Sensors for Space Applications

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SENSORS FOR SPACE APPL I CATI ONSM E. Koni gerMesser schm t t - Bol kow- Bl ohmTechnol ogy Resear ch ZTAPost box 801109, D- 8000 Munchen 80

ABSTRACT SENSORS FOR ATTI TUDE AND ORBI T CONTROLSol i d st ate sensors compri s i ng severaldet ector e l ement s or h i ghl y i ntegr ated det ec-to r a r r ays pl ay an i mpor t ant ro l e i n a l l spa -cecr af t , be t hey manned vehi cl es or unmannedsate l l i t es . Sun sensors , ear t h sensors andst ar t r ackers are empl oyed f or att i t ude andorbi t cont ro l o f t he vehi c l es . I n r endevouzand docki ng manoeuvres of vehi cl es m cr owavesensors , l aser rangef i nders and opt i cal sen-

SOKS are used f or t he d i f f erent phases. Depen-di ng on the m ssi on r equi r ement s of t he space-c ra f t , ma i nl y opt oel ect r oni c sensors opera t i ngi n numerous spec t r a l ranges ( v i s i b l e , near I R,m dd l e I R and f a r I R) and sensors f or X- rayand y- r ay measurements ar e put t o work. Theexact , re l i abl e opera t i on o f t hese sensors a ta l l t i mes i s a pr e requ i s i t e f o r the successof t h e m ss i on.

I NTRODUCTI ONMany ki nds of sensors pl ay an i mpor t antr ol e i n spacecr af t be they manned vehi cl es orunmanned sat el l i t es. For exact att i t ude andorbi t cont r o l o f t he vehi c l es ma i nl y sun sen-s o r ~ ~ar t h sensors and s ta r t rackers a reempl oyed.

I n r endevouz and docki ng manoeuvres of 2 spa-cecraf t or unmanned pl at f or ms, m cr owave sen-so rs , l aser s and opt i cal sensors a re used orbei ng devel oped f or t he gui ded phase, f orhom ng and f or t he f i nal approach.Dependi ng on t he m ss i on r equi r ement s of t hespacecra f t , opt oel ect ron i c sensor s opera t i ngi n t he v i s i b l e , ne ar I R, m ddl e I R and f a r I Rand sensor s f or x- r ay measurements pl ay adom nant ro l e. Most l y t he det ector e l ement sar e ar r anged i n l i ne or a r r a y f o r m i n t hef ocal p l ane of cameras or spect r ometers . Thesei nstr ument s ar e used f or eart h observat i ons( spa t i a l ma pp i ng , e x pl or a t i on o f t e r r es t r i alr essour ces, l and use, moni t or i ng of t he vege-t a t i on) and fo r ast r ophys i cal research i n t heI R and x- r ay regi ons.I t i s t he i nt e nt i on of t hi s cont r i but i on t ogi ve an over v i ew over several sensor appl i -cat i ons i n t he past , present and f uture spacepr ogr ammes .

Eart h SensorAs an exampl e a s ensor whi ch was devel op-ed fo r a t t i t ude cont ro l o f 3 ax i s s t a bi l i s edcommuni cat i on sat e l l i t es i s d i scussed /l/. I ti s a sensor wi t h very l ow z er o dr i f t . I t ss t andard vers i on i s des i gned fo r o rb i t s wh i chdo no t dev i a te subst ant i a l l y f r omt he geost a -t i onary or bi t . The sensor i s made of pyroe-l ec t r i c mat er i al ( L i T aO, ) and ope r a t e s i n t h e

