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The GPS Network
The Global Positioning Network is a complex network costing over $12 billion to build. The network itself is composed of 3 basic components:
  1. Satellites
  2. Communications Stations
  3. Receivers.
Satellites
The Satellites are often referred to as the space segment of the Global Positioning System. The satellites are set up in orbit to send signals from space. These satellites are set up in a "constellation" of 24 and they are positioned to provide signal coverage over all of the earth. Each satellite orbits the earth in a period of 12 hours. There can often be more or less than 24 satellites as each satellite has a limited useful life and the Department of Defense replaces satellites with new ones periodically to maintain the functionality of the network. The orbital altitude of the satellites ensures that each satellite repeats the same track and crosses the same point roughly every 24 hours. There are 6 orbital planes, or areas in which a group of satellites orbit. Each orbital plane is inclined at about 50 degrees with respect to the equatorial plane, spaced 60 degrees apart, and contains 4 satellites. This network configuration ensures that between five and 8 satellites will be visible from any one point on the earth.

Communications Stations This portion of the network is composed of a system of communications or monitor stations around the world. In addition to each of the stations around the world, there is one "Master Control" facility located at Schriever Air Force Base in Colorado. Each station is equipped with sophisticated computer equipment that allows the station to measure signals from each satellite. These measurements are then used compute the exact orbital data and clock functions of the satellites. The "Master Control" facility uploads the orbital and clock data to each of the 24 satellites to ensure that they are all synchronized. The satellites then send this data to the GPS receivers as radio signals.

Receivers
GPS receivers are devices that can be installed in ships, vehicles, airplanes, or can he carried by individuals. GPS receivers convert the signals from the satellites into precise position, velocity, or time estimates. In order to compute this information accurately, signals from four satellites are required. Handheld receivers can be small enough and light enough to carry in a person's hand, pocket, or bag.

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