How a Radio Altimeter Work

Summary

This video explores the radio altimeter, a crucial flight instrument used to measure an aircraft's height above the ground. It explains how the system works with antennas, a processing unit, and the radio wave emission/return process. The video covers the radio altimeter’s role in navigation, its accuracy, limitations, and various applications, including automatic landing systems and instrument approaches. It also highlights important considerations such as residual height, irregular terrain effects, and the role of callouts in aiding pilots during landings. Additionally, the video discusses how interference, pitch, and bank limits may impact its functionality.

Takeaways

• 😀 The radio altimeter (RA), also known as radar altimeter, measures the aircraft's height above the ground level (AGL) in real-time.
• 😀 It consists of two antennas: one for transmitting and one for receiving radio waves, as well as a central processing unit and an instrument in the cockpit.
• 😀 The radio altimeter works by emitting a radio wave from the aircraft, which bounces off the ground and returns to the aircraft, allowing the system to calculate the altitude.
• 😀 Modern radio altimeters use frequency-modulated continuous wave (FMCW) technology, where the phase shift between transmitted and received waves is used to calculate altitude.
• 😀 The system relies on the speed of light for accurate, real-time altitude measurement, updating multiple times per second.
• 😀 Residual height refers to the difference between the radio altimeter’s antenna and the aircraft's landing gear, which affects the instrument's reading, especially during landing.
• 😀 Radio altimeter readings may vary when flying over uneven terrain, as the instrument measures the height relative to the ground directly below the aircraft.
• 😀 The radio altimeter is calibrated to show zero feet when the main landing gear touches the ground, and it can display negative values if the residual height is incorrectly calibrated.
• 😀 Radio altimeter callouts are part of the Ground Proximity Warning System (GPWS) and automatically announce important altitude thresholds, such as 2500 feet, 1000 feet, and minimums.
• 😀 In Category 2 and 3 Instrument Landing System (ILS) approaches, the radio altimeter helps pilots by providing decision height (DH) in relation to the ground rather than mean sea level.
• 😀 The system can also be used for automatic landings (Auto Land), where it guides the aircraft from a certain height using radio altimeter data when the glide slope is no longer effective.
• 😀 Limitations of radio altimeters include sensitivity to pitch and bank angles, with errors occurring if the aircraft exceeds typical pitch (30 degrees) or bank (60 degrees) limits.

Q & A

What is the main purpose of a radio altimeter in an aircraft?

The main purpose of a radio altimeter is to measure the height of the aircraft above the ground, specifically its height in relation to the terrain directly beneath it, or AGL (Above Ground Level).

How does a radio altimeter calculate the height of an aircraft?

The radio altimeter calculates the height by emitting a radio wave from a transmitting antenna located on the aircraft. The wave bounces back after hitting the ground and is received by a receiving antenna. The time it takes for the wave to return is measured, and using the known speed of light, the height of the aircraft is calculated.

What is the key difference between modern radio altimeters and traditional ones?

Modern radio altimeters use a frequency-modulated continuous wave system, where the frequency of the radio wave changes continuously. The system calculates the phase shift between the transmitted and received frequencies to determine the height, as opposed to traditional altimeters that measure the time it takes for the wave to return.

What is residual height, and why is it important in the operation of a radio altimeter?

Residual height is the difference in height between the aircraft's radio altimeter antenna and the main landing gear. It is important because the radio altimeter is calibrated to indicate zero feet when the main landing gear touches the ground during landing. This helps ensure accurate height readings during the landing phase.

Why might a radio altimeter show negative values after landing?

A radio altimeter might show negative values after landing due to changes in the residual height. For instance, if the aircraft's landing gear is compressed after touchdown, the residual height may decrease, causing the radio altimeter to show a negative value relative to its pre-landing calibration.

What is a significant limitation of radio altimeters when flying over irregular terrain?

A significant limitation is that radio altimeters may provide inaccurate or fluctuating readings when flying over irregular terrain. This happens because the height above the ground varies constantly as the aircraft moves over mountains, valleys, or other uneven surfaces, even if the aircraft's altitude remains constant.

What is the function of the height reference knob on an analog radio altimeter?

The height reference knob on an analog radio altimeter allows the pilot to set a specific reference height, such as the decision height for landing. When the aircraft reaches this height, the warning light illuminates, signaling the pilot to make a decision about continuing the approach or performing a missed approach.

How does the radio altimeter assist during ILS category 2 and 3 approaches?

During ILS category 2 and 3 approaches, the minimums are specified in terms of radio altitude instead of barometric altitude. The radio altimeter provides the reference for the decision height, and pilots use it to decide whether to continue landing or initiate a missed approach.

What are radio altimeter callouts, and why are they used?

Radio altimeter callouts are pre-programmed voice alerts that inform the pilot of the aircraft's altitude above the ground at specific intervals during an approach. These callouts, such as '500 feet' or 'minimums,' are used to help the pilot stay aware of the aircraft's position without constantly needing to look at the altimeter.

What are some common interference sources that can affect the performance of a radio altimeter?

Common interference sources for radio altimeters include nearby antennas, devices operating in the same frequency band, and technologies like the 5G mobile network, which uses frequencies that overlap with those of radio altimeters, potentially causing signal interference and inaccurate readings.