the integration and testing of xmm

r bulletin 100 — december 1999

Configuration and scheduleThe XMM satellite (Fig. 1) is configured in amodular manner, described in detail elsewherein this issue. The Focal-Plane Assembly (FPA)can be fully integrated and tested independentlyfrom the Service Module (SVM). However, it isreadily apparent that splitting the satellite intoService Module and Payload Module (PM) doesnot solve the problem of size limitations inenvironmental testing facilities, since thepayload is distributed throughout the length ofthe spacecraft.

For environmental test purposes, thespacecraft is therefore split at roughly mid-height. This requires the introduction of aninternal bolted interface between the Lowerand Upper Telescope Tubes (LTT and UTT), butresults in a configuration which lends itself toaccommodation in the largest existingEuropean test facilities, namely those atESTEC.

The Lower Module (LM) consists of: SVM withMSP, Mirror Modules, RGA, Lower TelescopeTube and a closure plate. The Upper Module(UM) consists of FPA and Upper TelescopeTube.

The Lower Module and Upper Moduledimensions are compatible with:– the ESTEC Large Space Simulator solar

beam size (6 m diameter) for thermal testing– the ESTEC 280 kN electrodynamic shaker for

vibration testing– the ESTEC Mass Properties measurement

machine with its latest L-4600 arm.

The fully assembled spacecraft is compatiblewith ESTEC’s Large European Acoustic TestFacility (LEAF). Besides acoustic testing, theLEAF was also used to conduct a Modal

The XMM (X-ray Multi-Mirror) spacecraft, a spaceborne X-rayobservatory to be launched by Ariane-5, stands 10 m high andmeasures over 4 m in diameter in launch configuration, for a launchmass of just under four tons. Such a tall spacecraft challenges thecapabilities of existing European environmental testing facilities.Provisions were made in the design for a split according to geometryinto an Upper Module and a Lower Module for environmental testpurposes. Optical testing of the X-ray Mirror Modules – the coretechnological challenge – required the use of several existing andcustom-built test facilities. In the face of strict schedule requirements,spacecraft-level test flows were organised around extensively parallelflows and all tests were scrutinised for their potential for earlyproblem identification. This article briefly introduces the XMMconfiguration and schedule constraints, explains the spacecraft-levelmodel philosophy, discusses the consequences for each category oftest in terms of facility and test specimen configurations, andsummarises the spacecraft test flows and the results achieved.

Furthermore, analysis determined thatmechanical qualification of the SVM alone wasunrealistic. The mechanical behaviour of thelower part of the spacecraft is largelydetermined by the presence of the three MirrorModules (MMs), which together and with thetwo Reflection Grating Assemblies (RGAs)account for a mass of more than 1300 kgmounted into the Mirror Support Platform(MSP). Proper load introduction into the SVMfrom the MSP fully equipped with MirrorModules is indispensable.

The Integration and Testing of XMM

Ph. Kletzkine XMM Project, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

Figure 1. Exploded view ofthe XMM spacecraft. Fromtop to bottom: FPA thermaltent, FPA with 3 EPICcameras and 2 RGScameras, Telescope Tubeupper and lower parts, MSPwith 3 Mirror Modules andOptical Monitor, SVM

As soon as their respective test programmeswere completed, the STM and EM ServiceModules were separated from the rest of theirrespective satellites and delivered for re-use toIntegral, another ESA scientific satellite projectthat uses the same Service Module design.

The Proto-Flight Model is the actual satellite tobe flown. The structural qualification had beenacquired on the STM; the ‘Proto’ part of thename therefore concerns only limited electricalaspects and minor qualification gaps left byconfiguration changes. The PFM does not re-use any part of the STM nor EM models.However, because the flight Mirror Modules areextremely sensitive to contamination, testing ofthe PFM spacecraft was mostly carried out withthe three STM Mirror Modules installed. This ispossible because the STM Mirror Modules’representativeness is excellent in all respects,except of course in terms of optical properties.The flight Mirror Modules were installed at thelast mating of the two modules, after thermaland vibration tests and just before acoustictesting.

In addition, the RF Suitcase Model wasprovided to check RF compatibility betweenthe spacecraft and the ground stations. The RFSuitcase re-uses parts of the EM (Central DataManagement Unit and Transponder).

Test flows All three test flows make use of the scheduleoptimisation made possible by the modularsplitting of the spacecraft by conductingparallel testing whenever the two modules arenot assembled together. For each of the threespacecraft models, integration of the UpperModule and Lower Module took placeseparately.

