How does radar altitude differ from physical altitude?

• A. Radar altitude is altitude above mean sea level.
• B. Radar altitude is altitude above terrain measured by radar sensing; physical altitude is altitude above mean sea level.
• C. Radar altitude is altitude above terrain; physical altitude is altitude above ground level.
• D. Radar altitude is altitude above terrain measured by GPS; physical altitude is altitude above datum.

Answer: (B)

Radar altitude is the height of the aircraft above the terrain directly beneath it, measured by a radar altimeter using the round-trip distance to the ground. Because it references the ground surface, it changes with terrain elevation and is effectively height above ground level (AGL). Physical altitude, on the other hand, is height above a fixed reference datum, typically mean sea level, called true altitude in many contexts. So you can be high above sea level but low above the nearby terrain, or vice versa, depending on the ground beneath you. This distinction is why radar altitude is used for near-surface obstacle clearance and approaches, while MSL-based altitude is used for airspace, flight levels, and navigation.