DME accuracy is determined by what?

• A. Line of sight
• B. Signal type
• C. Altitude
• D. Temperature

Answer: (A)

The key factor is whether there is an unobstructed line-of-sight between the aircraft and the DME station. DME measures distance by timing how long a signal takes to travel from the aircraft to the ground beacon and back, so it relies on a direct path through space. If the path is blocked or heavily reflected, the measured time—and thus the distance—can be distorted, reducing accuracy. The Earth’s curvature and terrain limit line-of-sight, and while higher altitude can extend the visible range, the fundamental determinant of accuracy is having a clear line of sight. Other factors like signal type or temperature don’t set the intrinsic accuracy.