Learn how a radio altimeter measures height above terrain, differs from a barometric altimeter, and what can make its reading unreliable.
A radio altimeter, also called a radar altimeter, measures an aircraft’s height above the surface directly beneath it. It transmits radio energy towards the ground, receives the reflection and converts the signal’s travel time or frequency difference into radio altitude, chiefly for low-level warnings, approach guidance, flare and automatic landing.
In aviation and real-world flying, radio altimeter and radar altimeter mean the same instrument. Most transport-aircraft units are intended for low heights, often displaying up to about 2,500 feet, although the usable range and attitude limits depend on the equipment and installation.
How does a radio altimeter measure height?
The system calculates range by comparing a transmitted radio signal with the signal reflected from the surface below the aircraft.
- Transmission: An antenna under the fuselage directs radio-frequency energy towards the ground. Civil radio altimeters typically operate in the 4.2–4.4 GHz band.
- Reflection: Part of that energy reflects from terrain, water, buildings, trees or another substantial surface inside the antenna footprint.
- Range calculation: A pulse system measures the signal’s round-trip time. A frequency-modulated continuous-wave system compares the transmitted sweep with the delayed return; the resulting frequency difference is proportional to distance.
- Display and distribution: The processor applies installation calibration, produces a radio-altitude value and sends it to the cockpit displays and other aircraft systems.
Because the radio signal travels to the surface and back, a pulse system divides the measured round-trip distance by two. Many installations also compensate for antenna position so that zero corresponds to a defined aircraft datum near touchdown, rather than the antenna physically reaching the runway. The exact zero reference is aircraft-specific.
How is radio altitude different from barometric altitude?
Radio altitude is measured directly above the reflecting surface, while barometric altitude is derived from air pressure and a selected pressure reference.
| Feature | Radio altimeter | Barometric altimeter |
|---|---|---|
| Reference | Surface beneath the aircraft | Pressure datum, normally mean sea level when QNH is selected |
| Typical indication on a runway | Near zero, subject to installation calibration | Aerodrome elevation with the correct QNH |
| Pilot pressure setting | None | QNH, standard pressure or another authorised setting |
| Primary use | Low-level height, callouts, warnings and automatic landing functions | Flight levels, assigned altitudes, terrain clearance and approach altitudes |
| Coverage | Limited low-altitude operating range | Used throughout normal flight |
A common mistake is to expect both instruments to show the same number. At an airport 600 feet above sea level, for example, the pressure altimeter should indicate about 600 feet on the runway when correctly set, while the radio altimeter should be close to zero. Our guide to QNH and pressure-altimeter setup explains the pressure side of that comparison.
A radio altimeter needs no terrain database and does not know the airport elevation. It also does not look ahead; it reports range to the effective reflecting surface within its downward antenna beam. It is therefore not, by itself, a forward terrain-avoidance sensor.
What uses the radio-altimeter reading?
Radio altitude feeds several safety and automatic-flight functions when the aircraft is suitably equipped.
- Cockpit indication and callouts: The display may show radio altitude explicitly, while the avionics generate height callouts such as 500, 100, 50, 40, 30, 20 and 10 feet. The available callouts vary by aircraft and operator configuration.
- GPWS and TAWS: Ground-proximity logic uses radio altitude alongside descent rate, aircraft configuration and other inputs. Our explanation of GPWS modes and warnings covers how those inputs become alerts.
- Automatic landing and flare: Autopilot, autothrottle and flight-control computers may use redundant radio-altimeter inputs to initiate flare, retard thrust and adjust control laws close to the runway.
- Approach minimums: A decision height may be based on radio altitude where the published procedure, aircraft approval and operating rules permit it.
Decision height is not interchangeable with decision altitude or minimum descent altitude. Setting a DH or RADIO minimum changes the alerting threshold; it does not alter or calibrate the measured radio altitude.
When can a radio altimeter be inaccurate or unavailable?
A radio altimeter can become unreliable when the return signal, antenna geometry or aircraft installation falls outside its designed limits.
- Excessive height: Above its operating range, the indication may disappear, freeze behind a flag or be marked invalid. This is normal behaviour, not necessarily a failure.
- Steep terrain or large bank and pitch angles: The antenna can receive a return from an offset part of the surface. The result is an effective beam range, not a perfect vertical measurement.
- Abrupt surface changes: Flying over a cliff, building, treetops, bridge or shoreline can produce a rapid change even though the aircraft’s barometric altitude remains stable.
- Multipath reflections: Signals bouncing from several surfaces can interfere with one another and produce unstable or rejected data.
- Antenna or wiring faults: Damage, contamination, moisture, poor connections or failed processing hardware can cause flags, fixed values or intermittent readings.
- Radio-frequency interference: Strong unwanted emissions near the operating band can corrupt reception. Aircraft procedures and restrictions depend on the equipment, certification and aviation authority involved.
Modern systems run plausibility checks and normally flag invalid data, but a believable-looking false value is more hazardous than a blank display. Pilots use the aircraft checklist and cross-check other instruments rather than treating an intermittent reading as valid terrain clearance.
Why is the radio altimeter blank or wrong in a flight simulator?
A blank simulated radio-altitude indication usually means the aircraft is outside the sensor’s range, lacks a modelled radio altimeter or has not powered the relevant avionics bus.
In some add-ons, the instrument simply calculates height above the simulator’s terrain mesh. It may ignore rendered trees and buildings, while other aircraft model additional surfaces or sensor limitations. Scenery elevation errors can therefore cause jumps, negative values or a reading that disagrees with what appears outside.
We use three quick checks: confirm that the aircraft type is actually fitted with a radio altimeter, descend within its documented operating range, and compare the indications over a known runway. On touchdown, radio altitude should be near its calibrated zero while the pressure altimeter remains near airport elevation. If those instruments are being confused, our MSFS instrument-reading guide shows how their roles differ in the cockpit.