50
Appendix
WAAS and DGPS
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
horizontally and vertically 95% of the time. Current tests have shown
the actual accuracy to be on the order of 2–3 meters. For more informa-
tion, 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) 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 a 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. Please 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.
