During the final seconds before touchdown, an airliner needs a very different kind of altitude information from the barometric altitude used during most of the flight. A barometric altimeter estimates altitude from atmospheric pressure. A radio altimeter — often called a radar altimeter — measures the aircraft’s height directly above the terrain beneath it by transmitting radio-frequency energy toward the ground and analysing the returned signal. The FAA describes radio altimeters as critical safety equipment that provides accurate height-above-terrain information to pilots and integrated aircraft systems, particularly for low-visibility take-off and landing operations. [1]
Radio altitude becomes especially important below a few thousand feet, where the difference between “altitude above sea level” and “actual height above the surface below the aircraft” matters most. Autoland, ground-proximity warning, flare logic, automatic callouts, thrust-reverser logic and other functions may use radio-altimeter information depending on aircraft design. In July 2026 the FAA introduced new performance requirements intended to ensure radio altimeters remain accurate in the presence of neighbouring C-band wireless signals. [2]
Barometric altitude and radio altitude are not the same thing
Barometric altitude is derived from atmospheric pressure. It depends on the selected pressure reference and on how the real atmosphere differs from the standard model. An aircraft on final approach to an airport 5,000 feet above sea level may therefore show a barometric altitude of roughly 5,500 feet while being only about 500 feet above the runway.
Radio altitude ignores sea level. It measures the vertical distance to the reflecting surface beneath the aircraft. Near the runway, this provides a much more direct indication of actual height above terrain than pressure altitude can provide. That distinction is why radio altitude is so useful for flare, low-visibility approach and terrain-awareness functions.
The basic measurement principle
A radio altimeter transmits electromagnetic energy downward from an antenna on the lower fuselage. The signal reflects from the ground and returns to a receiving antenna. The system measures information related to the propagation delay or frequency relationship between the transmitted and received signals and converts it into distance.
Modern low-range radio altimeters commonly use frequency-modulated continuous-wave techniques rather than a simple single pulse and stopwatch. The transmitter sweeps frequency in a controlled way; the delayed reflected signal returns with a frequency difference related to the round-trip propagation time. The electronics convert that difference into height. FAA AC 20-199 provides current installation guidance for low-range radio-altimeter systems used to comply with the 2026 interference-tolerance requirements. [3]
Why the measurement is so fast
Radio waves travel at approximately the speed of light. At a height of only 100 feet, the round trip from aircraft to ground and back covers roughly 200 feet, so the propagation time is only a tiny fraction of a microsecond. Directly timing that interval with sufficient precision is possible, but continuous-wave frequency techniques provide an efficient way to obtain accurate low-altitude measurements with compact avionics.
The system repeatedly updates the calculation, giving the aircraft a continuously changing radio-height value as it descends. This rapid update supports automated systems that need to know not just that the aircraft is low, but exactly how quickly the measured distance to the surface is changing.
Why the antennas are on the underside
The system needs a clear electromagnetic path toward the surface below the aircraft. Transmit and receive antennas are therefore installed on the lower fuselage in locations selected to minimise blockage, structural interference and multipath effects. Their spacing, orientation and cable installation are part of the certificated system.
The antennas are not ordinary communications aerials. The radio-altimeter installation has to produce a well-defined downward-looking radiation pattern and reject unwanted energy from other directions. FAA installation guidance treats antennas, transmission lines, receiver performance and interference susceptibility as a complete system rather than independent components. [3]
Why two radio altimeters are common
Large transport aircraft often use redundant radio-altimeter channels because the information is safety-critical during low-visibility operations. Autoland and other functions may compare independent channels or use voting and monitoring logic so that one failure does not automatically create an undetected false height.
The exact number varies by aircraft. Some installations use two; some complex fail-operational landing architectures use additional redundancy or integrate the signals with multiple flight-control computers. The certification objective is to ensure the required landing capability remains available or fails safely according to the approved operational category.
Why a wrong radio altitude can be dangerous
A radio altimeter does much more than move one number on the primary flight display. If a system falsely reports that the aircraft is higher than it really is, flare or terrain-warning functions could occur too late. If it falsely reports that the aircraft is lower, warnings or automatic modes could trigger too early.
Because the same data can feed several systems, an erroneous but plausible value can be more serious than an obvious failure flag. Modern avionics therefore monitor reasonableness, compare channels and use system-specific cross-checks to detect disagreement where possible.
Automatic altitude callouts
The familiar automated calls heard during landing — such as radio-height announcements in the final part of the approach — are typically generated from radio-altimeter data. The exact callout set differs by aircraft and operator. They give the pilots a precise auditory picture of the remaining height while their visual attention is directed outside and toward the flight instruments.
