What is WAAS?

The Wide Area Augmentation System, or WAAS, is a system of ground stations and satellites that provide GPS signal corrections for improved positional accuracy. WAAS is being developed by the Federal Aviation Administration and the Department of Transportation, to provide precision flight plan positioning information for aviation. Although it has not yet been approved for aviation, WAAS is already finding wide use for boaters and recreational GPS users.

A Little History
The Global Positioning System (GPS) was built by the Department of Defense (DoD) and was originally intended for military applications. The inherent accuracy of the system is about 15 meters 95% of the time. Prior to May 2000, the DoD degraded the signals available to civilians, using Selective Availability (SA). SA reduced the accuracy of the system to 100 meters 95% of the time.
To improve the safety of navigation in close quarters with SA in place, the Coast Guard, another branch of the same government, created a system of differential beacons called differential GPS or DGPS. The beacons, which were built using existing radio beacon stations, send error correcting signals at around 300 kHz, that improve the accuracy to about 5 meters. To utilize DGPS, it is necessary to have a separate differential beacon receiver and antenna, or a GPS unit with differential beacon receiver built-in. WAAS is not dependent on a land-based system of beacons and operates on the same frequency as GPS, so that no additional receiver is needed.

How WAAS works
WAAS is based on a network of about 25 ground stations, which continually monitor GPS satellite data. These precisely surveyed stations receive the GPS signals and determine if any errors exist. Errors are relayed to a wide area master station, which calculates correction algorithms and assesses the integrity of the system. A correction message is then uplinked to a geostationary communications satellite, where it is rebroadcast on the same frequency as the GPS signals (1575.42MHz). This eliminates the need for a specialized receiver and allows the communications satellite to act as an additional navigation satellite. WAAS corrects for satellite orbit errors and clock drift, as well as signal delays caused by the atmosphere and the ionosphere, resulting in a positional accuracy of about 3 meters.

Should you upgrade to a WAAS capable receiver?
It depends. The WAAS accuracy of 3 meters is impressive when compared to the inherent accuracy of 15 meters for a non-WAAS GPS receiver. However, with SA disabled, we believe the actual accuracy is 8 meters 95% of the time. Do you really need to know your position to an accuracy less than the distance that it takes to turn your boat? It is possible that you do, if you are a dredge operator maneuvering with cables, or you need extreme accuracy for some research measurements, but most recreational users will find that 8 to 10 meters is accurate enough.

However, modern GPS receivers may have features which absolutely are a cause for upgrading. The software is improved with user customizable screens for ease of use. 12 channel receivers are standard, providing quicker first fixes and better reliability in areas with obstructions, such as forests and canyons. Displays are greatly improved, offering higher resolution and features like the TFT daylight readable color displays. There have also been several enhancements to cartography including faster redraws and Garmin's new BlueChart marine cartography. Most mapping receivers come standard with a built-in worldwide base map. So you see, there are several good reasons to upgrade, not just for the WAAS capability.



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