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GPS Tracking System

Short for Global Positioning System, a worldwide MEO satellite navigational system formed by 24 satellites orbiting the earth and their corresponding receivers on the earth. The satellites orbit the earth at approximately 12,000 miles above the surface and make two complete orbits every 24 hours.

The GPS satellites continuously transmit digital radio signals that contain data on the satellites location and the exact time to the earth-bound receivers. The satellites are equipped with atomic clocks that are precise to within a billionth of a second. Based on this information the receivers know how long it takes for the signal to reach the receiver on earth.

As each signal travels at the speed of light, the longer it takes the receiver to get the signal, the farther away the satellite is. By knowing how far away a satellite is, the receiver knows that it is located somewhere on the surface of an imaginary sphere centered at the satellite.

By using three satellites, GPS can calculate the longitude and latitude of the receiver based on where the three spheres intersect. By using four satellites, GPS can also determine altitude.

GPS was developed and is operated by the U.S. Department of Defense. It was originally called NAVSTAR (Navigation System with Timing and Ranging). Before its civilian applications, GPS was used to provide all-weather round-the-clock navigation capabilities for military ground, sea, and air forces.

GPS has applications beyond navigation and location determination. GPS can be used for cartography, forestry, mineral exploration, wildlife habitation management, monitoring the movement of people and things and bringing precise timing to the world.

History Attempting to locate where you are or to determine the best way to reach a destination can be a particularly challenging task - especially if you are unfamiliar with the area and the terrain is rough.

A good map can help, however, it can even be difficult to determine where you currently are on the map. GPS is a technology that is ideally suited to both navigation and positioning.

GPS technology originated in the US military. The Defense Department recognized the need to have a precise positioning technology to locate soldiers, vehicles, enemies, and supplies on a battlefield and monitor their movements.

The Global Positioning System was developed in response to fulfill this important military need and included a satellite network, ground communications stations, and receivers that cost over $12 billion to build.

On March 29, 1996, President Clinton a Presidential Decision Directive (PDD) that changed the categorisation of GPS into an international information utility. The Presidential directive included the following relevant points:

1. The U.S. government will continue to operate, maintain and provide basic GPS signals worldwide, free of direct user fees.

2. The U.S. will advocate the acceptance of GPS and it's augmentations as a standard for use by initiating international discussions in agreement with Japan and Europe.

Currently the Global Positioning system is operated by the military, but is used by both military and commercial users.

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

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.

2: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.