Astrionics System I CHAPTER 6 RADIO COMMAND SYSTEMS TABLE OF CONTENTS Section Page ................................. . 1 INTRODUCTION 6.1.1 ................ . 2 INSTRUMENT UNIT COMMAND SYSTEM 6.2.1 .......................... .2.1 General Scheme 6.2-1 ...................... .2.2 Modulation Techniques 6.2.2 ..................... .2.3 Command Word Format 6.2.3 ........................ .2.4 Command Receiver 6.2.6 ........................ .2. 5 Decoder Operation 6.2-8 ......................... .2. 6 Data Verification 6.2.12 ............ 3 SECURE RANGE SAFETY COMMAND SYSTEM 6.3.1 ........................... .3.1 Overall System 6.3.1 .......................... .3.2 Message Format 6.3.1 ......................... .3.3 Ground Equipment 6.3-2 .................... . 3 . 4 Vehicle-borne Equipment 6.3.3 6. 4 SATURN COMMAND A N D COMMUNICATION SYSTEM (SCCS) (To be supplied at a later date) .......... 6.4.1 6 . 5 RANGE SAFETY COMMAND SYSTEM ................................. A N/DRW -13) 6 . 5 . 1
The Saturn Vehicles ca rr y two different typesof radio command sys tem s; fo r range sa fety and fordata transmission to the Instrument Unit.
The Instrument Unit command system providesdigital data tran smissio n from ground stations to theAstr ion ics System in the S-IVB/IU Stage. Thi s sy s-
tem will be used to up-date guidance information orcommand c er ta in functions in the S-IVB/IU Stage.The Instrument Unit command system will not be usedduring powered flight phases.
The range safety command system providesa means to term ina te the flight of the vehicle by radiocommand fr om the ground in cas e of emergency situa-tions in accorda nce with range safety requirements.
Each powered sta ge of the vehic le is equipped withtwo command receiv ers/de coders and the necess aryantennas to provide omni-directional receiving c har -act eris tics (range safety requirements). The com-mand destruct system in each stage must be com-pletely s ep ar at e and independent of t hose in othersta ges . In ca se of vehic le malfunctions which ca us etrajectory deviations larger than specified limits,the vehicle will be destroyed by the range safetyofficer by means of the ran ge safe ty command syste m.The range safety system is active until the vehiclehas achieved orbit. After succe ssful insert ion intoearth orbit, the destruct system is deactivated (safed)by command fro m the ground. The earl y Saturn IBand V Vehicles will be equipped with a tone commandsystem (AN/DRW-13) which will be replaced in latervehicles by a secu re range safety command system.
The Saturn Instrument Unit command systemis used to tran smit digital information from groundsta tio ns to the Launch Vehicle Digital Computer inthe Instrument Unit.
Figu re 6.2-1 shows a block dia gr am of theover all command system fo r Saturn V. The commandmessage is transmitted in the S-band using a carrierfr equ enc y of 2101.8 MHz. The command me ssa ge is
modulated on a 70 kHz su bc ar ri er , which in turn is
modulated on the 2101.8 MHz ca rr ie r. The signa lfrom the ground station is received through the S-bandtran spond er of the Saturn communication and commandsystem in the IU. The recei ver separ ates the tra ns-mitted message f rom the car r ie r and feeds the resul t -ing signal to the IU Command Decoder whe re decodingi s accomplished. Fro m the decoder, th e command
message is sen t through th e LVDA to the LVDC. Ve ri -fication of the messa ge received is achieved by tr an s-mitting a signal over the IU-PCM telemetry systemback to the ground station.
See A of Figu re 6.2-3). The 1 kHz tone and the phasemodulated 2 kHz tone a r e algebraica lly summe d toproduce the composit e waveform shown in B of Fig ure6.2-3. Thi s composite waveform then modulates anRF car ri er for transmission to the vehicle. In theSaturn IB IU command syst em, the 450 MHz RF ca rr -i e r will be frequency modulated *60 kHz. In the
Saturn V IU command system, the composite base-band waveform will frequency modulate an intermedi-at e ca r ri er (su bca rri er) of 70 kHz, which will in turnphas e modulate the 2101.8 MHz S-band c ar ri er . Th efor mer modulation scheme is ref err ed to a s PSK/FM(at 450 MHz), and the latt er i s cal led PSK/FM/PM(a t 2101.8 MHz).
6.2.3 COMMAND WORD FORMAT
The command word in the T[J command syste mi s composed of 35 information bits which ar e use d asfollows:
Three bits for vehicle addre ss
Fourteen bits for decoder addre ss
Five bit s of cont rol information -
One computer mode command bit
Two bit s fo r computer "sync" signal
Two bits f or "interrupt" signal
Thirt een b its of data to the guidancecomputer
Each of t hes e 35 information bit s i s encodedinto 5 sub-b its (tota l of 175 sub-bits). The vehic leaddress is encoded into a different sub-bit patternthan the following 32 informa tion bits.
Since thi s is a non-return to zero system,the re i s no time inter val ("dead time") between bits,and individual period synchronization is obtainedfrom the 1 kHz subcarrie r. The bit transmissionr a t e is also derived from the 1 kHz timing system;therefore, the sub-bit rate is 1 kHz(O.001 second)and the information bit r at e is 200/second (0.005
second).
The sub-bit patterns a r e chosen for optimumdifferentiation between information "onesff and"zeros" to provide maximum er ro r detection capa-bility. For any par tic ula r flight, only 4 of the pos-sible 32 (25 = 32) 5-bit patte rns a r e used. Two ofthese five-bit patterns a r e used for vehicle addres sand two ar e used for the remaind er of the me ssage.
To simplify the following discussion, the sub-bits will be disregarded unless particular reference ismade to the baseband PSK modulator or demodulator.
