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. DME.
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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.
DME Distance (Slant Range)
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)
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