What is GPS
GPS is an acronym for Global Positioning System.
The Global Positioning System (GPS) is a space-based radio navigation system, consisting of 24 satellites and ground support, that provides accurate, three-dimensional position, velocity, and time, 24 hours a day, everywhere in the world, and in all weather conditions. It was initiated in 1973 and the first GPS satellite was launched on 21 February 1978.
Currently there are two main systems in the world used for Global Positioning and navigation, that of the United State's NAVSTAR GPS and that of the Russians, called the GLONASS (GLobal Navigation Satellite System). Due to the inherent limitations and relative inaccurate positioning obtained from the GLONASS , all commercial and most military use is based on the United State's GPS system.
The GPS Global Positioning system consists of three main components.
1. GPS Ground control stations. (operated by the US Defence force)
The ground control component includes the master control station at Falcon Air Force Base, Colorado Springs, Colorado and monitor stations at Falcon AFB, Hawaii, Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, and Kwajalein Island in the South Pacific. The control segment uses measurements collected by the monitor stations to predict the behaviour of each satellite's orbit and atomic clocks. The prediction data is linked up to the satellites for transmission to users. The control segment also ensures that GPS satellite orbits remain within limits and that the satellites do not drift too far from nominal orbits.
2. GPS satellites (The space component)
The space segment includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral in Florida, United States. GPS satellites orbit in circular orbits at 17,440 km altitude, each orbit lasting 12 hours. The orbits are tilted to the equator by 55º to ensure coverage in polar regions. The satellites are powered by solar cells to continually orientate themselves to point the solar panels towards the Sun and the antennas towards the Earth. Each satellite contains four atomic clocks.
Delta Rocket launching NAVSTAR GPS Satellite
Look at the Delta Rocket Boosters that fell in the Western Cape !
3. GPS receivers (what you buy and use to determine your position and support your personal navigation)
When you buy a GPS, you are actually buying only the GPS receiver and get free use of the other two main components, worth billions of dollars - compliments of the Government of the United States. (If you don't have a GPS, BUY ONE NOW, before losing out on the free use of billions of dollars of technology !)
The ground stations send control signals to the GPS satellites, The GPS satellites transmit radio signals and the GPS receivers, receive these signals and use it to calculate its position.
The calculations used to determine your GPS receiver's position is based on very small time differences, from when the satellite transmitted the signal, to, when the GPS receiver received the signal. These small differences are then used to calculate the distance from the receiver to the satellite. However, when receiving only one signal, we can only calculate how far away from the satellite we are. When receiving two signals, we can determine two likely positions where we are. We need three satellite signals to determine our exact position on the earth's surface. (2D/2 Dimensional positioning). When more than three satellites are "visible" to the GPS receiver, it will also calculate the altitude of the receiver (3D/3 dimensional positioning).
Your GPS receiver requires signals from at least three satellites to determine your unique position on the earth's surface. With a fourth signal your altitude can also be determined. Receiving signals from more than four different satellites, the position of the GPS receiver can more accurately be determined. (See accuracy hereafter)
The GPS satellite constellation is designed in such a manner as to guarantee that at least 4 satellites are visible from any place on earth at any moment in time. Most of the time (+95%) however, you should have at least 6 satellites visible. Many commercial GPS receivers can receive and process signals from 12 satellites for increased reliability and accuracy.
GPS satellites carry atomic clocks that measure time to a high degree of accuracy. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the range to the satellite. The receiver must account for propagation delays caused by the ionosphere and the troposphere. With three ranges to three satellites and knowing the location of the satellite when the signal was sent, the receiver can compute its three-dimensional position.
To compute ranges directly, however, the user must have an atomic clock synchronized to the global positioning system. By taking a measurement from an additional satellite, the receiver avoids the need for an atomic clock. The result is that the receiver uses four satellites to compute latitude, longitude, altitude, and time.
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