The f i rs t 3 bit s of ea ch command word re pr e-sent a vehicle address. (These bits a r e not coded inthe sa me sub-bit patt ern a s the following 32 bits.)They will be re fer red to a s "X" bits. The 3 bits ofvehicle ad dre ss a r e applied to distinguish betweencommands fo r the Saturn I U and commands for theApollo Spacecraft. Both messages ar e t ransmit tedover the same carrier frequency. The vehicle ad-dr es s may be different for each flight. The thr eelfX" bits ar e also used to r ese t the onboard decoderpr io r to rece ipt of the following 32 bit s of th e me s-
sage. Fourt een of the other 32 bit s in the word a r eused as a decoder address . These bi ts ar e dis t r i -buted throughout the word as shown in Figure 6.2-4.The 14 addre ss bits a re compared with a prewired
Pr es en t plans ca ll for the LVDA to be capableof re ceiv ing seve n different types of me ssa ges,although this can be expanded to many more ifnecessa ry. The seven different messages ar e asfollows:
8 Update LVDC
8 Execute update
8 Ente r Switch Selector mode
8 Enter closed-loop test
8 Execute subroutine command (e. g. ,te lem ete rflight control measurements )
8 Memory se ctor dump
8 Telemeter s ingle memory addre ss
The type of command being tran sm itt ed isdete rmin ed by the fir st 35-bit word of the mes sage.Th i s f i r s t word is always a mode command word, andthe mos t significant 6 bit slo ts of the 13-bit datagroup a r e coded to esta blis h which one of the seve ncommand types is being sent. The next 6 bits arecomp lem ents of the prece ding 6 bits. The last bitin the data group is not used. The second and sub-sequent 35-bit words of each m essa ge ar e alwaysdata command words except for command Types (2),(4), and (5) , which a r e disc rete commands requiringonly the mode command word. Command Type (1)requ ires a t leas t four data command words per LVDCword. If additional LVDC words a r e required, mor e
data command words a r e sent, but always in multiplesof four. As many a s 10 group s of four dat a command
words may follow the mode command word to com-plete one Type (1) command. Command Types (3) an(6) re qui re only two data command words permessage. Four data command words a r e requiredfor Type (7).
Table 6.2-1 sum mar ize s the composition ofea ch of th e 7 type s of commands. Fig ure 6.2-6 showsthe format for a mode com mand word. The data com-mand word form at depends on the part icul ar commandtype, and si nce all of t hes e form at s have not beenfir mly decided, only the proposed form at for a Type(1) data command word is shown (se e Fig ure 6.2-7).
Table 6.2 -1 Number of Words Transmi tted forDifferent Commands
I DCW - Data Command Words
Figu re 6.2- 6 Mode Command Word Form at and Coding
6.2-5
Information Bits ----)
Bit Function --+
~ i + c ~ d i ~ ~ - - - - ,
tMost
?Least
Legend: Signif icant Signif icant1 - Interrupt Bit BitY - Sync0 - ModeD - Data
CB - No t Used* - Undetermined** - These 6 Data Bits Designate One of
The Saturn IB IU command sys tem use s theMCR-503 Re cei ver with a frequenc y ran ge of 406 to450 MHz. In the Saturn V Vehicle the IU commandis rece ived by the S-band transpo nder of the Saturncommand and communication system. Thi s S-band
rece ive r is desc ribe d i n Section 6.4.
A block diag ram of the MCR-503 Rece iver isshown in Figure 6. 2-8. The receiver R F s ignal iscoupled fro m the RF input, J1, t o a fixed-tuned, low-pa ss fil ter which has a cutoff frequen cy of approxi-mate ly 570 MHz. The output of the low-pass fil teris connected to a cr itic ally coupled, double-tunedbandpass filter, which is tunable over the requ iredfre que ncy ra ng e of 406 MHz to 450 MHz. The band-pas s f i l te r output is amplified by the R F amplifierstage. This amplifier provides additional rejectionto sign als outside the rece iver passband and is tun-able over the require d frequency range. The low-pas s f i l te r, bandpass f i l te r, and RF ampl if ier ar econtained in the pres elec tor assembly. The -3 dbbandwidth of the complete preselector assembly isapproxim ately 4 MHz. The gain is approximately5 db.
The pres elec tor R F amplifier output iscoupled to the fi rs t mixer. Here the multiplied out-put of the loc al oscill ator is heterodyned with the R Fsignal to produce the f i r s t IF s ignal. The f i rs t IFs ignal is amplified by the fi rs t I F amplifier and ap-
plied to the second mixer, where it is heterodynedwith a signal at the local oscilla tor frequency to pro-duce the 10.7 MHz second IF signal. The local oscil-la tor is cry sta l controlled and has a tuning range of100 to 120 MHz. During fac tor y alignmen t, theoscillator, multiplier, f irs t mixer, and fir st IFamplifie r a r e tuned to provide maximum sensitivity.
The fir st and second mixer, fir st IF amplifier, crys -ta l oscillato r, and frequency multiplier a r e containedin the fi rs t IF assembl y. The -3 db bandwidth of thisassembly is approximately 1. 5 MHz and the gain isapproximately 43 db.
The 10 .7 MHz output of the fi rs t IF ass embl yis coupled dire ctly to the I F bandpass filt er. Thispassive LC fil ter d etermines the overall receiverbandpass char acte rist ics. The nominal -3 db band-width of the fil te r is 340 kHz and the -60 db band-width is 1200 kHz. The filt er insertio n loss i s
approximately 10 db.
The bandpass fi lter output is fed to the secondIF assembly, which contains two feedback amplifierpai rs. Each pa ir has a gain of approxim ately 30 db.The output of e ac h pa ir is tuned to 10.7 MHz. Theoutput of th e second ampli fier pa ir f eed s both thelimiter-di scriminato r assembly and the signalstrength tele metr y circuit. The low-level signalstrength telemetry output is a dc voltage that is pro-portional to the recei ver RF input signal.
Word Bits 4
lnformatioBits
DCW Number_fLegend: Bit Code:
A - Word Time L - Last Word Group1 - " I l l1 s cc- "0-0"=1
D - Data M - Mod ule Address 0 - "0" "0- 1 9
I - Interrupt S - Sector Address y - " 0 1 ~ ""1 -0"=3
0 - Mo de CC- DCW Counters (Tag Bits) S,M,D,A,L - As Required "1-1 "=4
Y - Sync * - Unused
IBM B30
Figure 6 . 2 - 7 Data Command Word Group For mat and Coding for an Update comm and
Limiting is accomplished in the fi rs t l imiterby the back-to-back diodes in the tran sis tor collectorcircuit. The second lim ite r lim its by saturation andcutoff of the t ran sis tor and dri ves the Foster-Seele ydiscriminator. The discriminator sensitivity is
appro ximat ely 4 milli volts r m s per kHz of peak devia-tion. The emi tte r follower output sta ge is used forimpedance isolation between the disc rimin ator andthe audio amplifier.
