Proceeding of the 6th International Symposium on Artificial Intelligence and Robotics & Automation in Space: i-SAIRAS 2001, Canadian Space Agency, St-Hubert, Quebec, Canada, June 18-22, 2001.

ANDROMEDA, An Autonomous System for Attitude Determination from Stars: Current Development and

Future Trend

Andrea Bacchetta, Simona De Sanctis, Mario Montagna, Maria Teresa Ravazzotti, Gianfranco SechiAlenia Spazio

strada Antica di Collegno, 253, 10146 Torino – Italyabacchet@to.alespazio.itsdesanct@to.alespazio.it

mmontagn@to.alespazio.itmtravazz@to.alespazio.itgsechi@to.alespazio.it

Keywords: autonomy, data fusion, high output rate,high rate operation, in-flight experiment, lost in spacerecovery, mode management, multi-head management,on-board catalogues, true-sky tests.

AbstractANDROMEDA (Autonomous Network of DistributedRemote Optical Modules for Enhanced Determinationof Attitude) is an autonomous attitude determinationsystem for space vehicles, based on the informationfrom stars only. It is able to provide the completespacecraft attitude and angular rate in real time, startingfrom scratch, autonomously (namely without anysupport from ground or space based systems) and thencontinuously. The system consists of a set of one tothree CCD based Optical Heads and a single, internallyredunded Electronic Unit, running the attitudedetermination software including reduced starcatalogues, pattern recognition, estimation and self-calibration algorithms.

Main features of the system include high accuracy(within few arcsec even at high rotation rate), extendedoperating range (up to 10º/s), continuity of operationeven for reduced configuration, fast and reliablerecovery from lost-in-space condition (within 1s, 99%confidence level), high output rate (up to 30Hz).

ANDROMEDA is being developed upon internalfunding, based on a cooperation with leader Italianscientists of Eicas Automazione lasting since 1992. Theconcept has been demonstrated through intermediatesteps (SETIS, IST, AST), funded by the Italian andEuropean Agencies to validate the concept. The paperdescribes such intermediate steps, focusing on relevantresults and outcomes for next-future developments.

1 INTRODUCTIONThe ongoing progresses in on-board computing powerand memory sizes and in detector technology allowimplementing autonomous star tracker technologies,

through embedding of star catalogues and appropriatestar pattern recognition algorithms in their software,while real time measurement accuracy can increase byseveral times through appropriate filtering andcalibration techniques. As a consequence rate sensorscan be deleted at all (unless safe modes require verysimple hardwired solutions) as star sensors can becomefast and clever enough to provide continuously and inreal time the complete attitude vector and its firstderivative.

The AOCS design based on star sensors only canbecome an almost universal standard, applicable towhichever orbit and pointing target, the only necessarycomplement being a position GPS for LEO Earthpointing spacecraft or alti tude information for HEOEarth pointing spacecraft. Integrated autonomousnavigation techniques can be designed, encompassingattitude and trajectory determination features, aiming atsupporting interplanetary missions, requiring high levelsof autonomy for long cruise times.

2 ANDROMEDA: The ConceptMost of the autonomous star trackers offered on themarket are constituted of a single Optical Head connectedto a single Electronic Unit. Apart from basicconsiderations of accuracy (the one around the boresightis typically lower than the one around the CCD axes), theoccurrence of a failure of any of these units would implythe loss of the star tracker. Reliability issues, togetherwith those related to reduced availabil ity caused by skyoccultations, imply then the adoption on-board of morethan a single star tracker, demanding each form ofsupervision and monitoring to the AOCS.

The advantages of an integrated multi-head system,where multiple Optical Heads are operated together andmanaged by a suitable Application SW resident in acentralized Electronic Unit, can be synthesised asfollows:

� improved output performances (quaterniondetermination, accuracy, output rate), through

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dynamic data fusion of the information from morethan a single Optical Head;

� improved sky coverage, when passing frominformation provided by one OH to the other withoutneed for process initialisation;

� output availability at high rotation rate;� increased reliability.