STM testsOnce the two modules of the Structural andThermal Model (Fig. 2) were integrated, theywere mated, aligned, submitted to light-tightness and alignment checks, de-mated,and shipped to ESTEC for environmentaltesting.

Each Module underwent, in turn, massproperties measurement, thermal-balance, andsine-vibration testing. The rest of the testscould be carried out on the completespacecraft and therefore the Upper Moduleand Lower Module were mated and theassembled STM spacecraft underwent modal-survey testing, acoustic testing, clamp-bandrelease (which also served as a spacecraft-level shock test), and a mechanisms functionaltest.

integration and testing

Survey test on the fullyassembled spacecraft. Also with

XMM in fully assembled configurationin and close to the LEAF, fit-checks and

a clamp-band release test were performedwith the launch-vehicle adapter.

The spacecraft development schedule had tobe compatible with delivery of the Flight Modelexperiments to the spacecraft in 1998 and witha budget-dictated duration imposing thatlaunch take place no later than early-2000.Phase-B (preliminary design) was performed in1995, Phase-C/D (detailed design, manufacturingand test) was initiated in March 1996. Thistranslated into the obligation to perform theStructural and Thermal Model (STM) programmeand Electrical Model (EM) programme inparallel, and to start the Proto-Flight Model(PFM) assembly before the end of the STM andEM programmes.

Spacecraft-level model philosophyThe Structural and Thermal Model was used toqualify by test the complete spacecraft primarystructure and thermal design. The STM can beseparated into Lower Module and UpperModule as described above, and the structuraland thermal designs take into account the factthat the two are tested separately. The STMwas also used to prove the LM-to-UM mating/de-mating procedures, verify the behaviour ofthe LM-to-UM interface, verify alignment andlight-tightness design and test procedures,verify compatibility with the shock inputs, andexercise assembly and handling proceduresand Mechanical Ground-Support Equipment.

The Electrical Model was used to verify theelectrical design, internal interfaces, software,EMC/ESD, checkout procedures, and ElectricalGround-Support Equipment (EGSE). The EMunits are flight-model representative in ‘form, fitand function’, but are not required to meet asstringent part-reliability standards as the flightunits. The EM has no Telescope Tube and noMirror Modules. The EM Focal-Plane Assemblyand EM Service Module are each built aroundstructures representative of the flight layout, soas to achieve good representation for EMC andharness layout.

r bulletin 100 — december 1999

EM tests Testing on the Electrical Model proceededalong with integration in the classical sequence,i.e. electrical integration tests (pin-to-pin, signalpresence and shape) and Integrated SystemTests (ISTs: all functionalities) were conductedafter integration of each electronic unit andeach major subsystem. After completion of thetests on both the Lower and Upper Modules,the two were linked by a harness representativeof the harness running along the TelescopeTube in the PFM situation. Tests wereconducted on the EM satellite to verify Electro-Magnetic Compatibility (EMC) behaviour, bothradiated and conducted, and Electro-Static

Discharge (ESD) behaviour, also bothconducted and radiated. The software logicand code of all major subsystems werechecked, exercised and debugged, includingboth open-loop and closed-loop testing of theAttitude and Orbit Control Subsystem (AOCS)and ISTs of the scientific experiments. Thesophisticated Electrical Ground-SupportEquipment was also put to the test, thearchitecture and interfaces between the corecomputer, the various items of front-endequipment, subsystem checkout computersand scientific instrument stations wereexercised and their software debugged. After

completion of the EM programme, those itemsnot delivered to Integral as part of the EMService Module were refurbished as Assembly,Integration and Verification (AIV) spares.

PFM testsThe Proto-Flight Model test flow generallyfollowed the same principles as the STM andEM flows and combined them both. However,the PFM test flow was not a simple addition ofthe STM and EM test flows.