The audio ampli fier con sis ts of a voltageamplifier stage, a phase inverter, and a low imped-anc e output stag e. The nominal volta ge gain of theamplifier is 14 db. The lower -3 db freque ncy isapproximately 250 kHz. The overal l rec eiv er -3 dbbandwidth i s app rox ima tel y 600 Hz to 80 kHz. Thi sis se t by the output circu it of the discrim inato r. Theamplifier output fee ds two 47-ohm isolation re si st or s,ea ch one of which fe ed s an output connector. Th es ere si st or s allow one output to be s horted to groundwithout reducing the other output voltage more than3 db. One of t he se outputs will be used a s the inputfor the IU Command Decoder. The ampli fie r outputalso feeds a bandpass fil ter. This fil ter output is
fed to an AGC-controlled ampli fier and then to thehigh-level tele met ry detector circuit. The telem etryvoltage thus obtained will provide a useful measureof R F input sig nal up to 500 microvol ts.
In the ca se of the s ec ure range saf ety com-mand sy st em , one of the re ce ive r outputs will beused a s an input to the range sa fety decoder.
Chara cteri stics of the MCR-503 Receiver a regiven in Table 6. 2-2.
6.2.5 DECODER OPERATION
The decoder is the interface unit between theIU Command Rece iver and the LVDA. Data tr an s-mission is made through a 32-bit word which is pre-ceded by thre e vehicle a ddre ss bits ("Xfv-bits). Eachdata and address bit is compo sed of 5 sub-b its. Thetotal 175 sub-bit mes sag e must be decoded into theorigin al mes sag e configuration of 35 bit s before thedata can be tra nsf er re d into the LVDA for computer
acceptance.
The function s of t he IU Command Decoder areas follows:
Demodulate the PSK baseband su bca rri ers .
Recover the original 175 sub-bits.
Compare each 5 sub-bit group againstthr ee prewired bit codes to recover the35-bit command word.
Check that all 35 bits a r e received.
a Check that each bit is rece ived within the5-millisecond bit period.
Check that the 3-bit vehicle ad dre ss andthe 14-bit decoder a ddress a re c orrect.
Inhibit any fur th er decoding if any of thechecks a r e invalid.
Pre sen t the 18 information bits to theLVDA in par al le l form.
a Present an "address verification" signalto PCM telemetry.
a Receive the "LVDC r es et ff signal fro mthe LVDA and present thi s signal to PCMtelemetry.
Table 6.2 -2 Cha rac ter ist ics of the MCR-503Receiver
Frequency range 406 to 450 MHz
Frequency deviation *30 kHz (for 1 V rm s output)
Quieting 15 db at 10 microvolts
Maximum RF input 2 .0 V rm s
Input VSWR 1 . 5 :1 maximum
Tuning s tabil ity *30 kHz
Oscillator Crysta l controlled, singlecrysta l
RF bandwidth (-3 db) 340 ~t 0 kHz
RF bandwidth (-60 db) 1200 kHz
Type of output Audio, two is ol at ed output s
Audio bandwidth (-3 db) 1 to 80 kHz
Audio distortio n Less than 5 percent
Audio output level 1.4 V r m s into 75 ohms(with two tones, + 30 kHzdeviation per tone)
The opera tion of the decoder ca n best be ex-plained by breaking th e deco der into two logical func-tional pa rt s a s shown in Figure 6.2-9.
PSK SUB-BIT DEMODULATOR
The onboard receiver demodulates the RFcarrier and presents the composite baseband signal(Fi gur e 6.2-3) t o the IU Command Decoder. The pur-
po se of the PSK demodulator is to sep arate the 1 and2 kHz s ignal s, comp are the phase of th e 2 kHz tonewith the 1 kHz r efe ren ce tone once each millisecond,and generate a pulse on the "1" output circuit or the"0" output circuit, a s the cas e may be. Figure 6.2-10
i s a synchrog ram of the variou s waveforms generatedin the PSK demodulator by the input signal. Figur e
6.2-11 is a simp lifie d block dia gra m of the PSK demod-ulator. Fo r the purpose of illustration, a tra ns-mitted s igna l consisting of 3 sub-bits in 0- 1-0 ord eris shown in A of Fig ure 6.2-10. This compositesignal is amplified and fed to an e mitter-followerstage t o provide sig nal driving power for the tunedfil ter s. The overall gain of these fi rs t 2 stage s isapproximately 3. This gain varies slightly over thetem per atu re ran ge due to the action of a temp era-tu r e s ensi t ive r e s i s to r in the emitter-follower stage.This gain change compensates for other temperatur e-caused gain changes in the detector.
A sharp ly tuned fil ter network reco vers the1 kHz component of th e composi te input waveform .The output of th is network is shown in B of Fi gu re
6.2-10.
The 1 kHz si ne wave is capacitive coupled toa sha per circuit which acts to "square up" t he posi-
Fig ure 6.2 -9 IU Comman d Decoder Fun ctionalBlock Diagram
18 Lines To
LVDA
tiv e half cycl e of th e input waveform ( se e C of Fig ure6. 2-10). Th is wave form is differentiated and thepositive pulse used to trig ger a monostable multi-vibra tor. The trail ing edge of the monostable out-put waveform is differentiated, shaped and amplified,and fed a s one input to each of two AND gat es. Theoutput of the amp lif ier , shown in D of Fi gu re 6. 2-10,
is a very narrow pulse appearing at a 1 kHz rate, andwill hereinafter be referred to as the "sampling"pulse.
Audio lnput
Ma in Reset From'
ecoder LVDAFrom
Receiver -)
1 kH z Filter Out
(Inverted by
lnput Ampl )
L eset to T/M
Addressb Verification
To T/M
IB M J 2
PS K
Demod. -
1 kH z Shaper Out C 1I
I
Sampling Pulse
2 kH z Filter Out
(Inverted bylnput Ampl)
2 kH z Shaper Out
Inverter Out
" 1 " Mono Out
"0" Mono Out
Time Base: 1 ms/Division
Amplitude Scale: ArbitraryIBM B3 3
Fig ure 6.2-10 PSK Sub-bit Dete ctor Sync hrogr am
also applied to a 2 kHz tuned filt er. In contrast toth e 1 kHz filter , the 2 kHz filt er is broadly tuned,thus providing greater sensitivity at low receiver RFinput levels. The output of the 2 kHz fi lt er i s shownin E of Fig ur e 6.2-10. The 2 kHz waveform is capa-citive coupled to a shap er circuit. The output of this
shaper is shown in F of Fig ur e 6.2-10. The shap eroutput is applied to 2 places: f irst , it i s applied to aninverter stage, and second, it is applied to the "1"
AND gate. Wavef orm D of Fig ur e 6.2-10 shows thesampling pulse input to the "1" AND gate, and F ofFig ure 6.2-10 shows the shaped 2 kHz input to thesa me gate. When the 2 kHz waveform is positiveand tim e coincident with the sampling pulse, the "1"
AND gat e will produc e an output pulse. This pulsewill trigger the "1" monostable multivibrator to the"ON" stage. The tim e perio d of this monostable is
200 microseconds * 10 percent. Waveform H ofFi gu re 6.2-10 shows the monostable output, which
is fe d to the input of th e sub-bi t decoder.