The baseline configuration (shown in figure 2-1)relies upon a single internally redunded EU, where theBasic and Application Software are embedded,managing up to three non-coaligned, medium FOV,OHs, each driven by a dedicated Read-out Electronics.

Figure 2-1 ANDROMEDA High Level Architecture

The Application Software (ASW) includes theAADS (Autonomous Attitude Determination System)Library developed by Eicas Automazione, on the basisof the experience gained on the ESA Hipparcosprogram: it is a collection of modules constituting self-standing software able to manage autonomously all theinternal operating modes and transitions, applicable,with a minimum tuning effort, to a wide range ofmission requirements and hardware constraints. It isprovided with autonomous fault detection and internalredundancy management. It aims at minimising on-board software size and execution time.

The ASW is organised through Operating Modes,scheduled according to the transition diagram of figure2-2. Real-time Operative System primitives (includingtime-out, watchdog and semaphores) are used tomanage both Hardware and the overall tasks. Executionof initial attitude determination (Pattern RecognitionMode) and fine attitude and angular rate estimation(Normal Mode) are based on access to smart on-boardcatalogues (image and reference star cataloguesrespectively). Rate Only Mode is enabled as long as too-

high rate is detected with respect to Pattern RecognitionMode operating conditions. The system uses anestimator to improve attitude accuracy, to estimateangular rate and to fuse measures coming from camerasat different measure times. The estimator parameterschange autonomously on specific spacecraft andcameras status (angular rate, star measure accuracy,camera availability, star background variations), whilecan be tuned at design level depending on missionrequirements. The estimator works in prediction inorder to recover delays due to frame readout and imageand star measure processing. Prediction is used to giveattitude and rate estimates also when star measures aretemporarily not available. Robustness against non-starobjects in FOV is an intrinsic feature of the AADSLibrary algorithms. External commands are available toimplement specific user requests (such as f.i. non-autonomous initialisation of attitude). The ASWincludes also an orbit propagator, supporting therelativistic aberration correction.

The ASW is furthermore provided with standardauxiliary modes (Stand-By Mode, Uploading andDownloading Modes, CCD Window Dump).

Figure 2-2 Autonomous Mode Transition Diagram

3 Simulation CampaignThe multi-head concept has been submitted to anextensive simulation campaign in the frame of the ESAContract for AST (Autonomous Star Tracker)Algorithms. The basic features of the AADS Libraryhave been integrated with mission oriented FDIRcapabilities and a specific Head Manager module hasbeen developed. The environment is built on a UNIXcommercial not real-time operative system (the contractwas awarded to Alenia Spazio and Eicas Automazione).

The demonstrator installed at ESTEC allowsexercising of all the basic features of the AST softwareprototype, providing accurate modelling of both realworld to be encountered once on orbit and of the ASTReference Configuration (see figure 3-1). It is generalenough to allow assessment of the behaviour forspecific satellite environment and physical and

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operating conditions. In particular it includes thefollowing main segments:

� Dynamics: to support dynamic analysis of a bodyunder the effect of the perturbations affecting theprocess of star coordinate measurement and attitudedetermination estimate. It includes the completerotational dynamics for either rigid or flexible bodies,where flexibili ty is treated in a user-friendly beam-likecantilevered-free approach. The system referencevelocity and acceleration are specified by the user tostudy different kinematic conditions. Externaldisturbances are modelled through perturbations onthe acceleration profile. Disturbance torque spectrumstandard deviation and actuator quantisation levels areuser input.

� Ephemerides: to compute the Sun and the satelliteposition and velocity vector in its motion around theEarth .

� Camera: in charge of1) all the geometric computations required to project

the stars on the CCD’s, accounting for OHorientation and interference with celestial bodies;

2) the image generation on the CCDs: the modelincludes blooming, heavy ion and radiationdamages, optical system and focal planedeformations.

REAL WORLD APPLICATION SOFTWARE

star sensorpixel data stream

. s/c kinematics

. Sky

. Autonomous Attitude Determination

attitude and ratedetermination

k in em atic in p ut

SW

OPTICAL HEAD

ELECTRONIC UNIT

. Basic software

CCD Commands

framed starcoordinates

UNIX UNIXor TARGETCOMPUTEREMULATOR

Figure 3-1 – All SW Simulator

The system performances have been evaluated overstatistically significant samples and for realisticdegradation conditions. Table 3-1 shows nominalperformances.

