Communication and Navigation Systems Practice Test

Radar (radio) altitude is used to determine height above the ground in GPWS operation. This type of altimeter sends a radio pulse to the terrain and measures how long it takes to return, giving a direct reading of the distance to the ground. That ground-referenced measurement is essential for assessing terrain proximity and triggering warnings as the aircraft descends.

Barometric altitude, by contrast, is based on air pressure and tells you height above mean sea level, which can vary with weather and terrain and doesn’t reflect the actual distance to the ground beneath the aircraft. GPS altitude can be less reliable near the ground due to signal errors and update rates, and AU altitude isn’t a standard measurement used for terrain awareness.