The output of t he inv er te r (G of F igure 6.2-10)is appli ed to the "0" AND gate along with the sam p-ling pul se (D of F igu re 6.2- 10). In a manner si mi la rto that descr ibe d previo usly, the "0" AND gate willproduce an output pulse and tr ig ger the "0" mono-stable (see J of Figu re 6.2-10). Note tha t both ANDgates cannot have an output at the same time.
The synchrogram shows an approximate phaselag of the 1 kHz waveform, with respec t to to, ofabout 45 degrees . The 2 kHz waveform also exhibitsa phase lag, although the degr ee of lag is difficult tomeasure unless all "1's" or all "0's" ar e being tran s-mitted. The RF link is a contributor to this phase lag,a s well as the fi lt er s in the PSK sub-bit detector. Iti s obvious that too much of a rel ati ve phase dif feren cebetween the 1 kHz and the 2 kHz components will causethe detector to malfunction (lose data). In order toprovide maximum insensitivity to this relative phasediff erence, the timing res is to r of the sampling mono-
stable i s selected so that the sampling pulse falls a s
Fig ure 6.2-11 PSK Sub-bit Detect or Block Diagr am
32-bit counter, and (3) send a re set pulse to the out- Fi rs t of a ll, the Manned Space Flight Networkput flip-flop. After the thi rd "X" i t has been recog- is designed to provid e a high degree of data tran smi s-nized, the count-of -three output fr om the 3-bit counter sion accura cy from the originating station in Houston,will provide a di rect-co upled signal to AND 3 and AND Texas, to the down-range DCS unit , o r data proc es-4. As long a s this signal is pr ese nt, AND 3 and AND4 sing computer, which actually tra nsmi ts the mess agea r e "open", that is , they will allow valid "0" and "1" to the vehicle. Secondly, in orde r to tran spos e a "1"
bits to pass through. bit to a "0" bit o r vice -ve rsa, ever y one of the fiv e
The next 32 bi ts will be coded with "0's" and"1's" a s required. Each time a count-of-five st ateis reached by the 5-bit counter, a comparison is madeto deter mine if the bit is a "0" o r a "1". If i t is a "0"o r a " IT1 , pu lse will be produce d a t the output of OR 5,
thus keeping the missing -bit clock running. The se 32bits will als o be shifted and written into the 32-bitshift regi ste r, and counted by the 32-bit counte r. Uponthe count of 32, a 1-milli second monostable multivibr a-t o r is tri gg ere d "ON". Th is signal opens AND 5 andallows an add res s comparison to be made. If the hd-d r e s s is col rect , a signal will pa ss through AND 5 and
sub-bits must be transposed to i ts compliment; andthis must be done in multiples of five in synchronismwith the bit ra te (200 bits pe r second) or the me ssag ewill be rejecte d. Thirdly, 17 bits of each 35-bitt ransmiss ion must be correc t ( there is no possibilityof an undetected er r o r here ) before the 18 informationbits ar e presente d to the LVDA. Last of all , a veri-fication loop util izing te leme try a s a down-link is
employed to verify the 18 information bits for crit icalcomma nds (the LVDC update commands). The other
types of commands ar e not so cri t ica l in nature and donot req uir e bit-for-bit verification.
trig ger the output fl ip flop (a bistable multivibrator) to The verification loop works a s follows. If th ei t s s e t s t a t e. In addition, the two 60-mil liseco nd 14-bit address is correct , an addr ess ver i f ica t ionmonostable multivibrators will be triggered "ON", pulse is sent to te lemetry. This pulse is decommu-providing an indication to teleme try that the add ress tated in rea l t ime at the ground station and present edwas valid. Two multivi brators a r e used fo r reliabili ty. to the message acceptance pulse circuitry. see
The se t output of the fl ip flop also res et s the3-bit counter (through OR 4), and provid es an enablingvoltage to the 18 data output driv ers. The binary infor-mation sto red in the 18 dat a sta ges of the 32-bit s hiftr e g i s t e r is thus t r ansf erre d in para l le l for m to theLVDA. When the LVDC acc epts thes e data-bi ts fr omthe LVDA, a re se t puls e acknowledging recei pt is sent
Fig ure 6. 2-15. A second pulse is a l s ~ erived in thevehicle and sent to telemetr y. This is the LVDC re-se t pulse which signifies that the LVDC has receive dthe 18 information bits. This pulse is also decommu-tated in real t ime and routed to the message acceptancepulse c i rcui t ry. The address verification pulse trig-ge rs a 200-millisecond monostable multivibrator to
through the LVDA to the de coder. This pulse wi ll pa ssTable 6.2 -3 Cha ract eris tics of the IU Commandthrough OR 8 and will cle ar the 32-bit shift regis ter, System
re se t the output flip flop thus disabling the 18 data-output driv ers , and tri gge r two 60-millisecond mono-s table mul t iv ibra tors . The outputs of t he se 2 multi-v ibra tor s ar e sent to te lemetry to indicate receipt ofthe LVDC res et pulse. If, for so me reason, the LVDCre set pulse does not ar ri ve pri or to the beginning ofthe next message, the second "Xu bit of vehicle ad-dr es s will provide an output fro m the 3-bit counter torese t the decoder c i rcui ts .
Cha rac teri stic dat a of the IU command sy ste mar e summa rized in Table 6 .2-3 .
6.2.6 D ATA V E R I F I C AT I O N
The application of th e IU command sys te mand the information transmitted through the sys temrequires a high probabili ty that a correct commandmessage will be receive d in the vehicle. This highprobabili ty is obtained by the use of sev era l differenttechniques throughout the system.
