Radio Altimeter
A radio altimeter, often known
as a radar altimeter, is a device that measures the distance between the
aeroplane and the terrain underneath it. It's mostly utilised for instrument
approaches and low-altitude or night flights below 2500 feet. The primary
altitude information for landing decision height comes from the radio
altimeter. It has an adjustable altitude bug that gives the pilot a visual or
audible warning when the plane reaches that altitude. When the decision height
is reached and the runway is not visible, the pilot will usually abort the
landing.
Radio altimeter frequency & Aircraft Operation
A radio altimeter transmits a carrier wave at 4.3 GHz from the aeroplane to the ground using a transceiver and a directional antenna. The wave travels at a constant speed and is frequency modulated at 50 MHz. It collides with surface objects and bounces back toward the aeroplane, where it is picked up by a second antenna. The transceiver processes the signal by measuring the elapsed time the signal travelled and the frequency modulation that occurred. The display indicates height above the terrain (ground) also known as above ground level (AGL).
Radio altimeters working principle
Radio altimeters work
on the principle of electromagnetic wave pulses being reflected by the terrain
(earth's or sea's surface). These waves are part of the radio spectrum. Because
electromagnetic waves travel at the speed of light, calculating the distance is
almost instantaneous. Radio altimeters are a reliable and precise way of
measuring height, despite being impacted by surface irregularities which create
deviations in the radio signal.
Radio Altimeter Calculation.
The time it takes the
wavefront to travel from the aircraft to the surface and back is used to
compute altitude.
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