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What is WAAS?

The Wide Area Augmentation System (WAAS) is an F.A.A. funded 

project to improve the overall integrity of the GPS signal. It is a space-

based system that broadcasts integrity information and correction data as 

determined by ground reference stations (see DGPS below). At this time 

the system is still in the developmental stage with a goal of providing 

reliable signals with an accuracy of 7 meters (21-22 ft.) both horizon-

tally and vertically 95% of the time. Current tests have shown the actual 

accuracy to be on the order of 2-3 meters. For more information, go to 

What is Differential GPS (DGPS)?

The U.S. and Canadian governments (among others) have set up 

Differential GPS (DGPS) stations to transmit correction signals. They are 

operational in coastal areas and on many navigable river systems. The 

DGPS system is available for use without fee. 

In addition to compensating for errors in the position, these correc-

tions signals can also correct for deliberate degradation of the accuracy of 

GPS by the U.S. Department of Defense under their Selective Availability 

(SA) program.

Refer to the U.S. Coast Guard’s web site: http://www.navcen.uscg.

gov/ for locations and the status of the differential sites. Using DGPS 

requires additional equipment.