Transmiss ion f requencyband - - - - - - - - - 406 to 450 MHz
Transmission sub-bitr a t e - - - - - - - - - 1000 per second
the "ON" sta te. The LVDC re se t pul se will occur ashor t t ime the re dt er and t r igger a 10-mil l isecondmonostable mult ivib rato r to the "ON" stat e. If theLVDC reset pulse does not appear during the "ON"tim e of the add res s verification pulse multivibrat or,then it will not appea r at al l for a given transmissio n.The 2 multivibr ator outputs ar e logically-combined in
an AND gat e, which will have an output only when bothmul t iv ibrators ar e "ON" a t the sam e t ime.
The 1 0-millisecond pulse thus produced iscalled the message acceptance pulse and is fed to theDCS, or data proce ssin g computer. The DCS, o r dataprocess ing computer, use s th is message acceptancepulse as a "next-message- t ransmit" pulse . Uponrecei pt of th is pulse, the DCS, o r data processingcomputer, will trans mit the next message (if any).
If the messa ge acceptance pulse does not appear with-in a cer tai n length of t ime ( approximatel y 400 milli-seconds fro m sta rt of messa ge), the DCS, or dataprocessing computer, will then ret ran smi t the mes-sage .
To complete the data verification, the LVDCsto res a l l LVDC update messages (as many as for ty18-bit groups) and non-destructively rea ds out thes edata bits to telemet ry. Upon receipt by the groundstation, these bit s a re sent to Houston via the MannedSpace Flight Network and compared with those origi-nally transmitted. If al l b i t s ar e veri f ied, an 'Texecuteupdate" command is sent to the DCS, or da ta proce s-sing computer and thence to the vehicle. Only uponrece ipt of t his m ess age will the LVDC act upon the"update" data bits .
Fig ure 6.2-14 Example of Wiring Between Shift Regi ster and Sub-bit Comp ara tor s
A different approach of messa ge verificationis used for all mode command words and the datacommand words fo r command Types ( 3 ), ( 6 ), and( 7 ) . Fo r the se commands, one-half of al l data bit sar e transmitted a s required in "true" form; and theother half in complimentary form. This is calledduplex coding. The LVDA will reject a commandunless the compliment bits check out to be the inverseof the tr ue bits. Such a proc edure provides a gr ea te rprobability of receiv ing the sam e message a s wastransmitted.
The se cur e range safety command system pro-vides a high deg ree of protectio n against intentionalinterrogation by unfriendly intruders and against unin-tentional interrogation (false al arm s) by noise. The
security against intentional interrogation is measuredby the fraction of the tota l code combinations that an
intelligent unfriendly int errog ator, having unlimitedtechnical resources, would be able to transmit duringequipment acc ess time. It is assumed that all systempa ram et er s a r e known to him except the code-of-themission. The security against unintentional or ran -domly generated false ala rms is, a s i t should be,much gr ea te r than again st intentional, intelligent in-terrogation. All hardware is unclassified except theactual flight code plugs.
A simpl ified block diagr am of the se cu re rangesafety command system is i l lustrated in Figure 6.3-1.Each address character is chosen by the code plug.
Two ident ical code plugs, one in the ground encoderand the other in the flight decoder, a r e used. This
removable, inexpensive, easil y stored code plugmini mizes the actu al operatio nal difficulties of hand-ling the security aspec ts of the system , thus elimi-nating the necessit y of classifying the syst em hard -ware (decoder and encoder).
6.3.2 MESSAGE FORMAT
The message transmitted to the vehicle con-si sts of 2 words; an ad dre ss word and a function orcommand word. The add res s word consi sts of 9
characters of a high-alphabet s ystem , and the functionword con sis ts of 2 charac ters; thus the total messagecomprises 11 characters .
Each ch ar ac te r con sis ts of two simu ltane oussymbols, which a r e audio-frequency tones between7.35 kH z and 1 3.6 5 kHz. Each cha racte r can be tonecoded by choosing fr om a symb ol alphabet of 7 tones.
Since 2 tones a r e used simultaneously, the chara cteralphabet is established at 21 possibili t ies or choices(no choice is repeated within a given word). Thus
the encoding scheme is logically called high-alphabet.
/The secu rity of the mess age is realize d mainly
fr om the fac t tha t only 9 out of 21 pos sibi liti es a r e
used per address , and that these 9 address charac-te rs a re pos it ion coded. The limited tim e of a cc ess
to the equipment is the final factor in the sec urity,for any code can be broken if acce ss t im e is unlimited.
The function word con sist s of 2 ch ara cte rs .Since there ar e 21 character-coding ar rangementsavailable fo r the fir st function ch ara cter and 20 forthe second, 420 character/position codes a r e avail-able fo r the function word. Only 5 functions ar e re -qui red of the system . To provid e the maximum lan-gauge diffe renc e between any two of t he se functions ,5 codes were chosen a s commands in a spacing schemewhich provides maximum sec urity against commandtransla tions. Thes e commands and their codes a r elisted in Table 6.3-1.
Each char ac ter period, including "dead time",is approximately 8.6 milliseconds in duration, exceptthe eleventh, which is three t i mes a s long.
Each char act er of the 9 character a ddress wordis unique within a given address, and therefore the newsy ste m can oper ate without the necessity of a trans mit -ted clock or re fere nce synchronizing information. The
symbols are , however, synchronous with the charact er
Table 6. 3-1 Coding Scheme for Function Cha rac ter s
2. Arm/fuel cutoff (charg-ing of the EBW f i r ingunit and thrus t te rmina-
3 . MSCO/ASCO (Saturn
4. Sp are (No. 2)
5. Safe (command sys tem
interval period (and with each other) and ar e spacedand phased to minimize intersymbol i nterfere nce, whichcould creat e unwanted c hara cter s.
The main R F c ar ri er (450 MHz) is frequencymodulated by a subc arr ie r syste m which in turn em-ploys a multiple freque ncy shift technique. The te rm
frequency shift keying is also used to descr ibe thetechnique. The portion of th e baseband oc cupied bythe subca r r i e r sy s t em is approximately the same asthat now reserv ed and used for range safety purposes(see Figure 6.5-2). The nominal tone frequencies,
however, a r e not the sam e as the in ter- range ins t ru-mentation group tone channel frequen cies. The tone-frequency spacing is by design an integral multipleof the ch arac ter repetit ion rate . This makes poss i -ble a simple phase-coherent tone keying scheme.
