The Global Positioning System (GPS) consists of a constellation of 24 NAVSTAR satellites that provides accurate position coordinates worldwide. This system is owned, managed and controlled by the United States Department of Defense (DoD). GPS receivers use a sophisticated ranging technique—much like land surveyors—which measures the time delay for radio signals to reach the GPS receiver from orbiting NAVSTAR satellites. This time delay is converted into a distance measurement.

Due to national security reasons, the DoD degradates the signal by introducing errors. This intentional scrambling of the GPS signal is called selective availability. As a result, accuracy is limited if uncorrected. By using a GPS Base Station, whose geographic location is precisely known, corrections to the measured coordinates can be made. This technique is referred to as differential GPS (dGPS). When these corrections are made in the field they are called real-time corrections, while those made subsequently upon returning to the office are referred to as post-processing corrections.