Talking about GPS

Listen to BBC Radio Solent Interview with Graham Collins, Proprietor of ESL, talking about GPS

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What is Differential GPS ?

DGPS, Differential GPS is the use of differential correction data for the satellites in view, to remove the errors in the position measured by a GPS receiver due previously to SA (selective availability), and to other effects. The correction data is generated by a base station, which is a high quality GPS receiver with a good antenna sited at an accurately known location. The correction data is derived from the satellites pseudo range data and the base station's position. Effectively correcting the timing errors artificially introduced into the atomic clock data by SA, or atmospheric effects. The correction data is formatted in the industry standard RTCM SC-104 and is available on a continuous basis.

There are various ways that this data can reach the mobile or field users GPS receiver. Some services use an encrypted radio link, which means that you have to buy a special receiver and pay a subscription, usually annual, for use of the service.

During 1998 the UK Coastal beacon transmitters previously operated in this way were taken over by Trinity House and are now operated free of charge to users. These UK differential beacons now join the many other European Beacons offering a free differential service. All you need is a Differential ready GPS receiver and a beacon receiver to take advantage of the 5 metre accuracy that may be achieved.

How Accurate is it ?

Previously with SA on the error could be as much as 100 metres. Differential GPS would enable you to obtain fixes within 10 metres of the actual position. The errors that were left after applying corrections were truly random noise, so by averaging the corrected position as a static point, for about 3 - 5 minutes, the position obtained was usually within 3 metres of the actual position. Most GPS receivers have a waypoint averaging facility which lets you make these averaged survey type measurements. Now that SA is off the situation is different. We have recently run scatter plots, with and without differential corrections, at a previously surveyed point. We had an open sky view and were within 51Km of a UK DGPS Beacon. Conditions were good and we were using between 6 and 8 satellites in both the DGPS and GPS solutions. The receiver was a Garmin GPS 25 with a GBR-21 beacon receiver supplying the differential corrections. The GPS 25 is a 12 channel parallel engine representative of all the newer 12 channel Garmin GPS receivers. The following results were obtained:

Points were collected for about 20 minutes in each case. Without differential correction, (left hand image), the furthest excursion from the surveyed point was about 23 mtrs. The green dashed circle is 15 mtr radius from the surveyed point.

With differential corrections applied, (right hand image), the scatter is within a 5 metre radius, (inner green circle). Averaging some of these points, even for a few minutes gives a position not more than 1 or 2 metres from the actual surveyed position.


Typical GPS scatter (SA off)

Typical DGPS scatter (SA off)

Our conclusion is that for most navigation applications it is now unnecessary to use differential GPS. However for peace of mind in extreme conditions the use of differential GPS could still be an advantage and would give more reproducible results. For survey applications such at location of dive targets and accurate sonar datalogging, the use of differential is essential.