What is WAAS (Wide Area Augmentation System) ?
Mabe you have heard the term WAAS or seen it on packaging and ads for Garmin GPS navigation products, and maybe you even know that it stands for Wide Area Augmentation System. Okay, so what the heck does that mean? Basically, it is a system of satellites and ground stations that provide GPS signal corrections to yield a better position accuracy. How much better? How about an average of five times better!? A WAAS capable receiver can give you a position accuracy of better than three meters 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS.
The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision air 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 disturbances in the upper atmosphere (ionosphere), timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.
WAAS consists of about 25 ground stations positioned across the United States that monitor GPS satellite data. Two master stations located on the east and west coasts collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit, clock drift, and signal delays caused by the atmosphere and ionosphere. The corrected differential message is broadcast to one of two geostationary satellites (satellites with a fixed position over the equator). The information is compatible with the basic GPS signal structure, which means that any WAAS enabled GPS receiver can read and interpret the signal.
Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is best suited for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.
Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.
The Evolution of GPS Accuracy:
100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government imposed Selective Availability (SA) program.
15 meters: Typical GPS position accuracy without SA.
3-5 meters: Typical differential GPS (DGPS) position accuracy.
Less than 3 meters: Typical WAAS position accuracy.
List of Garmin WAAS enabled products:
The famous Garmin eTrex Legend !!
eTrex Legend C
eTrex Vista C
GPS 15H and GPS 15L
GPS 18 OEM
GPSMAP 168 Sounder
GPSMAP 188 Sounder
GPSMAP 238 Sounder