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Subject Code: AVI2041
Malaysian Institute of Aviation Technology
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AFD 31202
AV2240
AIRCRAFT PULSE
AVIONICS SYSTEMS
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Malaysian Institute of Aviation Technology
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CHAPTER 2
Distance Measuring Equipment
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Table of Contents
LEARNING OBJECTIVES
INTRODUCTION
PRINCIPLES OF DME NAVIGATION
Slant Range
Frequencies
Pair of Pulses
Random Spacing
Audio
DME SYSTEM
DME System Description
DME Navigation Procedures
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DME TRANSCEIVER SYSTEM
Functional Description
Transceiver System Operation
Rockwell Collins 860E5 DME Transceiver System
Rockwell Collins 860E5 Operating Modes
DME INDICATORS
TACAN
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Learning Objectives
Upon completion of this chapter, you will be able to:
State the principle of Distance Measuring Equipment navigation system,
includes Slant range, Frequencies, Random spacing and Audio
Describe the use of radio pulse signals to measure flying aircraft.
Recognize the concept of aircraft distance measurement from air to ground.
Describe the configuration of DME navigation system.
Explain the DME system and navigational procedure.
State the DME functional description.
Explain the DME transceiver system operation.
Explain the operation of DME indicator.
State the principle of TACAN.
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Introduction
In order to measure the distance from the aircraft to a radio tower on the
ground, which may be located at the airport, a radio signal is transmitted
from the aircraft to the ground.
When a ground transmitted radar pulse is transmitted into a "clear" sky,
the reflection or echo call is detected.
For aircraft transmitted pulse system to work, the ground tower must have
a receiver and then must retransmit a return signal to be received by the
aircraft.
The aircraft avionics transmitter and receiver and time measuring avionics
equipment is referred as the DISTANCE MEASURING EQUIPMENT or
DME.
The primary function of airborne DME is to calculate the aircraft's distance
to or from a selected enroute VOR navigation station or an approach ILS
facility.
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The displayed DME distance and VOR bearing are used by the pilot to
determine the aircraft's polar coordinates or distance from a fixed point (ground
station) and direction from a fixed line (radial from magnetic north).
In addition to providing nautical mile distance, most DME systems also
compute and display:
The aircrafts velocity in respect to the ground station, known as ground-
speed
The time required for the aircraft to intercept the ground station
A DME station is typically located at a VOR station, is known as VOR/DME
facility;
near an instrument landing system (ILS) at an airport, is known as
ILS/DME.
When a VOR or LOC frequency is selected on the VHF navigation control
panel, the frequency of the associated DME station (if there is one) is
simultaneously selected.
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In military aircraft, VOR/DME is replaced by TACAN (TACtical Air Navigation)
stations for obtaining both bearing and distance information.
Civil aircraft use only the distance measuring portion of the TACAN signal.
Bearing information for civil aircraft is not provided by the UHF TACAN station
since civil aircraft are equipped with VHF omnirange navigation receivers to
obtain the radial bearing.
A ground station equipped with both VOR and TACAN is known as a
VORTAC station.
Military aircraft obtain both bearing and distance information through the
TACAN system,
while civil aircraft use the VOR station for bearing information and the
DME portion of the VORTAC for distance measurement.
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Principles of DME Navigation
The majority of airborne DME systems automatically tune their respective
transmitter and receiver frequencies to the paired VOR/LOC channel (in the
same manner that the glideslope is automatically tuned to its paired localizer
frequency).
The DME or TACAN station is identified by a 1350-Hz coded audio tone
which is transmitted every 30 seconds.
An airborne DME transmits and receives pulse-modulated signals in the UHF
frequency range of 960 MHz to 1215 MHz.
The effective range of the airborne DME transceiver in nautical miles can be
calculated by multiplying 1.23 by the square root of the aircraft's altitude in
feet.
For example, an aircraft at 30,000 feet would be able to receive a ground
station 200 nautical miles away.
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Typical DME navigation system block diagram
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The principle of the DME operation is based on the airborne system
transmitting paired pulses, known as interrogations, to the DME or TACAN
ground station.
The ground station receives the interrogation signals and replies (after a 50
microsecond delay) by transmitting paired pulses that are synchronous to the
interrogation pulse pair back to the aircraft DME system on a different
frequency.
The time required for the round trip of this signal exchange is measured in
the airborne DME transceiver and is translated into distance in nautical miles
from the aircraft to the ground station.
This distance is then displayed on the DME indicator.
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Once the ground station reply signal is received, the airborne DME transceiver
will initially go into a search mode, and will examine all signals received which
have a regular time relation with respect to its own transmitted pulse pair.
When the search circuit determines which received pulses are due to its own
interrogations, it will lock on to them and go into a track mode, at which time,
the slant range distance will be displayed on the DME indicator.
