WAAS (Wide Area Augmentation System) was developed by the Federal Aviation Administration to expand the Global Positioning System to ensure the accuracy of improving the integrity and availability. WAAS was originally designed to enable aircraft using GPS for all phases of flight, Including precision approach to the airport to leave within its coverage area. WAAS provides usually more than 1.0 m laterally and 1.5 meters vertically in most of the neighbors and the United Statesmost of Canada and Alaska. This accuracy is provideing with aircraft capable of precision approaches to safe flight and navigation in all weather conditions. WAAS integrity of the information is not more than three seconds of incorrect data for each year, the system allows for secure FAA Instrument Flight Rules.
Although originally developed for aviation, WAAS is not limited to air transport in the GPS receiver, which is to receive the WAASSignal will be able to take advantage of the data are correct, so That the more accurate GPS positioning. Indeed, a WAAS-enabled GPS receiver and can pave the road in the lane on the road by car (provided that the program is supported Maping "Lane Assist" directions). Why the Wide Area Augmentation System has quickly become the industry standard in GPS, most new GPS receiver today, WAAS-enabled. Just as with traditional GPS WAAS does not come with an extrause of cost or charges. All that is required is that GPS receivers are WAAS-enabled, so you can receive and decode the position fixes a date for the position is. Currently, WAAS service is only for USA, Canada, Alaska and Hawaii. Although independent of WAAS, Europe and Asia are working on their GPS systems additional correction. Europe 's Euro Geostationary Navigation Overlay Service (EGNOS) and Japan working on his Multi-Functional SatelliteAugmentation System (MSAS).
Since the WAAS Works
WAAS uses a network of 25 terrestrial wide-area reference stations (WRS) in North America and Hawaii, to measure small changes in GPS satellite signals in the Western Hemisphere. These ground stations monitor carefully examined and gather information about GPS and send their data to the three wide-area master stations (WMS). The WMS is to create two different types of corrections: fast, slow. The rapid bug fixes that are changing rapidly and is a priority for the current GPS satellite positions and clock errors. Corrections are independent of the user's location, which can be used immediately by a WAAS receiver in the transmission. The fixes are slow to an estimated long-term ephemeric clock error and ionospheric delay information.
After these corrections are generated, the master station sends two pairsforward from Ground uplink stations (GUS) that the messages of correction in a series of geostationary satellites, which broadcast the correction data to Earth. Then, WAAS-enabled GPS receivers use this information to the GPS fixes signial original thing WAAS-enabled GPS receiver position more precise. GPS receivers use the information transmitted from GPS satellites to determine their position and the current time. Depending on the GPS device, a GPS receiver needs onlya signal receiving 3-4 satellites (of the 31 satellites currently transmiting civilan a sign that users are able to) is a calculation of the position. In addition to the GPS signal, a WAAS-enabled GPS receiver, the signal from the WAAS geostationary satellite.
The two different types of messages from the WAAS correction system (fast and slow) are used by the GPS receiver in different ways. The fastest way to correct the correct data includes satellite tracking and timeto determine the dates of their current position using standard GPS calculations. Once an approximate location fix is obtained, the GPS receiver starts the slow corrections use to improve their accuracy. Slow-Contains the data of the ionosphere delay. If the GPS signal from satellite receiver to travel, it must pass through the ionosphere. The receiver calculates the position where the signal is pierced and the ionosphere, where she received an ionospheric delay value for this site, youcorrect the problem that created the ionosphere. In contrast to the data transfer speed is slow data needed to be updated frequently, not because the ionospheric Conditions are changing rapidly. During the slow data can be updated every minute, are only updated every two minutes and are valid for a maximum of six minutes.
Limitations of the WAAS
(1) WAAS is currently only for U.S. and most ofCanada and Alaska, but it plans to expand the system to other countries and continents.
(2) Since the WAAS satellites are geostationary broadcasting brings them ° N to less than 10 ° above the horizon for locations north of 71.4. This means that aircraft in the territories of Alaska and northern Canada may have difficulty maintaining a lock on the WAAS signal.
(3) To calculate the ionospheric delay grid point, this point mustSituated between a satellite and a reference station. The small number of satellites and ground stations, the number of points which can be calculated.
(4) conducting WAAS approaches must Aircraft GPS receivers are certified.
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