What is GPS?
Posted: 2006-10-30
Author: Michael Welsh
Manufacturer: N/A
Source: Geeks.com

The Global Positioning System, commonly known as GPS, is a network of satellites that determines a ground based user’s specific latitude, longitude, and altitude. GPS devices have become more integrated in our daily lives through navigation computers in cars to emergency locators in cellular phones. GPS can even keep the time accurate on watches and VCRs. Portable GPS receivers are becoming increasingly popular for young and old alike.

For the GPS to work there are twenty-four satellites in Earth’s orbit. Each satellite is spaced so that at least four can be used by any given receiver on the ground. The satellites each contain a computer, radio, and an atomic clock. Because it uses the frequency of atoms rather than quartz or gears to keep time, an atomic clock is more accurate than a normal clock. The satellites transmit radio signals to a receiver on the ground and the receiver calculates its position in relation to the satellites. A minimum of three satellites are needed to provide latitude and longitude information using a process known as triangulation. If a fourth satellite is in range, then the user’s altitude can also be determined. The more satellites that are available, the more accurate the receiver's calculation will be. GPS receivers come in a variety of technologies, from handheld devices to geological monitoring stations.

The History of GPS

Global positioning grew from the use of radar tracking at the end of World War II when radar stations were placed along the coastlines and inshore for ships and planes to orient themselves. But data and distance were limited and subject to weather and atmosphere interference. With the launch of the Sputnik man-made satellite program by the Soviet Union in the late 1950s, American scientists learned they could precisely locate the satellite by the frequency of its signal. The Doppler effect (define) of the signal grew stronger as the satellite approached and grew weaker as it flew away. They also realized that the opposite would be true and several satellites would be able to precisely locate items on the ground.

Through the 1960s, the first navigation systems went into orbit but were limited in function and accuracy. In 1978, the first of several NAVSTAR GPS (NAVigation Signal Timing And Ranging Global Positioning System) satellites was launched with military use as the primary purpose. NAVSTAR proved to be more accurate and resilient than radar and unlike radar, NAVSTAR did not rely on ground stations being nearby. It was not hampered by weather and was also harder for enemy units to jam the signals. The full constellation of satellites as we know them today was achieved in 1994.

Although originally designated for military use only, GPS was later opened for civilian use with restrictions on usage and accuracy.

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