Dme Theory

1DISTANCE MEASURING DISTANCE MEASURING EQUIPMENT (DME)EQUIPMENT (DME)

ObjectivesObjectives

Students will be able to :Students will be able to :

Describe the basic components of Distance Describe the basic components of Distance Measuring Equipment.Measuring Equipment. Understand operation of DME.Understand operation of DME.

2DME BasicsDME Basics

It shows pilot a distance from a ground station It shows pilot a distance from a ground station (usually VOR station) in Nautical Miles (NM).(usually VOR station) in Nautical Miles (NM). It operates in UHF band between It operates in UHF band between 978 978 --12131213

MHz. Its range is limited to line of sight.MHz. Its range is limited to line of sight. Its range is about 250 NM depending on the Its range is about 250 NM depending on the

altitude of station and aircraft.altitude of station and aircraft. DME stations are usually coDME stations are usually co--located with VOR located with VOR

stations.stations. The distance shown is The distance shown is slant distanceslant distance..

Tactical Air Navigation (TACAN) Tactical Air Navigation (TACAN) SystemSystem

It is a military system which gives both It is a military system which gives both bearing and distance like VOR/DME.bearing and distance like VOR/DME. Its ranging part is exactly the same as DME, Its ranging part is exactly the same as DME,

but with more channels.but with more channels. It operates between 962It operates between 962--1213 MHz.1213 MHz. Civil aircraft may use this facility.Civil aircraft may use this facility.

3Slant RangeSlant Range

Ground Distance

Slant RangeError increases as aircraft nears station

Station

At the stationAt the station

Altitude=6100 ft.

Distance reads as 1.0 NM, eventhough the aircraft is directly overthe station!

Station

4Slant rangeSlant range

Basic operationBasic operation

5Measurement of rangeMeasurement of range

Aircraft DME unit measures the time elapsed Aircraft DME unit measures the time elapsed between transmission and reception of the signal.between transmission and reception of the signal.

R = (tR = (t--d)/12.36d)/12.36 R R = Slant range to station (NM)= Slant range to station (NM) t t = Total elapsed time (= Total elapsed time (s)s) d d = Fixed delay of ground station (50 = Fixed delay of ground station (50 s)s) 12.36 12.36 s = Typical time for signal to travels = Typical time for signal to travel

1NM and return.1NM and return.

Interrogation and replyInterrogation and reply

6Interrogation (airInterrogation (air--toto--ground) signalground) signal

DME has 100 channels with 1 MHz spacing.DME has 100 channels with 1 MHz spacing. TACAN has 126 channels between with 1 TACAN has 126 channels between with 1

MHz spacing.MHz spacing. Interrogation signal is made of series of Interrogation signal is made of series of

pulse pairs (p1pulse pairs (p1--p2, p3p2, p3--p4, p5p4, p5--p6, etc.)p6, etc.) X or Y mode (channel) operationX or Y mode (channel) operation 55--150 pulse pairs per second150 pulse pairs per second

X mode (channel) interrogationX mode (channel) interrogationp1 p2 p3 p4 p5 p6

3.5 s Random Random

12 s

3.5 s

Fixed

Fixed Fixed

Length of each pulse = 3.5 s (fixed)Distance between each pulse = 12 s (fixed)Distance between pulse pairs = variable (random)

7Y Mode (channel) interrogationY Mode (channel) interrogationp1 p2 p3 p4 p5 p6

3.5 s Random Random

36 s

3.5 s

Fixed

Fixed Fixed


Reply (GroundReply (Ground--toto--air) signalair) signal

Reply is transmitted 63 MHz above or Reply is transmitted 63 MHz above or below interrogation signalbelow interrogation signal It is made of series of pulse pairs.It is made of series of pulse pairs. X or Y mode operationX or Y mode operation Channels are numbered 1X, 1Y, Channels are numbered 1X, 1Y,

2X,2Y,2X,2Y,. both in interrogation and reply. . both in interrogation and reply. (For example, 962 MHz is known as 1X or (For example, 962 MHz is known as 1X or 1Y channel)1Y channel)

