Radio Altimeter (RA): FMCW Principle, Integration, and Failure Impacts

022-12-08Pablo Asensio Martínez2026-03-272 min

Overview

The Radio Altimeter (RA), or Radar Altimeter, is a self-contained onboard system that measures the True Height of the aircraft above the terrain immediately below it (Above Ground Level - AGL).

Operating Principle

  • Frequency Range: Operates in the SHF band (4200 MHz - 4400 MHz).
  • Method: Transmits a Frequency Modulated Continuous Wave (FMCW) downwards.
  • Calculation: It measures the frequency difference between the transmitted wave and the received wave reflected from the ground. This difference is proportional to the time delay and thus the height.
  • Operating Range: Typically indicates from 0 to 2,500 ft. Above this altitude, the display is usually blanked.

Calibration and Residual Height

  • Antenna Location: Antennas are located on the underside of the fuselage.
  • Residual Height: The vertical distance between the antennas and the lowest part of the landing gear (main wheels).
  • Indication: The system is calibrated to indicate 0 feet when the main wheels touch the ground in the landing attitude.

System Integration

The Radio Altimeter is a critical sensor that provides data to multiple aircraft systems:

  1. AFCS (Autopilot & Flight Director):
    • Essential for Autoland operations (controlling Flare and Rollout).
    • Disconnects Glideslope signal typically at 50 ft.
  2. Autothrottle (A/T):
    • Triggers the "RETARD" mode during landing (reducing thrust to idle at specific heights, e.g., 20-30 ft).
  3. GPWS / TAWS:
    • Primary source of height data to calculate closure rates with terrain.
  4. TCAS / ACAS:
    • Used to inhibit certain Resolution Advisories (RAs) near the ground (e.g., "Descend" RAs are inhibited below 1,100 ft AGL).

Operational Implications of Failure

  • Autoland Capability: Loss of an RA typically degrades the Autoland capability (e.g., from Fail-Operational to Fail-Passive or prohibiting Autoland entirely).
  • False Readings: A faulty RA indicating a low height (e.g., < 50 ft) during cruise could cause the Autothrottle to erroneously enter RETARD mode, reducing thrust to idle.