An Efficient Coding System For Deep Space Probes With .
NASA T ECHNICAL NOTEN A S A --T N .-D -4105cC, IA N EFFICIENT CODING SYSTEM FORDEEP SPACE PROBES WITH SPECIFICAPPLICATION TO PIONEER MISSIONS by Dule R. Lnmb und Larry B. Hofmun? ,Ames Reseurcb CenterMoffett Field, Cui NATIONAL AERONAUTICS AND SPACE A D M I N I S T R A T I O NWASHINGTON, D. C.0AUGUST 19676b
TECH LIBRARY KAFB, NMAN EFFICIENT CODING SYSTEM FOR DEEP SPACE PROBES WITHSPECIFIC APPLICATION TO PIONEER MISSIONSBy Dale R. Lumb and L a r r y B. HofmanAmes Research CenterMoffett Field, Calif.NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONFor sale by the Cleoringhouse for Federal Scientific and Technical InformationSpringfield, Virginia 22151CFSTl price 3.00-
TABLE OF CONTENTS.INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .NOTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .CODING SYSTEM CONSTRAINTS F O R P I O N E E R MISSIONS . . . . . . . . . . . . .SELECTED C O D I N G S Y S T E M . . . . . . . . . . . . . . . . . . . . . . . . .C o d e D e t e r m i n a t i o n and Performance . . . . . . . . . . . . . . . . . .Spacecraft Encoder C h a r a c t e r i s t i c s . . . . . . . . . . . . . . . . . .Decoder Characteristics.GROUND OPERATIONAL EQUIPMENT . . . . . . . . . . . . . . . . . . . . . .Demdulator/Synchronizer . . . . . . . . . . . . . . . . . . . . . . .C o m p u t e r Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . .TAPE PROCESSING S T A T I O N OPERATION. .EXPERIMENTAL DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . .SUGGESTED SYSTEM IMPROYEMl3NTS . . . . . . . . . . . . . . . . . . . . . .CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A P P E N D I X A .P I O N E E R PROGRAM . . . . . . . . . . . . . . . . . . . . . .APPENDIX B .DECODING ALGORITHM . . . . . . . . . . . . . . . . . . . . .APPENDIX c .DEMODULATOR/SYNCHRONIZER PERFORMANCE TESTS . . . . . . . . .REFERENCES.TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SUMMARY*mPage11235589991010111112141922232427
AN EFFICIENT CODING SYSTEM FOR DEEP SPACE PROBES WITHSPECIFIC APPLICATION TO PIONEER MISSIONSBy Dale R. Lumb and Larry B. HofmanAmes Research CenterSUMMARYRate 1/2 convclutional encoding with sequential decoding has beeninvestigated for application to deep space probe telemetry links which arecharacterized by a gaussian channel with coherent matched filter detection ofphase shift keyed signals. A (50, 25) code was optimized and its performanceevaluated by means of computer simulations. Data for these studies wereorganized into blocks of 224 information bits where 14 bits were known at theend of each block for purposes of synchronization and to aid in decoding.Results show that the basic coding technique yields a gain of at least 6.5 dBover no coding.For error detection, a technique of reverse decoding was investigated.This technique, used in conjunction with extending the known sequence at theend of each block to 21 bits, permits an additional O.9-dB gain.A large portion of the report is concerned with the implementation ofthis coding scheme on Pioneer missions. Since Pioneer is an operationalprogram, the mission-dependent equipment is analyzed to determine themodifications required for introducing error correction coding to a no-codedtelemetry link.INTRODUCTIONCoding is introduced into the space communications telemetry link topermit more information to be transmitted at a given power within a maximumallowable error rate. Coding increases the number of data bits transmittedby the insertion of parity bits into the information bit stream. For thecoding process, the encoder takes the information data and computes, accord ing to a fixed rule, the parity bits to be transmitted. The data rateincrease caused by the addition of parity bits is distinguished from anactual information rate increase afforded by a coding gain.Quite obviously, if a fixed amount of power is to transmit a giveninformation rate in an otherwise fixed communications system, the introduc tion of additional data (parity bits) reduces the available power per trans mitted data bit. Hence, for fixed power, the bit-error rate from the datademodulator will be greater with coding than with no coding. Thus, the taskof the decoder, which is inserted following the data demodulator, is to cor rect errors made by the data demodulator. The insertion of the encoding and
