-4, ,%

:. NASACONTRACTO_REPORT %__

: NASA,i,<-170909

; LIGHTNINGMAPPERSENSORDESIGN STUDY

&

By L. R. Eaton, C. W. Poon,

: J.C. Shelton, N. P Laverty

and R. D. Cook -

]" TRW Space _ Technology Group :.'

Alil, S_-CR-17O909} LIGII_I_G _&PPBR SEI_S(DB _i84-137.%2 "

" DE$1GN STUDI ¥inal Be_ort (_BW Space -"

Technolog7 La_s,} 15.3 p HC AOS/_F AOI

CSCL 0_ Uncla£ -

G3/47 q260|

" Final Report ._

September 1983 ..

_" Prepared for

" NASA - Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

!

https://ntrs.nasa.gov/search.jsp?R=19840005664 2018-05-14T12:41:52+00:00Z
TECHNICAl. REPORT STANDARD TITLE PAGE

[1. REPNO.ONASA RT CR-170909 12. GOVIalNIq4_NTACCESSIONNO, 3. RECIPIENT'S CATALOG NO.

4. TITLE AND SUBTITLE 5. REPORT DATE

September 1983

Lightning Mapper Sensor Design Study s. PERFORMINORGANIZA G TIONC_E

7. AUTHOR(S) S. PERFORMING ORGANIZATION REPORt

L. k. Eaton t C. W. Poon t J. C. Shelton a N. P. Lavertv and R. D. Cook

9. PERFORMING ORGANIZATION NAME AND ADDRESS liO. WORK UNIT NO.

TRW Space & Technology Group

One Space Park 1. CONTRACTORGRANTNO.

.: Redondo Beach, CA 90278 NAS8-34_4_

13. TYPE OF REPOR', & PERIOD COVERED

• • 12 SPONSORING AGENCY NAME AND ADDRESS

National Aeronautics & Space Administration Contractor Report

Washington, D. C. 20546 14. SPONSORINGAGENCYCODE

15. SUPPLEMENTARY NOTES

Final Report

_ Technical Monitor: Hugh Christian, Atmsopheric Science Division, Systems Dynamics

Laboratory, Marshall Space Flight Center, Alabama

| 16, ARSTRACT

_; World-wide continuous measurement of lightning location, intensity, and time

during both day and night is to be provided by the Lightning Mapper (LITMAP)

instrument. This study is primarily a technology assessment to determine if the

: LITMAP requirements can be met using existing sensor and electronic technologies.

The baseline concept discussed in this report is a compromise among a number of

opposing requirements (e.g., ground resolution versus array size; large field of

-:: view versus narrow bandpass filter). The concpet provides coverage for more than

80 percent of the lightning events as based on recent above-cloud NASA/U2 lightnlng

_. measurements.

?_

,

'! !

!

• 17. KE_ WORDS 16. DISTRIBUTION STATEMENT

Lightning mappec Unclassifted-UnlJ_tted

G. F. McDonough

Director, S stems D cs Lab_

; ,. SECURITVCLASS,..(" _ .., _AS.,.. _ ,_'" --)_ .. P.,CE

Unclassified . Unclassified 155 NTIS IIIrc - Fern $|N (ly t_'

F_ roleby Ni_o_l _ _fm.mtlos Ikmft_,l_i_la_L vbrsb_ BI II

i

i
:1

_CKNOWLEDGMENT

_e work reported herein was supported by the National Aeronautics and

Space Administration, Marshall Space Flight Center, Systems Dynamics Laboratory,

Atmospheric Sciences Division, under NASAContract NAS8-549,1 and under the

sponsorship of Dr. James C. Dodge, Mesoscale Atmospheric Processes Program

Manager, NASAHeadquarters, Washington, D.C.

Special thanks are given also to Dr. H. J. Christian (scientific monitor of

this study) and other NASA and university lightning research team members who

provided technical advice and input to the final documentation.

®
TABLEOF CONTENTS

I. OVERVIEW I

l.l STUDY 1

1.2 DRIVERS l

1,3 CONCEPTSELECTION 2

1.4 BASELINEDEFINITON 3

1.5 FINALREPORTCONTENTS 3

2, SYSTEMSPARAMETERS 5

I 2.1 REQUIREMENTS 5

2,2 DESIGNPARAMETERS 6

2.3 SUM_RY 9

3. SYSTEMSANALYSIS 11

3.1 DAYTIMEOPERATION 11 .

3.2 NIGHTOPERATION 11

3.3 ADJUSTABLETHRESHOLD 13

, 3.4 PARAMETRICTRADES 14

3.5 KEY SENSORPARAMETERS 15 ,

3.6 SOLUTIONSTO SYSTEMDRIVERS 16

, 3.7 SUf@IARY 20

• 4. CONCEPTDESCRIPTION 21 '

4.1 GENERAL 21

4,2 OPTICALCONCEPT 22

4.2.l Requirements 22

4.2.2 TelescopeConcepts 23

4,2.3 OpticalPartitioning 25

4,2.4 FilterDesign 29

4,2,5 Transmission 31

4,2.6 BaselinePerformanceSummary 31

4.3 DETECTOR 32

4.3.1 Selection of Detector Type 32 .

4.3.2 FrameTransfer Methods 37

4.3,3 FocalPlane Configuration 39

4,3.4 RequiredModifications 42

iii
TABLEOF CONTENTS(Continued)

4.4 SIGNALPROCESSING 46

4.4.1 DesignRequirements 46

4.4.2 BackgroundSubstraction 48

4.4.2.1 AnalogVersusDigital 49

4.4.2.2 DigitalApproach 50

4.4.2°3 AnalogApproach 52

4.4.3 ProposedSystem 52

4.4.3.1 DataProcessor 54

: 4.4.3.2 ControlLogic 63

4.4.3.3 Data Formatter 64

4.4.3.4 PowerControl 66

4.4.4 A_ternateDesignApproaches 66

! 4.4.5 OtherDesignConsiderations 67

_- 4.4.5.1 ProductDesign 67

4.4.5.2 SpacecraftInterface 68

4.5 RADIATIONEFFECTSIN SILICONDEVICES 69

4.5.1 General 69

4.5.2 Soft Damage 69

4.5.3 TransientEffects 69

4.5.4 HardDamage 70

" 4.5.5 LITMAPElectronicsHardeningApproaches 70

: 5.0 TECHNOLOGYASSESSMENT 73

" 5.1 CCD REQUIREMENTS 73

5.2 SENSORCONCEPTS 74

5°3 CURRENTLYAVAILABLECCD's 76

5.4 DISCUSSIONWITHMANUFACTURERSREGARDINGDEVICEOPTIMIZATION 7g

5.4.1 Texas Instruments 79

." 5.4.2 RCA ElectroOpticsand DevicesSolidStateDivision 80

5.4.3 RCA DavidSarnoffResearchLaboratories 81 .

5.4.4 HughesAircraftCo., IndustrialElectronicsTechnologyCenter ,

. 5.4.5 WestinghouseAdvancedTechnology Laboratory 83 b

5.4.6 Fairchild CCDImaging 83

' 5.5 CONCLUSIONS 83

6. REFERENCES 85

, iv

Lq )
.

TABLEOF CONTENTS

APPENDICES

A. PARAMETERSPECIFICATIONS 87

A.I INTRODUCTION 87

A.2 FIELD OF VIEW: FOV 87

A.3 AREA PER PIXEL: Ab 93

A.4 SOURCE CHARACTERISTICS 94

A.4.1 SourceArea: As 94

A.4.2 GeometricSplittingFactor: v 95

A.4.3 LightningPulseDuration: TL 9B

A.4.4 PulseTimeSplitting:C 100

A.4.5 SourceEnergyStrength: Es,X 101

A.5 SIGNAL/NOISERATIO: SNR 103

t A.6 BACKGROUND: Ib, ALBEDO 105

- A.7 OPTICS:F/NO.,K 107

A.8 FILTER: _,n 108

A.9 SENSORCHARACTERISTICS 110

r A.9.1 QuantumEfficiency:Q 110

A.9.2 PixelDimensions:6 112

.: P

A.9.3 SensorNoise:c 114

" A.g.4 IntegrationTime: Ti 115

) A.g.5 FullWell Capacity: FW 117

A.lO DYNAMICRANGE 121

A.ll SUMMARY 122

V

®
!

APPENDICES(Con't)

B. PARAMETRICTRADES 125

B.I INTRODUCTION 125

B.2 BASELINEPARAMETERSET 125

B.3 OPTIMISTICSET 129

B.4 LIGHTNINGPULSEDURATION 130

B.5 FILLFACTOR 131

B.6 PIXELSIZE 133

B.7 READOUTNOISE 135

B.8 WAVELENGTH 138

B.9 FILTERBANDPASS 139

B.lO EVENTSOURCESIZE 140

B.ll EXISTINGDEVICES 141

B.12 SUMMARY 145

vi
I

FIGURES

1.1 LIGHTNINGMAPPERTECHNICALELEMENTS 1

2.1 PRIMARYSYSTEMELEMENTSAND ASSOCIATEDPARAMETERS 8

3.1 LASELINEDEVICEPIXELAND FULLWELL REQUIREMENTS 16

4.l CASSEGRAIN 24

4.2 REFRACTIVE 24

4.3 FOURCCD'SWITH INDIVIDUALOPTICSAND SPECTRALFILTERS. 25

4.4 QUADRI-FURCATEDFIBEROPTICS 26

4.5 MINIFICATIONOF FIBEROPTICSIMAGE 27

4.6 IMAGESLICER,TWO WAY 27

4.7 FOUR-WAYIMAGESLICERCONCEPT 28

4.8 FILTERHALFPOWERBANDWIDTHVERSUSMAXIMUMANGLE OF INCIDENCE 29

4.9 SOLC FILTER 30

4.10 GENERICDETECTORALTERNATIVES 33

4.Il MULTI-ANODEMICROCHANNELARRAY 35

4.12 ANAfIORPHICOPTICALSUBSYSTEM 36

4.13 ORGANIZATIONOF 5040 AREAARRAY 37

4.14 WESTINGHOUSE5040 38

4.15 LITMAPSYSTEMCONCEPTUALBLOCK DIAGRAM 47

4.16 PROPOSEDLITMAPELECTRONICSSYSTEMBLOCK DIAGRAM 53

4.17 BACKGROUNDSUBTRACTIONWITH RUNNINGAVERJ_GE 55

4.18 DATABUFFERFUNCTIONALBLOCK DIAGRAM 61

4.19 DATABUFFERMEMORY STAGE FORMAT 62

4.20 MASTERCONTROLLOGIC 63

4.21 DATA FORMATTERFUNCTIONALBLOCK DIAGRAM 64

• 4.22 DATA FORMATTERMEMORYSTORAGEFORMAT 65

4.23 RADIATIONDOSE VERSUSSHIELDINGTHICKNESS 72

5.I SENSORCONCEPTS 76

vii
FIGURES

APPENDIX

A.l Key GeometricDimensionsfor a GeosynchronousObservationVantage B8

Point (B) and GeometricOverlayof SensorArray onto Earth's

Cross-section(A).

A.2 DistanceAlong Earth'sSurfaceas Functionof SensorObservation 90

Angle (A)and Earth'sLatitude(B).

A.3 Linearand Area Variations(forequal steps)Normalizedto the 91

RespectiveDimensionat the SubsatellitePointRelativeto Sensor

ObservationAngle (A)and Earth'sLatitude(B).

A.4 Variationof Earth'sLatitude(fixedlongitude)with Observation 92

Angle.

i

A.5 Distributionof HorizontalCloud Dimensions. 95

A.6 AveragellluminanceWithina Pixelas a Functionof PixelSize 6B 96

NormalizedRelativeto EventSize 6S PixelFocalPlane.

A.7 EffectivePulseDuration(peakenergy/integratedpower)(seetext).98

A.8 MaximumRadiantEnergyvs. MaximumPeakPower (A)and all Strokes(B).

A.g Time FactorEffecton SNR 101

A.IO EnergyDistributionfor Lightning. 102

A.ll FAR vs. SNR. 104

A.12 Probabilityof Detectionvs. SNR. 106

m A.13 Cloud Top Altitudes. 106

A.14 AngularShiftfor InterferenceFilters. 109

A.15 ImageSpreadingin Array Detectors. III

A.16 Resolution vs. Aperture. 112

A.17 Aperture vs. Pixel Dimensions. 113

A.18 FocalPlanaPartitioningvs. SampleTime. 115

A.I9 FullWell Capacity. 118

A.2O DynamicRangevs. Allowable Noise. llB

i

A.21 DynamicRangeas Percent of Full Well. 119

A.22 Absolute Noise Limits Relative to Full Well Capacity. 120

.< A.23 Relattve DynamicRangevs. Probability. 122 :{

"l

,{ viii

.{
FIGURES

APPENDIX(Continued)

B.] ThresholdEnergyfor 8683A as a Functionof IntegrationTime for the126

Baseline:Setof ParametersGivenin Table2.2.

B.2 ComputerPrintoutFormatfor Baseline/Day 127

B.3 ComputerPrintoutfor Baseline/Night 128

B.4 Baselinewith (---)and without(---)Spatialand Time SplittingFactors

: B.5 OptimisticPerformance 130

B.6 PulseDurationEffects 131

B.7 FOV Effects- Day 132

L

) B.8 FOV Effects- Night L33

' B.9 ThresholdDependencyon FOV. Baselineconditionsand Ls= 11 km. 134

_ B.IO Pixel SizeEffects- Day 136

B.ll PixelSizeEffects- Night 136

B.12 AmplifierNoise Effects- Day 137

B.13 AmplifierNoise Effects- Night 137

B.14 WavelengthSelection 138

B.15 OpticalBandpassEffects 139

B.16 SourceSize Effects- Day 140

B.17 SourceSizeEffects- Night 141

B.18 ActualDevices- RCA,TI 142

B.19 Actual Devices - 8683A 142

• B.20 Actual Devices - 7774A 143

B.2I RCA:8683A and 7774A (SID 501D) 143

B.22 Westinghouse: 8683A and 7774A (5040) 144

B.23 Hughes: 86_3A and 7774A (H4068) 144

ix
TABLES

2.1 Statementof Work Specification 5

2.2 Definitionof Terms 7

3.1 ParametricAnalysisSummary 14

3.2 Summaryof SensorConfigurations 18

4.1 RCA SID 5OlD Performance 43

4.2 TI 800 x 800 Performance 45

4.3 ArraySize and Data RateTrades 48

5.1 Summary- Characteristicsof AvailableImagingCCD's 77

-' APPENDIX

I

• A.I ObservationParametersfromGeosynchronousOrbit 89

k

A.2 SNRvs. FAR 105

A.3 Filter/Optics Figure of Merit 108

! A.4 Parametric Analysis Selection Summary 123

B.1 Parametric Trade Results 145 -

__ |

i , ....
I. OVERVIEW

1.I STUDY

World wide, continuous measurement of lightning location,

intensity (or energy) and time during both day and night is to be

provided by the Lightning Mapper (LITMAP) instrument. This study

was primarily a technology assessment to determine if the LITMAP

requirements can be met using existing sensor and electronic

)

technologies. The baseline concept discussfd in thi final

i report is a compromise among a number of opposing requirements

(e.g., ground resolutlon versus array size; large field of view

versus narrow bandpass filter). Th( _oneept does provide

& coverage for more than 80% of the lightning events as based on

recent above cloud NASA/U2 and satellite lightning measurements.

1.2 DRIVERS

The elements making up the Lightning Mapper (LITMAP) sensor

system are illustrated in Figure 1.1. Those elements enclosed

FIGURE 1.1 LIGHTNING I_,PPER TECHNICALELENENTS

. j
within solid lines fall within the bounds of the study statement _

of work (SOW). The primary development risk areas are indicated

with an asterisk and include focal plane preprocessing, focal

plane readout and residual data processing. The remaining

elements enclosed in the dashed lines, (source characteristics,

optics, telemetry and utilization) did not have to be defined in

detail as an official part of this study.

The prime scientific drivers are rapid frame time coupled

with high spatial resolution and oper tion in a night and high

background day environment while maintaining a low energy

detection threshold. These scientific requirements translate

into the engineering requirements of very high data rate and

! pixel by pixel background subtraction. Additional technology

drivers identified during the system trades were: focal plane

array type and size, narrowband filter and a wide field of view.

1.3 CONCEPT SELECTION

As with any application, there are multiple approaches that

can be taken which must then be evaluated against schedule, cost

and risk factors. A minimum cost, low risk approach for LITMAP

was selected as the baseline design from which parametric systems

.' trades were made. An approach using a single mosaic array could

i be implemented but it would have up to severa] tens of kilometers

- undetected strips where the mosaic elements are butted together.

Operationally, slight field of view directional adjustments could

provide the desired full coverage. An alternate approach for a

P

single optical train is to use ftoeroptlc field splitting. Both

of these approaches face a large field of view (FOV) versus :*

narrow band filter technololy problem. This technology problem

can be solved by either reducing systems performance requirements

(25A bandwidth rather than 5A) or higher development risk I

(blrefrintent instead of interference tilter). Thus, there Is

latitude to implement alternate varlatJons of the baseline

concept as driven by flnal user consideratio.s, trades and

desires.

!

