WAAS (Wide Area Augmentation System) is a form of differential GPS (DGPS) giving enhanced position accuracy developed primarily for aeronautical navigation but usable by other users. WAAS was developed by the Federal Aviation Administration to augment the Global Positioning System to improve its accuracy, integrity, and availability. WAAS was originally intended to enable aircraft to rely on GPS for all phases of flight, including the precision approach to airport’s within its coverage area.
WAAS GPS
How The WAAS System Works
WAAS uses a network of approximately 25 ground based Wide-area Reference Stations (WRS) in North America and Hawaii, to measure small variations in GPS satellite signals in the western hemisphere. These precisely surveyed ground stations monitor and collect information on the GPS signals and send their data to the three Wide-area Master Stations (WMS). The WMS’s generate two different sets of corrections: fast and slow. The fast corrections are for errors that are changing rapidly and are a primary concern to the GPS satellites instantaneous positions and clock errors. These corrections are user position independent, which means they can be applied instantly by any receiver in the WAAS broadcasting area. The slow corrections are for long-term ephemeric and clock error estimates and ionospheric delay information.
Limitations of the WAAS System
(1) The WAAS system is currently only available to United States and large parts of Canada and Alaska but there plans to expand the system to other countries and continents.
(2) Because the WAAS broadcasting satellites are geostationary causes them to be less than 10° above the horizon for locations north of 71.4° latitude. This means aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal.
(3) In order to calculate an ionospheric grid point’s delay, that point must be located between a satellite and a reference station. The low number of satellites and ground stations limit the number of points which can be calculated.
(4) Aircraft conducting WAAS approaches must possess certified GPS receivers.
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