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WAAS (Wide Area Augmentation System) is a differential correction system for the GPS receivers that is available in North America. It is a project funded by the FAA and is intended to increase position accuracy and integrity for aircraft. There are approximately 40 base stations around North America, all of which monitor the GPS satellites. Correction data is sent to several data centers for processing, then sent to satellite uplink facilities, and then rebroadcast from several satellites.

As of 5/14/2011, WAAS correction data is being broadcast from the following satellites:
- PRN #133 at 98W (longitude is approximately the same as the middle of Texas)
- PRN #138 at 107.3W (longitude is approximately the same as the western tip of Texas)
- PRN #135 at 133W (over the Pacific ocean between Hawaii and California)

Note that all three of these satellites are geostationary, which means they sit directly above the equator and rotate around the earth once every 24 hours, thus they always appear at the same spot in the sky to us. Since they are above the equator, they are located in the southern part of the sky for those of us in North America.

Most GPS receivers will auto-select the best WAAS satellite to receive correction data from. However, if you need to specify which satellite to use, you will ideally want to use the satellite that is the highest in the sky for your location, which will result in the least amount of signal blockage from trees and buildings.
135 is the best option if you are West 120.15W (Reno, NV or Los Angeles, CA)
133 is the best option if you are East 102.65W (Rapid City, SD or Amarillo, TX)
138 is the best option if you are between the above locations.


Last updated: May 14, 2011

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