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A typical radio-altimeter installation consists of...
  • A
    three antennas placed longitudinally: one for transmission and two redundant receive antennas.
  • B
    one antenna that is alternately transmitting and receiving radio pulses.
  • C
    four antennas: two longitudinal and two lateral antennas that use Doppler techniques.
  • D
    two antennas: one continuously transmitting and the other continuously receiving a radio signal.
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 indication is typically from the ground up to 2 500 ft. The height measurement is based upon a frequency modulation wave, for which the frequency difference between the transmitted wave (by a first antenna) and the received wave (by a second antenna) after ground reflection is measured. The transmitted and received It provides data to the TCAS, the GPWS, and the Autoland system.

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