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What Does a Radio Altimeters Do?

A radio altimeter is an electronic device that measures the altitude of an aircraft above the ground immediately below the aircraft. It determines the altitude by sending a radio signal from the aircraft to the ground where it reflects and travels back to the aircraft. The altimeter makes note of the time it took the signal to travel to and from the aircraft and converts that into a figure representing the altitude. The altitude measured by the radio altimeter is called “absolute altitude” and refers to the constantly changing measurement of the distance between an aircraft and the ground. The speed of the radio altimeter’s signal is necessary to provide the pilot with real-time readings of altitude.

The radio altimeters’ primary use is during approach, low level flight, and night flight below 2500 feet. It provides key information for landing decision height, and can be adjusted to provide the pilot with a visual or audio warning when the landing decision height has been reached. The landing decision height is the lowest altitude in the approach stage of landing, and the last chance a pilot has to decide to carry out a landing or initiate a missed approach. To do this, the radio altimeter sends a signal at 4.3 GHz directly toward the ground. The signal’s frequency is regulated at 50 MHz and travels at a standard speed. Upon reaching the ground, the signal bounces back toward the aircraft where a separate antenna receives the signal. The signal is processed by measuring the elapsed time of travel is displayed as a figure representing height above ground level (AGL).

Radio altimeters are much more accurate and responsive than air pressure altimeters when it comes to AGL information, especially at low altitudes. Large aircraft incorporate altimeter information into a ground proximity warning system which alerts the pilot to terrain that is dangerously close to the aircraft. Radio altimeters are critical tools in the minimization of controlled flight into terrain (CFIT). CFIT occurs when an airworthy aircraft under control of the pilot is unintentionally flown into terrain, water, or an obstacle. CFIT usually happens because the pilot’s sight is compromised and they are unaware of the danger. The use of radio signals prevents this by alerting the pilot to surrounding terrain even if it is not clearly visible.

Radio altimeters have a variety of uses in both civil and military aviation. In commercial aircraft they are used for approach and landing, particularly in low-visibility conditions. They also play an important role in automatic landings, in which the landing process is fully automated by a system and only supervised by the pilot. These systems are in place to allow commercial flights to land in weather conditions that would otherwise be too dangerous to operate in. Radio altimeters also allow military aircraft to fly at low altitudes over land or sea to avoid radar detection and targeting by anti-aircraft weapons or surface-to-air missiles. Not present on civilian or commercial aircraft, some military aircraft such as the F-111 Aardvark are equipped with further antennae that allow forward-looking radar to supplement the radio altimeter, which is only capable of looking directly downward.

The radio altimeter kit is an important part of any navigation system. At AFR Enterprises, owned and operated by ASAP Semiconductor, we can help you find all the radio altimeter kit parts for the aerospace, civil aviation, and defense industries. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, email us at sales@afrenterprises.com or call us at 1-714-705-4780.


January 8, 2020

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