After completion of its integration along themechanical and electrical integrationprocedures validated on STM and EM, the PFMLower Module was tested for conducted andradiated EMC, then shipped to ESTEC where itfirst underwent sine vibration testing atacceptance levels in the axial (i.e. longitudinal)direction. Thermal tests (thermal balance, andthermal vacuum at acceptance temperaturelevels) in the ESTEC Large Space Simulatorfollowed. The Lower Module was opened topermit the removal of several electronic units.They underwent minor modifications as a resultof either component alerts or non-conformances, or hard-wired logic changes inthe power subsystem decided upon afterconsideration of the lessons learnt from therecent in-orbit problems experienced by thejoint ESA/NASA Solar and HeliosphericObservatory (SOHO). Other activities includedthe removal of one scientific experiment, theOptical Monitor telescope, to exchange thetelescope optics for a higher-performance set;and to exchange, as scheduled, the STM MirrorModules for the FM Mirror Modules. Exchangeoperations for the Optical Monitor and MirrorModules took place in a Class-100environment because of the sensitivity of theoptics to contamination. After all flight units hadbeen mechanically and electrically re-integrated, the Lower Module was mated to theUpper Module and the assembled spacecraftunderwent acoustic testing.

The integration schedule for the PFM UpperModule was driven by the delivery schedulesfor the five focal-plane scientific cameras. TheUpper Module went through conducted EMCand was shipped to ESTEC. For reasons oftest-facility availability, it first went to thermaltesting (thermal balance, and thermal vacuumat acceptance temperature levels) in theESTEC Large Space Simulator. Massproperties, limited to weighing anddetermination of centre-of-gravity offset to thelongitudinal axis, were measured. A sinevibration test in the lateral direction followed.Two electronic units were removed and weremodified, for the same reasons as describedfor the Lower Module. Meanwhile, software

Figure 2. The XMM STMLower Module (left) andUpper Module (right) at

ESTEC

Figure 3. The XMM PFMLower Module in the LargeSpace Simulator at ESTEC(January 1999). TheTelescope Sun Shield isdeployed. The three MirrorModule doors and theOptical Monitor door areopen

upright (as it will stand on top of the Ariane-5launcher) inside the chamber. The LowerModule was ‘upside-down’, with the 2700 kgmass of the Service Module and MirrorModules on top of the lower half of anextremely lightweight telescope tube. Thisunusual set-up (Fig. 3) offered the possibility ofsimulating very realistically the thermalenvironment of the bottom part of thespacecraft where the Mirror Module apertureshad an unobstructed view to cold space. Thecorrect simulation of the heat fluxes lost intospace was of paramount importance for theverification of the temperatures and gradientsof the Mirror Modules and the Mirror SupportPlatform. This would not have been possible ifa more conventional mounting of the spacecraftby means of its launch-vehicle interface flangehad been selected.

Due to the architecture of XMM, it was simpleto simulate the thermal interface provided bythe missing spacecraft module. Because of thelow thermal conductance of the long thin-walled Telescope Tube entirely made ofcarbon-fibre composite, the two modulescannot exchange heat by conduction. The fluxexchanged by radiation was simulated bycontrolling the temperature of a plate inside thetest adapter. The two modules were mountedby means of the same test adapter on the LSSgimbal stand, which provided the possibility ofchanging the spacecraft’s attitude with respectto the solar beam direction as required by thesimulation of the various orbital phases.

debugging of the scientific camerasproceeded. The two modules were thenmated, as described above.

Beyond the electrical integration tests andsubsystem-specific Integrated System Tests,the PFM was submitted to functional testingbetween the major environmental tests and atmajor system milestones. To cut down onredundant testing, all flight procedures (bothnominal and emergency) were distributed intosix ‘Spacecraft Functional and Performance Test’ (SFPT) series that were used instead ofspecific functional tests. The potential risk wasthat tests before and after an environmentaltest or integration step were not always one-to-one identical, potentially making test-resultcomparison more difficult. All six SFPT serieswere run, and this drawback has notmaterialised; the few test deviations have beencorrectly diagnosed. The advantages of thisapproach are a significant time saving and thepossibility to stagger the very labour-intensivepreparation and verification of flight proceduresoftware in an efficient manner. One of theSFPT series was run during thermal-balance/thermal-vacuum testing in addition tosubsystem-specific ISTs.

Communication interfaces with the groundsegment, located in this case at ESA’sEuropean Space Operations Centre (ESOC) inDarmstadt, Germany, were verified by use ofthe RF Suitcase Model, without having had towait for completion of the PFM spacecraft.Communications were also checked atintervals by allowing the ground segment tolisten-in on the PFM electrical testing beingperformed at ESTEC. These Listen-In Testswere followed by System Verification Tests(SVTs), i.e. full-fledged end-to-end (spacecraft-to-Mission Operations Centre) tests to exerciseall telemetry and telecommand and all flightprocedures.

