Distance Measuring Equipment (DME): Slant Range and Use

062-02-04Pablo Asensio Martínez2026-04-012 min

DME (Distance Measuring Equipment) is a secondary radar system that provides the distance between an aircraft and a ground station. It operates in the UHF band (962 - 1213 MHz).

Operating Principle

It is based on measuring the travel time of radio pulses:

  1. The interrogator (airborne) sends pairs of pseudo-random pulses.
  2. The transponder (ground) receives the pulses, waits 50 microseconds, and retransmits them on a frequency offset by 63 MHz.
  3. The airborne equipment measures the total time, subtracts the 50 $\mu$s delay, and calculates the distance.

Slant Range

DME Slant Range Diagram

DME measures the direct distance between the aircraft antenna and the station, not the horizontal distance over the ground. This is known as Slant Range.

  • The error is maximum when flying directly over the station, where the DME will indicate the aircraft's altitude (in NM).
  • 1 NM $\approx$ 6080 feet.

Operating Modes

  • SEARCH: The equipment sends up to 150 pulse pairs per second (ppps) to lock onto the station.
  • TRACK: Once locked, the pulse rate drops (typically to 24-30 ppps) to maintain tracking.
  • MEMORY: If the signal is momentarily lost, the equipment maintains the last rate of change of distance for 8-10 seconds before returning to search mode.

Association and Frequencies

  • The DME is usually associated with a VOR or ILS. The pilot tunes the VOR/ILS VHF frequency, and the equipment automatically tunes the paired DME UHF frequency.
  • If associated, the DME Morse identifier is heard every 30-40 seconds (with a higher pitch) interleaved with the VOR identifier.

Beacon Saturation

A DME beacon can serve a maximum of approximately 100 aircraft simultaneously (about 2700 ppps). If this limit is exceeded, the beacon becomes saturated and prioritizes stronger signals (usually from closer aircraft).

Derived Calculations

  • Groundspeed (GS): The equipment calculates groundspeed based on the rate of change of distance. It is only accurate if flying directly towards or away from the station.
  • DME Arc: Flying maintaining a constant distance. In this case, the GS indicated by the DME will be zero.