NAVSTAR GPS (NAVigation System with Time And Ranging Global Positioning System): is a satellite-based radio system providing 3-D position, navigation and time information to suitable equipped users, at global scale and in any meteorological condition.
Primarly designed for military purposes (since 1973), GPS consists on
satellites transmitting signals at two L-band frequencies,
at 20,200 km of altitude in circular orbits at 6 different orbital planes
with an inclination of 
degrees and a rotation period of half day:
At least 4 satellites are visible in any time and place...
GPS is primarily a navigation system, with this foundation:
The satellite transmits at two L-band frequencies ( 
GHz and
GHz) pseudorandom codes.
This allow to the user "observe" the apparent travel time or delay suffered by the signal between the transmitter j and the receiver (navigation model, other terms neglected):
Then we need at least 4 observations to solve by Least-Squares the
equation to estimate the receiver 3-D position (x,y,z) and the receiver
clock bias 
:
Geometric interpretation: intersection of "spheres"...
The Global Positioning System is usually understood as three segments:
SPACE segment (the satellites)
CONTROL segment (the 5 control stations)
USER segment (the receivers)