Radio altimeter explained: how radio altitude is measured, the 4.2–4.4 GHz band, antennas, cockpit uses, limitations and simulator faults.
A radio altimeter, also called a radar altimeter or radalt, measures an aircraft’s height above the surface directly beneath it. It transmits radio energy downwards and analyses the reflected signal to calculate radio altitude, mainly supporting low-height indications, terrain warnings, approach callouts, flare logic and automatic landing systems.
In aviation and real-world flying, radio altimeter and radar altimeter describe the same type of equipment. Cockpit labels and manuals may abbreviate it as RA, RAD ALT or RADALT.
What is radio altitude?
Radio altitude is the measured distance between the aircraft and the effective reflecting surface inside the altimeter’s downward-looking antenna beam.
It is commonly described as height above ground level, but that shorthand has limits. The system does not consult a terrain database or know the elevation above mean sea level. It measures a radio return from terrain, water, trees, buildings or another substantial surface beneath the aircraft.
The result may therefore represent the top of a forest canopy or building rather than bare earth. Over steep slopes or during large pitch and bank angles, it can be closer to a slant range than a perfectly vertical height measurement.
Many transport-aircraft installations provide radio altitude from roughly 2,500 feet down to touchdown, although the certified range varies by system and aircraft. Some helicopters, military aircraft and specialist installations use equipment with different ranges.
How does a radio altimeter work?
A radio altimeter determines range by comparing a transmitted signal with the delayed signal reflected back from the surface.
- Transmit: The transmitter sends radio-frequency energy through a downward-facing antenna or aerial mounted beneath the aircraft.
- Reflect: Part of the energy strikes the surface inside the antenna footprint and returns towards the aircraft.
- Receive: A receiving antenna collects the weak reflected signal. Its delay depends on the distance travelled down to the surface and back.
- Calculate: The processor converts that round-trip delay into range and applies the aircraft installation’s calibration.
- Distribute: The resulting radio-altitude value is sent to cockpit displays, warning computers and automatic-flight systems.
Most conventional civil radio altimeters use frequency-modulated continuous wave, or FMCW, technology. The transmitted frequency sweeps at a known rate. Because the reflected signal arrives slightly later, its frequency differs from the signal being transmitted at that instant; the resulting beat frequency is proportional to the round-trip delay and therefore the height.
Pulsed radar-altimeter designs instead measure the travel time of discrete pulses. Their basic relationship is distance equals the speed of the radio wave multiplied by round-trip time, divided by two.
The indicated zero is installation-specific. Cable delay, antenna position and the aircraft’s landing attitude may be accounted for so that the display reaches its defined touchdown reference even though the antennas remain physically above the runway. A small residual or negative indication can consequently be normal on some aircraft.
What is the radio altimeter frequency band?
Civil aviation radio altimeters are designed to operate in the 4.2–4.4 GHz band, equivalent to 4,200–4,400 MHz.
An individual unit does not necessarily transmit across that entire 200 MHz allocation. Its waveform and occupied bandwidth depend on its design and approval. The reflected signal is extremely weak, so interference affecting the receiver can cause invalid, unstable or potentially misleading data even when the unwanted transmission is not itself a radio-altimeter signal.
Why does a radio altimeter usually have two antennas?
A typical radio-altimeter installation uses separate transmitting and receiving antennas on the underside of the fuselage.
Separation helps the receiver detect the ground return while the transmitter is operating. The antennas are normally low-profile units positioned to maintain a useful downward beam across the aircraft’s approved pitch and bank envelope. An aircraft with two independent radio altimeters may consequently have multiple antenna pairs.
Antenna type, location and cable length are part of the approved installation, not interchangeable accessories. Damage, moisture, unapproved paint, incorrect cables or altered cable lengths can affect signal strength or calibration and require proper maintenance action.
How is radio altitude different from barometric altitude?
Radio altitude is measured from the surface beneath the aircraft, whereas barometric altitude is calculated from static air pressure and a selected pressure reference.
| Feature | Radio altimeter | Barometric altimeter |
|---|---|---|
| Reference | Effective reflecting surface below the aircraft | Selected pressure datum, such as QNH or standard pressure |
| Typical runway indication | Near its calibrated zero | Aerodrome elevation when the correct QNH is set |
| Pilot pressure setting | None | Required |
| Normal operating range | Primarily low altitude | Used throughout normal flight |
| Main uses | Low-height awareness, warnings, callouts and automatic landing functions | Assigned altitudes, flight levels, approach altitudes and planned terrain clearance |
A mistake we see constantly in flight simulation is expecting both instruments to agree. At an aerodrome 600 feet above mean sea level, a correctly set pressure altimeter should read about 600 feet while the aircraft is on the runway; the radio altimeter should be close to zero. Our explanation of how static pressure produces the barometric-altimeter reading covers the other side of that comparison.
Radio altitude is also not the same as GPS height or a terrain-database calculation. Most importantly, a conventional radalt does not look ahead. It cannot by itself warn that rising terrain lies in front of the aircraft.
What uses the radio-altimeter reading?
Radio altitude supplies low-height data to cockpit indications and several safety or automatic-flight functions, depending on the aircraft.
