The Global Positioning System (GPS) is a network of 24 Navstar satellite aorbiting Earth at 11,000 miles. Originally established by the U.S. Department of Defence (DOD) at a cost of about US$13 billion, access to GPS is free to all users, including those in other countries. The system�s positioning and timing data are used for a variety of applications, including air, land and sea navigation, vehicle and vessel tracking, surveying and mapping, and asset and natural resource management. With military accuracy restrictions partially lifted in March 1996 and fully lifted in May 2000, GPS can now pinpoint the location of objects as small as a penny anywhere on the earth�s surface.

GPS System Segments

    The Global Positioning System is comprised of three segments: satellite constellation, ground control/monitoring network and user receiving equipment. Formal GPS Joint Program Office (JPO) programmatic terms for these components are space, operational control and user equipment segments, respectively.

1. The satellite constellation contains the satellites in orbit that provide the ranging signals and data messages to the user equipment.
2. The operational control segment (OCS) tracks and maintains the satellites in space. The OCS monitors satellite health and signal integrity and maintains the orbital configuration of the satellites. Furthermore, the OCS updates the satellite clock corrections and ephemerides as well as numerous other parameters essential to determining user position, velocity, and time (PVT).
3. Lastly, the user receiver equipment performs the navigation, timing or other related functions (e.g. surveying).

Location of GPS ground stations

The two possible locations

GPS System Operation

    The basic idea behind GPS is to use satellites in space as reference points for locations on earth. With GPS, signals from the satellites arrive at the exact position of the user and are triangulated. This triangulation is the key behind accurate location determining and is achieved through several steps.