GPS And How It Works

by Smithsonian Institution
GPS And How It Works

GPS stands for Global Positioning System, a satellite navigation system with twenty-four satellites in orbit. These satellites were put in place by the United States Department of Defense for military uses, and were called NAVSTAR. The first satellite was launched in 1978 and the twenty-fourth and last satellite was finally in place in 1994. In 1980 NAVSTAR was made available to the general public for commercial use.GPS works 24 hours a day in any weather. The satellites orbit the earth twice a day in a specific orbit that is about 12,000 miles above us. In orbit, the satellites travel as fast as 7,000 miles an hour. As they are orbiting, they transmit information to receivers on earth. The receivers use this information to calculate the user's location. This calculation is made by determining the difference between the time a transmission was made and when the receiver received it. This is then used to calculate the distance and the position is displayed on the receiver.For a receiver to calculate a latitude and longitude position is to receive information from three satellites. To calculate latitude, longitude and altitude a receiver must be able to receive information from four or more satellites. After position is known the GPS can then tell the user information about speed, trip distance, the distance to a desired destination, sunrise and sunset times, bearing and other information.While in orbit, the satellites are powered by solar energy. They also have backup batteries that are used in the event of no solar power such as an eclipse. The energy is used to power small rockets on the satellites that keep them in the proper orbit. At any one time only about 50 watts of power or less is used to transmit information. The satellites are designed to last about ten years, and the U.S. Department of Defense is constantly making and launching replacement satellites. Each satellite is about 2,000 pounds and seventeen feet across when the solar panels are out.GPS receivers are generally accurate within 15 meters. Other than investing in a receiver there are no fees or other equipment required to access the GPS signal. If very accurate readings are needed, Differential Global Position Systems (DGPS) will provide accuracy within three to five meters. The United States Coast Guard operates the most popular DGPS.Two power signals are transmitted and are referred to as L1 and L2. The L1 frequency is used for civilian purposes. These signals are relatively low power signals and travel by line of sight, so they can go through clouds, glass, and plastic, but not solid objects like buildings or mountains. In every transmission the satellite sends three types of information, its pseudorandom code, ephemeris data and almanac data. The pseudorandom code is an I.S. code that identifies which satellite the information is being sent from. Ephemeris data tells the receiver where the satellite should be at any time of the day, and almanac data sends information

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