The system demonstrated furthermore to survive toaccelerations up to 5°/s2 without loosing contact and towork within specifications also in case of disturbancesfrom flexible appendages. It performs well with one or

even two failed OHs, presenting a degradation which isconsistent with the decrease in the number of averageavailable stars. It is robust against OH blinding, beingable to work with a reduced configuration too. In caseof occultation of all the three OHs, the system canpropagate its own attitude, with a degradation ofperformances that increases with the angular rate and isa function of the disturbances acting on the satellit e. Itdemonstrated to be pretty insensitive to false stars andlarge objects in FOV. Sensitivity increases at lowerrates.

Code size has been estimated for the executable ofthe AST SW (simulator excluded), running on thedemonstration workstation and created withoutdebugging and optimization options:

Code size 266 KBytesData size 39 Kbytes.

The size of both catalogues together is 138 KBytes.

AST PERFORMANCEOperMode

Ang.Rate

Timeto fix

Updatefreq

Accuracy

3°/s 1s - 0.004°/s

5°/s 2s - 0.005°/sROM10°/s < 5s

30Hz- 0.01°/s

PRM 0÷3°/s 1s <260” -< 0.5°/s 1” 0.34” /s

1°/s 1.3” 0.76” /s

2°/s 3.6” 3.9” /s5°/s 9.5” 25” /s

NM

10°/s

0.03s 30Hz

15” 55” /sTable 3-1 AST Prototype Simulation Campaign Results

4 Ground Test: True-Sky Campaign

4.1 SETIS-IST: the HardwareIST stands for Italian Star Tracker: it is the name for theexperiment set-up to demonstrate the concept ofAutonomous Attitude Determination which is the coreof ANDROMEDA. It is based on a single-headprototype, developed by Alenia Difesa Officine Galil eoas SETIS (Stellar and Extended Target Intelligent Sensor)under two separate ESA and ASI contracts (phase 1 and.phase 2).

SETIS-IST consists of :� one OH, provided by Alenia Difesa, Off icine Galileo;

it has the same optical characteristics as the OH chosenfor the reference configuration of AST (see table 4-1); itis provided with a baff le adapted from a design for theCassini mission star tracker;

� one EU, based on an ERC32 chip-set processor,provided by Laben, whose main characteristics areshown in table 4-2;

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� the SW, including the BSW, provided by Alenia DifesaOff icine Gali leo, and the ASW provided by AleniaSpazio and based on the AADS Library.

SETIS-IST outputs the attitude qauternion andangular rate estimates with a frequency of 10Hz. Table 3-3 shows the prototype mass-power budget.

OpticalSystem

Double Gauss Modified, radiationhardened glasses

FOV 8.23° x 10.96°Focal Length 46 mmF# 1.5Bandwidth 550 to 950 nm (OG550)CCD TH7863M (288x384 pixels, 23µm)Cooling Peltier (3 stages)IFOV 103 arcsecPSF size 2.4 ÷ 3.2 pixels (95% energy)Exposure time 10 ms ÷ 1 s, 10 ms stepADC 16 bit, 100KHzRead-out time(CDS)

35 ms (10 windows of 10x10pixels), 1.76 s (full CCD)

OperativeTemperature

-20°C ÷ 50°C

Table 4-1 SETIS-IST Experiment OH Characteristics

CPU based on Temic ERC32 chip set (SPARC V,32-bit RISC, 10 Mips) 12 MHZ

� 8 Kbyte start-up PROM� 2 Mbyte RAM (EDAC protected)� 512 Kbyte EEPROM� 8k x16 bit FIFO (EU-OH interface)External Bus Interface STD-MIL-1553BComponent grade: MIL-883DC/DC converter for CPU board and OH powerCurrent driver for Peltier controlTable 4-2 SETIS-IST Experiment EU Characteristics

