In 2 words, the GPS is a satellite positioning system, capable of giving n 'anywhere on the globe a position between a few hundred meters and centimeters, day or night. The visible part is a small electronic unit (GPS receiver) that regardless of the time and place, indicates the exact location, altitude, speed, time, and with it quickly and accurately .
The GPS system was originally a satellite navigation system, designed by the U.S. military DoD (Department of Defense), which controls and fully fund the project. Originally the GPS was designed to provide U.S. forces with a global tracking system and very good accuracy. Gps systems similar to the time (SATNAV, etc.). Had not the availability and security requisent for operational use in the armed forces. The specifications of the time imposed the GPS to provide an authorized user information accurate position, velocity and time, anytime and anywhere on the globe.
GPS: What's the point?
For us civilians, the GPS is used primarily to locate themselves in unfamiliar surroundings, know where north is, and how to reach a specific location if one knows its polar coordinates. But there are many other functions that I would explain later.
The NAVSTAR concept is the result of studies undertaken in 1965 by the Army Air and USN.
The dates that marked the realization:
1965: First concept of GPS
1972: Preliminary Feasibility Study of GPS.
1974 - 1979: Validation of the concept (the first shot in February 1978)
1979-1986: Assessment Development
1986-1994: Operational implementation phase interrupted between January 1986 and February 1989. The launches of satellites BLOCK II does in fact have begun in February 1989.
The signals from GPS satellites
Each satellite transmits two carrier frequencies simultaneously, one at 1575.42 MHz, the other at 1227.6 MHz. The signal is modulated by phase modulation. Each GPS satellite transmits a navigation message, containing all necessary data to the receiver to do all the navigation calculations. These data include:
Information to the health of the satellite
Information necessary for the acquisition of the message code
The accuracy information of satellite
Information concerning the propagation delay due to the ionosphere