The NAVSTAR User Segment is composed of GPS receivers and the GPS user community. This NAVSTAR segment uses broadcast data from the NAVSTAR Space Segment (see below) to determine precise position location. It does this by receiving precise timing and satellite position data from a minimum of four (4) NAVSTAR satellites and performing 3-D triangulation to determine ground location and altitude. There are literally thousands of GPS users; GPS hardware is available from numerous military and commercial sources. As a rule, elements of the NAVSTAR User Segment have no direct communications with either of the other two GPS segments.
The NAVSTAR Space Segment consists of a consellation of satellite vehicles (SVs) in 12 hr orbits. This constellation consists of a total of 24 SVs -- 21 active and 3 in-orbit spares. The NAVSTAR Space Segment provides broadcast data, consisting of accurate time signal and SV ephemeris data, that allows NAVSTAR users to determine their location on earth by 3-D triangulation. NAVSTAR SV orbits are arranged such that they pass within communications of one of the NAVSTAR Control Segment monitor and/or ground stations (discussed below) every 24 hours.
The NAVSTAR Control Segment controls and coordinates the operation of the NAVSTAR Space Segment. The NAVSTAR Control Segment consists of four major components: five (5) Monitor Stations, a Master Control Station, three (3) Ground Stations, and supporting communications.
The NAVSTAR Monitor Stations are used to accurately measure signals received from NAVSTAR SVs, preprocess these signals, and send the resulting data to the NAVSTAR Master Control Station. These Monitor Stations are located at Falcon AFB, Colorado; Hawaii; Ascension Island; Kwajalen Atoll; and Diego Garcia.
The NAVSTAR Master Control Station performs overall coordination and control of the NAVSTAR system. Data from Monitor Stations is used by the NAVSTAR Master Control Station to calculate required corrections to current SV ephemeris and clock data; the Master Control Station also maintains status on all SVs and performs mission planning functions.
NAVSTAR Ground Stations have the ability to communicate with NAVSTAR SVs. There are three NAVSTAR Ground Stations; they are colocated with the NAVSTAR Monitor Stations at Ascension, Kwajalen, and Diego Garcia. Correction information generated by the Master Control Station is transmitted to NAVSTAR SVs by the NAVSTAR Ground Stations.
Additional information on the NAVSTAR system may be found here, here, and here. Each of these links has good information on the NAVSTAR system, as well as numerous links to other GPS-related sites.