10.6.5 Global positioning system (GPS):
A GPS is a radio navigation, timing and positioning system based on a constellation of 24 satellites in orbit around the earth at altitudes of approximately 20000 km. These satellites emit continuous electromagnetic waves coded on two frequencies (L1 = 1.2 GHz and L2 = 1.5 GHz). If the positions of the satellites on their orbits are precisely known and if the antenna collects at least four satellites, the receiver can solve by trilateration the three unknown factors (longitude, latitude, and height, or X, Y, Z coordinates) defining its position.
For deformation monitoring, the GPS can be used in two different modes. The first method involves high precision static methods such as Continuously Operating Reference Systems (CORS) that are used to monitor regional scale deformations such as crustal dynamics, subsidence and geotechnical movements. These continuous systems are normally combined to form permanent networks. The second class of GPS technique is the use of episodic GPS data commonly used for monitoring on a smaller scale (with baselines up to a few kilometres). The use of the episodic technique commonly includes the monitoring of dams, open-pit mine walls and landslides. The primary technical differences between the two GPS monitoring classes are the permanency of the GPS receiver locations and the processing strategies employed to obtain deformation solutions.
GPS does not require direct line of sight between stations. The antennas, however, must have good sky visibility, to receive the satellite signals without interference. It can work regardless of weather conditions, and may be used with rain, mist or fog, strong sunshine, or at night. It can easily cover larger areas than conventional surveying methods, with high precision.