DGPS
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Contents
Overview
Differential GPS (DGPS) is a technology used to correct the various inaccuracies in the GPS system, pushing its accuracy even farther. DGPS can yield measurements good to a couple of meters in moving applications and even better in stationary situations. That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes more than just a system for navigating boats and planes around the world. It becomes a universal measurement system capable of positioning things on a very precise scale.
DGPS radio beacon systems operate in many parts of the world. These stations—part of a large network that covers coastal areas, navigable rivers, and, more recently, inland agricultural areas—are used for marine navigation. However, these beacons have a range of a few hundred kilometers inland and can provide free real-time differential accuracy in the one-meter range, depending on the GPS receiver and the distance from the radio beacon.
DGPS in maritime equipment (input from ACR)
- For many maritime applications, basic GNSS accuracy (say 5 to 10m) is good enough (EPIRBs, AIS, DSC etc)
- The cost of adding DGPS to equipment often outweighs the benefits of having it
- SBAS may be a better option (if improved accuracy is needed) as you in effect get it for free in the GNSS Receiver, without the need for a secondary receiver
- Many maritime specifications don’t have an integrity requirement - if they do its usually just RAIM, and even then all you usually have to do is report if its working or not. Shipboard AIS is the only system where DGPS is required, but even then its only by receipt of a Message 17 from an AIS Base Station on shore, there is no requirement for a DGPS receiver as part of the AIS equipment and you still send position even if its poor (e.g. a satellite is giving poor data).
- DGPS and / or SBAS are important for some things at sea, like precise positioning, but they are much more useful on land, where a couple of metres error can make a lot of difference.
How it works
The underlying premise of DGPS is that any two receivers that are relatively close together will experience similar atmospheric errors. DGPS requires that a GPS receiver be set up on a precisely known location. This GPS receiver is the base or reference station. The base station receiver calculates its position based on satellite signals and compares this location to the known location. The difference is applied to the GPS data recorded by the second GPS receiver, which is known as the roving receiver. The corrected information can be applied to data from the roving receiver in real time in the field using radio signals or through postprocessing after data capture using special processing software.
Real-time DGPS
Real-time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal if the source is land based or via a satellite signal if it is satellite based and applied to the position it is calculating. As a result, the position displayed and logged to the data file of the roving GPS receiver is a differentially corrected position
Correction message format
RTCM Special Committee 104 (RTCM SC-104) format recommendations define the correction message format. Each correction message includes data about the station position and health, satellite constellation health, and the correction to be applied. Using real-time differential corrections allows navigation to within one to two meters of any location depending on the service and the GPS receiver.
