Radarhoogtemeters

Radarhoogtemeters, or radarhoogtemeters in Dutch, are avionics sensors that measure an aircraft’s height above the terrain directly beneath it. They provide precise height information at low altitude, supporting approaches, landings and other manoeuvres where accurate altitude above ground is essential. They are used in civil aviation, helicopters and many unmanned aerial systems to maintain safe clearance from terrain or obstacles.

Principle of operation: A radarhoogtemeter sends downward-directed radio pulses and detects their reflections from the ground. By measuring the round-trip travel time of the pulse and applying the speed of light, the system computes height above ground level (AGL). The resulting altitude is displayed to pilots and fed into flight computers or autopilots for altitude control and decision-making. Modern units may use multiple frequencies or beams to improve accuracy and reduce clutter.

Construction and performance: Most radarhoogtemeters operate in microwave bands such as X- or Ku-band and may incorporate digital signal processing, dual antennas, or multiple channels. The usable range typically extends from near zero up to several thousand feet AGL, with accuracy influenced by surface type, terrain slope, weather, and electromagnetic interference. Proper installation, calibration and maintenance are required, and readings are often supplemented by barometric altitude measurements.

Applications and limitations: In flight, radarhoogtemeters support precision approaches, short-field landings and low-altitude operations, and assist autopilots during descent. They are less effective over cresting terrain, water with waves, snow, or highly reflective surfaces, which can cause erroneous or noisy readings. Therefore, they are normally used in conjunction with other altitude sensing systems rather than as a sole reference.

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