Real time differential GPS (DGPS)employs a second stationary GPS receiver at a precisely measured spot established using traditional survey methods.This receiver corrects or reduces errors in GPS signals. A DGPS station is able to do this becauseits computer already knows its precise locationand can easily determine the amount of error provided by GPS signals.In order for DGPS to work properly,both the user's receiver and the DGPS station receiver must be accessing the same satellite signals at the same time.