5.1 DGPS
Learning Objectives
On the successful completion of this chapter you should be able to:
- explain what a differential technique is
- explain what the different methods of DGPS are
- explain the difference between real time and post processed corrections
- explain the difference between DGPS and point positioning
- explain the errors associated with DGPS and how to minimise them
- debate the need for, and explain what constitutes redundant or check observations with DGPS.
In the beginning…
Not satisfied with one GPS receiver, some surveyor somewhere figured out that using two receivers could help cancel out some of the errors that impact point positioning.
Enter differential GNSS, which has stubbornly hung onto the name differential GPS or DGPS in the majority of cases.
DGPS uses two GNSS receivers using code observable to reduce the errors that impact point positioning, and can be used in real time, or the correction process can be saved for later when back in the office.
If one GNSS receiver is good, two MUST be better right?!
Note: There are other differential techniques, like RTK, that rely on the carrier phase component of the GNSS signal. These are not covered in this module, they are covered in Chapter 6.
DGPS
Point positioning provides a quick and cheap method for getting autonomous positions, however, the errors that limit its accuracy, limit its applications outside of navigation.
Unless you have more than one GNSS receiver.
Differential GPS, most commonly known as DGPS, is a technique that uses code observable to make point positioning more accurate, by using two GNSS receivers in different locations. DGPS has a variety of applications in asset management, basic data collection and improved navigation.
DGPS utilises two GNSS receivers in different locations, but same area, to model errors and apply corrections to observations. One receiver is set up over a mark (usually a permanent survey mark) that has a known coordinate – this receiver is called the base station, as it stays in place for the entire time observations are being undertaken by the second receiver. The second receiver is called the rover, as it is moving around taking the GNSS observations.
By observing a point of known coordinates with a base station, we are able to determine the difference between the known coordinates and the collected coordinates. Essentially a base station tells us the difference between where a point position says it is, and where it knows it is. This difference is then used as a basic model of the errors for that area, and this model can be applied as corrections to the data collected by other GNSS receivers in the same area at the same time.
The errors that impact an area aren’t random, and can generally be modelled as relatively smooth movements over time. Any time a GNSS receiver is collecting positioning data, it is creating a position and a time record for a point, and by matching the time of a correction to a rover measurement, we can