6.3.3 GROUND EQUIPMENT
A block d iagr am of the ground sys te m is i l-lustrat ed in Figure 6. 3-2.
When the range safety officer decides to ini-tiate a command from his console, he actuates ahooded toggle switch. The output of t he enc ode r is
then routed in paralle l for m to a tone remoting tr ans -mit ter which processe s the message for t ransmiss ionover the 8 km distance to the tran smit ter sit e. Atone remot ing receiver a t the t ransmit ter s i te demo-dulates the message and feeds it to a modulator thatconverts the pa rall el information to the high-alphabet11-character format . Th e 11 dual- tone burs ts ar e
then fed to the AN/FRW-2A Tra ns mit ter System andto the vehicle. Fo r reliabili ty, a completely redun-dant backup system is provided, with a continuouslymonitoring error detector that provides automatictran sfer to the backup sy stem in ca se of a fail ure inthe pri mar y chain (this er ro r detection and switchingc i r cu i t ry is not shown in the diagram).
The range safety equipment incorpora tes apriority-interrupt scheme that will interrupt any com-mand being transm itted and tra nsm it any higher-prio rity command selected by the range safety officeror origin ated by the computer. After completion ofthe tran smi ssio n of the high er-p rior ity command, thetran smi ssio n of the interru pted command will re sum e.
The equipment at the downrange sites is i l -lust rate d in the block diagr am of F igu re 6. 3-3. Alldownrange sites are connected by cable, and whenthe range safety officer pre ss es a command switchon his console, the command pulse will be trans mit-ted over the cable via the supervisory control system
(this sys tem will be replaced by a digital remoting received at the proper uprange site, converted tosyste m in the near future). All si te s will receive the paral lel form, and fed to the modulator. The modu-command, and the transmit ter on the ai r at the mo- lator will convert the binary-coded message to thement the command is received t ransmits the message high-alphabet 11-character forma t. This messageto the vehicle (to keep the onboard receiv er captured, is then transmitted to the vehicle.one t ransmit t er is always radiating).
-)
When the vehicle must be interrogated from 6.3.4 VEHICLE-BORNE EQUIPMENTan uprange site, the signal processing and routing isslightly different. As the paral lel data leave the en-
ANTENNAS- -
coder a t the cape central control, they are changed toser ia l fo rm by a para l le l - to-ser ia l conver ter and then The type and number of ante nnas used will be
applied to a serial data remoting transmitter. After such that their radiation pattern m eets the require -
being checked for e r r or s , the ser i a l message wil l be ments of range safety at the E astern Tes t Range.
I
Cape Central Control Cape Transmitter SiteRSO= Range Safety Officer IBN B4O
+
D/R
FunctionRemoting
IT O Downrange Sites I+via Subcable I
I
Fig ure 6.3- 3 Downrange Station
6 . 3 - 3
-
I
FromCape +
Kennedy
D/RFunctionRemoting
RSO
D i g i t a lt r - ) Mod ula tor Transmi tteremoting -1 Remoting Encoder System
The command signal fr om the ground stationis received in the vehicle with the MCR-503 Receiver.The sa me type of r ece ive r is used als o in the Instru-ment Unit command system and is described inSection 6.2.
DECODER
A logic dia gra m of the vehicle-borne decoderis illustrated in Figure 6. 3-5. The front end consi stsof the fil ter dri ver , optimum decision circuitry, seventone filters , and seven threshold detectors. The opti-mum decision circuitry will be discussed in moredetail later, but operationally, the circui t can be con-sid ere d a s a hybrid, fas t-acting AGC circuit. Thepurpose of each filt er is tb detect a pa rticu lar tone,and the purpose of the thresh old detec tor s is to estab-lish the decision level and to amplitude standardize
the output of e ach fil te r into a well-shaped pulse suit-able fo r us e in the cir cui try which follows.
8 The AND matr ix consi st s of 21 ANDgates and interconnections to the inputof the code plug. The pur pos e of themat rix i s to for m the main alphabet ofcha rac ter s from the sub-alphabet ofsymbols (tones).
8 The code plug con si st s of a mul ti-pin plugwired in the chosen code-of-the-missionconfiguration. The code plug can be
wi re d in a number of unique ways. Thepur pos e of the code plug is to unscramblethe code and apply the 9 add res s charac-ter outputs to the sequence register. Asecond purpose of the plug i s to pre sen tthe 12 unused ch ara cte rs to OR 1.
8 The sequence regis ter or addres s memoryconsists of: ( 1 ) eight electronic s witches(SW1 through SW 8), ( 2 ) one delay elem entconsisting of a monostable multivibrator(DELAY MVB 1) and a tr aili ng-ed ge dif-ferentiator, and ( 3) eight AND gates with
inhibiting inputs (AND 1 through AND 8).In gen era l, the purpos e of the sequenceregis ter is to provide a pulsed signal tothe input of AND 9 i f each of the 9 cha rac -te rs of the add res s word is received inthe proper sequence. A second functionof the re gi st er is to provide a pulsed s ig-nal (up to eight) to the input of OR 1 if oneor more c harac ters a r e received out of
sequence. The r egi ste r will not providea signal to the input of OR 1 if a charac-te r is repeated, which is desirable sinceradar blanking could cause a friendlychara cter to be interrupted. The purposeof the eight pass ive time-dela y elem ents( T ) n the regis ter i s to prevent an un-
friendly intruder from gaining any advan-tage by transmitting more than 2 tonessimultaneously. If 3 tones a re t ransmit-ted, 3 cha rac ter s will be formed. Becauseof the s hor t time delay introduced by ( T ,2 of the 3 cha ra ct er s will be recognized a sbeing out of sequence, even if they have thecorrect tone coding.
8 The false -char acte r action. circuit consistsof OR 1, OR 3, CLEAR MVB, F F 9, andAND 9. The pur pos e of thi s circ uit i s toprovide a pulse whenever, ( 1 ) one of the
unused characters is received, or ( 2 ) acharacter is rec eiv ed out of sequence. Thepulse ap pe ar s at the output of OR 1 and isrouted to 3 places. Firs t , i t trigge rsF F 9 to its set state, which inhibits AND 9s o that the ENABLE MVB cannot be tu rnedON fo r the duration of the m ess age in ter val ,re ga rdl es s of what occ urs elsewhere in thedecoder. Second, it pa ss es through OR3and cle ars (rese ts) the sequence regis ter.A third action also to be initiated, providedthat the cha ract er corresponding to pointA has not yet occurr ed, i s the trigg ering of
the interval timer (a monostable multi vi-brator).