Thermal testingIn total, four environmental spacecraft-levelthermal tests were performed, all in the Large Space Simulator (LSS) at ESTEC, the largestsolar-simulator facility in Europe: thermal-balance tests on the STM Upper and LowerModules, and thermal-balance/thermal-vacuum tests on the PFM Upper and LowerModules.

The purpose of the thermal-balance tests wasto validate the thermal mathematical modelsand to verify the ability of the Thermal ControlSubsystem to keep payloads and spacecraftequipment within specified temperature limitsunder simulated extreme expected orbitalconditions. The Upper Module was mounted


Because of the stringent cleanlinessrequirements imposed by the optics of thetelescope system, a pure nitrogen purge linewas located inside the test adapter and usedfor directly venting the interior of the telescopetube during the re-pressurisation phases. Thecryo-panels inside the facility were used to trapcontaminants. The STM thermal-balance testalso had to verify the effectiveness of thecleanliness measures and procedures adoptedin providing the cleanliness level required for thethermal-vacuum testing of the PFM spacecraft,which was then performed with the same set-up, configuration and adapter.

The objective of the PFM thermal-vacuum testswas to verify that the fully integrated spacecraftperformed correctly in all operational modes atthe expected extreme temperatures induced bythe orbital conditions. In addition, somethermal-balance test phases were inserted intothe thermal-vacuum programme in order toverify the Thermal Control Subsystemperformance after minor modifications hadbeen introduced between the STM design andthe final FM design. For cleanliness reasons(even though eventually the cleanliness levelsachieved were very good and well withincontamination budget), the tests were carriedout with the STM Mirror Modules installed in thePFM spacecraft Lower Module, instead of theFlight Model Mirror Modules. In addition torevealing the need for minor trimming ofradiators and minor repairs to defective heaterlines, the thermal tests have been fullysuccessful in verifying the thermal-controlperformance and the thermal predictions.

Structural testingStatic strength testsStrength verification of the primary structurewas achieved by statically loading each of themajor constituents (SVM central cone, SVMupper and lower platforms, upper and lowerTelescope Tube) separately at their own level bytheir respective manufacturers, i.e. beforesystem-level structural testing. These testswere performed at qualification levels on theSTM elements, and at acceptance levels on thePFM elements.

Vibration testsOne test objective was to validate the structuralmathematical models used to predict thespacecraft’s behaviour during test and in flightas calculated by the Launch vehicle DynamicCoupled Analysis. Another test objective wasto provide proof-of-strength for those parts thatdid not see a strength verification beforehand,namely: Focal Plane Platform, Service Moduleequipment panels and their interfaces to the

equipment, Focal Plane Assembly secondarystructure, Service Module shear walls, ServiceModule secondary structures such as thrusterbrackets and Telescope Sun Shield (TSS).

In each of the three orthogonal axes, each STMmodule was sine-vibration tested following theclassical sequence: low-level, intermediate levelto define notch profiles, qualification levelfollowed by low-level again in order to checkthat modal characteristics had not beenaffected by the tests.

Shaker input levels for the STM Upper Modulecould not be taken directly from the Ariane-5User’s Manual because of the transfercharacteristics of the Lower Module. A system-level response analysis was run to determinethese transfer characteristics and the resultinginputs from the Lower Module into the UpperModule at the interface between the LowerTelescope Tube and the Upper Telescope Tube.These levels were used as inputs for the UpperModule testing.

For the STM Lower Module testing, the inputlevels to be found in the Ariane-5 User’s Manualwere taken. Despite the absence of the UpperModule, the Lower Module has many modescorresponding to the complete system dynamics,so that a system-level notch profile could beestablished. This notch profile was acceptablealso to the launch authorities.

The testing of both STM modules hasdemonstrated that the desired response levelshave been reached at the resonances asforeseen.

For PFM acceptance testing, the levels weredetermined by first dividing the levels actuallyachieved during qualification on the STM by afactor of 1.25. Manual and automatic notchlevels were then corrected down in two narrowfrequency bands to account for possibleshaker control overshoot, thereby making suresensitive flight hardware was not endangered.The levels were then checked against theresults of the Launch Vehicle Dynamic CoupledAnalysis and it was verified that acceptancelevels showed positive margins throughout thefrequency spectrum. This conservativeapproach both ensured safety of the flighthardware and conserved significant margins tothe flight environment, at the time known onlyas measured values on the first two Ariane-5flights. Later, the results of measurementsaboard the third Ariane-5 flight confirmed thesuitability of this approach.