8X mode (channel) replyX mode (channel) replyp1 p2 p3 p4 p5 p6

3.5 s Random Random

12 s

3.5 s

Fixed

Fixed Fixed


Y Mode (channel) replyY Mode (channel) replyp1 p2 p3 p4 p5 p6

3.5 s Random Random

30 s

3.5 s

Fixed

Fixed Fixed


9Summary of interrogation and replySummary of interrogation and replysignalssignals

30 30 ss3.5 3.5 ss12 12 ss3.5 3.5 ssReplyReply (ground to air)(ground to air)

36 36 ss3.5 3.5 ss12 12 ss3.5 3.5 ssInterrogationInterrogation (air to ground)(air to ground)

Pulse Pulse spacingspacing

WidthWidthPulse Pulse spacingspacing

widthwidth

Y channelY channelX ChannelX Channel

Signal typeSignal type

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Segments of DMESegments of DME

Aircraft segmentsAircraft segments Ground segmentGround segment

Aircraft segmentAircraft segment

TransmitterTransmitter ReceiverReceiver IndicatorIndicator AntennaAntenna

Transceiver

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Aircraft segment External BlocksAircraft segment External Blocks

NAVcontrolhead

DMETX/RX IND

AudioSuppression RNAV

Bus

TransceiverTransceiver

Transmitter continuously transmits its unique, Transmitter continuously transmits its unique, randomly spaced interrogation signalrandomly spaced interrogation signal Receiver is able to recognize its own signal Receiver is able to recognize its own signal

when retransmitted back by the ground station when retransmitted back by the ground station with the same spacing. Then it measures the with the same spacing. Then it measures the range.range. Suppressor bus prevents other radar transmitter Suppressor bus prevents other radar transmitter

from operating when one of them is working. from operating when one of them is working.

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Transceiver modes of operationTransceiver modes of operation

JitterJitter SearchSearch TrackTrack Memory Memory HoldHold

JitterJitter

It is the process of placing random time It is the process of placing random time spacing between the pulse pairs.spacing between the pulse pairs. It is unique to each aircraft and randomly It is unique to each aircraft and randomly

produced.produced. For example, if a DME unit sends 25 pulse For example, if a DME unit sends 25 pulse

pairs per second (called pairs per second (called pulse repetition pulse repetition frequencyfrequency) average random spacing ) average random spacing between pulse pairs will be between pulse pairs will be

1 sec / 25 = 0.040 sec = 40 millisecond1 sec / 25 = 0.040 sec = 40 millisecond

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SearchSearch

It is the process in which DME tries to match the It is the process in which DME tries to match the transmitted signals to the received signals to transmitted signals to the received signals to recognize its own signals, when it is turned on.recognize its own signals, when it is turned on.

During the search period a warning flag in DME During the search period a warning flag in DME indicator is in view, showing that DME system is indicator is in view, showing that DME system is not operative yet.not operative yet.

Pulse repetition rate is about 120Pulse repetition rate is about 120--150.150.

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TrackTrack

Normal operating modeNormal operating mode Continuous range is given as the aircraft Continuous range is given as the aircraft

moves towards or away from the station.moves towards or away from the station. Pulse repetition rate is about 24Pulse repetition rate is about 24--30.30.

MemoryMemory

It becomes active when DME unit loses It becomes active when DME unit loses reply signals from the ground station.reply signals from the ground station. There are two memory mode :There are two memory mode : Static memory : The indication remains Static memory : The indication remains

steady showing the steady showing the last rangelast range before signal before signal is lost.is lost. Velocity memory :The range continues to Velocity memory :The range continues to

change at the last known ratechange at the last known rate.. Memory period lasts about 10 sec.Memory period lasts about 10 sec.