spect r a l r ange 14- 16 pmOpt i c al pr i nc i pl eI nf ra r ed rad i a t i on o f t wo oppos i t e hor i zons oft he eart h i s f ocussed by a Ge obj ect i ve l ensand reaches a c i r cul ar chopper d i sc at t hef oca l pl ane o f t he l ens (F i g .1 and 2 ) . Af t e rpass i ng the osci l l a t i ng d i sc wh i ch has t hesame di ameter as t he eart h i mage, t he radi antf l ux f r om bot h o ppo s i t e e ar t h ho r i z ons i sd i r ected v i a secondary opt i cs to t he pyr oel ec-t r i c de tect o r . I t s ent rance wi ndow ac ts asf i l t e r de f i ni ng the opt i cal bandwi dt h of t hesyst em Onl y chopped I R r adi at i on i s convert edi n to an e l ect r i cal s i gna l , wh i ch i s a measur eof angul ar m sal i gnment . Automati c corr ect i ono f ext e rnal (e .g . seasona l va r i a t i ons) and i n-t e r n al ( t e mpe r a t u r e g r a di e nt s ) r a di at i on di f -f erences i s possi b l e v i a chopper ampl i t udemodul at i on.lens 1I

/concave mirror

Fi g.1 Cr oss sec t i bn o f t he opt i cal pa th3 - 6 8

CH2704-5/89/0000/3068/$01OO 0 989 IEEE


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, 40 1 km f 2 1001 10 1 l m l do;k,phases

interspacecraft

guided homng final approachphase

I I II I I

Fi g. 4 Mai n RVD m ssi on phases l aserr angef i nder i s under devel opment at MBB f undedby an ESA cont r act / 2 / . The oper a t i ng pr i nc i p-l e i s a s f o l l o ws ( F i g. 5) . A sho r t pul se em t -ted by a l aser d i ode t r avel s f r omt he chaser( approachi ng spacecr a f t ) t o the t a rget sa te l -l i te and i s r e f l ect ed by a cor ner cube re f l ec-tor . The accur acy of t he rangi ng syst emi s ofthe order o f severa l cent i meters .A f i el d of v i e w o f 30 x 30 deg can be scannedby 2 de f l e ct i o n m r r o r s i n r o ughl y 10 sec .Tracki ng can be done f ast er si nce onl y a smal lf i el d of vi ew need to be cover ed. The r angeval ue toget her wi t h az i muth and e l evat i onangl e y i e l d t h e l i ne- of - s i ght v ec t o r a nd i t sde r i v at i v es gi v e t h e r e l a t i v e v el o ci t y ( r a ngerate and angul ar r ates) .

30I

,1(large anglescan)11 ,I //

SAS' t = 1 S(smll1 anglescan)

IFOV, 0.30

4-@-o1xoMSM T = O 5 S(mebsuremeht)

F ig .5 Oper a t i ng p r i nci p l e of l aserr angef i nder

EARTH OBSERVATI ON FROM SPACEFor t h i s k i nd o f observat i on t he pushbr o om pr i nc i pl e ( F i g. 6) i s n ow uni v er s a l l yempl oyed. I t i mp l i es el ec t r oni c scann i ng ac -ross f l i ght pat h wi t h a l a rge number o f d i sc -re te photosens i t i ve e l ement s ( CCD l i ne ar r ays)d i r ec t l y cor r espond i ng t o sub- t rack g roundpi x el s .

Scanning Principle("Pushbroom")

fliqhtdirection sensitiveCCD detectorelementsrefractive4 1728-4048)\ IFOV

surface

image swath strip

Fi g. 6 Push broom pr i nc i pl eI n Fi g. 7 a dual l ens a r r angement f o r scan l i neext ensi ons beyond one CCD l i ne i s shown.Up t o 6 sensor s chi ps have been opt i ca l l ybut t ed t o f o rm one l ong cont i nous l i ne ar r ay .Thi s has been r eal i zed i n t he MOMS- 01 camer asystem ( Modu l a r Optoel ec t r oni c Mu l t i spec t r a lScanner) /3/. The syst em has been devel oped byMBB under t he sponsor shi p of t he M ni st r y ofResearc h and Technol ogy.MOMS was success f u l l y t ested severa l t i mes onShut t l e f l i ght s i n 1983 and 1984. A t o t a l a r e aof 3 ,s x lo6 km2 was cover ed.

