Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that uses DGPS Surveyors a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudo ranges and actual (internally computed) pseudo ranges, and receiver stations may correct their pseudo ranges by the same amount. The correction signal is typically broadcast over UHF radio modem.
DGPS Surveyor techniques are called the Ground Based Augmentation System and Ground based Regional Augmentation Systems. Both of these systems broadcast corrections via the aviation VHF band.