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.
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.