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GPS is making news all over the world and hence how it works is what everyone wants to know. The main basis of GPS is trilateration which is to know one's exact position based on the distance calculated. The Global Positioning System has several units which work together to bring the required effects. The most important component of GPS is the satellites. There are about 29 satellites at present, which are 20,000 km above earth's surface. They circle the earth twice daily. These satellites transmit signals in the microwave part of radio spectrum, on two different frequencies.
After the signals are transmitted, they are received by the GPS receivers. GPS receivers generate their own signals. After receiving the signals from the satellites, the receivers match it with the signals they have generated. When they match, the receivers are able to calculate the time taken by the signals to reach them from the satellites. Since light and radio waves travel at the same speed, it's easy to calculate distance by using the speed of light. After the receivers picks up signals from four or more satellites, they are able to determine the distance and location. This data helps the receivers to synchronize their time with the standard GPS time. They also calculate altitude, longitude and latitude based on this data. The GPS receivers receive signals passively. They do not bounce or transmit signals off the satellites, thus enabling the satellites to support a large number of both civilian and military users. This system is maintained by the US Air Force and is used by both the civilians and the military. The US Department of Defense runs these satellites. The signals transmitted are different for civilians and military. Separate codes and receivers are used for the civilians and the military. The civil GPS receiver has access to the C/A code which is the coarse/acquisition code transmitted on the L1 frequency. The military can also access an additional code called the P or Precision code which is transmitted on L1 and L2 frequencies. Some receivers can access the P code directly. The C/A code is generally transferred to the P code by the military. The use of this system by the civilians is free of cost. This system is controlled by a joint board in the US.
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