Aircraft Navigation System (GPS)

Aircraft Navigation System (GPS)

Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and ground stations. In fact, with advanced forms of GPS you can make measurements on more than one centimeter! In a sense it as giving every square meters on the planet a unique address. A typical GPS receiver calculates its position using signals from four or more satellites GPS. 1 Four satellites are needed since the process requires a very accurate local time, rather than any normal clock can provide, to the receiver within the critical time, as well as position.

Each GPS satellite has atomic clocks, and continually transmits messages containing the current at the beginning of the message, parameters to calculate the location of the satellite (in ephemeris), and overall system health (the almanac). The signals travel at the speed of light in space, and slightly slower through the atmosphere. The receiver uses the arrival time to calculate the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry. 2

Although four satellites are needed for normal operation, fewer may be needed in some special cases. If the variable is already known (for example, marine vessels knows its altitude is 0), the receiver can determine its position using only three satellites. Moreover, in practice, receivers use additional keys (the Doppler shift of satellite signals, last known position, dead reckoning, inertial navigation, etc.) to provide answers degraded when less than four satellites are visible. The flight path of satellites tracked by the U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, as well as monitor stations operated by the National Geospatial-Intelligence Agency (NGA). Tracking information sent to the Air Force Space Command Master control at ...