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Forensic Files



Global Positioning System (GPS)


The Global Positioning System, or GPS, is a network of satellites encircling the globe that transmits signals accessible by an unlimited number of ground-based receivers. These signals can be used to aid in navigation and to pinpoint the position of an object—a car, bus, airplane, animal, person, etc.—with amazing accuracy. In fact, the GPS is so accurate and reliable it could one day replace all current navigational tools. Its applications have become so widespread that, in a short time, use of the GPS will be as commonplace as using a telephone. Additionally, second generation satellites and more advanced and less costly receivers have made the system easier to use and have further broadened the scope of potential applications.

Twenty-four satellites continuously circling the Earth in elliptical orbits comprise the Global Positioning System, also called NavStar (navigation system by timing and ranging). Four satellites are in each of six precisely defined and uniformly spaced orbits, at an altitude of 10,890 nautical miles above the Earth. They stay in these orbits due to the combined gravitational attraction of the sun, the Earth, and the moon. Each satellite circles the Earth every 11 hours and 58 minutes. The four satellites in each orbit are irregularly spaced to ensure continuous coverage and acceptable performance in case of satellite failure.

Signals emitted by the satellites and integrated by a receiver can be used to locate every point on Earth with an accuracy of about 8 meters, or 32 feet. The United States Department of Defense originally developed the GPS for military use. It envisioned the GPS as a worldwide, continuous, all-weather positioning system. With the launch of the first GPS satellites in 1978, some of the signals produced were already available for civilian use. Although the signals accessible to the military provide greater accuracy than those available to civilians, the number of civilian applications continues to increase. Many industries are taking advantage of the GPS, and it is in use by pilots and mariners for navigational purposes, by dispatchers, police, and fire and public rescue crews, and by surveyors, architects, planners, and engineers. Among the more recent applications of GPS is its use in automobiles for pinpointing and plotting a course to a particular destination.

Each GPS satellite carries four atomic clocks that keep precise time. Only one clock is in use at a given time, but all are available. These clocks are important because it allows the receiver to compute the time it takes for a signal to travel from the satellite to the receiver, known as the elapsed transit time. The receiver knows the location in space of each satellite within a few meters at any given time. With this information, the receiver can then determine the range from the user to the satellite. The range is equal to the elapsed transit time multiplied by the speed of light (186,000 miles/sec). An error of 1 billionth of a second is equal to about 1 foot. Using data from four or more satellites, the receiver determines lines of position from the satellites to the Earth and then calculates the point of intersection of those lines to pinpoint a location.

These orbiting atomic clocks provide precise information simultaneously and continuously that enable a user to determine an object's location, altitude, time, speed, heading, attitude, and distance. This information could be used in many ways. For example, a trucking company might use GPS data to monitor drivers' hours and route performance, and delivery services might use it to improve response times. Transit operations can reduce waiting times, farmers can improve land use, and developers can reduce excavation costs using GPS. In addition, new applications continue to emerge, such as using GPS to track endangered species.

For other techniques used in Forensic Files, click here.

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