Optoelectronic Concept

Fi g. 7 Dual l ens ar r angement

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MOMS T ec hni c al spec i f i c at i o n of t he opt i c almodul e****************A

SensorNumber of CCD' s perl i nePi xel s i z eChannel No. 1Channel No. 2Gr ound p i xel s / l i neGr ound pi xel s i zeLi ne f r equencyRadi ometr i c r es.Ga i n f act o rFocal l engthRel . l ens aper t ur eI nst ant aneous FOVTotal FOVSwat h wi dt hDi s t o r t i on

Ret i con CCPD 1728416 pm x 16 pm575 ... 625 nm825 ... 975 nm691220 m ( 3 00 k m o r bi t )350 Hz 5 10 %128 ( 7 bi t )1 or 2237. 2 mm1: 3. 567. 5 pr ad26. 2 deg138 km ( 300 kmorb . )+ 8 pm max. ( TFOV)-

t ypi cal i mage t aken by the NONS camer a i sshown i n Fi g. 8 ( Vi l l a Bl anca i n Sout h Ameri canear t he Pac i f i c Ocean).

Fi g. 8 I mage t aken by the NOMS camer aA syst em si m l ar t o MOMS has been devel oped i nFrance. I t s name i s SPOT whi ch i s an acr onymf or " Syst eme Probat o i r e d ' observat i on de l aTer re" .

New devel opment s ai m at devel opi ng CCDl i ne arr ays wi t h phot odetect ors whi ch oper ateat l onger wave l engths , i . e . i n the SW R (Shor twave i nf ra r ed) spect ra l r ange / 4 , 5, 6/.Par t i cul a r l y wave l engt hs a t 1 ,6 pm and 2 , 2 pmare of i nt e rest . These spect r a l bands ar epr o m s i ng f o r a gr i c ul t u r e mo ni t o r i ng, f orexpl o r i ng m ner a l and o i l depos i t s , andobservat i on o f eco l ogi cal p rocesses .Di f f erent l i ne arr ay sensors are under deve-l opment. At Thomson- CSF a hybri d concept ass o-ci at i ng I nGaAs phot odi odes and Si - CCDs wassel ected. The phot odi odes are connected t o t heCCDs by wi r e bondi ng. At MBB/ I MEC a monol i t hi csol uti on i s under devel opment sponsor ed by anESA contr act . The monol i t h i c ar r ay compri sesPt Si Schot t ky bar r i er di odes and CCD r eadouton st andar d MOS- I C grade si l i con. A cr osssect i ona l v i ew o f a det ector p i xel and a

guard ri ng transfer-gate 3-phase-CCD

t .--I '-7p - S i

mxm aluminium - oly-SimF ig .9 Cross sec t i ona l v i ew of P tS idet ect or and 3 phase CCD3 phase CCD st r uctur e i s shown i n Fi g. 9. Thear r ay shows excel l ent uni f o rm t y . At themoment t he but t abi l i t y of severa l l i ne a r r aysi s i nv es t i ga t e d.

Te chni c al d at a ( pr e l i m na r y )* L i ne a r r ay, but t abl e* Bac ks i de i l l um nat i on* Temper at ur e of* Pi xel s i ze 18 x 39 pm* Number of pi xel s* Quantumef f i c i ency

opera t i on L i qui d n i t rogeni n one l i ne arr ay 256at 1, 6 pm - 12 %at 2, 2 pm - 10 %

