When is DME (Distance Measuring Equipment) considered most accurate?

Distance Measuring Equipment (DME) is considered most accurate when an aircraft is flying in a straight line to the station. This is primarily due to the fact that DME measures the time it takes for a radio signal to travel from the aircraft to the ground station and back. When the aircraft is approaching the station in a straight path, there is less potential for error related to terrain or atmospheric variations which could distort the signal path.

High altitudes can introduce a range of errors due to the larger distance the signals must travel and potential reflections from the atmosphere or other structures. When operating low and far from the station, while still possible, accuracy may be affected by terrain and other obstructions that could interfere with the signal. Therefore, the ideal conditions for DME accuracy exist when the aircraft is on a direct approach path to the ground station, as this minimizes the potential for disruption and allows for the most precise distance measurement.