Potential Ionospheric Limitations to GPS Wide-Area Augmentation System (WAAS)

Abstract

Navigation and positioning using the FAA's GPS Wide-Area Augmentation System (WAAS) with single-frequency receivers suffers potentially from the unknown spatial variability of ionospheric range delays (e.g., spatial gradients in ionospheric delays) between ground locations where dual-frequency measurements from GPS satellites are being made. By deploying a sufficient number of dual-frequency GPS code and/or codeless reference receivers, it is possible to correct for most of the ionospheric range delay in a given large region using WAAS real-time prediction algorithms. The statistics of differences in range delay over station separations from approximately 350 km to over 1,600 km are presented, using ionospheric data collected from a number of stations in North America. The results illustrate large ionospheric gradients during periods of high magnetic activity. Fortunately, these events are infrequent. For the midlatitudes, the accuracy goal is to keep ionospheric range delays to only a few meters for at least 99 percent of the time so that WAAS can be used in precision approaches. To illustrate the possibility of achieving this goal, statistics of estimation errors in ionospheric range delay using this data set are also presented.