The radio altimeter

Navigation · Chapter 30 · DGCA CPL and ATPL

How a frequency-swept radio beam measures true height above the ground, its band, range and accuracy, and what the display shows.

Remember

True height above the ground directly below; self-contained; no setting.
FMCW: the echo lags the sweep; the gap ∝ height. 4 200–4 400 MHz (SHF, ~7 cm).
Range 0–2 500 ft (overlapping returns above). Accuracy ± 3 % or ± 1 ft, the greater.
Mask above 2 500 ft or fault; flag for no signal; DH light and tone.
Reads main-wheel height (zero at touchdown). Feeds the GPWS and autoland.

True height, straight down

Straight down • A radio altimeter (also called a radar altimeter): two small aerials under the belly send a radio beam straight down and hear it come back. • So it gives your height above the ground below you. • No setting needed. It works alone.
Over hills • Fly level over rising ground: this reading falls. • The normal altimeter does not change. It measures pressure, not ground.
Three users • The pilot's screen, autoland, and the GPWS all use it. • It sees only below you, not ahead.

Frequency-swept, not pulsed

Two horns • One horn sends (pink), the other listens (blue). It transmits all the time. • Here they are, under the belly.
A sweep • The pitch (frequency) of the signal slides up and down all the time: fmcw, frequency-modulated continuous wave. • The echo comes back on the pitch sent a moment ago.
Higher = bigger gap • Drag the height. The blue line slides further behind the pink. • The gap between them is the height.
The band • 4 200–4 400 MHz: a wavelength of about 7 cm.

Why only 2 500 ft

This lesson is part of the Navigation plan. See plans

Display, DH and wheels

This lesson is part of the Navigation plan. See plans

Test yourself

This lesson is part of the Navigation plan. See plans