SENSORS FOR I NFRARED ASTRONOMYI n s i t u measurement s f r om space vehi c l esare t he mai n r eason f or t he enor mous pr ogres swhi ch can be achi eved wi t h I R ast r onomy i ni nt e rs t e l l a r phys i cs and chem st ry and i n t heexpl orat i on of p l anetar y at mospher es.I R- ast r onomy wi t h t he I SOPHOT photomet er du-r i ng the p l anned I S0 m ssi on ( ESA' s SpaceObser vat ory) wi l l use ext r i nsi c photoconduc-t o rs ( doped Ge or S i ) f or ast r ophys i cal re -search at wavel engths r angi ng f r om 3-200 pmThe I SOPHOT exper i ment consi st s of f our d i f f e-r ent subuni t s. An overv i ew concerni ng t hei rcapabi l i t i es i s shown i n the tabl e .I sopho t - P , a multi-aperture/multi-band phot opol ar i meter , wi l l cover t he wavel ength r ange3- 110 pm usi ng 3 s i ngl e-p i xel det ector s. Ge: Gawi l l be used at X > 30 pm the mater i a l s f o rt he 2 short wavel ength channel s wi l l besel e ct ed f r o m S i : I n, Si : Ga, Si : As , Si : Sb a ndSi : P. Ext r emel y l ow NEP val ues i n the l ow10- 1' W Hz - l 12 range and respons i v i t i es ofsevera l A W -1 at t he peak wavel engt h can beobt ai ned /I/.

r sophot - C, a f ar I R camera f or i magi ng and3 - 71


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Capabi l i t i es of I SOPHOT

ISOPHOT-P ISOPHOT-C ISOPHOT-AC50 ClOO C200

Wavelengthrange ( 3.. 30.. 52.. 120.. 25110 55 118 200Spectralresolution 1.5.. 4 at 4 at 5 at 3FOV ..3 1.6x 2.1x 2.9x 2.8x(ar cmi n 2.1 2.1 2.9 2.8

10 352. 7 0 r m 135rm

Spatial res.(arcsec) 5 33 84 168 21Min.det.fluxF m x m ( m J y ) 2.6 8.8 24.6 42.3 3(S/N-lO,lOOs)at at at at25rm 35rn 70rm 135rm

10% 10% 10 % io\ l o aPolarimetry 5.. 5.. 5.. 5.. 5..

ISOPHOT-Ss1 s2

2.5.. 6.0..5.0 1290 90

0.4x 0.4x0.4 0.424 2434 7 3 8 8

pol ar i metr y, uses 3 doped Ge ar r ays, i n par t i -cu l ar Ge: Be f or A < 55pm Ge: Ga f or A < l l 8pmand Ge: Ga f or X < 200pm Se ns i t i v i t i es cl os eto f l i gh t spec i f i ca t i ons have been achi evedso f a r .I sophot - A, a camera f or i magi ng and pol ar i -met r y , was o r i g i nal l y p l anned t o cons i s t o f3 l i near a r r a ys of Si : I n, Si : Ga and Si : P wh i c hwer e meant t o produce maps dur i ng cont i nuoussc ans . F or r e asons o f s i mpl i c i t y , h owe ve r , i twas deci ded t o r eal i ze onl y st ari ng mode ope-r at i on i n the r ange 20- 30pmThe dual channel I sophot - S spect r ometer wi l luse l i near S i : Ga a r r ays of 6 4 el ements . A NEPof 5 x lo- W Hz-l/* and a responsi v i t y up to25 A W-l at t he peak wavel engt h of 15pm havebeen achi eved.Nom nal operat i ng t emper at ures of t he I sophotexper i ment i s 3, 2 K. Al l sensors i nc l udi ngel ectr oni cs must be abl e to oper ate at t h i sextr emel y l ow t emper atur e l evel .

X- RAY SENSORSX- r ay spect r oscopy i s a very powerf u l me-t hod for d i agnost i cs i n the study of manyast r ophysi cal phenomena. Spect r al measur ementsi n t he X- r ay domai n provi de the basi s f or aquant i t i ve assessment of t he physi ca l st ateand evo l ut i on o f t he ho t t est obj ect s i n t heUni verse. The dynam cs of many of t he mostenerget i c phenomena l i ke f l a res , bur s t s , pul -sat i on, beam ng and shocks can be st udi ed.x- r ay em ssi on i s a common f eat ure of a l lc l as ses o f a s t r onom c al o bj e ct s , b ut det ai l e dspect ra l i nf o rmat i on i s up t o now onl y avai -l abl e f or sev er a l of t h e br i ght e s t sour c e s . I ti s expected that aft er t he ROSAT-m ssi on,whi c h i s a n al l - s ky sur v e y pr o j ec t , i n par t i -cul a r t he XMM- m ss i on wi l l p rovi de hi gh qual i -ty spec t ra of m l l i ons o f obj ec t s r a ngi ng f r o mt he nearest st ars t o the most d i st ant quasar s.

