What the hell is GPS?
Global Positioning System
(In some places call: Global Positioning Satellite)
or Castilian: Global Positioning System.
Above our heads (about 20,000 km) there are 24 satellites orbiting the earth, each of these carry an atomic clock and are mutually synchronized.
Using Radio Frequency , they emit a signal to the world (heh, sounds crazy, but is literal, "everyone"), something like a ping (" broadcasteado "), the GPS receivers are as the Garmin , other brands, and my beloved Nokia N95 , only receive satellite signals that are "visible" (clear sky), with 4 satellites and can begin to calculate the location.
Navstar, one of the satellites
As you do this?
It measures the time it takes me to the ping (calculated at the speed of light, but some kind of error) of each satellite, but when each of these, this type of method is called triangulation , which said incidentally, also applies to cell phones, with a margin of error much more clear.
With these calculations determines: Latitude, Longitude and Height.
Constellation of satellites
Then the service is free?
Yes!, And you got no fucking no dependence on the telcos 
Because the GPS device is a mere receiver and this is responsible for the calculations.
The system owns the U.S. defense offered it freely to the civilian world, (they also have another system but military) on the other hand the European Union is doing its own system called " Galileo "was launched in 2015.
'll Tell you and show you how it works with my N95
chup.
Official website: http://www.gps.gov/
Links: http://es.wikipedia.org/wiki/Sistema_de_posicionamiento_global
the gps galileo of American union is already running and is only for civilian use, the Russian republic of glonnas also is working and military use.