The interval timing circuit consi sts ofOR 2 , the interval timer multivibrator,and a differentiating c irc uit which pro -duces a pulse a t the end of the timed in ter -val. This pulse re se ts all decoder circ ui-tr y to the proper condition for receivinganother message. Note that the prese nceof any of the 21 pos sib le c ha ra ct er s williniti ate the tim er. The pur pos e of OR 2is to allow either the first character
(point A) or any of th e other 20 cha ra ct er s(the output of OR 1) to initiate the time r.The function of OR 6 and AND 16 is toestablish a fixed recovery time for theinterval timer MVB.
8 The enabling cir cui t con sis ts of DELAYMVB 2, the ENABLE MVB, and the powerswitch. The purpose of this cir cui t i s to
prevent the function rel ays f rom beingactivated until after the add res s has beenverified and the 11th chara cter has a r-rived. Only when the 9 ad dr es s chara c-t e r s have been sent in accordance withthe chosen code, will a pulse appear a tthe input of AND 9, find it uninhibite d, and
trigger DELAY MVB 2. When this multi-vibrator completes its cycle, the -d/dtcirc uit tri gger s the ENABLE M V B ON forthe function interval. The "ON" state ofthe ENABLE MVB closes the power switch,thus providing power to the function relaysand associated logic circuitry.
a The decoding inhibit circ uit con sis ts ofOR 4, OR 5, and AND 15. In general, thepurpos e of this cir cui t is to prevent decod-
ing during the "clear" inter val of the s e-quence regis ter. The circ uit does not func-tion during the reception of a friendl y mes-sage and therefor e will not be discussedfurther.
The function rela ys and logic cir cui tryrep res ent the remai nde r of the decoder.The purpos e of th is cir cui try is to allowthe proper command transmission to closethe appropriate relay(s), which supplies28-volt power to the desired outputfunctions.
a Signal tracing a friendly message. Thecode jack (Fi gur e 6. 3- 5) has 21 inputsand 9 addr ess outputs. For purposes ofanalysis, assu me that the code plug is
Based on such a code-plug wiring and onthe message format previously mentioned,the total transmitted message will appeara s shown in Figure 6. 3-4 (bottom) if acommand is to be effected. (The commandi s assumed to be des t ruct . )
The demodulated composite audio signal cor-responding to the chosen tone pa irs is fed from thereceiver to the seven tone fil ter s in the decoder. The
fil ter outputs ar e matrixed t o the 21 AND gates, thusproviding the 21 req uir ed combinations. The outputsof the 21 AND gate s a r e wire d to a connector thatfe eds the input of the remo vabl e code plug. Assumingthe code-plug wir ing shown in Figur e 6. 3-4, the fi r s ttone pair ( 1 and 2) will cause a pulsed s ignal to appearon the output si de of t he code plug at point A. The sig -nal a t point A is distribute d to 2 points: ( 1 ) it turnsON1 the first electronic switch (SW 1) in the sequencere gis ter , which in turn supplies a voltage to the in-
hibi ting input of AND 1; and ( 2) i t passe s throughOR 2 and AND 16 and tu rn s ON the inte rval tim er .Approximately 8. 5 milliseconds af ter the st ar t of thef i r s t character, the second character ar r ives . Tonepa i r 1 and 3 corres ponds to point B, and a pulsedsignal at this point is distributed to 2 places: ( 1 ) itis applied a s a signal input to AND 1, but cannot passthrough becaus e of th e pr es en ce of t he inhibiting volt-ag e from SW 1; and ( 2 ) it turns ON SW 2. In addition,the output of SW 2 sup pli es a voltage t o the inhibitinginput of AND 2.
The third through the eighth characters ( C
through H) cre ate res ult s analogous to those of A andB. When the ninth-character pulse arrives (point J ),
it tr ig ge rs DELAY MVB 1 t o its "ON" state. Whenthe multivibrator completes its cycle, the -d/dtcirc uit ap plies a pul se to AND 9. Since the inter valtim er is in its "ON" sta te and F F 9 i s in the "reset"sta te , AND 9 will not be inhibited. Thus this pulsedsignal will be allowed to pa ss through and trigge rDELAY MVB 2 to it s "ON" sta te. Eight and fivetenths milliseconds later, the ENABLE MVB will betrig gere d to its "ON" sta te. Note that a t no time dur-ing the ad dre ss word is t her e an output fro m OR 1.
This is because the address i s the correct address
and an output fr om OR1 will occur only if the addressis not correct . After the ninth-character pulse turnsON the ENABLE MVB, the volt age thus o btained is( 1 use d to turn ON the power switch, which in turn
l ~ a c h l e c t r o n i c s w i t c h i n th e s e q u e n c e r e g i s t e ra s s u m e s th e O F F s t a te w he n B t i s r e m ov e d . T h e r e -
f o r e , w h en t h e d e c o d e r i s f i r s t t u r n e d O N , a l l
s w it c h es a r e O F F .
supplies 28-volt power to the rel ay arma tur es, and( 2 ) used a s the supply voltage input to SW 10 and SW11. Since we have a ssum ed a destruct command, thetenth-cha racter pulse will be form ed by tone pai r 1and 2. Th i s pul se will tur n ON SW 10. The output ofSW 10 i s applie d a s a c ontr ol voltage input to AND 10.Note that even though the time pe riod of the tenth
char acte r is over, neither of the 2 re lay s is as yetenergized. When the eleventh- charact er pulse ar -r ives , it pass es through AN D 10 and is used to turnON SW 11, thus energizing both relays in the destructchannel. Since both re la ys a r e now closed, 28 voltswill be applied to the a pprop riat e pin of the output plugand routed out of t he decoder to the rang e safe ty sys -tem contro ller, which then takes over and initiatesthe destruct action and any preliminary critical func-tions (such a s payload jettison, if applicable). Assoon a s the ENABLE MVB completes its cycle, thepower switch will turn OFF , thus turning OFF SW10 and SW 11. A shor t t ime la ter, the in terval t ime r
will complete it s cycle, and the -d/dt cir cui ts will re-set all decoder circ uitry to the proper state.
Much attention has been given to preventingexec ution of a catastrophic command should one com-ponent (tra nsis tor, rela y, etc.) fa il during flight. Inmany ca se s of component fa ilu re , the decoder designwill still allow reliable interrogation.
