Nuclear Instruments and Methods in Physics Research A329 (1993) 329-336North-Holland

Measured single event upset rates in low earth orbit for the ROSATWide Field CameraM.R. SimsX-ray Astronomy Group, Physics Department, University of Leicester, Leicester LEI 7RH, UK

AT SimsSpace Sector, Defence Research Agency, Famborough, Hants GU14 6TD, UK

R.D. Bentley and P.R. GuttridgeMullard Space Science Laboratory, University College, Dorking, Surrey RH5 6NT, UK

J.A. GourlaySpace Science Department, Rutherford Appleton Laboratory, Didcot, Oxfordshire OXI1 OQX, UK

Received 27 November 1992

Single event upset (SEU) rates in orbit have been measured for the ROSAT satellite Wide Field Camera's (WFC) Commandand Data Handling System (CDHS) memory . The WFC was launched as part of the German/UK/USA ROSAT satellite missionon the 1st of June 1990 . In over 600 days of operation 40 SEUs were detected . Most appear to be associated with the SouthAtlantic Anomaly. Agreement between predicted and measured rates is discussed. Problems associated with such analysis arehighlighted including the need for more relevant test data for electronic devices used in orbit . Detection technique is describedalong with several features of the WFCoperations, CDHS software and construction which directly relate to SEUs . No incidencesof multiple bit upsets were detected .

1. Introduction

On 1st June 1990 ROSAT was launched on a DeltaIl rocket from Cape Canaveral. The ROSAT X-rayastronomy project is an international collaboration be-tween Germany, the United Kingdom and the UnitedStates of America. The satellite carries a payload oftwo coaligned imaging telescopes . ROSAT's primaryscientific objective was to perform, over a six monthperiod, an all sky survey in the X-ray (6-80 k) andXUV (60-200,A,) [1] bands. This is now complete andROSAT is still operating 800 days plus after launch,carrying out pointed observations on selected sources.The ROSAT XUV Wide Field Camera (WFC) wasprovided by the United Kingdom [2] ; Germany pro-vided the X-ray telescope (XRT). The USA (under theauspices of NASA and via the USAF) provided thelaunch vehicle and the High Resolution Imager of theXRT.

The WFC is controlled by a Command and DataHandling System (CDHS). This consists of two redun-

0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved

NUCLEARINSTRUMENTS&METHODSIN PHYSICSRESEARCH

Section A

dant NSC800 8-bit microprocessors (CPUs) plus inter-faces to ROSAT and all the WFC electrical subsys-tems . All CDHS code is stored in Programmable ReadOnly Memory (PROM) with a central secure code(with limited functions) which runs in PROM only .After the CDHS is switched on, the main code iscopied to RAM (on command) where it is run. Thissystem allows the code to be reconfigured (patched) toallow for software or memory problems detected inorbit . All RAM areas can be reloaded by commandfrom the ROSAT ground station .

The WFC CDHS active memory devices are subjectto Single Event Upsets (SEUs) [3-5]. SEUs or bit flipswithin the memory can cause unexpected behaviour,CPU crashes, or can even be benign . The WFC activelydumps its memory contents in a trickle type modewhen science data is not being taken (for exampleduring near earth radiation belt passages) and criticalareas are subjected to an onboard checksum and auto-matic on-board safing . Both these features enable SEUsto be detected . In the latter case the time when the

33 0

SEU occured is available from the WFC telemetry andorbital position can be measured .

Measured rates have been compared to a radiationmodel of the onboard memory .

2. The WFC CDHS memory and the in-orbit detectionof SEUs

The WFC CDHS memory is divided between anumber of 6116 (16 384 bits per chip) RAM IntegratedCircuits (ICs) positioned on various cards within thesubsystem. Table 1 gives the memory map of the WFCand the location of the ICs with respect to card num-ber. Table 2 gives a basic summary of the use of thememory area .