In practically all radio equipment using an oscillator, stability is much to be
desired and is made as good as possible.
The aircraft DME system, however, utilizes one oscillator (to control spacing of
pulses) whose stability is deliberately made very low.
This is the secret of success for a DME system since this oscillator provides
the spacing of transmitted pulse pairs in a random manner.
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When an aircraft is flying at altitude, the direct distance to the station will be
the slant range or line-of-sight distance.
The difference between the measured distance on the surface and the DME
slant range distance is called the slant range error.
The error will be maximum when the aircraft is directly over the ground facility,
at which time, the DME will display altitude in nautical miles above the station.
Slant-range error is minimum at low altitude and long range, and is negligible
when the aircraft is one mile or more from the ground station for each 1,000
feet of altitude above the elevation of the station.
The Figure below illustrates the effect of altitude and range on slant-range
error.
Slant Range
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The basic concept of DME slant-range distance
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The airborne DME computes slant-range distance to or from the station as
follows:
D = (T – 50 microsec) / 12.359
where:
D = slant-range distance in nautical miles
T = time in microseconds between transmission of the interrogating
pulse pair and the reception of the corresponding reply pulse pair
50 microsec = delay in DME ground station between reception of initial
interrogation and transmission of a reply
12.359 microsec = time required for RF energy to travel one nautical mile
and return
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The DME computes the aircraft ground-speed as the rate of change of the
distance with respect to time.
Since this computation is measured as a function of slant range, the ground-
speed would read zero when the aircraft is flying over the station or in a circle
at a constant distance from the station.
In both of these cases, the distance is not changing as would if the aircraft
was flying directly to or from the DME station.
The aircraft receiver has a complex task in determining which response is
from its own transmission before making the calculation of distance to the
tower.
Slant Range is different from the GROUND RANGE due to the triangle
determined by the altitude.
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If a computer, or the pilot, knows the altitude, then the ground range can be
calculated.
The computer can now also calculate the GROUND SPEED by knowing the
difference between distance and the time for a few readings, but only if flying
directly to the tower.
DME slant range and ground range calculation
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The VOR frequencies range between 108.00 MHz to 117.95 MHz with
different frequencies for different towers.
The VOR frequency spacing is 50 kHz between channels with a resulting 200
channels.
The DME uses the UHF frequencies between 960 MHz and 1215 MHz.
Because the DME and VOR systems share the same station, it was then
natural to have a VOR transmitted frequency "paired" with a DME receiver
frequency.
Thus the pilot would have to only set-in the VOR Frequency and the DME
transmitted frequency would be automatically set.
The DME effective range of operation is also in the VHF line-of-sight and at an
altitude of 30,000 feet would be 200 nautical miles.
Frequencies
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The DME signal sent by the aircraft is a Pair of Pulses, which are Amplitude
Modulated or keyed.
Pair of Pulses
DME air to ground pulses
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The aircraft uses a 12 microseconds pulse spacing for 100 Mode X
Channels or 36 microseconds apart for Mode Y channels.
The DME tower receives these pulses and sends back the same pulse pairs
with an added fixed 50 microsecond delay to provide a fixed ground station
delay for accurate calculations.
The ground transmitter returns 12 microsecond pairs for incoming 12
microsecond pairs, and for incoming 36 microsecond pulses, returns pulses
with a lower 30 microsecond pulse spacing.
The aircraft equipment can now measure the delay from transmission to
reception, subtract 50 microseconds, and calculate the distance, D, from the
ground to the aircraft.
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Because the DME tower receives signals at only one frequency and there are
many aircraft sending pulse pairs at this same frequency and at the same
pulse spacing, there are numerous returned pulse pairs, and the aircraft
would have a difficult task to determine which of the returns are in response to
its own transmission.
In order for the aircraft to make this determination, each aircraft generates a
different "random" spacing between pulse pairs and then only looks at pulse
pairs that are returned with its own "random" spacing.
Random Spacing
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With an aircraft transmitted nominal spacing between the two pulses in the
pair of 12 microseconds, the ground DME transmits the reply pulses also at a
spacing of 12 microseconds but at an RF frequency that is 63 MHz higher.
Spacing with pulse pairs
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An alternate aircraft mode is to transmit pulses with a 36 microsecond
spacing between pulses in the pair.
In this case, the ground DME transmitter responds with a reply spacing of 30
microseconds but at an RF frequency that is at 63 MHz lower than the
receiver frequency.
The "random" spacing between the pairs is how the aircraft can determine
the reply to its own transmission.
Since the random spacing of the pulse pairs is unique to each DME unit, the
aircraft is able to recognize its own signal when retransmitted by the ground
station, and to distinguish it from other DME transmissions in the same area.
