Radar altimeter

A radar altimeter is a little machine that tells you how high you are from the ground. It works kind of like a bat using sound waves to "see" where things are - but instead of sound waves, it uses radio waves.
When you're in a plane, the radar altimeter sends out radio waves towards the ground. The waves bounce back up towards the plane, and the machine measures how long it takes for the waves to come back. The longer it takes, the further away from the ground you are.
It's important to know how high you are when you're flying a plane so that you don't accidentally run into anything on the ground, like buildings or mountains. That's why radar altimeters are so important!