The WFC is operated mainly by two types of com-mands, time tagged commands (deferred commands)and on-board stored sequences called Relative TimeSequences (RTSs). The latter can be called by theformer. In addition real time commanding is possiblewhile the spacecraft is within range of the groundstation located at Weilheim near Munich in southernGermany. Spacecraft operations are controlled by theGerman Space Operations Control Centre (GSOC) .The spacecraft is in a 53° inclination orbit at 575 kmheight, and hence is only visible from the groundstation 5-6 times per day for 6-10 minutes each time,although some additional coverage is available fromNASA ground stations . The WFC is consequently op-erated mainly via the deferred command store.

When the WFC is not collecting science data, whilethe microchannel plate detector's [2] HV is off, duringpassages through the earths radiation belts (AuroralHorns and South Atlantic Anomaly) and some otheractivities, the memory is trickle dumped in the sciencedata stream . Over a complete 24 h period the memory

Table 1WFC memory map and IC location

M.R. Sims et al. / SEU ratesfor the ROSAT Wide Field Camera

Table 2WFC memory usage

IC type

Memory start Memory stop Useaddress address

6616 PROM 0000

03CF

Kernel Code6616 PROM 03CF

059F

System RTSs6616 PROM 05AO

07FF

ROMcopy code6116 RAM

0800

OAFF

Parameter area6116 RAM

OB00

OFFF

DeferredCommands

6616 PROM 1000

27FF

ROMcopy code6116 RAM

2800

425F

Main CodeArea

6116 RAM

4260

47FF

Code Patches6116 RAM

4800

4FFF

Spare RAM6116 RAM

8800

8CFF

Buffers6116 RAM

8D00

8EFF

RTS Commands6116 RAM

81700

8FFF

Stack

contents are downlinked to the ground at least onceand can be compared with a reference data set todetect SEUs. Detected SEUs are corrected by real-timepatching of the memory during ground contacts toallow near continuous operation of the instrument . Notall memory locations are used and others change dy-namically, therefore some sections are not dumped .Thus SEU rates in the undumped sections of memoryare unknown. Consequently the WFC CDHS isrestarted or "rebooted" at periodic intervals (2-3months) to avoid accumulation of undetected SEUs .

Because the instrument is operated by deferredcommands calling mainly RTSs, the two areas of mem-ory containing the deferred commands and RTSs aredeemed critical, and hence on-board checking has beenimplemented post launch by patching the CDHS soft-ware . When a SEU is detected in these areas via achange in a calculated checksum, a flag is raised in thehousekeeping data stream and the WFC is automati-cally safed via a RTS command sequence . Because aflag is raised in telemetry it is possible to determinethe time of the SEU to an accuracy of ±8 s and via anorbit trajectory model the position of the spacecraftover the earth (to an accuracy of approximately ± 1° inboth longitude and latitude).

3. Detected SEUs and rates per integrated circuit

Table 3 gives all SEUs detected between launch andFebruary 1992. Forty SEUs were detected in 609 daysof operation. For each SEU the date, memory locationand contents change is given (where noted by theoperations team). If either the deferred or RTS com-mand store were hit, the memory address is replacedby the store name . On one occasion the SEU caused a

IC type Memorystartaddress(hex)

Memorystopaddress

IC location CDHS cardnumber

6616 PROM 0000 07FF CPU CARD 26116 RAM 0800 OFFF CPU CARD 26616 PROM 1000 17FF CLOCK CARD 46616 PROM 1800 1FFF CLOCKCARD 46616 PROM 2000 27FF CLOCKCARD 46116 RAM 2800 2FFF CLOCKCARD 46116 RAM 3000 37FF I/O CARD 76116 RAM 3800 3FFF I/O CARD 76116 RAM 4000 47FF 1/O CARD 76116 RAM 4800 4FFF I/O CARD 76116 RAM 8800 8FFF CPU CARD 2

Table 3WFC SEU detection log

CDHS crash, hence the non availability of one address.In cases when the SEU time could be determined thespacecraft position on the earth is given in latitude andlongitude. The other columns related to attemptedcorrelations with solar flares and proton fluxes and willbe dealt with in the next section . Table 4 gives thefrequency of SEUs per integrated circuit . Fig. 1 showsthe position on the earth of the 10 positionable SEUs

