Differential Global Positioning System (DGPS)

                                           What is DGPS?

Differential Global Positioning System (DGPS) is a method of providing differential corrections to a Global Positioning System (GPS) receiver in order to improve the accuracy of the navigation solution. DGPS corrections originate from a reference station at a known location. The receivers in these reference stations can estimate errors in the GPS because, unlike the general population of GPS receivers, they have an accurate knowledge of their position. As a result of applying DGPS corrections, the horizontal accuracy of the system can be improved from 100m (95% of the time) to better than 10m (95% of the time).

More importantly, the reference stations provide integrity monitoring, warning users to disregard a satellite which is operating outside of specification. With DGPS, this warning happens within a few seconds of the satellite becoming 'unhealthy', compared to GPS warnings where some hours can elapse.

The most common form of DGPS used internationally for maritime navigation operates in the MF Radiobeacon band (285-325 kHz) and conforms to the RTCM Recommended Standards for Differential Navstar GPS Service for the transmission of data. This form of DGPS uses pseudorange corrections and range-rate corrections from a single reference station which has sufficient channels (typically 12) to track all satellites in view. Pseudoranges (distance measurements) are simultaneously measured to all satellites in view, and using the known (surveyed) position of the receiver's antenna and the positional (ephemeris) data from each satellite, the errors in the pseudoranges are calculated. These errors are converted to corrections and are broadcast to user receivers.

The users GPS receiver applies the corrections to the pseudoranges measured to each satellite used in its position calculation. The GPS receiver always applies the latest (ie. newest) corrections received.

Using this method, and depending on the user-to-reference station separation and the age of the corrections being applied, accuracies better than 10 metres (95%) are achievable.

As the user-to-reference station separation increases, the signal from the satellite to the user takes a different path through the atmosphere compared to the signal from the satellite to the reference station. Due to variations in the atmosphere, there is a different signal delay at the user receiver compared with the delay at the reference station.

The greater the user-to-reference station separation, the less the satellite ephemeris errors are corrected. This is because different line-of-sight vectors to the satellite from the reference station and from the user produce different perceived ephemeris errors.

As a result of the range decorrelation of the atmospheric and ephemeris errors, the further the user is from the reference station, the less accurate the navigational accuracy can be.