Distance Measuring Equipment (DME)

Wing Commander Rajagopal

Distance Measuring Equipment (DME) using principle of Secondary Radar

Distance Measuring Equipment (DME) is a radio navigation system that measures the slant-range distance between an aircraft and a ground station. It works by transmitting UHF radio pulses from the aircraft and measuring the time taken for the ground station to reply. DME helps pilots determine distance for en-route navigation, instrument approaches, and position fixing.

Distance Measuring Equipment

  • Distance Measuring Equipment works on the principle of secondary radar.
  • Provides accurate slant range.
  • Combination of VOR and DME is an ICAO-approved equipment for airways.
  • TACAN is military equipment equivalent to DME.
  • Installed in military airfields.
  • Indications in the aircraft are the same as DME.

Radar Mile

  • Radar Mile is the time taken for a pulse to travel 1 NM and return back to the radar.
  • Radar Mile can be calculated using the speed of radio waves.
  • 1 Radar Mile = 12.36 Microseconds
  • Distance of the target can be calculated using Radar Mile.
  • One-way Distance (NM) = Time Taken for Pulse to Return ÷ 12.36

Principle of DME

  • DME equipment onboard the aircraft sends a pair of interrogation pulses.
  • Ground transmitter responds after a 50-microsecond delay.
  • The time difference between the pulses gives the slant range in the aircraft.
  • Slant range is the straight-line distance of the aircraft from the DME.
  • Slant range is calculated using the speed of radio waves.

Slant Range and Plan Range

  • Plan range is the range of the aircraft on the ground.
  • Plan range can be calculated from slant range using Pythagoras’ Theorem.
  • Plan Range² = Slant Range² − Height²
  • Slant range is considered equal to plan range when the distance exceeds three times the height.
  • If the aircraft is close to the beacon, slant range must be converted to plan range.

DME Operating Frequencies

  • Frequency range: 962–1213 MHz (UHF Band).
  • 1 MHz spacing between two DME channels.
  • Accommodates 252 channels.
  • DME uses two different frequencies for transmission and reception.
  • The two carrier frequencies are spaced 63 MHz apart.
  • If the aircraft transmits at 962 MHz, the ground station responds on 1025 MHz.

Echo Protection Unit

  • Transmission frequencies of the aircraft and ground station differ by 63 MHz.
  • This frequency difference eliminates certain problems.
  • Prevents aircraft from receiving its own pulses reflected by clouds or the ground.
  • Prevents repeated self-triggering of the transponder due to cloud reflections.
  • DME is equipped with an Echo Protection Unit.
  • The Echo Protection Unit provides a 50-microsecond delay to avoid ground-reflected waves.

Jittering of Pulses

  • DME uses the Random Pulse Repetition Frequency (PRF) technique.
  • This technique is called Jittering.
  • The aircraft transmits pulse pairs at random intervals.
  • The receiver gates open only to accept the corresponding response.
  • This prevents confusion between replies intended for different aircraft.

Electronic Gates

  • Twin pulses are transmitted to avoid accepting matching randomised single pulses from ignition systems or other radars.
  • Electronic gates are established by the aircraft transponder as part of jittering.
  • These gates open the receiver only for the correct response pulse.
  • Only responses matching the transmitted PRF are accepted.

Operating Modes and Features of DME

Search, Track and Memory Modes of DME

Search Mode

  • Active during initial contact.
  • Aircraft transmits 150 pulse pairs per second.
  • After approximately 100 responses are received, transmission is reduced to 60 pulse pairs per second until lock-on.

Tracking Mode

  • Activated after contact with the ground station is established.
  • Aircraft transmits 25 pulse pairs per second.
  • Uses the lock-and-follow technique with electronic gates.
  • The electronic gates continuously adjust according to the changing distance from the station.

Memory Mode

  • Operates during temporary loss of DME responses.
  • DME display shows calculated distances based on ground speed.
  • Airborne equipment retains memory for approximately 10 seconds.

Beacon Saturation

  • Assumes 95% of aircraft are in tracking mode and 5% are in search mode.
  • Average requirement is 27 pulse pairs per second per aircraft.
  • Ground station average output is 2700 pulse pairs per second.
  • Can accommodate approximately 100 aircraft.
  • Beacon saturation occurs when the ground station exceeds 2700 pulse pairs per second.
  • Occurs when more than 100 aircraft interrogate the beacon.
  • The ground station reduces receiver gain, excluding distant aircraft.

Associated VOR/DME

  • VOR and DME are selected using the same frequency selection in the aircraft.
  • Associated Terminal VOR-DME stations are located within 100 ft (30 m).
  • Associated Route VOR-DME stations are located within 2000 ft (600 m).
  • Non-associated VOR/DME may be located at the same place.
  • Non-associated VOR/DME stations separated by 1 NM have the third ident letter Z.
  • Example: VOR BBB may have DME BBZ.
  • Non-associated VOR/DME stations more than 6 NM apart have different identifiers.

Accuracy of DME

  • DME accuracy is based on a 95% probability.
  • Narrow Spectrum DME (DME/N): ±1.25% of range or ±0.25 NM.
  • Precision DME (DME/P): ±0.20 NM.
  • Ground speed computation becomes less accurate when directly overhead the station.
  • Ground speed indication is valid only when flying directly toward or away from the station.
  • ILS/DME distance is measured from the runway threshold.
  • This distance is accurate only during the approach.

Designated Operational Coverage (DOC)

  • Maximum range is based on the line-of-sight equation.
  • Practical range is approximately 250 NM.
  • Designated Operational Coverage (DOC) protects DME from range errors caused by co-channel interference.
  • DOC 20/240 indicates coverage of 20 NM at 24,000 ft.