aerospace telemetry

A VERSATILE AEROSPACE TELEMETRY SYSTEM

G. James Blaine and G. E. BrechlingSystems Engineers

Electro-Mechanical Research, Inc.Sarasota, Florida

ABSTRACT

In general, the trend toward telemetry systemsof increased flexibility and versatility for aero-space vehicles has yielded unacceptable powerand processing efficiencies and questionablereliabilities.

This paper describes the concepts, organization,and areas of application of a core-memory-pro-grammed telemetry system which has beenspecifically developed, in its entirety, to providethe versatility required for aerospace vehicularapplications and environments. It is felt thatthis versatility has been achieved, since thetelemeter can reasonably be applied to data-processing requirements in such diverse areasas deep-space probes, orbiting spacecraft, oravionic data systems. Although format genera-tion is its primary mode of operation, modularexpansion of the basic system permits the ex-panded telemeter to perform as an integral data-compression system.

The paper also describes the development of alaboratory model of this versatile telemetrysystem, which has been constructed to physical-ly implement the various system functionsdescribed herein.

INTRODUC TION

In recent years there has been a marked trendtoward systems of increased flexibility for aero-space data processing and telemetry. In aero-space vehicular data systems, this tendencytoward flexibility is demonstrated by the replace-ment of hard-wired programmed telemeters,capable of generating only fixed telemetry for-mats, with magnetic-core-memory-programmedtelemeters in which many formats can be stored,selected as required, and modified in place ifdesired.

This has resulted in an extremely versatile tele-meter which does much more than simply ac-quire and transmit data. The presence of acore-memory store and limited arithmetic cap-ability permits it to perform in a variety ofapplications completely beyond the scope ofstandard telemetry systems.

It is important to bear in mind, however, thatfor the great majority of aerospace applicationsit is not sufficient to merely adopt the system-and component-design practices of ground data-processing and computer systems. Almost cer-tainly, unacceptable power and processing effi-ciencies and questionable reliabilities will resultif this is done.

Any expansion of telemetry capabilities must beaccomplished within the normal constraints ofthe aerospace environment. This means that theversatile telemeter must provide increased flex-ibility and yet remain competitive in terms ofsize, weight, power consumption, and systemreliability.

This paper describes the concepts, organization,and design of a core-memory-programmed telem-etry system designed specifically for aerospacevehicular applications. Its makeup is very sim-ilar to that of a special-purpose digital computer,in that an existing configuration can be adaptedto perform a variety of functions.

The versatility of the telemetry system is demon-strated by the fact that it can be used efficientlyin many diverse applications, such as in deep-space probes, satellites, orbiting spacecraft,and avionics data systems. The area of applica-tion will be a strong factor in determining thetelemeter's role in the overall vehicular system.For example, in a deep-space-probe application,a strong case can be made for employing a singlecentral control unit to accomplish a variety offunctions previously performed by a multiplicityof special-purpose system elements. Experi-ments programming and control; engineering andscientific telemetering; data compression;guidance, attitude, and power control; commanddecoding; navigation computations--all are tasksthat can be performed by a single system posses-sing a memory store, arithmetic, and sufficientinput-output capability. Since the format-gener-ation rates are relatively slow and the telemetrysystem normally has access to the various datasources required to accomplish the above tasks,it is a logical choice to function as the heart ofthe deep-space data system.

An entirely different situation exists in an orbit-ing-spacecraft data system. Normally, a rela-

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tively large and powerful central-control digitalcomputer exists for performing the tasks previ-ously mentioned. However, the high-speed data-acquisition rates required would place an unrea-sonable burden on the central computer if it hadto generate the data format. Using the versatiletelemeter for format generation frees the centralcomputer from this routine task and therebygreatly simplifies the design of the spacecraftdata system.

In a large-data-system application the telemetrysystem, in addition to performing essentially asan input-output device for the central computer,can perform additional tasks by a modular expan-sion upon the basic configuration. Data com-pression and monitor-and-alarm are routinefunctions that can be accomplished by an expand-ed telemetry system.

In all of these expanded telemetry applications,the common denominator is the routine nature ofthe task. A small expansion in the telemeterhardware permits a proportionately greater sim-plification in compute r s oftwa re.