The PFM Lower Module (with about 80% of thetotal mass, the heavier of the two) was sine-

r bulletin 100 — december 1999 bull

Figure 4. The XMM PFMLower Module during axialsine-vibration testing on theESTEC 280 kN shaker(December 1998)

acoustic test performed on the completespacecraft.

Acoustic testingAcoustic testing particularly involved thestructures with low mass per surface area suchas the Telescope Sun Shield, Service Moduleupper and lower platforms, Telescope Tubeand Focal-Plane Assembly secondarystructures.

For STM qualification, dummies representedthe solar arrays. Flight solar panels weresubmitted to separate acoustic tests. TheTelescope Sun Shield had gone through anacoustic verification at unit level.

Responses at the level of the Service Moduleunits were recorded for comparison with theunit-level specifications.

The Ariane-5 specified launch environment(plus 4 dB qualification margin for the STM test)determines the qualification and acceptancetest levels. Additional STM qualification runs

vibration tested with input only in thelongitudinal direction (Fig. 4). This savedsignificant test time and cost. This approachwas possible because the successfulexperience acquired during STM qualificationhad drawn attention to the fact that cross-coupling alone was sufficient to induce theresponses that would have been sought in atest with lateral input. Also, correlations of STMtest results with mathematical test predictionswere very good and provided confidence in themodelling. The modifications from STM to PFMwere few and minor, except for one change oflocation for one of the two batteries; even thischange was not significant at spacecraft leveland the panel affected was tested separately tovalidate the change locally.

The PFM Upper Module was sine-vibrationtested only with input in the lateral (z-axis)direction. The rationale was similar to thatapplied for the Lower Module, with thedifference that the results of the Launch vehicleCoupled Dynamic Analysis show lesssubstantial margins in lateral accelerations thanin axial. It was therefore decided to test in themore critical lateral direction.

Modal surveyThe modal-survey testing was performed onthe STM spacecraft by a team from DLR-Göttingen (D). It has shown that the overalllateral mode corresponds very well withcomputer predictions (11.7 Hz measured,against 11.8 Hz calculated) and has confirmedthe recurrence, on the complete spacecraft, oflocal Service Module modes as found in theLower Module test. The objective of identifyingbelow 100 Hz all modes with effective massabove 5% of the total mass has been met. Thistest has been rounded off with a so-called‘boosted’ run in which high lateral inputs weregiven to the Focal Plane Platform such thatresponse levels reached flight levels times aqualification factor. This dwell test at 11.7 Hzdemonstrated the load capacity of the fullybuilt-up central core in both lateral directions,as well as the stability of the first lateralresonance under increased loading. This alsoconfirmed qualification of the Lower-to-UpperModule bolted interface.

The excellent results obtained from the STMModal Survey, together with the fact that thechanges from STM to PFM were minor andwith the availability of sine-vibration results for both the PFM Lower and Upper Modules, ledto a decision not to perform such a modalsurvey on the PFM spacecraft. Theworkmanship of the Lower-to-Upper Moduleinterface was checked by inspection (thebolted flange is of a simple design) and by the


* To account for uncertaintiesconcerning the frequencyspectrum and level of shocksimparted by Ariane-5 duringthe flight, a thoroughcomplementary unit-levelshock analysis and testprogramme was conductedon EQM units in parallel withthe launch campaign, as adouble-check.

were performed to solve facility controlquestions, to assess margins available and totake into consideration the acoustic environmentmeasured on the first two Ariane-5 launches.The maxima measured in each octave on thetwo flights were taken as flight environmentplus margins for uncertainty and for qualification.This approach was thus conservative, butrealistic in view of flight experience. Comparedto the Ariane-5 User’s Manual, it led to anincrease of several dBs in the low frequencybands, but also to a substantial decrease in thehigh-frequency bands, where the originalUser’s Manual specification was unnecessarilyconstraining. The launch-vehicle authority alsowelcomed this approach, since it adequatelycovered all concerns about uncertainties abovethe User’s Manual specification in the lowfrequency bands.

This STM spacecraft-level acoustic-test serieswas successful in demonstrating qualificationof the structure and also in identifying thoseunits for which more unit-level qualification datahad to be acquired, which was subsequentlydone.

For PFM spacecraft acceptance, the acoustictest was performed on the complete PFMspacecraft, including FM solar arrays andTelescope Sun Shield, with the same realisticspectrum, but of course without the addition ofthe 4 dB qualification margin.

