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 DME (Distance Measuring Equipment ) 

• Frequency Band:

 –  Airborne: 1025 MHz –  1150 MHz

 –  Ground : 63 MHz below Tx frequency 1025 –  1087 MHz63 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 alsoto 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 MHz63 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

• 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 thentransmits 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 anddivides by 2.

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 DME

• This is very simple but gets more complicated when wewant to service more than one aircraft

• We need a method of distinguishing among the signals fromup to 100 aircraft.

• This is done essentially by generating a random set of pulses and correlating with the replies to determine thecorrect 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

Ground Range

Al   t  i   t   u d  e 

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 DME Ground Station

SQUITTER

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 itssensitivity so that the weakest interrogations get ignored

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 DME Ground Station

SQUITTER

Using squitter has the following advantages:

• The transmitter average output power is constant

•The receiver AGC has a constant average signal towork 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 towork 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 poweroutput 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%