M.R. Sims et al. / SEU rates for the ROSAT Wide Field Camera 33 1

overlaid on a contour map of proton flux with anenergy of greater than 3 MeV (lowest energy thresholdavailable in reference) at 600 km orbital height [61,ROSAT's approximate orbital height . Nine out of the10 SEUs appear to occur within or be associated withthe South Atlantic Anomaly (SAA) as was expectedprelaunch (see section 7) . It is interesting to note thattwo ("Flare" associated) (See section 4) events occur

Num-ber

Date Daysafterlaunch

Addresseffected(hexa-decimal)

Expectedcontents

Found Solarflaresmaximumdate

Flux(PFU)(protonfluxunits)

SEUlatitude(degrees)

SEUlongitude(degrees)

GOES-7protonfluxdates >8.7 MeV> 10-1 CM-2s - I sr -I

SEUtype?

1 03-06-90 2 RTS2 10-06-90 9 34123 28-06-90 27 28713 CB C34 18-07-90 47 4244 9 195 29-07-90 58 3733 77 57 26-07-90 21 26/3007-90 F6 14-08-90 74 34EA E2 EA 30-07-90 230 12/1508-90 F7 19-09-90 110 2810 FB F98 05-10-90 126 32013 99 D99 14-10-90 135 38113 E6 C6

10 16-10-90 137 RTS -17 -4611 17-10-90 138 35113 AF EF12 20-10-90 141 2A9E 3E 3C13 22-10-90 143 31AF 32 2214 30-11-90 182 deferred -30 -6615 16-12-90 198 30DF 29 2116 25-12-90 207 2CC1 BO 9017 27-12-90 209 2DID 117 SF18 16-01-91 229 deferred -32 -4119 04-02-91 248 31783 2A 6A 31-01-91 24020 06-03-91 278 2EC1 46 4221 24-03-91 296 21338 0 10 22-03-91 43000 23/3103-91 F22 28-03-91 300 deferred -32.5 -156.5 23/3103-91 F23 21-05-91 354 29CF 0 1 13-05-91 350 21-05/20-06-91 F24 27-07-91 421 deferred -33 -6025 10-08-91 435 21304 59 49 11-08-91 F'???26 15-09-91 471 2AF9 9 8927 16-09-91 472 37A2 9 2928 07-10-91 493 313913 32 2229 17-10-91 503 RTS -31 -25 .530 26-10-91 512 3206 13 3331 30-10-91 516 2817E C8 E8 28-10-91 94 28-10/1-11-91 F32 10-11-91 527 31378 9 1933 11-11-91 528 21733 DE DF34 29-11-91 546 21346 27 3735 04-12-91 551 21372 92 B2 -20 -4736 06-12-91 553 crash -35 -4337 07-12-91 554 deferred -20 -58.538 12-12-91 559 284E 50 54 12/13 12-91 F39 27-12-91 574 deferred -53 +19 25/2712-91 F40 23-01-92 601 31374 F2 B2

332

M.R. Sims et al. / SECratesfor the ROSAT Wide Field Camera

Table 4SEU frequency per integrated circuit and SEU rates

' Only RTS and deferred command SEU's are detected from these ICs, rest of chip is not dumped . (see section 5) . Total exposuretime in orbit to 31st January 1992 : 609 days ; number of "active" days : 156; number of solar quiet days : 453; integrated circuitnumber relates to number in terms of position in memory address list, location of the IC inside the CDHS is given in table 1 .

6 Checks on deferred commands were only started 98 days after launch, total exposure time 511 days, number of solar quiet daysexposure 380, active days 131 .

` Time waited average, as amount of memory dumped and checked from this IC varied during mission, 4872 bits 90 days exposuretime, 6168 bits 174 days exposure, 6752 bits 313 days exposure .

outside the contours for the SAA. Events just outside

quacies in the model used . This proviso cannot applythe SAA contours may of course be SAA induced,

to SEU 22 .

either due to errors in the orbital elements, or inade-

Tables 3 and 4 show that SEU's occur throughout

PROTON FLUX CONTOURS E>3MeV Altitude = 600 km

Fig. 1 . Positionable WFC SEUs plotted on a world map overlaid with the proton flux contours for all protons with a energy greaterthan 3 MeV [6] .

180°W 150 , 120' 90° 60' 30' 0° 30' 60 , 90 , 120 , 150, 180°EN

8 75 75°

60° ~rrrr ~-~ ~r f~ ~ ~ 60°,

45'45°

30'30'

15'15'

0.0 .

