GIS Development in Romania

Global Positioning System (GPS)

The Global Positioning System (GPS) is a technology using a constellation of 24+ satellites in low-earth orbit to provide precise, all weather, 24-hour, three dimensional positioning information anywhere on the earth. GPS is maintained by the U.S. Air Force and, although it was originally developed for the U.S. Military and its NATO allies, GPS is freely available to anyone with a receiver.

Civilian use of GPS has escalated in recent years as the cost of GPS receivers has decreased and new applications have been developed. These include numerous applications in fields such as navigation, surveying and resource management.

The U.S. Department of Defense has imposed accuracy restrictions on the civilian use of GPS by incorporating deliberate errors into the GPS satellite broadcasts: this is known as "Selective Availability" (SA). The system currently provides accuracy to the civilian user of about 100 metres or better. This positional accuracy can be improved by a number of techniques.

Post processing

The 100 metre accuracy quoted above is for a single measurement taken with a GPS receiver. If a sequence of measurements is taken over a period of time and averaged, the locational accuracy can be improved to approximately10 metres.

Differential GPS

If a GPS receiver is operated within approximately 200Km of a known reference point (a point whose coordinates are accurately recorded), then it is possible to measure the relative positions to approximately 3 metres. Thus , it is possible to calculate the measured position to within 3 metres +/- the accuracy of the reference point.

This accuracy can be increased to better than 1 metre if the radio carrier phases are compared.

Dual frequency operation

In addition to the above techniques, it is possible to use dual frequency operation - to receive the GPS signals simultaneously on two separate frequencies. This technique reduces the errors which derive from ionospheric distortion.

Dual frequency operation is applicable to all three types of measurement: typically, the accuracy can be increased by a factor of 5 for this mode of operation.

So, providing a number of reference points are known, sub-metre accuracy is readily available to the civilian population, even in the presence of Selective Availability

For further information, please contact Geo Strategies at:

	Geo Strategies Ltd		Geo Strategies SA
	St John's Innovation Centre		Str G-ral V. Milea, 10A
	Cowley Road		Sibiu 550331
	Cambridge CB4 0WS		Romania
	United Kingdom

	Tel:	+44 (0)1223 205080	Tel:	+40 (0)269 210832
	Fax:	+44 (0)1223 205081	Fax:	+40 (0)269 211165
	Email:	maps@geo-strategies.com	Email:	maps@geo.strategies.ro