2
1.4 BASELINE DEFINITION

The low risk baseline sensor concept discussed in the

following sections is built around existing imaging CCD arrays

which require minimal design and development. Since the optics

aperture is only around ten centimeters, separate telescopes are

used to minzmize the FOV of each array thus permitting the use of

state-of-the-art narrow band interference filters. The focal

plane must be partitioned to handle the data rate, but RCA has

suggested to TRW an approach which leaves the focal plane sensors

intact and only moUifies the output mux, hence minimizing

development cost and risk.

1.5 FINAL REPORT CONTENT

Section 2 summarizes three sets of data: a. LITMAP design

goals as extracted from the statement of work (SOW); b. baseline

design parameters and; c. an optimistic parameter set which

should be attainable with only a slight increase in risk. The

optimistic parameters set could be pushed even further if a

development effort is implem nted to combine the best attributes

of the various technologies and manufacturer's techniques.

Details defining these parameters are provided in Appendix A.

: Section 3 highlights the system parametric trade

conclusions. The supportive details for this section are

provided in Appendix A.

Section q provides the concept selection and baseline

; concept description. The objective was to provide sufficient

concept definition to permit design trades, t-chnology

assessments and flight development costing.

The technology assessment is given in Section 5.0 where

device availability, cost, schedule and complexity are evaluated.

Primary emphasis was placed on the focal plane array with

parallel evaluations for the optics, narrow band filter and

advanced processing techniques being performed under separate

IR&D activities

The flight hardware development and manufacturing cost are

covered in a separate volume.

3
1 OP_IC-ffU_LP_CE IS

2. SYSTEMS PARAMETERS OF POOR QUALITY

2.1 REQUIREMENTS

Exhibit A entitled "scope of work" (SOW) for the lightning

mapper (LITMAP) sensor design study provided specification goals

and lightning characteristics as a general guide for the

technology assessement of large array sensors and background

handling techniques. The primary LITMAP program objective is to

provide measurements of the energy or power of individual

lightning strokes along with their time and location of

occurence. These measurements are to cover the full disc with a

90% probability of detection and a false alarm rate of less than

10%• A summary of these goals and characteristics is provided

in Table 2•I.

From preliminary analysis, the primary system drivers are in

three areas: very high data rate resulting from full time, high

resolution coverage coupled with a few milliseconds of framing

time; background clutter during daytime operation due to solar

TABLE 2.] Statement of Work Specification

JEER ]TEN VALUE UNIT CATEGONT CORREWTS

_uroe Size _urae Size-Disaster 5 km Cherzeterletie Typical Cell Size

Resolution 5 kn Gaol Fully Illuminated Cell

Field of View Goal Full Dtee& Comus

_uree Power Dynamic Nen|e 107-1012 Watts Goal Tote1 Optics1 Power ,

Power Distribution 10T Watts Characteristic 9OS Greater Than

_reshold 107 Watts Goal 50S Prob. Detection

Statlobioa _teotlce trrie_snoy 90 Percent Goal of AI__I Events

Fells Rlarl Rate (10 Percent Gool of Torsi Events

Pules Duration (1 Nileee Cheraeteristi¢

• _her Wavslen|th e683 An|sireN Charenterietio NI (1)

7771 An|sirens Chereeteriatle OI (1)

Data Dissemination (S Minutes Goal Near Roe1TL_

Rink-Cost/Schedule Goal Keep an low el Possible

_tput LI_AP Location Gaol Nap Or L, I, T. ,

Intensity

Time

scattering from clouds; and the dynamic range of events. These

factors are intercelated and must be weighed carefully when

5

PRECEDING PAGE BLANK NOT FILMED

i®
ORIG._'IAL' 'J"

OF POOR QUALITY

optimizing the system. The overall objective of the system

trades and concept definition is to show that daytime lightning

detection can be accomplished.

2.2 DESIGN PARAMETERS

The primary focal point for a systems analysis of an

optical/detector system performance capability is the signal to

noise ratio (SNR). This SNR relationship relates the various

parameters of the detection system through which system

optimization can be pursued via parametric analysis and

performance trades. The SNR is defined as follows.

The number of optical signal generated carriers, Ns, in one

sensor element (pixel) is given by:

t

2 Es k Ns = _ &s K Q _ v _ •

,, 4 (f/No)2 EpHOT

The terms are defined in Table 2.2. Two factors have been

added to accommodate the statistical lightning event splitting:

v

spatial (v) and temporal (_). These factors will be _iscussed

later.

4

The noise generated carriers come from several sources

including those due to the signal (Ns) and the background (Nb),

(e.g. solar illiminated clouds). The detector also provides

several contributions including amplifier (Namp,rms), and dar_

• noise (Ndn , rms). These sources are assumed to be uncorrelated,

hence will add in quadrature to give the total rms noise Nn of:

, |

_,i, "n " V "s + "B + "_;+ Nd2. " V N_ _ I + (Ns + N_p+ Nd_)/NB

b

:L

' "V'NB Vl+c

'i where the latter form is useful during parametric analysis

6

i

ill II n " n I ir --
®

_1_ ORIGtMALPAGE iS

OF POORQUALITY

whenever <<I. The number of carriers created by the solar

background irradianee is given by:

NB = x _p2 K Q IB Ti AX A1b •

4 (f/No)2 EpHOT

TABLE 2.2 Definitionof Terms

OPTINI S'TIC

IR_ IaWllcxn (cx_o_ _z_ olscaxProa

t &lb 0.7 -- Albedo (scene reflectance)

i AID 121 x 100 12 Lflhtnln s Source Area

121 x 10 s m2 Bech|ro_md Aires

: C 3.00 s 10 8 Bin Vacuum 81_ed of LLl_t

4 x 10 -6 J/ai-sr Bourne Radiance (goal)

_ot g.20 x 10"lg J Znurl_r per Photon - hcl_

rtl 1 -- Effective Pll

b 0,83 x 10-34 Je Planks Conetaut

18 300 Wlm2-ur-_s Suckllroued (solar) Irradlance

E 0.4 0.8 Optical YrunmLsslon

q 0.36 0.S Detector Quantum Efficiency

T I 4 x 10 -3 lee Intqretlon TIDe

Tb 5.4 s 10 -4 eec |vent Width FWHN

40 3 s 10 "5 • Source lm_ Size

_p 3 n 10 "S 4 x 10 -5 • Detector Pixel Size

n 0.15 LtzhtninE Spectral Efficiency st

x 8.883 z 10 -7 • Center Wnvelensth

6_ S x 10 "4 MI Spectral Ehsndpane

v 0.02 -0 Geometric Split Factor

C (I - TL 13Ti ) .. Temporal 8pllt hctor

Nsmp ISO e'8 Resdout Noise

R 3.56 x 107 • Sensor Altitude

The signal to noise ratio is defined as the peak signal to

the rms noise and is given as:

SNR -= Ns/N

= Ns/ V NB (1 + c)

which upon rearranging becomes

V _T _rl_'T P Ei (1)_ _L/3T_ 1

dEpNDT I8 A|b"

• ® ® ® ® ®

SOURCE OPTICS/FILTER SENSOR SOURCE/BACKGROUNDTIME OTHER

FILL STRENGTHIN PHOTONS NOISE

FACTOR SOURCES

7

®
I

ORIGINAL PA_E IS

OF POOR QUALITY

The factors in the SNR expression can be grouped in a number of

ways, one of which is illustrated above with the segregation

being based on the physical origin of each term (c.f. Figure

2.1). Factors 1,2 and part of 6 are source/object related. Item

3 _ primarily optics/filter related; and 4 and 5 are dictated by

-_en_,)rcharacteristics. Items 6 along with the total pixel count

FIGURE 2.1 PRIMARY SYSTEM ELEMENTS AND ASSOCIATED PARAMETERS

A° P_V Array 81ze

|m I Q

Iplio t " •

IJCIIfI OPTICI! 8IISOR PNOCISOIWG

Albedo -i

IB T!

ABIPLxOI

IIACIOIlUOUI_

" and sensor array partitioning quantifies the post detection

electronic processing bandwidth.

Each term in the SNR relationship is discussed and

quantified in Appendix A and summarized in Table 2.2. With the

. practical constraints defined, performance trades can be made

with the o_,_ective of optimizing the design based on a given set

- of crit_-ia (c.f Table 2.1). 1

_ince a desired SNR is a priori known, i.e, it is derivable !

.i_ fr_:_ the desired average false alarm rate (FAR) and event pulse

_'i curation (TL) , _he chosen approach was to solve the SNR equation

! for the equivalent attainable threshold energy. The parametric

' trades :he,_ focused on determining the practical combination of

8

_..4
parameters which will satisfy as many of the requirements in

Table 2.1 as possible.

The following section summarizes the results of these

parametric trades.

2.3 SUMMARY

Table A.4 summarizes the results of each parameter evaluated

in Appendix A. This set of data provided the baseline

configuration input to Table 2.2 as well as the "optimistic" data

set. Two (often neglected) terms, geometric and time splitting,

have been included in this analysis. As shown in Section 3 and

Appendices A and B, these factors can influence the threshold

l

performance capability by a factor of two or three, a very

significant consideration when the total system is pushing the

technology limits as LITMAP is.

!

|

!

I

i
ORIGINAL PAGE IS

3. SYSTE:4S ANALYSIS OF POOR QUALITY

3.1 DAYTIME OPERATIONS

As stated in Section 2, the SNR is determined by the FAR

specification. Thus the approach used for the parametric study

was that of determining the attainable spectral threshold energy

for each set of parameters. The SNR relationship can be inverted

to give the threshold energy, NEst , for a discrete spectral line

providing:

nEst--- SNR _'AB _'_ _X Z(f/NO)1 VEpHoT IB'Alb" "_t

_'_S %/ ¥'K _ 6p (I_TL/3Tt)

L

Section 2 and Appendix A discuss and define the limits for

each of the terms in this relationship. The above form of the

expression is most useful when the sensor system noise is

dominated by solar irradiance for which case £-.0. As amplifier

noise becomes more prominent, the threshold value is modulated by i

the _ term. A practical worse case condition would be when the

non-solar noise sources equal the solar contribution for which

" _=i and _ : 1.41,i.e., a forty-one percent rise in the lower

threshold limit relative to the condition where solar is the only

major "noise" contributor.

_ Two groups of variables appear in the E s relationship" the

4 linear more influential parameters (SNR, f/No.) and the those

whose influence is somewhat suppressed via the square root

j dependancy. Background/source area (modified by the geometric

splitting factor) and the time (with time splitting) lie

somewhere between these two groups.

3.2 NIGHT OPERATION

r By definition of SNR

!

m

11 PRECEDING PAGE BLANK NOT FILMED
ORIGINAL PAGE IS

OF POOR QUALITY -- i i

_,_- .,o/--'V"_., + ,o+_,,

Day:

The relative day-to-nlght signal electrons (for constant SNR) is

given by:

!

i

L

But near threshold Ns << N2amp + Nso I in day

'- and Ns << N2amp at night, hence

Nsd/Nsn -. "_/N2AMP ,+ Nsol / NAMP

If Namp .__/3 then

-. Nsd/Ns" = 4Nsol(intr/NAMp (ms) •

If N2amp = NsoI (a probable worse case design condition)

NsD/NSN _ Q-2 = 1.41.

AS indicated previously, the nighttime threshold would be about

q15 lower than the daytime. This small improvement is due to the

large magnitude o£ the amplifier noise contribution.

i I£ N2amp _ Nsol/IO then

f

NSD/NSN =_/Namp : 3

that is, the nighttime threshold would be a factor o£ three lower

:!

! 12

• V _Jw-:_ -- - - J
than the daytime value. For a value of Nso I : 50OK, the

amplifier noise would have to be less than 236 electrons rms.

This noise level appears to be attainable. An upper limit of 500

e rms has been projected (Hughes, 5 MHz operation, custom device)

and would give a NsD/NSN = 1.73, i.e., a 73% improvement for

nighttime performance over daytime operation.

If a significant improvement in nighttime operation is

desired, the non-solar background contributions to the overall

noise must be minimized and an automatic threshold adjustment

incorporated into the electronics.

3.3 ADJUSTABLE THRESHOLD

The results of the system analysis of available technology

against LITMAP requirements indicate that the solar background

! will be a severe limitation during the daytime. Reaching the

goal of 4 u _I/m2-sr at the 90% probability of detection with less

than I0% FAR is very demanding. An array designed with large

pixel size (hence large aperture) and large full well (FW) (hence

. longer integration time) will drive the system capability in the

right direction.

There is another technique which would help optimize the

threshold setting versus data output rate. The average sensor

array threshold can be adjusted based on event throughput rate.

The throughput rate would be set at a value which would be

! compatible with system constraints such as the down-link rate,

i onboard storage capability, etc. As the background energy

decreases the throughput event rate will decrease for a fixed

threshold setting. The threshold can be lowered until the event

rate is back up again (a lower limit may also be desirable).

When the storm activity increases, the threshold would be

automatically adjusted to avoid overloading the down-link, etc.

In this way, maximum sensitivity can be maintained which, under

some conditions, may not meet the flase alarm (FAR) or

probability (Pd) specifications but would provide the maximum

amount of data consistant with the systems constraints. The data

could be further reduced on the ground using sophisticated

13
®

i

ORIG_AL PAGE 15

OF POORQUALITY

analysis techniques. For scientific purposes this may prove to

be very beneficial in that aspects of the FOV nonunlformities

could be further suppressed, hence permitting extracting

information which may have been lost if a fixed threshold had

been utilized. This type of threshold adjustment is being used

on a pixel by pixel basis for the newer satellite surveillance

systems. A combination of fixed minimum plus an adjustable

threshold level would contribute toward optimization of the

LITMAP sensor system.

3.4 PARAMETRIC TRADES

The primary functional dependency of the detector threshold

L setting on most of the parameters can be deduced from the Es

relationship given in Section 3.1. Once an absolute baseline

• value is established, effects of other parameters such as F/No.,

_ , and SNR can be determined by direct scaling (linear,

i

TABLE3.1 Paramtrlc Analysts Summary

pJ_T ._e_osx_ sus_v

Im_ltN •

ldo@l (B_JOIIN) X Bvent eplittiq (tiJmlopoeteel) is

lmpoFtsnt,

OPteUstic X lues_eood porforumaee etuJ be sttaisod

st seine iscroue is rlok ud pooOible

out.

_L I hloe du_stio8 _d Ctsm oplittimJ

drteea the iateqtzatloe tUmo toesrdl

Nv0r_l edlltNeNds.

: Ptll _tetor • J 0it sedir 91emtq gtll trove • tbSmed_-

tmld dog_datioe of up to • tutor

ot ebeett 8.

Ptml 8too X • taUlJO pizel aim to _ry (_PJ0 urn).

llflfl p J • dmplltter mlN muoe N Item eiptfte_tlF tmlee tim Nlsr ooelrtlmtioa

it otdmleod liSM porto_ is

: (I.e., NfdP (rum) _gool (too)It).

' 1 • hlsttge mNla|tudoo of tll_Je spectral

ltm are seeded.

a_ I Junto beadp_m 10 Neeomr7 (IS_).

Ooueee |tie I I 890et rune sise _ be better

quutttted.

:' _omNeeisl Devteoo I I_totta8 emmeeeisl devieeo _aa 8ppeeeeb

tim LI55J_P begeJJmo porfoJm_ee. 0pttmllstiN Md pertitJoeiJ8 ere reqwtred.

i DA
!

i square root etc.), For parameters such as time and spatial

i splitting, the effects are not as easily visualized, hence

j' parametric computations, graphically presented, become very

useful. Table 3.1 summarizes some of the parametric analyses

performed and Appendix B contains further discussions.

3.5 KEY SENSOR PARAMETERS

Although a number of parameters can be varied (e.g., F/No.,

SNR), the two primary adjustable parameters for the sensor array

are pixe_ size and full well capacity. As indicated in Appendix

A, these two parameters are somewhat related in that the larger

pixel sizes have larger full well capacities. The relationship

between these parameters and the minimum resolvable lightning

threshold setting is given by:

FW(min) : Bp2(mln) = (DR Ns(mln ) + Ns2(mln)/SNR 2)

max signal max noise

6p(min) : B"I (DR Ns(min) + Ns(mln) 2/SNR2)

Ns = SNR 2 Z/2 + SNR qSNR 2 Z2/4 + N2am p

Z = (I Alb/nEs) 

B = 1.1 1015(max), 0.37 1015(mtn)

Figure 3.1 provides_E s versus pixel size and Full well

requirements for several dynamic ranges. The shaded area for a

100:1 dynamic range accounts For the variability of existing

devices. This variation is primarily related to device geometry

and can be varied by design for a given configuration. The

dashed lines illustrate that for a given threshold, larger pixel

sizes (and FW capabilities) are required for larger dynamic

ranges. Between 30 to 50 micron pixel dimensions with a full

well (FW) capacity of 106 electrons are required to permit a

source or _ lO-6J/m2-sr to be detected t'rom geosynohronous orblt

using the baseline system specifications in Section 2. Smaller

lS
!

ORIGINAL PAGE IS

OF POOR QUALITY

FW capacities result in high thresholds. Thls figure illustrates

that the NASA/Galileo 15 micron plxel configuration is not

compatible with the baseline LITMAP full dtse requirements.

Existing Westinghouse, Texas Instruments (TI) and RCA devices

come closer to the desired specifications but 3t111 require

partitioning to accommodate the short frame time requirement.

