An Overview of WAAS
The Wide Area Augmentation System (WAAS) is a system that provides for precise approach guidance to a runway environment, similar to an Instrument Landing System (ILS). Before WAAS, (ILS) availability was needed for a precision approach in low-visibility conditions. WAAS uses satellite-based navaids for precise lateral and vertical approach guidance—similar to ILS operations—without the need for ground-based navaids of any kind.
WAAS Origins
The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.
How WAAS Works
There are approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations create a GPS correction message from the collected data. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. One of two geostationary satellites then broadcast the corrected signal.
The Benefits of WAAS
North America is currently the only place where WAAS satellite coverage is available. GPS users in South America, for example, cannot use WAAS even though they can receive it. Since there are no ground reference stations in South America, the signal has not been corrected and thus would not improve the accuracy of their unit. Having the satellites positioned over the equator makes it difficult for some users in the U.S. WAAS signal reception is ideal for open land and marine applications. Unlike DGPS, WAAS does not require any additional equipment.
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