MASSEU: 3 KgOH: 2.5 KgBaff le: 0.6 Kg

POWER 18.9 @ 0°C OH temperature26.0 @ 30°C OH temperature

SIZE EU: 250 x 210 x 96 mm3 (L x W x H)OH: 237 x 127 x 127 mm3 (L x W x H)Baff le: 245 x 161 mm3 (L x Diam)Bracket 140 (L) x 206 (W) x204(H)mm

Table 4-3 SETIS-IST Experiment Budgets

4.2 The Ground Test Set-upGround tests have been carried on during seven brightnights during summer 1999, at Saint-Barthelemy, avill age on the Italian Alps, 1700 m high. The number ofnights dedicated to the tests was constrained by the tighttime schedule of the SAC-C Program and by the need to

match nights not il luminated by the moon (for at least 4-5 hours) and with favourable weather conditions.

IST has been fixed inside a box filled with nitrogenand equipped with a lens to compensate the atmosphereeffect. The resulting container has been mounted on a 3-axis mechanical actuation system,with two axes controlled by a PCvia stepper motors (see figure 4-1and 4-2). The actuation systemdesign was based on the use ofcommercial components.

Figure 4-1 SETIS-IST Adaptation for Ground Tests

True-sky data were collected for off -line analysis inthe simulated environment while on-line processing wasperformed by the flight SW integrated on the EU.

Figure 4-2 Ground test set-up

3.3 True-Sky Tests ResultsThe results collected during the observations are ingeneral of good quali ty and of high interest. Theanalysis activity has been limited to the need to validatethe SAC-C IST on-board SW. A “bank” of frames hasbeen collected, for possible future use. The followingsessions have been performed:

� Simulator tuning: mainly dedicated to the imagegeneration model validation.

� Star detection and background estimate algorithmtuning.

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� OH photometric and astrometric calibrationvalidation

� Pattern Recognition Mode verification throughforced recursive attitude acquisition

� Normal Mode verification through on-lineprocessing of long data sets

� On-line calibration assessment.The system demonstrated to work according to the

expectations and continuously even in presence ofatmosphere and disturbances induced by the actuationsystem. As an example, figure 4-3 shows the estimatedpath output from a conical scanning of the sky: theEarth movement is clearly evident through a sample 1.5hours long.

Figure 4-3 SETIS-IST Output Attitude Estimate(Ground Tests Result: Conical Scanning)

5 Flight Experiment

5.1 The Host SatelliteSAC-C is an international Earth observing satellit emission conceived as a partnership of the CONAE andNASA and with considerable additional support ininstrumentation and satellit e development from theDanish DSRI, the Italian ASI , the Brazili an INPE, andthe French CNES.

Table 5-1 collects a short technical sheet relevant tothe SAC-C mission. Figure 5-1 shows the layout ofSAC-C upper platform, where IST OH and EU areindicated. Figure 5-2 shows the instrument integrationon the satellit e.

Name Satelite para Aplicaciones Cientificas CCustomer Argentinean Space Agency (CONAE)PrimeContractor

INVAP

Orbit Sun-synchronous (10:15am DNC)Height 702 KmConstellation EO-1, LANDSAT7, TERRA, SAC-CSun to orbit 26°

planeAttitude Earth PointingAttitudeaccuracy

1.5°control, 0.2° reconstruction onground

AOCSfrequency

1 Hz

Launch date November the 21st, 2000Lifetime 4 years

Table 5-1 SAC-C Mission Characteristics

Figure 5-1 SAC-C Upper Platform Layout

Figure 5-2 IST Integration on SAC-C

5.2 In-Flight Experiment ResultsThe IST experiment was switched on earlier than whatforecasted, (on 24/11/00, during first satellitecommissioning) after request by CONAE, as thebaseline SAC-C star tracker was not performing asspecified: in particular, it was unable to provide attitudeout of eclipse, and even then it was subject to suddenoperation interruptions.

The first analyses showed a good behaviour of theSETIS-IST system, able to provide attitude for longconsecutive periods without fallback, even in presenceof a light background much higher than what expected.Main consequences induced by high background figureand not-uniformity are:

� a decrease in the star detection capabili ty

Trajectory

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� a reduction in the star image coordinate andmagnitude estimate accuracy.