Adapter fit-check and clamp-band releaseAn Arianespace team performed this test, withthe complete STM spacecraft clamped to itslaunch-vehicle adapter. After a fit-check withthe adapter, it involved the pyrotechnic releaseof the 2624 mm-diameter clamp band. Oneobjective was to prove correct fit to the adapterincluding accessories (e.g. clamp band, clamp-band extractors and catchers, umbilicalconnectors, purge ports, release springs,separation switches). Another objective was todemonstrate the feasibility of mating theTelescope Sun Shield to the spacecraft afterclamp-band installation and to show properclamp-band release without interference withany part of the spacecraft, including the SunShield. A third objective was to measure theshock levels induced by the clamp-bandpyrotechnic release on both sides of theseparation plane and further at selectedequipment levels. Subsequently, a release ofthe Telescope Sun Shield was performed toverify proper functioning of its deploymentmechanism, even after clamp-band releaseshock and under adverse thermal gradients.This series of STM qualification tests wascompletely successful and the results gave riseto no particular concerns.

Shock testing had been performed at unit level,on EQM of FM units as determined on a case-by-case basis, on all those units of the LowerModule for which susceptibility to shock couldnot be excluded simply by design. UpperModule units are located too far from thelaunch-vehicle interface to be of any concern.

For PFM spacecraft acceptance, another fit-check with the flight adapter was performed. Apyrotechnic clamp-band release on the PFMwas not performed, since all of the usefulinformation that it could provide had beensuccessfully gathered during the STM test*.

Physical checksMass propertiesThe mass, the Centre of Gravity (CoG) andMoments of Inertia (MoI) of both the Lower andUpper Modules (separately) were measured onthe STM spacecraft along all three axes. Thesemeasurements agreed very well with thepredictions.

For PFM acceptance, the mass, CoG locationin the horizontal plane and MoI around thelongitudinal axis of both Lower and UpperModules (separately) were measured. This was just to double-check that no gross error hadslipped into the calculations and to correct the inevitable small errors due to, for example, testharnesses or minor equipment exchanges. Thiskept the test configuration simple. Valuesaround the other two axes, while much morecumbersome and costly to measure, need notbe known with high accuracy. The STM testinghad sufficiently validated the prediction of theirvalue.

Alignment and light-tightnessAt regular intervals between STM satellite tests,checks have verified that the spacecraft wasable to maintain full integrity, alignment and lighttightness – which are crucial to the scientificmission – throughout the gruelling qualificationenvironment (Fig. 5). Custom-designed equip-ment was built to meet the size, configurationand accuracy requirements of XMM, for both the alignment and the light-tightnessmeasurements.

Between major environmental steps, alignmentand light-tightness were checked on the PFMspacecraft in accordance with the proceduresverified during STM qualification. Because ofthe excellent performance of the structure, thechecks were somewhat less extensive than onthe STM. The major alignment activityconsisted of positioning the five scientificcameras located at the telescope focal plane,while taking into account the measuredcharacteristics of the Mirror Modules and


Figure 5. The STM XMMspacecraft assembled onthe alignment rotary table.The alignment stand is onthe right

apart. In electromagnetic terms, the activeelectronics represent EMC sources, while theinterconnecting harness acts like an antenna.Additionally, the parallel routing of differentsignal cabling could be susceptible to cross-talk. On the other hand, radiated couplingbetween the Focal-Plane Assembly and theService Module is minimal.

On the EM satellite, the Lower and UpperModules were connected by a harness.Conducted emissions and susceptibility werechecked. Electrostatic discharges were tested,first conducted (which uncovered malfunctionson two units, later corrected) and then radiated.Radiated emissions and radiated susceptibilitywere then measured.

All of these tests, even the radiated ones, wereperformed in a clean room and not in ananechoic chamber. This was only possiblebecause the environment had been measuredand verified to be quiet and because the testwas performed during the evenings, with littleactivity around. Despite the very low limitimposed by the launch-vehicle compatibilityrequirements in the critical 420–480 MHz band, the influence of ambient noise was

Reflection Grating Assemblies. This was doneat the time of the mating of the two satellitemodules to form the assembled spacecraft,with the actual flight-model Mirror Modulesinstalled.

The accuracies required are only millimetric, butthe large size of the spacecraft and thecriticality of the positioning – and its stability –for the scientific mission make the time-consuming and delicate alignment activitiescritical for PFM acceptance. Light-tightness ofthe telescope is similarly critical, since evenminute amounts of stray light would blind theexquisitely sensitive CCD detectors of theexperiment cameras, which are able to countX-ray photons one by one and are notcompletely insensitive to visible light.