3.6 SOLUTIONS TO SYSTEM DRIVERS

,. The parametric trades Indicate that the system drivers man

be eased by implementing some or all of the following (full disc,

gaosynehronous orbit assumed):

P

! 16
Background Clutter

o Narrow band optical + less background noise: <5A

filter

o Large aperture in SNR: Plxel > 20 um 

o Larger full well _ permits larger

capacity aperture: >5 105 e's

o Smaller ground _ larger aperture,

resolution lower background: < 7 km

o Background removal _ eases dynamic range: frame

subtraction

Dynamic

o Larger plxel _ larger full well: >>20 um

'. Data Rate

o Device selection + high clock frequency: >5 MHz

o Partitioning _ lower rate per port: >16 ports

o Larger integ-ation _ longer frame time: >4 msec

time

o Taylored output readout noise: <300 e/pixel - 

amplifler Q

These factors are discussed in this report

" The degree to which the full potential of the imaging CCD

, technology can be realized depends on a number of factors. The

" _ain sensor factors are pixel size _oupled with total pixel

B

coLmt. Other subsystem constraints include wide FOV cot, oled with

narrowband filter and larKe pixel count coupled with short

' framing time.

?

; As dt,_cussed in this report, there are a number of acceptable

approaches which can be taken to implement the LITMAP instrument

, with minor modifications to existing devices. Table 3.2

summarizes some of these configurations. The first entry Is the

baselined approach discussed in this report from which all

parametric trades were made. It provides the full disc coverage

: wlth the best performance/cost ratio with the large-FOr/narrowbandwidth-filter combination betnl a key driver.

t

-|

17
18
!

Custom imaging CCD arrays with single optical trains are

covered in the second group of configurations. Although this

group is within the present technology, the costs and risks are

hi_her due to the requirement for larger CCD arrays which are not

now in production and also due to the requirement for a SOLC

birefringent filter to handle the large FOV and narrow bandwidth

requirements. The sensor and filter can cost a factor of 2 to 3

above the corresponding baseline items. This cost/risk pena]ty

may be deemed warrented by the ultimate users.

The third group contains two approaches which &re not deemed

suitable or practical for the full disc LITMAP application. The

last group does not provide continuous full coverage but can

provide _ar continuous coverage of specific storm areas using

unmodified existing devices with a considerable decrease in

electronics.

Section B.11 evaluates a few existing commercial imaging CCD

devices which contain the necessary on focal plane frame storage.

, The results for a 4 millisecond integration time are:

Device uJ/m2-sr Device uJ/m2-sr

RCA 9 (x2) RCA 11 (x3)

Tt 201 12 (x3) Westinghouse 12 (x3)

. Westinghouse lq (x3+) TI 201 17 (x4)

_ TI 202 20 (xS)

Hughes }

The (x2) refers to a threshold which is about a factor of two

above the NASA measured 4 uJlm2-sr. A (x2) to (x3) would be

-t tolerable if the signal dynamic range is I00:1 or greater.

,i

_ The large pixel, RCA device gave the best daytime

sensitivity. Array partitioning and taylorlng of the output

amplifier for high speed operation are needed. The TI devices

P

operate at TV rates (> 5 HHz) but array partitioning is still

required to get the desired framing time. The Westinghouse unit

is small (100 x 100 x 2) and would require a mosaic of 64 to 100

$9
units to handle full disc coverage from geosynchronous orbit.

This unit would be suitable for a Shuttle or U2 operation.

The RCA or TI sensor devices would provide the minimum risk

and cost (< IM) for implementii_g the LITMAP instrument using

multiple optics.

Special larger imaging array devices can be produced which

match the LITMAP requirements more closely with an increase in

development risk and cost (> 2M + risk). For a single optical

train, the large FOV may require a birefringent narrowband filter

which may apprJach a million dollar development cost.

TI has had experience in producing the larger imaging CCD

arrays followed by RCA. Both of these companies are proparing

arrays for the commercial TV market. Hughes, Fairchild and

Westinghouse also produce imaging CCDs. Hughes has provided a

preliminary estimate to TRW for the development of sensor arrays

which are compatible with the baseline configuration discussed in

this report (c.f. fourth entry in Table 3.2).

3.? SUMMARY

The primary conclusion based on a) 905 of the above cloud

data is above 4 u J/m2-sr and b) the intensity dynamic range

between 105-905 is at least 100:1 is that significant lightning

data can be collected from geosynchronous orbit. Existing

commercial devices can approach the LITMAP performance

requirements of sensitivity (pixel slze/aperture) and dynamic

range (full well capacity). The integration and cycle time can

only be attained within the present technology via securing

partitioned sensor arrays with the total elements per port on the

order of 10,000 to 40,000 pixels. This partitioning is quite doable but requires custom devices.

2O
®

!

4. CONCEPT DESCRIPTION

4. I GENERAL

This Section deals with baseline hardware selection and

concept definition. Trade discussions are briefly repeated to

support the selection of a given hardware implementation

approach. The details leading to the selection of the baseline

set and the parametric trade results are summarized in Sections 2

and 3 with the bulk of the data given in Appendices A and B

respectively.

The previous discussion of lightning characteristics,

scientific requirements and signal/noise considerations have

L

illustrated the challenge surrounding the design of an instrument

"- to map lightning from synchronous orbit over the full disk of the

-" Earth. The following sections discuss a proposed concept which

satisfies the lightning mapper objectives.

The four major considerations are:

o The optics, including the telescope, bandpass filter, any

.: possible image splitting, and the detector at the focal 4

plane. In the following section on optics, the issue of :_

interference versus birefringent filters is discussed and

. three schemes for partitioning the image among multiple

" detectors will be weighed.

: o The detector. This section is primarily devoted to

possible CCD formats with alternatives discussed briefly.

The emphasis will be on creating a device which satisfies

"i the requirements with minimal modification to existing

designs. An argument is made for using multiple sensors

: to cover the full earth disk.

o Signal processing includes that part of the instrument

which takes the raw signal from the detector, decides

when a lightning event has occured and returns a digital

signal proportional to lightning intensity. The

discussion will cover the background suppression

technique, choice of analog or digital background memory,

and a method of adaptively setting the lightning

detection threshold to allow improved nighttime

performance.

i

o Data formatting. For every lightning event, a data word I

is created into which is packed the lightning intensity,

i

location, and time of occurence. These data are saved in

a small local memory for telemetry at a low data rate.
4.2 OPTICAL CONCEPT

The LITMAP performance criteria impose significa_t design

requirements on the optical suOsystem. Thus the optimi_ation of

the total sensor system requires an interactive trade between the

optical and sensor constraints. The optical design effort was

not a part of this NASA study but was evaluated under a separate

IR&D activity. The following provides an overview of this

effort.

4.2.1 Requirements

The desired field-of-view is a square 17.4 degrees on a side

covering the entire earth disk. For a subsatellite ground

resolution of about 13.6 km, an instantaneous angular FOV of 3.8

x 10 -4 rad is required. This roughly corresponds to 800 x 800

elements.

Use of existing arrays limit the size of the focal plane to

2.4 cm with a pixel size of 30 um. The effective focal length of

the system must then be 7.8 cm. Concurrently, the radiometric

calculations for the system indicate that an entrance pupil of

15.2 cm is necessary. The resultant system has an F/No of 0.5.

Although computer simulations indicate that such a design is

possible for monochromatic light, in practice it would be

prohibitively expensive. Section 2 defines some of the optics

parameters which provide a workable compromise between scientific

requirements and technology constraints. The filter transmission

: bandwidth of the system must be less than 5A to obtain sufficient

: background supression.

" The table below summarizes the above ideal requirements and

also the baseline specifications presented in Section 2. The

following sections provide a description of some of the

alternatives considered for the filter and optics.
Parameter Ideal Baseline

Aperture 15.2 cm 9.7 cm

Effective Focal Length 7.8 cm 9.7 cm

Total FOV 0.303 rad 0.256

(17.4 deg) (14.9 deg)

Instantaneous FOV 3.78xi0 -4 rad 3.21xi0 -4 rad

Spectral Bandwidth < 5 A 5 A

Spectral Line Center 868 _ 8683

4.2.2 Telescope Concepts

A catadioptic front end telescope (c.f. Figure 4.1) is

typically used for this type of application. Clear apertures up

to 15 cm were evaluated via computer. The image from the

telescope of Figure 4.1 was minified 1:4 using a lens clu_ter.

All goals can be met except for the spectral bandwidth. The best

that an interference filter could do in such systems is 30A

bandpass which is a result of the large FOV requirements. The

filter constraints are discussed later, i

An all refractive approach to a wide field telescope was

also designed (c.f. Figure 4.2) for which 80% of the energy falls

within a blur diameter corresponding to approximately one half of

a resolution element. This approach also provides most of the

desired features except for the filters. Again the F/No of the

internal telescope combined with the FOV limits the bandpass of

interference filters to more than 20A.

To accommodate the filter bandwidth constraint, various

schemes for splitting the FOV after the primary images were tried

and are discussed later. In all the splitting schemes the relays

were mounted on an axls of symmetry for the partial FOV resulting

in angular shifts of the filter bandpass on the order of 5 A.

23

®
OF pOOR QUALITY

; FIGURE4.1 CASSEGRAIN

,C

; F/3 OBJECTIVE . I/4.5 TRANSFER LENS

' FIGURE 4.2 REFRACTIVE

The simplest approach to the FOV/bandwtdth problem is to

, ! design a refraQting telesoope with 1.28 red (7.4 deg) total FOV.

Then a oluster of four of these 10 om diameter telesoopes would

cover the entire FOV (e.t'. Figure 4.3). Several advantages are

i apparent. First, existing 400 x 400 CCD arrays oan be easily

:" used at the four independent fooal planes, ieoond, filters may

ii t
I OR!GINAL PACE IS

OF POOR QUALITY

be placed over the apertures or at intermediate surfaces in the

primary lens cluster and have maximum wavelength shifts of only 5

A for a required bandpass of about 3 A if the center wavelength

and filter shape are placed properly. The design of the filter

will be discussed in the section on filters. This latter

approach (4-cluster) was selected as the baseline concept.

4.2.3 Optical Partitionin_

The lightning mapper is a system in which the optical field

splitting clearly improves performance and the feasibility of

fabrication,.

The three approaches selected for inclusion in the study

were:

[

I. Four telescopes,

2. Knife edge image slicing, and

3. Fiber optics.

Each was shown to be completely feasible but possessed differing

degrees of complexity and risk.

o The four telescope approach was chosen as the baseline

design as mentioned previously. The choice was based on

minimizing the risk associated with system development.

4

• /

- t

FIGURE 4.3 FOURCCD'$ WITH INDIVID_L OPTICS AND$PECT_L FILTERS,

i 2s

i

]

b 
ORIGi!_kL P;,_ IS

OF POOR QUALITY

The alternate approaches are as follows.

o Galileo Electrooptics indicated that _ quadrifurcated

image plane 10 x 10 cm could be fabricated with 20 micron

spatial resolution, The primary image plane Figure 4.4

would be divided into four implemented focal planes each

4.8 cm on a side. Depending on signal processing needs

each focal plane could be imaged onto an 800 x 800 array.

A further division into 16 separate channels would allow

use of 16 600 x 400 arrays to fill the fiber optics area.

This would achieve twice the ground resolution compared

to the quad-telescope and hence enhance the threshold

capability of the system. The alternative to using more

detector arrays is to relmage the output from the fibers

onto the 400 x 400 arrays with a 1:4 mlnlflcatlon (Figure

4.5) as in the basic telescope. Only a single collector

is required wlth either 4 filters or a single wide FOV

filter.

!

o The image slicer is a knife edge mirror in the focal

plane of the primary optics (c.f. Figure 4.6). Since

this only divides the images into two pal ts additional

1:1 relay lenses and knife edges are required for the

final division into 4 fields (Figure 4.7). Each field

must be minified and imaged as before and utilizing

either 4 filters or a single wide FOV filter.

OPTICS

: IMAGE PLANE

•, I

- FIBER

_, BUNDLE

1/4 OF

I FOCAL

,. / PLAN

OlrrlCAL

-" IMAGE

PLANE

- :

"

t

!i

2' '

" FIGURE 4.4 QUADRI-FURCATEDFIBER OPTICS

26

i

J

i

i,, i ii i ....
EI_D OF OF POOR QUALITY

FLIER OPTICS 1:4 RELAY , LENS BUNOLE

__i_.__ _____ DETECTORS

FIGURE 4.5 HINIFICATION OF FIBER OPTICS IHAGE

FOCAL PLANE

! -

q

I RELAY LENS "

.a.. ! /

I

' SOLC FILTER _ jFIELD LENS

_ .,,KNIFE EDGE

FIGURE 4.6 IHAGESLICER, TWOWAY

._ _

.! 27
OF pOOR QUALITY

ETC

I

I

COLLECTOR

ETC... / MIRROR

x

-<-

L _

_ MIRROR

1:4 LENS

1:1 RELAYLENS

SLICER

MIRROR

LENS

FIGURE4.7 FOUR-WAYZHAGESLTCERCONCEPT

i

;_ 28
_._.,_'., _/_= "_

OF POOR QUALITY 4.2.4 Filter

The threshold performance of the LIPMAP, when the scene is

sunlit, pivots on background radiation rejection. Consequently,

the spectral bandpass of the system must match the line shape of

the Nitrogen line at 8683 A as closely as possible.

Unfortunately, with interference filters and birefringent

filters an increase in the cone angle and/or field-of-view for a

system increases the effective bandpass of the filter. The ideal

situation would be to have all the light incident on the filter

at a single angle. The limitations of the two filter types were

investigated both theoretically and with potential manufacturers.

Multi-cavit_ Interference Filters The two cavity

i interference filters provide both a narrow spectral bandwidth

• with reasonably steep slopes and good out of band blocking. By

_hifting the filter toward the short wavelength side of the line

{i.e., < 8683 A) the filter bandwidth may be minimized. The

filter will be designed for the specifics of the optical design.

Figure 4.8 shows the wavelength shift as a function of angle and

the approximately minimum filter bandpass attainable for the 4-

o

: cluster configuration.

2

g

_o- i

i

....... i

o i I I I

0 2 4 6

ANGLE OF INCIDENCE |DEG)

FIGURE 4.8 FILTER HALF POMERBANDWIDTHVERSUS

IqAXllqJIqANGLEOF XNCIDENCE

29
O_i,_i:=ALPAG_ :3

OF POOR QUALITY

It is evident that mean angles of incidence greater than 4

deg will not allow the _ystem bandpass requirements of 5 A to be

met. The peak transmission of the 5 A filter may be as much as

40%. The interference filter was chosen for the baseline

configuration.

Birefrin_ent Filters The classic filters in this category

are the Lyot-Ohman or Sole filters. A Sole filter was chosen for

analysis since it is much more 3ompaet for the same FOV and

bandwidth.

The Sole filter can easily achieve a bandpass from 1.5 to 3

A over incidence angles of _ 10 deg. Consequently a single

filter may be used even in a wide FOV collector optics train to

L suppress background radiation. The estimated peak transmission

of a 5A filter for LITMAP is 25%. Figure 4.9 illustrates the

design of a Solc filter. The use of Solc filters in a narrow FOV

telescope is simplified since the filters are smaller and the

angles of incidence are less. Under these conditions it would be

relatively easy to achieve 1.5 A bandpass.

_ BIREFRINGENT

_-_'_- PLATES

 ji.

"_ FIGURE 4.9 SOLCFILTER

3O

........................................... J_
®'

!

4.2.5 Transmission

The optics wlll be antireflection coated with "V" coatings

on each surface. These coatings will assure an inband

transmission of 0.99 per surface. For the baseline design the

overall transmission would be <0.99)22 e 0.8. When coupled with

an interference filter transmission of 505, the system

transmission will be _ 405. In the case of the Solc filter the

transmission will be approximately 225.

q.2.6 Baseline Performance Summary

The system performance of the baseline optics design is:

Clear Aperture 9.7 cm

t Effective Focal Length 9.7 cm

: Pixel Size 320 urad

m

Resolution < 140 urad

Fields-of-View (q) 7.q deg x 7.4 deg

Filter Bandwdlth < 5 A

Transmission " 405

Filter Interference

Configuration Four cluster

i

J

w

31
|

4.3 DETECTOR

Detector selection is a key element of the LITMAP design

process. It is here that the twln sets of constraints from the

optical side and the electronlc sldp converge, and it is from

here that the logic of the design must flow, upstream towards the

telescope and the physlcal appearance of the instrument and

downstream to the signal processing and data formatting. There

are some fairly general arguments that lead to the choice of a

CCD as a detector, and some specific arguments that indicate the

use of multiple CCD's to cover the ful! Earth disk. These are

discussed in the following sections.

4.3.1 Selection of Detector Type

First, the minimum signal of interest, the worst ease solar

. background, the size of a pixel on Earth and the satellite

altitude are given. Selection of the pixel size at the detector

fixes the focal length of the telescope. Placing a reasonable

limit on the numerical aperture, say f/l, fixes the telescope

diameter. Physical and manufacturing considerations described in

Sections 2, 5, and Appendix A, constrain the optical tilter

bandwidth and losses. At this point in the loEic Lhe number of

? signal photons available to a detector that corresponds to the

minimum lightning event is fixed as is the number of photons per

second at a detector pixel due to the maximum solar background.

