An Introduction to Differential GPS (DGPS) or Differential GNSS (DGNSS)
In order to establish a position, GNSS receivers use timing signals from at least four satellites, and any number of errors or delays can occur during the signals’ transit to earth. Differential Global Positioning System (DGPS), now called Differential Global Navigation Satellite System (DGNSS), is an enhancement to GNSS that was developed to correct these errors and inaccuracies in the GNSS system, allowing for more accurate positioning information. In general, access to this correction information makes differential GPS and GNSS receivers much more accurate than other receivers; with these errors removed, a GNSS receiver has the potential to achieve accuracies of up to 10 centimeters.
The hypothesis on which this system is based is that any two receivers within close proximity to one another (e.g. within a few hundred kilometers) will experience the same atmospheric errors. For this reason, differential GPS or GNSS makes use of at least two GNSS receivers. One receiver must be located in a precise, known location; this receiver is used as the base, or reference station, and the other is known as the roving receiver. The base receiver calculates the difference between its position as calculated by GNSS satellites, and its actual, known position. The difference is an error correction factor, which is then transmitted to the roving receiver (or multiple roving receivers) in order to correct its measurements. The corrected information can be applied in real-time in the field using radio signals, or through post-processing after data capture using special processing software.
Differential GPS or GNSS can use a network of fixed, ground-based reference stations to send the difference between the positions broadcasted by the GNSS satellites and the known fixed positions. DGPS/DGNSS can refer to any type of Ground Based Augmentation System (GBAS). There are many operational ground systems in use throughout the world.
A similar system that transmits corrections from orbiting satellites instead of ground-based transmitters is called WAAS (Wide Area Augmentation System or Wide-Area DGPS (WADGPS). Sometimes used synonymously, the Satellite Based Augmentation system (SBAS) can include orbiting satellite systems that have been implemented in other parts of the world such as EGNOS, MSAS, QZSS, and GAGAN.
Today, most commercial GNSS receivers support a form of differential corrections with SBAS.