II l l I , , , , , ,decoding functions into a communications system is illustrated in figure 1.To achieve a coding gain, the error rate after decoding must be less than itis for no coding.Several approaches for introducing coding into the space communicationstelemetry link have been investigated at Ames Research Center and elsewhere.From an implementation standpoint, the coding state of the art for the deepspace channel is typified by the 8-bit biorthogonal telemetry system builtand evaluated at Goddard Space Flight Center (ref. 1). The gain of thisbiorthogonal system is 4 to 5 dB over no coding. The coding system reportedhere gives at least 6.5 dB over no coding. Although the work was done forapplication to the Pioneer program,' most of the results are general and aresummarized in this report. This report describes:(1) The gains both in information rate and data accuracy afforded by thecoding system; and(2) The modifications of the present Pioneer system necessary forrealizing these gains.Any alteration to an operational system must be examineu from the view point of its effect on all parts of the system. To that end, the ratherminor addition to the Pioneer spacecraft, the changes to the ground equipment,and the Ames Research Center data processing system, as well as operationalconstraints, are analyzed and reported herein. Rather stringent constraintswere imposed in order to minimize the modifications to an operational system.The further improvements in coding performance that could be made by removingcertain restrictions are also discussed.NOTATIONE/Nosignal energy per transmitted bit per noise spectral densityEb/Nosignal energy per information bit per noise spectral densityPe,Pe bit-error probability for NRZ-L and NRZ-M transmission, respectivelybPSbits per secondDSSDeep Space StationDTUDigital Telemetry UnitD/SDemdulator/SynchronizerGOEGround Operational Equipment-. . - A' brief discussion of the basic Pioneer communication system is givenin appendix A.2
I&DIntegrate and DumpNRZ-LNon-Return-to-Zero LevelNRZ-MNon-Return-to-ZeroMarkPCMPulse Code ModulationPSKPhase Shift KeyingTPSTape Processing StationTWTTraveling Wave TubeCODING SYSTEM CONSTRAINTS FOR PIONEER MISSIONSThe Pioneer deep space probe series is an approved NASA program for fivespacecraft, with two spacecraft (Pioneer VI and VII) now in orbit about thesun. In order to introduce coding into future Pioneer missions, several con straints are necessarily imposed or appear appropriate. One constraint isthat coding would be commandable to permit no-coding operation wheneverdesired. That is, all spacecraft data formats and ground data processing,both real-time and off-line, will be the same as now employed on Pioneer VIand VII, f o r the no-coding option. This commandable mode may be appropriatefor any space mission that might consider using coding (other than a simpleparity check code). During the launch phase and for the first few stationpasses of any deep space mission the signal strength will probably be quitehigh and coding of the data will be unnecessary. Also, due to the necessityof early assessment of spacecraft soundness, orientation maneuvers, and otheroperational considerations, it may be desirable to minimize the number of com ponents in the telemetry link. Thus, a spacecraft could be launched in ano-coding mode just as the scientific experiments are not turned on untilsometime after launch.A second constraint is that there will be no internal modifications tothe spacecraft Digital Telemetry Unit (DTU) for the coding function. Anyrequired pre-encoding data conditioning, other modulation frequencies, etc.,will be provided by the spacecraft encoder. This is primarily imposed tominimize spacecraft modifications.It is also highly desirable to have the Scientific Data System (SDS)920 computers at the Deep Space Stations (DSS) do most of the decoding inorder to provide near real-time data to the Mission Control Center. However,an alternate approach for decoding using additional ground equipment is beinginvestigated.Pioneer VI and VI1 data are organized into 7-bit words consisting of6 data bits and a seventh parity check bit. This is a simple encoding schemethat is used for error detection only. Figure 2 shows the theoretical per formance of this parity error detection scheme with the demodulator bit-errorrates shown as a parameter. Under present Pioneer criteria, bit-rate change3
i s m d e when t h e b i t - e r r o r r a t e of t h e ground Demodulator/Synchronizer2 (D/S)A t t h i s p o i n t , t h e undetected word e r r o r r a t e i s about 7 . 0 1 0 ' exceedsand t h e d e l e t i o n r a t e ( p a r i t y e r r o r r a t e ) i s 4.0 10' . (See f i g . 2 ) .During t h e code s t u d y and code s e l e c t i o n phases of work ( r e f . 2 ) a mini mum t a r g e t g a i n of 3 dB w a s s e t as a c r i t e r i o n f o r performance improvement.The g a i n comparison w a s referenced a g a i n s t t h e nominal performance of PioneerV I and VI1 f o r t h e b i t - r a t e change c r i t e r i o n given above. It should be notedh e r e t h a t t h e c r i t e r i a f o r comparison a r e r a t h e r important. For example, i fno coding of any kind were used as r e f e r e n c e , t h e d e s i g n minimum coding g a i nwould be increased over t h e above s p e c i f i e d t a r g e t of 3 dB by about 2.6 dB.This i s i l l u s t r a t e d b y t h e comparison between word e r r o r p r o b a b i l i t y f o r nocoding versus t h e word e r r o r p r o b a b i l i t y f o r t h e simple p a r i t y d e t e c t i o nscheme as shown i n f i g u r e 3. I n t h i s f i g u r e t h e 6 - b i t word