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A radio altimeter can be defined as a:
  • A
    self-contained on-board aid used to measure the true height of the aircraft
  • B
    ground radio aid used to measure the true height of the aircraft
  • C
    self-contained on-board aid used to measure the true altitude of the aircraft
  • D
    ground radio aid used to measure the true altitude of the aircraft
The radio altimeter can be defined as a self-contained on-board aid used to measure the true height of the aircraft. The RA operates between 4200 MHz and 4400 MHz and the maximum range for the indication is typically from the ground up to 2 500 ft. Therefore, besides the autopilot (for obvious reasons) it is the primary source of information during an automatic landing. The transition from one phase to another during the autoland is based on the height measured by the radio altimeter. For example, at 50 ft AGL the glideslope signal is disconnected, and between 50 ft AGL and touch down, the autopilot will keep a vertical speed depending on the radio altimeter height, as this is more precise than the barometric altitude. The higher the VS the sooner the aircraft will start to flare.
 

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