How Radio Altimeters Work: True Height Measurement in Aircraft

Radio altimeter is a crucial instrument that enables an aircraft to determine its actual height above the ground, not just its altitude above sea level. This device uses a radio signal to measure the precise distance separating the aircraft from the surface directly beneath it.

Why Aircraft Need a Radio Altimeter

The radio altimeter is an electronic device designed to measure the vertical distance between an aircraft and the ground below. Unlike the standard barometric altimeter, which estimates altitude based on atmospheric pressure, the radio altimeter directly measures the distance to the surface using radio waves.

This is why radio altimeter readings are often referred to as relative or true altitude above ground level. While flying over flat terrain at a constant barometric altitude, the radio altitude remains steady; however, it fluctuates when passing over hills or valleys, even if the altitude above sea level is unchanged.

This principle is especially important during descent and landing, when even minor errors in estimating the distance to the surface can be critical. The radio altimeter functions independently of atmospheric pressure, providing the aircraft's systems with up-to-date height data throughout the flight.

Radio altimeter data is not just displayed for the crew; it is fed into multiple onboard systems including the autopilot, autoland system, ground proximity warning systems, and other avionics. On modern airliners, familiar voice callouts during landing such as "fifty," "forty," "thirty" are generated based on radio altitude readings.

The terms radio altimeter and radar altimeter are often used interchangeably, as both measure the distance to the ground using radio waves. However, in aviation, the term "radio altimeter" typically refers to devices for low-altitude measurements.

The barometric altimeter is still essential, as each device serves a different purpose: one provides altitude based on atmospheric pressure, the other gives the true distance to the ground. Modern aircraft use both instruments in tandem.

How the Radio Altimeter Works

The operation of the radio altimeter is based on transmitting a radio signal downward, which reflects off the ground and returns to the aircraft. The system calculates the distance to the surface by measuring the difference between the transmitted and received signals.

Most modern aviation radio altimeters use Frequency-Modulated Continuous Wave (FMCW) technology. The transmitter sends a radio wave whose frequency continuously varies according to a set pattern. The reflected signal returns with a slight delay, causing its frequency to differ from the current transmitted frequency.

The electronic unit compares the original and reflected signals. A greater delay results in a larger frequency difference, indicating a greater distance to the ground. Knowing the speed of radio waves, the system converts this difference into a height value.

How Radio Signals Measure Ground Distance

The process begins with the transmitting antenna, usually mounted on the underside of the fuselage, directing a radio signal toward the ground. The radio wave travels at nearly the speed of light, hits the surface, partially reflects, and returns to the aircraft.

The receiving antenna captures the reflected signal and sends it to a processing unit, which analyzes the delay and frequency shift to calculate the distance between the aircraft and the ground.

At low altitudes, the time for the signal's round trip is extremely short-just microseconds for a few hundred meters. Accurately measuring such brief intervals requires high precision, so the frequency-based method provides more reliable readings for small distances.

The radio altimeter measures the distance to the ground directly below the aircraft, not relative to a geographic reference point. This means readings can change rapidly when flying over hills, ravines, buildings, or other large objects.

Continuous Measurement During Flight

The radio altimeter operates continuously, updating its measurements throughout the flight. The aircraft does not need to hover over a single spot; new signals are constantly sent to the ground and processed almost instantaneously.

As the plane descends, the radio altitude decreases. When climbing, it increases. While flying horizontally over uneven terrain, the radio altitude can change even if the barometric altitude remains constant.

The high refresh rate makes the radio altimeter invaluable during critical phases of landing. Onboard systems receive nearly real-time data on the remaining distance to the ground, supporting automatic control, crew alerts, and voice callouts for height above ground.

Components and Signal Processing

A typical radio altimeter consists of several main elements: a transmitter, receiver, antennas, and an electronic processing unit. In modern aircraft, the system is integrated with the avionics, so the calculated altitude is instantly available to pilot displays and automated control systems.

The transmitting antenna, usually located on the underside of the fuselage, sends the radio signal downward. A separate receiving antenna captures the reflected wave, minimizing interference from the transmitter and making it easier to detect the weak returning signal.

The transmitter generates a continuous, frequency-modulated signal. After reflecting off the ground, the signal returns with a slight delay. The receiver sends it to the processing unit, where it's compared with the current transmitted frequency.

The core computational module determines the frequency difference and converts it into distance. The output is filtered to prevent brief interference or unstable reflections from causing sudden jumps in readings.

Once processed, the radio altitude data is sent to the aircraft's network. Pilots see it on their primary displays, and other systems use the same data automatically. For example, radio altitude can affect autopilot operation, ground proximity warnings, and autoland modes.