The aircraft DME constantly (although intermittently) transmits its uniquely,
randomly spaced pulse pairs to the DME ground station.
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After a short delay, the DME ground station retransmits these pulse pairs at a
frequency either above or below the frequency of the airplane transmitter.
Random spacing of the pulses makes it possible for each DME system to
discriminate between its own retransmitted signals and those of other aircraft.
The random spacing identification also makes it possible for the aircraft
receiver to determine elapsed time between transmission from the aircraft
and the receiving of that signal retransmitted from the ground station.
Since this time interval is a function of the intervening distance between the
airplane and the ground station, the aircraft system can display the distance
between the airplane and the ground station.
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Airborne receivers for DME are provided with an audio system that
receives identification codes from DME stations.
This makes it possible for the pilot to identify positively the station that
the DME has locked onto.
In the majority of VOR/DME receivers, when a particular VOR frequency is
selected, the associated DME frequency is automatically selected for that
station.
Like most other radio navigation aids, the signal received from the ground
also has an Audio tone at 1350 Hertz which indicates the Morse Code for
the particular station so that the pilot will not be heading to the wrong
station.
Audio
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Typically, The DME tower at Cedar Rapids Iowa has a VOR frequency of
117.60 MHz and call letters of CID with the corresponding Morse Code of
(dash-dot-dash-dot, then dot-dot, then dash-dot-dot).
The pilot would fly to the tower at this airport until a distance of 15 miles is
reached and then the aircraft would turn in an arc until the landing heading is
reached.
The DME outputs would be connected to the Audio Panel as well as to data
processing Computers.
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DME Systems
The operation of the airborne DME system is based on the transmission of
paired pulses that are received and replied to by a ground station.
The spacing between the pulse pairs transmitted by the airborne DME is
either 12 or 36 microseconds apart, depending on the operating mode.
The ground station reply also consists of paired pulses, spaced either 12 or
30 microseconds apart;
however, the reply is transmitted on a different frequency.
The time required for the round trip of this signal exchange is measured in
the airborne DME transceiver, translated into nautical miles from the aircraft
to the ground station, and displayed on the DME indicator.
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DME system configuration
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A block diagram of the DME ground station and airborne DME is shown in
Figure below.
The antenna is switched back and forth by a duplexer arrangement in the
antenna lead-in.
The frequency selected at the VHF NAV control panel also controls the
receiver and transmitter frequencies selected in the DME interrogator.
DME System Description
DME system block diagram
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The transmission exchange cycle begins when the airborne DME transceiver
transmits pulse pairs on the receive frequency of the ground station; this may
be any one of 200 channels ranging from 1,041 MHz through 1,150 MHz.
Upon reception of the coded pulse pair interrogation, the ground station
decodes the received signal and transmits a pulse pair reply (after a 50
microsecond delay) on a frequency offset by 63 MHz from the interrogation
signal.
The airborne DME receiver operates in the frequency range of 978 MHz
through 1,213 MHz. The reception of the ground station pulse pair by the
DME receiver concludes one complete DME cycle.
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The 50 microsecond delay in the reply from the ground station is added to
eliminate the possibility of uncoordinated operation when the aircraft and
ground station are at close range.
Without the delay, the airborne DME could still be transmitting its second
pulse when the first pulse of the reply was received.
The Ground Based system receives a pulse pair at its own designated
frequency and, after a 50 microsecond delay, transmits the pair at either the
12 or 30 microsecond spacing and at the frequency which is 63 MHz higher
or lower than its receiver frequency.
When no aircraft pulse pairs are received, the DME Ground Station sends
out its own series of random pulses, called "Squitter" to show the aircraft
that it is active and on the air.
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It also sends out these Squitters between responses to aircraft interrogations
to maintain a constant pulse repetition frequency (PRF) rate of 2,700 pulse
pairs per second.
Squitter provides filler pulses between replies to interrogations to maintain the
ground station transmitter at a constant duty cycle.
As the number of interrogations increases, the squitter will be replaced with
reply signals.
In addition to transmitting squitter and reply pulse pairs, the ground station
also transmits a 1,350-Hz coded audio identification signal every 30 seconds.
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The aircraft uses its receiver to wait for a signal from a DME tower to see that
it is in range of the station.
It waits and receives signals only in this AUTOMATIC STAND-BY MODE.
When it does receive DME's signal, it switches to a SEARCH MODE and
sends out its pulse pairs at 90 pulse pairs per minute.
The receiver then looks at the responses corresponding to all distances from
0 to 390 miles, and varies its own rate in a "fixed random" manner until it is
certain that the received pulse pairs are its own.
It then only looks at a specific range of "time delays" for what corresponds to
its "distance. This time range is called the WINDOW or RANGE GATE for its
own reply.
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When the computer electronics in the DME is certain that it has the correct
range, it calculates the Distance and Ground Speed and then switches to a
TRACK MODE and sends out the pair of pulses at a reduced rate of 22.5
pairs per second.
It displays the DISTANCE to the pilot.
It continues to look in the time Window or Range Gate for the pulse pairs.
If three consecutive pulse pairs are not detected, or if 7 out of 15 pulse pairs
are not detected, it reverts back to the Search Mode.
In actual operation, a given ground station will be interrogated simultaneously
by a number of aircraft which are within range and tuned to the station's
frequency.
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The ground station will then reply to all interrogations, and each aircraft will
receive the sum total of replies to all aircraft.
However, if the ground station receives more than 2,700 interrogations per
second, it will reply only to the stronger interrogations rather than increase its
PRF rate.
To prevent interference from replies to other DMEs, it is arranged that each
DME's interrogation pulses occur at a rate which intentionally varies, within
limits, in an irregular manner.
This effect is caused by permitting a non-stabilized multivibrator circuit to
exercise gross control over the interrogation rate.
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In order that the DME may distinguish replies to its own interrogations from
squitter or replies to other DMEs, a "stroboscopic" search process is
employed in the distance circuit.
Stroboscopic refers to a technique wherein a particular set of recurring pulses
is located at a point in time by matching to their periodicity an adjustable
electronic time-gate.
The strobe locates the proper reply pulses by finding the fixed time delay,
measured from its own previous interrogation pulse pair, at which a reply
pulse is repeatedly received.
The strobe progressively scans various time delay intervals by means of a
sliding ‘time slot' or "range gate".
It quickly tests each time slot position for the number of successive reply
pulses received.
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Issue No : 000 Pulse sequence process
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If no replies or only sporadic replies are received, the strobe advances the
range gate to test a slightly longer time delay interval.
When, at some particular time delay interval, a sufficient number of recurrent
pulses are detected, the strobe's search will be completed and the range gate
will be locked-on to that particular time interval.
Upon completion of the search mode, the DME will enter the track mode, at
which time; the slant range distance will be displayed on the DME indicator.
In the track mode, the airborne DME reduces its PRF rate to reduce the load
at the ground station, and the delay setting of the strobe's range gate
automatically and continuously follows any normal variations in the time delay
of the proper reply pulses.
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Such variations will occur if the aircraft's distance is actually changing as a
result of its flight path.
If the received reply signal is momentarily interrupted, a memory circuit will hold
the display at the last reading for approximately 10 seconds until the signal is
again recovered before reentering the search mode.
The majority of DME systems operate on 200 channels.
These consist of 100 X-channels and 100 Y-channels.
When operating on X-channels, both the airborne DME and the ground station
use and recognize 12 microsecond transmitter and receiver pulse pair spacing.
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Y-channels differ from X-channels in that the airborne transceiver transmits
pulse pairs that have a 36 microsecond spacing and listens for ground station
replies with pulse pair spacing of 30 microseconds.
Correspondingly, in Y-channel operation, the ground station transmits 30
microsecond pulse pairs and listens for 36 microsecond pulse pair
transmissions from the airborne system.
Since it is possible for both airborne and ground stations to transmit on the
same frequency using X- and Y-channels, pulse pair time separations are
used.
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The polar coordinates of the aircraft may be determined by comparing the
distance and bearing information available from the airborne DME and VOR
systems.
An approach procedure is also available that combines the use of VOR
radials and DME information for interception of the localizer course.
This type of "arc" approach provides the pilot with a smooth transition onto the
approach path by eliminating much of the vectoring commonly used in
interception.
Basically, when the DME arc approach is available, the pilot maintains a
constant specified distance from the selected station by flying a circular
pattern around it.
When a predetermined VOR radial is intercepted, the pilot initiates an inbound
turn to provide a smooth transition to the ILS approach course.
Using the DME in this manner eliminates the need for a procedure turn.
DME Navigation Procedures
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DME Transceiver System
The S-TEC DME-451 system is a typical
example of a DME system used in private and
light corporate aircraft.
The DME-451 system consists of:
TCR-451 transceiver
IND-451 indicator
ANT-451 antenna
The IND-451 indicator provides a continuous
Light Emitting Diode (LED) readout of DME
distance in nautical miles in the top display,
while the bottom display is controlled by the
display selector control.
Functional Description
S-TEC IND-451 DME indicator
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The information that may be selected for
readout on the bottom display consists of
any one of the following:
Ground-speed in knots, time-to-station
(TTS),
Elapsed time (ET),
Greenwich Mean Time (GMT), and
Estimated time of arrival (ETA).
TTS, ET, GMT, and ETA are displayed in
minutes.
The ET pushbutton starts, stops, and
resets the elapsed time display each time
the button is pressed.
S-TEC IND-451 DME indicator
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The NAV mode control on the IND-451:
Applies power to the system and
Provides selection of which VOR/LOC
navigation receiver is to be used to control the
frequency selection for the TCR-451 transceiver.
Provides a DME frequency hold (H) function to
allow the navigation receivers to be tuned to an
alternate frequency
The RNAV position on the mode control allows
distance information to be displayed from the Area
Navigation (RNAV) System.
S-TEC IND-451 DME indicator
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Issue No : 000 S-TEC TCR-451 transceiver block diagram
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A block diagram of the TCR-451 transceiver is illustrated in Figure above.
The 2-out-of-5 frequency control information supplied by the VIR-351
navigation receiver or comparable VHF control head is applied to the TCR-
451 transceiver for DME channel selection.
Within the TCR-451, the 2-out-of-5 control logic is decoded into DME
channels
Determine the transmit and receive frequencies necessary for compatible
operation with the selected station.
Distance information, as well as velocity and flag data, is provided to the
IND451 indicator from the TCR-451 transceiver.
Distance data is computed on an analog 40 millivolt per nautical mile signal,
and velocity is calculated on a 20 millivolt per knot analog signal.
Functional Description (cont’d)
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Time-to-station is computed in minutes by an analog divider contained within
the indicator.
A system failure warning flag that blanks the indicator display is also supplied
by the TCR451 transceiver.
The TCR-451 receiver, shown in Figure below, consists of:
a low-pass filter
diplexer
preselector
double-conversion IF amplifier
The reply pulse pair from the DME ground station is received at the antenna
and applied through a low-pass filter to the diplexer junction.
Functional Description (cont’d)
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Issue No : 000 S-TEC TCR-451 receiver block diagram
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In the receive mode, the pin diode is reverse-biased and appears as an open
circuit to the RF.
This forces the received signal to flow into the preselector.
In the transmit mode, the pin diode conducts and connects the transmitter to
the antenna through the low-pass filter.
The transmission line quarter-wave section between the diplexer junction and
the preselector reflects the low input impedance of the preselector as an open
circuit at the diplexer junction,
therefore, inhibits power flow into the preselector, and
provide a low impedance at the preselector input to ensure proper diplexer
operation
Functional Description (cont’d)
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Issue No : 000 S-TEC DME-451 system interconnect wiring diagram
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In the installation of DME equipment in an aircraft, the location of the antenna
is critical.
The antenna is a short stub, approximately 2.5 in. [6.35 cm] long, usually
mounted on the bottom of the fuselage.
Care must be taken in locating the antenna, because it can be blanked out
easily by obstructions such as landing gear or other antennas nearby.
It is recommended that manufacturer's instructions for installations in similar
aircraft be observed when making a new installation.
Functional Description (cont’d)
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Transceiver System Operation
The Rockwell Collins 860E-5 DME transceiver, commonly used in airline
operations, conforms to ARINC Specification No. 568-5.
Rockwell Collins 860E5 DME Transceiver System
The Rockwell Collins 860E-5 DME transceiver transmits coded interrogation
signals to the ground station.
The ground station receives the interrogation and returns a coded reply signal
for each interrogation.
Upon receiving the reply signal, the DME computes the slant-range distance to
or from the ground station.
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Issue No : 000 Rockwell Collins 860E-5 DME transceiver block diagram
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Issue No : 000
Rockwell Collins 860E5 DME Transceiver System (cont’d)
Measurement of the slant-range distance from the aircraft to a VOR/DME,
ILS/DME, TACAN, or VORTAC ground station begins with the selection of the
corresponding VHF frequency on the navigation frequency control unit.
The 2-out-of-5 logic supplied by the control unit is applied to the video
processor in the 860E-5 DME.
Within the video processor, the 2-out-of-5 logic is converted into a BCD number
representing 1 to 126 channels.
Each of the 126 channels may have X or Y spacing, thus producing 252
available DME channels.
The 860E-5 transmit frequency range is 1,025 M Hz to 1,150 M Hz and the
receive frequency range is 962 M Hz to 1,213 MHz.
The 860E5 has an extended lower frequency range, thus providing 52
additional channels in comparison with the S-TEC DME-451 system previously
discussed.
58. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 DME Transceiver System (cont’d)
The channel number, in BCD format, is applied to the SMO board that produces a
tuning voltage to the varactors contained within the voltage controlled oscillator.
The VCO generates an RF signal in the frequency range of 256.25 MHz to 287.5
MHz.
This signal is multiplied by four to produce both a pulsed transmitter drive
signal, and a receiver injection frequency in the 1,025 MHz to 1,150 MHz
range.
The DME interrogation period begins with a pair of RF pulses being transmitted.
Following the interrogation, the receiver portion of the DME listens for any ground
station replies.
The time of the interrogation period is dependent upon the DME mode of
operation.
In search mode, a nominal 90 pulse pairs per second are transmitted. In track
mode, a nominal 22.5 pulse pairs per second are transmitted.
59. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 DME Transceiver System (cont’d)
During the transmit phase, the X/Y mode signal from the video processor is
applied to an encoder in the range computer to control the pulse pair spacing (12
microseconds for an X-channel, or 36 microseconds for a Y-channel).
A PRF generator in the range computer initiates the pulses that are applied to the
encoder.
The two encoder outputs, modulation trigger and driver trigger, are applied to the
modulator assembly and driver board assembly, respectively.
During the time that the modulation output signal from the modulator assembly is
applied to the power amplifier (PA) assembly board, the 1,025-MHz to 1,150-
MHz signal from the driver board assembly is amplified in the PA assembly
board.
These amplified RF pulse pairs are routed through a circulator and lowpass
filter to the antenna.
60. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 DME Transceiver System (cont’d)
The circulator acts an electronic microwave switch to provide the RF output to
the antenna while isolating the RF from the input of receiver preselector.
Whenever a pulse pair is transmitted, a suppression pulse pair is
simultaneously sent to the internal receiver and external Air Traffic Control
Transponder or the other DME (if a dual DME system is installed).
These suppression pulses protect the receiver portion of the Transponder
or other DME from damage due to possible reception of the high-powered
RF energy from the DME pulse transmission.
After the interrogation pulse pair has been transmitted, the receiver portion of
the DME becomes active.
The 962-MHz to 1,213-MHz signal received from the ground station is routed
through the circulator and lowpass filter to the preselector assembly.
Within the preselector assembly, the RF signal is routed through varactor-tuned
filters that receive their tuning voltage from the curve shaper board.
61. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 DME Transceiver System (cont’d)
The RF signal from the filters is mixed with the local oscillator
signal from the driver board assembly to produce the 63-MHz IF
signal.
Within the IF amplifier board, the IF signal is mixed, amplified, and
detected to produce the IF video signal.
The IF video signal is applied to a pulse-pair decoder in the video
processor that produces a decoded video pulse for properly
spaced pulses.
The decoder determines if the pulses have sufficient amplitude and
are properly spaced for the channel selected (12 microseconds for
an X-channel, or 30 microseconds for a Y-channel).
The decoded video signal is applied to the range computer.
If the IF video signal contains a 1,350-Hz audio identification code
signal, it is amplified in the video processor and applied to the
62. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 DME Transceiver System (cont’d)
The outputs from the range computer are routed through the video processor
and made available to an external DME indicator for display.
The distance data is available in either a pulse-pair format or by means of a
digital data bus.
The distance measurement from the pulse pair output is represented by the
spacing between the pulses.
In both the two-wire and six-wire data bus outputs, the distance
measurement is represented by a 32-bit data word.
The range rate output consists of a series of pulses, one pulse being
outputted for each 0.01 nautical mile change in distance.
The presence of the flag output indicates that a fault exists in the equipment.
All data outputs are inhibited whenever a fault appears.
63. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 Operating Modes
While the aircraft is on the ground, the DME system may be in the MANUAL
STANDBY mode, as selected on the control head.
In this mode, the DME transmitter is inhibited and the receiver is operative.
If the ground station signal is received during the standby mode, the identification
code will be audible but the digital distance indicator will display four dashes.
When the aircraft is airborne, the DME starts in the AUTOMATIC STANDBY
mode.
In this mode, the DME transmitter is inhibited and the receiver is operative.
The DME will remain in this mode until the receiver determines that the antenna
is receiving more than 450 squitter pulse pairs per second from a ground station.
When this occurs, the DME switches to the SEARCH mode.
64. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 Operating Modes (cont’d)
In the SEARCH mode, the DME interrogates the ground station by
transmitting pulse pairs at a PRF of 90 pulse pairs per second.
After each interrogation, the DME receiver searches the ground station
signals for a reply pulse pair that is synchronous to the interrogation pulse
pair.
The receiver searches during the time a signal would be received from a
ground station located between zero and 390 nautical miles away.
The range computer counts the time from the interrogation pulse pair to the
decoded reply pulse that it locates and stores this time.
Once the DME has located a decoded reply pulse, it waits until the next
interrogation pulse pair is transmitted.
It then counts out to the time at which the last decoded replies pulse was
received and develops a range gate.
65. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 Operating Modes (cont’d)
Presence of a decoded reply pulse in the range gate means that, twice in a
row, the DME has found a pulse located at the same time interval after the
second interrogation pulse.
When this occurs, the DME continues to develop a range gate at this same
point in time for consecutive interrogation pulses.
Location of seven decoded reply pulses in fifteen consecutive interrogation
periods is the criterion necessary for the DME to switch to the PRE-TRACK
mode.
Development of the range gate, at one point in time for fifteen consecutive
interrogation periods, can be terminated.
This termination will occur if the DME fails to find a decoded reply pulse
during three consecutive interrogation periods, or if it loses the 7-out-of-15
decision.
66. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 Operating Modes (cont’d)
When a termination occurs, the DME begins to search outbound from the
previous distance to 390 nautical miles, and then from zero to 390 nautical
miles, until if finds another decoded reply pulse.
When another decoded reply pulse is found, the range gate is then developed
at that period of time.
This process continues until the DME finds a point in time at which seven
decoded reply pulses occur within fifteen consecutive interrogation periods.
The DME will then switch to the PRE-TRACK mode.
In the PRE-TRACK mode, the DME determines the ground speed, or relative
velocity of the aircraft with respect to the ground station.
This is accomplished during the four-second PRE-TRACK mode by a velocity
accumulator which fine-positions the range gate so that the reply pulses are
centered within the range gate.
67. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 Operating Modes (cont’d)
The velocity accumulator determines both the direction of range gate
movement, either inbound or outbound, and the slew rate of the range gate
to track the reply pulses.
During PRE-TRACK mode, the DME continues to interrogate the ground
station at 90 pulse pairs per second and valid data is displayed on the
indicator.
After the four-second PRE-TRACK mode, the DME switches to TRACK
mode.
During TRACK mode, the interrogation rate is decreased to 22.5 pulse pairs
per second and the velocity accumulator and error detector continue to keep
the reply pulses centered in the range gate.
The criterion for maintaining track is that the DME continues to find at least
seven synchronous decoded replies for every fifteen interrogation periods.
If this criterion is not satisfied, the DME will go into MEMORY mode.
68. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Rockwell Collins 860E5 Operating Modes (cont’d)
The nominal 11.4 second MEMORY mode is entered when a temporary or
permanent loss of reply signal occurs.
During MEMORY mode, the DME continues interrogations at the 22.5 pulse
pairs per second rate, and distance is displayed as if the station were still
being tracked.
If the signal is re-acquired during MEMORY mode, the DME returns to TRACK
mode.
If the signal is lost for a length of time greater than 11.4 seconds, the DME
reverts back to SEARCH mode.
69. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
DME Indicator
The DME equipment mounted in an airplane consists of timing circuits, search
and tracking circuits, and the indicator.
The timing circuits measure the time interval between the inter-rogation and
the replay, thus establishing the distance of the ground station from the
airplane.
The search circuits cause the airborne equipment to seek a reply after each
challenge, a function accomplished by triggering the receiver into operation
after each interrogation.
When the receiver picks up a reply of the correct code, the tracking circuit will
operate and enable the receiver to hold the received signal.
The time interval is measured and converted into a distance reading, which is
then displayed on the DME indicator.
70. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
If the airborne receiver picks up a signal with an incorrect code (that
transmitted from another aircraft), the equipment automatically rejects that
signal.
Any airborne DME receiver will accept only signals that were originally
transmitted by its own equipment.
This means of signal discrimination allows several aircraft to navigate
using the same DME ground station.
DME distance indications are displayed digitally on one or more panels or
instruments.
Figure below shows how a radio magnetic indicator (RMI) has been combined
with DME indicators in an instrument called a digital distance radio magnetic
indicator.
71. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
A digital distance radio magnetic indicator
72. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
As an airplane equipped with DME is approaching a DME station and is
receiving DME information, the distance readout will continue to change as the
distance from the station changes.
The rate of change is fed to a computer that produces a ground-speed
indication.
In many of the advanced navigation systems, the time required to reach a
given station or waypoint is also displayed.
This is shown in the photograph of a DME unit and indicators in Figure below.
73. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
DME remote unit and indicators
74. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
The computer does not only calculate the RANGE and SPEED, but can use
these factors to also calculate the Estimated Time to Arrival, ETA.
The indication of Distance can be in "Statute" Miles or usually Nautical Miles,
in Ground Range or usually in Slant Range.
The frequency is selected in the VOR NAV panel as a frequency pair for the
VOR and DME.
A typical digital readout is on the DME Indicator and can include:
Air Speed
Estimated Time to Arrival
Elapsed Time of Flight
others
75. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
For the selected VHF NAV frequency of 108.10 MHz, the DME frequency is
1042 MHz and the aircraft pulse pair spacing is 12 microseconds.
Frequency select & DME indicator
76. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
A distance-measuring and bearing-indicating system similar to the VOR/DME
system described above is called TACAN.
