Differential GPS (DGPS) Explained
To make GPS even more accurate, the U.S. Coast Guard created a radio beacon system based upon differential GPS techniques. This system is a series of more than 60 ground-based stations that transmit a correction or differential signal on marine radio beacon frequencies. This signal can make a GPS receiver accurate from 1-3 meters by applying additional 'corrections' to the existing GPS signal.
What effects the signal?
There are hosts of natural and man-made interferences to the GPS signal. Basically, anything that will not allow the receiver to have a clear view of the sky will block the signal. This includes buildings, trees, mountains and the body of your car. This is why the receiver needs to be outside for optimum reception. The signal can be effected by water vapor in the upper atmosphere and even by the earth's own sounds.
 How it works
The GPS receiver will receive the signal and calculate your position with the standard method. At the same time, the ground station receives the same signal at a known spot. The difference between what is received and the actual known spot is calculated and broadcast as the DGPS signal. The properly equipped GPS receiver will make the necessary calculations to correct itself. All this takes place in seconds.
Where is this available?
Currently the DGPS signal is available on the Coasts of the U.S., Canada and Australia and around the Great Lakes. These reference stations do broadcast to a large area that extends far inland. To find out if it is available in your are click on one of these links: US Coverage, Canadian Coverage, Australian Coverage
 How can I receive the signal?
Most GPS receivers, including the handheld models, will accept the DGPS signal from a DGPS receivers like the Garmin GBR 23. The receiver must be set to the DGPS input mode and connected with the proper cables. This includes Magellan and Lowrance GPS receivers.
References:
For more information about differential GPS, have a look at some of these web resources:
US Coast Guard
Canadian Coast Guard
Australian Maritime Safety Authority
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