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How Differential GPS Works

 Differential Correction  removes signal errors caused by satellite and receiver timing errors, and atmospheric interference.   An uncorrected signal is accurate to 7-8 meters or or about 25-30 feet.  There are two differential correction methods: postprocessing (above) and real-time (below.)  The concept is the same.  A base station is surveyed to 1-3cm accuracy.  When the satellite signals reaches a base station,  the base station recognizes the difference between its accurate surveyed position and the error position sent by the satellites.  The correction is either saved to a hard disk for postprocessing or communicated real-time to the GPS receiver. 
.Real-time is valuable for navigation.  Postprocessing is more accurate.  Consider this information when purchasing a Trimble GPS receiver. 

There are over 200 Trimble base stations world-wide for postprocessing.  Navigation beacons and satellite systems are available for real-time differential correction.