The Wide Area Augmentation System (WAAS) will provide real‐time differential GPS corrections and integrity information for aircraft navigation use. The most stringent application of this system will be precision approach, which requires the use of differential ionospheric corrections. WAAS must incorporate information from reference stations to create an ionospheric correction map. More important, this map must contain confidence bounds describing the integrity of the corrections. The difficulty in generating these corrections is that the reference station measurements are not collocated with the aviation user measurements. This is not a problem for an undisturbed ionosphere over the conterminous United States (CONUS), as the ionosphere is nominally smoothly varying. However, a concern is that irregularities in the ionosphere will decrease the correlation between the ionosphere observed by the reference stations and that seen by the user. It is essential to detect when such irregularities may be present and increase the confidence bounds.

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.

Satellites belonging to the Global Navigation Satellite System (GNSS) emit radio signals that are used routinely to determine position. Over the past two decades, satellite-based augmentation systems (SBAS) have been developed to render GNSS position estimates safe and reliable for aircraft navigation. As a radio signal propagates through the ionosphere, it experiences delay due to the presence of charged particles along the signal ray path. Disturbances of the ionosphere can cause this delay to increase dramatically. Indeed, the ionosphere currently remains the largest source of positioning error for single-frequency GNSS users. From a GNSS perspective, the threat to positioning accuracy posed by an ionospheric disturbance depends upon its magnitude. This paper examines how the Wide Area Augmentation System (WAAS), the augmentation of the United States'

The Wide Area Augmentation System (WAAS) achieved Initial Operating Capability (IOC) in July 2003. At IOC, WAAS had 25 reference stations in the Conterminous United States, Hawaii, Alaska, and San Juan, Puerto Rico, two master stations, and four uplink stations supporting two narrowband L1 only Geostationary Earth Orbit (GEO) satellites. Today, WAAS has 38 reference stations including four in Canada and five in Mexico, three master stations, and six uplink stations supporting three wide band L1/L5 GEO satellites. In addition to the architectural expansion, the Federal Aviation Administration (FAA) continued to evolve and maintain the algorithms, hardware, and software. With WAAS now turning 15 years of age, this paper takes stock of these modifications and details the improved service and integrity that has occurred since 2003.

[1] We provide an enhanced model of the errors induced by deviations of ionospheric delays from those estimated by the planar model used by the GPS-based Wide Area Augmentation System (WAAS). To a first approximation the nominal ionospheric spatial decorrelation of vertical equivalent signal delays, s decorr nom , is roughly constant over the whole of the WAAS service volume. However, significant gains may be achieved by including a more detailed description of s decorr nom as a function of various metrics such as geomagnetic latitude, time of day, as well as the quality of the planar fit as characterized by the radius, relative centroid of the fit, and the density of delay data. We take the first step in the development of this more sophisticated model by determining which of these parameters is best suited for use as a metric for determining s decorr nom . This allows us to construct a first-order model of the ionospheric decorrelation which depends on the local density of ionospheric pierce points. Our preliminary study indicates that this first-order model will result in a better than 20% reduction in the values of broadcast grid ionospheric vertical errors (GIVEs) within the coterminous United States. We also observe a better than 50% reduction in trips of the ionospheric irregularity detector in the Alaska region, which will lead to significant improvements to continuity, although this comes at the cost of a roughly 20% increase in the median GIVE in the Alaska region.