Radio Altimeter (RA): FMCW Principle, Integration, and Failure Impacts
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:
- AFCS (Autopilot & Flight Director):
- Essential for Autoland operations (controlling Flare and Rollout).
- Disconnects Glideslope signal typically at 50 ft.
- Autothrottle (A/T):
- Triggers the "RETARD" mode during landing (reducing thrust to idle at specific heights, e.g., 20-30 ft).
- GPWS / TAWS:
- Primary source of height data to calculate closure rates with terrain.
- 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.