compatibility and performance testing of communications systems

SUPPLEMENT TO IEEE TRANSACTIONS ON AEROSPACE / JUNE 1965

COMPATIBILITY AND PERFORMANCE TESTING OF COMMUNICATIONS SYSTEMS

Howard C. KyleNASA Manned Spacecraft Center

Houston, Texas

ABSTRACT

During the normal progress of design, fabrication,and integration of communications subsystems forspacecraft and for ground installations, every ef-fort is made to assure that the equipment meetscertain specifications relating to performance, en-vironment, reliability, and interface capability.These specifications are based on the best avail-able definition of requirements and interfacecharacteristics of complementing subsystems. Fre-quently, in the field of manned spaceflight, thespacecraft subsystems, the launch vehicle subsys-tems, and the ground systems must be designed andconstructed concurrently. This means that the op-erating and interface characteristics of one sub-system are not available for use by the engineersin the design of the other subsystems. Close tech-nical liaison among the various engineering groupsis essential in the accomplishment of overall sys-tems' integrity. Component and subsystem testinghas been developed to a high degree, but the re-sults of these are necessarily limited. They can-not validate the overall systems' performance andcompatibility.

It is considered mandatory that the interfacingsubsystems be mated to form a complete system in acontrolled test environment as early as practicablein any program, especially in one involving commun-ications systems as new and as complex as those forApollo. This must be accomplished at such a phasein the program that corrective engineering detailscan be fed back to the cognizant design, fabrica-tion, or integration groups involved in time fornecessary modifications prior to the beginning ofthe flight phase. Essentially these tests, accom-plished by using early models of actual spacecraft,network, and control center hardware, are designedto evaluate and demonstrate the capability of allcommunications systems to interface properly andto meet all operational requirements.

This paper discusses the test philosophy, typicalfacilities, test planning, and the optimum utili-zation of test results.

INTRODUCTION

The success of the manned space program is contin-gent on the capability of the spacecraft and groundsystems to operate effectively as an integratedunit. The spacecraft and ground systems for theseprograms are being manufactured and tested, inmany cases, independently of each other by variouscontractors and subcontractors. Therefore, inorder to assure compatibility between spacecraftand ground systems, it is mandatory that a programof integrated system tests be initiated. Theprime objective of such a program is to verify thecompatibility between the space vehicle and groundsystems to be used during operational missions andto determine the capability and performance ofthese combined systems under expected operationalsituations.

In such a test program, combinations of communica-tion and electronic equipment and facilities areinterconnected to form systems capable of performingspecific functions. For the Apollo program, testinghas been divided into three phases consisting ofground, aircraft, and space vehicle tests. Thispaper discusses the salient features of these tests,while emphasizing the ground test.

COMMUNICATIONS REQUIREMENTS

The communications requirements which must be satis-fied in the current manned space programs may beclassified as follows:

VoiceTelemetryTrajectory measurementsTelevisionUp-Data

Each of these requirements represents a group withinwhich there is a multiplicity of specific require-ments, varying at frequent intervals with the pro-gram requirements and with the phases of a givenmission.

Two-way voice is required between Earth and all man-ned vehicles, between the vehicles themselves, be-tween extra-vehicular astronauts, and between themand their parent vehicles.

Telemetry constitutes a very large number (perhapsthousands) of individual measurements, with widelyvarying requirements for frequency response andsampling rate. The entire space vehicle, includingpropulsion units, with all the complex systems andthe crew, must be monitored--some in near-real timefor flight control, others only for post-flightanalysis.

Trajectory measurements include those of severalvehicles from Earth stations, as well as those madefrom the vehicles themselves. The latter includemeasurements to aid in rendezvous and in lunarlanding.

Transmission of data to space vehicles has become anecessity, even those with a human crew aboard. Itis a means of doing what the crew cannot do, eitherbecause they haven't the time, the capability, orthe equipment, or because they are incapacitated.The most stringent requirement is for transmissionof complex data such as those needed to up-date on-board computers and navigation systems. Intervehic-ular data link systems, not in appreciable use now,may soon be required.

