Methodology and Case Studies of Signal-in-Space Error Calculation Top-down Meets Bottom-up

Signal in space (SIS) errors are a major error source for the Global Positioning System (GPS). They are defined as any errors related to satellite transmission, mainly satellite position and clock errors. A better understanding and characterization of the signal in space errors are essential for GPS integrity, because the SIS errors are a metric to determine satellite outages or failures. The statistics of the SIS errors are an important factor to monitor the system performance in terms of integrity. We present two methods to calculate SIS errors. One is called top-down, which is based on high data rate dual frequency measurements obtained from the Wide Area Augmentation System (WAAS) or the National Satellite Test Bed (NSTB) networks. The SIS errors of a satellite are obtained by stripping off all non-SIS errors from the total pseudo-range error. We apply our algorithm to L1/L2 measurements now as an intermediate step to migrate to L1/L5 measurements when L5 signals are available in the future. The other way of characterizing SIS errors is the bottom-up method, which builds up the SIS errors by summing the satellite position errors and satellite clock errors, etc. The satellite position and clock errors are calculated by differentiating broadcast and precise ephemerides obtained from the International GNSS Service (IGS) network and the National Geospatial Intelligence (NGA) network, respectively. The top-down and bottom-up methods well complement each other. In the second part of the paper, we apply the top-down and bottom-up methods to two actual satellite outages from 2007. The results show that the two methods match well no matter whether a satellite is faulted or not. The discrepancies of the two methods are currently within +/4 meters and are independent of the carrier smoothing filter length.