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 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 ? General Principle: – Airborne transceiver transmits a pair of pulses –(spaced at 12 μ s for mode X and 30 μ s for mode Y) – Ground transmitter receives the pulses, waits 50 μ s and then transmits another pair of pulses back to the aircraft – Airborne transceiver measures the time between transmission and reception, subtracts the 50 μ s, multiplies by the speed of light and divides by 2. DME ? This is very simple but gets plicated 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 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. SLANT RANGE DME Distance (Slant Range) Ground Range Altitude DME Ground Station The ground station simply receives a pulse pair, inserts the 50 μ s delay and retransmits it. To reduce the effects of reflections it will not reply to another interrogation for about 60 μ s (dead time) 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 pul