These callouts are not barometric flight levels. They represent measured height above the reflecting surface. On sloping terrain before the runway threshold, the value can therefore change in ways that differ from the barometric descent profile.
Autoland depends on radio height
Automatic landing systems need to know when the aircraft is entering the very low-altitude phase in which control law changes from approach tracking to flare, touchdown and rollout. Radio altitude provides a direct height reference for that transition. EASA CS-AWO identifies radio height as part of the terminology and alert-height framework for Category III systems. [4]
The exact flare initiation height and control logic are aircraft-specific. It would be unsafe to publish a universal number and imply every autoland begins flare at the same radio altitude. Aircraft size, landing-gear geometry, control laws and certification data all influence the design.
Ground-proximity warning systems
Traditional Ground Proximity Warning Systems use radio altitude with other parameters to identify dangerous proximity to terrain. Enhanced systems add a terrain database and position information, but radio altitude still provides valuable direct measurement of height above the surface.
Modes involving excessive descent rate, terrain closure, altitude loss after take-off or unsafe configuration can use radio-height thresholds depending on system design. The result is that one radio-altimeter fault can affect more than landing automation, which is why its health is carefully monitored.
Why radio altitude can change over uneven terrain
Unlike barometric altitude, radio altitude responds to the surface below. If an aircraft flies level over a ridge, the radio altitude decreases as the terrain rises. Over a valley it increases. During an approach over sloping ground, the measured height may not follow a smooth geometric line even though the aircraft is descending normally toward the runway.
Systems that use radio altitude therefore account for the operational context. The measurement is valuable precisely because it follows terrain, but designers cannot treat it as if it were always measuring distance to the runway surface.
What surface actually reflects the signal?
Radio energy can reflect from soil, rock, buildings, water, snow and runway pavement. The strength and character of the return depend on surface properties, antenna geometry and height. The radio altimeter must work across the range of surfaces expected in certified operation.
At very low height over a runway, the return is generally strong and well defined. At greater height or over complex terrain, the received signal can include multiple reflections. Signal processing and antenna design are used to obtain a stable estimate within the intended low-range operating envelope.
Why it is called a low-range radio altimeter
Transport-aircraft radio altimeters are optimised for low-altitude precision rather than for measuring cruise altitude. Barometric and air-data systems handle tens of thousands of feet efficiently. The radio altimeter becomes operationally important in the lower part of the flight where direct height above terrain matters.
The display or system may stop providing a normal usable value above its designed range. Exact upper limits differ by equipment. The key point is that radio altitude is a landing and terrain-proximity sensor, not a replacement for the barometric altimeter throughout flight.
Why 5G created a radio-altimeter problem
Radio altimeters operate in spectrum near frequencies allocated for terrestrial wireless services. The aviation concern has not been that a mobile phone somehow transmits on the radio-altimeter frequency intentionally. The issue is that powerful signals in neighbouring bands can enter a receiver that was designed decades ago with insufficient rejection of out-of-band energy.
If the receiver is desensitised or produces an erroneous height because of interference, integrated aircraft systems may receive unreliable data. This became a major regulatory issue as C-band wireless deployment expanded.
The July 2026 FAA requirements
In July 2026 the FAA announced new regulations requiring radio altimeters used in the United States to meet defined next-generation interference-tolerance performance. The agency stated that the rules are coordinated with the FCC’s allocation of additional Upper C-band spectrum and are intended to ensure accurate altitude readings despite neighbouring wireless signals. [1]
The FAA also issued AC 20-199 on 24 July 2026 to provide installation-design guidance for low-range radio-altimeter systems intended to comply with the new requirements. This is substantially more current than the temporary mitigation measures used during the earlier stages of C-band deployment. [3]
Interference tolerance is a receiver-design issue
A better radio altimeter uses filtering, receiver linearity, antenna characteristics and signal processing capable of rejecting strong signals outside the intended band while continuing to detect the weak reflected signal from the ground. The challenge is difficult because the desired return can be extremely small compared with a nearby high-power terrestrial transmitter.
Certification therefore has to test the complete installed system under defined interference conditions. Simply replacing one software line or adding a cockpit procedure is not equivalent to a receiver whose hardware and installation meet the required tolerance.
Why legacy altimeters were not necessarily “bad”
Many older radio altimeters were certificated in a spectrum environment that did not include the same neighbouring high-power wireless deployments. Their receiver design could be entirely adequate for the conditions assumed at certification while still having limited rejection of a new out-of-band environment.
The 2026 requirement therefore represents a change in the external radio-frequency environment as well as an evolution in aviation equipment standards. It should not be interpreted as evidence that previous systems had always been unsafe in the conditions for which they were designed.