The pur po se of the DELAY MVB 2 is to delaythe tr igg eri ng of t he ENABLE MVB fo r 8. 5 millisec-onds. This is necessa ry if complete freedom of
choice for the ninth- characte r coding is to be main-tained, for observe what would happen if the ninth,tenth, and eleventh char act ers we re coded a s follows(assum ing that DELAY MVB 2 was not pre sen t):
9th 10th 11th- - -5 a n d 6 1 and 2 1 a n d 3v
estruct command
Since the ninth character corresponds to thefi rs t function-characte r of the safe command, and
since the ENABLE MVB would be turned ON uponrecogni tion of the ninth cha ra ct er , SW 14 would tu rnON. This would degrade reliability and incr ease theprobabilit y of a command tran slat ion, sin ce the in-tended command is dest ruct . Delaying the turn-ONof the ENABLE MVB unti l the ti me pe rio d of theninth c hara cter is over, solves the problem withoutre str ict ing the choice of the ninth char ac te r codingto one of the codes not used for a tenth cha ra cte r.
CLOSED-LOOP CHECKOUT Table 6.3-2 Cha rac ter ist ics of the Command Decoder
Until such time that a full s ecuri ty require-ment i s plac ed on fligh ts of the Satur n Vehicle, thecode-of-the-mission will be openly transmitted dur-ing the prela unch countdown. Thi s will be done tofacili tate the pad checkout procedures.
If i t i s ever necessary to ut il ize the ful lsec uri ty capabilities of the system, then a closed-loop prelaunch checkout procedur e will be used inlieu of the open-loop trans mis sio n. In anticipationof such a requi reme nt, a closed-loop proce dure isbeing developed which will be compatible with therange procedure.
The char ac ter ist ics of the Command Decodera r e given in Ta ble 6.3-2.
Audio (75 ohms)
Audio input level 1 V rm s per tone (1.4 V r m sper tone pair)
Input dymanic range At le ast 12 db (0.56 V rm sto 2.2 V rms)
An over all d iagra m of th e range sa fety com-mand-destruct system is shown in Figu re 6.5-1. Th eground portion of th e syst em is the responsibility ofthe Atlantic Miss ile Range (USAF). The thr ee rang esafety commands applicable to Saturn flights ar e:
a ARM/ FUEL CUTOFF - Arming of theexploding bridge wire and thrusttermination.
The commands a re originated by a rangesafety officer who monitors the vehicle's poweredflight with the help of trac king equipment. In orde rto keep the FM rece ive r "captured" during flight,the vehicle is constantly illuminated by an RF c ar -rier until the last command is transmitted. This is
accomplished by tra nsm itt ers at sever al downrangestat ions when the vehicle is beyond the ra dio hori zonof t he Kennedy Space Cente r. The commands ar etrans mitt ed by frequen cy modulating the command
a DESTRUCT - Firing of the eXP1oding bri*e tra nsm itt ers (at the launch sit e and downrange sta -wire. (Pro pell ant Dispersion Command) tions) with sele cte d combinations of audio tones.
a SAFE - Disconnecting the command de-coding equipment fro m the battery.
RSO -EBW -
AN/DRW -13
DecoderReceiverI
FiringUni t
Range Safety OFficerExploding Bridgewire
Fuel Cutoff
To EBW
I + A N / D R W - 1 3Power Off
VEHICLE STATION (2 Systems Per Live Stage)- - - - - - - - - - - - - -
The R F c a rr ie r i s g enera ted by ~ ~ ~ A N / F R W - ~ ATransmitter whose usable modulation bandwidth isfr om 600 Hz to 80 kHz. This audio-frequency base -band has been divided into 20 channels known as inter-range instrume ntation group channels. The spacing ofthe channels is such that second harm onic s of a givenchanne l will fall ou tsid e the bandwidth of any of th e
other channels, thus minimizing interference. Th ebaseband sp ectr um, with the center freque ncies of the20 inter-range instrumentation group channels desig-nated, is shown in Fig ure 6. 5-2. Although al l 20channels a r e avai lable for range safe ty use , theactual prac t ice has been to use only the lower base-
band f requencies f or range safe ty and to res erv e theremainder of the baseband for other uses.
On most vehicles launched thus far , rangesafety has uti l ized only thr ee of the res erv ed tonechannels (channels 1, 2, and 5) although the Sa turnprog ram now uses channels 6 an d 7 for a commandsyst em SAFE function (deactivation). To providesom e insurance agains t noise or t rans ient RF pulsesaccidentally triggering a function, 2 tones a re a lways
tran smit ted simultaneously. Thus, two audio tonesar e requi red to compose one command s ignal.
Range safety commands have priority overall other u se rs of the AN/FRW-2A baseband; there -for e a l l o ther us er s a re automat ica l ly s ilenced dur ingtransmission of a range safety command.
The c a r r ie r frequen cy of the AN/FRW-2A
Tr a n s m i t t e r is frequency-modulated by the two simu l-taneous inter- rang e instrumentation group tones, and
the resul tant s ignal is tran smit ted to the vehicle. Thet rans mit te r f requency can be s e t anywhere be tween
406 MHz to 550 MHz, although the anten nas pre se ntl y
use d limit the high end to a 500 MHz capability. Theeffective radi ate d power of the sys tem is over 100 kW.
The antenna system on the vehicle receivesthe R F s ignals and channels them to the CommandReceiver. Saturn Vehicle s use the AN/DRW-13 Com-
mand Receiver. Figure 6.5- 3 is a simplified blockdiagram of this receive r. The audio tones repres ent-ing the command a r e rec over ed by demodulation of the
rece ived R F carr i e r. The tones ar e applied to band-pass fi l ters which detect whether a particular tone is
present . Each fi l te r output is applied to the coil of arel ay which is energized only when the filter outputdetects a tone. The contacts of the rela ys a re wireds o that only the two co rre ct simultaneous to nes willcomplete the se r ie s c i rcui t and perf orm the des i r ed
range safety function.
The AN/DRW-13 is a dual-conversion FM
rece iver with ten audio channels, each channel (fi l ter)tuned to a different tone frequency in the modulationbandwidth of the tr an sm it te r. Charac te r i s t ic da ta of
the rece iver is given in Table 6. 5-1.
Table 6.5-1 Range Safety Command Receiv er/Decoder (AN/DRW- 13) Characteristics