For a detector ptxel size of 30 um, an optical bandwidth of 5A

and an overall optics transmission of 0.4 these numbers are 3952

photons of signal and 1.5 108 photons/see of background. It is

the high magnitude of the photons per second for solar baekKround

,, that eliminates whole classes of detectors.

For a fixed telescope f/number, changing the detector pixel

size changes the telescope size. The number of photons received,

_I both of sisnal and background, is proportional to the square of

:! the detector pizel size. For the LITMAP arplieation_ photon I

statistics are the limiting factor :n attaining a useable signal/noise ratio and that th particular the instrument can be

:_ made to work with a plxel size of 3_ micrometers and not with a

32

a__.. i __ i III i ,111
pixel size of 15 micrometers (c.f. Appendlx B). This result will

have important consequences later.

Figure 4.10 shows a family tree of detector alternatives.

"FP" on this figure stands for focal plane. The first level of

classification of sensors divides clocked or integrating

detectors from random event or non-integrating detectors.

Integrating detectors, e.g. CCD's or vidicons, have a definite

frame time: incoming photons free charge carriers which

accu_.;ulate until the next periodic readout time. Non-integrating

or random event detectol3 read out only when an event occurs. A

one-dimensional example of this would be a photomultiplier in a

photon counting configuration or (to sense lightning events

instead of photon arrivals) any single detector/preamp followed

by a high pass filter and pulse discriminator.

• LIPMAP

SPECS

• , [

! 1

L_ I [ ' I "

LOCKED SYSTEM RANDOM EVENT

! ' "

',. l , I J " 1

!_ "_1 ' I l_ ! i I

:! OMMERCI L CT ADVANCE CTD LECTRICAL OPTICAL

: • OFF FP • SOME FP • FPPROCESSING • FPPROCESSING

PROCESSING PROCESSING • ELECTRICAL • OPTICAL

'i READOUT READOUT

-I [ ELECTRO- OPTICAL

"HYBRID"

• FP PROCESSING

• FP READOUT

OPTICAL: LOCATION

ELECTRICAL: INTENSITY

FIGURE 4.10 GENERICDETECTORALTERNATIVES

33
!

I

f

A true two dimensional lightning event detector with the

background/lightning separation performed on the focal plane, is

not now available and a significant developaent effort would be

required to provide it. Much effort is currently being expended

on the background suppression problem, particularly for infrared

arrays, but no devices are planned with the size and frame rate

needed for LITMAP. The following detector concepts were

evaluated for use with off-focal-plane background suppression.

CTD: Charge transfer devices (CTD's on the chart) and

vidicon-like devices are integrating devices. Silicon target

vidicons have similar optical and detector properties to CCD's

; but do not have the capability of multiple parallel readouts to

keep the data rate down and are not in general suited to

millisecond frame time applications. The only charge transfer

device that might be considered for LITMAP besides a CCD is a

: CID, or charge injection device. ACID, with its ability to

.p

randomly access pixels, might be useful in a lightning mapper

that only looked at a small, electrically steerable portion of

the field of view, but its higher noise makes it inferior to a

straight CCD for full time processing of the full Earth disk.

MAMA: A two-dimensional example of a photon counter is the

- Multi-Anode Microchannel (MAMA) system (Figure 4.11). It

' consists of a photocathode, a microchannel plate electron

multiplier and two orthogonal sets of stripe anodes connected to

coincidence logic. The MAMA system is an extremely effective 2D

photon counter, but it counts photons, not lightning events and

the maximum counting rate per pixel of less than 650 per second

is many orders of magnitude short of the 220 million per second

:! background rate.

!

t _. _....... j
ORIGINAL PAGE |_

OF POOR QUALITY

FEEDBACK-FREE

MICROCHANNEL PLATE

COINCIDENCEANODE

ARRAY

UPPER PLANE

CODING ELECTRODE

UE

PHOTOCATHODE

QUARTZ INPUT-FACE SUBSTRATE ELECTRODE (*,2000V)

FEEDBACK-FREE

MCP (C-PLATE)

LOWER PLANE

CODING ELECTRODE OUPUT-FACE ELECTRODE (OV)

OUTPUT CHARGE PACKET UPPER PLANE

(*,106 ELECTRONS PER PULSE)_ /CODING ELECTRODE

"--..' ./c-75v)

LOWER PLANE ",,,=-¢c'_ _. ..-_L__---,

CODING "L_I_. ;_.,_;_/_."/_._/.7_f- .'-_---"_--.- SiO2 INSULATING

ELECTRODE ("75) _'_'QUARTZ_... -- LAYER

SUBSTRATE

FIGURE 4.]l MULTI-ANODE MICROCHANNEL ARRAY

Anamorphtc: The anamorphic concept has an appealing

sZmplicity (Figure 4.12) and consists of two or possibly three

linear arrays each with a cylindrical lens. A vertical or

horizontal stripe from the field of view is focussed onto a

single detector element. Each detector element has a preamp,

high pass filter (to perform the background supresslon) and

threshold circuit. The (digital) signals from the two axes are I

correlated to sense the location of a lightning strike. Because !

each detector element sees a whole stripe's worth of solar i

background but only a plxel's worth of lightning, the

35

E
I

URiGi;_,:_PAGE L IS

OF POOR QUALITY

_ DETECTOR ARRAY

(AMBIGUITY RESOLVER)

_ CYLINDRICAL FOCUSING

OPTICS

BEAM SPLITTER

• f

DETECTO R AR RAY

(ELEVATION)

L RICAL FOCUSING

• OPTICS '- MODULATOR

DETECTOR ARRAY

(AZIMUTH)

FIGURE 4.12 ANNIORPHIC OPTICAL SUBSYSTEM

signal/noise ratio required for this scheme to work is quite =

high. This high signal to noise ratio could be attained in a

near earth (e.g., Shuttle) orbit but is not possible at

. synchronous orbit. It is difficult to suppress even one plxel's

solar background from synchronous orbit, much less 800 times

that.

o From the above brief discussion, the most promising

detector for full Earth daytime lightning coverage from

; geosynchronous orbit is a CCD. These devices are now

discussed further.

:i 36
OR_G!NAL PAGE IS

4.3.2 Frame Transfer Methods OF POOR QUALITY

Frame Transfers Figure 4.13 shows schematically how a

typical CCD is organized and Fisure q.14 is a photograph of a

CCD. This CCD is a frame transfer device; there is an imaging

area which is placed at the detector focal plane and there is a

separate image storage area. Photoelectrons accumulate in the

imasing area durin8 an lntesration time, then the entire image

(stored electrons) is rapidly shifted intact to the image storage

, |ELECT " OUTPUT

,- bNJECI_,,_ INPUTMUX " FET

'

1

.... J3--;

--J4--

--JI-- y

-- J2-- /

' -- J3- |

100 x '100 -- J4-

' IMAGING _1v

AREA _

I00 x 1I0 IMAGINGAREA I

• _1'__ ....

'

_ ,

CROSSHATCHING

DENOTES LIGHTSHI O J

IELECT OUTPUT

INJEC1• _I_ OUTPUTMUX " FET

FIGURE 4.13 ORGANIZATIONOF 5040 AREAARRAY

37
FIGURE4.14 WESTINGHOUSE5040

section. The image is then read out sequentially, pixel by

pixel, a line at a time during the next integration time. Thus

: during an integration time one frame is being formed in the

imaging section and the previous frame is being read out from the

" image storage section. A typical shift time for frame transfer

devices is 0.3 milliseconds. This is an appreciable fraction of

the q millisecond integration time needed for LITMAP and will

cause some image smear of the stronger lightning flashes when

they occur during the transfer interval.

J

: Line Transfer -There are also line transfer and interline

, transfer CCDs. Line transfer devices have no on-chip memory and ii

: must use an external shutter to block the image while they are i

' being read out. These are useful only in astronomical ,

' I

.::_ applications and the like where the significant readout time can I

I

be tolerated. Interline transfer devices have columns of opaque

_": memory cells interleaved with columns of photosensitive cells. [

• _ On command the accumulated electrons are shifted sideways one i

- line into the interline memory columns where they are read out :

<"/ :

: 3B i
during the next integration time. This type of CCD has much to

recommend it for high frame rate applications: for frame

transfer, the image moves over only one pixel instead of down a

whole column and so transfers faster in the ratio of the number

of pixels per column. The disadvantage of interline transfer

devices is the lower effective quantum efficiency (0.12)

resulting from having half the sensing area opaque for the frame

storage function. Recently, rows of tiny cylindrical lenses

have been manufactured with the sensor chip to redirect most of

the focal plane light onto the optically sensitive portions

resulting in increasing the effective detection quantum

efficiency.

4.3.3 Focal Plane Configuration

Four CCD focal plane configurations were addressed to

provide an assessment of alternate approaches for satisfying the

LITMAP objective of obtaining lightning data over a full disc

from geosynchronous orbit. These configurations ranged from

severely pushing the existing technology to an approach which

utilizes existing, unmodified hardware. Approach B is the

baseline used for the concept definition and costing tasks of the

study. Briefly these configurations are:

A: Two Chip - 800 X 800 Array - Modified

, This is the most discussed approach using two

, sensor arrays such as the TI 800 X 800 array that is

used by astronomers and by NASA for the Galileo

project. A single simple prism split could be used to

avoid any loss of information along the mating edge.

' No array exists which can handle the data rate

; required. Primary modification of existing devices

" would be array partitioning and the adding of output

,. buffers and amplifiers to provide between 8 to 16

outputs per array, each operating in the 10 MHz regime

,_ to provide a two to four millisecond frame readout

_ time. System analysis indicates that a pixel size

considerably greater than the 15 microns of the

existing 800 X 800 TI device is needed to handle the

daytime background and to provide the dynamic range.

TI has indicated that the yield for such a large device

would be so low that its production could not be

J Justified.

Mosaic: An alternative for the single focal

39
plane is to further sub-divzde the focal plane and butt

the arrays together intc a single mosaic. Loss of

significant information and the problem of getting a

narrow band filter for the single wide FOV are

significant added complications.

Itek/Bell Northern has been able to precisly cut

and butt mount (along two opposite edges) sensor arrays

to form a long continuous array. This technique has

been extended to three sided buttability by several

sensor manufacturers with RCA claiming that they should

be able to do four sided butting with their PtSi sensor

technology. In all cases to date, the dead space is at

least 50 microns. For LITMAP this results in loosing

one to three pixels. The affect on a mosaic sensed

image is the loss of information along strips which are

between 10 to 30km wide at the sub-satillite point.

Depending on the mosaic arrangement and satellite

location, this loss of information could be

intolerable. An operational approach could be used

which slightly repositions the sensor FOV whenever

coverage under the normally missed areas is desired.

Although the mosaic approach could be used to

cover the full FOV with a single optical system with no

: auxillary image splitting, there remains the problem of

_" providing an optimum narrow band filter to cover the

wide FOV as discussed eleswhere. The mosaic is a

viable alternative and must be considered during

-: detailed instrument design.

B: Four Chips - TI, RCA, Fairchild - Modified

_ Optically subdividing the total FOV into four

sections relaxes the FOV and pixel cell size

, constraints to the point of being cost effective and

compatible with the present optical and sensor

technologies. Additional array partitioning is still

required to accomodate the necessary data rate. Such

partitioning, although fairly straight forward, will

still require on the order of a million dollar

development activity with about a year lead time. This

is the baseline approach that is primarily focused upon

in this final report.

The FOr splitting can be done with a single

telescope coupled with four way prism image splitting

or by using four independent telescopes for the four

sub arrays. The required low F/No makes the prism

splitting very difficult hence the four telescope

approach Is baselined. (A mosaic and single focal

plane was discussed in the first configuration above.)

40
C: . Four Chips - RCA - Unmodified

A RCA chip exists which can provide the required

exposure time via the following procedure. The basic

frame time through the existing output port is 17

milliseconds. The image section can be exposed for 13

milliseconds and then dumped to a drain at the opposite

edge away from the output mux. The next 4 millisecond

exposure is transferred to the on chip frame storage

for output during the next 17 milliseconds. The

dumping/output cycle can be repeated giving a 3

millisecond (or any other desired integration time)

sample every 17 milliseconds.

This time sampling approach can provide statistics

on total energy/stroke distribution but there will be

no means of separating first return stroke data from

other data. For an operational system such as a severe

storm indicator, this mode of operation may be

satisfactory. For scientific objectives, the relative

roles of the strokes (first versus subsequent) will not

be retrievable.

A potential variation of this time sampling

" approach would be to, during the readout cycle, rapidly

transfer m rows (One or two MHz rate) to the output

mux which will result in a loss of this data. The next

n rows are transfered and processed at the normal rate

. (sub MHz row transfer). The remaining rows would be :

rapidly dumped, again with the total cycle time being

the allowable 4 milliseconds (or whatever the selected

integration time). The same n rows could be processed

for a period of time (a few seconds would give hundreds

of sampling cycles) then shifted to the next n rows.

Repeat measurements over the same area would occur

every few minutes providing a time sequence for each i

, active area hence providing full coverage on a time-

. share basis. This approach would preserve the

distinction between first and subsequent strokes which

has scientific value.

D: Single (or two) Small Arrays With Mechanical Scan -

Unmodified

; i

., A Westinghouse 2 X 100 x 100 chip exists which can :

provide the frame time required. Since only a few

percent of the total field of view will contain active ii

storms at any time, a smaller array combined wlth a b

mechanical scan mechanism can be used to cover the ;

desired area. A given sub area can be observed for I

several seconds giving a few hundred frames and then ,

another area is measured. On the order of lOG i

positions would cover the full field of view, hence a il

recycle over a given area can be done every few

minutes. Alternately, a given storm can be fixed upon

and followed over a period of time with only an

i occasional scan over the full area to determine if

:_

I 41
i (;Y

other severe storm areas are developlng.

The above summarized four focal plane concepts which can provide

lightning data. Concept B will provide the best full time

coverage at the expense of special sensor development. Approach

D provides an off-the-shelf concept which can provide good storm

coverage at the expense of an added mechanical tracking

mechanism. Approach B was chosen for the baseline technology

assessment.

o Approach B was selected for the concept definition and

• costing tasks of this study.

4.3.4 Required Modifications

Table 4.I illustrates a slgnal/noise calculation for a RCA

CCD type SID 501D. The three left-most columns of the day and

night tabulations on the bottom half of the page provide the

: minimum detectable and maximum non-saturatlng lightning intensity

tabulated as a function of frame time. The units of the

lightning intensity are uJ/m2-sr. These numbers can be compared

I

-" with the MSFC NASA/U2 data taken in the _ummer of 1982:

Lightning I.tensity Percent of Events

(Max integrated Below this

radiance/event) Intensity

: uJ/m 2 sr

1.8 1.6

" q.2 7.2

5.6 12.0 =

7.5 22.3

' 13.3 50.9

42.2 80.2

56.2 89.7

, 100.0 98.q

A look at Table 4.1 shows that a frame time o£ two

milliseconds gives excellent performance at night, loslog only

, about 2 percent of lightning events at the low end and none at

_ the high end. The daytime performance for the same integration

: time is adequate: a loss of 12 pc;cent of lightning events at

_ the low end and again, none at the high end.

The picture with a four millisecond frame time is a little

less optimistic: the system performance is essentially unchanged

6_

42

-4 P
ORIGINALPAG_ ;_;

OF POOR QUALITY

TABLE4,1 RCASID 501D Performance

e • • L|TM/_P S/N * * e

This iea calculatlon of the LITMAP minimum detectat)le slgnal

, and maxzmum non-saturating szgnal e>:presseO zn terms o_ l_ghtnlng

," intensity at the cloud tops in (uJ/m 2 st).

Required S/N ratio = 5.9

Psxel slze on Earth (edge of square) (km) = I0

E_fective source area (l.m_2) _ 100

Average stroke duration (ms FWHM) - .54

Satellite altitude (km) = 35b00

CCD type - RCA SID 501D

*" P_el size of CCD (um) • 30

Quantum ef_Iclency = .28

"_" Reader noise (rms ell) = 150

Full well (electrons) = 250000

Telescrpe _/nueber - 1

Telescope 4ocal length (ce) = I0.68

Telescope diameter (ca) - 10.68

Solar lrradzance (te/m_2 st) = 301

CI_Q 41bedo • .8

Optics transmxlllon, lnCludlng 411ter = .5

Wavelength (nm) = 868.3

Filter Dandwidth (nm)• .b

Day (worst calm lun):

Frame Lightning Pulse Solar Min 8kgrnd Readout Total

Time M1n Max Splitting 8k grnd Signal Noise Noise Noise

(Ira) (uJ/m_2 It) Less (ell) (ell) (rms electrons)

0.25 6.9 1385 0,39 15614 1169 I."3 150 198

" 0.50 5.0 791 0.64 31227 17495 177 lW._ 235

I.O0 4.9 529 O. 82 62455 1737 250 150 294

2.00 5.8 :318 0.91 124909 2283 353 150 387 ,

4. O0 7.5 0 O. 96 249818 30_6 500 150 525

8. O0 10.1 • O. 98 499636 4281 707 150 72b ,_

lb. (X) 14.0 • 0.99 999273 5981 1000 150 1014

: t

• Solar background exceeds _ul1 well capacity.