TELEMETRY-SYSTEM APPLICATIONS

Format Generation

The core -memory -programmed telemeter differssignificantly from the hard-wired system in themethod used for data acquisition. In the usualhard-wired system, a variety of binary countersare advanced by clock pulses at their respectiveframe or subframe rates, and the contents ofthe counters are decoded by logic gates which,in turn, actuate the data-source switches. How-ever, in the versatile telemeter, the countersare replaced by a core memory in which isstored the address code of the individual dataswitches. If 2n data sources constitute the sys-tem, a code of n binary bits provides a uniqueaddress for actuating each data-source multi-plexer switch. The address code is independentof (1) the character of the data, be it digital orhigh- or low-level analog; (2) the location of thedata source in the format, and (3) the samplingrate. A format is generated by obtaining se-quentially from the memory the addresses ofthe multiplexer gates for the data sources to beincluded in the data frame.

In a typical application this telemetry systemwould utilize one of several preplanned dataformats stored in the core memory. The appro-priate format is called up via a command link,a manual selector switch, or by means of anadaptive process and is initiated by locating in

memory the starting address of the first formatword. As the memory is sequenced from thispoint, address words appear sequentially in theoutput register and are presented to the systemmultiplexers to effect data-switch selection.

The telemeter operates in a manner similar toa single-address digital computer. Each wordstored in memory is an ordered set of digitswhich are processed as a unit. A fixed numberof bits constitute the operation-code portion ofthe word, and other bits are used for indirectaddressing and indexing. The remaining bits(normally designated the operand) are the data-switch or memory data addresses.

In its most basic configuration, the core-mem-ory-programmed telemeter has an instructionrepertoire limited to those commands requiredfor format generation. Memory-referencedinstructions for this application include:

Data Out- -Provides in the operand portionof the word the address code of the data chan-nels which are sampled at the basic frame rate.

Memory Out--Provides in the operandportion of the word the memory address of data,such as frame synchronization codes, to beinterlaced with the output PCM bit stream.

Subcommutate--Used to obtain addresscodes of data channels which are sampled atless than the frame rate.

Jump--Used to transfer program controlto a randomly selected core-memory location.

Load Program Counter-..Provides thestarting address of a given format.

Load Index Counters--Re-enters intocounters, at the completion of a subframe,data indicating the size of the respective sub-frames.

Instructions which do not reference memoryare designated "augmented" since the operationcodes are augmented, or extended, by causingvarious operations to occur as a function of thebits normally reserved for channel addressdata. The operation is a micro-program inthat multiple operations can be specified andperformed simultaneously according to theoperand data bits. Typical uses for the aug-mented instructions are: bit-rate selection;word-length selection; format-number com-pare, search, and verify; operating-modeselect; register transfer, and input and outputtransfer.

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SERIAL PCM

IPOWERSUPPLY

Fig. 1. Basic Configuration For Format Generation

The data formats are normally loaded into mem-ory off-line via assembled paper tape or magnet-ic tape, although manual entry or modificationof data is also provided. However, to provideadditional flexibility, the system is capable ofentering format data while the telemeter is on-line, either from an associated on-board com-puter or directly through a command link. Theentry of data is random access; therefore, theindividual data words which establish the systemconditions can be modified to effect changes inbit rate, word length, etc. As a precautionagainst entering incorrect format data into mem-ory, each data word that is written undergoes averification cycle before the next word writecycle is initiated.

The block diagram of Figure 1 illustrates thebasic elements of the versatile telemeter in the

format-generation mode. Additional capabilitiesof the telemeter will be illustrated in subsequentsections.

Monitor -and-Alarm

The versatile telemeter is well suited to performthe basic monitor-and-alarm functions requiredin aircraft and manned spacecraft. A standardmonitor-and-alarm (M-and-A) system acquiresdata representing the physical status of the air-borne or spaceborne environment and equipment,then compares these data against preplannedstored signals of a given value and tolerance.The value represents the expected situation; thetolerance represents its normal variation fromthe mean condition. Any out-of-tolerance datasignal is assumed to represent a fault conditionrequiring an alarm indication to the crew, a

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display of the fault, and possibly a diagnosticresponse to assist in correcting the situation.

Figure 2 is a block diagram of a typical monitor-and-alarm system. Since a large portion of theM-and-A system is devoted to acquiring data,the basic configuration of the versatile telemetercan be directly adapted to this task, and the M-and-A multiplexers are sequenced in accordancewith the format-generation program stored inthe core memory.

is programmed to repeat the acquisition-and-testprocedure in order to avoid raising unnecessaryfalse alarms due to simple bit errors. If thefault indication persists, the fault detector causesthe normal format-generation program to beinterrupted, and a special memory fault routineis initiated. At this time, data which serve toconvert the channel address of the data-sourcefault to a physical identification are obtainedfrom memory. This parameter identification isthen applied to the display unit, which may range

