How do aircraft communication and navigation radios work?

Aircraft communication radios transmit and receive voice or data on selected channels; navigation radios interpret coded signals from ground stations or satellites to provide bearing, course deviation, distance or position. Both need electrical power, the correct frequency or channel, a suitable antenna, valid signal coverage and the right cockpit audio or display selection.

For readers using our Aviation & Real-World Flying material, these real-aircraft principles also explain most simulator radio stacks. In avionics, navigation and communication equipment may share controls and screens, but each system still has its own signal, receiver logic and operational purpose.

What is the difference between COM and NAV radios?

A COM radio carries messages between people or systems, while a NAV radio turns aviation radio signals into guidance or position information.

COM1, COM2, NAV1 and NAV2 describe separate radio units or functions. Integrated avionics may place them on one screen, but selecting a frequency on one receiver does not automatically route its audio, indication or transmission through the rest of the cockpit.

How does an aircraft communication radio system work?

A conventional civil VHF aircraft communication radio selects an aviation channel, receives amplitude-modulated voice on that channel and transmits when the pilot presses the push-to-talk switch.

Civil VHF voice channels occupy roughly 118.000 to 136.975 MHz. The receiver filters out other channels, demodulates the selected signal and sends the audio through the aircraft's audio panel to a headset or speaker. Pressing push-to-talk changes the selected COM transceiver from receive to transmit.

VHF aviation voice is normally simplex: everyone shares the channel and only one station should transmit at a time. Two simultaneous calls may produce a squeal, blocked transmission or unreadable mixture. Amplitude modulation can sometimes reveal that another station is transmitting, but it does not make overlapping speech intelligible.

Most radio heads have active and standby windows. The pilot enters a channel in standby and transfers it to active; the audio panel then determines which receiver is heard and whether COM1 or COM2 is used for transmission. Tuning COM2 alone does not mean anyone is listening or transmitting through it.

Channel spacing depends on the region and airspace. Equipment may support 25 kHz or 8.33 kHz channel assignments, and an 8.33 kHz display can show a channel designator that is not simply the carrier frequency. Enter the published value rather than attempting a manual conversion.

Controllers use different frequencies for clearance, ground, tower, approach and area services. Our practical explanation of frequencies, call signs, phraseology and ATC readbacks covers how that shared voice channel is used operationally.

What communication equipment is fitted to an aircraft?

An aircraft communication system normally includes transceivers, audio controls, microphones, speakers or headsets, antennas and the electrical connections between them.

• COM transceivers select, receive and transmit on the assigned channel.
• The audio panel chooses the transmitting radio and the receivers the crew can hear. It may also control intercom and marker audio.
• Push-to-talk switches and microphones key the transmitter and provide speech audio.
• Antennas and coaxial feed lines carry radio-frequency energy between the equipment and the outside air.
• HF, satellite and data-link equipment support communication beyond normal VHF coverage when fitted and authorised.

A transponder and ADS-B equipment also exchange or broadcast information, but their primary role is surveillance rather than crew voice communication. An emergency locator transmitter is a separate distress beacon, not a normal two-way COM radio.

Why does aircraft radio range change with altitude?

VHF communication and most VHF or UHF navigation aids are mainly line of sight, so range usually increases as the aircraft climbs above terrain and the radio horizon expands.

Mountains, buildings, the Earth's curvature, antenna position and aircraft attitude can block or weaken a signal. Transmitter power and receiver sensitivity matter too. Hearing a powerful ground station does not prove that the ground station can receive the aircraft's weaker or obstructed transmission.

Interference affects different aviation radio signals in different ways. Electrical noise and thunderstorms are particularly troublesome for ADF/NDB reception; terrain and reflected signals can disturb VOR; vehicles or structures near an ILS may distort its tightly aligned beams. HF can cover great distances through ionospheric propagation, but reception quality varies more than ordinary VHF line-of-sight communication.

What is radio navigation in aviation?

Radio navigation in aviation is the use of signals from ground stations or satellites to determine bearing, distance, position or deviation from a defined route or approach path.

