2 -dme

FOR TRAINING PURPOSE ONLY

Subject Code: AVI2041

Malaysian Institute of Aviation Technology

Issue No : 000

AFD 31202AV2240

AIRCRAFT PULSEAVIONICS SYSTEMS




Issue No : 000

CHAPTER 2

Distance Measuring Equipment




Issue No : 000

Table of ContentsLEARNING OBJECTIVES

INTRODUCTION

PRINCIPLES OF DME NAVIGATION

Slant Range

Frequencies

Pair of Pulses

Random Spacing

Audio

DME SYSTEM

DME System Description

DME Navigation Procedures




Issue No : 000

DME TRANSCEIVER SYSTEMFunctional DescriptionTransceiver System Operation

Rockwell Collins 860E5 DME Transceiver SystemRockwell Collins 860E5 Operating Modes

DME INDICATORSTACAN




Issue No : 000

Learning ObjectivesUpon 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.




Issue No : 000

IntroductionIn 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.




Issue No : 000

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-speedThe 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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

Typical DME navigation system block diagram




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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




Issue No : 000

The basic concept of DME slant-range distance




Issue No : 000

The airborne DME computes slant-range distance to or from the station as follows:

D = (T – 50 microsec) / 12.359where:

D = slant-range distance in nautical milesT = time in microseconds between transmission of the interrogating

pulse pair and the reception of the corresponding reply pulse pair50 microsec = delay in DME ground station between reception of initial

interrogation and transmission of a reply12.359 microsec = time required for RF energy to travel one nautical mile

and return




Issue No : 000

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.




Issue No : 000

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




Issue No : 000

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

97811049791105980110698111079821108983110998411109851111........

1041104110421042104310431044104410451045104610461047104710481048........

17X17Y18X18Y19X19Y20X20Y21X21Y22X22Y23X23Y24X24Y.......

(ILS)(ILS)

(ILS)(ILS)

(ILS)(ILS)

(ILS)(ILS)

108.00108.05108.10108.15108.20108.25108.30108.35108.40108.45108.50108.55108.60108.65108.70108.75...........

DME Reply (MHz)

DME Interrogation (MHz)

DME ChannelVHF Nav Frequency (MHz)

10171143101811441019114510201146115710311158103211591033........1086121212131087

10801080108110811082108210831083109410941095109510961096........ 1149114911501150

56X56Y57X57Y58X58Z59X59Y60X60Y61X61Y62X62Y.......125X125Y126X126Y

(ILS)(ILS)

111.90111.95112.00112.05112.10112.15112.20112.25112.30112.35112.40112.45112.50112.55...........117.80117.85117.90117.95

DME Reply (MHz)

DME Interrogation (MHz)DME ChannelVHF Nav Frequency (MHz)




Issue No : 000

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




Issue No : 000

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.




Issue No : 000

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




Issue No : 000

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




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

DME system configuration




Issue No : 000

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




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000 Pulse sequence process




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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.




Issue No : 000

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




Issue No : 000 DME arc procedure




Issue No : 000

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 transceiverIND-451 indicatorANT-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




Issue No : 000

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.





Issue No : 000

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.





Issue No : 000 S-TEC TCR-451 transceiver block diagram




Issue No : 000

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)




Issue No : 000

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.





Issue No : 000 S-TEC TCR-451 receiver block diagram




Issue No : 000

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





Issue No : 000 S-TEC DME-451 system interconnect wiring diagram




Issue No : 000

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.





Issue No : 000

Transceiver System OperationThe Rockwell Collins 860E-5 DME transceiver, commonly used in airline operations, conforms to ARINC Specification No. 568-5.

Rockwell Collins 860E5 DME Transceiver SystemThe 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.




Issue No : 000 Rockwell Collins 860E-5 DME transceiver block diagram




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.




Issue No : 000


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.




Issue No : 000


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.




Issue No : 000


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.




Issue No : 000


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




Issue No : 000


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.




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.




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.




Issue No : 000


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.




Issue No : 000


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.




Issue No : 000


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.




Issue No : 000


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.




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.




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.




Issue No : 000 A digital distance radio magnetic indicator




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.




Issue No : 000

DME remote unit and indicators




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




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




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




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.




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.




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.




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.




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.




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.




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.




Issue No : 000

TACAN antenna showing rotating directors




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.

Business

2 -dme