What is a GPS?
The GPS device that you can buy at a store is a part of the user segment of the Global Positioning System. The other two parts of the Global Positioning System are the Space and Control segment respectively. The space segment are the satellites orbiting the earth. Currently there are 30 GPS satellites orbiting the earth in six different orbital planes. The control segment of the Global Positioning System are the ground bases that keep the system running. These bases are located in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado.
How does it work?
The GPS system is all about time. Their are atomic clocks on board each of the GPS satellites that are accurate down to one second in one million years. The GPS satellites are continuously sending out signals containing the time when those signals are sent out according to the atomic clock. Now when the receiver gets that message it uses an equation to figure out the distance between the receiver and the satellite. That equation is distance equals the time traveled times the velocity of the signal. The velocity is a known constant so all the receiver has to do is take the time the signal took to get to the receiver and times that by the known velocity. The GPS receiver has to do this for at least three satellites to get an actual position on the earth. The more satellites the GPS receiver is in contact with the more and better information will be provided. In actuality the equation is more complicated because the receiver is deal with the path of the satellite and can compensate for known and unknown error.
History of the GPS:
The very first precurser to the Global Positioning System was radionavigation. Radionavigation used radios to detect ground based transmitters. This was limited because your ship had to be in range of the transmitters to get a signal. In 1967 the Navy made available to the public at program called Transit. Transit consisted of 7 low level polar orbiting satellites. Receivers for the transit system would find their position by measuring the Doppler shift of signals from the polar orbiting satellites. The transit system is still in use today. Timation was the next stage in development leading to the GPS system. Timation was two satellites that were launched into space and then used to test and develop the technology. Some of the technology tested by Timation was high stability clocks, time transfer, two dimension navigation and even three dimensional navigation. Later Timation satellites had atomic clocks and the last two Timation satellites became the first two GPS satellites.
Today's GPS recievers:
The Global Positioning System is being used everywhere it seems like today, in cell phones, in cars, not to mention hand help recievers. Most good recievers have between 12 and 20 channels so they can keep track of as many GPS satellites as possible. WAAS or Wide Area Augmentation System is available on most GPS recievers and increases the accuracy of GPS.
