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ME and TACAN DME = Distance Measuring Equipment ( provides only distance information ) TACAN = Tactical Air Navigation ( provides bot distance and bearing ) DME and TACAN are secondary radar systems DME is similar to te distance measuring part of TACAN and vice versa Any craft equipped !it DME equipment can ma"e use of range facility of

TACAN

CONSTRUCTION

#ystem consists of $%E T'AN#MTTE' and 'ECEE' ( calledinterrogator) carried in te craft

Pulse Receiver Transmitter System( called a TRANSPONDER) at fi*edposition on te ground

WORKING Te INTERROGATOR transmits rf pulses periodically at frequency f+ Tese are received by te receiver of te transponder, amplified,

demodulated and made to trigger te transmitter after a fi*ed delay Te frequency of te Transmitter ( f- ) is different from receiver n craft, te receiver is tuned to f-, receives tese pulses Ten te delay bet!een te transmitted and received pulses is measured to

obtain te distance of te transponder from te craft

$'./&EM# .0 A #EC.NDA'1 'ADA' n a $ulsed secondary radar a number of aircrafts can interrogate te

transponder in te same cannel (f+) Tis simultaneous interrogation creates some problems Transmitter is sub2ected to varying loads A3C is employed ere and also !it te nterrogator

%se of A3C introduces some problem n te Transponder, it prevents te response to !ea" signals !en tere arelarge number of aircrafts

n te interrogator, variation of 3AN migt prevent te reception ofresponses to its o!n interrogation

Tese - disadvantages can be overcome by operating the transponer ata constant pulse rate !hich is inepenent o" the num#er o"interrogating aircra"ts

Tis is called$ONSTANT D%T& $&$'E OPERATION

DISAD(ANTAGE ) * 4en very fe! aircrafts are interrogating, te gain of te receiver in te

transponder goes up and some noise pulses may trigger te transmitter

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DISTAN$E +EAS%RING E,%IP+ENT

n DME, constant uty cycle operationis acieved by ma"ing te noisepulses trigger te transponder transmitter at a constant rate

4en te interrogations are received, tese replace te noise pulses and"eep te D%T1 C1C&E constant

-RE,%EN$& RANGE5 +6-78++76 M9: +-; Cannels spaced +M9: apart

0or eac cannel, a pair of frequencies f+ and f- !ic differ by ;< M9: areallotted

;< M9: is te 0 in te 'eceivers /ot te NTE''.3AT.' AND T'AN#$.NDE' operate !it $%E $A'#

consisting of t!o pulses +- sec apart Tis is done because te probability of te transponder triggering to spurious

noise pulses is reduced Te gain of 'eceiver no! is made ig $ulse 'ate in transponder = - $ulses must be received +- #ec apart-> /ot pulses must ave amplitude e*ceeding te trigger level Te pulses received from te NTE''.3AT.' satisfy bot of tese conditions /ut only some noise pulses satisfy tese conditions and teir number

depends upon te triggering level /y ad2usting tis trigger level, te number of "iller pulsescan be obtained A = nterrogation from an aircraft and rest are noise pulses $ulse $airs /, C, D, E, 0 arise from noise and satisfy bot above conditions 3, 9 $ulse $air do not trigger te transmitter as tey do not satisfy te

conditions + and -

/) output of te trigger circuit C) $ulse pairs generated by tese !ic furter modulate te transmitter Te Noise 3enerated $ulses are regularly spaced in time and are said to ave

S,%ITTER

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AIR./ORNE D+E INTERROGATOR

Te transmitter rf cain consists of a+> crystal oscillator 0giving one of +-; frequencies around ?7 M9:),-> a frequency multiplier and A po!er amplifier

Te output po!er range5 76 4 to +6664

Te po!er amplifier is modulated by a pulse modulator !ic is timed by!ave forms obtained from ranging circuit

A common antenna is used for te receiver and transmitter Te transmitted energy is prevented from getting into te receiver by

selective elements in te T& !ic acts as !ave traps "or transmitter"re1uency

A part of te .scillator output is mi*ed !it te incoming signal An 0 = ;< M9: is generated because of te ;< M9: separation bet!een teTransmitted and received frequencies

