 WAAS Explained
Standard GPS methods produce a position accurate to about 10 meters (32.8 feet). The US Federal Aviation Administration desired to increase this accuracy and allow aviators to use GPS for more precise guidance. As a result, they developed a system similar to DGPS (Differential Global Positioning System) but on a much larger scale. This is the WAAS (Wide Area Augmentation System). Although still in its early roll-out stage, this system can improve a GPS receivers accuracy to about 3 Meters (9.8 feet) or less.
How it works.
As with DGPS, the WAAS system uses base stations at known reference points to calculate the accuracy of the GPS signal. This is accomplished at each of the 25 ground reference stations (Currently only in the US) receiving a standard GPS signal. A set of correction data deetermined from the difference between the GPS calculated position and the known position is transferred to one of two ground control stations that then uplink the data to the WAAS satellite. The WAAS (InMarSat) satellites then transmit this information back down to the GPS user using a GPS-like signal complete with the correction information. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.
 Where can it be used?
Currently the system is only accurate in North America and primarily in the United States. However, the signal can be received over half of the world on the Inmarsat AOR-W and POR satellites. This means that in parts of the world not covered by the base station corrections, you will get a WAAS signal, but the corrections will place you well off your mark in Australia, South America and Europe, for example. This is why it is best to turn off the WAAS reception outside North America.
The Future
Because the WAAS program is still in the early stages, there will be many changes to come. There are plans for many more base stations and control stations that will be located throughout North and South America. Additionally there are plans for similar systems to come on line in Europe (Euro Geostationary Navigation Overlay Service - EGNOS) and Japan (Multi-Functional Satellite Augmentation System - MSAS).
References:
For more information about WAAS, have a look at some of these web resources:
Garmin International
Federal Aviation Administration
Magellan GPS (Thales Navigation)
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