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 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

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

DME Paired Frequencies with VOR/ILS 63MHz

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

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

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 ()

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

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

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

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

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

DME - Failure mode On modern DME receivers the display goes blank

Echo protection Radio Navigation – Chapter 5

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

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

Q. Height of aircraft = 18 000 ft, DME range is 5 NM 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

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

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

Types of ground stations

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

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”

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

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

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

Identification DME 60 VOR VOR VOR VOR 45 15 VOR VOR 30 DME

Flying the arc. Radio Navigation – Chapter 5

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

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

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

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

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)