It is well known that ionospheric scintillation has the potential to affect all types of GPS receivers, even dual‐frequency military precise‐positioning service versions. In a previous effort the degree of degradation to Wide Area Augmentation System (WAAS) operation caused by scintillation, on the basis of simulated data input to an actual WAAS reference receiver under carefully controlled laboratory conditions at Space and Naval Warfare Systems Center, was analyzed and reported [Morrissey et al., 2000]. This degradation is manifested in increased errors for carrier phase and code range measurements and in a higher probability of loss of GPS signal track. However, the results supported the assessment that scintillation should not be a problem for WAAS receivers in the conterminous United States, except perhaps during the very rare occurrence of a “severe geomagnetic storm.” The previous work was briefed in a number of fora, and a detailed report was widely distributed throughout the ionospheric community along with a request for identification of any “gaps” in the results that could be addressed with further testing. From the feedback received, the following tests were conducted: (1) tests with long‐duration deep amplitude nulls, corresponding to the GPS signal moving with the ionospheric disturbance; (2) tests with phase scintillation waveforms derived from 50 Hz ionospheric scintillation monitor (ISM) data previously collected by the U.S. Air Force (Philips Laboratory) at Antofagosta, Chile; (3) tests of a modified military single‐frequency receiver (Enhanced Miniaturized Airborne GPS Receiver (EMAGR)) side‐by‐side with the WAAS receiver, with emphasis on maintaining lock at L1; and (4) tests at values of input carrier‐to‐noise ratio (CNR) lower (i.e., down to 36 dB Hz) than those used in the original tests. The tests with deep amplitude nulls were reported by Morrissey et al. [2002], and the tests with realistic input phase scintillation waveforms were reported at the Ionospheric Effects Symposium (IES) 2002. The current paper is based upon the IES 2002 results as modified by recent testing. Both these amplitude and phase results include EMAGR and low CNR performance.

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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.

Multipath is the primary range error constraining the accuracy of differential GPS systems. This has been an important motivating factor in the development of receiver and antenna technologies to mitigate multipath errors. This paper describes the results of a study of GPS receiver multipath error envelopes, measured employing a high-resolution technique for use with laboratory-environment GPS signal simulators, and their ( ) consistency with multipath errors observed at the Federal Aviation Administration's FAA Wide Area Augmenta-( ) tion System WAAS reference sites. Using the WAAS reference receiver, the effects of receiver bandwidth, correlator spacing, very short or extended multipath delays, autocorrelation function sidelobes, and multiple specular reflections were explored. Test results showed good agreement among theoretical, experimental, and field-observed multipath error estimates, and illustrated the characteristics and impact of the variety of multipath mitigating receiver technologies employed in the WAAS receiver. The results are widely applicable to GPS receiver designs in general, and to other differential systems, such as the FAA's Local Area Augmentation System. Results confirmed that measurements from the WAAS reference sites are dominated by close-in multipath and multiple reflections. New multipath mitigation techniques, such as ultra-narrow correlator spacing and refined receiver algorithms, coupled with new antenna designs, have the potential for further reducing multipath errors at differential reference sites.