Fig. 2. Configuration For Monitor-And-Alarm

Each channel-address instruction is accompa-nied by the retrieval from memory of the ex-pected data value and tolerances associatedwith the given data source. The acquired dataare processed through the data-conversionequipment and applied to a fault detector whichhas as its reference input the data-value bandrepresenting the expected condition. The faultdetector performs arithmetic operations to testthat the acquired data signal falls within theexpected range; if an out-of-tolerance conditionodcurs, the format-generation program is per-mitted to advance normally,. If an out-of-toler-ance condition is encountered, the test sequence

in complexity from a simple projection displayto a printout, microfilm, or cathode-ray display.At the same time, the conversion factors asso-ciated with the data-source fault are obtainedfrom memory and applied, together with thedata-source amplitude, to an arithmetic unit.Scaling and conversion of the data-source elec-trical amplitude to a physical quantity is accom-plished in the arithmetic unit, and the resultantsignal is applied to the display unit. The latterprovides visual identification of the fault condi-tion and an indication of the magnitude of thephysical parameters involved.


Depending upon the size and complexity of thedata system and the degree of safety required,several program options are now possible. Ifthe system is simple and the fault is of a minornature, it may be sufficient to raise a visual oraudible alarm to alert the crew to the situation.If the system is complex and the fault is consid-ered serious, several additional procedures maybe programmed into the fault subroutine.

Once the nature and magnitude of the fault havebeen established, the memory is available toinitiate an associated data-acquisition subroutine.A major data fault may have a format-generationprogram which interrogates associated datasources in order to provide additional data per-tinent to the condition. In a simple system, itmay be sufficient to make these data availablein the form of a printout to help the crew trouble-shoot. In a more complex system, a requestfor diagnostic service may be made of the cen-tral-control computer, and the associated datacan be entered into a diagnostic program. Usingthese related data, the computer can resolveany possible ambiguities in the fault conditionand/or establish and display corrective andmaintenance routines to be followed. Locationand identification of spares or backup equipmentmay also be displayed.

As an alternative approach, the analysis anddiagnosis of a fault condition can be made by thecontrolling ground complex. In this mode ofoperation, a major fault condition would causea telemetry-format-change request to be initiated.The telemeter would sequence and transmit sev-eral frames of fault-related data and then revertto its original format. Since the telemeter haslimited format storage, the fault-condition for -mat should be general purpose; hence, an identi-fication tag must be added to each fault-formatdata word.

Data Compression

Frequently, applications for which the versatiletelemeter is well suited can utilize some form ofdata compression efficiently. Data systems fordeep-space probes are typical of such applica-tions. Several forms of data compression arereadily accomplished with an expanded versionof the versatile telemeter.

The zero-order predictor, which exemplifies thefloating-aperture compression technique, is themost well known and probably the most widelyemployed compression device. Generally, thistechnique is employed after the analog data areconverted to digital words. The zero-orderpredictor is a form of run-length coding in whicha tolerance band is placed about the data samples.Briefly, the predictor takes the first sample inthe data frame, sorts it, addresses it, and com-pares successive samples of the same channel.No information is transmitted until the samplebeing examined is outside the error band, atwhich time the original sample magnitude, itsaddress in the frame, and the frame number aretransmitted to the buffer store. The samplewhich exceeded the error band then becomes thenew basis for comparison, is addressed, andthe procedure is repeated.

Figure 3 is a block diagram of the expandedversatile telemeter used to perform as a zero-order predictor. It can be seen that a greatsimilarity exists between this system and themonitor-and-alarm system previously described.Either task can be performed by essentially thesame hardware with different programming.

SYSTEM DESIGN

Laboratory-Model Development

After completion of the subroutines for a detectedfault, the M-and-A system returns to its originalsearch-and-check mode. Since the normal rateof scanning of physical parameters may be rel-atively low, modest fault subroutines can becompleted with little or no interruption of themonitoring process.

The on-line write capability of the M-and-Asystem's memory is an important factor in over-all system performance. During a flight it isusually necessary or desirable to modify valuesand tolerances of the fault-comparison databased on in-flight computations or on data re-ceived via a command link. Such modificationis readily accomplished through random accessto any memory location.

A laboratory model of a representative versatiletelemetry system has been developed at EMRthrough a company-funded program. Figure 4is a photograph of this model.

The program consisted of a study phase followedby an implementation phase. As a result of thestudy phase, major emphasis was placed onmemory design. The system requirements dic-tated a memory design to achieve high reliability,low power consumption, high-speed readout,and relatively high-speed selective writing. Aword-organized linear-select memory conceptwas chosen and implemented with basic readout(NDRO) and nonvolatile storage.


