How does a radio altimeter differ from a barometric altimeter?

A radio altimeter measures altitude using radio waves, which is the key distinction from a barometric altimeter. Specifically, a radio altimeter operates by sending out a radio signal from the aircraft to the ground, measuring the time it takes for that signal to bounce back. This time delay is then converted into altitude information, providing real-time data about the aircraft's height above the ground.

In contrast, a barometric altimeter gauges altitude based on the atmospheric pressure, which decreases as altitude increases. This method is affected by weather conditions and does not give precise readings at low altitudes, especially during landing.

The other options describe aspects that are not accurate for radio altimeters. Radio altimeters are not limited to measuring air pressure, nor are they exclusively used during takeoff. They can function effectively during approaches and landings as well. While radio altimeters may have limitations in specific conditions, they are not rendered unusable in bad weather; rather, they can provide critical information in scenarios where visibility may be limited.