Radio altitude and the landing gear
The radio-altimeter antennas are mounted on the fuselage, not at the wheel axle. The aircraft therefore knows height from the antenna reference point, while landing systems need to control the position of the main wheels relative to the runway. Aircraft geometry is built into the certification and control laws.
A large widebody cockpit may still be several storeys above the runway when the main wheels touch. Automatic callouts and flare control are calibrated to the aircraft installation rather than representing the pilot’s eye height directly.
Why banking changes the geometry
In a bank, the antenna beam no longer points geometrically straight toward the surface in the same way as in level flight. Terrain slope and aircraft attitude can therefore influence the return. The system’s certified accuracy and operating envelope account for realistic approach attitudes and manoeuvres.
Extreme attitudes or unusual terrain can produce measurements different from the simple textbook picture of an antenna pointing vertically at a flat runway. This is another reason system users rely on the certified avionics behaviour rather than manually converting radio altitude into a geometric drawing.
What happens after touchdown?
As the aircraft reaches the runway, measured radio altitude approaches the installation’s zero reference. The signal can contribute to air/ground logic and rollout functions depending on aircraft design, but landing-gear compression switches, wheel-speed signals and inertial data are also commonly used.
No single sensor should be assumed to determine every “on ground” function. Aircraft use multiple cues because transitions such as spoiler deployment, reverser availability and braking are too important to depend casually on one measurement without system-specific safeguards.
Radio-altimeter faults
A fault may be detected internally by built-in test, by disagreement between channels or by another aircraft system recognising inconsistent data. The cockpit can then receive a failure flag or capability downgrade. In low-visibility operations this can affect whether the aircraft still meets the required landing-system status.
EASA Category III guidance requires the pilot to be able to determine system capability at alert height and requires failures that demand a missed approach to be positively annunciated. Radio-altimeter health is one part of that integrated capability picture. [5]
Why maintenance cares about antenna cables
At radio frequencies, antennas, coaxial cables, connectors and bonding are part of the measurement system. Damage, moisture, poor shielding or incorrect replacement components can alter attenuation or introduce interference. Maintenance therefore follows approved installation and test procedures rather than treating the altimeter computer as the only critical component.
The 2026 interference-tolerance requirements make installation quality even more important because electromagnetic compatibility depends on the performance of the whole receive chain.
The radio altimeter is an input to many other systems
Modern avionics are deeply integrated. Radio altitude can influence automatic landing, terrain awareness, flight-director modes, automatic callouts, configuration warnings and other aircraft-specific functions. FAA statements on the 2026 rule explicitly note that accurate radio-altimeter data are supplied both to pilots and integrated safety systems. [1]
This broad integration is why radio-altimeter interference created operational restrictions on certain approaches during the earlier 5G transition. The concern was never merely that one cockpit number might flicker; incorrect height could cascade into systems that assume the data are trustworthy.
Why GPS cannot simply replace it
Satellite navigation can provide excellent three-dimensional position, but it measures aircraft position relative to a global reference, not direct instantaneous clearance above the local surface. Turning GPS altitude into exact height above the runway would require accurate terrain/runway elevation data, integrity monitoring and different failure assumptions.
Radio altitude is independent of terrain databases and directly measures the reflection from below. That makes it especially valuable in the last part of landing and for terrain-proximity functions. GPS and radio altitude therefore solve different navigation problems.
A direct measurement at the moment precision matters most
For most of a flight, pressure altitude is the language of vertical separation and flight levels. In the last few hundred feet, the aircraft needs something more immediate: how far is the surface below right now? The radio altimeter answers that by transmitting downward, receiving the reflection and converting the radio propagation relationship into height.
That one measurement supports some of aviation’s most demanding functions. It tells pilots and automation when the ground is approaching, helps autoland transition into flare and rollout, supports terrain warnings and feeds low-altitude logic. The FAA’s 2026 interference rules underline how important that information has become: a radio altimeter is not merely an extra gauge, but a foundational sensor for modern low-visibility and terrain-protection systems. [2]
Verified Sources / References
- Federal Aviation Administration — Requirements for Interference-Tolerant Radio Altimeter Systems. Current 2026 FAA explanation of the new interference-performance requirements.
- FAA Statement on 5G, 22 July 2026. FAA explanation of radio-altimeter importance and the new rule.
- FAA Advisory Circular 20-199 — Installation of an Airborne Low-Range Radio Altimeter System. Issued 24 July 2026.
- EASA CS-AWO Issue 2 — Category III terminology and radio-height/alert-height framework.
- EASA CS-AWO — Category III indications and alerts.
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