Nl ght:

b

Frame Lightning Pul le Sol ar fli n Dkgrnd ReadDut Total

Till Plln Plan Splitting Bkgrnd Ill gnal Noi Ilie Noise Nolle

,' (el) (UJ/I"2 It) Loll ills) (ell) (rml electrons) i

O. 2_ B. 3 1477 O. 39 0 903 0 154) 153

't 0.50 3.3 903 0.64 0 . IK'3 0 150 15_ : I • O0 2.5 705 0.82 0 _03 0 150 153

2.00 2.3 636 0.91 0 903 0 150 153 !

4.00 2.2 606 0.96 0 903 0 150 15:

8.00 2.1 _S92 O. qtl 0 903 0 150 153

¢ 16.00 2.1 S85 0.99 0 903 0 150 153

( i

v

' [

! 43 I

•

i p
at night, but in the daytime the CCD is saturated by the solar

background. Even if the full well capacity were higher, the low

end threshold has risen to the point of losing 20 percent of the

lightning events. Hence the framing time needs to be kept as low

as possible and the full well capacity as high as possible (see

Appendix A for further analysis).

Commercial CCD's have frame rates appropriate to TV

applications or slower, i.e. 50 or 60 per second or less, and can

be driven less than a factor of two faster than that before

significant performance de@radation occurs. A frame rate of 250

frames per second (4 millisecond integration time) is required.

The restriction is the speed at which a CCD can be read out which

is limited primarily by the output amplifier. Image partitioning

with the data sent to multiple output ports resolves this

bottleneck. The manufacturers of the sensors have proposed

various schemes for doing the partitlontng (c.f. Technology

Assessment, Section 5).

The electronics to deal with the multiple data streams

emanating from the detector must be made in multiple parallel

versions also. This electronics and how the background

subtraction Is to be implemented are discussed in the next

section.

Table 4.2 is a slgnal/nolse calculation for the device

originally proposed by the Optical Science Center, University of

Arizona, for a ]ightning mapper, the Texas Instruments 800 x 800

array. The threshold levels are above 25 uJ/m 2 - sr due to the

5.4 cm aperture which is determined by the small 15 micron plxel

size. Comparison of thls 15 micron device results wlth the 30

micron device results above (c.f. Table 4.1) indicates that a

large pixel slze Is required to meet the LITMAP requirements

(c.f. Appendices A, B, and Sections 2 and 3 for further details).

J

o Integration time must be kept low _q mtlltsec nominal).

f

o Array partitioning Is required to keep the port

operating frequency within technolosy limits.

o Large plxel sizes are needed to attain low threshold

.: capability.

44

_ IIII[
ORIGINALPAGE IS

OF POOR QUALITY

TABLE4.2 TI 800 x 800 Perfomance

• • • LITFIAP SIN • • •

Th;m iS • calculation 04 the LITI_P minimum detect&Die mlgnal

and maximum non-saturattnO signal expressed in t_mS 04 llghtnAng

; intensity at the cloud tops in (,,Jim-2 st).

; Requ;red BIN ratio - 5.9

P/x•1 size on Earth (edge o$ square) (ks) m 10

E44ecttve IKlurce area (ks*2) a 100

Average stroke duration (dis Irbli_1) * ._4

Eat•liSt• altitude (km) m 3_00

CCD type • Texas Instruments 800 x •00

Ptxel size o4 CCD (_m) = IS.2

. Ouantum e44xcilmcy m .13

RFueladl outwell no;l(_ee]ectrons (rag) eli) - • IOWO00 150

- Telescope 4 Inue/aer • 1

Telescope 4o¢81 length (ca) = ._.4112

Telescope dLametor (cm) m _.4112

Solar irra_Jlance (W/m^2 It) a 301

•_ C1 oud a'- bed• m • 8

'" Optics transmzsslon, tnclu_in 0 4ilter = .S

kJavelength (nm) a 068,3

Fzlter bandN;dth (ha) = .b

Day (_=rst case sun):

Frame Llghtnln_ Pulse Iolar Mtn |kgrnd Readout Total

_- Tlme Iq;n Me, Spl ittf, ng Ikgrnd Signal Noise No;s• Noise

(ms) (uJ/m'2 St} LoSS (mill) (el•) (ras electrons)

' O. _ 46.5 531 1 O. 39 llkbl 9311 43 1_0 159

- 0.50 29.5 3192 0.64 3722 973 61 ISC 165

. 1. O0 24. & 2404 O. 82 7444 | 039 I_ 1_0 17 •

2.00 24.7 2007 0.91 141110 11_i0 122 l_O 19•

4.00 27.11 1610 0.96 _ 1366 173 150 232

• . O0 33. • 982 O. 9• _ 1708 244 150 2119

16. O0 43. qP • O. 99 119101 223B 345 150 379

• • klar background exc08_lll Sull _11 CePKlty.

• NIght :

_ Fr_ Ll 9htni nO Pul _ 8ol •r I% n Ikgr hd RedKimJt Total

Time Nln Re, lllDlitttng llkgrnd |lanai Noise Not_e Nuise

:_ (am) (u_/e*2 _r) loss (eli) (eli) (rN eleCtrons)

L

0.2_ 44.7 _1404 0.39 • 903 0 150 153

O._lO 27.4 330_J 0.64 • 903 • lS0 153

I. O0 21 • 4 2_80 O. 12 0 903 • 1_10 133

2.00 19.3 2325 •.91 • _ • leo 15 _

4.00 11.3 2215 0.96 • 903 • leo 15_,

! O.O0 17.9 2164 O.qffd • 903 • 180 153

16.00 17.7 2140 0.99 • qJ03 • lS0 153

": 45
4.4 SIGNAL PROCESSING

The signal content of each pixel consists of various

components of which e desirable component is the lightning

stroke intensity integrated over the sample time. The unwanted

components consist of integrated background irradlance 3nd the

detector noise. Etch plxel signal arriving from the detector

must be tested for the presence of a lightning stroke. The

detection, extraction and measurement -f the lightning signal

from the detector output data stream is the primary function of

the data processor. The data processor will perform the

following functions:

a. Detect lightning stroke with time and location tag

b. Measure lightning energy

c. Format and return the data

The baselined 800 X 800 CCD imagine portion of the array

: consists of 6.4 X 105 pixels which must be individually

integrated for the presence of lightning at the sampling rate of

q milliseconds or less per frame. Thls transforms to a total

data rate of at least 160 mega samples per second for the total

detector array. This data rate through a simple port exceeds the

present electronic processing as well as the CCD technologies.

The data rate can be reduced to an acceptable level for

processing by using multiple output ports from the detector array

(see previous section). Table 4.3 gives the various combinations

of array size, output port number, sampling time and the

resulting sample rate. Each output port will then be serviced by

a signal processor circuit. The extracted event data from each

data processor, however, can be streamlined into a sin31e data

formatting circuit with reduced data handling rate capability

?-

because the lightning oocurance rate is sufficiently low. This

parallel processinl approach reduces the complexity of the system

wlthov_ sacrificing the performance.

_._.1Deslsn Requirements

The data processor must be desilned to perform the three

46

D
®

FIGURE 4.15 LITMAP SYSTEM CONCEPTUALBLOCK DIAGRAM

|

primary functions llsted in the previous section. In order to _

facilitate the evaluation of various design approaches and

concepts, baseline design requirements were established. The

basellne system block diagram in Figure 4.15 shows the primary

functional blocks for a data processor servicing one of the ,_

output ports of the CCD. The total size and power requirement o£ _

the system is very much dependent on the number of output ports

which results in the duplication of this data processer section.

Based on past experience each data processor is estimated to

occupy approximate half of a 7" X 7" circuit board with about

eighty 16-pin dual-ln-llne packages. Therefore a system with 16

output ports can be fitted into less than one cubic foot volume

which is comparable to other flight systems of similar

complexity. From Table 4.3, a 16 channel system wlth an 800 X

800 CCD device at 4 ms sampling interval is equivalent to an

effective data rate of 10 mega samples per second. A reasonable

power budget based in similar flight hardware, is in the range of

47
)

ORIGINALPAGE IS

OF POOR QUALITY

TABLE 4.3 ArraySizeand Data Ratelrade.

MMY SIZE _ INTERVAL NO.OUlq_UT SNgL£ O/tTAMT£ B_CKEROUNO Iq_lONT INWCESSINET:I_

(nSEC) PO_TS PtR po_r (sPs) sxzE e[R PORT m StaPLE

(sA_ES) (J_c)

MOOI 810 3 16 13.3 x 106 40000 7S

IIM I MJO 3 32 6.7 X 106 ZOO00 1_0

880 X mO 4 16 10 X 106 dl(XX)O 100

810 Xm 4 32 S X tO6 20OOO ZOO

; JO Xm S 16 8 x 106 4OO00 12s

gJl I NO S 32 4 X 106 ZOO00 FJO

m x (mo_ 3 16 10 X 106 30000 100

me X m 3 3z S X 1o6 1_00 20o

OOOX 100 S 16 6 x 106 30000 167

_ 800 I go0 S 32 3 _ 106 15000 333

100 I 100 3 1 3.3 x 106 10000 30(,

!_ 100 I 100 3 4 0.8 x 106 z500

lo0 X 100 S 1 Z X 106 10000 500

:_ 100 X 100 S 4 O.S X 106 _O(X)

;:

30 to 50 watts.

" In summary, the baseline electronic subsystem requirements

are:

I. Data rate - 10 mega samples per second per output port

2. Power - 30 to 50 watts

3. Size - approximately I cubic feet

, 4. Weight - approximately 60 pounds

4.4.2 Background Subtraction

The lightning mapper system is required to detect lightning

flashes during day and night. Since the background conditions

-

vary drastically between day and night and over cloud top, land

and sea, it is necessary to design the system to handle the worst

:I 48

1

l

'
case background illumination conditions.

There are essentially three methods for extracting weak

signals from a strong background. These are spectral, temporal,

and spatial filtering. The spectral and temporal filtering may

be implemented at the optical and analog signal stages of the

process. They may be considered predetection processing

techniques. Spectral filtering is essential to the performance

of the system when the clouds are sunlit. The use of an optical

filter has bee_ discussed in detail in an earlier section and

will not be presented here.

Spatial filtering uses an averaged background level from

some adjacent pixels which is subtracted from the incoming signal L

in order to isolate possible lightning signals. This technique

requires that the background level be relatively uniform over

adjacent areas which often is not a valid assumption. Therefore

the use of spatial filtering for background subtraction will not

: be considered for the LITMAP application.

Temporal filtering depends on the background remaining

static for t_o or more successive frames. Successive frames may

be subtracted to elliminate the background signal from each pixel

: with the drawback that the overall noise will increase by a

factor of _2. A running average technique for the background

, statistically can reduce the _2 factor significantly. Further

discussion of this technique will be presented _n a later

section.

4.4.2.1 Analog vs Digital

The baseline conceptual block diagram of the lightning

, map_, system is shown in Figure 4.15. Temporal background

subtraction is employed. The present frame pixel signal S(i) is

_ , being fed into the background subtraction amplifier. The

_ : subtrahend B(i) is equivalent to S(i-1) (S(i) from the previous

i

frame) plus some contribution from the past due to averaging.

This scheme can be realized by using analog or digital hardware,

i or a hybrid. These approaches will be analyzed against the

; technology and design requirements a_d then, a data processor

system design concept will be discussed.
?=%

f

4.4.2.2 Digital Approach

The digital background subtraction system can be visualized

from Figure 4.15 by installing the A/D converter for the digital

system as shown. The input signals from the focal plane CCD

detector are preprocessed by the A/D to provide digital data.

Subsequent data processing such as multiplication and background

subtraction is carried out in digital circuitry• The three

; important circuit components to be considered in this digital

approach are: the A/D converter, the background memory and the

associated logic circuitry that controls the processing. In

order to develop a feasible flight system the critical factors

that need to be considered are size, weight, power and technology

[ availability.

The A/D converter is a very important component in this

approach. Based on previous o_ta and calculations, the

background irradiance can occupy as much as 20% of the t J full

well capability. IF the minimum lightning signal is taken into

account, the A/D converter will require better than nine bits of

resolution and the background memory will require eight bits for

: background information storage. The A/D resolution and range

: requirement can only be satified simultaneously by flash type A/D

converters which have rather high power requirements (on the

: order of one watt). Lower power devices are available with less

, bit resolution but would sacrifice resolution and therefore can

only be considered as an alternative. Another way to reduce the

A/D resolution is by front-end signal compression using a

logarithmic amplifier. Application of the log amplifier at this

point, however, will induce large inaccuracy in the system where

background signal is a significant portion of the entire useful

, signal range. This inaccuracy is worse at low level lightning "

signals and consequently will tend to reduce the quantity and

quality of useful data. The signal compression technique,

however, will be useful after the background subtraction to

reduce the required resolution of the A/D converter for the

analog approach that is discussed in a following section.

:I The data processing technique that is proposed in this ,

f 50

...... -,|rlm-_ : -- -
system consists of background averaging, limiting, subtraction

and adaptive threshold adjustment. In order to realize these

functions digitally, high speed digital processing circuitry and

components have to be utilized which tends to significantly

increase the power consumption of _r= syztem.

The background storage memory is a digital storage device

and can be a module made up of an array of random access memory

devices (RAM) with the associated logic control circuitry. The

proposed system requires that the signals derived from the pixels

within one frame be processed within the next frame of time.

This results in less than 100 nsec data time per pixel based on

10 MSPS data rate. In order to store the current digitized frame

information, the background memory in a digital system will

, require a storage capacity of 40000 X 8 bits (for 8 bit A/D

- conversion) at an access time of much better than 100 nsec.

Commercially available bipolar or highspeed MOS devices generally

can operate with better than nsec access time for read or write

cycles. A module of 40000 X 8 bit memory will typically consists .p

of 20 4k X 4 bit RAM chips in addition to the associated logic i

circuit for addressing control, and signal processing. When

translated into 16 parallel output channels the total electronics

size, weight and power will exceed the proposed design

requirement by a wide margin. Therefore, alternate methods or

. devices have to be sought.

One method of reducing the per channel power requirement is

to use low power high density MOS type RAM memories. Static RAMS

with 8k X 8 memory size and I/4 watt per chip power dissipation

J

are available but are generally slower. The most straight

i forward approach to oversome the slow speed is to increase the

number of output ports along with the number of data processors.

If the 10 MSPS data rate is reduced to 5 MSPS by increasing the i

number of output ports to 32 for a 800 X 800 CCD array, the

processing time is increased to 200NS and the background memory

size is reduced to 20000 elements per data processor. This 1 I

translates into a total of 3 memory chips and 3/4 watts power

requirement per data processor. The power reduction however, is

51
offset by an additional 6 data processor modules. An alternate

wayof utilizing these lower power, higher density devices is to

multiplex the data to two or more memory modules per channel to

increase the effective storage area at a slower speed. Ho:_ever,

all of this will require additional logic control circuitry and

will increase overall size and power.

To summarize, the digital approach is best if size and power

are not constraints. Since the objective is to design a system

possessing reasonable size and weight, digital method appears to

be a questionable approach. The future trend is toward high

speed, lower power and high density. It is passible that in the

future a system based on a digital app-oach would be more

compatible with the power constraints wk_ile providing the

necessary processing speed.

4.4.2.3 Analo_ Approach

An analog approach to background subtraction can be realized

by storing the previous frame pixel signals in a CCD memory. A

CCD device developed for the focal plane can be used for this

purpose by masking out the photosensitive area. On chip input

: and output ports equal in number to the focal plane output ports

can be implemented. In addition the approach is a cost effective

way of utilizing existing technology. Analog approach involves

preprocessing the time multiplexed analog signal with fast analog

circuitry. This type of circuitry currently exists and

therefore the approach represents a lower risk than the digital

approach. The total size, weight and power requirements are also

substantially lower than for the digital approach. Further

discussion of this technique will be presented below as part of

the baseline approach.

4.4.3 Baseline System

Figure 4.16 shows the proposed data processor system block

diagram. The focal plane sensor array consists of one or several

large array CCD detectors, each with built in multlple-output

ports. The system will be operated in the staring mode with

52
ORIGINAl- PAGE III

OF POORQUALITY

_FOCAL PLANE [ BACKGROUND SUBTRACTION CIRCUIT "_ _'_

CCD DETECTOR I _ I

r---qi

I I_]1 ,I---I f-'_ I---i I_'_R'_7_-- ouTPUT I I [ ,J,_APLITUDE .JMULTIPLIERLII_ . _d_IMULTIPLIER__ / |

| r7 BUFFER I I- "ILIMITER vl k I Vt " _ 1 k r_'_l - / J

# .......

: I I I.cK'

I ...... I_ _,ACKGROUNOL----J II

I_:_iI ,...o_,_o,_ _ I

_ _

i I PRO'CE'_,, C,RCUNG,T I

J INTERFACE

I

I

TF.RERO" S"O'r_GCORONT_OLUJ'NO T.RESHOLODETE"--!CLOTOGR,C

" ,TAT,ON I "" ,f

, -L____. _._... ..I

J

: FIGURE 4.16 PROPOSEDLITMAPELECTRONICSSYSTEMBLOCKDIAGRAM

.-' frame rate in the two to five milliseconds duration to optimize

SNR. Detail discussion of the device and the SNR were presented

in earlier sections and will not be repeated here. Each output

port is serviced by its own analog data processer (DP).

Background subtraction and threshold detection are carried out

within the DP. The subsequent lightning intensity data, along

with the tlme and location tags, will be fed to a single data

formatter (DAF). The extremely low lightning occurance rate

allows the use of a single DAF approach without compromising

system throughput while minimizing system complexity and

53
maximizes system efficiency. This multiple output-port approach

also permits scaling the system (number of output ports) to match

the array size with minimal modification of the baseline system.

