Differential GPS

Surveying and other work that demands a high level of precision use Differential GPS (DGPS) to increase the position accuracy of a GPS receiver. A stationary receiver measures GPS timing errors and broadcasts correction information to other GPS units that are capable of receiving the DGPS signals. Consumer GPS receivers that support DGPS require a separate beacon receiver that connects to the GPS unit. Consumers can receive DGPS signals from free or commercial sources.
Unless you’re doing survey or other specialized work, you really don’t need DGPS capabilities. For consumer use, the increased accuracy of DGPS has mostly been replaced with WAAS.

Coast Guard DGPS
DGPS signals are freely broadcast by a series of U.S. Coast Guard stations in the United States. Whether you can receive these Coast Guard broadcasts depends on your location.
For more information on DGPS, including coverage maps, pay a visit to
www.navcen.uscg.gov/dgps/coverage/default.htm.

Commercial DGPS
DGPS services are offered commercially for the surveying market. You can rent or purchase electronic and radio equipment for gathering precise location information in a relatively small area.