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Distance Measuring Equipment DME

Distance Measuring Equipment DME. DME - Principle of Operation. Secondary Radar Operates at UHF (960 - 1215MHz) Signal travels one way Requires an interrogator unit in the aircraft and a responder unit on the ground. DME - Principle of Operation.

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Distance Measuring Equipment DME

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  1. Distance Measuring EquipmentDME

  2. DME - Principle of Operation • Secondary Radar • Operates at UHF (960 - 1215MHz) • Signal travels one way • Requires an interrogator unit in the aircraft and a responder unit on the ground

  3. DME - Principle of Operation • The aircraft calculates its distance from the beacon by measuring the time between transmitting a signal and receiving a reply. • There is a 50s delay before the beacon replies

  4. DME - Principle of Operation • Unique signal - double pulse transmission (P0N) • 2 frequencies: one for transmitting, one for receiving 63MHz apart • UHF channels paired/ganged with VHF frequencies

  5. DME Paired Frequencies with VOR/ILS 63MHz

  6. DME - Propagation • UHF • Direct Wave (Line of sight) • “Blade” type antenna on aircraft • Signal may be obscured whilst turning

  7. DME - Why two frequencies? • Ground station can send & receive simultaneously • To prevent “re-triggering” of the ground station caused by reflections

  8. DME - How is the correct reply identified? 1. By “jittering”(dithering or varying) the pulse interval (PI) between double pulses 2. By locking on to pulses coming back at a constant time interval ()

  9. P1 100s P2 85s P3 110s P4 90s P5 Constant time interval A “Jittering” the PRF

  10. DME - Pulse Repetition Frequency • The initial interrogation rate is 150 ppps • If, after 15 000 pulse pairs have been transmitted, the DME has not locked on, the rate drops to 60 ppps • However, once locked on and the DME is in “track” mode, the interrogation rate drops to about 5 ppps

  11. DME - Ground station capability • The average interrogation rate from all aircraft in view is about 30 ppps • The beacon capability is 3000 ppps • Therefore, the beacon is theoretically capable of handling 3000 30 = 100 aircraft

  12. Distance readout is in nautical miles Maximum indicated distance is limited by line of sight range and “gate” circuitry in the DME receiver. DME Range 0 50 200 NM “GATE” 20s wide

  13. DME - Interruption of Signal • If the signal from the beacon is lost for more than 10 s the receiver will go into memory mode • There are two types of memory: • Static memory - last measured slant range is retained for 10s before receiver goes into search mode again • Velocity memory - last measured rate of change is retained

  14. DME - Failure mode • On modern DME receivers the display goes blank

  15. Echo protection Radio Navigation – Chapter 5

  16. The range measured by the DME is the distance along the direct path between the interrogator and the transponder (slant range). Radio Navigation – Chapter 5

  17. PR2 = SR2 - Height2 • PR =  SR2 - Height2 • The ground distance (plan range) can be calculated from Pythagoras’ theorem: SR2 = PR2 + height2 2 height2 2 Radio Navigation – Chapter 5

  18. Q. Height of aircraft = 18 000 ft, DME range is 5 NM. What is the ground range from the station? A. 18 000 ft  3 NM, PR =  (52 - 32) =  (25 - 9) =  16 = 4 NM 2 height2 2 Radio Navigation – Chapter 5

  19. Accuracy of DME • ± 3% of Slant Range or ± 0.5 NM, whichever is greater • E.g. If indicated range is 20 NM, what is the accuracy? • 20 x 3% = ± 0.6NM • If indicated range is 10 NM, what is the accuracy? • 10 x 3% = ± 0.3NM  Accuracy = ± 0.5 NM

  20. Accuracy of DME • Note: If indicated range is 16.66 NM, what is the accuracy? • 16.66 x 3% = ± 0.5NM • If range > 162/3 NM, accuracy = ± 3% of SR If range < 162/3 NM, accuracy = ± ½ NM

  21. Types of ground stations

  22. Co-located & Associated DME- AERODROME Co-located: within 30m/100ft Call signs the same . VOR/DME

  23. Co-located & Associated DME- AERODROME Associated: not within 30m/100ft “WTM” 108.40 . VOR TACAN/DME “WTZ” 108.40 (Ch 21X) Last letter of DME call sign becomes “Z”

  24. Co-located & Associated DME- AIRWAY Co-located: within 600m/2000ft Call signs the same . VOR/DME

  25. Co-located & Associated DME- AIRWAY Associated: not within 600m/2000ft . DME . Last letter of DME call sign becomes “Z” VOR

  26. Identification DME (Tone = 1350Hz) 60 VOR VOR (Tone = 1050Hz) VOR VOR 45 15 VOR VOR 30 DME

  27. Identification DME 60 VOR VOR VOR VOR 45 15 VOR VOR 30 DME

  28. Flying the arc. Radio Navigation – Chapter 5

  29. ILS/DME • The accuracy of ILS/DME is ± 0.1 NM, provided: • The aircraft is on the centre line of the ILS 27 . DME Distance will read ZERO at the THRESHOLD

  30. Questions • What does DME actually measure? • Slant range • What kind of signal is a DME transmission? • Double pulse modulation • In which part of the radio spectrum can DME transmissions be found? • UHF (decimetric) • How do you tune DME? • By selecting a paired VHF frequency • What is the capacity of a DME beacon? • 100 aircraft

  31. Questions • How is an individual aircraft transmission identified? • By “jittering” or variation of the PI/PRF • When does the pulse rate drop to 60 ppps? • After 15,000 pulse pairs if the DME has not locked on • Why do reflections of transmitted signals not re-trigger the DME beacon? • Because the transmit & receive frequencies are different • What is the frequency separation between transmit & receive frequencies? • 63 MHz

  32. Questions • The reply to an DME transmission takes 124s. Ignoring the ground delay, what is the range of the aircraft from the beacon? • 124 s ÷ 12.4 = 10NM • On an aerodrome, a TACAN beacon’s call sign is “BEZ” and the VOR is “BEN”. What is the relationship, if any, between them? • They are associated, not co-located • What is the maximum distance between a VOR & DME to be considered co-located? • 30m/100ft on an aerodrome; 600m/2000ft on an airway

  33. Questions • What is the accuracy of DME/N? • 3% of SR or ±0.5NM • How accurate is an ILS/DME (DME/P)? • ±0.1NM (on the centreline)

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