Example of this would be a small array focal plane lightning

mapper system with a single DP that could be flown on Shuttle

mission in lower earth orbit. A detailed description of the

system is presented in the following sections. The resulting key

features for the signal processing are:

I. Multi-port detector array

2. Multi-channel analog data processor

3. Background subtraction by temporal technique

4. Background smoothing by running average technique.

5. Post detection digitizing using A/D converter

6. Single data formatting circuit.

4.4.3.1 Data Processor

Ou_gut Buffer. The proposed data processor (DP) as

described earlier is based on an analog approach. The CCD output

signal is buffered and the data sampled and held. This S/H

function is utilized to sample the data at the approoriate time

: to avoid transients and switching glitches. The buffer also

serves to extract the signal from the DC bias voltage of the CCD

output amplifier. Commercially available instrumentation and

operational amplifiers and sample and hold amplifiers can be used

to implement this curcuit. However, at 100 ns processing time

per pixel, the bandwidth requirement for these devices are well

• above 10 MHz. Commercially available devices which can operate

_ to this specification generally have high power requirements

relative to the total system power budget established in Section

' 4.4.1. Consequently, it is necessary to design circuitry based

on discrete components or hybrid elements which also provides

additional size savings.

The buffered signal is fed into the background averager and

• the background subtraction amplifier for further signal

processing. The description and design approach of these

y
functional blocks will be discussed next.

Background Subtraction. The function of the background

subtraction circuitry is to isolate lightning signals from the

background and thus reduce the dynamic range required for the A/D

converter. Averaging techniques are used to increase system

responsitivity and performance. A simplified functional block

diagram is shown in Figure 4.17. Subtraction is carried out in

the subtraction amplifier with inputs S(i) and B(i). S(i) is the

buffered output from the detector and B(i) is a conditioned

derivative of S(i) from previous frames.

The input signal S(i) is fed to the amplitude limiter which

i prevents excessive lightning signal amplitude passing to and

contaminating the background data. The limiter provides a coarse

limiting of the signal pulse with further smoothing provided by

time averaging the background samples. An adaptive feedback

based on the average background is used to optimize for both day

and night operations.

The background averager consists of the multipliers k and

: R(i)

,. S(i) ! MULT ULT _"

• ccOUTPUT o_o BACKGROUND

;, _ JA(i) SUBTRACTION

ADAPTIVE

THRESHOLD ;

.-: BACKGROUND I B(i)

MEMORY I

4,

I

,. j

FIGURE4.17 BACKGROUND SUBTRACTION WITHRUNNINGAVERAGE

i

t

_ 55
l

(l-k), the adder and the background memory. The background

memory is essentially the same type of CCD device used in the

focal plane detector. The function of this device is to store

the analog data arriving from the adder, and to output on a

flrst-in flrst-out (FIFO) basis the stored data for the

background subtraction and for the averaging routine. The

coefficient k is confined between zero and one. In general, the

sum of the two multipliers k and (l-k) should be less than unity

to prevent the system from saturating. The input to the

background memory A (1) is

A (i) : K S (1) : (I-K) B (1)

wlth B (i) = A (i-I) l

where i and i-i refer to present and previous frames

respecitvely.

If K:O, there is a straight throughput with no subtraction.

On the other hand, k:1 corresponds to a simple frame-to-frame

differencing. For 0 < k < I, the subtrahend S(i-1) is a "fading

moving average". Frames are filtered with a time constant

inversely proportional to k, i.e., the relative contribution of

each sample decreases with age. Let S(1) contain some noise NF.

?

Then R(i) will contain some noise NR also given by:

NR/N F : (2/(2-k)) I/2

Some representative values of NR/N F are:

K NR/_ N

: 0 1.000 m No subtraction

0.001 1.000 1000

, 0.125 1.033 7.49 Selected N

0.25 1.069 3.q6 Another ADD for

(l-k) = 3/4

i 0.5 1.155 1.44 Additional ADD

0.75 1.265 < 1

1 1.414 < 1 FIFO simple two

: frame subtraction

56
where N is the effective number of frames being averaged as

defined by

(l-k) N = e-] or N f I/ in(I/(1-k)) .

Conversely

k = ] - e -I/N.

As the number of background frames being averaged increases the

noise approaches that of the original frame (assuming incoherent

noise). Simple frame to frame subtraction (k=1) will increase

noise by _2, hence increasing the threshold setting. Background

averaging (five to eight frames) permits the background to be

removed while minimizing signal degradation.

The overall background substraction circuitry can be

realized in hardware by an integrated rather than a modular

design. The multiplexers k, k-1 and adder can be implemented by

a single summing OP AMP thus reducing the component eount and

therefore power requirements. Further size reduction can be

achieved by hybridization of the entire baekground subtraction

cireuitry (excluding the background memory). It was mentioned

earlier that the wide bandwidth requirement tends to drive up the

power consumption of an OP AMP. It Es therefore necessary to

select the device with the best power to bandwidth trade but

?

still be able to meet the performance specification. Commercial

OP AMP components will be selected where available while

transistor circuits using single supply source will be desig,,ed

when necessary to minimize power consumption.

Another important design consideration is the circuit

timing. The high data. rate requirement provides very little

excess processing time to allow for delay, transients and

settling. Therefore, it is necessary to minimize these timing

errors by optimally applying high speed circuit design techniques

such as minimizing load capacitance, path and lead length, and

transistor saturation.

57
Background Memor Z The background analog memory consists of

several CCD devices similar to those used in the focal plane.

The devices are opaque and have electrical input ports to accept

the analog data, The background memory and supportive circuitry

can be built on a single elrcuit card.

Input and output buffers are required for signal

conditioning and signal level matching. On the input side of the

memory a sample and hold (S/H) amplifier samples the output from

the adder during the allowable time window to avoid transients.

Amplification raises the background signal to a level which

efficiently fills the full well capacity of the CCD memory to

minimize the noise contribution from the analog memory. The

output ports of the background memory have similar S/H

amplifiers.

•- Post-Subtraction Processing. Post-subtraction processing

consists of digitizing the lightning signal and buffering it with

the appropriate location tag for distribution to the data

formatter. Data compression using logarithmic amplifier may be

implemented here to reduce the resolution requirement of the A/D

converter. A threshold detector Is utilized to establish the

false alarm rate prior to digitization thus minimizing the load

on the A/D. The functional block diagram of the post subtraction

processing is shown in Fig. 4.16.

• The primary function for an automatic background adaptive

b

threshold control is to vary the threshold for day and night

conditions, which occurs fairly slowly. The bandwidth

' requirement of this circuit Is estimated to be much less than the

: focal plane data rate. Therefore, the design of thls circuitry

does not present any critical problems. Manual ground control

I circuitry can be implemented by feeding a digitized telemetry

signal through a D/A converter and applying the analog voltage to

the threshold detector circuitry.

; The output of the background subtraction ampllfler consists

of pulses coinciding wlth the occurrence of llghtnlng strokes.

The average occurrence rate Is extremely low. The maximum rate

anticipated is In the order of several strokes per frame (entire

S8

t
focal plat,e). The temporal spreading of a single stroke across

several integration intervals is treated as two lightning events

occuring in different sample frames and, therefore, poses no

difficulty to the post-subtraction processing circuitry. The

spatial straddling, however, will require the post-subtraction

processing circuitry to be able to handle bursts of data packe

associated with consecutive pixels. At 10 MSFS data rate the

post-subtraction precessing circuitry is required to complete

each single event processing within 100 nsec. Hence, the

bandwidth requirement on this circuitry is similar to the

background subtraction circuitry except that it will be in a

quiescent state most of the time due to the low average event

rate. The low data rate also results in a lower powe r level than

the similar circuitry used for the background subtraction.

Data Conversion. The output of the threshold detector is

a one/zero sign a'i to indicate the presence or absence of a

lightning signal. This signal will be used to command _he A/D

converter to initiate conversion upon positive detection of _

lightning signal, and to inform the control logic for timing and

location tagging functions.

£s mentioned earlier, the detected lightning stroke

intensity is compressed by a logarithmic amplifier and a sample

and hold amplifier freezes the output signal from the background

subtraction amplifier. The bandwidth requirement of this circuit

will still be dictated by the high event burst data rate as

discussed in the last section, hence it will be implemented with

operational amplifier technique using discrete components.

The A/D converter performs the final digitizing function on

the log-compressed signal. The dynamic range requirement of the

signal is 100:1. It is necessary to have adequate resolution in

order to extract useful statistical data for the lower lightning

intensities. The log amplifier compresses the intensity data

without sacrificing the low level signal resolution. The lowest

bit resolution of the A/D converter will depend on the loi

amplifier output level. In order to utilize the bit counts of

59
the A/D converter over the dynamic range efficiently, it is

necessary to set a minimum bit resolution level. The minimum

signal level based on a 4 msec frame time and 5A filter bandwidth

is about 1800 electrons. Based on minimum bit resolution £qual

to 600 electrons and full well capacity of 250k electrons, a 6-

bit A/D converter is capable of delivering an adequate

performance. The table below lists the bit resolution for

various compressed signal levels from 6, 7, and 8-bit A/D

converters:

SIGNAL LEVEL RESOLUTION AT STAT£D SIGNAL LEVEL

6-BIT _r_

250k el (Full well) 22828 el 11597 el 58q5 el

110k el 10044 el 5103 el 2572 el

,/ 10k el 913 el q6q el 23q el

1830 el (Min. signal) 167 el 85 43 el

Each add_tlonal bit doubles the resolution capability of the

output.

In order to have adequate slew rate capability, the A/D

converter must have a sampling frequency doubIe that of the data

rate. Presently, off-the-shelf 6-BIT A/D converters ca_able of

delivering this performance are available. RCA and Siemens z,'e

deliverin8 devices requiring less than 112 watt. These devices

(and their next 8eneration spin-offs) will be the primary

candidates selected to meet the deslgn requirement.

Data Buffer. The function of the data buffer (Fig. 4.18)

is to temporarily store the A/D output d_t¢ and the location

information from the control loEic. This information will be

.- extracted by the data formatter for downlink to the ground

station.

As discussed earlier, each data processor (channel) will

interface with some 40,000 plxels during each frame of sampling

time. The location of these 40,000 pixels can be labelled by a

16-blt word. The proposed system will cycle throuE_; the qO,O00

location labels in synchronization with the sequence of signal

6O

4,,D, .... -_
ORIGINALPAGE IS

OF POORQUALITY

A/OCUTPU[---- T _ _

(P8IT BUS)

'_ DATA LINE

r_ (_BIT BUS) I%_ F1F0

i// MEMORY TO DATA FORMATTER

LOCATION ID

FROM MASTER

CONTROL LOGIC _-_

(2 8-81T BUS)

! •

LATC H

THRESHOLD ._ ___

DETECTOR TIMING AND

_= STATUS CONTROL DATA FORMATTER HANDSHAKE

.

FROM MASTERgCONTROL LOGIC

": FIGURE 4.18 DATABUFFERFUNCTIONALBLOCK DIAGRAM

!

processing. When a lightning signal is detected by the threshold

detector, its location label will be latched into the data buffer

memory in 8-bit words, followed by the A/D output word. Thus, I

there will be three 8-bit data words to be stored in the data

buffer for each pixel containing a lightning signal (Fig. 4.19).

Based on the occurance rate of lightning it is estimated that no

more than four events will occur wzthin the same 40,000 pixels in

the same frame. Therefore, the size of the memory required is

about 12 x 8 bit. The speed of the memory depends on the way the

data words are latched into the memory. An 8-blt parallel data

configuration will require an access time better than one third I

of the processing time, or better than 33 nsec. One method to

implement this is to use two low power, hlgh speed CMO$ (such as

J
qptGtNAE PAGE 18

!

_V _V v 'qp, IV _r _v 'qV

A/D OUTPUT

LOCATION - MSB

LOCATION - LSB

A/D OUTPUT

LOCATION - MSB

'•_ 4-BIT " 4-BIT

; FIFO ; FIFO "

£_

I

I LOCATION - LSB

AID OUTPUT

LOCATION - MSB LOCATION- LSB

°

-_. FIGURE4.19 DATABUFFERNEHORY STAGEFORMAT

QMOS) FIFO memories, Due to the low average occurrence rate, the

i FIFO will essentially be at low standby power. The data formatter will ha dle the channel tag and the time tag. Further

discussion of the data formatter function will be presented in a

_ late r section.

62

i
r

ORIGINAL PAGE IS

OF POOR QUALITY

4.4.3.2 Control Logic

The control logic consists of the master control logic

(MCL) and the control logic (CL) circuits associated with the

data processor. The MCL (Fig. 4.20) will perform the following

functions:

i. Generate a high frequency clock pulse for sub-processing

time synchronization

2. Generate clock pulses for frame transfer and output of

the focal plane CCD detectors

3. Generate clock pulses for S/H actuation in the data

processor

4. Generate clock pulses for input and output transfer of

the background memory

5. Generate clock pulses to synchronize the data buffer

6. Generate clock pulses for data formatter

7. Interface with the ground-based threshold detector

adjustment input.

8. Generate location labels to data buffer

• BUFFER/

,- COUNTER TO FOCAL PLANE CCD DETECTOR

.__ SUFFER COUNTER/ _-_ TO BACKGROUND SUBTRACTION CIRCUIT

._ SUFFER/ _ TO BACKGROUND MEMORY CIRCUIT COUNTER

3RYSTA L MASTER

3LOCK _ COUNTER

" 3ENERATOI_ _ _

SUFFER/

COUNTER TO DATA FORMATTER

'

-_ COUNTER BUFFER/ _-_ TOCIRCUIT POST_USTRACTION PROCESSING

, i COUNTER i

' i

• '"'"'°°-'t H !--" i

• " CONTROL DATA LATCH DIA AND THRESIIOLO ADJUST FROM GROUNO BUFFER

; t STATION

E

I •

ii FIGURE 4.20 MASTERCONTROLLOGIC !

1
4'

I ORIG_#JPL'"": _;°

OF POOR QUALITY

The control logic circuits provide buffers for all the clock

pulses and signals from the MCL to the appropriate destination.

The MCL and control logic can be implemented using low power QMOS

devices.

4.4.3.3 IData Formatter

The data buffer functions as a temporary storage of the

lightning i_tensity and location data. The Data Formatter (DAF)

(Fig. 4.21) will extract the data from all the data buffers.

, Due to the low lightning occurrence rate, the data formatter has

a relatively low data rate requirement. It will have the frame

sampling time (milliseconds) to extract data out from the

r buffers. A time tag for each frame sampling period will be

generated by the MCL and latched into the DAF memory.

2_

L

TIMETAGFROM

MASTERCONTROL"-----VL

' ii

' 'll DATABUFFER2 I---'_ OUTPUTDATA

TOTELEMETRY

• _ LINK • MEMORY _ CODE ",• " I _ CONVERTER

4 •

i

' "i " DATABUFFER1S I" --'_

DATASELECT i

l[ : THRESHOLD 1 "-Ib COUNTERAND DOWNLINK

DETECTOR 2 CONTROL HANDSHAKE

STATUS 3 _ i

15 "-I)

IB --)

m

DECODER "r" FROM I

MASTERCONTROL .... I

i

FIGURE 4.21 DATAFORMATTERFUNCTIONALBLOCKDIAGRAM

64
Subsequently, all the data extracted into the DAF memory from the

same time frame will be identzfied by this time tag. The segment

or channel identification will be latched into the DAF along with

its data. A 4-bit word representing 16 channels will be

adequate to identify the channel numbers. Therefore, the total

number of data bits allocated to location identification is equal

to twenty (Fig. 4.22).

TIME TAG BYTE 2

i

TIME TAG BYTE 1

L TIME TAG BYTE O

A/D OUT 1 INTENSITY

• } '

LOCATION MSB LOCATION

LOCATION LSB

CHANNEL ID BYTE . i CHANNEL k2

, A/D OUT 2 INTENSITY AT M2

%

LOCATION MSB

LOCATION M2

LOCATION LSB ]

A/D OUT 1 INTENSITY AT M 1

LOCATION MSB / LOCATION M1 LOCATION LSB J

CHANNEL ID BYTE CHANNEL k1

TIME TAG BYTE 2

TIME TAG BYTE 1 ' TIME TAG OF FRAME n

TIME TAG BYTE 0

FIGURE4.22 DATA FORMATTERMEMORYSTORAGEFORMAT

65
The resolution of the time tag will be equivalent to the

frame rate, which is 4 msec as indicated in the requirement

section. The time tag for a 12 hour counter at 4 msec resolution

will be equivalent to 10.8 x 106 counts or 24 bits. The DAF is

interfacing with the down-link telemetry. At some time interval

the DAF will perform a data dump to the telemetry link, and the

cycle will repeat as commanded. During a high lightning

occurance interval, the data dump will speed up to avoid

overflowing the DAF memory. The byte counter will perform this

function by signaling the downlink interface when the DAF memory

is filling up to a threshold level prior to the programmed data

dump time.

The total capacity of the DAF memory will depend on the data

dump rate and the lightning occurance rate. However, since the

occurance rate is low the memory is not required to be a high

speed device. Consequently, it is possible to use CMOS type RAM

to conserve power.

4

4.4.3.4 Power Control

, The power control circuit converts the bus raw power to

secondary power sources for the electronics. Due to the low

, power allocation it is necessary to maximize the efficiency of

; the power control circuit. Commercial DC-DC converters qualified

for flight generally have low efficiency (50 percent to 65 i

percent). Past experience with flight systems similar to LITMAP

' indicates that it is possible to design converters with more than

85 percent efficiency and will also provide fusing and transient i

: protection for proper operation of the system.

I

4.4.4 Alternate Design ApRroaches

There are various design approaches based on the proposed

i

66

¢ i
i

system described earlier that can offer some power and size

advantages. These approaches, however, will require further

study and analysis when more detailed and specific system

requirements are specified. The following paragraphs summarize

these approaches:

I. The effective data rate bandwidth and power requirement

of the post-subtraction processing circuitry can be

reduced by sharing their work load among themselves.

All the outputs from the subtraction amplifiers can be

fed into a multiplexing circuitry with proper sample and

hold function and routed to several post-subtraction

i processing circuits. This approach will allow the

reduction of the number of post-subtraction circuits as

; well as reducing their effective bandwidth requirement.

This approach is feasible due to the low lightning

occurrence rate.The trade-off is in the complexity of

l the multiplexing circuitry, the timing control and the

location identification method.

2. A variation and subset of the above method is in

multiplexing after the logarithmic compression to reduce

the number of A/D converters. This method will allow

the use of an A/D converter with higher resolution.

This approach tends to increase resolution at the

expense of reducing data rate handling capability.

However, at low data rate this approach seems to offer

advantages in lower part counts and size without

compromising capability.

3. The background averaging technique can be treated as an

option to provide improved system SNR and performance.

There are alternate methods of arranging the various i

" functional blocks to reduce part count and consequently

' reducing the power.

4. Methods to reduce average power consumption of the

: system can be implemented by operating the system during

= severe thunderstorm weather, or selected seasons and

hours of the day only. The system can then be commanded

. to a power-off or standby mode during the quiescent and

• idling period.

m

'_ 4.4.5 Other Design Considerations _

'7 There are other design considerations for the LITMAP system

in addition to the approach and the hardware realization

; discussed above.

t

4.4.5.1 Product Design

, The packaging of this system is important in providing a ,

= !

t 67
sound mechanical support and thermal as well as radiation

protection. TRW has wide experience in desig_,ing flight systems

operating under similar stringent requirements and as such the

design approach will be given the most careful consideration.

4.4.5.2 Spacecraft Interface

The LITMAP system as a payload will be interfacing with the

various capabilities of the bus, The interface design and system

management will be an important design and development task.

Detailed implementation and design will be carried out when

specific design requirements are established.

L

o

58

,|
4.5 RADIATION EFFECTS IN SILICON DEVICES

4.5.1 General

The synchronous orbit radiation environment can produce a

number of effects/defects within silicon based components which

can broadly be grouped into two categories:

CATEGORY PRIMARY EFFECT SOURCE RESULT

soft electronic ionization e, p, gamma free electrons

hard atomic collision fast neutrons displaced atoms

References such as Chapter 6 of Barbe (1980; discuss these

effects. Some brief comments can be made as follcws.

! 4.5.2 Soft Damage

The soft damage is often transitory, but under certain

conditions can result in a cumulative effect, e.g., trapped

charge accumulation at a dielectric interface. Positive bias and

surface devices are most susceptible to this trapped charge

effect. Designs using negative bias and buried channels help

minimize this effect. Gamma rays and a few MeV electrons can

also sometimes cause hard damage referred to as point defects.

The point defects usually are very limited in extent and are not

= of a major concern.

4.5.3 TransienL Effects

A transient effect that must be considered is that of a soft

e_ent producing enough free charge within a given pixel which is

accumulated and interpreted as a lightning event. A radiation

,; dose _ at which the CCD pixel well will become saturated can be :

estimated by

7 = NFW/E7( L + W)A i!

where i

!

NFW = full well capacity in number of electrons t

g = electron/hole pair (ehp) generation rate

, A = surface area of p_xe! accumulation site i

I 69 .

1
W = depth of pixel accumulation site

L = diffusion length

For a buried channel device with NFW = 5 x 105 electrons, L

= 50 urn, W = 12 urn, A = 90 um 2 and g = 4.3 x 1013 ehp cm-3 tad

i (Si) -I, a well is predicted to saturate after a dose of 0.2 tad

i (Si). There are about 10 8 seconds in a five year period. For a

_ 10 6 rad (Si) accumulated dose, this gives about 10 -2 rad/sec,

about 4 x 10-5 rad/frame or about 6 x 10-11 rad/pixel-frame.

{

i Although a single alpha particle could generate 106 ehp or a J

cosmic ray event (> 100 MeV) around 105 ehp, the event rates are

' so low as to be lost in the nominal background induced false

t

alarm rate.

The CCD recovery time T R after a transient pui-_ of ionizing

radiation can be approximated by

¢

TR = g_ VcolI/NFw_

where Vcoll is the well collection volume. For a 107 Hz clock

rate, T R = 10-7 sec. Actual measurements on devices has

indicated that recovery times vary from less the 0.1 milliseconds

p to over a second. Device design and selection (c.f., SIRTF,

: 1978) seems quite reasonable to hold the recovery time to below a

single frame time for these soft effects.

' 4.5.4 Hard Damage

Hard damage is long lasting and results due to fast neutron

collisions within the bulk material causing a displacement of

, atoms from their normal positions in the semiconductor crystal.

z This displacement damage leads to significant decreases in i

_ carrier concentration, carrier mobility and minority carrier

" lifetime. These effects become significant for greater than

: I011, I013, and 1015 neutronslcm 2 respectively where about 50_ of

their energy goes into these "cluster (displacement) defects".

i

4.5.5 LITHAP El.ectronics Hardening Approaches t

L i

Two approaches can be taken in order to tolerate the

I

radiation environment. One is to harden the devices so they can |

7o i

A
withstand the exposure and the second is to shield the devices so

that the effective exposure levels are dropped to a tolerable

range.

Figure 4.23 (Space Systems and Technology Workshop II, 1982)

shows the trapped particle dose as a function of aluminum

shielding thickness with synchronous orbit solar particles

contributing an additional few thousand fads. CMOS

microprocessors, which are currently planned for future missions

are able to withstand up to approximately 105 fads. Local

shielding of 150 mils of aluminum will reduce tile exposure to

below this level, hence permitting 5-10 year missions.

Currently available unhardened CCD's with high sensitivity

and low noise are available which can perform up to about 104

fads radiation exposure. Localized shielding of 300 mils of

aluminum will reduce the expected dose to below this level. This

will allow required performance for missions in the 5-10 year

range.

TI and Hughes have been developing radiation hardened CCD

structures. Presently Hughes has some multiphase, buried channel

devices which perform fairly we. _ up to 3.5 x 105 rad (Si)

(Chang, 1980). The TI virtual phase devices have been tested

(McGarth, 1981) and performed fairly well up to 106 rad (Si). If

these hardened devices are available in LITMAP configurations,

then the shielding requirements can be relaxed. For the

synchronous equatorial orbit application, the spacecraft material

plus a little shielding should provide the necessary reduction in

I

radiation to permit five to ten year lifetimes. Hence, although i

the radiation environment must be considered, it does not appear

to be a serious problem for the LITMAP application which does not

have to survive a low, high inclination earth orbit or a

thermonuclear event.

i

l®

5.0 IMAGING CHARGE-COUPLED DEVICE TECHNOLOGY ASSESSMENT

in considering suitable types of imaging charge-coupled

devices for the Lightning Mapper application, discussions were

held with domestic manufacturers to determine the applicability

of currently available devices and also to determine the

possibility of developing custom devices which would be optimized

for this application.

Discussions were held with bne following respresentatives of

the manufacturers listed:

a) Dr. Dean Collins, Texan Instruments, Central Research

Laboratory

b) Dr. E. D. Savoy, RCA Electro Optics and Devices Solid

State Division.

c) Dr. Garry W. Hughes, RCA David Sarnoff Research

Laboratories

d) Dr. David A. Robinson, Hughes Aircraft Co., Industrial

Electronics Group Technology Center.

e) Dr. Rudolph Dyck, Fairchild CCD Imaging

5.1 CCD REQUIREMENTS

The primary requirements of the CCD focal plane array for

the Lightning Mapper sensor application are:

a) Large format, to obtain wide angular coverage with high

spatial resolution.

b) Frame transfer configuration, to enable rapid transfer

of induced electron charge from the imaging area of the

CCD and thus almost continuous integration of radiation

from the observed s_.ene.

c) High quantum efficiency in the near-infrared range c.r

the spectrum, to permit observation of energy in either

the 777 nm 02 band or the 868 nm N2 band.

d) A high frame rate, in the order of 3 to 5 msee, to '

permit an optical time history of the lightning flashes

to be obtained.

e_ Partitioning of the CCD to use multiple parallel output !

ports and preamplifiers to enable rapid readout of the i

on-chip storage se:tion and high frame rate.

t

f) A large pixel size, with large full well (electro;i !

n !

i®
charge) capacity, permitting use of a lerge optical

system for a given angular subtense of the pixel. The

combination of the large optic and high full well

capacity results in a high S/h ratio.

5.2 SENSOR CONCEPTS

As previously di&cussed in Section 4.3, the CCD focal plane

array and sensor concepts which have been developed in this study

consist of the following (Figure 5-I):

a) Two Large Format CCD'S

Two large format (800 X 800) pixel imaging CCD's

modified to p_ovide on-chip frame storage and multiple

output ports and preamplifiers to enable fast

(parallel) readout and thus a ver7 high frame rate.

b) Four CCD's with One or Four Optic::

Four individual CCD's modi "led by partitio,_ing to

obtain multiple output ports and high frame rate. The

optical fields of view might be combined by optical

beam splitting, or individual optical systems could be

used for each of the four CCD's. The former approach

is difficult to accomplish using an optic with a low fnumber and presents problems in design of the narrowband _pectral filters due to wide field of view of the

q-chlp array. The latter approach eliminates optical

beam splitting and results in a reasonable angular

fieldv J: ",Jew for the narrow-band spectral filters.

c) Four _J_: _. ,ith Intermittent Integration Sampling

Four RCA frame transfer imaging COD's, in either of two

sensor configurations as discussed in b) (above), in

which intermittent sampling of the incident optical

radiation would be accomplished by the use of a short i

Irtegratlon Interval in conjunction with a longer

readout time of the on-chip _torage section. The short

exposure time would be obtained by dumping most of the I

induced charge in the tm_ge section into a guard ring

; structure above the l_age section of the CCD, and using

the remainder of the normal exposure interval for

' integration in the conventional manner. Thus, with

operation at a frame rate of 60 Hz, a useable

integration time of q msec out of the normal 17 msec

integration tire cou'_d be obtained.

d) One or Two Small CCD',q With Small FOV i

One or two CCD's with a single optical system covering

only a sm_ll field of view. Here currently available

CCD's would be used, but only very limited areal

:_ coverage would be obtained with a mechanical storm :

t tracking mode being appropriate, t

• _ -- _ i i i fillii...... J
!

OR,C:._AL t t_ PAGE IS

OF POOR QUALITY CCD #1 ,tl=

4, 8, OR 16 OUTPUT PORTS

400 x 800 PIXEL STORAGE

SECTION 400 x 800 PIXEL IMAGE

SECTION

/ / CCD #2 400 x 800 PIXE'L IMAGE

// SECTION

400 x 800 PIXEL STORAGE

SECTION

NARROW

BAND OBJECTIVE LENS 4, 8, OR 16 OUTPUT PORTS

FILTER

; A) TWO LARGE FORMAT CCD'$

° REFLECTIVE /_

, /

., B) FOUR CCD'sWITH EITHER A COMMON OPTICAL SYSTEM AND SPECTRAL

FILTER OR INDIVIDUAL OPTICSAND SPECTRAL FILTERS

;",. C) SAME AS B) BUT WITH SHORT EXPOSURE TIME OF CCD

't

• |

i. :

!i '

1 - 75
ORIGINAL PAGE IS

OF POOR QUALITY

OBJECTIVE LENS (_X_ " // -,J

AND

/ _ "_/_._,Y __J [ ELEVATION

I"-.1/ / f" ' "'X c.I/

AZIMUTH

; D) ONE SMALLCCD WITH A NARROW FIELD OF VIEW

,- GIMBALL_D ON TWO AXES

FIGURE 5.] SENSOR CONCEPTS

5.3 CURRENTLY AVAILABLE CHARGE-COUPLED DEVICES

Table 5.1 lists the eharacterlstlcs of currently available

imaging CCD's which can be considered for the Lightning Mapper

application. These are indicative of the technological

capability of the manufacturers listed in terms of CCD

configuretion and performance. Some of the devices can be

directly applied to the LITMAP application, while others would

require modification. Each device in Table 5.1 will be briefly

discussed.

f

:, The largest format in a single CCD is that of the 800 X 800

pixel device from Texas Instruments. The device is in a line

transfer configuration, and modification to a frame transfer

configuration with multiple readout ports = .uld be required. TI

is also developing a ;024 X 1024 device for NASA, but this is

still in the early developmental stage. A disadvantage of this

i

! 76

u,i. .......... It J
ORIGINAl..PAGE IS

OF POOR QUALITY

•_ _ _ _ _ _ _ _ ,_, E: _ _'-

I

N _

i

_. _ _ _ _" _. -_ _ o , _ ,-

i •

-=_ _ _ _' _ "_% i_ _ _ " "-= "_

.... _,_,_

_ g" '_" _ne

77

®
CCD is the small pixel size, limited by the size of a device that

can be produced with reasonable yield. Yield is limited by the

degree of purity (number of defects) in the silicon base

material.

Two CCD's are in development by TI for use in commerical

television, the TC 201 (for U.S. standards) and the TC 202 (for

European standards). The primary difference is that the U.S.

version does not use blooming control, eliminating the need for

anti-bloom drains within the imaging format, resulting in a more

contiguous imaging area (higher packing density) and increasing

the effective quantum efficiency. Pixel sizes of the two devices

are comparable.

The SID 501D has recently been announced by RCA, replacing

. the previous SID 53612. The former has an overlapping gate

_ structure and improved preamplifier, while the latter has a

planar gate structure. This device is listed primarily to

indicate the current capability of RCA in this field. This

. device uses a thinned substrate (laminated to glass and backside

illumination to obtain high quantum efficiency. This sometimes

.. causes problems in the near-lnfrared range of the spectrum due to

interference fringe patterns occurring in both the glass laminate i

and silicon which result in high fixed pattern noise. Antireflection coatings and adjustment of substrate thickness may

help resolve these problems. For the LITMAP application,

frontside illumination and thick CCD substrate maybe preferable

to eliminate the problems associated with deeply penetrating

' infrared radiation. RCA has previously manufactured such devices

, for use in CCTV cameras.

c The salient characteristic of the Hughes H4068 CCD is that

of radiation hardness, being processed to withstand both natural

and weapon-induced radiation. This device has been developed for '

use by TRW in star trackers used for satellite attitude control

under the MADAN contract to the USAF Space Division. The

m limitation of this device is that of the line transfer

configuration, without on-chip data storase. However, it is

feasible to add on-chip frame storage to this type of device.

78
The Westinghouse type 4068 was developed under NASA funding

in !977. Although small in format, the device has the advantage

of high quantum efficiency due to the use of SnO gates, which are

very transparent in the visual range of the spectrum. This

device would be applicable for the LITMAP sensor configuration

where only a limited optical field of view is used.

Fairchild is well advanced in development of the CCD 222.

This @evice is used in both commercial CCTV cameras as well as In

USAF military aircraft and helicopters. Considerable Navy

funding has resulted in the development of integrated CCD logic

and driver modules, as well as modules for video processing,

resulting in versatility in available camera configurations. The

primary limitation of the CCD for the LITMAP application is the

use of opaque interline frame storage registers in the image

section, which block out 50% of the imaging area. However, this

limitation might be overcome by the use of a faceplate consisting

of a series of miniature cylindrical lenses precisely registered

to the CCD imaging area, similar to the method suggested by Cross

et al (1982)

5.4 DI2cUSSIONS WITH MANUFACTURERS REGARDING DEVICE OPTIMIZATION

5.4.1 Texas Instruments

The present 800 X 800 Galileo Project CCD uses an amplifier

which is optimized for a slow readout rate (50 KHz) with the CCD

at low temperature to reduce dark current. For the LITMAP

application the preamplifier would be replaced with a wideband

: preamplifier of the type used on the TC 201 and TC 202 devices.

Partitioning the device for parallel readout with multiple

preamplifiers presents no problem. Four, 8 or as many as 16

output ports could be used. Clocking can be performed at rates

as high as 17 MHz without loss of charge transfer efficiency.

However the use of correlated double sampling at this high

readout rate is not feasible, as both sampling and transfer

pulses must occur within the pixel readout interval. Without

double sampling, a noise level in the order of 125 electrons

79
rms/sample should be realized with a readout of 6 MHz.

TI would want to maintain the small (0.6) mil pixel size, as

the large format results in a large chio (2.4 cm 2 including I/O

provisions) and yield is limited by chip _ze. Some reduction in

spatial resolution would be expected due to the roll-off of the

modulation transfer function in the near infrared range of the

spectrum doe to small pixel size. This would be more critical at

the 868 nm wavelength than at the 777 nm wavelength.

