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Your WAAS Questions Answered

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Below are some frequently asked questions regarding the new Wide Area Augmentation System (WAAS).

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.


Q:  Why do I even need differential correction (DGPS) of any kind since selective availability (SA) has been turned off?
GPS_SA_Event_QAs.pdf
  A:  The government disabled SA in May of 2000, but DGPS systems will continue to provide improved accuracy for standard GPS. Although GPS without SA has a horizontal accuracy of approximately 20 meters, systems augmented with differential correction have a horizontal accuracy of 3 meters, which results in safer navigation. Additionally, DGPS systems offer warnings of interruptions to the service to users, whereas uncorrected GPS systems do not.

Q:  Why should I choose a unit with WAAS instead of beacon differential?
  A:  WAAS provides a more extensive coverage area and is less susceptible to weather, signal interference and range limitations.  For example, in the Bahamas and other areas at the fringes of beacon differential  coverage, WAAS will provide better positioning.
 

Q:  Will I be able to receive WAAS in my area? 
  A:  Our testing shows that WAAS is received reliably for navigation purposes along the east coast from Maine to the Bahamas and on the west coast from Mexico to northern California. See the map for the total coverage area. Soon a Pacific satellite will also broadcast the WAAS correction, enabling the Pacific Northwest and Alaska to have full WAAS coverage.

Q:  I've heard that WAAS is not yet operational and that reliability is not good enough. Is that true?   A:  The FAA has declared that WAAS is available for VFR aviation and other applications such as boating, recreation, agriculture and surveying. WAAS continues to be developed to provide the necessary integrity for safety-critical aircraft landing applications, and until then WAAS is not approved by the FAA for aircraft IFR navigation.

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