[1] Ionospheric scintillation is a rapid change in the phase and/or amplitude of a radio signal as it passes through small-scale plasma density irregularities in the ionosphere. These scintillations not only can reduce the accuracy of GPS/Satellite-Based Augmentation System (SBAS) receiver pseudorange and carrier phase measurements but also can result in a complete loss of lock on a satellite. In a worst case scenario, loss of lock on enough satellites could result in lost positioning service. Scintillation has not had a major effect on midlatitude regions (e.g., the continental United States) since most severe scintillation occurs in a band approximately 20°on either side of the magnetic equator and to a lesser extent in the polar and auroral regions. Most scintillation occurs for a few hours after sunset during the peak years of the solar cycle. Typical delay locked loop/phase locked loop designs of GPS/SBAS receivers enable them to handle moderate amounts of scintillation. Consequently, any attempt to determine the effects of scintillation on GPS/SBAS must consider both predictions of scintillation activity in the ionosphere and the residual effect of this activity after processing by a receiver. This paper estimates the effects of scintillation on the availability of GPS and SBAS for L1 C/A and L2 semicodeless receivers. These effects are described in terms of loss of lock and degradation of accuracy and are related to different times, ionospheric conditions, and positions on the Earth. Sample results are presented using WAAS in the western hemisphere.

A scintillation signal model and a Global Positioning System (GPS)-Wide Area Augmentation System (WAAS) receiver model are developed. The scintillation signal model is based on a Nakagami-m distribution for intensity and a Gaussian distribution with zero mean for phase. The GPS-WAAS receiver model includes Link 1 (L1) GPS and WAAS carrier-and C/A-code-tracking loops, as well as semicodeless Link 2 (L2) carrier and Y-code tracking capabilities. The results show that noncoherent delay locked loops (DLLs) typically used for code tracking are very robust to both amplitude and phase scintillation. Carrier-phasetracking loops are much more susceptible to scintillation, and the signal-to-noise threshold for reliable carrier tracking is very dependent on the scintillation strength. Fortunately, it appears that the worst case scintillation encountered at midlatitudes, including the United States, does not significantly impact L1 carrier-tracking performance. Semicodeless tracking of the L2 carrier is shown to be very fragile. Even weak scintillation can cause loss of L2 carrier lock for low-elevation satellites. 1996; Aarons and Basu, 1994]. This effect could cause a receiver to "lose lock" on the ranging signals broadcast by Wide Area Augmentation System (WAAS) [Loh et al., 1995] geostationary or GPS satellites, potentially causing a short service outage for one or more aircraft [Pullen et al., 1998]. Scintillation occurs most frequently during the peak of the solar cycle. Scintillation may be severe in equatorial regions (geomagnetic equator + 15 ø) after sunset and, to a somewhat lesser extent, the polar and auroral regions. Scintillation typically has minimum impact in midlatitude regions, e.g., the conterminous United States (CONUS). The aviation community is interested in the answers to the following questions regarding scintillation: (1) For what percentage of time will GPS and WAAS receivers lose lock for one satellite, two satellites, etc., in each of the regions noted above? (2) What is the impact of scintillation on the availability of WAAS (and GPS in general) in the United States and worldwide? This paper will try to answer the first question. The second question will be answered in a future paper because it requires the incorporation of a scintillation model into a WAAS service volume model.

The Federal Aviation Administration (FAA) Satellite Program Office is developing a GPS Wide‐Area Augmentation System (WAAS) to support a precision approach capability down to or near the lowest Category I (CAT I) decision height (DH) of 200 ft. In one of the candidate architectures under development, a vector of corrections is sent to the user via geostationary communications satellites (e.g., Inmarsat). This correction vector includes components for ionospheric, clock, and ephemeris corrections. The purpose of this paper is to evaluate the performance of the grid‐based algorithms and other real‐time ionospheric algorithms that could be implemented at the ground ionospheric reference stations, as well as at the airborne receiver. Results show that all of the ionospheric algorithms used in this paper (grid‐based, least‐squares, and spherical harmonics) provide roughly equivalent performance. Based on an extensive data collection program, the error in estimating ionospheric delay is derived. An analysis of WAAS accuracy performance is also presented.

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.

[1] This paper describes a one-directional iterative technique which converts from slant Total Electron Content (TEC) to vertical TEC using information describing the current state of the ionosphere. The method combines the well-known Chapman function of electron density with a spherical harmonics representation of the peak density over a spherical surface. Several parameters either have restricted movement or are kept constant in order for the problem to remain manageable. This technique is compared to the standard thin shell model obliquity factor in order to assess the degree of improvement in accuracy and the conditions under which this occurs. The Parameterized Ionospheric Model (PIM) is used as a truth model.