Fig. 3. Configuration For Data Compression

Fig. 4. The Laboratory Model

A significant reduction in power consumption isrealized with the NDRO, word-organized mem-ory since data restoration does not require awrite cycle with each read cycle. This featureallows high-speed operation in the "read-only"mode, which is the normal mode of operationfor format generation. In addition, the NDROand nonvolatile properties of the memory en-hance system reliability through integrity of thestored data. Memory operation is either sequen-tial or random access. Random access permitsthe on-line modification of memory data withoutdisruption of system operation. Because pri-mary emphasis was placed on system conceptsand organization, only moderate data-channelcapacity and sample rates were specified forthe laboratory model. It should be noted thattelemetry-system capacity is limited only bythe capacity of the memory unit.

The laboratory model consists of the basic ele-ments shown in Figure 1. These elements are a256-word-by-12-bit core memory, a controlunit, analog and digital multiplexers, and ananalog-to-digital converter. The pertinent de-tails of each of these elements are described inthe following sections.


Core Memory

The basic storage element selected for the corememory is a multiaperture, orthogonal-flux-path ferrite core. It was selected to satisfy theaforementioned constraints of low power, non-volatile storage, nondestructive readout, andhigh speed.

As is shown in Figure 5a, the ferrite elementcontains an interrogate hole and a storage holewhich are mutually orthogonal and nonintersect-ing. Information is stored in the element bysetting the flux around the storage hole to satur-ation in one direction for a "zero" and the otherdirection for a "one. " A "ratchet-write" (1)current is applied through the interrogate hole,time-coincident with a "one' or "zero" partialwrite current through the storage hole. Thisresults in a flux in the common volume betweenthe holes which remains in a quiescent "one" or'zero"' state after the currents are removed.

Interrogation of the element is accomplished bythe application of a current pulse through theinterrogate hole. The current pulse causes anelastic magnetic-domain reorientation whichproduces a voltage on the storage line. Theoutput is a bipolar pulse that corresponds to thestate of the core. Upon termination of the cur-rent pulse, the domain reverts to its originalorientation, thus effecting a true nondestructivereadout. (2)

The output waveforms corresponding to a stored"one" and a stored "zero" are shown in Figure5b. Typical output levels of ±8 mv are obtainedfor interrogate currents of approximately 250 ma,bit-write currents of +110 ma, and two-pair"ratchet-write" currents of ±400 ma.

The physical and electrical characteristics ofthe ferrite element permit a simple array con-figuration for the memory unit.

An m-word-by-n-bit array, as shown in Figure6, eliminates the need for "weaving" lines. Twolines through the interrogate hole of each coreare used so that unipolar current drivers canperform the "ratchet writing. "1 The particulararray utilized in the core, memory for the labor-atory model consists of 256 twelve-bit words.The array is organized in an 8 x 32 matrix con-figuration to permit optimal decoding of wordaddresses. An overall block diagram of thememory unit is shown in Figure 7.

(1)Baldwin, J.A., Jr., "Theory of Ratchet Writ-ing in Thin-Wall Multipath Memory Cores, "INTERMAG Conference Proceedings, pages 6. 4-1to 6.4-8; April 1965.

_ STORAGE LINE

STORAGE HOLE

BASIC ELEMENT

STORED 1I "

BIPOLAR OUTPUT

a

STORED "O"

b

Fig. 5. Basic Core-Memory Element and OutputWaveforms

M WORDS

BIT I

BIT 2

N BITS

BIT N

Fig. 6. Basic Array

Read and write commands to the memory aresynchronized to the system clock. A read com-

(2)Beveridge, R. W., "The Micro-biax MemoryElement, " WESCON Proceedings, pages 2.4-1to 2.4-5; August 1965.

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r - - - - - - - - - - -- - - - - -- - - - - --

INTERROGATE

WRITE

N TERROGA TESITS 6, 7, 8 mAR

ITUS

0 STAGES R)

SX lSTAGOSCES

CLEAR LOAD

INTERROGATEPULSE

Fig. 7. Block Diagram of Memory Unit

mand initiates an interrogate cycle. The dataaddress is decoded to select the proper currentdriver and word gate, and the memory data reg-

ister (MDR) is cleared. A gated current source

provides the required level of current to the in-terrogate line, resulting in an output on thesense lines. An integrated-circuit comparatoramplifier is used as a detector on each sense

line. The amplifier output gate is strobed atthe proper time to load the data into the MDR.The complete read cycle is accomplished inless than one microsecond.

The write cycle is initiated by the write com-

mand. The desired location is again selectedby the memory address, and the desired dataare loaded into the MDR. The MDR controlsthe polarity of the bidirectional bit-currentsources and hence the polarity of stored infor-

mation.