BASIC SYSTEMS

The spacecraft and ground systems utilized in thefirst U.S. manned space program (and to some extentin the current Gemini Program) were those basicallyavailable from previous aircraft and missile programs.

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A large amount of redesign and repackaging had to bedone in order to meet weight, space, power, and heatload limitations; but there were no new or untriedbasic concepts. The voice system was VHF (and HF)Amplitude Modulation, similar to existing militaryand commercial aircraft systems. The telemetry sys-tem was VHF FM/FM and PAM/FM/FM, used extensively inaeronautic and missile research and development.Trajectory measurements were accomplished by conven-tional instrumentation radar equipments, using coop-erative transponders, or beacons. The up-data linksystem was an adaptation of the "range safety" tonemodulation system, using the same ground transmitters.

However, the programs now being planned and developed,such as Apollo, place much more stringent require-ments on the communications systems. The multiplic-ity of requirements, the desired system flexibilityand reliability, and the requirement for satisfac-tory performance margin at great distances have dic-tated a new approach to systems' design. No longercan we afford to have an independent system to per-form each function; instead we must conserve weight,space, and power by making maximum utilization of a'1common"i communications system. Such a philosophyis partially followed in Apollo, using a UnifiedFrequency System. (Since it operates at S-band fre-quencies, it is commonly termed the Unified S-bandSystem, or USB.) All the primary spacecraft-Earthfunctions are accomplished by modulations on a singleup-carrier and two down-carriers. (The two down-carriers are necessary only because of modulation in-compatibilities. Television, for example, beingbroad band, is not compatible with narrow-band PhaseModulation; hence television utilizes Frequency Mod-ulation.) These systems, for both spacecraft andground, plus systems for intervehicular communica-tions and for crew extravehicular communications,must be designed and developed, with new and unprovenconcepts, components, modules, and subsystems. Inspite of this, these systems must maintain a factorof reliability far in excess of that normally attain-ed in electronic equipment. Obviously, an exhaustivetesting program is mandatory.

TEST PHILOSOPHY

It is first necessary to define further the testingto be discussed. There are a great many types oftests, varying with the status or phase of design,development, implementation, and operation; and withthe test variable, such as physical environment orelectrical environment. The testing to be describedhere is based on the following assumption:

The entire communications system (space-vehicle andground subsystems) with all its components, modules,and other elements has previously been thoroughlytested to assure compliance with specifications re-lated to physical environment, electrical environ-ment, performance, subsystems compatibility, radiofrequency interference, electromagnetic interference,etc. There remain then two primary test objectives:

1) Evaluation of overall space-vehicle/ground systems'compatibility

2) Evaluation of total systems' performance and capa-bility of satisfying mission requirements

This testing for Apollo has been divided into threephases:

1) Ground tests -

Conducted in a controlled electrical environment,using actual space vehicle and ground hardware.

2) Aircraft tests -Conventional aircraft are at least partiallyequipped with spacecraft hardware. This test islimited in dynamic and r.f. simulation. Primaryfunction is station checkout and operator train-ing, but may uncover systems' or operational in-compatibilities.

3) Space vehicle tests -Conducted in early vehicles in short range Earthsuborbital or orbital flights. Other provenequipment will be used to perform the operationalfunctions on these flights.

The objective of all three test phases (though to aconsiderably lesser degree in Phase 2) are as statedpreviously--compatibility and performance. Eachphase is conducted when the preceding phase has pro-gressed to such a state that analysis and evaluationof test data indicate that compatibility and per-formance results are satisfactory, determined withthe desired level of confidence. Each test, withits unique configuration and environment, increasesthe overall confidence level, so that after a satis-factory space vehicle test flight there is 100-percent confidence that the various systems and sub-systems have the capability of performing their as-signed functions during all phases, nominal and con-tingent, of an operational mission. Only then willthe communications systems attain operational status.

Each of these test phases will be discussed in briefdetail.

GROUND TESTS

Here the space vehicle hardware and ground hardwareare mated together for the first time. But beforethis "marriage" can be performed the individual sub-systems must be thoroughly calibrated and validated.This is essential for two reasons:

1) It documents reference or background data on theentire space vehicle subsystems and ground sub-systems separately.

