Wide Area Augmentation System - How It Works
Unlike traditional ground-based navigation aids, the WAAS covers nearly all of the National Airspace System (NAS). The WAAS provides augmentation information to GPS receivers to enhance the accuracy and reliability of position estimates.
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The signals from GPS satellites are received across the NAS at many widely-spaced Wide Area Reference Stations (WRS) sites. The WRS locations are precisely surveyed so that any errors in the received GPS signals can be detected.
The GPS information collected by the WRS sites is forwarded to the WAAS Master Station (WMS) via a terrestrial communications network. At the WMS, the WAAS augmentation messages are generated. These messages contain information that allows GPS receivers to remove errors in the GPS signal, allowing for a significant increase in location accuracy and reliability.
The augmentation messages are sent from the WMS to uplink stations to be transmitted to navigation payloads on Geostationary communications satellites.
The navigation payloads broadcast the augmentation messages on a GPS-like signal. The GPS/WAAS receiver processes the WAAS augmentation message as part of estimating position. The GPS-like signal from the navigation transponder can also be used by the receiver as an additional source for calculation of the users position.
WAAS also provides indications to GPS/WAAS receivers of where the GPS system is unusable due to system errors or other effects. Further, the WAAS system was designed to the strictest of safety standards users are notified within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate.