i ...

TACAN SYSTEM

GENERAL

The Tactical Air Navigation (TACAN) systems are used to determine the bearing and distance of a selected surface beacon station from the air­craft. Continuous bearing and distance information is displayed on in­dicators as a navigational aid, during cross-country flights.

- -AmCRAFT INSTALLATION

Two complete and independent AR!'i- 21C TACAN systems are installed in the aircraft. Each system includes the following components:

• Receiver-Transmitter • Control Panel • Instrumentation Coupler • Antenna Selector • Antenn.l Swit~·h • T op A:~:<: :1n.:t

• Bottorn Ant~nna

The Receiver-Transmitter (RTJ units and instrume ntation coupler; a..r~ ic::~:.d.h::J i:1 :.;:~ left-ha:-td a·:ior.ics·eq:.:.ip:-::e:1: r::t.ck tt.:::.::erd-.:<.' kJ. T~!::

antenna selectors and antenna switches are iocatcd to t!:e l:?it and :;ll.;:::l~·

aft of the left-hand avionics equipment rack. The control panels are in­stalled in the center console; one is on the pilot' s side of :ne cente!· co:->­sole and the .;,ther is on the copilot's side. A stub-ty!)e antenna ior c::c~ system is mounted on the top centerline of the fusel:lge. The bottom antenna for each system is flush-mounted on the bottom centerline of the fuselage. !15-volt, A-C power is applied through circuit breakers on the avionics circuit breaker panel. No. 1 TACAN receives A-C power from navigation bus No. 1 and D-C power from the main D-C avionics bus ;o.;o. 1. The No. 2 TACAN is supplied in an identical manner from the No. 2 buses.

VOL. V:l 11-1

' I

--I N

< 0 I:"' . < ....

U CA II 110. 2 --

TACAII 110. 1

lll AVIOIIICI [QUI ~M[NT IIACII

.. c .. •

TACAN S YS TEM COMPONENT LOCATIONS

1

< 0 r

< -:

-I ""

AC NAY II US

MAIN DC

AVI ONI CS IIUS

,--- -- ·-·+-·---

~------·

RT -2/'0C/ AP.rl-2 1

ll ( \... IIV{A

lR ANSHilt(A

POW(A

UOT fOH

I tiS TRUMEN­TAT I ON COUPLC R

F--------·----- -1

T/R® RH ~ IL

orr ·

VOl

•'

COMPASS HEA DING

DISTANCE

BEAR IHG

COMPASS

BE AR ING DISTANCE N[AOINQ INDIC AT OR

StLtCTto COUttS[

lltAAING

DISTAN C

(PILOT'S, COPILOT'S AND NAV INST PAN ELS )

COMPASS HOG

AND flAG

L--- ··-- - ________ _J

(.0Hfflt')L t' AH[ l

IDENTifiCATI ON TO I NTtRPHON£

HOR IZ ONTAL SITUATI ON IN DICATOR

( P ILOT 1 S AND COPILOT ' S INST PANELS )

IIII I I 0 l 0 (. 1\ O l l CR lM A R N -21C

.•··

· .. ....

SYSTEM OPERATION

The two systems are designated as the No. 1 (pilot's) TACAN and the No. 2 (copilot's) TACAN. The No. I TACAN obtains compass reference from the No. 1 C-12 Compass system while the No . 2 TACAN is referenced to the No. 2 c - 12 compass. Cross-connection between the two compass systems, through tlie pilot's and copilot's instrument panels, provide increased reliability in the event of failure in one of the compass systems. The TACAN systems may be selected for use by the flight director systems to provide lateral guidance in­formation. TACAN beam deviation signals may be supplied by the No. 2 TACAN system to the All-Weather Landing System (AWLS) autopilot. Bearing and dis­tance signals may be furnished by the No. I TACAN system to provide a source from which the AN/ ASN-24 navigational computer may develop ground track in­formation. For each TACAN system there is one top and one bottom antenna. Since aircraft attitude during maneqvers may blank out one of the antennas, an antenna selec tor monitors the TACAN receiver output and automatically selects the antenna which is receiving adequate signal.

NOTE: For the remainder of this chapter, unless otherwise indicated, only one TACAN system will be discussed, since both systems are iden­tical in composit~on and operation.

All controls necessary for operation of the system are on the Control Panei. Two rotary switches al!ow selection of ar.y channel from l through 126. Tv;o modes cf operation are available: "REC" (receive) and "T / rr " (transmit/ re­ceive). Wber.. "REC ··operation is selected, ~he receiver circuits cf the TR :.mit a re energized, and ~!le sys:em provides bearing information. When tl:e '"!"/ :t" moce is selec ted , both r ecei•;e::- :l!':.d :.ransmitter circ:.Uts are e nergized, a1:.d ~he system pro\·ides ~eari!:g a.~:.c distance ir.!ormation . T!le "VOL" (v;,;ume) ~Otlt!"~l var ies the at.:C:ic v;,.;.::::>ut to :he :\.X/AlC-1~ .-\ inte:-?hor:e syste::::.

~:v .. :'len t!'le r~ceiver ~.U:cui:::; :.re ~oergized , signals frc !ll t."le Oea~on :Statio!'l .are processed to pro,·ide bearbg and station identification signals. The bearing Si:5-l<ll :S , t=:~nsrr.it~eci by the be:>.con st:J.tion, consist of four :nodulated components: o.::nJ l~· Hz (hertz.} si;:1:l.~s c~c.c! r•••) ~:5 :iz si;n~l.;. On~ I~ Hz ~ignal h~s ~~ ··nxed'' \!'eferenc~ } t)hase rega:-cll~ss o~ t.~e cear1:1g cf the a.1;--:rait :o the beac~n atation. It varies degree-for- degret:! with the magnetic bearing of the aircraft, with re­s;:>ect to the beacon station. The 15 Hz v:u- iable phase is a ::-e su!t of the !'otation of the cardioid ;:>attern of the ground station antenna. The 135 Hz si.g:lals also consist of a reference phase and a variable phase. The 15 Hz signals provide coarse bearing information, ar.d the 135 Hz signals pro,·ide fine bearing in;.'or­ma:ion. These bearing si.g:'lals are appli~d to r eference and variable bearing detectors in the receiver. The refer ence signals an.:l the variable bearing sig­nals are separated by the detectors and compared in phase comparison circuits.

11-4 VOL. VI

·· ... •

The phase difference (determined by the position of the aircraft, relative to the beacon station) between the signals is converted into bearing information.

The bearing information is displayed on Bearing-Distance-Heading Indicators (BDHI). There is one BDHI on the pilot's instrument panel, one on the copilot's instrument panel, and two at the navigator's station. BDHI selector panels, on the .pilot's and copilot's instrument panels, are used to select the information displayed on their respective BDHI's. Two navigation selector panels enable the pilot and copilot to select TACAN signals for their flight director systems. When TACAN signals are selected for use by the flight director, distance to the beacon station is displayed on Horizontal Situation Indicator (HSI). Two HSI's are provided; one on the pilot's instrument panel, one on the copilot's instrument panel. To fly a TACAN radial, the course arrow of the !iSI is manually posit­ioned to the desired TACAN course. The selected course information is com­pared with the TACAN bearing signals to develop course deviation informati-on. This comparison takes place in the TACAN instrumentation coupler. The course deviation information is displayed by the course deviation indicator of the HSI. Deviation of the indicator in either direction (left or right) shows that the aircraft is not on the selected course, and it must be flown in the direction of the indi­cator deviation to get on course. The "to-from" indicator of the HSI shows whether· the aircraft is flying on a radial toward or away from the beacon station. The deviation indicator alarm flag is used to monitor the reliability of the TACA.'I signal. When the flag is in view, the information being displayed is unreliable . TACAN bearing is displayed by the HSI bearing pointer when TACA::-; is selected at the NAVIGATION SELECTOR panel.

The radar principle is used :o measure distance . It is based on the cons :aot s peed of RF energy as it travels through space.

c-:CTE: RF energy trn.ve:s cr,e nau tic:l! mile ir. approximately 5 .13 :::i~!'03<·C0:1ds .

W!ten the "T/ R" mode oi o;eration is selected, the RT unit t1'3J\SI:lits a 5ig:J.ai to :.he ground station. Narrow, " idely-spaced interrogation pulses are repeatedly transmitted . These pulses are picked up by the surface beacon and applied to the bea~on receiver . 7:-e rE-cei\'e :- dev~lvps a signal which caus~s tr.e beacon transmitte r to send out ~ coply pulses. T.~e reply pulses are picked u;> by the airborne receiver. Timi:tg circuits in the receiver measure the interval be-tween the original transmission and the reply from the surface beacon. This time interval is converted into distance information.

Distance information is displayed on t.'le navigator's two BDHI's. The No. 1 BDHI displays distance information from the No. I TACA~ system anc! the ::-<o. 2 BDHl is used with the No. 2 TACAN. The pilot's BDHI displays No. l TACAN distance and the copilot's BDHI displays No. 2 TACAN distance. When TACAN signals are selected for use by the flight director systems, distance information is also displayed on the HSI 's .

vor:. VI .. 11-5

....

The antenna selection is determined on the basis of a minimum usable signal. When the system is not receiving a usable signal, the antenna selector operates, causing the antenna switch to alternately connect the top and bottom antennas to the receiver. When an antenna which is receiving an adequate signal is connec­ted to the receiver, the receiver applies a signal to the antenna selector unit. This stops the ·switching action and keeps the selected antenna connected.

SPECIFICA 110NS

. ARN-21C TACAN

11-6

CHARACTERISTIC

Frequency range

Low band (channels 1- 63)

--High band (channels 64- 126)

Intermediate ~<·equency (IF}

Receiver sensi;iv ity

T~ansmitter pulse power output

Pulse pair transmission rate (PRF)

Channel selection time

..

SPECIFICATION

962 MHZ to 1213 MHZ

Transmitter: 1025 l\llHz to 1087 MHz Receiver: 962 MHz to !024 MHz (63 MHz below trans­mitter frequency) ·

Transmitter: !088 MHz to 1150 rv1Hz Receiver: !151 l\llHz to 1213 MHz (63 :\1Hz above trans­mitter frequency)

63 :V!Hz :!: 70 KHz (trans­mi:ter frequency used for receiver injecuon)

-90 db triggering level

t K'~\· n~ tnin1un-., 2. 5 K\\>. maximum

Search: 120 to !50 ppps (varying) Track: 22 to 30 ppps (varying)

12 seconds maximum

VOL. VI

. ' ..... _.

SPECIFICATIONS (continued) ARN-21C TACAN

CHARACTERISTIC

- System accuracy

Beacon identity tone

Operating range

Power requirements

---

SPECIFICATION

Azimuth: :1: 0. 5 degree Range: ± 0 •. 1 nautical mile plus 0. 2 percent of distance

Morse coded audio tone at 1350Hz

195 nautical miles at 40, 000 feet .

115 volts, AC , 380-1000 Hz, single phase 115 volts, AC, 380-420 Hz, single phase 28 volts, DC

BLOCK DIAGRAM THEORY OF OPERATION

BEACON OPERATION

The surface beacon station is a pulsed transmitter-receiver system. Approx­imately 3, 600 paired pulses per second are transmitted. Approximately 25 pe r cent of these pulse-pairs are used as reference bearing signals . The remain­ing pulses are randomly-spaced filler pulses and distance reply pulses in a ratio determi:led by the nwnber of aircraft interrogating the beacon. The sur­face beacon also transmits an identification signal at 37.5 second intervals. This signal is keyed with three international Morse code characters, configured to provide a three-letter identification of the surface beacon. The transmitter is capable of :-esponding to inter:-ogations f::om as many as l20 airborne syste r:1.s simultaneously. When distance information is desired, interrogo.tion pulses ue sent out by the airborne transmitter . This interrogation is detected by the sur­face beacon and used to trigger the beacon transmitter. Some of the random pulse-pairs are now synchronized with the interrogation pulses. These syn­chronized pulses are called reply pulses. The airborne receiver measures the time between the inter rogation and the reply. This interval is converted into distance information.

voL. vr .. 11-7

..... ·

CENTER

ANTENNA

E~EMENT---._

ROTATING

CY~INDER

WITH ONE

REF~ECT lNG

t~EMENT---._

PU~SER

P~ATE --..__1

1-35Hz

SlUGS

T Y P I C A L

-----G ~ 0 U ~~ ::>

STA TIONARY

AN TENNA

~-- COVER

ROTATING

CYL lNOER

WITH N I N£

REF~ECTING

E~ENEtHS

WIRE

lMBEDOED IN

FIBERG~ASS

CY~INDER

15Hz REFER ENCE

~-- s~uc

8E AC O II A ~~ T E N N A

T!le su:-:ace bdacon st~:..~cn a:-:.te~:'l:l 1 .,;.::; ~· • • :J! ' ~·.; :::. ; :.>r t)a.:.~:J . :; c e:-~i.?:: ~!e~~1e:~: ~

su,r :- ou:.d·~C :)y an inne!" :t:~::l ~:l ·:..\.:.:· .. . ~ :·:..; ·.:r;!~s .: \.:y ::::r.:e=, :l;:.: ~ ~ ~.~. .; e~~ :'1:::.!:. Thes e ~or:1pouents dev :;lor;: a di:':~~ : iVL:l~ !"''l·.: : ;. .. t:vt: iJ;i. ~\.er:: . T :".t= ~ ~::te:.· eie!":': ; :!; is excited by the RF e:~erj,')' f~·o~• t b :r:1.ns rr.n ter. This e leoent is s;.a•ionary a nd is omni-directional in ~!1.: ~0ri:~~::~l p:":~e . Aro;,;.nd tb~ e!e:ne r,t l S the i::~er :·i::,erg~ass .:..:y lmde-r v .:1 ~;: ::~~ z. .- ~ ·:i.: :... . .. · .;. .Ki:..:.: ~i·:~ ·:;:~:~ ~:.: ~e:.:tet:: Hi ::-.~.\

iiberglass. This wil:e ac ts .i$ a rel~~ctur tu c;.:..st~n·~ the nor1nal ratllGi.uo.~ pa:tern into a cardioid. The cyl1:.c er is ro~.::.:cd o.t 15 r evolutions pe~ secoo.d to cause the entire cardioid pattern •o ro:.at.: . A;; :1 :·esult of the pattern rota t ion, the signal received in any ·llrection from :he 'oeacon will have sinusoida l variations in strength as a function of tim e . The signal received by the airborne .;ystem will vary in amplitude at a 15 Hz rate . This equals one cycle for each antenna revolution. For each degre<: of geog:::lpnic bearing change, there i s a one de­gree change in the electrical phase of the 15 Hz variable phase signal. The phase of the signal is used to deve lop coarse bearing information. It is neces­sary to have a fixed reference of the same frequency for phase measurement.

ll-8 .. VOL. VI

····"

COM&IN(O PATTERN

INNER CY~ IIIOER

~E,.TER ER E~EMENT CY~I NOER

GROUi'iD oEACON

..

... '· : ,.

:, . .. .. -~. ·' .. ·' ..

A N i E N N A

The fixed reference must be received in all directions around the beacon with identical phase. Therefore, pattern passes through"east" (referenced to a magnetic compass), a series of pulses is transmitted. This is the r efer ence phase 15 Hz signaL This signal is commonly r eferred to as the :\lain Refercn!:c Burst (?vlRBj .

The vo.riable phase 15 Hz signai and tht> :I!RB a.rc compared in the airborne receiver to form the coarse bearing signal of the TACAN system.

VOL. VI ..

each time the maximum point of the cardioid

CENTER ANTENNA

HE><ENT

INNER CY~INOER

1 5 H z

MAG NORTH

t ---~ R PS

P A T T E R N

11-9

12 PULSE PAIRS

1 5 H z REFERENCE CODE GROUP

FINE BEARING INFORMATION

A A ,._____]"L...l~'L

A fine bearing feature is used to produce greater accuracy than is possible with the cardioid antenna pattern system. T he outer cylinder of the surface beacon antenna is used to produce the fine bearing signal. The cylinder has nine verti­cal conductive wires embedded in the fiberglass. These wires are spaced uni­formly at 40 degree intervals. The wires distort the cardioid pattern. The resulting·composite pattern maintains the overall cardioid variation pattern resulting in sinusoidal · variation in amplitude of the received signal in any direction from the beacon transmitter . The received signal has the basic 15 Hz amplitude variation as before. The :.ine minor lobes produce nbe cycles o! amplitude va:· iation with each an­tenna revolution. Since the pattern rotates at 15 revolutions pe r second . the nine lobes produce a 135 Hz signal (15 x 9 ;

CENTER

ANTENNA

tLEMENT

MACON ET I C

NOI!TH

• I

OUTER X ,_.., ~ .CYLINDE:R ~

1 3 5 H z P A T T E R N F' I N E ~ E A R I N G

I N F' 0 R M A T I 0 N

135). To furnish a reference phase for the 135 Hz component, another series of coded pulses - the Auxiliary Reference Burst (ARB) - is transmitted. The ARB's are spaced 40 degrees apart. T he first ARB appears 40 degrees after the maxi­mum point of the antenna radiation pattern rotates past east. Comparing the ARB's and the 135 Hz variable bearing signals provides fine bearing information. In a 40 degree arc of bearing, which is the width of a minor lobe, the phase of the received 135 Hz signal will vary through a complete cycle of 360 degrees.

11-10 •• VOL. VI

Thus each degree of bearing change results in a nine degree change in the phase of the 135 Hz signal. The ratio of nine electrical degrees to one rotational degree gives a pro­no~ced magnifying effect in the pro­cess of detecting changes in bearing.

·The fi>IRB's and ARB' s are controlled by the pulser plate (reference code disc) on the surface beacon antenna assembly. The plate consists of a rotary aluminum disc with soft iron

14ACNtT I C

slugs embedded in the outer edge. The c 0 M a 1 N E 0 P A T T E R N disc rotates at 15 revolutions per second. The slugs pass through a pickup coil, producing two sets of voltage (trigger) pulses. One pulse occurs once for each 360 degrees of disc rotation; the other occurs at 40 degree inter­vals thereafter. These pulses are synchronized with the 15 Hz and 135 Hz modu­lation components, and are used to trigger the transmitter which generates the

1 3 5 H z

6 ~V\. S( '*• I RS

136.,uc

R £ F E R ~ N C E C 0 0 E G R 0 U P

~i~3·~ J.r.<l .-\.!"'.3 's . T:·.: :.Lq £' ;; and .-\RB 's have pri~ rity over distance r eply, idemi.iication, :lnd ran.::<> r.lly-sp::~cec filler pulses.

T!le Receiver an:l T,-::~nsmitter have the same frequency selection system, tun­able to any of the 126 cb::~nr.el frequencies. These channels are spaced one Mega­hertz (:\1Hz) apart. The transmitter operates in the frequency range of I, 025 through l, 150 MHZ with a peak pulse power output between 1. 0 and 2. 5 kilowatts. The receiver oper:l.tes from 962 through 1, 02~ MHz in low band ·and from I, 151 through I, 213 MH.z in high band. The receiver frequency is 63 MHz below the transmitter frequency for low band (channels 1-63) operation and 63 M;-iz above the transmitter frequency for high band (channels 64-126) operation. This .

VOL. VI .. 11-11

ONE CYCLE OF 15Hz VARI ABLE SIGNAL EVERY 360°

ONE CYCLE OF 135Hz VARIABLE BEARING SIGNAL EVERY 40°

130" no· 210' 2'50. m· • 330 • 10 OI S TANCt •t~LY AN O

Sli'ACtO II' ULS U COMPOS I T E R E C E I V E 0 5 I G N A L

-~ '" I T H A IRCRAFT DIRECT-LY

E A 5 T 0 F G R 0 U N 0 5 T A T I 0 N

enables the transmitter frequency to qe used as the local oscillator frec;uency for the receiver. Operating frequencies are de\· eloped from a crystal-controlled oscillator. The oscillator output is multiplied in h equency by a factor of 27 to

develop a signal for transmitter excitation and receiver oscillator YOltage.

CHA~NEL SELECTI0::-1

Channel sel~ction is accomplished by selectin~ the proper oscllia•vr crys:a: and tuning all of the RF circuits to accept the operating frequency . Channel selection is controlled by two balanced-bridge servo loops. One bridge is com­posed of a precision-resistive-network, connec ted across a coarse frequency selector switch in the control panel , and a potentiometer located iii tt:,; RT ut~>. The fine frequency selector switdh is part of a similar bridge circuit. ~electing

a new frequency on the control panel unbalances one or both of the bridge cir­cuits. A voltage appears across the bridge, providing an input sig~al to the servo amplifier which causes the tuning servo motor to drive. The motor ad­justs the potentiometers in the RT unit to balance the bridge. The motor also positions the tuning mechanism to select the necessary oscillator-crystal and tune the RF circuits to accept the selected frequency. The coarse bridge error causes the tuning mechanism to drive to within 10 channels of the desired fre­que.ncy; then the fine bridge error drives the tuning mechanism to the selected channel.

11-12 ' .. VOL. VI

!

r ---T- ---;---~---. ..., MODULATOR I

I I MODULATOR I I PULSE I I I PULS£' I I \ I ~7

I ! 1 025-1150MHz I ~ ! I I I

Rr TRIPPLER Rr 2ND Rr 3Ro Rr ~

.. 1ST - ...... AMP L AMPL AMPL AMPL AMPL n-;.oou-'1 t ~RECEIVER INJECTION I LA·TOR ~ 113.9-1 27MHz 37.96-42.59MHz I PULSE

! XTAL OSC l XTAL I TRIPPLER Rr SELECTOR AMPL AND

341.'7-AMPL 126 XTALS I TRIPPLER

383.3MHz '-- ---- ...:.--,__ I

---+--- ---+ - _..J

A

N

-

-

c 0 A R s E

r I

N £

FREQUENCY

I I

COARSE ERROR

AMPL

COARS £­

FIN£

ERROR AMPL

L __

MOTOR f-.- CW ~ RELAY

CONTROL L.... C'J

~-A·M~~-L--~r- RELAY

G E N E R A T I C N

When the controi panei frequency is changed by as much as 10 ch::::neis, :1 : o-:t!"3-! error :-esults. The signal is a mplified by the c oa rse enor am pii.f: ,,r J.:::! :.;;?, i:C: to the coarse-fine error ampli!ier. Any fine bridge error signal is :.l3o ~p:;! 1e d to the coarse-fine error ampli!ier. The output of this stage is a;:>plied to :1 n:v­tor control amplifier. The amplifier operates either a C lod:wise (CW) or a Counter-Clockwise (CCW) relay. The relays determine in which direction :he tuning motor will drive. As the motor drives, both tWling bridges will balance. As balance occurs, the error signal decreases to zero and the motor stops. Tuning is now complete .

RANGE CffiCUITS-TRA.'<SMITTER

The frequency multiplier and transmitter-preselector subassemblies contain tht! circuits which generate and amplify the desired RF output. The frequency mul­tiplier contains the crystal oscillator and two frequency tripler circuits. The oscillator operates at the frequency of the crystal selected by the tuning system.

' VOL. VI .. 11-13

l There are 126 cr ystals which operate in the range of 37. 96 - 42. 59 MHz. The oscillator output frequency is multiplied in the two tripler circuits, providing

MOO PU~SES HV MOO

TO DR IVER POWER TRANSFORMER .. SUPP~Y TRANSM ITTER

I t !_JU\_

1 '"'I/IIIJ\r PUUE

JVL .,-4046Hz FORMI NG CO INC IDENCE TR AN SFORMER ~

osc C IRCU I T GATE ~ .•

l ,-

..JL 12 .0.. S<:C ~

DELAY !.• •

.. 1 I NPIJT

f. " FR OM RANGE PRF ,. 2ND PU LS~ L- L CIRCU ITS_... MV ...

J GC:<t RA TCR j

. • P R F C I R C U I -:-

a sign:l~ :.o t~e tran.;mit:f: r ranging from 3-IL 7 - 3S3 . 3 ).L'iz. ".Vi;!ilr. t:,~. ;,>re­:;;c!ector , ~ :ri;:: ler-an~pH.~!er c!evelops :h! ~=-~:-a::;.rrnt:c:· vu :~~t :r. •::-: fr·.: ~~~ .-.~:

~ange Jf 1, •ns - 1, 15•1 :IlHz . . \ ocwtion of tl1rs si;;nal :~ ao~:i~'! ir. ~ ~ece!·:·;!" i~ ­jection. !:: "T / R. ·· ope . .t'~:::~.m , rug:-. ' '?l :.:lge' ;Ju!s•t ;')n i. !· ~ ~re :1;:pL:..·~ ~~ :~v ~ ? :.;!rc ·.1i~.s . !'h~: se :.:..t !s~s .1:.·~ ".k:vc~cpaC b\' ~::~ :nvduia:o~· c~rct..: it::t ~.; .,nat:c: · .. ·· t::a nsmitt.e:· :C deve io~ .;ui:lcient R.F po·.ver .; ,._put. The s bol't !';.:r~ :~ vi rtF e nerg-J, provided by t.'le transm!:t!!r, are the b :e rrogation pulses for :!'lt> ~ur­lJ.ce bt!acon ,; :.atlvn .

The time r efer ence for the interrogation is a c rystal- controlled oscillator wh:ch generates a 4 , 046 Hz sine ·.vave. The time of one com plete sino! wave is eqaiv­alent to 20 radar miles . ·A :;harp pulse is generated fro m eac.:h cyc!e of t.'l e 4 , 046 Hz s ine wave t :: a pulse gt'ne rator cir cuit . These puises are a pplied to a coincidence gate whe r e t1 lower fr equency pulse is generated. The Pulst! Rep­e tition Frequency (PRf) is permitted to var y (within limits) in :.l:l ·irrC'gular or random manner. Thill is achieved by employing a non-stable multivi t..rator to control the output of the coincidence gate . When the 4, 046 l:.l z pulse and the multivlbrator gating pulse are coincident, the coinc'idence gat,e provides an out­put pulse. These pulses, which are time-coincident with one of the 4, 046 Hz

l l-14 . ., VO L. \'I

timing pulses, establish the repetition rate of the modulator trigger and the in­terrogation pulses. The modulator trigger pulse is coded by a 12 microsecond delay line and a second pulse generator to supply a second pulse for each input from the coincidence gate. The modulator, therefore, receives two pulses which are 12 microseconds apart. These pulses trigger the modulator circuits to sup­ply the high voltage required for transmitter operation. The transmitter ra­diates pulses of RF energy, 3 . 5 microseconds wide. These pulses are trans­mitted in pairs, spaced 12 microseconds apart. After the transmission of each

· pulse pair, time is allotted for the surface beacon station reply.

RANGE CffiCUITS-RECEIVER

The received replies from the surface beacon station are applied to the receiver preselector, which consists of four tuned RF cavitie~ arranged in pairs. One pair is used for low band reception and the other pz.ir is for high band. T i:e channel selection circuits tune the cavities and select which pair is to be used. The output of the pres elector is applied to a mixer, where the RF signal is com­bined with an injection signal (1, 025 - 1,150 MHZ) from the transmitter circuits. The difference frequency of 63 MHz is applied to a six IF amplifier stage. The output...of this stage is applied to a crystal diode detector circuit which removes the IF component of the signal. The detected pulse output is applied directly to a pulse dec<X!er and through a 12 microsecond delay line to the pulse decoder. When properly spaced pulses are applied, the pulse decoder develops a video pulse for each pulse pair. The video pulse coincides with the second pulse i:1 the pulse pair. The decoder output is amplified and applied to identiiication, bearing, and range c ircuits .

The range circl4its mus: search fo!", find, and track :.he b~ac:on dis~nce rt?~ly pulses which corres:x>nc:! to :J.!l inte.rrogation. '!'hese re?lY pulses a.rrive a ; 2.

c~:tain fi:{ed tL'"='e :~..f;.e:- e~ch inter-ro&ation, .:::? !"r ~sponcli;-:.~ to ~e ~is~a:1c e be­r.vee~ t!le !llr::raf: and c!1e su:-i~ce Ueacon. Th':" ~racki~; t:·i:=:.:::s ~=-~ ~ i~vpt-r:.:: ·:e

a~ ail times e;~cept during a ti..~e interva .i centering around the arrtva! of t!1e cor.rect reply pulses. In this way, the tracking circuits can i~o:-e t!le repiy puises of othe !" airc!"a!~ ir.terrogat:ng the sur!z.c~ beaccn. Thls is ~ccomplis~e<.i

by generating gates to coincide with the reply pulse arrival. When the system is first turned on and a surface beacon station selected, the system has no way of predictbg when reply pulses will arrive. Therefore, it must search for the correct reply pulses. The pulse which triggers the modulator and causes t!:e transmitter to int:?.rrogate the surface beacon is also used to start a variable­wid:h gate. The width of the gate is made to progressively increase (search) with each inte.rrogation. The trailing edge of the :;ate is use1 to start a selec­tor pulse. As the system searches, the selector pulse is progressively delayed to effectively lengthen the search distance. This process continues until a de lay corresponding to the surface beacon distance is produced. At this time, the reply pulses and the selector gate will coincide, and the circuits will assume the "track" mode of operation. The position of the selector gate will then be

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controlled by circuits which compare coincidence of the reply pulse and the gates. The gate will thereby be made to coincide with the reply pulse for the duration of the track mode.

In the range circuits, a signal from the 4, 046 Hz reference oscillator is applied to the rotor of the range resolver in the instrumentation coupler. The phase­shifted output from the resolver is used to develop a reference pulse for each sine wave cycle. These pulses appear 20 nautical miles apart. One revolution

' of the resolver produces a change in range of 20 miles. These reference pulses ace applied to a coincidence gate to be compared in time with the selector gate signal. To develop the selector gate, a timing pulse from the modulator is applied to a phantastron. The modulator pulse causes the phantastron to gener­ate a range gate. A distance potentiometer, located in the instrumentation coupler, applies bias to the phantastron to determine the width of the gate. One revolution of the potentiometer produces a 200 mile change in range. When the range resolver and the distance potentiometer are set to zero, the duration of the range gate is 50 microseconds. When the indicated range is 195 miles, the range gate duration is 2, 400 microseconds. A 50 microsecond gate is required at zero range because an intentional 50 microsecond delay is introduced at the surface beacon station between cistance interrogation pulses and distance reply pulses. The range gate is. applied to the selector gate generator. A selector gate is produced by the trailing edge of the range gate. The selector gate and the 4, 046 Hz reference pulses are applied to the coincidence gate.

T~e first reference pulse occuring within the selector gate will produce a 12 microsecond pulse in the .output of the coincidence gate. This pulse is applied directly to the early gate stage and, through a 10 microsecond delay !ine, to the late gate stage. These stages are gated on for approximately 12 microseconds each. Video pulses (distance reply pulses) !rom the receiver are a!so applied to these stages. Since the combined "o~" t ime of the two gates is 24 micro­seconds, the tracking circuits function :o accept reply pulses from :10 area ap­proximately 2 miles wide. In"search"operation, the range resolver and the dis­tance potentiometer are being driven to cause the gate signals to be progressive­ly delayed aiter each interrogation. This action causes the tracking circuits to

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continuously "step out" in range until distance reply pulses fall within the gating time interval. When the video pulses occur midway between the early and late gate pulses, both stages conduct simultaneously. Their outputs are rectified and connected to produce an output of zero. When the video pulses are coinci­dent with either the early or late gate, a polarized D-C output will be developed. The polarized output is amplified and used to control the servo motor. As the m.otor drives, the range resolver and distance potentiometer are rotated to change .. the gating circuits and center the early and late gate stages to keep the system in track operation when reply pulses are momemtarily lost. When the gating circuits lose their output for a period greater than 10 seconds, the memory circuit causes a relay to switch the circuits back to search operation. It also causes the distance indicator warning flag to cover the distance indicators when the system is searching.

