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.

GPS and ionospheric scintillations

“… Forte [2005] and Beach [2006] demonstrate that failure to adequately consider irregularity dynamics may lead to false conclusions. Given the various effects of phase fluctuations as GPS receiver tracking (see section 3.3 and 6) and the significance of ionospheric gradients on WAAS integrity [ Klobuchar et al , 1995], a better or multiple definitions of σϕ are called for, both to separate the refractive and diffractive components and to be relevant for the user community.…”

Impact and mitigation of ionospheric anomalies on ground‐based augmentation of GNSS

“…Prior to 2002, it was believed that, during unusual ionospheric activity, ionospheric spatial gradients would not be more than 5–10 times greater than the value of 4 mm/km that was derived as a conservative one‐sigma bound on nominal zenith ionospheric spatial gradients during “active” ionospheric conditions at solar maximum [ Lee et al , 2007]. At the very worst, it was thought that known ionospheric anomalies, including ionospheric storms and the potential impacts of scintillation in equatorial and auroral regions, could not produce a spatial gradient larger than roughly 50–75 mm/km [e.g., see Klobuchar et al , 1995], which would not be a significant threat to LAAS users. However, WAAS data analysis of gradients that occurred in the northeastern quadrant of the United States during the 6−7 April 2000 ionospheric storm showed gradients perhaps as large as 320 mm/km moving in a pattern similar to that of a tropospheric weather front and with a varying propagation speed [ Datta‐Barua et al , 2002].…”

Statistical analysis on spatial correlation of ionospheric day-to-day variability by using GPS and Incoherent Scatter Radar observations

“…Perfectly correlated data will have correlation coefficients equal to 1, dropping to 0 for no correlation between the data, and -1 for perfect anti-correlation. The horizontal correlation of ionospheric climatology has been studied by several researchers using ionosonde, GPS, and TOPEX observations (Bust et al, 2001;Gail et al, 1993;Huang, 1983;Klobuchar et al, 1995;Nisbet et al, 1981;Rush, 1976;Saito, 1978). But most of these papers investigated the horizontal correlation of the mid-latitude area.…”