GPS uses underground sensing tubes which are buried 25 to 30 cms deep and 1 to 1.5 metres apart.
The tubes are filled with pressurised liquid and detect the differential change in pressure between them, caused by an intruders approach.
The main features of these types of system is that they are invisible and can follow the local terrain and are able to function under many surfaces, therefore difficult for an intruder to locate.
Physical barriers are not required and the system is not constrained by "Line of Sight"
Visit the GPS Perimeter Systems Ltd website for more information on GPS Plus