A first activity was then dedicated to backgroundestimator parameters tuning, aiming at improving thereaction of the model to the actual backgroundvariations. The success of the operation is demonstratedby the data collected from then onward, where fulladequacy of the system to the expectation isdocumented. SETIS-IST is one out of three star trackerson-board SAC-C, but it is the only one successfullyoperating. As a consequence, CONAE has funded theSETIS-IST BSW upgrades needed to interface directlythe instrument with the C&DH computer. Now, theSETIS-IST output quaternion is made available on-board to both the optical payloads, requiring attitudeinformation to correlate their measurements, and to theAOCS, requiring attitude fixing to monitor operatingmode management. The SAC-C baseline star tracker hasin fact being excluded at all from the overall processand switched off .

Figure 5-3 Estimated SETIS-IST Absolute Attitude

Figure 5-3 shows the estimated attitude of theinstrument boresight during one of the Delta-V firingsrequired to form the constellation: the overallmanoeuvre starts with the blue trace on the top left, ittakes four orbits to stabili se attitude again and endswhen the brown trace tends to superimpose to theoriginal blue one (right). The system followedcontinuously many other maneuvers, even if highangular rates (more than 0.5º/s, see estimates of figure5-4) were reached; figure 5-5 compares the estimateddeclination over right ascension tracks relevant to twoconsecutive orbits when an orbit raising occurred.

Figure 5-4 Estimated Angular Rate

Figure 5-5 Continuity in SETIS-IST Output

The SETIS-IST SW is now able to work properlywith a higher than expected background and even withpartial Moon in FOV. Figure 5-6 shows the backgroundestimate along the orbit, corresponding to some 20,000e-/s/pixel over the il luminated Earth, some 40,000 e-

/s/pixel when the sun approaches the limit angle fromthe instrument boresight (about 80º). The presence ofMoon in FOV within some 5º from the boresight doesnot interrupt continuous operation (it contributes withsome 25,000 e-/s/pixel to the estimated background).

0

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20

30

40

50

60

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200

400

600

800

1000

1200

1400

1600

Sun/Ecl

Series1

BGN

Sun background effect

Illuminated Earthbackground effect Moon in FOV

moon in FOVBoresight-Moon angle[deg] (Series 1)

Eclipse Eclipse

Figure 5-6 SETIS-IST Background Estimate

Right Ascension Vs. Declination

30

40

50

190 200 210 220 230 240 250

Deg

Deg

∆V

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Some pictures of Moon in FOV have beendownloaded: figure 5-7 shows as an example an image(200 ms exposure time), thresholded at 10,000 e-/s/pixelfor representation only: the ASW works on the contrarythrough an adaptive background estimator. Pictures willbe collected in the future with lower exposure times togain representativeness.

Preliminary system accuracies have been computedover 4 orbits, based on the internal quality indextelemetry, as reported in table 5-2 below.

Figure 5-7 Moon in FOV (10,000 e-/s/pixel threshold)

X Y Z

Average 11.7” 3.9” 1.6”

Min 8.7” 2.0” 1.1”

Max 19.3” 7.0” 2.2”

Table 5-2 Attitude Accuracy (X is the boresight)

6 Current Development andConcept Evolution

Alenia Spazio is currently pursuing the development ofthe baseline ANDROMEDA configuration, as per figure2-1, based on an EU developed by Laben (as an upgradeof the SETIS-IST prototype) and running the completeAutonomous Attitude Determination software thatmanages up to three remote OHs developed by Sira (anEnglish engineering company with a consolidatedexperience in the field of star sensors and opticalpayloads in general, and with specific expertise inadvanced radiation tolerant components), alreadyquali fied and ready to be flown on the PROBA mission.

The two (or three) OHs can be used in a coordinatedmode or in an independent mode (in hot or coldredundancy).

Design performances are collected in table 6-1;furthermore the system will provide a lost-in-spacerecovery time of 1 s (with a confidence level greater than

>99.8%) and an output rate of up to 30 Hz, according tothe timing sketched in figure 6-1. The ASW will includestar catalogues containing approximately 4500 Imagesand 2500 Reference Stars.

