The idea behind differential GPS is to remove as much errors as possible from the range measurements

The idea behind differential GPS is to remove as much errors as possible from the range measurements by establishing these errors at a reference site. In its most simple setup, a GPS receiver is located at a well surveyed position and its (pseudo) range measurements are compared with the actual calculated range from this receiver to the SV s. The differences between measured ranges and calculated ranges at the reference receiver are applied as corrections to the ranges measured by other receiver(s) close by.