geographically fixed radio transmitter that radiates coded signals in all directions to enable aircraft and ships to determine their bearings. An aircraft or ship can determine its line of position and drift if it knows its bearing relative to the radio transmitter and the geographic location of the transmitter. By taking successive bearings on two or more radio ranges the craft can determine its geographic position. Radio ranges are usually unattended; they emit either repeated call letters or steady signals that are periodically interrupted by station identification letters in Morse code. The aircraft or ship obtains its bearings relative to the radio range by picking up these signals with a receiver having a directional antenna, usually a loop antenna. The strength of the signal received depends on the orientation of the antenna relative to the radio range. By varying the orientation of the antenna and observing the changes in signal strength, the bearing of the vehicle can be obtained. When the antenna is driven automatically, the instrument is called an automatic direction finder (ADF). Both manual and automatic direction finders are also called radio compasses, although in aircraft the radio compass usually means an ADF. Another type of radio range called an A-N range transmits two coded signals via directional antennas so that a pilot on one of four fixed courses hears a continuous tone in his or her receiver when the craft's bearing is correct; if it veers off course either a Morse A or N is heard depending on the direction in which the error is made. A very-high-frequency (VHF) omnidirectional radio range transmits a reference signal and another signal that varies from the reference according to the bearing of the receiver. Radio ranging is being made obsolete by the Global Positioning System (GPS), which uses a network of orbiting satellites to precisely locate the position of an aircraft or ship.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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