A radio altimeter is an aircraft instrument that measures height above the ground directly below the airplane. This is different from a pressure altimeter, which estimates altitude above sea level using air pressure. Radio altimeters are most important during the final part of flight, when pilots and flight computers need very accurate height information near the runway.
They help make approaches, landings, and low altitude warnings safer.
Understanding Aviation: The Radio Altimeter
Inside the system, antennas on the lower fuselage send radio energy toward the surface. Another antenna receives the weak return. Many modern units use a continuously changing radio frequency rather than a single short pulse.
The transmitted signal is compared with its echo. A larger delay creates a larger difference between the two frequencies, so the electronics can calculate distance.
This method must work extremely quickly because an aircraft can move several metres while the instrument is making repeated measurements. The display therefore updates many times each second.
The measured path is close to vertical only when the aircraft is level. During a bank, the antenna points somewhat away from the vertical direction. The signal may then reach a point beside the aircraft rather than the point directly under it.
Over flat ground this makes little difference at small bank angles. Over steep hills, cliffs, buildings, or uneven runway surroundings, the reading can change suddenly. The returning energy can be weak or scattered by rough terrain.
Water, snow, vegetation, and airport structures reflect radio energy in different ways. Designers use signal checks to reject doubtful echoes, but pilots must understand that no sensor can make rough ground behave like a smooth flat surface.
The number becomes especially useful when it is connected to other aircraft systems. Flight computers use it to judge when an aircraft is approaching the runway surface and when to begin the flare, the gentle nose-up movement before touchdown. Warning systems combine it with airspeed, descent rate, landing gear position, and aircraft configuration.
A fast descent near the surface is more concerning than the same descent much higher up. The instrument can therefore support warnings that are based on the aircraft's actual situation rather than on one number alone. On a low visibility approach, this information helps automated equipment follow a carefully planned sequence close to the runway.
Students should keep the reference point clear in their minds. A radio altimeter reports separation from whatever surface lies below the aircraft. It does not know whether that surface is the intended runway, a ridge, a tree-covered slope, or a building.
An aircraft flying toward rising terrain can lose radio height even when its pressure altitude hardly changes. Pilots compare several sources of information, including charts, navigation data, visual cues, and pressure altitude. They treat any unexpected low reading seriously, especially outside the airport area.
It is useful to practise simple travel-time thinking. Since the signal makes a return journey, the measured time must be split in half before it represents the one-way distance to the ground.
Key Facts
- Radio altitude measures height above ground level, not height above sea level.
- Distance from time delay: h = ct/2, where c is the speed of the radio wave and t is the round trip time.
- Radio waves travel at about c = 3.00 x 10^8 m/s in air.
- The factor of 2 appears because the signal travels down to the ground and back up to the aircraft.
- Typical aircraft radio altimeters operate during low altitude flight, often from about 0 to 2500 ft above ground level.
- Radio altimeter data can feed autoland systems, ground proximity warnings, flare guidance, and automatic callouts such as 100, 50, and 10 feet.
Vocabulary
- Radio altimeter
- An aircraft instrument that uses radio waves to measure the aircraft's height above the ground directly below it.
- Above ground level
- A height measured relative to the local ground or runway surface instead of relative to sea level.
- Time delay
- The time between sending a radio signal and receiving its reflection back from the ground.
- Autoland
- An aircraft system that can guide and land the airplane automatically using sensors, computers, and flight controls.
- Ground proximity warning system
- A safety system that warns pilots when the aircraft is too close to the ground or descending dangerously.
Common Mistakes to Avoid
- Confusing radio altitude with pressure altitude. Radio altitude is height above the ground below the aircraft, while pressure altitude is based on air pressure and may be referenced to sea level or a standard atmosphere.
- Forgetting to divide the radio wave travel distance by 2. The measured time is for a round trip, so using h = ct instead of h = ct/2 gives twice the true height.
- Assuming the radio altimeter works like radar for long range navigation. It is designed mainly for accurate short range height measurement near the ground, not for mapping distant objects.
- Treating the ground as perfectly flat in every situation. Sloped terrain, uneven surfaces, or obstacles can affect what the reflected signal represents, especially away from a smooth runway environment.
Practice Questions
- 1 A radio altimeter signal takes 1.00 microsecond to travel to the ground and back. Using c = 3.00 x 10^8 m/s, calculate the aircraft height above ground in meters.
- 2 An aircraft is 300 m above the runway. What round trip time should its radio altimeter measure for a signal traveling at 3.00 x 10^8 m/s?
- 3 A pressure altimeter and a radio altimeter show different values during final approach. Explain why both readings can be correct and which one is more useful for landing height above the runway.