Differential GPS (also known as DGPS) is an enhancement to Global Positioning System that provides improved location accuracy with the use of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the GPS (satellite) systems and the known fixed positions, says Wiki.
This supporting maritime system becomes more and more irrelevant with every new launch of high-end GPS satellites with its innovate equipment pack. The system founded in 1990 had worked with 40-70 meters of GPS accuracy while the newest satellite provides very precise locating up to 5-8 meters.
Last week, U.S. Coast Guard announced that its DGPS system will be scaled-back. This government service wants to reduce the number of receivers from actual 84 to 22 units. Now Coast Guard control their own ground sites and work with DGPS infrastructure belonged to the Department of Transportation and Army Corps of Engineering. All of this will be almost cut during next months.
Differential GPS systems are also used in Canada, Australia and some European countries for maritime navigation and land surveys.