The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth. It was developed by the U.S. military and after a while, the U.S. government opened it up for the public.
The GPS receiver receives signals coming from 32 satellites orbiting the Earth and from those signals the GPS receiver can calculate its absolute position of Earth. These satellites circle the Earth, making two complete rotations everyday. There are only 24 satellites that are operational, the additional are meant for backup in case one of the 24 fails.
GPS is a system. It’s made up of three parts: satellites, ground stations, and receivers.
Satellites act like the stars in constellations—we know where they are supposed to be at any given time. The ground stations use radar to make sure they are actually where we think they are. A receiver, like you might find in your phone or in your car, is constantly listening for a signal from these satellites. The receiver figures out how far away they are from some of them. Once the receiver calculates its distance from four or more satellites, it knows exactly where you are. From miles up in space your location on the ground can be determined with incredible precision.
Aircraft, ships, submarines, trains, and the space shuttle all use GPS to navigate. Many people use GPS when driving cars. The GPS receiver plots the car's constantly-changing location on an electronic map. The map provides directions to the person's destination. Some hikers use GPS to find their way, especially when they are not on marked trails.
Other satellite navigation systems which are either in use or under development are:
GLONASS – Russia’s global navigation system. Works worldwide.
Galileo – A global system which is being developed by members of the European Union and other partner countries.
COMPASS – China’s global navigation system. Will be operational by the year 2020.
IRNSS – India’s regional navigation system, covering mainland India and the northern parts of the Indian Ocean.