15 ,15'

30°30'SEU 22

-

45°45° y

60°60°

75'75°

85°S , '~~~ 85°S

180°W 150 ° 120' 90° 60° 30° 0° 30° 60' 90 , 12 150, °E

IC number Startaddress

Stopaddress

CDHS cardnumber

Number ofSEUs

Number ofbitsdumped

Numbersolar flarerelated?

Total rateSEUs[bit- ' day-']

Solar quietrate

1 ' 0800 OFFF 1 6 6 10240 2 1.2X 10-6 1.0 X 10 -1,2 2800 2FFF 4 15 16384 4 1 .5 X 10-6 1 .5 X 10-6

3 3000 37FF 7 9 16384 2 9.OX 10-7 9.4 X 104 3800 3FFF 7 5 16384 5.0X 10-7 6.7 X 10-75 4000 47FF 7 1 6307 ` 2.6 X 10-7 3.4 X 10 -76 4800 4FFF 7 not dumped7 ' 8800 8FFF 1 3 4096 1.2X 10-6 1 .6 X 10-6

unknown 1Total

/average 40 69795 8 9.3X10-7 1 .0X10 -6

the memory, most however were relatively benign anda reasonably small amount of operating time (less than127 h) was lost by SEU induced problems in the timeframe studied. Only 13 SEUs actually caused loss ofobservation time [7] . It is of note that all SEU changesin memory contents are explained by a single bit changeor flip (e .g ., CB to C3 requires a 1 to 0 flip in bit 3) .There are no incidences of multiple-bit upsets .

4. Correlation with solar flares and other data

An attempt has been made to correlate the de-tected SEUs with "active" periods on the Sun . Solaractive days have been defined as days on which a solarflare occurred and the proton event was detected inearth orbit, or a day on which the proton flux meas-ured by the GEOS-7 spacecraft increased significantlyabove background level [8] . This was defined as ameasured flux of greater than 10-1 protons cm -Z s-1sr -1 above 8.7 MeV. A proton flux unit is defined as 1proton cm - ` s - I sr -1 , measured over a 5 min averagein geosynchronous orbit and is the "standard" unit forquoting proton events associated with solar flares .These events are identified in table 3 . Eight SEUsoccured during the 156 "active" days and are labelledas possibly flare related in the last column of the table .Table 3 lists the dates in the format : day or days(/),month, year for both the solar flares, and the days onwhich the GOES-7 measured flux significantly in-creased, during which or close to which, an SEU wasdetected .

To directly associate SEUs with solar flares oneneeds to remove all SAA induced upsets, then test theremainder which occur on active days and those whichoccur on quiet days against a null hypothesis using aChi-squared test . This unfortunately requires a knowl-edge of satellite position for all events, which is for themajority of SEUs is not available. If the system wassusceptible to solar flare induced SEU, a numbershould have followed the large solar flare of the 22ndMarch 1991 . In fact the rate did not change apprecia-bly. The available evidence indicates that the devicesare not susceptible to solar flare induced SEU, andhence the "solar flare related" label given in table 3 isprobably irrelevant or just coincidence . Locatable SEUsoutside of the SAA are probably due to galactic cosmicray interactions . With the available data nothing can bestated about solar flare induced rates .

5. Calculation of in-orbit rates

Table 4 gives the number of SEUs per IC, numberof bits dumped and a derived rate, in-orbit SEU eventrates are given in SEUs per bit per day. The rate is

MR. Sims et al. / SEUrates for the ROSAT Wide Field Camera

calculated either by using the total in-orbit exposuretime or by removing the possible solar flare relatedevents (which may not be valid depending on theprobability of a flare causing one of these SEUs) andthen dividing by the number of quiet solar days . A rateof approximately 9 X 10 -7 SEUs bit - ' day - ' is meas-ured . There appears to be some variation in SEU ratesbetween the different ICs; this is discussed in a latersection . 44 456 bits of WFC CDHS are not dumpeddue to their dynamic contents or non-use in activecode . Therefore, approximately another 24 undetectedSEUs could have occurred, highlighting the need forregular restarting or rebooting of such a CDHS system .