ROTRATE

Att itudeEstimate

Err or

Angular r ateEstimate Err or

0º/s 2” 8” /s

2º/s 4” 15” /s

5º/s 7” 25” /s

Table 6-1 ANDROMEDA Accuracy (1σ) - each axis

Figure 6-2 Multi-Head Timing

Concept evolutions towards on-board autonomyincrease are currently being pursued, both internally andwith the support of ESA. In particular, some areas ofinterest are being exploited, such as

� integration of attitude determination and controlfunctions, in a single AOCS dedicated computer;

� integration of attitude and orbit or trajectorydetermination functions, in a single AOCS dedicatedcomputer, with the further support of an integratedsensor hardware, too;

� interfacing with advanced detector hardware, such asthe one derived from the APS (Active Pixel Sensor)technology, to improve radiation resistance andsystem competitiveness.

7 AknowledgementsThe Authors wish to thank prof. F. Donati (from EicasAutomazione), for his fundamental support in theconcept development and implementation: the AADSLibrary stands over his brilli ant algorithm conception. Athanks to ESA - in particular to Phil Airey (Responsibleof the AST Contract) for his valuable suggestions andcontinuous thrust – and ASI for having supported theAST and SETIS-IST contracts, to Off icine Galileo andLaben – for the work they did with the SETIS-IST HW– and to Sira – mainly to David Purll , for his support inplanning the next-future developments.

Expo 1 Expo 1 Expo 1

Expo 2 Expo 2 Expo 2

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Expo 3 Expo 3

Read Out 1 Read Out 1

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NM 1 NM 1 NM 1

NM 2 NM 2

NM 3

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ANDROMEDA DATA SHEET

Single-head: 2 arcsec [pitch-yaw] 17 arcsec [roll ] (pointing) 12 arcsec [pitch-yaw]

100 arcsec [roll ] (5°/s slew)

Attitude Determination Accuracy (filtered NEA, 1σ,EOL)

Multi-head: 2 arcsec (pointing) 7 arcsec (5°/s slew)Single-head: 8 arcsec/s [pitch-yaw] 66 arcsec/s [roll ] (pointing) 36 arcsec/s [pitch-yaw]

300 arcsec/s [roll ] (5°/s slew)

Angular Rate Determination Accuracy (1σ, EOL)

Multi-head: 8 arcsec/s (pointing) 25 arcsec/s (5°/s slew)

Output rate Independent Single Head 10 Hz

Coordinated Multi-Head up to 30 Hz

Attitude recognition from scratch 1 s, 99% (0 - 1°/s)20 s, 99% (up to 5°/s)

Max operative rate 5°/s (nominal performances)10°/s (degraded)

Required input on/off command (optional: command to theactuators, linear velocity, Earth position)

Output Boresight quaternion, absolute attitude rate vector insensor axes (optional: estimated external torques,single star centroids and magnitude, attitude and ratewrt Earth)

On-board catalogue size 2500 stars, 4500 images (100% sky coverage)

Optical System Refractive, radiation hardened glasses

Sensitivity mI = 6.2 (pointing, 200ms)mI = 3.9 (5°/s slew, 20ms)

Number of stars tracked Up to 10 each OH

Accuracy on raw attitude measurement (1σ)

(without estimator usage)

Bias < 5 arcsecNEA = 4 arcsec

Error on magnitude 0.15 mI (calibrated)

Calibration On-line

Operating Voltage 28 V ± 5

Mass EU 3.35 kgOH 1 kg eachBaff le 0.76 kg (40° SAA, 30° EAA)

Power < 28.5 W (< 35.7 W, Peltier included)

Envelope (indicative) EU: 250 x 200 x 150 mm3 (L x W x H)OH: 87 x 90 x 90 mm3 (L x W x H) eachBaff le: 284 (L) x 320 (W) mm3 each

Operating Temperature Range OH: -20°C, +50°C; EU: -20°C, +60°CSurvival Temperature Range OH: -50°C, +60°C; EU: -40°C, +60°CLifetime > 5 years

Data interface MIL-STD-1553B (RS 422, 1355, MACS)

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