Gps - A Brief Overview
Filed under: Communications
The Global Positioning System (GPS) is a satellite-based navigation system owned and developed by the United States Department of Defense. It is officially named NAVSTAR (NAVigation Satellite Timing And Ranging) GPS. During the 1980s, the United States government made the GPS available for public use for free. It can be used from anywhere in the world and is operational 24 hours a day. The GPS consists of three segments: space, control and user.
The space segment includes a network of 24 GPS satellites that is managed by the United States Air Force. The satellites are spaced so that at least four satellites will be above the horizon from any point on Earth. These satellites are enough to determine the location of a GPS receiver anywhere on earth. They orbit the earth every 12 hours at an altitude of about 12,000 miles. They travel at speeds exceeding 7,000 miles an hour.
The control segment consists of a master control station, monitor stations and ground antennas. Monitor stations track the orbits of the satellites and this data is sent to the master control station. The master control station regularly uses the ground antennas to send updated orbital path information to every one of the satellites.
The user segment consists of the GPS receiver. The receiver has an antenna that allows it to acquire the signals sent from the satellites. A receiver can have between 12 and 20 channels. The number of channels indicates how many satellites it can monitor simultaneously.
Here’s how all these segments work together. Each GPS satellite contains a computer, atomic clocks and a radio. Each satellite constantly broadcasts its current time and current location. All of the satellites broadcast their information at the same time. A GPS receiver can calculate its exact position by determining the distance between itself and three or more satellites. The time between a signal transmission and reception will allow the receiver to determine the distance to each satellite. When the receiver gets the distance measurements from three or more satellites, the exact location of the receiver can be determined. This process is known as triangulation. If the receiver can display a map, its position can be shown on the map. The position can be determined within 10 to 100 meters. If you have special military-approved equipment, the position can be determined within 1 meter. In addition, the clock in a receiver is not as accurate as the atomic clocks in the satellites. This is why receivers track one or more additional satellites to correct any clock errors.
A GPS receiver can calculate its two-dimensional position (latitude and longitude), if it acquires signals from at least three GPS satellites. A GPS receiver can calculate its three-dimensional position (latitude, longitude and elevation), if it acquires signals from four or more GPS satellites. The receiver can not only determine its exact location; it can also determine speed, direction of travel, distance to destination, estimated arrival times and more.
If you want to ensure the highest accuracy, a GPS receiver should have a clear line of sight (unobstructed view) of the GPS satellites. This means that it should be used in the outdoors. A GPS receiver will not work indoors, underground or underwater. Buildings, overpasses, forested areas, or electronic interference can shield the receiver from a satellite. This can weaken the signal from the satellite and cause the receiver to give unreliable readings. Another problem that may occur is multipath interference. Multipath interference occurs when a signal from a satellite to a receiver is delayed because it was forced to bounce off of tall buildings or large rock surfaces.
Michael Russell
Your Independent guide to GPS
Tags: Global Positioning System, GPS
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