DME

DME (Distance Measuring Equipment) is a radio navigation technology that measures the slant range distance between an aircraft and a ground station by timing the propagation delay of UHF radio signals. It operates on frequencies between 962 and 1213 MHz.

How it works:

1. The aircraft's DME interrogator sends paired pulses to the ground transponder
2. The ground station receives and replies after a fixed delay (50 μs)
3. The airborne equipment calculates distance based on total round-trip time minus the known delay

Important considerations:

• DME provides slant range, not horizontal distance — the difference is significant at close range and high altitude
• DME is often co-located with VOR (VOR/DME) or ILS (ILS/DME)
• Accuracy: typically ±0.2 nm or 0.25% of distance, whichever is greater
• Maximum range: approximately 199 nm

DME is being retained as a backup to GNSS in many countries' minimum operational networks, as DME/DME can provide position fixing independent of satellite navigation.