Airplane Navigation Systems

Types

The magnetic compass displays the aircraft's current magnetic heading: the aircraft's direction of travel relative to the globe's magnetic field. At the Earth's poles, magnetic compasses are unreliable. Further complicating polar navigation are converging meridians at the poles. A type of navigation called grid navigation is typically used in place of magnetic navigation when flying over the polar regions. With grid navigation, a separate grid system is applied over the polar regions to aid in navigation.

Radio transmitters, commonly referred to as nondirectional beacons (NDBs), are the radio equivalent of a lighthouse. These beacons send out an amplitude modulation (AM) radio signal. The signal itself does not include a direction. However, equipped with an automatic direction finder, the aircraft can locate the direction of a radio source; the ADF automatically and continually displays the relative bearing from the aircraft to a radio station. The effective range of these beacons varies from 15 to 75 nautical miles, depending on the transmission power of the beacon.

VOR is another type of ground navigational aid using short-range radio navigation. VOR enables an aircraft to determine its position and maintain course by receiving radio signals from a network of ground beacons. The bearing is displayed in the aircraft. The line of position is the “radial” from the VOR. The intersection of radials from two different VOR stations on a chart provides the aircraft position. VOR stations have a relatively short range, typically 200 miles or less.

TACAN is a navigation system used by military aircraft that provides the aircraft with bearing and distance, known as “slant-range” to a ground or ship-based station. It is a more accurate version of the civilian VOR/DME system. The DME (distance measuring equipment) portion of a TACAN is available for civilian use at VORTAC stations where VOR is combined with TACAN to provide VOR/DME information. Distance from the station is provided by measuring the time required for the signal exchange from the station to the aircraft. Aircraft equipped with TACAN can use this system for en route navigation and nonprecision approach landings.

Variations of the ADF/VOR/TACAN and VORTAC displays include horizontal situation indicators (HSIs), radio-magnetic indictors (RMIs), and bearing-distance-heading indicators (BDHIs), all of which overlay bearing information onto a compass card to display relative bearing to a radio source with respect to the aircraft's heading.

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