This system was developed by the Navy for use on aircraft carriers and other
Navy air installations.
The word TACAN is a shorten version of the descriptive term Tactical Air
Navigation.
The TACAN distance-measuring facility is now utilized for civilian air
navigation, as well as for the military.
The combination of VOR and TACAN to give both bearing and distance
information is called VORTAC.
To utilize VORTAC, an aircraft must be equipped with UHF radio units that
can operate on the TACAN frequencies.
TACAN
77. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
VORTAC ground station.
VOR transmitting antenna on roof of shelter emits VOR bearing signals.
TACAN, primarily a military system, is used by civil aircraft for the DME function.
78. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
The low TACAN band has receiving frequencies from 1025 to 1087 MHz and
transmitting frequencies from 962 to 1024 MHz.
The high TACAN band has receiving frequencies from 1088 to 1140 MHz and
transmitting frequencies from 1115 to 1213 MHz.
The DME/TACAN transponder transmits an average of 2700 pulse pairs every
second.
TACAN modulates the amplitude to provide bearing information.
As in VOR, the bearing of the aircraft is determined by measuring phase angle
between a reference and variable signal.
TACAN has two variable signals.
One is sine wave modulation of the DME pulse amplitude, with a 15 Hz
sine function.
A second sine function at 135 Hz is the 9th harmonic of the 15 Hz sine
function.
79. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
The phase of these modulations is a function of aircraft position relative to
the ground station.
This is the same as in the VOR and modulation is generated by a rotating
antenna pattern.
Assume that only the 15 Hz sine modulation is applied to the DME pulse
amplitudes.
After pulse detection in the receiver, the phase is compared to a reference to
determine the TACAN bearing of the aircraft.
The reference must be unaffected by the position of the aircraft and is
transmitted on a subcarrier and omnidirectionally, as in the VOR system.
The reference in TACAN is generated by arranging a group of pulses in the
pulse modulation of the DME pulses.
80. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
A burst of pulses called the north reference burst is generated.
The same technique to create an ident pulse is used for the main reference
group.
Twelve pulse pairs have nominal spacing:
12 ms for X channels and 30 ms for Y channels between pulses in a pair;
30 ms between pairs, make the north reference burst.
To distinguish the reference burst from random squitter pulses, the reference
group has regular, not random, spacing between pulses.
The north reference burst occurs when the antenna points north, which occurs
15 times each second.
81. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
If phase is measured relative to the 15 Hz sine function, the angle is equal to
the TACAN bearing.
The system works, for all practical matters, exactly as the VOR system and
accuracy is similar to that of VOR.
TACAN has a second sine function with a frequency of 135 Hz, which is the 9th
harmonic of the 15 Hz modulation.
Eight auxiliary bursts are a reference for making phase measurements to this
135 Hz sine wave.
The required ninth reference burst is shared with the main reference burst.
The auxiliary reference group consists of six pulse pairs with the constituent
separation for X or Y channels.
The pulse pairs are separated by 24 ms.
82. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
One cycle of the 135 Hz sine function only covers 40 degrees of the TACAN
bearing.
To determine the correct 40 degree segment of the TACAN bearing, the 15 Hz
sine function is used as a coarse measurement.
All other factors remaining the same, the accuracy of a TACAN system should
be nine times better than a VOR system.
In practice, accuracy is on the order of three times better.
The main purpose of TACAN, which provides the same information as VOR
and DME, is the compact design of portable ground stations for the military.
The wavelength, which is about one tenth that of VOR, requires smaller
antennas.
83. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
In the case of VOR it was pointed out that a rotating antenna was not
physically possible because of size and rate of rotation.
Because of slower rotational velocity and shorter wavelength, TACAN
antennas can be mechanically rotated, as shown in Figure below.
The TACAN antenna consists of:
A central radiating element
Nine rotating elements
The rotating elements act as directors in a directional array and are mounted
on a dielectric drum-shaped structure.
The antenna rotates 15 revolutions per second or 900 RPM, which is not
unusually high.
84. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
TACAN antenna showing rotating directors
85. FOR TRAINING PURPOSE ONLY
Subject Code: AVI2041
Malaysian Institute of Aviation Technology
Issue No : 000
Unlike the VOR, the antenna is only about ½ meter in diameter.
The 135 Hz fine modulation is generated by embedding 9 director elements in
the rotating drum.
The shorter wavelength of TACAN also aids in siting problems.
TACAN can operate aboard ships, where VOR would have extreme difficulties
with multipath.
TACAN can also do air-to-air ranging with airborne transponders.