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IndicatorIndicator

StylesStyles self containedself contained integratedintegrated

Warning flagWarning flag Self TestSelf Test DigitalDigital

AntennaAntenna

UHF bandUHF band Short (2 inch)Short (2 inch) Vertically polarized Vertically polarized Blade typeBlade type belly on the CLbelly on the CL

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AntennaAntenna

Ground stationGround station

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Operates with the same principle of aircraft Operates with the same principle of aircraft unitunit When it receives interrogation signal from When it receives interrogation signal from

aircraft it waits 50aircraft it waits 50s.s. Then retransmits back to aircraft at a Then retransmits back to aircraft at a

frequency 63 MHz above or below of frequency 63 MHz above or below of incoming signal.incoming signal.

Ground station operationGround station operation

TX

RX RX

TX

Computer

50us delay

Indicator

ETA

GS

NM F=+/- 63Mhz

P1 & P2 @ dedicated freq. and spacing

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Each ground station can handle up to 100 Each ground station can handle up to 100 aircraft simultaneously.aircraft simultaneously. It is assumed that 95% of aircraft in track It is assumed that 95% of aircraft in track

mode and 5% is in search mode.mode and 5% is in search mode. Thus, to serve 100 aircraft at the same time, Thus, to serve 100 aircraft at the same time,

the station should answer app. 2700 the station should answer app. 2700 interrogation signals (pulse pairs) in a interrogation signals (pulse pairs) in a second. second.

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Where did we find 2700 Where did we find 2700 ppspps??

Number of signals from 95 aircraft in track Number of signals from 95 aircraft in track mode (24mode (24--30 30 ppspps) :) :95 x 25 95 x 25 ppspps = 2375 = 2375 ppspps Number of signals from 5 aircraft in search Number of signals from 5 aircraft in search

mode (120mode (120--150 150 ppspps) :) :5 x 125 5 x 125 ppspps = 625= 625 TOTAL = 3000 TOTAL = 3000 ppspps It is taken as 2700 It is taken as 2700 ppspps in design of ground in design of ground

stations stations

Constant duty cycle operationConstant duty cycle operation

A ground station is designed to reply 2700 A ground station is designed to reply 2700 interrogation signal (pulse pairs) always interrogation signal (pulse pairs) always even if the number of aircraft is less than even if the number of aircraft is less than 100.100. This is known as constantThis is known as constant--duty cycle duty cycle

operation.operation.

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Advantages of constant duty cycle Advantages of constant duty cycle operationoperation

The station is automatically maintained in its The station is automatically maintained in its most sensitive condition.most sensitive condition. The circuit always has a constant number of The circuit always has a constant number of

pulses to work on, thereby simplifying its pulses to work on, thereby simplifying its design.design. In case of interrogation more than 100 In case of interrogation more than 100

aircraft, it allows to maintain service to the aircraft, it allows to maintain service to the closest aircraft and neglect the farthest .closest aircraft and neglect the farthest .

SquitterSquitter

Ground station is designed to emit 2700 Ground station is designed to emit 2700 reply signals per second (constant duty reply signals per second (constant duty cycle) assuming 100 aircraft in the region.cycle) assuming 100 aircraft in the region. If there is less than 100 aircraft, the station If there is less than 100 aircraft, the station

continues to emit 2700 signals, but some continues to emit 2700 signals, but some of them are fake (noise) signals. This type of them are fake (noise) signals. This type of signals are called of signals are called squittersquitter.. SquitterSquitter are not used for range are not used for range

measurement, but may be used for measurement, but may be used for identification of the station by the aircraft.identification of the station by the aircraft.

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SquitterSquitter

A ground station also emits an identification signal A ground station also emits an identification signal in Morse Code at 1350 Hz.in Morse Code at 1350 Hz. Thus, it can be said that the ground station signals Thus, it can be said that the ground station signals

are composed of three types :are composed of three types : Real reply signalsReal reply signals SquitterSquitter (noise) signals.(noise) signals. A continuous identification signalA continuous identification signal If the number of aircraft interrogation the station is If the number of aircraft interrogation the station is

100, then all ground signals are real reply signals.100, then all ground signals are real reply signals.