6. Description of the theoretical model

7. Predicted SEU rates

33 3

Upset rate calculations for cosmic ray and solarproton induced SEUs were performed using the Cos-mic Ray Effects on Microelectronics (CREME) soft-ware package [91 implemented at DRA Farnborough.For a given satellite orbit, the flux of cosmic rays andsolar protons is calculated as a function of energy andLinear Energy Transfer (LET), taking into account theeffects of geomagnetic shielding. The susceptibility of agiven device to SEU is characterised by a curve ofupset cross section versus LET, obtained from bom-bardment of the device by heavy ions . Such upset crosssection curves are combined with predicted LET spec-tra to give the expected number of SEUs per bit perday for the device in question .

An upset cross section curve was available [10] forthe SRAM device IDT 6116 . The device cross sectionwas normalised to a "per bit" crossection by dividingby the number of bits in the device . The limiting crosssection was determined by estimating the cross sectionthat the curve approaches asymptotically and a value of3.04 X 10 -' cm' was used . The upset rate productionfor heavy ions is insensitive to small errors due to theflux spectrum, where the number of particles with LETvalues exceeding 20 MeV mg- ' cm - ' is small com-pared to fluxes of particles with a lower LET.

All cosmic ray (CR) and solar flare (SF) inducedupset rates were calculated assuming a spacecraftthickness of the equivalent of 6 mm of aluminium.Other work (see section 8) shows that the equivalentthickness is actually 4.6-6 mm hence the predictedrates are probably very slightly low.

Upset rate calculations were made for a quiet envi-ronment of background cosmic rays only [9], and asevere solar flare superimposed on the quiet back-ground . For the quiet environment a rate of 5 .1 X 10 -H

334

Y

M.R. Sims et al. /SEUratesfor the ROSATWide Field Camera

is predicted whilst for the solar flare a rate of 1 X 10 -5SEUs bit -1 day-1 is predicted.

The predicted error rate due to cosmic rays is oneorder of magnitude lower than observed rates, indicat-ing that the dominant cause is probably trapped pro-tons, as confirmed by fig. 1 .An attempt hasbeen made to predict the upset rate

induced by interactions in the device due to trappedprotons encountered in the SAA. The predictionmethod used was developed by Bendel and Petersen[111, and involves fitting a upset threshold parameter(A : which describes the proton induced upset crosssection as a function of incident proton energy) to ameasured cross section. Unfortunately test data wasnot available for the 6116 device only the SRAMdevice IDT 71256, where the limiting cross sectionused above is 7.62 X 10-7 cm2, giving rates of 7.5 X10' and 1.92 X 10-6 , respectively, for the two casesdiscussed above.

Table 5Predicted SEU rates for trapped protons in SEUs bit -day-1 for the IDT 71256 device

WFC BULKHEADSAND STRUCTURE

WFC DETECTORS

FOCAL PLANEELECTRONICS

Fig. 2. Cross section through "Solid" model used in ESABASE software to calculate shielding and radiation dose for each IC.Calculation allows for each solid surface or volume shown. ICs are located in CDHS near centre .

Tables of total dose test data were used, which giveproton induced upset cross sections of between 4 X

10-13 cm2 and 3 X 10-14 cm2 for an incident protonenergy of 61.4 MeV. Using Bendel and Petersen [11] avalue of 24 to 26 MeV is deduced for the A parameter.Greater confidence could be placed on the method ifan upset cross section was available for higher energyprotons.

Upset rates were calculated using two values of theA parameter and three spacecraft thicknesses, 2, 4 and6 mm of aluminium. An orbit averaged proton spec-trum has been used . Table 5 gives the predicted rates.

As can be seen the predicted rates indicated thattrapped protons dominate the SEU rate . This is will bethe case for the 6116 device, comparing relative num-bers .

In summary the theoretical predictions indicate thatthe SAA trapped proton rate dominates in a quietsolar environment. Exact predictions require protondose data for the real device and data at the appropri-ate energies .

