Global Positioning System (GPS)
A satellite-based navigation system operated and maintained by the US Department of Defense. GPS consists of a constellation of 24 satellites providing worldwide, 24-hour, three dimensional (3D) coverage. Although originally conceived for military needs, GPS has a broad array of civilian applications including survey, marine, land, aviation, and vehicular navigation. As a satellite system, GPS is immune from the limitations of land-based systems such as Loran. By computing the distance to GPS satellites orbiting the earth, a GPS receiver can calculate an accurate position. A 2D position calculation requires three satellite ranges. A 3D position calculation, which additionally provides altitude, requires four satellite ranges. GPS receivers can also provide precise time, speed, and course data, which is beneficial for marine navigation.
See also Differential GPS.