Radio Altimeter Principles and Limitations

Principle of Operation

A radio altimeter transmits a frequency-modulated continuous wave (FMCW) signal towards the ground. The signal reflects off the terrain and returns to the aircraft, where the system compares the frequency difference (beat frequency) between transmitted and received signals. This frequency difference is directly proportional to the aircraft's height above ground level (AGL). Modern systems operate in the 4200–4400 MHz range and are most accurate below 2500 ft AGL, with precision typically within a few feet at the lowest altitudes.

Purpose and Use

Radio altimeters are essential for providing precise Decision Height (DH) information during precision approaches, especially in Category II and III operations. They supply critical data to systems such as autothrust (for flare/retard cues), Ground Proximity Warning Systems (GPWS), and Terrain Awareness and Warning Systems (TAWS). The radio altimeter's output is also recorded by the flight data recorder and displayed on the Primary Flight Display (PFD).

Limitations and Errors

Radio altimeters only measure the vertical distance to the ground directly beneath the aircraft. They are ineffective over water, uneven terrain, or in the presence of tall obstacles not directly below the fuselage. Their range is limited—typically up to 2500 ft AGL. Errors can arise from sloping or rough terrain, banked attitudes, or interference. A failure in the radio altimeter can disable GPWS alerts and autothrust flare cues, increasing risk during approach and landing.

Comparison with Other Altimeters

Barometric (pressure) altimeters—simple, sensitive, or servo-assisted—measure altitude based on atmospheric pressure and are subject to errors from pressure setting, temperature deviations, and instrument lag. GPS altitude can serve as a cross-check but is referenced to a mathematical model of the Earth's surface (WGS-84 ellipsoid), not the actual terrain, and is less precise for low-level AGL measurements.

Operational Implications

A faulty radio altimeter can compromise automatic landing systems and terrain warning protections. Pilots must recognize symptoms of radalt failure and understand alternative references, such as barometric or GPS altitude, while being aware of their limitations.