What kind of distance do DMEs measure?

• A. Horizontal distance
• B. Vertical distance
• C. Slant range
• D. Great-circle distance

Answer: (C)

DMEs measure slant range—the straight-line distance from the aircraft to the DME ground station. The system uses the time it takes for an interrogation to travel to the station and for the reply to return, converting that round-trip time into distance in nautical miles. Because you’re up in the air, the line of sight isn’t just along the ground; the distance includes altitude, so it’s longer than the horizontal ground distance (and could be related to horizontal distance with a simple Pythagorean relation if you know the altitude). Horizontal distance, vertical distance, and great-circle distance describe other concepts, not the specific measurement DMEs provide.