8. Correlation of measured SEU rate with shielding

As noted in section 5 there is some variation in thenumber of SEUs per IC. It was thought that somevariation, apart from statistics may be due to the vary-ing amount of shielding for each IC location . Shielding

Bendel A Shield thickness [mm]parameter 2 4 6[MeV]

24 .0 9.64X 10 -7 9.1 X 10-7 8.6X 10-726 .0 2.96X10 -7 2.8X10-7 2.65X10 -7

wzYU

a

ALTHICKNESS (mm)

"

AL THICKNESS (mm)RADIATION DOSE (PROTONS) RADS

u

RADIATION DOSE (PROTONS) RADS

MR Sims et al. / SEU ratesfor the ROSAT Wide Field Camera

335

vO

z

v

EE

U)wzYUS

Q

103

10 2

10 °10°

101

102

10°

10110 2NUMBER OF SEUS (NORMALISED)

NONFLARE SEUS (NORMALISED)

O1

g

vnO

v

Fig . 3 . (a) Modelled equivalent aluminium thickness and radiation dose for each IC plotted against normalised number of SEUS .Number of SEUS normalised to number per 16384 bits by scaling measured numbers . (b) As (a) but SEU number is "non-flare"

related SEUS .

considerations are only relevant for devices which aresusceptible to trapped proton induced SEUS .

Two tests have been performed on the data, firstlyan analysis of the statistics and second a detailedshielding calculation . If Poisson statistics are assumedit is possible to calculate the probability of obtainingthe measured number of SEUS per IC, assuming amean rate derived from all the data. Table 6 gives theprobability of obtaining the measured number usingthe mean with the appropriate corrections . It is stronglyindicative of some inherent variation between the ICs,with regard to the data for ICs 2 and 4.

To investigate shielding considerations a ("solid")computer model was constructed for use in ESA'sESABASE RADIATION software suite which calcu-lates radiation dose for a given geometry and set oforbital conditions [12-17]. Fig. 2 shows a cross sectionthrough this model, the radiation dose was calculatedfor each IC location taking into account only alu-minium and other large metal structures surrounding

Table 6Probability of obtaining measured SEU rates derived frommean rate and Poisson statistics (corrected for bits dumpedper chip, and exposure time)

the ICs. The program suite derives a radiation dose fora simple (spherical) geometry for a given orbit, which isthen used as input for a program which calculates theequivalent amount of material surrounding a target bya ray tracing method and solid angle calculation andhence derives an actual radiation dose . Only structuralitems close to the rear half of the WFC (where theCDHS is located, centre of fig . 2) were taken intoaccount. No details of the actual circuit boards wereinput in order to make the solid model manageable .Radiation dose and "equivalent aluminium" thicknesswere calculated for one years worth of the ROSATorbit and for trapped i .e . SAA protons only . Figs . 3aand 3b plot the normalised number of SEUS for eachchip verses equivalent aluminium thickness and radia-tion dose in rad for all SEUS (3a) and non-flare relatedSEUS (3b) . The number of SEUS has been normalisedby scaling the data to the number per 16 384 bits, toremove the effect of differing amounts of memorydumped from each IC.

The number of SEUs appears independent of radia-tion dose as all ICs receive the same dose to within10%, and hence the difference in numbers and ratesprobably reflects some intrinsic variation between theICs or fine details of the shielding like nearby compo-nents etc. This latter explaination is thought unlikely asexamination of the board layout shows no large compo-nents close to any of the ICs. Hence, variations in thenumber of SEU's per component type appear possiblefor a given radiation dose . This is not too surprising asthe proton dose versus depth curve has begun to flat-ten out at the energies and depths appropriate forproton induced SEUS . Hence inherent differences be-tween devices will have a greater effect than localshielding, and relatively large variations in radiationperformance between devices are to be expected given

IC Number(reference table 4)

Number of SEUSdetected

Probability of thisnumber occuring

1 6 0 .142 15 0.023 9 0.134 5 0.055 1 0.1 .6 not dumped N/A7 3 0.2

33 6

current device manufacturing techniques and toler-ances.

9. Conclusions

Measured SEU events for the ROSAT WFC appearto be consistent with theoretical predictions derivedfrom device upset cross sections . SAA protons inducedSEUs dominate the measured rates. A variation innumbers per IC is found which does not correlate withshielding and is probably outside the statistically ex-pected variation, hinting at a variation in SEU rate perdevice . This indicates batch testing of devices for radia-tion effects is required to assess inherent variationsbetween devices of the same type .