AZIMUTH CffiCUITS

Two types of bearing signals are applied to the bearing circuits. The coarse and fine reference bearing signals (15 Hz and 135 Hz) are applied to a limiter stage to remove amplitude variations. Removing amplitude variations removes any variable bearing signal. The composite coarse and fine variable bearing signals ar.e. also supplied a burst eliminator which removes any reference bear­ing signal components. The output of the burst eliminator is applied to a peak riding detector circuit where amplitude variations are detected to provide an output containing 15 Hz and 135 Hz amplitude variations. This composite signal is applied to two filters where the 15 Hz and 135 Hz components are separated. The 15 Hz (coarse) signal is shifted in phase by a bearing potentiometer in the instrumentation coupler. The 135 Hz (fine) signal is shifted in phase by a bear­ing resolver in ~he instrumentation couple:-. The two sigr.als are then COr:lpare<.l with the reference bearing signals . The ccarse signals are com[.)arec i:-l the ~eference signal gate and the fine signals are compared in t.'le ph:\se disc!'imi­nato:-. The pi:ase s!:!ited 15 Hz vJ.ri:lble ~ear~:1g 3ig~al 1s usee t~~ produce a 40 degree gate. In se:1r ch oper~t~on, the bt!a:u:g ci:-cuits are con:..o.r.uC1.1S i.;; be;.:1g cl!.~iv en anc! the ~v de-;ree gate is contiauO~.$ly being shifted ur..!il ~he g:.te and ~!1e

15 Hz reference be:aring signal coincid". At coincidence, the indicated bearing is within 20 degrees of the actual aircr:lft oearing.

When the 15 Hz reference signal is coincident with the 40 degree gate, the ref­erence signal gate stage provides a signal to actuate the search-tract: relay . '!'he relay transfers cor.trol of the indicator bearing servo loop :o the phase dis­criminator which is ;:ontrolled by the fine bearing signals. Square waves are produced from the l35 Hz variable bearing signal and applied to the phase dis­criminator. When the two signals are not properly in phase, the discriminator develops a polarized error signal. The error signal is amplified and used to drive the bearing servo motor. As the motor drives, the 135 Hz variable bear­ing signal is shifted in phase by the bearing resolver to bring the variable bear­ing signal and the reference bearing signal into the proper phase relationship.

11-20 . .. VOL. V1

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As this occurs, tbe discriminator output decreases to zero and the bearing servo motor stops. The phase shift necessary to maintain this coincidence is measured by a synchro transmitter to provide an accurate indication of the ground beacon station bearing.

INSTRUMENTATION COUPLER

Th&- instrumentation coupler receives all range and bearing information signals from the TACAN RT unit and distributes them to the BDHI's and HSI's. Bear­.ing, distance, and distance flag signals are fed directly to the appropriate BDHI's . No. 1 instrumentation coupler outputs are sent to the .pilot's BDHI and the navigator's No. I BDHI. No. 2 instrumentation coupler outputs go to the copilot's BDHI and the navigator's No. 2 BDHI. TACAN outputs are also pro­vided by the No. 1 instrumentation coupler for use by the AN/ ASN- 24 navigational computer. These outputs consist of distance and bearing information. In addit­i~n. outputs of the No. 2 instrumentation coupler are provided for use by the AWLS autopilot system.

-A bearing resolver within the instrumentation coupler is used in conjunction With a resolver in the HSI to provide deviation and to-from signals. The resolver in the instrumentation couplet" is positioned as a function of TACAN bearing, while the HSI resolver is positioned as a function of selected course. Both re­solvers are con­nected to deviation and to-from detec­tor circuits in the instrumentation coupler. These circuits measure any existing difference between the selected course and the actual course. When the two sig­nals do not agree, the deviation

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circuits develop a D-C voltage which is applied to the course deviation indicator of the HSL This indicator presents lateral deviation of the aircraft to the right or left of the desired course. It also presents the desired track; when the deviation bar is to the left of center, the aircraft is right of the selected course.

VOL. VI 11-23 ..

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The instrumentation coupler also provides a signal to the to-from indicator of the HSI and reliability signals to the deviation warning flag.

SYSTEM PECULIARlTIES

· AltbQugh the channel selector switches on the control panel do not give an indi­cation of channel frequencies, it is always possible to compute both the trans­mitter and receiver frequencies for any selected channel. Since there are 126 selectable chaMels which Increase from 1 - 126 in 1 MHz steps. knowing the transmitter frequency for only channel 1 will suffice for computing the frequencies of any other channel. Example: the channell transmitter frequency is 1025 MHz. To compute the transmitter frequency for channel 39, merely add 38 MHz to the channel 1 frequency (1025 plus 38 = 1063 MHz). To find the channel 119 transmitter frequency, add 118 MHz to the channell frequency (1025 plus 118 = 1143 MHz).

In computing the receiver frequencies, it must be remembered that the receiver operates in two bands. Channels 1 - 63 use the low band input; channels 64 - 126 use high band. in low band, the receiver frequency can be determ:ned by sub­tractlng'-63 MHz (IF) from the transmitter frequency. Example: channel 39 transmitter frequency minus IF eqU2ls receiver frequency (1063 minus 63 = 1000 MHz). In high band, the receiver frequency is computed by adding 63 MHz (IF) to the transmitter frequency. Example: channell22 transmitter f requency plus IF equals r~eiver f::-equency (!1-!3 plus 63 = 1206 1fHz).

11-26 VOL. VI