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Radiotelephone (two way voice radio) systems have been installed in aircraft since before World War II, and have been widely used for mission coordination and air traffic control. Early systems used vacuum tubes, and because of their weight and size, were installed out of the way with only a control head in place in the flight deck. Standardization on VHF frequences occurred shortly after World War II, and transistor radio systems replaced the tube-based systems shortly afterward. Only minor changes have been made to these systems since 1960s.
The earliest navigation systems required the pilot or navigator to wear headphones and listen to the relative volume of tones in each ear to determine which way to steer on course.
Later, navigation systems developed along three separate paths:
The NDB (non-directional radiobeacon) was the first electronic navigation system in widespread use. The original radio range stations were high-power NDBs, and followed nighttime routes previously delineated by colored light beacons. NDBs use the LF and MF bands, and are still in use today (2003) at smaller airports because of their low cost. DF (direction finder) and ADF (automatic direction finder) avionics can receive signals from these. A needle shows the pilot the relative heading toward the station compared to the centerline of the aircraft.
The VOR system (VHF omni range) is less prone to interference from thunderstorms, and provides improved accuracy. It is the backbone of the air navigational system today (2003). VOR receivers allow the pilot to specify a radial, that is, a line extending outward from the VOR transmitter at a particular angle to magnetic north. Then, a course deviation indicator (CDI) shows the amount by which the aircraft is off the chosen course. Distance measuring equipment (DME) was added to many VOR transmitters and receivers, allowing the distance between the station and the aircraft to be shown.
The instrument landing system (ILS) is a set of components used to navigate to the landing end of a runway. It consists of lateral guidance from a localizer, vertical guidance from a glideslope, and distance guidance from a series of marker beacons. Optional components include DME and a compass locator, the name given to an NDB placed at the start of the final approach course.
For a time, LORAN systems, which provide navigational guidance over large areas, were popular particularly for general aviation use. They have declined in popularity with the commercial availability of GPS service.
GPS receivers have made some inroads into the flight deck but remain a supplemental source of navigation.History
Non-Directional Radiobeacon
VHF Omni Range
Instrument Landing System