Analysis of such satellite data is currently domi-nated by poor statistics and lack of test data at relevant(high) proton energies . Radiation analysis would begreatly aided by the presence of; carefully positioned,carefully designed integral radiation monitors on thespacecraft (as is planned for several future satellitemissions); device test data at relevant energies ; andknowledge of the orbit position at which effects occur.The first and third requirements require detection (andsafing) to be an integral part of the system, electronicand software design of both spacecraft subsystems(from which such data is collected) and any SEU orother radiation effects space based dedicated experi-ments.

Acknowledgements

The Wide Field Camera has been built by a consor-tium of UK space research groups from University ofLeicester, University of Birmingham, Rutherford Ap-pleton Laboratory, Mullard Space Science Laboratoryand Imperial College of Science Technology andMedicine funded mainly by the SERC . We would liketo acknowledge the contributions of the many mem-bers of these groups who have been involved in thisproject. Data for this paper has been provided by theWFC operations team based in the UK and Germany.The success of WFC has also been dependent on theefforts of the German teams in building ROSAT and

M.R. Sims et al. / SEU rates for the ROSAT Wile F:zld Camera

operating the ground system, and the USAF and NASAfor the DELTA II launch .

References

[1] J. Trumper, Physica Scripta T7 (1984) 209.[2] M.R . Sims, M.A . Barstow, J.P . Pye, A. Wells, R Will-

ingale, G.M . Courtier, B.J . Kent, D. Reading, A.G .Richards, R.E . Cole, C.V . Goodall, T.J . Sumner, andG.K. Rochester, Opt. Eng. 26(9) (1990) 649.

[3] J.C . Pickel, and J.T . Blandford, IEEE Trans. Nucl . Sci .NS-25 (1978) 1166 .

[4] J.F. Ziegler, and W.A. Lanford, Science 206 (1979) 776.[5] E. Petersen, IEEE Trans. Nuc. Sci . NS-28 (1981) 3981 .[6] E.G . Stassinopoulos, World Maps of Constant B, L, and

Flux Contours, NASA SP-3054 (1970) 82 .[7] G.S Pankiewicz, WFC Quick Look Facility, Max Panck

Institute Garching, Munich, Germany, Private communi-cation (1992) .Solar Geophysical Data Part 1 (Prompt Reports) Na-tional Geophysical Data Center, NOAA, 551-570 (1990-1992).

[9] J.H . Adams, Cosmic Ray Effects on Microelectronics,Part IV, NRL Memorandum Report 5901, WashingtonD.C . (1986) .

[10] P.R . Guttridge, IDT, Device Proton Test Data, privateCommunications (1990) .

[11] W.L . Bendel, and E.L . Petersen, IEEE Trans. Nucl . Sci .NS-30 (1983) 4481 .

[12] ESA, ESTEC, ESABASE version 90 .1 .[13] ESA Contract 6479/85/NL/JS Matra Espace .[14] S.M . Seltzer, SHIELDOSE: A Computer Code for Space

Radiation Shielding Methods, NBS Technical Note 1116,National Bureau of Standards (May 1980). Modificationsto the original program made by ESA/ESTEC.

[15] H.Y. Tada, J.R . Carter, B.E . Anspauth, and R.G . Down-ing, Solar Cell Radiation Handbook, 3rd edition, NASAJPL 82-69 (1982) ; Modifications to original programmade by ESA/ESTEC.

[16] AP and AE series of trapped particle models ; AP8:Sawyer and Vette (NSSDC/WDC-A-R-&S 76-06) AE5:Teague and Vette (NSSDC/WDC-A-R &S 74-03) AE6:Teague, Chan and Vette (NSDCC/WDC-A-R_&S 76-04) AE17 : Hills, Chan, Teague and Vette (undocu-mented).

[17] IGRF (International Geophysical Reference Field) mod-els, see FOS, 62(49) (1981) 1169;Barraclough, Harwood, Leaton and Malin, Geophys. J.R. Astr . Soc. 43 (1975) 645.