Overview of GPS

 The Navstar GPS is a constellation of orbiting satellites that provides navigation data to military and civilian users all over the world. The system is operated and controlled by members of the 50th Space Wing located at Schriever Air Force Base (AFB), Colorado.

GPS provides 24-hour navigation services which includes:

    * Extremely accurate three-dimensional location information (latitude, longitude and altitude), velocity and precise time.
    * Passive all-weather operations.
    * Continuous real-time information.
    * Support to an unlimited number of users and areas.
    * Support to civilian users at a slightly less accurate level.

GPS satellites orbit the earth every 12 hours emitting continuous navigation signals. With the proper equipment, users can receive these signals to calculate time, location and velocity. The signals are so accurate, time can be figured to within a millionth of a second, velocity within a fraction of km per hour and location to within a few feet. Receivers have been developed for use in aircraft, ships and land vehicles as well as for hand carrying.

GPS promises to significantly enhance many of the functions being provided by current positioning and navigational equipment and will result in greater accuracy at lower cost. Such functions as mapping, aerial refueling and rendezvous, geodetic surveys, and search and rescue operations will benefit from GPS capabilities.

Such capabilities were put to the test during the US involvement in Operations Desert Shield and Storm. Allied troops relied heavily on GPS to navigate the featureless Saudi Arabian desert. Forward air controllers, pilots, tank drivers and even cooks used the system so successfully that several US defense officials cited GPS as a key to the Desert Storm victory.

The Delta II expendable launch vehicle is used to launch GPS satellites from Cape Canaveral Air Station, Florida, into nearly 11,000-mile circular orbits. While orbiting the earth, the systems transmit signals on two different L-band frequencies. Their design life is 7.5 years.

The GPS Master Control Station (MCS), operated by the 50th Space Wing's 2nd Space Operations Squadron at Schriever AFB, Colorado, is responsible for monitoring and controlling the GPS satellite constellation. The GPS-dedicated ground system consists of five monitor stations and four ground antennas located around the world. The monitor stations use GPS receivers to passively track the navigation signals on all satellites. Information from the monitor stations is then processed at the MCS and used to update the satellites' navigation messages.

The MCS crew sends updated navigation information to GPS satellites through ground antennas which are also used to transmit commands to satellites and to receive satellites' state-of-health data (telemetry).

A beginners guide to GPS