DISTANCE MEASURING EQUIPMENT

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DISTANCE MEASURING EQUIPMENT

• DME

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PRINCIPLES

• Unlike VOR and ADF which are passive systems, DME
  is an active system.
• The aircraft equipment transmits UHF pulses.
  (interrogating signal)
• The ground station replies with its own UHF
  pulses.
• The aircraft equipment measures the time
  difference between signal sent and signal
  received. Radio waves travel at a constant speed,
  allowing a distance to be computed.

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• In order for this process to occur both ground
  station and aircraft are equipped with a
  transmitter and receiver. (transceiver)
• A DME ground station must be able to
  differentiate between multiple interrogation
  signals. (numerous aircraft in the area)
• This is accomplished by the aircraft transmitting
  two pulses spaced in a pattern unique to the
  unit. (jitter)

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• The ground station responds with a pulse pair
  that matches.
• The aircraft unit scans all replies until it
  locates the one matching its own pulse signature.
• Once located it ignores all other signals and
  concentrates on the match.
• This Lock On process creates a delay from the
  time the DME frequency is tuned until information
  is displayed to the pilot.(1-3sec)

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OPERATION

• DME is possibly the easiest of all navigation
  equipment to use.
• Basic units must be tuned separately, while other
  units are paired to the VOR receiver.
• Most DME ground stations are collocated with a
  VOR facility.
• Most DMEs automatically tune to the DME frequency
  associated with the VOR.
• Simply set the VOR receiver to the required
  frequency and the DME will tune itself.

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• If more than one VOR receiver is installed NAV 1
  or NAV 2 must be selected on the DME.
• A HOLD function allows DME information to be
  retained when the VOR receiver is reset to a
  facility without DME.
• Caution it is easy to lose track of what
  distance information is being displayed NAV 1,
  NAV 2, or a previously held frequency. Descending
  or turning based on the wrong distance
  information could have catastrophic results.

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SLANT-RANGE DISTANCE

• The distance displayed by a DME is subject to
  slant-range error.
• DME measures the straight line distance
  (slant-range) between the aircraft and the ground
  facility not ground distance.
• The altitude of the aircraft has an effect on the
  distance information.
• An aircraft directly over the DME ground facility
  would display a DME equal to aircraft altitude in
  nm.
• This error is negligible when the slant range is
  more than the altitude in 1000s of feet.

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• Slant-range can be converted to ground distance
  by this formula ground distance?(S²-A²)
• Sslant range nm, Aaltitude nm
• As a rule of thumb slant-range error can be
  disregarded when the aircraft is more than a mile
  away for every thousand feet AGL. Ex more than 5
  miles away at 5000 feet AGL.
• DME system accuracy is /-0.5NM or 3 of the
  distance whichever is greater.

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DERIVATIVE INFORMATION

• Although DME only provides distance directly, it
  can also derive groundspeed and time to station.
• The DME computes groundspeed by timing the
  interval between distance changes.
• Time to station is calculated from groundspeed
  and distance.
• This derivative information is limited by the way
  the information is obtained. It is accurate only
  when the aircraft is traveling directly towards
  or away from the station.

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• If an aircraft were to fly a constant DME arc
  around a ground facility the DME would determine
  the aircraft is standing still, because the
  distance from the station remains constant.
• This is an important concept to understand when
  utilizing DME derived groundspeed or time to
  station.

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AUTO-STANDBY MODE

• DME is subject to line-of-sight reception.
• An aircraft below the horizon (out of
  line-of-sight range) will not receive any signals
  from the ground facility.
• Whenever the aircraft DME is not receiving ground
  signals it enters auto-standby mode and stops
  transmitting. (saves electricity and wear and
  tear on unit)
• As soon as the aircraft is in range of a ground
  facilities signal it returns to normal operation
  and begins to transmit once again.

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COLLOCATED FACILLITIES

• DME facilities can exist alone but they are
  usually collocated with a VOR.
• Although VOR and DME operate on different
  frequencies, the two are integrated through
  paired frequencies.
• Only one frequency must be selected (VOR) and the
  other (DME) is automatically selected.
• Care must be taken to ensure the two facilities
  are actually collocated, separately located
  facilities will result in distance discrepancies.

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VOR/DME, VORTAC, TACAN

• VOR/DME collocated facilities only useable by
  civilian aircraft.
• TACAN a military installation giving azimuth
  information on a UHF frequency as well as DME
  information.
• The DME may be used by civilian aircraft, however
  any apparent track information must be ignored.
• VORTAC a civilian VOR collocated with a TACAN,
  civilian aircraft receive track information from
  the VOR and DME from the TACAN.

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