Determining one's altitude has been a critical part of flying since aviators first took to the skies. As such, the barometric altimeter has been long employed to give pilots an accurate measurement of the current height they are flying at. Its invention allowed aircraft to fly in conditions with less visibility while still providing a precise altitude. As aviation instrument technology advanced, the radar altimeter has become commonplace on many modern aircraft. It works with the barometric altimeter to provide pilots a more accurate representation of their altitude. In this blog, we will discuss the history, function, and application of radar altimeters.
Barometric altimeters calculate altitude based on the change in air pressure, owing to the fact that the pressure decreases as altitude increases. These devices have long been used to help determine a plane's altitude, but not height. The distinction is very important because if a pilot were heading towards a mountain, their barometric altitude would not change, but their height above the ground would. Therefore, engineers sought to implement a device that could give an accurate and changing elevation to aid pilots in landing all while providing inclement visibility. In 1924, a well-known inventor, who helped create the coaxial cable, presented his radar altimeter device to Bell Labs. Nearly 14 years later, the first radar altimeters were finally installed in an aircraft, and the world of aviation changed.
Radar altimeters work by sending a beam of radio waves down from a transmitter, then waiting for them to bounce off the earth and return to the plane. Radio waves travel at the speed of light, which is over 300,000 kilometers per second, so the altimeter is able to receive real-time information. Once the altimeter has received the returned waves, it multiplies the time by half the speed of light, in kilometers, to produce altitude. Another class of radar altimeters are frequency-modulated altimeters. These devices provide a similar level of accuracy and work by calculating the change in frequency between the signal sent and received since the shift is proportional to the altitude.
Radar altimeters are widely used in commercial aircraft during approach and landing, especially in low visibility. Many automatic systems like landing gear on an aircraft are automatically activated when a certain altitude has been reached. Their implementation is crucial to instrument-landing systems, creating an FAA mandate that every airliner have at least two onboard. Additionally, fighter jets and other military aircraft that fly low to the ground to avoid radar detection rely upon radar altimeters extensively to avoid dangerous terrain.
While radar altimeters are arguably the most reliable elevation technology available today, there are a few limitations. Namely, they can only give information about the ground directly beneath the aircraft. Therefore, pilots must have additional terrain warning systems or general knowledge of the area in which they are flying over to avoid sudden obstacles. Additionally, most radar altimeters only give accurate measurements up to 2,500 feet above the ground. However, the weather radar can be pointed towards the ground, allowing for readings at altitudes up to 60,000 feet.
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