Garmin GPSMAP 76CS - Manual (Page 104)

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GPSmap 76CS
Appendix
WAAS and DGPS
What is WAAS?
The Wide Area Augmentation System (WAAS) is an FAA funded service to improve the overall integrity of the GPS signal for users in
North America. WAAS is just one service provider that adheres to the MOPS (Minimum Operational Performance Standard) for global
Satellite Based Augmentation Systems (SBAS). Eventually there will be several services of worldwide geostationary communication satel-
lites that broadcast integrity information and differential correction data as transmitted by ground reference stations.
At this time WAAS is still in the developmental stage with a goal of providing reliable signals with an accuracy of 7 meters (21-22 feet)
both horizontally and vertically 95% of the time. Current tests have shown the actual accuracy to be on the order of 2-3 meters (7-10
feet). This accuracy is comparable to DGPS (as explained below), but without the need for a separate beacon receiver. Since all SBAS
systems use the same receiver frequency, any SBAS system should be capable of providing your GPS unit with increased accuracy at any
location in the world when those systems (EGNOS and MSAS for example) reach operational status.
In addition to satellites in orbit over the Earth, ground based reference stations must also be in place near the region you are located.
Enabling WAAS in regions that are not supported by ground stations, which correct position data, typically, does not improve accuracy
even when receiving signals from an SBAS satellite. In fact it can degrade the accuracy to less than that provided by GPS satellites alone.
For this reason, your Garmin GPS receiver, when in the “WAAS Enabled” mode, uses the method that achieves the best accuracy. To
enable/disable WAAS, refer to the System Setup Page of your unit. For more information, go to http://gps.faa.gov/Programs/WAAS/waas.htm.
What is Differential GPS (DGPS)?
The U.S. and Canadian governments (among others) have set up Differential GPS (DGPS) beacon stations along seacoasts and
inland waterways to transmit correction signals. The DGPS system is available for use without fee.
Refer to the U.S. Coast Guard’s web site: http://www.navcen.uscg.gov/ for locations and the status of the differential beacon sites.
Using DGPS requires interfacing of your unit to additional equipment in the form of a beacon receiver compatible with the RTCM
format sentences supported by your Garmin GPS. Refer to the section titled “Interface Setup.”
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