What Is DGPS?
What is GPS?
The Global Positioning System is a satellite radionavigation system deployed by the U.S. Department of Defense (DOD). At power on, a GPS receiver roughly determines which satellites are visible and where in the sky the’re located. The receiver tracks these satellites, and down loads precise satellite-position data from each one. Using range measurements to each satellite, the GPS receiver calculates position, time, and velocity. Because of intentional DOD signal degradation, civilian GPS horizontal accuracy is limited to about 100 m, with velocity of about 1 to 2 KT (95 percent). This intentional degradation is called Selective Availability (SA) and is a normal characteristic of civilian GPS.
What is DGPS ?
The U.S. Coast Guard has installed a nationwide system of DGPS base stations to null-out the intentional degradation effects of SA. Each base station is a high-accuracy GPS receiver placed at a precisely surveyed location. The base station then tracks all GPS satellites in view. As the base station knows its location, it calculates satellite range corrections to line up its known position with the position reported by the GPS satellites. These corrections are then broadcast to remote GPS receivers, which apply them to their satellite measurements. The end result: Northstar DGPS receivers provide horizontal accuracy of 3 meters and 0.1 KT (95 percent).