The term DGPS is misused quite a bit in the UAV industry. DGPS, which stands for differential GPS, refers to any system that provides corrections to a GPS receiver based on a second GPS receiver at a known location. The GPS receiver providing the corrections is called the “reference station” and the GPS receiver using the corrections is the “rover.” Because the reference station knows it’s location it can calculate the amount of error in the signal from each satellite. The errors are transmitted to the rover which uses them to improve the accuracy of its position fix.
Any SBAS (Satellite-Based Augmentation System) such as WAAS, in the USA, or EGNOS In Europe are a form of DGPS. The accuracy improvement provided by these systems is modest. These GNSS receivers still have several meters error in position even after the corrections are applied.
RTK GNSS systems (also called carrier phase) are quite different. An RTK GNSS system uses the phase of the radio signal broadcast by the satellite when it is received to calculate a much more accurate position fix than traditional GPS receivers. An RTK GNSS receiver can calculate a position fix to an accuracy of a centimeter or so while a traditional GPS receiver has an accuracy of a couple meters.
One drawback of an RTK GNSS receiver is that it must have corrections from a reference station. This is due to the fact that the phase angle the GNSS receiver measures repeats at multiple locations spaced one wavelength of the carrier frequency apart. The receiver needs the corrections from the reference station in order to figure out which location corresponds to the receiver’s position.
As RTK GNSS receivers must have corrections in order to operate, often the term DGPS is used instead of RTK or carrier phase. Since DGPS refers to a wide range of GNSS receivers, and not just RTK GNSS receivers, it can lead to confusion; so, it’s best to be aware that DGPS is often used when the term RTK is more appropriate.