Global Positioning System devices can identify the location of an object anywhere on The Earth. They do this by transmitting an accurate position and time signal via wavelength. These devices could locate the route of your journey from the Cliffs of Moher in Clare to the GPO in Dublin, the location of a ship at sea or even the precise boundary of your home.
The 24 satellites on the 6 orbits at 20,000km above the Earth. These satellites are constantly moving on these planes. When a person is collecting GPS data a minimum of four satellites in their vicinity is required. But, by the time their hour survey is complete more than four satellites have helped obtain the GPS readings.
Satellites were initially used solely for military intent during The Cold War but they evolved to become used for civilian purposes.
When GPS devices are used to record the boundary of a school building or a new road, for example, the field surveyor needs to walk around the building while taking coordinates at least at every corner. Coordinates are x and y numbers that help us locate an area on a map. While the field surveyor is recording his coordinates various satellites come in and go out of range of the GPS device. The United States is committed to maintaining the availability of at least 24 operational GPS satellites, 95% of the time. These aid the recording of GPS coordinates so that field boundaries, roads, rivers and buildings can be noted and displayed on an OSi map.