2) It verifies that these two subsystems are repre-sentative of operational space-vehicle andground subsystems.

The actual tests should follow some logical sequencefrom the simplest, such as that involving a single,one-way, unmodulated r.f. carrier, to the full sys-tem, involving several carriers with multiple modu-lations, two-way, in all operational modes.

AIRCRAFT TESTS

As soon as the ground network subsystems are in-stalled and tested against their respective speci-fications, there need to be dynamic data flow teststo assure confidence in station readiness and oper-ator proficiency. Many of these may be accomplishedusing local devices, such as test sets, prepareddata and voice tapes, and boresight or collimationtowers. However, a large void exists between thelevels of confidence and proficiency attained andthose desired for actual space flight. An aircraft,equipped with real or simulated space vehicle equip-ment, flown against the station using various pat-terns and procedures, can provide a very definiteincrease in the level of confidence that the equip-ment and personnel can support the space mission.The goal is complete confidence that they can sup-port the Phase 3 tests, the space vehicle testflights.

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SPACE VEHICLE TESTS

These are the final tests before committing the sys-tems to operational use. As early as possible inthe flight program the communications subsystems--spaceborne and ground--will be exercised and eval-uated. All modes will be tested--up and down data,ranging and doppler tracking, television and two-way voice. These tests, for instance, will permitverification of (and any necessary modifications to)established techniques and procedures for carrier,angle, and data acquisition. Since most uncertain-ties center about signal levels at or near thresholdconditions, these will be simulated by appropriateattenuation on the ground. At the conclusion ofthese tests, the systems' status should be as fol-lows:

The flight hardware operates satisfactorily in a

near-space environment.The communications system concept is valid, andthe system will support the lunar mission.Complete end-to-end data acquisition, processing,display, recording, and reduction exercises in-dicate overall systems' compatibility.

TEST PLANNING AND UTILIZATION OF TEST RESULTS

To insure the success and usefulness of a test pro-gram such as the one described, a great deal ofplanning must be accomplished prior to actual test-ing. Among the areas which must be considered are

schedule effectiveness, test phases, test require-ments, facilities capabilities, and utilization oftest results. The enormity of such considerationsis self evident, thus only a brief discussion ofthese areas can be presented here.

SCHEDULE EFFECTIVENESS AND TEST PHASES

For the aircraft "fly-by" test phase the test re-quirements dictate an entirely different type offacility. Here the actual tracking station is used,and a portion of the spacecraft equipment will behoused in an aircraft. These tests must provideadequate information to verify the operational read-iness of the ground station and provide a capabilityfor personnel training. The utilization of testresults will be primarily as a verification of equip-ment readiness; however, it is quite feasible thatsome hardware changes might still be omdocated atthis time.

For the "space vehicle" test phase, the facilitiesquite obviously become the entire Manned SpaceFlight Network and the space vehicle itself. Allsystems will encounter the actual mission conditionswhich have thus far been simulated. If the othertest phases have been completely successful, thetest results obtained will verify the compatibilityand performance of this enormous complex of electron-ic equipment and technical personnel.

TEST FACILITIES

The ground receiving and transmitting equipment islinked to the spacecraft equipment via r.f. hardline,through an R.F. Path Simulation System. The pathsimulator provides attenuation to simulate pathloss, doppler shifts, signal delays, and dynamicantenna pattern effects. Additional ground equip-ments include demodulators, data processing and dis-tribution subsystems, recording and display subsys-tems. Test equipments are generally grouped in twocategories. These are the Test Control and DisplayConsole and the Data Evaluation and ComparisonSystem.

NASA -S-65-2313

The effectiveness of the test program is almost en-

tirely dependent upon adherence to a logically es-

tablished test schedule. To provide this necessaryeffectiveness, the "ground test" phase must bescheduled concurrently with the development phasesof the system. The aircraft "fly-by" test phasemust be scheduled to insure that operational veri-fication of all ground tracking stations is accom-

plished prior to orbital missions. The "spacevehicle" test phase must be scheduled after "fly-by"test to insure that all spacecraft systems meet op-erational requirements in a space environment. Allof these tests must be accomplished prior to util-ization in a space mission.