The "ratchet write" is accomplished by switchingthe write-current source from the (-) to the (+)interrogate lines through two cycles. The writecycle is less than five microseconds.

Integrated circuits are utilized for the registersand the decoders. Discrete transistors are usedto switch the drive currents. The data and ad-dress input-output levels are compatible with thelogic utilized in the control unit. Low power

consumption has also been achieved. The stand-by, or quiescent, power is less than five watts;the power required during a read mode is lessthan eight watts, and the power required duringthe write mode is less than 24 watts.


WRITE

St"SE AMPS 112 TOTAL

DATA FRtASSEMBLYREGISTER

DATA OUTPUT

READ

WRITE

I-MHzCLOCE I L- - LOAD

I

369

System Electronics

Figure 8 illustrates the functional elements of thelaboratory model. All digital circuits utilizeDTL logic in the form of monolithic microcircuitsand are contained on double-sided printed-circuitcards. The cards are organized functionally.Two basic printed circuits, in conjunction withrear-side jumper wires, provide the approxi-mately 20 different logic functions required inthe system. Analog circuits in the analog-to-digital converter and the various signal-condi-tioning amplifiers employ both analog micro-circuits and discrete components.

is identical, regardless of data rates or thesubsystem in which the unit is used. Thisequipment standardization results in savings indesign qualification and in the amount of on-board equipment, and enables the use of rela-tively few diagnostic tests in checking the per-formance of a large portion of the systemcomponents.

The use of an address code to implement thedata-switch selection also permits the multi-plexers to be located remotely from the systemelectronics. Placing multiplexeis in proximityto the associated transducers results in

DATA* SOURCES

oATASOURCES

Fig. 8. Block Diagram of Versatile Telemeter Laboratory Model

The analog multiplexers have been designed toserve 64 data sources. Becastse of the random-access nature of the multiplexer switch-selectionprocess, the configuration of all multiplexers

increased system accuracy due to reducednoise pickup. It also provides significantsavings in weight due to reduced spacecraftwiring. The address code is normally

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presented in parallel to all system multiplexerssimultaneously. If system cabling is to be keptto an absolute minimum, the address codes maybe shifted sequentially into multiplexer registers;however, design requirements are eased if theaddress is presented broadside. The laboratory-model multiplexers have been designed to oper-ate such that in the nonaddressed, or quiescent,signal condition all power, with the exception ofmultiplexer code-recognition circuitry, isswitched off. In a large system this featureresults in significant power savings since onlythe multiplexer addressed is activated by a pow-er strobe, and then only for a time sufficient toeffect an analog -to -digital conversion. Addition-al bits can be provided in the channel-selectaddress code to designate a multiplexer ampli-fier voltage-gain setting, thereby performingsignal scaling, or full-scale range changing, inthe case of a programmed adaptive system.

The address code presented to each multiplexeris entered into a register in each unit, which inturn is presented to a logic network for switch-selection decoding. In the event of memoryfailure, it is possible to operate these multi-plexer registers as binary counters, therebyproviding data--however inefficiently and illorganized--on an emergency basis if desired.

CONCLUSIONS

The techniques described in this paper are a

direct natural development of airborne datasystems. In addition to the applicationsdescribed herein, other system functions suchas data storage, events programming, equip-ment fault detection and adaptive repair, andpreflight checkout are reasonable applicationsof the versatile telemeter.

The laboratory-model system is presentlybeing used to format and process telemetrydata. In addition, it has been interfaced withan in-house digital computer programmed toperform the functions of the expanded telemeterin the areas of monitor-and-alarm and zero-

order-prediction data compression. Develop-ment of the telemeter is continuing, and modu-lar additions are being designed as standardhardware.

The multiplexer configuration permits greatflexibility in the choice of a data switch. Todemonstrate this, the laboratory model hasbeen fabricated with an equal mix of transform-er-driven chopper switches, N-channel junctionFET switches, and MOS FET switches.

The laboratory model operates in the format-generation mode at bit rates of up to 500 kilo-bits per second and is primarily limited at thistime by the analog-to-digital conversion periodof ten microseconds. An A/D conversion to tenbinary bits at an accuracy of 0. 1So is providedas standard, but the output word is selectableby program control in any length from seven toten bits. Bit-rate selection, in increments ofpowers of 2, is selectable by a program-statusword.

A control panel displays the contents of themajor system registers and provides data entryinto these registers. A decommutation mode,in which any single channel can be selected anddisplayed on the control panel, is available.The format can be selected either with a thumb-wheel switch or by loading the program counterdirectly. The control panel also provides directmanual read or write of data into any memorylocation on a random-access basis.

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