e r r o r r a t e i st h e s i g n a l energy per information b i t . Deletion r a t ep l o t t e d versus %/No,i s shown as a parameter which, of course, i s z e r o f o r t h e no-coding case.The coding scheme discussed i n t h i s r e p o r t exceeds t h e 3-dB t a r g e t g a i nand i n f a c t provides an a d d i t i o n a l g a i n of a t l e a s t 0.9 dB which w i l l beregarded as an engineering s a f e t y f a c t o r . The d e t a i l e d coding performance i sdiscussed i n t h e s e c t i o n on code determination and performance.It i s suggested t h a t on t h e f i r s t launch i n which coding i s used, thecommunication system modifications be kept t o a minimum; t h i s d i c t a t e s t h a tthe f u l l p o t e n t i a l of the coding scheme n o t be immediately u t i l i z e d . S p e c i f i c a l l y , it i s proposed t h a t t h e system be capable of r e v e r t i n g t o a "no codingTT3o p t i o n with e x a c t l y t h e c a p a b i l i t i e s t h a t now e x i s t . This impliest h a t for no coding, t h e modulation index w i l l be s e t f o r a match of c a r r i e rand d a t a power a t t h e 8 bps maximum range. With t h i s c r i t e r i o n , o n l y one mod u l a t i o n index w i l l be a v a i l a b l e , which i s s e t b e f o r e launch. I n t h i s casethen, maximum range cannot be increased much due t o t h e degradation producedby t h e ground r e c e i v e r phase l o c k loop when i t i s operated beyond t h e designedmaximum range. It follows t h a t a 3-dB coding g a i n can be used t o (1) improved a t a q u a l i t y ( r e c e p t i o n accuracy), and ( 2 ) extend each b i t - r a t e range duringt h e mission except t h e maximum one. Except d u r i n g t h e "no-coding" mode, t h e8 bps information r a t e w i l l not be used s i n c e t h i s e n t i r e range can be handledby coding with an information r a t e of 16 bps. A normalized mission p r o f i l eshowing b i t - r a t e changes f o r t h e Pioneer p a r i t y check case and t h e codingscheme (assuming a 3-dB g a i n ) i s given i n f i g u r e 4.For t h e l a t t e r missions u t i l i z i n g coding, i t would be d e s i r a b l e t o extendthe maximum mission range, e i t h e r w i t h two modulation i n d i c e s (changed uponcommand) or with one index which would p u t l e s s power i n t h e d a t a channel.The l a t t e r a l t e r n a t i v e has t h e disadvantage of reducing the b i t - r a t e rangesf o r o t h e r than the lowest b i t r a t e , whereas a v a r i a b l e modulation index wouldnot r e q u i r e t h i s t r a d e - o f f between maximum and i n t e r m e d i a t e b i t - r a t e ranges.'The D/S i s t h e Pioneer u n i t which provides t h e o p t i m G d a t a demodulationand b i t synchronization of t h e biphase modulated t e l e m e t r y d a t a s u b c a r r i e r .'In r e l a t i o n t o Pioneer, "no coding" r e f e r s t o t h e simple p a r i t y checkscheme without any f u r t h e r d a t a conditioning.4
IIt is proposed in general that "listen only" stations4 receive data inthe uncoded form. That is, the spacecraft should be commanded from the pre vious GOE station to a "no-coding" mode as well as a lower bit rate, ifrequired, if the next station to view the spacecraft is a "listen only" sta tion. There are several reasons for suggesting this procedure. First,results of computer analysis of test tapes from the Goldstone DSS show thatthere is considerable degradation while demodulating taped data in subcarrierform at the Pioneer Tape Processing Station (TPS). Second, the .natureof theerrors is changed; specifically, the phase reversal error, which seldom occursin GOE demodulated data, becomes a predominant error source. A high frequencyof this type of error causes considerable difficulty in decoding non-return to-zero-level (NRZ-L) data. (This is the data form to be transmitted whenoperating in the coded mode.)A final reason for commanding the spacecraft to a "no-coding" mode over"listen only" stations is that, should a "listen only" station receive codeddata, the decoding will have to be done in the TPS at Ames Research Center.This will lengthen the processing time in the IBM 7094 system, which isexplained as follows. The DSS-generated magnetic tapes containing the noisysubcarrier telemetry data are normally demodulated at the TPS with playbackat 16 times real time. These data are formatted by an SDS 910 computer ontoa digital tape for subsequent decommutating, data quality tagging, etc., onthe IBM 7094 computer. Because of the high-speed playback, the decoding willhave to be done by the 7094 instead of the 910. If the longer processing timerequired for decoding is not considered significant, then another commandablemode could be provided that would convert the output of the encoder to a non return-to-zero-mark (NRZ-M) bit stream before modulation. This would allevi ate the phase reversal problem, but the stated gains produced by coding wouldbe reduced. The reduction in gain is