In relative positioning, also known as differential positioning, one tries to remove some of the sources of systematic error by taking into account measurements of these errors in a nearby stationary reference receiver that has an accurately known position. By using these systematic error findings for the reference receiver, the position of the target receiver of interest can be determined much more precisely.
In an optimal setting, the reference and target receiver experience identical conditions and are connected by a direct data link, allowing the target to receive correctional data from the reference. In practice, relative positioning allows reference and target receiver to be 70–200 km apart; they will experience essentially similar atmospheric signal error. Selective availability can also be addressed in this away.
For each satellite in view, the reference receiver will determine its pseudorange error. After all, its position is known to a high degree of accuracy, so it can solve any pseudorange equations to determine the error. Subsequently, the target receiver, having received the error characteristics will apply the correction for each of the satellite signals that it uses for positioning. In doing so, it can improve its accuracy to within 0.5–1 m on the basis of code measurements only.
With relative positioning techniques using carrier pahse measurements, a horizontal accuracy of 2 mm–2 cm can be achieved. This degree of accuracy makes it possible to measure tectonic plate movements, which can be as large as 10 cm per year for some locations on the planet.
The discussion above assumes we needed positioning information in real time, which called for a data link between reference and target receiver. But various uses of satellite-based positioning do not need real time data, making post-processing of the recorded positioning data suitable. If the target receiver records time and position accurately, correctional data can be used later to improve the accuracy of the originally recorded data.
Finally, mention should be made of the notion of inverted relative positioning. The principles are still the same as above, but with this technique the target receiver does not correct for satellite pseudorange error, but rather uses a data link to upload its positioning/timing information to a central repository, where the corrections are applied. This can be useful in cases where many target receivers are needed and budget does not allow them to be expensive.
Differential positioning