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

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

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

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.

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.

DME AIRBORNE TRANSPONDER

DME PULSES

DME OUTPUTS

•Distance

•Speed

•Time to StationNotes:

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.

SLANT RANGE

DME Distance (Slant Range)

Ground Range

Altitude

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)

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

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.

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.

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.

IntegrityDME 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

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%

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%