EMC testingAll units were fully EMC tested, radiative andconductive, for emissions and for susceptibility.XMM is not a particularly difficult satellite EMC-wise, as it does not carry very powerfulsources. The results from unit-level tests hadshown considerably wider margins than therequired 6 dB between worst-case emissionsand susceptibility. However, because ofspacecraft size, just as for environmentaltesting, full-fledged EMC testing in an anechoicchamber would have been next to impossiblefor the complete satellite, at least if cleanlinessrequirements were to be observed. It would stillhave been very cumbersome even if performedon the two separate modules. Nevertheless,self-compatibility and compliance with thelaunch-vehicle radiative environment had to bedemonstrated.

The active electronics are located on the Focal-Plane Assembly and Service Module. TheTelescope Tube along which the harness isstrapped holds the FPA and SVM about 7 m


Figure 6. An XMM MirrorModule

demonstrated to be small enough to obtainclear and positive results. This is indeed trueprovided the measurements are performed in asufficiently narrow bandwidth, i.e. 100 kHz asrequired by the Ariane-5 User’s Manual.Compliance is also made easier by the fact thatduring launch few units are on, namely thebatteries, main supply bus and regulationequipment and telecommand receivers.Susceptibility testing also took into account therelatively high field strength measured at thelaunch base and originating from varioussources other than the launch vehicle itself.

For the PFM testing, the Electrical Modelapproach was reproduced and even simplifiedsomewhat. For schedule and configurationsreasons, it was impractical to perform the testson the assembled satellite because it wouldhave meant putting off the EMC tests until theend of the programme. For the purposes ofacceptance, and in view of the good resultsobtained on the EM plus good knowledge of allequipment from unit-level tests, PFM testswere carried out separately on the twomodules. In addition, to take into account therequirement to verify radiated emissionstowards the launch vehicle, the PFM LowerModule radiated EMC test was carried out onthe Lower Module equipped with the TelescopeTube harness and the two FPA units that will bepowered during launch preparations. These arethe FPA Remote Terminal Unit (RTU) for datahandling, and the FPA Power Distribution Unit(PDU) for power. Similarity to the behaviourobserved on the EM was confirmed for bothmodules. The measured radiated emissionscomply with the launch-vehicle requirements.Performing the measurements in the usual

clean room at quiet times again providedusable results at a comparatively low cost.

ESD testing would have been risky on the flightmodel; it could have caused inadvertent failuresor reductions of lifetime. Therefore ESD testingwas not performed on the PFM.

Mirror module testingThe core of the XMM X-ray focussing optics ismade up of three highly nested Wolter-1grazing-incidence X-ray telescopes. Theyprovide a large photon collecting area: each1420 cm2 at 1.5 keV and 600 cm2 at 8 keV.Their spatial resolution is better than 16 arcsec.To obtain such an area and resolution while stillkeeping the mass reasonable, it was necessaryto develop the technology for manufacturingthin mirror shells, assembling them intotelescopes, called Mirror Modules (Fig. 6) in thiscontext, without loss of performance, testingthem thoroughly, and assembling them into aspacecraft while keeping contamination low soas to avoid performance degradation.

It was soon realised that a large amount ofMirror Module testing had to be performed. The‘Panter’ X-ray facility at the Max Planck Institute(MPE) in Neuried, Germany, was available toXMM. However, several aspects pleaded forthe creation of a new test facility. In addition tothe sheer amount of testing of the MirrorModules that was needed, the Panter facilitywas to be used for testing the scientificcameras of XMM. The fact that the MirrorModules tested at Panter had to be in thehorizontal position was not insignificant forsuch thin mirror shells, so parasitic gravityeffects could not be excluded. Most important


Figure 7. The XMM PFMspacecraft assembled andattached to the containerchassis before closure ofthe container lid

first one. For the STM Mirror Modules, theoptical tests were of course omitted, but theenvironmental tests cleared them for furtheruse in the spacecraft-level test programme asdescribed above. They also trained staff,procedures and equipment in advance of thedelicate testing of the Flight Models. After thesecond Flight Model, the test sequence wasoptimised to take advantage of the learningcurve achieved. The optical checks in-betweenvibration and thermal tests were omitted, someimage quality checks were speeded up, thenumber of time-consuming X-ray reflectivitycheck points was decreased, and the numberof thermal cycles was reduced from 6 to 3. Onthe other hand, extra test sequences wereadded to verify the behaviour and performanceof the Mirror Modules equipped with their X-raybaffles and, for two of them, with their ReflectionGrating Assembly. All Mirror Modules passedthe tests and demonstrated consistently better-than-specified performance. The testing alsoprovided the mass properties and alignmentvalues (focal length, orientation of optical axis)needed for the PFM spacecraft alignmentactivities. Since XMM carries three MirrorModules, two FM Mirror Modules are full-performance flight spares.