XMM i s due t o be l aunched i n t he 1998 t i mef r ame and i s char acter i zed by an X- ray f ocus-si ng opt i cs of unprecedented t hroughput i n thedomai n -0,l - 10 KeV.The f ocal pl ane i mager i nst r ument ( E P I C ) onXMM wi l l consi st of 3 mat ched camer as. Thesear e mount ed at t he f ocus of 3 t el esc ope modu-l es whi ch compri se t he fu l l XMM opt i cs.The X- r ay det ect i ng el ements wi t h each cameraar e C C D s whi ch prov i de t he des i red sens i t i v i -t y , ener gy band- pass, spat i a l and spectr a lr e sol ut i on.CC D c ha r ac t e r i s t i c sThr ee d i f f e rent C C D s ar e under devel opment atThoms on, EEV and MPE/ MBB i n Fr ance, t he UK andi n West Germany. Two of t hem uti l i se f r amet r ansf er MOS t echnol ogy, t he t h i r d one arenovel pn devi ces. Thei r t echni ca l per f ormancei s compar ed i n the tabl e. Al t hough t he 3 devi -ces under cons i der a t i on a re broadl y s i m l a r

CC D t echni ca l per f ormance

EEV Thomso n MPE/MBBParameters

Number of pixels 1152x7 70 1024x1024 200x64Pixel size ( p m ) 22.5x22.5 1.19Readout noise(electrons)

150x150(estim.)6 5 440 100eadout rate (k Hz) 40* Optical chargetransfer/pixel 0.99999 0.99999X-ray CTE/pixel 0.99998 0.99998Oper. temperature 175 R

Energy Resolution( FWHM eV) j100eV not meas. j100eVBackground (estim.)

(estim.)adiation

0.999990.9999917 5 R 17 5 RDepl. depth (pm) 30 5 28 0Dead layer ( A ) 5700 5100 j200

not meas. -99.999ejection ( % ) 99

Tolerance 14 krad not meas. 1 Mead(25% degr.)i n te rms o f t he i r use as X- r ay det ec to r s ,t h ei r c har a ct e r i s t i c s do di f f er i n det a i l . I nsummary t he charact er i st i cs of t he C C D s are asf ol l ows . T he l i m t i ng s pat i al r e s ol ut i on oft he devi ces r anges f r om 20 ,um ( MOS C C D s ) t o150 pm ( pn C C D s ) . The peak quant umef f i c i encyexceeds 90% An ef f i c i ency bet t e r t han 10 % i sachi eved over a wi de energy pass - band. Thespect ra l reso l ut i on ranges f rom 50 eV a t 1 KeVt o 150 eV at 7 KeV. I n t he nor mal operat i ngmode whi ch wi l l be appr opr i ate f or f a i ntsource s t udi es , f ul l f rame i magi ng i s pr ovi dedwi t h a t i me resol ut i on of 50 sec. I n t he caseof t he pn devi ces a t i me r esol uti on mode downto 16 psec over a r est r i c ted f i e l d o f v i ew i sposs i bl e .Wher eas t he MOS CCDS operat e i n a more or l essconvent i onal way the pr i nc i p l e of operat i on oft he pn C C D s i s no vel ( F i g. 11) /e/. A l a rgesem c onduc t o r c h i p of hi gh r e s i s t i v i t y n - t y peSi can be depl et ed f r om a smal l n+ ohm ccont act pos i t i vel y b i ased wi t h respec t t o t hepl anar p+ cont act s on both sur f aces. The dep-l e t i on zone wi l l expand f r omboth j unct i onssi mul t aneousl y and i s compl ete i f bot h r egi onsare t ouchi ng each ot her .

