Which instrument provides absolute height or vertical distance above terrain?

• A. RADALT
• B. Altimeter
• C. VSI
• D. ASI

Answer: (A)

Height above terrain is a direct measurement of how high you are above the surface below you. The radar altimeter provides this by sending a radio pulse to the ground, timing the echo, and converting that time into distance. That distance is your absolute height above the terrain, which is especially important during low-altitude flight and approaches.

In contrast, a barometric altimeter estimates altitude above mean sea level by sensing air pressure and applying a standard pressure-altitude relationship; this can differ from your height above the ground due to terrain, weather, and pressure settings. The other two instruments measure rate of climb/descent and airspeed, not height above terrain.