Posted on September 1, 2026 Kyle Salem
A radio altimeter is an aircraft instrument that uses reflected radio-frequency signals to measure the aircraft’s height above the surface directly below it.
Also called a radar altimeter or RADALT, it provides height above ground level rather than altitude based on air pressure. This direct measurement supports cockpit callouts, terrain warnings, automatic landing, and other safety-critical functions during low-altitude flight.
Unlike a barometric altimeter, which uses atmospheric pressure and a selected reference, a radio altimeter directly measures the aircraft’s clearance from the surface below.
Radio altitude is the measured vertical separation between the aircraft and the terrain or surface beneath it. It is generally expressed as height above ground level, or AGL.
This is different from altitude above mean sea level. An aircraft flying 2,000 feet above sea level might be only 300 feet above rising terrain. A radio altimeter is concerned with that 300-foot clearance, not the elevation referenced to sea level.
The FAA describes the radio altimeter as the only aircraft sensor that directly measures clearance height above terrain and obstacles. That is why its information becomes especially valuable during low-altitude flight, approach, and landing.
A radio altimeter works on a four-step radar principle:
1. Transmit → 2. Reflect → 3. Receive → 4. Calculate height
The transmitter sends radio-frequency energy through an antenna mounted on the lower part of the aircraft. The energy travels toward the terrain, runway, water, or another surface below.
Many civil aviation radio altimeters operate in the protected 4.2–4.4 GHz frequency band.
When the signal reaches the surface, part of its energy is reflected toward the aircraft. Runways, water, vegetation, snow, and buildings can produce different reflection characteristics.
A receiving antenna captures the much weaker reflected signal and sends it to the processing unit. The receiver must separate this return from electrical noise and unwanted radio-frequency energy.
The method used to calculate height depends on the radio altimeter design.
Pulsed systems can determine distance from the signal’s round-trip time. Many civil-aircraft installations instead use frequency-modulated continuous-wave technology. The equipment compares a changing transmitted frequency with its delayed return, and the resulting frequency difference corresponds to distance.
The processor converts that measurement into radio altitude and supplies it to the cockpit display and other connected systems.
The display is only the visible part of a larger installation. A typical system includes several components.
The transmitter generates the outgoing signal, while the receiver detects its reflection. Both functions may be contained in one line-replaceable unit.
Antennas are normally installed beneath the fuselage. Many systems use separate transmitting and receiving antennas. Their placement, cabling, bonding, and condition directly affect signal quality.
The processing electronics compare the signals, calculate height, monitor data validity, and distribute radio altitude to other aircraft systems.
Radio altitude may appear on a dedicated instrument or a primary flight display. Some installations also provide a selected decision-height alert.
Component descriptions such as a radar altimeter display or DIM unit may identify the indicating portion of a system, but the exact function still depends on the applicable equipment and aircraft documentation.
“Radio altimeter” and “radar altimeter” normally refer to the same aircraft system. RADALT and RA are also common abbreviations.
It should not be confused with weather or forward-looking terrain radar; its specific job is measuring clearance from the surface below.
Although both instruments contain the word “altimeter,” they answer different questions.
A barometric altimeter uses atmospheric pressure and an altimeter setting to indicate altitude relative to a pressure reference. It supports assigned altitudes, flight levels, instrument procedures, and vertical separation.
A radio altimeter measures the aircraft’s actual height over the surface immediately below it. It does not need a local barometric setting to make that measurement.
In practical terms:
The two readings may differ considerably over elevated or changing terrain without either instrument being faulty. GPS altitude is different again: it derives vertical position from satellite navigation rather than directly measuring the reflecting surface below.
Radio-altimeter information can support several functions during low-altitude operations.
During an approach, the system gives the crew and connected avionics a direct height indication. Many aircraft provide automatic callouts such as “fifty,” “forty,” “thirty,” “twenty,” and “ten” near touchdown.
The exact callouts vary by aircraft configuration.
Aircraft approved for automatic landing may use radio altitude to begin the flare, reduce thrust, and transition toward rollout. Unavailable radio-altitude data can therefore affect autoland capability.
Ground Proximity Warning Systems and Terrain Awareness and Warning Systems may use radio altitude to detect unsafe terrain proximity.
It can also support descent-rate warnings, landing-configuration alerts, windshear functions, and flight-control modes.
Because of these connections, radio altimeters are an important part of the integrated avionics installed across many civil aircraft, rather than a completely isolated cockpit instrument.
A radio altimeter measures a significant reflecting surface within its antenna coverage, not the elevation of a mapped point ahead.
Its indication normally changes smoothly over a runway but may vary over hills, valleys, buildings, trees, or uneven ground even at a constant barometric altitude.
The key limitation is that the system measures what is below the aircraft. It is not forward-looking terrain radar and cannot independently detect every obstacle or slope ahead.
Large bank or pitch angles can also change which surface falls within the antenna beam. Crews therefore interpret radio altitude alongside attitude, position, barometric altitude, and terrain information.
Radio altimeters are designed for dependable low-altitude operation, but their performance still depends on signal quality, installation integrity, and the environment.
Runways, soil, vegetation, snow, water, and structures reflect energy differently. Unusual or rapidly changing reflections may influence the indication.
Steep bank or pitch attitudes, slopes, cliffs, and irregular terrain can change the return path and the surface being measured.
Damage, contamination, poor bonding, moisture, or connector faults can weaken the signal. Troubleshooting must consider the complete signal path, not just the cockpit display.
Because the system detects weak reflections, unwanted energy near its operating band can be a concern.
The FAA has evaluated potential interference from certain 5G C-band deployments near the radio-altimeter band. This does not mean every 5G service affects every aircraft; risk depends on the frequencies, transmission characteristics, equipment filtering, location, and applicable mitigations. The FAA’s 5G and aviation safety information explains the issue.
If radio-altimeter data appears unreliable, crews follow approved procedures and compare other instruments and alerts. Maintenance teams may inspect:
An intermittent indication does not prove the receiver-transmitter is faulty; antennas, cabling, installation, or interference may be responsible.
Radio Altimeter Parts and Replacement Considerations
For maintenance teams and parts buyers, understanding the complete system makes replacement identification more reliable.
Radio altimeter components and replacement parts may include receiver-transmitters, indicators, antennas, control units, and related hardware. A shared description does not establish interchangeability.
The exact part number should be checked against the aircraft model, illustrated parts catalog, maintenance data, configuration, modification status, and approved supersession information.
Connector style or dimensions alone are insufficient because units may differ in interfaces, software, or certification basis.
Yes. In aviation, radio altimeter and radar altimeter generally describe the same system. RADALT and RA are common abbreviations.
No. It measures the aircraft’s clearance above the surface directly below it. A barometric altimeter normally provides altitude referenced to a pressure setting or standard datum.
Many conventional civil-aircraft radio altimeter systems have an upper operating range around 2,500 feet AGL, although the actual approved range depends on the specific equipment and aircraft installation.
Yes. Water can reflect radio-frequency energy, allowing the system to determine height. The quality and behavior of the return can vary with the surface, aircraft attitude, and equipment design.
It provides direct height-above-surface information used for cockpit callouts, terrain awareness, decision-height alerts, and automatic landing functions on suitably equipped aircraft.
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