Radio Altimeter | Overview of Radio Altimeter

radio altimeter image download

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).

For AGL information at low altitudes, a radar altimeter is more accurate and responsive than an air pressure altimeter. The indicator is normally located far away from the transceiver. On multifunctional and glass cockpit displays, decision height awareness is displayed as a numeric number with a bug, light, or colour change to notify when that altitude is reached. Large aircraft may use radio altimeter data to create a ground proximity warning system (GPWS), which audibly warns the crew of potentially dangerous terrain below the plane. On the EADI, a decision height window (DH) shows the radar altitude.

radio altimeter & transceiver image download
Radio altimeter equipment

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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