The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.
How GPS works?
Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. GPS operations depend on a very accurate time reference, which is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board.
Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions.
There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite’s orbit.
Determining Position?
A GPS receiver “knows” the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also “knows” it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect.
How accurate is GPS?
GPS answers five questions simultaneously:
“Where am I?”
“Where am I going?”
“Where are you?”
“What’s the best way to get there?”
” When will I get there?”
GPS is the only system today that can show your exact position on the Earth anytime, in any weather, no matter where you are!
The accuracy of a position determined with GPS depends on the type of receiver. Most hand-held GPS units have about 10-20 meter accuracy. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby. Observations made by the stationary receiver are used to correct positions recorded by the roving units, producing an accuracy greater than 1 meter.
When the system was created, timing errors were inserted into GPS transmissions to limit the accuracy of non-military GPS receivers to about 100 meters. This part of GPS operations, called Selective Availability, was eliminated in May 2000.
GPS Development:
GPS was designed by the U. S. military. The concept started in the late ’60s but the first satellite wasn’t launched until February 1978. In 1989 the Magellan Corp. introduced the first hand-held GPS receiver. In 1992 GPS was used in Operation Desert Storm. On March 1996 the President decided to make GPS free for civilian users.
GPS System Description:
GPS has three segments:
The space segment now consists of 28 satellites, each in its own orbit about 11,000 nautical miles above the Earth.
The user segment consists of receivers, which you can hold in your hand or mount in your car.
The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.
segments of gps system
GPS for Civilian Use
At first, the military did not want to let civilians use GPS, fearing that smugglers, terrorists, or hostile forces would use it. Finally, bowing to pressure from the companies that built the equipment, The Defense Department made GPS available for non-military purposes, with some restrictions. On May 1, 2000, President Clinton lifted the restrictions, and announced that the option to degrade civil GPS signals during emergencies would be phased out by 2010. The federal government is committed to providing GPS technology for peaceful uses on a worldwide basis, free of charge.