Measuring the gap beneath the aircraft
A radar altimeter measures distance to the surface directly below, rather than distance from a model of sea level. That distinction matters near a runway: an aircraft’s sea-level altitude can stay nearly the same while the ground rises beneath it, but the radio echo returns sooner as the gap shrinks.
Many modern systems transmit a continuous radio signal whose frequency changes in a controlled pattern. The reflected signal comes back slightly later than the one being transmitted; comparing the two reveals the delay, and therefore the distance. The instrument updates repeatedly, turning a series of echoes into a live reading of clearance above the terrain.
Why that reading matters on approach
As the aircraft descends, the radar-altimeter reading gives the crew and onboard systems a direct cue about how close the aircraft is to the ground. It can support low-altitude alerts and automated landing functions, where a small change in height is more useful than an altitude referenced to sea level. It is one input among several: pilots and aircraft systems also use barometric altitude, navigation data, and visual cues.
The measurement is not a perfect reading of the exact point under the nose. The radio beam covers an area, and over sloping or uneven ground its echo can represent a patch of terrain rather than a single point. That is why the instrument is especially useful as a close-to-ground clearance gauge, not as a complete description of the aircraft’s position. On final approach, its practical message is simple: how much ground is left beneath the wheels’ path.
