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WAAS Wide Area Augmentation System is a service of the FAA, similar to differential GPS (DGPS) used on inland and coastal waters. It can add enhanced accuracy to your position, but you must be in view of one of the satellites. They are in geosynchronous orbits over the equator. If using GPS for elevation on land, it is crucial to see one of these, otherwise the elevation data will be useless. A celestial navigator can calculate for you the elevation and bearing to them based on your Lat-Lon and the Lon of the satellites over the equator, or use the calculator in this article: WAAS and EGNOS: Satellite-Based (GPS) Augmentation Systems (SBAS) In Seattle area, you must have an unobstructed view to direction 160 to 180T at 35º above the horizon. In the ocean they should be visible. See footprints at the FAA WAAS site. See also the FAA WAAS general page The USCG NavCen has a lot on DGPS, but not many direct links to WAAS as it is a FAA/DOT program.

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