SquitterSquitter

If number of aircraft less than 100, then If number of aircraft less than 100, then some signals are real reply signals and some signals are real reply signals and others are noise signals.others are noise signals. Assume only one aircraft is in the region and Assume only one aircraft is in the region and

interrogates the station at a rate of 27 interrogates the station at a rate of 27 ppspps. . Then, the signal composition of the ground Then, the signal composition of the ground station will be;station will be;

27 real reply signals.27 real reply signals.27002700--27 = 2673 27 = 2673 squittersquitter signals.signals.

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Frequency pairingFrequency pairing

DME ground facility is usually coDME ground facility is usually co--located located with VOR station.with VOR station. Thus, when pilot selects a VOR frequency Thus, when pilot selects a VOR frequency

on the VHF navigation panel, the frequency on the VHF navigation panel, the frequency of DME located at that station is of DME located at that station is automatically selected.automatically selected. This is know as This is know as frequency pairingfrequency pairing. .

Frequency pairingFrequency pairing

Each NAV frequency Each NAV frequency is assigned two DME is assigned two DME frequencies.frequencies. One for transmitOne for transmit One for receiveOne for receive

By changing modes By changing modes and frequencies, the and frequencies, the each DME channel each DME channel can be used twice.can be used twice. This gives us a virtual This gives us a virtual

200 channels.200 channels.

VHFChannel

DMETX

DMERX

Mode

108.00 1041 978 X

108.05 1041 1104 Y

113.30 1104 1167 X

113.35 1104 1041 Y

You may find a complete table ina DME maintenance Manual.

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HoldHold modemode

If this mode is selected, the frequency If this mode is selected, the frequency pairing is destroyed. That is, in this mode, a pairing is destroyed. That is, in this mode, a change in VOR frequency, will not cause the change in VOR frequency, will not cause the DME frequency change automatically.DME frequency change automatically. The range and bearing information are The range and bearing information are

received from not a common station, but received from not a common station, but different stations. different stations.

VORTACVORTAC

Military aircraft are equipped with TACAN.Military aircraft are equipped with TACAN. It provides range and bearing.It provides range and bearing. In most countries, VOR stations are coIn most countries, VOR stations are co--

located with TACAN, not with DME station.located with TACAN, not with DME station. This type of combination is known as This type of combination is known as

VORTAC.VORTAC.

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VORTACVORTAC

DME flight line test setDME flight line test set

ATC-600A TEST SET

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DME flight line testsDME flight line tests

Flight line test tipsFlight line test tips

Calculation of "timeCalculation of "time--toto--station (TTS)" (in station (TTS)" (in minutes) : (Speed / distance)* 60minutes) : (Speed / distance)* 60 Due to memory feature, the test should be Due to memory feature, the test should be

started with low signal output, then it is started with low signal output, then it is increased until unit locks on. increased until unit locks on. The The squittersquitter on ATCon ATC--600A should be on or 600A should be on or

the DME unit may not search.the DME unit may not search. ATCATC--600 is used only for 17X and 17Y 600 is used only for 17X and 17Y

frequencies. frequencies.

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ATC-600A TEST FREQUENCIES

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DME bench test setDME bench test set

ATC 1400A TEST SET

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DME bench testsDME bench tests

DME bench test tipsDME bench test tips

Calculation TTS (same as flight line test).Calculation TTS (same as flight line test). Test should be started with low signal output.Test should be started with low signal output. SquitterSquitter must be on.must be on. Frequency can be set automatically set to the VHF Frequency can be set automatically set to the VHF

NAV channel.NAV channel. Decimal placement on range/speed/acceleration Decimal placement on range/speed/acceleration

should be watched.should be watched. Unit must be connected to a oscilloscope to for Unit must be connected to a oscilloscope to for

pulse width and spacing test. pulse width and spacing test.

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Objective checkingObjective checking

Can you check DME unit on all channels?Can you check DME unit on all channels? For what tests do you need oscilloscope?For what tests do you need oscilloscope?

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Dme Theory