Before we dig into this topic, we need to define some terms. Recall your private ground school where they talked about the different types of altitude measurement? Absolute altitude was one, and it’s the actual distance the aircraft is above the ground at any point, as if you stopped midair and extended a tape measure out the window. Once a popular retrofit in piston singles like Bonanzas and Mooneys, these are now typically only found in far more complex aircraft.
There are two techniques used to measure that absolute altitude. One is properly called a radio altimeter and the other is a radar altimeter. (More below.) The terms are often conflated, so we’ll just use the generic term rad alt when talking about the hardware, and abs alt when talking about the measurement they report.
