FMCW altitude measurement
Radar altimeters transmit an FMCW chirp straight down and listen for the ground bounce. Because the distance is short (tens of metres to a few kilometres), the beat frequency is low — easy to process. Accuracy is within a few centimetres, updated tens of times per second. The pilot sees a number that is true height, not an approximation.
Radio vs. barometric
Barometric altimeters are essential for separation above 2,500 ft, where everyone uses the same pressure reference. Below that, especially on final approach, radar altimeters take over. Autoland systems, terrain awareness (TAWS), and wind-shear escape manoeuvres all rely on radar altitude. The 737 MAX accidents showed how a single faulty AOA sensor can cascade; radar altimeters are equally flight-critical.
The 5G interference crisis
C-band 5G base stations in the 3.7–3.98 GHz range sit close to radar altimeter bands (4.2–4.4 GHz). Though not directly overlapping, 5G out-of-band emissions and receiver overload created real interference risks. The FAA and aviation regulators worldwide spent 2021–2023 analysing which altimeters were vulnerable and where 5G rollout had to be restricted near airports.
Next-generation altimeters
New designs use wider bandwidth, better filtering, and dual-frequency operation to reject interference. Some are exploring 60 GHz mmWave bands, far from cellular traffic, with even better precision. The mission is unchanged: tell the pilot exactly how much room is left.