Your WAAS Questions Answered

WAAS Coverage Maps
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Q:  What is WAAS?  http://gps.faa.gov/
  A:  WAAS stands for Wide Area Augmentation System. It is a network of ground reference stations located across the U.S. that monitors GPS satellite data. There are master stations and reference stations. The master stations collect information from the reference stations and, through a series of algorithms, correct the information received to account for standard GPS errors. The corrected message, now known as a differential message, is then broadcast through two Geostationary Earth Orbit (GEO) satellites. The correction is transmitted on the same frequency as the GPS signal itself.

Q:  How is WAAS correction different from the Coast Guard's maritime beacon system correction?
  A:  Beacon DGPS and WAAS DGPS operate on the same theory. Basically, the correction is provided when a monitoring station calculates the difference between where it knows it is and the place the GPS indicates it is. In beacon DGPS, this correction is broadcast on low frequencies from the Coast Guard beacon station itself. In WAAS DGPS, the correction is sent from the master stations to the satellites for broadcast on the same high frequency as the GPS signal.

Q:  What difference does it make if the DGPS correction is broadcast from a beacon tower or from a satellite?
  A:  The low frequency signals broadcast from the ground-based beacon stations are more susceptible to radio frequency and weather interference, and they have a limited range. The WAAS signal, since it is high frequency and satellite-based, is less susceptible to this kind of interference and will provide 3 meter Lat/Lon position accuracy over a large contiguous geographic area.