- Displays and callouts: A primary flight display or separate instrument may show radio altitude, while the avionics generate callouts such as 500, 100, 50, 40, 30, 20 and 10 feet. The available values and voice logic vary by aircraft and operator configuration.
- Ground-proximity warnings: GPWS and TAWS functions can combine radio altitude with descent rate, configuration, air data and other inputs to determine when to issue alerts.
- Automatic landing: Autopilot, autothrottle and flight-control computers may use redundant radio-altimeter channels for low-altitude mode transitions, flare and thrust-retard logic. Radio-altimeter faults or disagreements can downgrade or prevent automatic landing capability. We explain the wider autoland sequence from approach through flare and rollout separately.
- Flight-control and system logic: Some aircraft use radio altitude to schedule control laws, inhibit inappropriate warnings or determine when specific low-level functions become active.
Is decision height the same as radio altitude?
A decision height may use radio altitude, but the two terms are not interchangeable.
A published procedure and the aircraft’s approved operating method determine whether minimums are referenced to barometric altitude or radio height. A RADIO or DH selection normally sets an alerting threshold; it does not adjust or calibrate the radio altimeter. Decision altitude and minimum descent altitude are normally monitored on the barometric altimeter.
Use the source specified for the procedure rather than choosing whichever indication is closest. Our guide to interpreting and calculating charted instrument-approach altitudes explains the barometric side without confusing it with radalt height.
When can a radar altimeter be inaccurate or unavailable?
A radar altimeter can become unavailable or misleading when its height, attitude, reflected signal or installation falls outside the equipment’s valid operating conditions.
- Above its range: The indication may be blank, flagged or suppressed at cruise altitude. That is normal for many installations.
- Large pitch or bank angle: The beam may illuminate an offset area, produce slant range or lose a usable return.
- Cliffs and abrupt surface changes: Crossing a shoreline, building, bridge, forest edge or steep ridge can create a rapid radio-altitude change while barometric altitude remains steady.
- Weak or complex reflections: Smooth water at an unfavourable attitude, uneven terrain and multiple reflecting surfaces can weaken or confuse the return.
- Multipath: Several reflected signal paths can produce unstable data that the processor must resolve or reject.
- Antenna and wiring faults: Contamination, moisture, physical damage, poor connections or incorrect installation can cause flags, fixed readings or intermittent operation.
- Radio-frequency interference: Unwanted emissions can desensitise or disturb the receiver and corrupt its very weak return signal.
Modern systems perform validity and comparison checks, but a believable false value is more dangerous than an obvious blank display. If redundant units disagree, aircraft logic may remove associated automatic functions or present a fault annunciation. Pilots follow the applicable checklist and cross-check other instruments rather than using a suspect radalt for terrain clearance.
Why is the radio altimeter blank or wrong in a flight simulator?
A simulated radio altimeter is usually blank because the aircraft is above its modelled range, the equipment is unpowered or that aircraft does not include a functioning radalt.
Simulation depth varies sharply between default aircraft and add-ons. A simple implementation may calculate height over the terrain mesh, while a detailed aircraft may reproduce display limits, antenna geometry, system failures and automatic-flight dependencies. Rendered trees, bridges and buildings are not always part of the surface used for the calculation.
| Simulated symptom | Likely explanation |
|---|---|
| Blank at cruise altitude | Aircraft is above the radio altimeter’s display or operating range |
| Near zero on the runway while the pressure altimeter shows field elevation | Normal behaviour |
| Sudden change over a cliff, roof or shoreline | Realistic surface change or a scenery-mesh transition |
| Reading disagrees with visible trees or buildings | Those objects may not be included in the sensor calculation |
| Small negative value at touchdown | Possible aircraft calibration, datum choice or scenery elevation mismatch |
| Fixed or flagged value at low height | Power, failure simulation, aircraft logic or add-on fault |
How should you troubleshoot a simulated radalt?
Check the aircraft, operating range and power state before treating a strange radio-altitude indication as a scenery or software fault.
- Confirm the aircraft is fitted with one: Many light aircraft have only a barometric altimeter. A minimums selector does not prove that a radio altimeter is installed or modelled.
- Descend into its operating range: Test below the range stated in the aircraft documentation, with modest pitch and bank.
- Check electrical power: Make sure the relevant avionics and instrument buses are powered and that no simulated failure or pulled circuit breaker has disabled the system.
- Separate the display from the minimums alert: Changing
BARO,RADIOorDHminimums normally moves an alerting bug; it does not change the measured height. - Test over a flat runway: At touchdown, expect radio altitude to approach its aircraft-specific zero while the correctly set pressure altimeter remains near aerodrome elevation.
- Isolate aircraft and scenery variables: Repeat the test with an aircraft known to model radio altitude, then remove recently added scenery or aircraft modifications one at a time. Terrain elevation errors and incompatible add-ons commonly produce jumps or offsets.
If the pressure altimeter is the instrument that disagrees with the expected airport elevation, use our checklist for diagnosing an incorrect flight-simulator altimeter reading. QNH, standard-pressure selection and scenery elevation are separate from radio-altimeter calibration.