TEST REQUIREMENTS, FACILITIES CAPABILITIES, ANDUTILIZATION OF TEST RESULTS

The considerations for each of these areas differdepending on the particular test phase. For"ground tests," the test requirements must be estab-lished for a facility capable of conducting detailedengineering tests. The tests must provide adquateinformation to verify the compatibility and eval-uate the performance of the entire system. Herethe test results must be effectively disseminatedand applied as soon as they become available, sincethe entire success of developing a compatible sys-tem can be affected. The results may dictatechanges in the actual hardware and thus must beavailable before system development is too far ad-vanced.

DATA EVALUATION _AND COMPARISON

SYSTEMINTERFACE

EQU IPMENT

TO MCC-H

Fig. 1(a).- Basic Test Flow Diagram

Equipment required to link the Network systems withthe Mission Control Center (Houston) are in theform of data modems.

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1)

2)

3)


Figure l(a) shows a basic block diagram of the facil-ity. Figure l(b) is the same diagram with some ofthe equipments listed in the blocks.

iA%A.%A5.23IA

S/C EQUIPMENT MSFN EQUIPMENT

US: TRANSPONDER UGPRAATH ER T ROOUND STATION

PCl TELEMETRY ATTfNUATONS TEEMTR OUD TPUTIONFEUP-DATA SYSTEM COAXIAL SWITCH WD*ATAP AtECO"DECENTRAL TIMING MULTILEXEI DIGITAG COMDAND SYSTDATA STORAGE EQUIP POWER MONITOR DOWN RANGE UP-LINKAUDIO EQUIPMENT FRw-2 TRANSMI"EDVHf SYSTEME TES

EVALUATION EQUIPMENT_CENTRAL CONTROL

AND DISPLAY CONSOLE

ANALOG DISPLAYSDEClIMAL DISPLAYS

RECORDERS IHERATv MONITOR TTY

T COpMAND PromATRIX s HSPGM^T AND GET CLOCKSCOMPARISON CONSOLE WBDSERIAL SIT ERROR DETECTORANALOG COMPARATORPARALLEL SIT ERROR DETECTOR MCC-HAUDIO SYSTEM

Fig.s(b).- Basic Test Flow Diagram with Equibment

TYPICAL GROUND TEST

It is not possible here to present a detailed dis-cussion of all testing performed in the "groundtest" phase. However, in order to clarify the de-gree of testing being conducted, the tests of a

single subsystem are described.

TELEMETRY TESTS

The primary Apollo telemetry subsystem is Pulse CodeModulation (PCM) at 51,200 bits/second. The bitstream bi-phase modulates a 1.024 megacycle sub-carrier, which, in turn, phase modulates the S-bandcarrier. (There are other types and modes of telem-etry, but this is the primary one.) The purpose ofthese tests is to prove compatibility and determinethe performance, including error rates, of the PCMlink.

Prior to any operational testing, the entire system,including test equipment, must be validated and cal-ibrated. Validation here implies assurance that thespacecraft and ground equipment are adequately rep-resentative of operational systems, and that theassociated test subsystems do not disturb the in-tegrity of the "operational system" and are config-ured adequately to support the desired evaluation.Calibration carries its usual meaning--the collec-tion and documentation of all necessary static andbackground data for referencing the actual test data.These data include:

1)2)3)4)

5)6)7)8)9)

Various bandwidthsGainsDynamic rangesDigital-to-Analog onversion accuracy andlinearityModulation indicesReceiver thresholdsPower levelsR.F. path siT lator calibrationBaseband bit error rate

The System Operational Tests are conducted undersimulated mission (r.f. circuit) conditions, usingall modes of operation for one-way and two-way com-munucations.