• VOR: the receiver compares reference and variable signal components to determine the aircraft's radial from the station. The selected OBS course and TO/FROM logic then drive a CDI or HSI. The OBS selects the course to display; it does not change the radial on which the aircraft is located.
• ILS: the localiser compares overlapping 90 Hz and 150 Hz modulation patterns to show left-right displacement from the runway centreline. The glide-slope receiver uses a similar principle for vertical deviation. The beams are intended for a specific approach direction and should not be trusted outside published coverage.
• ADF/NDB: directional receiving antennas point the ADF needle towards a non-directional beacon. Depending on the instrument, the indication may be relative bearing or magnetic bearing. It indicates direction to the station, not a wind-corrected track.
• DME: the airborne unit sends timed pulse pairs and measures the delay before a ground transponder replies. It reports direct slant-range distance rather than horizontal or along-track distance. Near or above the station, vertical separation can account for much of the reading; our detailed DME operating guide explains the geometry and cockpit use.
• GNSS/GPS: the receiver calculates position and time from one-way satellite signals. It is radio navigation in the broad technical sense, although pilots often use “radio navigation” to mean terrestrial aids such as VOR, NDB, DME and ILS.

VOR, ILS, NDB and GNSS reception is passive. DME is different because the aircraft interrogates the ground station and waits for a reply. Many installations channel-pair DME with a selected VOR or ILS frequency, so the pilot may receive distance without tuning a separate visible DME frequency.

How do pilots tune and use communication and navigation radios?

Pilots obtain the correct published channel, tune it, select the required audio or display source, verify identification and cross-check the resulting indication.

1. Confirm electrical power. Check the battery, alternator or generator, avionics master and the relevant radio or audio bus. An illuminated flight display does not prove every radio has power.
2. Obtain the published value. Use the applicable chart, procedure, clearance or onboard database rather than memory.
3. Tune and transfer. Enter the value in standby and move it to active, or select it through the aircraft's integrated avionics. Our guide to COM, NAV, ADF and DME radio-stack controls explains the common knobs, transfer keys and audio selections.
4. Route the audio. Select the COM transmitter and any receivers to be monitored. Set usable volume and squelch. A NAV indication can operate even when its Morse identifier is not routed to the headset.
5. Identify the navigation aid. Compare its Morse-code or voice identifier with the published identifier. Treat a missing or incorrect identifier, or an active warning flag, as an unusable signal unless the applicable procedure and equipment explicitly provide another approved check.
6. Select the display source. Set the CDI or HSI to NAV1, NAV2 or VLOC rather than GPS when following a VOR or localiser. Enter the required course where applicable.
7. Cross-check before following it. Confirm that bearing, TO/FROM indication, deviation, distance and expected station position agree. Do not command the autopilot to follow an indication that has not been checked.

A mistake we see constantly in simulators is a correctly tuned NAV frequency paired with the wrong GPS/VLOC source. Another is expecting the course knob to retune or rotate a VOR radial; it only tells the indicator which course the pilot wants to follow.

Why is the radio tuned correctly but not working?

A correct frequency is only one link in the system; power, active-channel selection, audio routing, reception coverage, station identification and cockpit source selection must also be correct.

In a flight simulator, also check the aircraft's own avionics implementation. A detailed add-on may use custom controls that do not follow generic simulator key assignments. Assistance features can auto-tune or change the navigation source, while built-in ATC or multiplayer voice may operate separately from the modelled COM audio.

Navigation-data age is another common cause. A frequency shown on one chart may not match the simulator's installed database, and a visible antenna in scenery does not necessarily create a working navigation aid. Reception range, interference and station monitoring may also be simplified compared with the real aircraft system.

Do GPS and the FMS replace aircraft navigation radios?

GPS and an FMS reduce dependence on ground-based navigation radios, but they do not replace a radio aid when the procedure or installed equipment requires that aid.

GNSS supplies position; an FMS combines position sources with route, procedure and performance data to calculate guidance. Some integrated systems automatically tune and identify nearby VOR, DME or ILS facilities, but the underlying radio receiver and antenna still provide those measurements.

Use terrestrial radio guidance when flying a VOR, localiser or conventional ILS procedure, when practising radio-navigation skills, or when it provides an appropriate independent cross-check. Use GNSS/FMS guidance for an approved satellite-based procedure or route when the aircraft, database and operation support it. Our comparison of GPS with VOR, NDB, DME and ILS sets out that choice for simulator flying.

The autopilot does not correct a bad source selection. In NAV or approach mode it follows the guidance supplied by the selected GPS, VOR or localiser source, subject to the aircraft's mode logic. If GPS remains selected during an ILS interception, the radios may be tuned perfectly while the aircraft continues following the programmed route.