DME ( DISTANCE MEASURING EQUIPMENT (DME)

DME principle is like SSR RADAR but invert of this. Provide information of range/distance of the aircraft to the DME station. Frequency range 960 - 1215 MHz. Aircraft Transmits interrogate pulse and the ground DME station responds on interogate freq.+/- 63 MHz after preset delay.

Aircraft receiver calculates slant range from the replay.

Aircraft transmit pulsed interogation. Ground station replies after 50 microseconds. Elapsed this time is appropriately converted into a distance for display. [Time in microseconds - 50/12.359 = slant range in NM]

 

SIGNAL CHARACTERISTICS

- all pulse pairs

   interrogation (aircraft o ground)

- pulse width = 3.5 microsecond

- pulse spacing mode X = 12 microsecond

- Mode Y = 36 microsecond

- Radio frequency 1041 - 1150 MHz

- Channel spacing 1 MHz

   replay (ground to aircraft)

- pulse width = 3.5 microsecond

- pulse spacing mode X = 12 microsecond

- mode Y = 30 microsecond

- radio requency 987 -1213 MHz

- channel spacing 1 MHz

X and Y replay freq. are 63 MHz removed from introgation freqs.

E.G Ch 17X introgate freqs = 1041 MHz (the lowest) and replay freq = 978 MHz

17Y introgate freqs = 1041 MHz replay freq = 1104 MHz

There are 100 mode X channels for replay and 100 mode Y Channels for replay.

There are 160 VOR frequencys and 40 ILS frequencys

They are paired off

For example the lowest VOR frequency is 108.00MHz. The paires DME Channel is 17X. Acolocated DME, if fitted to ground beacon, receives only on 1041 MHz. If it receive a signal from an aircraft , it replies on 987 MHz

To ease selection in the cockpit, when the NAV receiver is set to 108.00 in the VHF band, the DME interrogator i automatically set to broadcast on 1041 MHz in UHF band and to listen out on 978 MHz