<P> WAAS uses a network of ground - based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere . Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS - enabled GPS receivers use the corrections while computing their positions to improve accuracy . </P> <P> The International Civil Aviation Organization (ICAO) calls this type of system a satellite - based augmentation system (SBAS). Europe and Asia are developing their own SBASs, the Indian GPS Aided Geo Augmented Navigation (GAGAN), the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-functional Satellite Augmentation System (MSAS), respectively . Commercial systems include StarFire and OmniSTAR . </P> <P> The WAAS specification requires it to provide a position accuracy of 7.6 metres (25 ft) or less (for both lateral and vertical measurements), at least 95% of the time . Actual performance measurements of the system at specific locations have shown it typically provides better than 1.0 metre (3 ft 3 in) laterally and 1.5 metres (4 ft 11 in) vertically throughout most of the contiguous United States and large parts of Canada and Alaska . With these results, WAAS is capable of achieving the required Category I precision approach accuracy of 16 metres (52 ft) laterally and 4.0 metres (13.1 ft) vertically . </P> <P> Integrity of a navigation system includes the ability to provide timely warnings when its signal is providing misleading data that could potentially create hazards . The WAAS specification requires the system detect errors in the GPS or WAAS network and notify users within 6.2 seconds . Certifying that WAAS is safe for instrument flight rules (IFR) (i.e. flying in the clouds) requires proving there is only an extremely small probability that an error exceeding the requirements for accuracy will go undetected . Specifically, the probability is stated as 1 × 10, and is equivalent to no more than 3 seconds of bad data per year . This provides integrity information equivalent to or better than Receiver Autonomous Integrity Monitoring (RAIM). </P>

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