Onwards to launch The spacecraft launch-preparation campaign isa continuation of the integration and testactivities. In this respect, operations such asthe assembly of appendages, battery charging,

for the measurement of optical performance, athird of the mirror shell surface could notphysically be properly illuminated because ofthe slight divergence of the X-ray beam. TheXMM Project Office therefore decided in 1994to complement the Panter facility by building acustom-designed, vertical facility equipped withan 800-mm EUV collimator and two thin X-raybeams, specially adapted to the dimensions ofthe XMM Mirror Modules. This facility, called‘Focal-X’, is located at Centre Spatial de Liège(CSL), in Belgium.

Nine Mirror Modules have been tested at thePanter facility and at CSL since the completionof Focal-X in 1996: one Qualification, threeStructural and Thermal, and five Flight models.Each Mirror Module underwent a sequence ofoptical tests (EUV full illumination image quality,X-ray local measurements of reflectivity andscattering). Specific tests in Focal-Xinvestigated stray-light characteristics forsources close (up to 7 deg) to the field of view.To validate the stray-light modelling of thetelescope, stray-light characteristics at higherangles were measured on two Mirror Modules(one of them equipped with its ReflectionGrating Assembly) in a custom-built test set-upat a Daimler-Benz Aerospace facility inOttobrunn, Germany. At CSL, the sequencecontinued with sine and random vibration testson the CSL shaker, thermal-vacuum tests inanother CSL vacuum chamber, and final opticaltests according to a sequence similar to the


Figure 8. XMM in flightconfiguration (artist’s

impression)

virtually all electrical checkout, alignmentstability check, light-tightness check, andcamera door checks have been performed justas they were performed during PFMacceptance. The complete end-to-endtelecommand and telemetry chain from theControl Centre at ESOC to the spacecraft hasbeen exercised during a System ValidationTest, similar to those performed when thespacecraft was still at ESTEC. However, anumber of operations have novel aspects:– The complete spacecraft was transported in

one piece (Lower and Upper Moduleassembled; Fig. 7) to the launch site in onevery large container, whereas all previoustransportations were in three parts, each intheir own container. The spacecraft and itsancillary equipment were transported by seaon a ship that also carried Ariane launch-vehicle stages, parts and equipment fromEurope to French Guiana.

– The Reaction Control System tanks are forthe first time fuelled with real hydrazine,rather than the water that was used once onthe STM spacecraft and twice on the PFMspacecraft to fill the tanks before thevibration and acoustic tests.

– The second flight battery has been installed(one was already installed in the spacecraftbefore shipment).

– The Telescope Sun Shield has been installedon the spacecraft after mating of thespacecraft to the launch vehicle, to allowaccess to the adapter and clamp band.

The eleven-week launch-preparation campaignis scheduled to lead to the spacecraft’s launch

in December 1999 and commissioning in early2000 (Fig. 8).

ConclusionLarge spacecraft such as XMM stretch orsurpass the capabilities of existingenvironmental test facilities in Europe. TheXMM test programme has combined testing onthe complete spacecraft wherever possiblewith modular testing where unavoidable. It hasmade optimal use of existing test facilities forboth environmental and electrical testing. Whilefull-illumination, collimated, end-to-end opticaltests on the complete satellite at X-ray energiesin representative flight conditions was notpossible, a combination of optical andalignment tests at Mirror Module level,scientific-camera level and spacecraft level hascome as close as possible to an end-to-endverification. This has permitted satisfactoryqualification and acceptance and has beenpossible thanks to the favourable split intomodules taken into account from the beginningof the XMM design process.

AcknowledgementsThe author would like to thank the staff of theESTEC Test Centre, the staff of Centre Spatialde Liège, and the XMM Prime ContractorDaimler Benz Aerospace/Dornier, whose workformed the basis for some of the material of thisarticle. The author also gratefully acknowledgesthe considerable contributions from the othermembers of the ESA XMM Project Team. r


the integration and testing of xmm

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