GPS utilises 24 medium Earth orbit satellites and is currently the only fully functional Global Navigation System (GNSS). Precise microwave signals are transmitted from the satellites, allowing a GPS receiver to determine its location, speed and direction.

The GPS reciever calculates its position by measuring the distance between itself and three or more GPS satellites. The distance to each satellite can be calculated by measuring the time delay between transmission and reception of each GPS microwave signal. This is possible beacause the signal travels at a known speed.

The reciever can compute its position using trilateration. Three spheres intersect at a point, there are 3 distances or ranges to resolve, latitude, longitude and height.

Receivers typically do not have very accurate clocks and therefore tracking four satellites is required in order to correct the receiver's clock error. Here there are four ranges to resolve, latitude, longitude, height and time.

Using just the receiver, accuracies are about +/- 10metres. The accuracy can be improved by using two or more receivers which will give ‘differential’ position (DGPS). It is possible to know the position of B in relation to A when the co-ordinates of A are known and the satellites are tracked simultaneously. In this situation, A would be called a reference station, or base station and B would be called a Rover. The data gathered at the base station can be broadcast by terrestrial radio to the rover and processed almost instantaneously. Thus positions and heights can be obtained in ‘real time’ - often known as Real Time Kinematic (RTK). It is possible to obtain millimetric accuracy using this technique. This brief explanation of GPS leaves out some of the more technical issues and solutions. For a more detailed description, please contact us.

Why use Merrett Survey Partnership?

Merrett Survey Partnership was one of the first land survey companies in the UK to acquire GPS. We have wide ranging experience in the application of this technology. For example, providing coastal monitoring services, ground control work, pipeline mapping, consultancy work and software solutions for transforming GPS data.

Merrett Survey Partnership has written consultancy GPS specifications for:-

600km pipeline survey - North Africa
PII Pipeline mapping work
Hydro electric projects - Myanmar / Pakistan
Kinematic survey - 29,400km for DPWH

Using an incorrect geoid model can result in large mistakes. High accuracy results are possible but only with great care. Spirit levelling measures heights relative to the Geoid (mean sea level). GPS measures heights relative to the ellipsoid (mathematical model approximating to the Earth’s shape). The geoid/ellipsoid separation must be determined and modelled to enable conversion of GPS (ellipsoid) heighting to geoid heights. (eg above OS datum).

It is important to use the correct ellipsoid so that positions and heights are accurate. The World Geodetic System (WGS84) is an ellipsoid which provides a best mean fit to the earth, however other elipsoids can be used such as the North American Vertical Datum (NAVD83), based on the GRS80 ellipsoid; an ellipsoid derived from satellite geodesy.

Merrett Survey Partnership also run a Geodesy Workshop, for more information, please contact us.