Tis is amplified, demodulated and ten applied to a Ranging $ircuit Tiscircuit trac"s te 'eceived signal and converts te delay into a istance

isplay Te receiver as t!o modes of operation

a) #earc Modeb) Trac"ing Mode

Te transmitter is initially in searc mode Transmitter operates at prf = +76 pulses @ second

OPERATION Te transmitter pulses trigger a delay circuit!ic in turn triggers te sort

duration -6 sec gate pulse generator 3ATN3 $%E moves from left to rigt because delay is increased from 6 to

ma*imum value by a potentiometer Te output pulse pairs of te receiver are converted to single pulses and

applied to a circuit !ic is gated by te !aveform (c) As 3ATN3 $%E moves to rigt, it allo!s te pulses !ic coincide !it it in

time to pass into a counter circuit Tere is a #udden increase in number of pulses due to pulses of response

appearing in te 3ATE along !it tose received from te beacon>

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a) Transmitter $ulseb) Te variable Delayc) 3ate 4ave 0orm

d) Te .utput of te demodulatore) #ingle $ulse generated from tem

Tis actuates te circuit !ic terminates the SEAR$2 +ODE and put tereceiver in teTRA$K +ODE

PR-of te interrogator is ten reduced to -7 pulses @ second and te 3ATEfollo!s te desired response

f reply falls in te part of te 3ATE, it advances f it falls in later part, te 3ATE is delayed Te position o" the aircra"tindicated by te elay o" the gating pulse is

displayed

D+E /EA$ON

#ame components as NTE''.3AT.' and a Transmitter and 'eceiver !it 0;

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TA$AN $rovides bot range and bearing info !it te same radiations .peration is divided in t!o parts

a) 'ange facilityb) /earing 0acility

'ange 0acility operates as in DME /earing info is provided by same metod li"e tat in .' Antenna 'adiation as a cardioid polar diagram !ic is made to rotate at +7

rev @ sec and provides a variable pase signal in te form of AM of pulse train A group of pulses is radiated every time te ma*imum of te cardioids passes

East (6)

IN$REASING A$$%RA$& A 8&obe pattern is superposed on te cardioids ma"ing te pattern appear

li"e tis

Polar Diagram Tis pattern as fi*ed pase relation to te cardoid, one MA of 8lobe

pattern coinciding MA of cardioids 'otation of tis antenna pattern at +7 rev@sec gives a signal !ic as

a modulation of +7 9: and +

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AD(ANTAGE5 'eduction of site errors caused form reflections from nearby ob2ects Te error in geograpical angle is reduced by a factor of Able to !or" on $..' #TE#

TA$AN E,%IP+ENT /eacon equipment is similar to DME e*cept for te provision of syncroni:ing

pulse groups

Antenna is common to te receiver and transmitter Duple*er isolates bot of tese by re2ecting te undesired signal in its tuned

cavities Antenna of TACAN is different from tat of DME

Te 'adiator is made up of stac" of D#C.NE ANTENNA# t gives a vertical polar diagram Around it tere are t!o coa*ial plastic cylinders in !ic parasitic elements

are embedded Tese modify te 9ori:ontal 'adiation $attern of te antenna from circular by

superposing on it t!o sinusoidal variations Te t!o cylinders are rotated togeter at 66 rev @ min by speed control

motor A Disc of non ferrous metal is coupled to te rotating cylinders #oft ron slugs inside tem are passed over a magnetic coil system and ten

tey generate $%E Tis pulse generates te group of timing pulses from !ic reference

sinusoids are generated in te TACAN 'eceiver

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.ne of tese pulse groups is emitted !en te ma* of te antenna patternpoints East

Tere is one suc group for every revolution Tese pulse groups consists of +- pulses spaced +- sec apart

TA$AN AIR/ORNE E,%IP+ENT DME nterrogator /earing 0acility added .utput to te bearing circuitry is ta"en from te decoder A3C is done to accommodate !ide variations of input signal !ile

preserving AM

RANGE -A$I'IT&

/EA'N3 C'C%T5

Te train of video pulses is applied to pea" riding detector !ic e*tracts teenvelope of te pulse train consisting of variable pase +7 9: and +

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4enever +