The Nature of Geographic Information

22. Differential Correction


Differential correction is a class of techniques for improving the accuracy of GPS positioning by comparing measurements taken by two or more receivers. Here's how it works:

The locations of two GPS receivers--one stationary, one mobile--are illustrated below in Figure 5.23.1. The stationary receiver (or "base station") continuously records its fixed position over a control point. The difference between the base station's actual location and its calculated location is a measure of the positioning error affecting that receiver at that location at each given moment. In this example, the base station is located about 25 kilometers from the mobile receiver (or "rover"). The operator of the mobile receiver moves from place to place. The operator might be recording addresses for an E-911 database, or trees damaged by gypsy moth infestations, or street lights maintained by a public works department.

Diagram showing base station and mobile receiver locations 25 km away
Figure 5.23.1 A GPS base station is fixed over a control point, while about 25 km away, a mobile GPS receiver is used to measure a series of positions.

Figure 5.23.2, below, shows positions calculated at the same instant (3:01 pm) by the base station (left) and the mobile receiver (right).

Actual and calculated positions of the base station (left) and mobile receiver (right). both are slightly off actual position
Figure 5.23.2 Actual and calculated positions of a base station and mobile receiver.

The base station calculates the correction needed to eliminate the error in the position calculated at that moment from GPS signals. The correction is later applied to the position calculated by the mobile receiver at the same instant. The corrected position is not perfectly accurate, because the kinds and magnitudes of errors affecting the two receivers are not identical, and because of the low frequency of the GPS timing code.

Using calculated error from the base station to correct the position of the mobile receiver. Points connected by a triangle
Figure 5.23.3 Error correction calculated at the base station is applied to the position calculated by the mobile receiver.
Photo of a GPS base station
Figure 5.23.4 GPS base station used for differential correction. Notice that the antenna is located directly above a control point monument.