Figure 2 shows a block diagram of a typical test set-up. Known analog and digital test signals are pro-vided by the sensor simulator and coupled to thesignal conditioning equipment for conversion and/orcoding. The coded signals are then time-multiplexedin the PCM equipment, producing a serial PCM bitstream. The basic commutation rate is 50 samples/second, with both subcommutated and supercommutatedchannels provided. The PCM bit stream is fed to thePremodulation Processor, then bi-phase modulates a1.024 megacycle subcarrier, which phase modulatesthe nominal 2287.5 megacycle carrier. This low-powered signal is transmitted via coaxial cablethrough the R.F. Path Simulator to the ground re-ceiving station. A data demodulator recovers thebit stream, which then is sent to the PCM groundstation signal conditioner, where the desired dataare extracted from the noise and a reconstructed bitstream is generated. This reconstructed bit streamis coupled to a bit error comparator, where a bit-by-bit comparison is made of these data with theoriginal data. Event counters record the numbersof errors and total bits. The bit stream is alsofed to the PCM decommutator and digital-to-analogconverters. The analog signals are compared withthe original signals using a calibrated differentialamplifier.

Certain of the original data were in the form ofparallel digital data. These are compared with theappropriately reconstructed parallel words comingfrom the decommutator. This comparison is accom-plished in a UNIVAC 1218 computer. This computer isprogramed to analyze all data and to generate suchparameters as r.m.s. errors and error probabilitydistribution.

NASA-S-65-2315

310MAL U$COPARTO 50 AT

FigNO. C PCMTelemetr Sst UIOPENTio lAC TOSStSIMULATOR " OING _0 EQUIp- LATION POtWER 00 SdiAT -AH

1)TypesOfTestFIE

rEQUIP6 MENT PROCESSOR AND & DOFPPLEDIGITAL-J MET TDlPtEXER SIMULATION

.REF BIT STrrEtvMs r

RATE)~ ~~~ECIE

|REF. DATA) Y -1 SIYTEMl

hSORAGE Ig ANallevel(and resu gIbtTER ro rat MCsaANDcePARAtI ANALOG COUllESrate)|EUIPMENT o NVERETOR I----- |ECIE||

t-1-- - g~~TSREAM JV-D I

DT EtMTRY m; ;COSIN ^ A IG^IPROCESSOR1t OutPuT AND 1, 1TIONER E"D

12111 | UFfER |MUTATOR |SYNCH-| AOR

LM~

Fig. 2.- PCM Telemetry System Operational Tests

The table below lists some of the tests, missionconditions, and operational modes to be considered.

1) Types of Test

a. Bit error rate vs r. f. signal levelb. Signal level (and resulting bit error rate vs

spacecraft roll rate)

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c. Bit error rate vs multipath effectsd. Bit error rate vs doppler offsete. Bit error rate vs rate of change of dopplerf. Parallel digital r.m.s. error rate vs r.f.

signal levelg. Parallel digital bit error rate vs r.f. signal

levelh. Analog r.m.s. error vs r.f. signal leveli. Analog channel frequency response

(dynamic data)j. Analog channel quantization error

(dynamic data)2) Mission Conditions

a. Multipath effectsb. Doppler frequency offsetc. Doppler frequency rate of changed. Space loss (r.f. attenuation)

3) System Operation Modesa. PCM only on down link; no modulation on

up linkb. PCM only on down link; voice only on up linkc. PCM only on down link; voice and digital data

on up linkd. PCM and ranging code on down link; ranging

only on up linke. PCM and ranging code on down link; ranging

and digital data on up linkf. PCM and voice on down link; no modulation on

up linkg. PCM, voice, and ranging code on down link;

ranging code only on up linkh. PCM, voice, and ranging code on down link;

ranging code and digital data on up linki. PCM, voice, and ranging code on down link;

ranging code, digital data, and voice on uplink.

SUMMARY

Only a very brief statement of the test plans, pro-cedures, and equipments has been presented. Thetests must encompass all systems and subsystems,all modes of operation including emergency, and allmission conditions. Only upon the satisfactory com-pletion of ground, aircraft, and spacecraft testscan the communications systems be committed to sup-port a lunar mission.

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compatibility and performance testing of communications systems

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