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What is the level of accuracy provided by radio altimeters in terms of altitude measurement?
  • A
    ± 500 ft
  • B
    ± 2 ft in the first 500 ft or ± 1.5%, whichever is greater
  • C
    ± 5%
  • D
    ± 50 ft

Refer to figure.
A radio altimeter is a self-contained onboard safety device that indicates the true height of the lowest wheels or skids (with oleos extended) above the ground.

Data supplied includes the distance between the ground and the altimeter.

Radio altimeters (with audio) are required equipment over water.

A continuous wave FM radio beam in the SHF band (4200-4400 MHz)36 is directed towards the ground in a 30° cone fore and aft, and 30° athwartships. The signal is reflected back to the aircraft.

It takes around 6.1 microseconds for a wave to be reflected from an object around 10,000 m away.

Low altitude radalts use FM and are used for precision approaches, with accuracy of ±2 ft between 0-500 feet or ±1.5%, whichever is greater, and are only active below about 2500 ft.

High altitude radalts work up to 50,000 feet above the surface.

Note:

  • The altimeter reads zero when the wheels touch down.

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