It's important to note that the radio altimeter measures the distance to the ground only in the area where the main reflection occurs. Over a smooth runway, the readings change smoothly; over uneven terrain, multiple surface points at different heights can influence the reflected signal.

The electronics must select the most appropriate signal and produce a stable altitude value. This is why antenna design, transmitter characteristics, and processing algorithms are crucial for radio altimeter accuracy, especially at low altitude.

Radio Altimeter vs. Barometric Altimeter: What's the Difference?

The radio altimeter and barometric altimeter measure different types of altitude and complement one another. The barometric altimeter is used throughout most of the flight, while the radio altimeter becomes vital near the ground.

Barometric Altitude

The barometric altimeter determines altitude by measuring atmospheric pressure. As the aircraft climbs, pressure drops, and the device converts this change into an altitude value.

This system requires a reference point, so depending on the flight phase, the crew sets a specific pressure, and the readings may be relative to sea level, the airfield, or standard atmosphere.

The barometric altimeter does not indicate what is directly below the aircraft. For instance, if the aircraft flies at a constant altitude above sea level but the terrain rises into mountains, the instrument's reading may not change significantly.

Radio Altitude

The radio altimeter measures the direct distance to the surface beneath the aircraft, so its readings depend on the terrain.

Imagine a plane flying at 1,500 meters above sea level over a plain at 200 meters elevation-the radio altitude will be about 1,300 meters. Flying over a mountain at 1,000 meters, the radio altitude may drop to around 500 meters, even though the barometric altitude remains unchanged.

This difference allows onboard systems to accurately assess the aircraft's position relative to the ground.

Why Both Instruments Are Needed

Barometric altitude is essential for navigation and maintaining safe separation between aircraft. It provides a common reference system for air traffic controllers, regardless of the underlying terrain.

Radio altitude serves a different purpose: it shows the actual proximity to the ground, which is especially useful during descent, landing, and low-altitude flight. For example, when approaching an airport in mountainous terrain, the aircraft may maintain a safe barometric altitude, but the distance to the ground can vary dramatically. The radio altimeter tracks this changing distance.

That's why pilots and automated systems use multiple data sources: the barometric altimeter for altitude in the flight reference system, and the radio altimeter for the true distance to the ground below.

Where and When the Radio Altimeter Is Used

The radio altimeter is primarily used at low altitudes, where knowing the true distance to the surface is crucial. At higher cruising levels, this information is less relevant, so radio altimeters are designed mainly for operation from ground level up to several thousand feet.

The device becomes especially important during landing. As the aircraft descends, the system continuously tracks the distance to the ground and supplies this data to the crew and automated systems. This ensures precise awareness of the aircraft's proximity to the runway, regardless of atmospheric pressure.

On many passenger aircraft, radio altimeter data is used for voice callouts before touchdown. When the crew hears "fifty," "forty," "thirty," "twenty," and "ten," the system is announcing approximate radio altitude in feet. This helps pilots judge when to begin the flare and control the final landing phase.

Radio altitude is also critical for autoland systems. The autopilot needs to know when the aircraft is approaching the ground to switch to the appropriate landing mode. Accurate distance measurement becomes a key part of the entire automated landing process.

Another application is in ground proximity warning systems, which use radio altimeter data to compare the aircraft's height above ground with other flight parameters, issuing alerts if the plane is descending too quickly or flying dangerously low.

Of course, the radio altimeter is only one of several sources of position information. Onboard navigation also uses satellite systems, barometric sensors, and inertial measurement units. You can learn more about the latter in the article How Inertial Navigation Works: Navigating without GPS in Modern Devices.

The accuracy of radio altimeter readings depends on surface characteristics and radio wave propagation conditions. A flat runway produces predictable reflections, while mountains, steep slopes, buildings, or other irregularities can create a more complex pattern of reflected waves.

The system also has a limited operational range. The radio altimeter is not intended to replace the standard altimeter at cruising altitude-its advantage lies near the ground, where precise knowledge of the distance to the surface is vital, not just altitude above sea level.

Conclusion

The radio altimeter enables an aircraft to measure its true height above ground, independent of atmospheric pressure. It sends a radio signal to the earth, receives the reflection, and calculates the distance based on the returned signal's parameters.

The main difference from the barometric altimeter lies in the reference system: the barometric instrument shows altitude above a pressure-defined level, while the radio altimeter provides the actual distance to the ground directly beneath the aircraft. This makes it especially important during descent, landing, and low-altitude flight.

In modern aviation, the radio altimeter is an integral part of the avionics system, providing data to pilots, autopilot, autoland, and ground proximity warning systems. The combination of multiple altitude measurement methods allows the aircraft to precisely control its position both in the air and relative to the ground below.