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.

is a Senior Associate with Zeta Associates Inc. and currently is working GPS receiver performance and system engineering issues for the FAA GNSS Program. He previously was president of Grass Roots Enterprises Inc. and worked for the U.S. Government. He received his B.S. in physics from Norwich University. Mr. Swen D. Ericson has been involved with GPS and WAAS system engineering for the FAA at Zeta Associates since 2003. Prior to joining Zeta, he was a navigation system engineer at MITRE CAASD and a geodesist at Sidney B. Bowne and Son, LLP. He has a B.S. in civil engineering from the University of Miami and an M.S. in civil engineering from Purdue University.

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