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n+- contact

tr ansfer di recti on/E 1

n-bul k I -Up+back contact

Fi g. 11 Pr i nc i pl e of o pe r a t i o n of a f ul l ydepl etabl e pn- CCDAppl y i ng a more negat i ve potent i a l at t he p+backs i de the e l ec t r on po tent i a l m ni mum i sshi f t ed t owards t he opposi t e s i de havi ng then+ ohm c cont act. By r epl aci ng t he l arge rec-t i f yi ng p+ contact on t he n+ si de by a s er i esof segment ed p+ st r i ps ( sh i f t r egi s t e r ) appro -pr i ate vo l t ages are appl i ed and l ocal pot en-t i a l m ni ma f o r el ec t r ons ar e crea ted a t adept h of approxi mat el y 20 Nm An adequat echange wi t h t i me of t hese vol t ages a l l ows adi s c r et e shi f t of t he stored si gnal char gest owar ds t he r eadout mode.Thi s concept coul d l ead t o s i gni f i cant i mpro -vement s f or X- r ay spectr oscopy. As t he chi p i ssens i t i ve to r adi a t i on over the ent i re t hi ck -ness of t he wafer (280, um, i t provi des anexcel l ent ef f i c i ency al so at h i gh energi es( 100%at 5 keV, 90%at 10 kev ) . The radi a t i onhas t o penetr ate onl y a very t h i n ( 100 A) deadl ayer on t he backsi de of t he chi p. No back-t h i nni ng process i s r equi r ed, because t he chi pi s f ul l y depl et e d. T hi s r esul t s i n a ve r y goodl ow energy sens i t i v i t y (>97%at 0. 1 keV) . Am ni mum i oni z i ng par t i c l e wi l l de po s i t a nenergy of mor e t han 80 keV; t here fo re t her e j e ct i on of t h i s ki nd of backgr ound i s prac-t i c al l y 100%

The exc l usi ve use of pn st r uctur es ( i nst ead ofMOS st r uctur es) makes t h i s t ype of CCD i ntr i n-s i cal l y radi a t i on hard. No s i gni f i cant degra -dat i on shoul d occur up t o 1 Mr ad. Exper i encesf r omhi gh energy physi cs experi ment s show aradi a t i on har dness up to 5 Mrad. P i xel s i zesof 150 x 150pm2 can be r eal i zed whi ch i s ani deal matchi ng wi t h the resol ut i on of t he XMMm rr or system ( t her eby reduc i ng- t he readoutt i me).I n order t o use t he l ow det ector capaci t ancef or f ast and l ow noi se appl i ca t i on a s i ngl esi ded gate n- channel J FET has been devel oped.Based on the same f abr i cati on process as f ort he CCD i t sel f t he new component can be mono-l i t h i c al l y i nt e gr a t e d cl ose t o t h e det e c t orr eadout node. Measur ement s on t hese devi cesl ead to an e l ec t r on noi se cont r i but i on o f 4e -( r ms) at a r eadout f r equency of 100 kHz and anoper at i ng t emperat ure of 200 K.

REFERENCES

/1/ G. Kul t erer , DGLR J ahr buch ( 1986, I ) 49/ 2/ ESTEC Cont r act NO. 6232, 035 CCN2, f i nalr eport , August 1988/3/ M Hof mann, Proc. I nter n. Geosci ence andRemot e Sensi ng Symposi um ( I GARSS ' 82)Ref . No. WP- 2/5 andDGLR J ahr buch ( 1983)/ 4 / H. E l abd e. a. SPI E I ntern. Tech. Conf . ,/5/ J . P. Moy e. a. , Proc. SPI E , Vo1. 865( 1987) 59/ 6/ J. von der Ohe e. a. , i b i d , 71/7 / J . Wol f e. a . , i bi d, 117/ 8 / L. St ruder e . a . , I EEE T ransac t i ons on

Geneva, Swi t zer l and, Apr i l 1983

Nucl ear Sc i ence, Vol . 35 ( 1988) , 372

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Sensors for Space Applications