due to the higher error rate in demodu lating NRZ-M data since all single D/S errors become two adjacent errors whenthe NRZ-M is converted to the original NRZ-L form. However, the exact amountof degradation under this condition has not been determined.SELECTED CODING SYSTENCode Determination and PerformanceBased on the studies reported elsewhere (ref. 2), the technique chosenfor coding is convolutional encoding with sequential decoding. (For a gener alized discussion see ref. 3.) The selection of the appropriate convolutionalcode was determined by several factors. Because of the D/S limitations (dis cussed later), low rate codes could not be used; because of rather extensivehardware modifications to accommodate clocking, rate l/3 codes were also dis carded, leaving the class of rate 1/2 convolutional codes. In general, thelonger the constraint length (the number of encoding shift register stages),the better a code will perform. However, certain practical constraints4"Listen only" stations are those not configured with Pioneer GroundOperational Equipment (GOE); the noisy data subcarrier is recorded on magnetictape for later processing.5
imposed by the encoder, decoder, and the format of the data fixed the m a x i mconstraint length at 25, requiring a shift register of 25 positions in thespacecraft encoder. The resulting encoder can be built within expected size,weight, and power restrictions. Also, one SDS 920 computer word can be usedto represent the entire shift register required by the decoder to duplicatethe encoder.A technique described by W. H. Lyne (ref. 4) was used to optimize theencoder connections for parity computations. Given the first N - 1 taps of theencoder shift register, this method decides whether to tap register positionN or to leave it untapped, according to which tap condition yields the mostdifferent set of 2N-1 messages that contain an error in position N. Thedifference of each of the messages is measured by the Hamming distance, whichis a count of all the bits in an error message which differ from the correctme s sage.The code selection algorithm was computer programmed to evaluate all theHamming distances for each tap choice at a given node and then to select thebest choice on the basis of the rules given in reference 4.This technique was used to find two codes whose connections yieldedidentical Hamming distances up through 21 positions. Because the number ofcomputations grows as a power of N, this technique consumes a prohibitiveamount of computer time for N greater than 21. At this point, one of thetwo codes, also found by Lyne, was selected because its tapped shift registerconnections appeared most symmetrical; thus the code would perform better ina reverse decoding technique to be described later.The 21-position code was then expanded to 25 positions by trying all thecombinations of code connections in the last four positions in a sequentialdecoder and selecting that set which gave the best performance. For thiscode, the parity bits are formed from the encoder register positions 1, 2, 4,6, 8, 9, 12, 14, 15, 16, 20, 21, 22, 24, and.25.The decoding algorithm, attributed to R. M. Fano of MIT (ref. 3), wasmodified to cause the decoder to force the bit decisions on certain knownwords and on the seventh bit for words which have a parity check bit providedby the Digital Telemetry Unit (DW) in the spacecraft. This technique effec tively provides an additional error-correcting capability to the code. Abrief description of the decoding algorithm is given in appendix B.Two of the decoder parameters, bias and threshold spacing, have beententatively set for simulation purposes, but are not necessarily fixed attheir final optimum values. Also, the decoder parameters have been set for agaussian channel in which the quantization points are set 0.7 standard devia tion apart. This arrangement provides a basis on which to conduct simulationtests. A program for use as a flexible research tool was written inFORTRAN IV language for the IBM 7094 on which all the preliminary tests havebeen made. In this program the quantization scheme and other decoding param eters can be varied by the investigator. The output of the program yields a6
h i s t o r y of t h e channel e r r o r s and decoder performance on a frame-by-frameb a s i s and a l s o a d e t a i l e d p r i n t o u t of t h e decoded information stream when"undetected" e r r o r s a r e committed by t h e decoder.I n e v a l u a t i n g decoder performance, one of t h e most important q u a n t i t i e st o be monitored ( i n a d d i t i o n t o t h e undetected e r r o r r a t e ) i s t h e amount oftime r e q u i r e d t o decode a given amount of d a t a . This i s i n d i
The Pioneer deep space probe series is an approved NASA program for five spacecraft, with two spacecraft (Pioneer VI and VII) now in orbit about the sun. In order to introduce coding into future Pioneer missions, several con straints are nec
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