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What is GPS?

GPS is an acronym for Global Positioning System, a worldwide navigation and positioning system developed by the US Department of Defense for both military and civilian use. GPS can determine the location of an object on earth with an accuracy ranging from tens of meters down to millimeters, depending on the receiver and signal processing technology. This technology permits military, civilian and scientific users to obtain three-dimensional locations near the Earth's surface with unprecedented precision. GPS has many applications; a navigational tool in boating and hiking, while the military applications cover a wide range of operational, strategic and tactical disciplines.

The Global Positioning System consists of a network of 24 broadcasting satellites orbiting the earth at a height of more than 20,000km. GPS also consists of receivers on the ground, which listen to and interpret the transmissions of the satellites. Stations on the earth carefully monitor the orbit of each satellite, maintaining a highly accurate record of the satellites instantaneous position. The knowledge of the precise position of the satellites allows them to be used as reference points, from which GPS receivers on earth can determine their position. This technique of determining the position of an object is called ranging.

The concept of ranging is best illustrated by example. Consider one satellite that is a distance of 25,000 kilometers from a person holding a GPS receiver.

Then the person's position is known to be somewhere on a sphere 25,000 km in radius, centered on the satellite. However, the exact location of the person on that sphere is yet unknown. If, at the same time, the distance from the person to a second satellite can be discovered to be 20,000 km, then a second sphere of radius 20,000 km on which the person is positioned can be determined. Thus the person must be on the circle formed by the intersection of the two spheres of position. A third satellite provides yet a third sphere, which narrows down the location of the person to exactly two points.

Many varied uses for Global Positioning System exist. GPS is able to measure position very accurately, and positioning signals are available to users around the world at all times. These features account for the popularity of GPS with many diverse groups.

The Global Positioning System was designed for precise, real-time military positioning. The military utilizes GPS in land, marine, and airborne navigation. In addition, GPS satellites are equipped with sensors to monitor and detect the detonations of nuclear weapons. However, navigation is the main function of GPS, with uses in all branches of the military. Some examples are photo reconnaissance, low-level navigation, target acquisition, command and control, en route navigation, and missile guidance.

Although GPS was designed for military use, civilian use of the navigation technology has dramatically increased with the advent of affordable, portable GPS receivers and the ability to increase the accuracy of civilian GPS readings. A major use of GPS is for surveying and mapping, including land, marine, and airborne surveying, local and global deformation monitoring, and geodetic control. Applications in transportation and communication include automotive navigation aids, with an automated display of the vehicle position on an electronic map. This is particularly useful for emergency vehicles and search and rescue missions. Monitoring the location and movement of vehicles such as taxis, trucks, and boxcars can also be achieved using GPS. Recreational activities have also become a large market for low-cost, portable receivers. Boating, backpacking, biking, and horseback riding are a few of the activities whose participants use fairly inexpensive, relatively low accuracy (good to tens or hundreds of meters) GPS receivers