N4JRI's Radio Pages: Aircraft Monitoring
Aviation Radio Equipment
Comm Radio - This is a radio (usually set up to simultaneously monitor 2 frequencies) used for voice communications. Frequency range is 118.000-136.975 in AM mode.
NAV Radio - Used for radio navigation while in flight and for Instrument Landing System (ILS) while landing. Frequency range for NAV is 108.000-117.975 MHz in AM mode. In flight, these radios tune in the VHF Omnidirectional Ranges (VOR's) which allow the pilot to triangulate his location by comparing the direction he's in from both VOR's. VOR signals are spin-modulated so that you can tell what your bearing is from the VOR.
Distance Measuring Equipment (DME) - Some VORs have a signal that can be interrogated by the plane's transponder to see how far away he is. If a pilot knows both his bearing and distance from a given VOR, he already has his location and has no need to triangulate it against the signal of another VOR. DME typically operates in the 960-1215 MHz band.
Instrument Landing System (ILS) - Accessed via the NAV radio. ILS signals include three essential elements and one optional element:
Localizer - Tells the pilot whether too far to the right or left of the runway he's approaching. Tuned in the 110-112 MHz range on the NAV radio. This automatically sets up the frequencies on the other receivers.
Glide Slope - Tells the pilot whether he's coming in too high or too low. Signal is in the 332-336 MHz range.
Marker Receiver - Receives 3 marker signals representing distance from touchdown. These are on 75.0 MHz in AM mode. Different tones are emitted by the Outer Marker, Middle Marker and Inner Marker, which can also activate colored lights on the marker receiver.
DME (optional) - Many ILS systems also have DME so that pilots know their distance long before they arrive at the outer marker. Note that DME isn't necessarily critical at large airports that have their own VOR. Often, the pilot will activate ILS on one NAV radio, and get his bearings from the airport's VOR on the second.
HF Radio - Most common in intercontinental airliners. Tuned 3-30 MHz using mostly the single sideband (SSB). Using these radios, planes can report their positions to the various ARINC stations around the world, speak to their countries via Long Distance Operational Control (LDOC), get VOLMET weather broadasts, or just listen to shortwave radio broadcasts while flying.
Audio Console - Mixes the sounds of all these devices into the pilot's headphones. A rotary switch or a series of pushbuttons usually determines which radio enjoys a direct connection with the pilot's microphone. There is also intercom circuitry so that pilot and passengers may converse within the aircraft using their headsets.
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