Global Positioning System (GPS)
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.
GPS has three segments: space, control, and user. The space segment includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral in Florida, United States. GPS satellites fly in circular orbits at 17,440 km (10,900 mi) altitude, each lasting 12 hours. The orbits are tilted to the equator by 55° to ensure coverage in polar regions. Powered by solar cells, the satellites continually orientate themselves to point the solar panels towards the Sun and the antennas towards the Earth. Each satellite contains four atomic clocks.
The control segment 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 clock. 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 satellite clocks do not drift too far from nominal behaviour.
User segment is a term originally associated with military receivers. Military GPS user equipment has been integrated into fighters, bombers, tankers, helicopters, ships, submarines, tanks, jeeps, and individual soldiers’ equipment. In addition to basic navigation activities, military applications include target designation, close air support, “smart” munitions, and rendezvous. GPS is found on the space shuttle.
With millions of GPS receivers, the civil community has its own large and diverse user segment. Even before a full complement of satellites was in orbit, surveyors were using GPS to save days or weeks over standard survey methods. GPS is now used by aircraft and ships for en route navigation and airport or harbour approach. GPS tracking systems monitor delivery vans and emergency vehicles to provide optimum advice on routes. In a method called “precision farming” GPS is used to monitor and control the application of fertilizer and pesticides. It is also available as an in-car navigation aid and is used by hikers.
The practical use of GPS by mariners still requires requires a plot to be kept – either electronically or manually. Electronic charts built into some GPS units still need cross checking against against the proper current chart for the appropriate area. Where there is no built-in electronic chart standard navigational techniques need to be applied while treating the GPS as an accurate navaid.