DME Errors and Limitations

Principle of DME Operation

DME works by timing the interval between an aircraft's interrogation signal and the reply from a ground-based transponder. The system operates in the UHF band (960–1215 MHz), using paired frequencies for interrogation and response. The measured distance is always the slant range—the straight-line distance between aircraft and station, not the horizontal ground distance.

DME Errors and Limitations Explained

• Slant Range Error: At long distances and moderate altitudes, slant range closely matches ground distance. However, when the aircraft is close to the DME station and at higher altitude, the slant range can significantly exceed the ground distance. This is most pronounced when nearly overhead the station.
• System Accuracy: For DMEs installed before 1989, the maximum permissible error is ±0.25 NM plus 1.25% of the measured distance. For newer installations, the error must not exceed ±0.2 NM (and in some cases, ±0.1 NM), excluding airborne equipment error.
• Non-standard Time Delays: Some DME stations use a non-standard ground transponder delay, which can deliberately alter the displayed distance for specific operational needs (e.g., serving parallel runways). This can lead to intentional range discrepancies outside the primary approach path.
• Signal Issues and Interference: DME relies on line-of-sight propagation, so terrain, obstacles, or atmospheric conditions can cause signal loss or multipath errors. Random pulse interference and propagation effects also contribute to minor inaccuracies.

DME Readouts: Ground Speed and Time-to-Station

Many DME systems display ground speed (GS) and time-to-station. The GS readout is derived from the rate of change of DME distance, but it only reflects the component of speed directly toward or away from the station. When flying a DME arc (constant distance from the station), the DME GS reads zero, even though the aircraft is moving.

DME in Procedure Design

DME arcs are flown by maintaining a constant DME distance, typically to transition onto a final approach. Pilots must understand that the DME readout is slant range, and that GS/time-to-station indications are only valid when flying directly toward or away from the station.

Key DME Limitations

• Slant range can overstate distance near the station.
• Accuracy is limited by system design, installation date, and environmental factors.
• Non-standard delays can create intentional errors for specific procedures.
• DME is line-of-sight; terrain or obstacles can block or degrade the signal.
• GS and time-to-station readouts are not valid when flying perpendicular to the station (e.g., DME arcs).