ephemeris estimation. Current upgrades call for the inclusion of data from six core National Imagery and Mapping Agency (NIMA) stations in ground processing as part of the Accuracy Improvement Initiative/Architecture Evolution Plan (AII /AEP). Previous analysis showed that these core NIMA stations will improve GPS accuracy by about 10% for typical broadcast users and improve filter performance (at zero-ageof-data) by as much as 40%. This paper addresses the potential benefits of adding five more NIMA stations beyond the six core stations to the GPS ground system. The analysis shows that filter performance will improve up to 20% more due to the five additional NIMA stations. The typical broadcast user will initiall y gain only about 3% additional GPS accuracy improvement since broadcast accuracy is currently driven by clock prediction error. Improved earth orientation parameters, better satellit e clocks, and reduced navigation message age-of-data all enhance GPS performance. The benefit of the five additional NIMA stations to the broadcast user approaches 15% if the navigation message update capabili ty is implemented. Adding NIMA stations also improves satellit e-monitoring capabili ty that is critical for timely, robust integrity determination. Since the AII /AEP software is already designed to handle up to 20 stations, the use of five more NIMA stations requires only the addition of dedicated communication lines, so significant accuracy and integrity improvements can be achieved at relatively low cost. INTRODUCTION The current GPS monitor system consists of five Air Force stations. GPS satellite tracking data from these stations is sent to the Master Control Station (MCS) at Colorado Springs. The MCS processes the ranging measurements in a Kalman filter every 15 minutes to determine satell ite ephemeris and clock corrections. Periodically, about once per day for each satellite, the MCS predicts the ephemeris and clock and forms a navigation message that is sent to the satellite for transmission to the user on the GPS signal. The primary factors that affect GPS signalin-space (SIS) user range error (URE) performance are the stabili ty of the satellite atomic clocks, the number and distribution of monitor stations, and the frequency of navigation message uploads. GPS has demonstrated a dramatic reduction in SIS error over the past decade (Figure 1) due to the establishment of a full constellation, better clocks on Block IIR satellit es, reduction of contingency upload thresholds, and enhanced Kalman filter tuning at the MCS. Current constellation SIS URE performance of about 1.5 meters root-mean-square (RMS) is four times better than the 1990 Systems Operational Requirements Document (SORD) requirement of 6 meters [1] and nearly meets the 2000 Operational Requirements Document (ORD) requirement of 1.25 meters [2].