DME (Distance Measuring Equipment )

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DME (Distance Measuring Equipment )

• Frequency Band
• Airborne 1025 MHz 1150 MHz
• Ground 63 MHz below Tx frequency 1025 1087
  MHz 63 MHz above Tx frequency 1088 1150 MHz
• This gives 126 channels but two codings are used
  (X and Y) which doubles the capacity

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DME

• As the name implies , DME provides information on
  the distance from the aircraft to the ground
  station
• Used to establish position along an airway and
  also to establish hold points

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DME

• Frequency Band
• Airborne 1025 MHz 1150 MHz (L band)
• Ground 63 MHz below Tx frequency 1025 1087
  MHz 63 MHz above Tx frequency 1088 1150 MHz
• This gives 126 channels but two codings are used
  (X and Y) which doubles the capacity

4
DME

• General Principle
• Airborne transceiver transmits a pair of pulses
• (spaced at 12µs for mode X and 30µs for mode Y)
• Ground transmitter receives the pulses, waits
  50µs and then transmits another pair of pulses
  back to the aircraft
• Airborne transceiver measures the time between
  transmission and reception, subtracts the 50µs,
  multiplies by the speed of light and divides by
  2.

5
DME

• This is very simple but gets more complicated
  when we want to service more than one aircraft
• We need a method of distinguishing among the
  signals from up to 100 aircraft.
• This is done essentially by generating a random
  set of pulses and correlating with the replies to
  determine the correct ones.

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DME AIRBORNE TRANSPONDER
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DME PULSES
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DME OUTPUTS

• Distance
• Speed
• Time to Station

Notes 1. The last two are valid only if the
aircraft is going directly towards or away from
the ground station. 2. The DME measures SLANT
RANGE to the station.
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SLANT RANGE
Altitude
DME Distance (Slant Range)
Ground Range
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DME Ground Station
The ground station simply receives a pulse pair,
inserts the 50 µs delay and retransmits it. To
reduce the effects of reflections it will not
reply to another interrogation for about 60 µs
(dead time)
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DME Ground StationSQUITTER
The ground station transmits 2700 pulse pairs per
second regardless of the number of aircraft
interrogating. The extra pulse pairs are called
squitter If there are not enough interrogations
to make up 2700 pulse pairs, the ground receiver
increases its sensitivity until noise pulses
trigger enough replies to make up the
difference If there are too many interrogations,
the receiver decreases its sensitivity so that
the weakest interrogations get ignored
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DME Ground StationSQUITTER

• Using squitter has the following advantages
• The transmitter average output power is constant
• The receiver AGC has a constant average signal to
  work with
• The ground receiver sensitivity is maintained at
  the optimum level
• In the case of overload, the aircraft farthest
  from the station are dropped off first.

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DME

• Using squitter has the following advantages
• The transmitter average output power is constant
• The receiver AGC has a constant average signal to
  work with
• The ground receiver sensitivity is maintained at
  the optimum level
• In the case of overload, the aircraft farthest
  from the station are dropped off first.

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DME as a Navaid
Accuracy The ICAO specification for DME is 0.5NM
or 3 of distance Tests done on Canadian DMEs
show that their errors are less than
30m. Integrity DME ground stations are equipped
with monitors which can detect erroneous delays
and out-of-tolerance power output levels. These
shut the system down if and error is detected
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DME as a Navaid
Availability As with most systems there is a
standby transmitter which takes over when the
main one fails. availability is well above 99.9
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DME as a Navaid
Availability As with most systems there is a
standby transmitter which takes over when the
main one fails. availability is well above 99.9