For a CCD of this large size the cost of a new set of

photomasks is in the order to $I00K, and total program cost would

be in $IM category. The duration of a development program of

this type would be I to 2 years, depending upon requirements.

k

Interest of TI management in committing resources to the

program would be based upon validity as a business proposition.

TI would appreciate sharing in development of the total sensor

design concept, rather than just accepting procurement

specifications for CCD development, as previous related

experience might be of benefit to the program.

Regarding t.e TC 201 and TC 202 types of devices, emphasis

is being placed upon initiating production for use in commercial

: CCTV applications. Devices are available at relatively low cost

with short delivery times. For this application the TC 201 would

, be preferred due to higher packing factor and quantum efficiency.

These devices could be used in the four-chip focal plane array

configuration with operation in a frame bransfer mode if somewhat

lower resolution is acceptable. Total format for four CCD's

would be 648 X 490 pixels. The TC 202 would provide a total

format of 780 X 584 for four CCD's

5.4.2. RCA Electro Optics and Devices Solid State Division

, RCA has discontinued work on all frontside illuminated CCD's f

at the Lancaster, Pa. facility. All devices are thinned and

backside illuminated. Major products are the SID 50q for CCTV

applications, with small pixels (20 x 20 um) and the SIP 501 with

large pixels (30 x 30 um) for scientific applications. Both

80
would require special attention for this application due to the

use of backside illumination with a very thin (10 um) substrate

which results in severe fringing when observing near-infrared

radiation. However, Dr. Savoy's group is considering development

of a thicker scientific CCD wi'h a substrate thickness of 30 um.

In combination with a higher resistivity material, this type of

device might be more suitable for use with red or near-IR

radiation.

For this operation, Dr. Savoy suggestea the following type

of operation for the LITMAP application. The image and storage

sections and output register of RCA CCD's are surrounded by a

guard ring shaped like a picture frame which can be used as a

drain to absorb unwanted photoelectrons from the image section.

Assuming a normal integration time of 17 msec with a 60 fps field

rate, the image section could be allowed to integrate optical

radiation for 13 msec. Presumably a large amount of charge would

be accummulated due to solar illumination incident on the earth.

This could then be rapidly clocked upward into the guard ring

(drain) and absorbed. The remaining 4 msec could be processed to

obtain measurements of storm activity, and the induced electron

charge in the image section would be clocked into the on-chlp

storage section and read out slowly in the usual frame processing

manner. This method of operation would permit use of an on-theshelf device in early feasibility tests of he LITMAP sensor.

In regard to accepting custom work in CCD development, Dr.

Savoy stated that management at RCA is placing extreme emphasis

on getting the SID 501 and SID 504 devices into production, and

that the Lancaster facility could not accept custom work for the

foreseeable future. This siAuation would probably change in a

year or two.

5.4.3 RCA David Sarnoff Research Laboratories

Dr. Gary W. Hughes was quite familiar with the requirements

for the LITMAP sensor, having previously worked with Dr. Wolfe of

the University of A, izona on this application.

He agreed with the concept of using frame transfer CCD's

81
with multiple output ports. The parallel transfer rate (from

image to storage sections) is limited to a maximum of 800 KHz by

CCD capacitance and driver capacity. Output noise would be in

the order of 200 electrons rms without correlated double sampling

and 35 to 40 electrons rms with double sampling at a readout rate

of 7.5 MHz. It may be possible to use double sampling at readout

rates as high as 10 MHz.

He has been involved in the design and development of the

SID 501 and 504 devices, currently in initial production at RCA

Lancaster. These devices are representative of the largest

format which has been developed by RCA.

If RCA were involved in development of a device for the

LITMAP application, design would be performed at the _esearch

laboratories and fabrication would be accomplished at the

Lancaster facility.

A rough order of magnitude estimate of cost of a development

program of this type would be $700K to $IM, with schedule in the

order of one year. Interest of RCA in bidding on a CCD

development program would be based upon business potential.

5.4.4 Hu_hes Aircraft C__ Industrial Electronics Technology Center

5 Hughes Aircraft Co. has been involved in the development of

radiation hard CCD's since 1975 under contract to NRL, DNA, and

the Special Projects Office of the Navy. To date imag{ng CCD's :

have been developed for both the Navy and TRW (under the USAF

MADAN project) with formats of 324 x 324 pixels in a line

transfer configuration.

Hughes proposes development of a device with a 400 x 800 :

pixel format, with half of the area being used for on-chip frame

= storage. The storage section would have eight output ports, each i

operating at a rate of 5 MHz, _esulting in a frame time of 4 i

msec, At this readout rate the noise level would be no larger

than 500 electrons rms. As with the MADAN devices, the full well {

capacity would be at least 106 electrons. Pixel size would be 20 i

x 20 micrometers and quantum efficiency at 868 nm would b_ 30

!

:i !

82

T
percent or higher.

A budgetary and planning cost estimate was submitted by

Hughes to TRW for development of both imaging CCD's and

breadboard electronics.

5.4.5 Westinghouse Advanced Technology Laboratory

Westinghouse is currently under subcontract to TRW for

development of an improved version of the 5040 imaging COD.

Details of the device are presented in Table 5.1. This device

would be suitable for use in the sensor concepts describ^d in

Section 5.2 d. In conversation with Dr. James A. Hall, it was

: determined that Westinghouse has not developed CCD's with very

large formats, although they would be interested in reviewing

procurement specifications for such a program.

[ 5.4.6 Fairchild CCD Imagin G

The Fairchild 222 has a favorable frame transfer time due to

its interline transfer and cylindrical lens arrays are being used

to increase their detection quantum efficiency. The devices are

used for a number of DoD applications. This device would be

suitable for use in the sensor concepts in Section 5.2c but

discussions for the development of a special device for LITMAP

with multiple ports have not been made. The device is being

. considered for inho_se high frequency evaluation of the LITMAP

, frame substraction concept.

5.5 CONCLUSIONS

Reviewing the requirements defined for the CCD in Section

5.1, we reach the following conclusions:

a) Format Size - The largest format which has been

developed to date is the 800 x 800 pixel device from

Texas Instruments. However, in order to maintain a

reasonable size of the chip to obtain an acceptable

yield in wafer processing, the pixel size is small (0.6

x 0.6 mils). This results in the following:

i o Small pixel size reduces the fuli well capacity,

which due to the use of the virtual phase

: structure is I09,000 electrons for this device

• 8:
o For a given angular field of view for each

pixel, a smaller pixel result_ in a shorter

optical focal length and a smaller lens

diameter. As less optical radiation is

collected, a reduction in S/N ratio results

o The smaller pixel size results in higher optical

cross-talk when observing radiation in the nearinfrared range of the spectrum. Due to the low

absorption coefficient of the silicon, the nearIR photons generate carriers below the depletion

region of the CCD, which migrate laterally to

adjacent pixels. This results in loss of

resolution which will be evident as broadening

of point source images.

Thus the combination of small pixel size, low full well

capacity, and increased optical cross-talk do not make a

large format virtual phase CCD attractive for this

application. None of the manufacturers surveyed were

, willing to consider development of a CCD with both a

very large format and very large pixel size.

On the other hand, the TI TC 201CCD looks attractive for

this application due to large pixel size, large full well

capacity, and high quantum efficiency. Format size when used in

the frame storage mode is 328 x 245 pixels. This device will be

applicable in a sensor configuration where multiple chips are

used.

b) Frame Transfer Configuration_ Partitioning, _nd Hi_

: Frame Rate - All of the manufacturers agreed that in

or-7-_ 1_--operate in a frame transfer mode at a high

frame rate, partitioning of the on-chip storage section

' of the CCD would be required. In the cas_ of the 800 x

800 plxel TI CCD, partitioning into 4, b, "" as many as

16 sections with a corresponding number of output

preamplifiers was suggesteJ.

c) Recommended Focal Plane Array Configuration - Only Texas

• Instruments has-'a's-_-emo--o'ns'_ra-c-t-e apabilit '_'-tyhe of producing i

very large CCD's (800 x 800 pixels). However, tnese

have disadvantages as discussed in item (a) above. TI,

RCA, and Fairchild have demonstrated the capability of

producing devices with a format in the order of 250 x t

300 to 400 x 500 pixels, with plxel sizes ranging from

": 12 x 18 to 30 x 3_ micrometers. A practical approach to i

the focal plane array for the LITMAP sensor would be to

use a mosaic of chips of this type with either a common I

: or independent optical system.

a

84
6. REFERENCES

Mcr,rath, R.D., J. L. Harris, M. M. Blouke, B. W. Keenan, H. H. Hosack

Final Technical Report for Draper Radiation Tolerance Imager Development,

CNN00030'-79-C-003_(for Charles Stark Draper Laboratory, Inc.),

Texas Instruments Incorporated,Central Research Laboratories, Dallas,

Texas, September 1981.

SIRTF, 1978: Shuttle Infrared Telescope Facili Z, Appendix A. Focal

Plane Instruments and Requirements Science Team, Ames Research Center,

April l, 1978.

Chang, C.P.: CCD Radiation Hardenin9, CN N00173-73-C-0185 (for NRL).

Hughes Aircraft Company, Newport Beach, CA.

Barbe, D. F., Editor: Charne Coupled Devices (article by J. M.

Killiany), Springer-Verlag,New York, 1980.

Cross, E. F. et al: Optical Techniques for Increasing Fill Factor

of Mosaic Arrays, SPIE Proceedings, Vol 366, Modern Utilization of

Infrared TechnolocLzVIII, August 1982, pp 10-18.

:|

35

D
A. PARAMETER SPECIFICATIOi

A.I INTRODUCTION

This appendix contains some details and rational for

selecting the parameters for the baseline concept. The system

analysis as summarized in Section 3 and Appendix B involved

performing parametric trades about these baseline specifications.

A summary of the results of this appendix is given in Section 2.

A.2 FIELD OF VIEW: FOV

One of the first parameters to be established is the field

of view (FOV). Full disc (FD) and continental United States

(CONUS) are given in Table 2.1 as design goals with emphasis

being placed on FD. Geosychronous orbit has also been specified

in the SOW. From this vantage point (approximately 5.56 earth

radii altitude) the absolute viewed disc will be less that the

full earth diameter. This geometric viewing coverage is further

restricted by the increasingly oblique line of sight (LOS) angle

at the outer edges of the viewed region which is du_ primarily to

the earths curvature. These factors will now be quantified.

I

The earth latitude (e), associated sensor viewing angle (9

) and distc.nces in a projection plane (tan = tangent at the

equator) are summarized in Figure A.I wlth represent=tive

numerical values given in Table A.I.

For equal sensor pixel size over the full , _ t;)e i

incremental observation angle_ ,is constant. As _ ;n, ea.-,-

i

the surface distano: (S) (hence area_ increases rapidly

illustrated in Figure _.2A. ?his normalized iner_ ,:-_ =

illustrated in Figure A.3. For eonst at icngltude, ori_ .,-1"_ng

at the sub-satellite point, the area observed by a pixez will

vary approximately linearly with the incremented change in the

distance along the earth's surface,AS . Along a diagonal, both -'

longitude and latitude are ohanging and the area increases in t

worse ease as approximately _$2. This latter change is

represented by the DA/DAo curve in Figure A.3. The relationship

between _ and e is shown in Figure A.4 whloh also

87

PRECEDINGPAGEBLANKNOTFII.M_ /,_ I

"W .................

ORIGINAL PAGE IS

OF POOR QUALrrY

T;,SLE A.I Observation Parameters from Geosynchronous Orbit

J S DL e Y z _ TAN D_ A&

(KM) _KM) (DEG) (RAD) (I_M) (KM) (DEG) (RAD) (KM)

0 12 12.1 .1 1.9E-0 , 12 6400 0 3.4E-04 12 1 1

10 115 12.! I .0179 115 1,3"_ .2 3.22E-0"3 11.74 I 1

20 217 12. I 1.9 .0339 217 _396 .3 6. IE-¢3 217 I.,_01 I.,_r)1

30 320 12. I 2.9 .05 320 6392 .5 8.988-03 32 1.002 l.Or_

40 423 12.1 3.8 .066 422 6386 .7 0118_ 422 1.003 1.4)L)b

5_._ 525 1_.1 4.7 .0921 525 6378 .8 ¢,1473 "_=.._1--, .r._4 L 4)r)9

60 629 12.2 5.6 .0982 628 6"369 1 01761 627 1.006 _ )',1_-

7(, 732 12.2 6.6 .1144 7"_0 6,_58 1.2 02")49 730 1.009 I "18

BO 836 12.2 7.5 .1306 833 6346 1._ 02_37 832 1.011 I 02--

90 94") 12.3 8.4 .146¢; 937 6._._I 1.5 02625 9_5 I 015 1 ,.'29

'OL', 1044 12." 9.3 .I63-_ 1040 6315 1.7 .0291_ 1037 1.018 I r'b

,IE, 1149 12.4 I0.'_ .1796 1143 6297 1.8 .03201 114(.)I.r)22 _,r_44

120 1255 12.4 11.2 .1961 1247 6277 2 .03489 1242 1.026 1.053

130 1"_61 12.5 12.2 ,2126 1350 6256 2.2 .0377_, I".4" 1.031 i Ob_

140 1467 12.,_ 1_. I ,2293 I_54 62_-,3 2.3 .04064 I'_:;-_1.0_b I.,.,74

150 1575 12.6 14.1 .24& 15,.39 6207 2.5 .04352 1,'J,*J). " 1.U42 1 •086

160 168_ 12.7 15.1 .2629 1663 6180 2.7 .0464 _653 1.)')48 1.099

1"70 1791 12.8 16 . 2799 1768 6151 2.S .04929 1756 •. L"_'_..," I.I 3

1£'0 1901 12.8 17 .297 1873 6120 3 .05216 1858 I.,:'62 I. 12w

190 2011 12.9 18 .3143 1979 6087 7.2 .05504 1961 1.0" I. 145

200 2123 13 19 ._,._17 2084 6051 3.3 .057_'1 2064 !.079 ". I_3

21(') 2236 1_ I 20 .'493 2190 6(.)14 _.5 .06,)'19 2157 1.08_ 1.18."

220 2349 I_.'_ 21 .3671 2297 5974 3.e .06367 2,".70 I.)'_90 _.205

230 2464 13.4 22. 1 .395 2404 5931 3.E .06655 237._ I. 108 I._28

240 2581 13.5 23. I .4032 2511 5687 4 .0694_ 2476 I. 12 I. 2,_'_

250 2699 13.7 24.2 .4216 2619 5640 4.1 .0723 2_79 1 132 1.28_

260 2818 13.9 25.2 .4403 2727 5790 4._ .07518 2682 i.145 I.'_11

270 2939 14 26.3 .4592 28,36 5737 4.5 .07806 2785 1. 159 I. _44

28(') 3061 14.2 27.4 .4783 2?46 5662 4.6 .00094 2899 1. 175 1.38

2_;,.) "_186 '4.4 28.5 .4978 3056 5623 4._ .09361 2991 1. 191 1.41r_

3_,0 331." 14.6 29.7 .5176 3167 5 "_'- 5 .09669 3094 1.209 1.462

310 344--" 14.9 30.t_ .5378 3278 ,er497 5.1 .08957 3197 1.229 1.51

320 357_ 15. i 3:_ .5504 3391 5426 5.3 .09245 3-.-:_ 1.25 i.562 i

330 3706 ?5.4 33.2 .579_ 3504 5_5_ 5.5 .¢'9532 _404 I. 27"/. I•62

340 3845 I.%.7 34.4 .6007 3616 5279 5.6 .0962 3507 I..298 1.685

,_50 .3985 16 35.7 .6227 ",,723 5199 5.6 .1010'9 36:1 1 •3_'6 I•757

,_L.) 4129 16.4 37 .6451 3846 5114 6 .10_96 _714 1.356 1.9_8

370 4277 16.8 38.3 .6662 3965 5024 6.1 .10663 3818 1.369 1.93

390 4429 17.2 39.6 .692 40i:_4 4928 6.3 .10971 _9.,.1 1.426 -_.0_4

390 4585 17.7 41 .7165 4203 482_ 6.5 •11259 4025 1.468 =.154

400 4747 18.3 42.5 .7418 4324 4719 6.6 .11546 4129 1.514 2.292

410 4915 16.9 44 .766 4446 4603 6.6 .11634 423_ 1.567 ".454

420 _ 19.7 45.6 .7954 4570 4480 6 9 .1,._121 433_ 1.6=7 ,_.646

430 527_,, 20.5 47.2 .kl2_9 4696 4346 7. 1 .12409 4440 1.696 2.876

440 5465 21 ," 48.9 .6539 4625 _205 7.3 .12696 4544 1.777 _. 157

450 5667 22.6 50.7 • _5 49_5 4050 7.4 . _.;984 4646 1 • 6",_'-, 3 5r)9 ;

460 5883 24.1 52.7 .9;-._ 50419 3881 7.6 .13271 4753 1._t9 3.96

470 6115 25.6 54.7 .qM_54 5226 34_95 7.R • |355_ 41;57 2. 1_5 4.56

480 63J_ 2/11.1 57 .q_47 5567 3486 7.9 ,1:646 4961 2. _2_ 5._96 i

490 b_k45 31.1 5t.5 1.0312 5514 3_50 8. '_ .14134 5065 2.577 6.64

500 6961 35.6 62.3 1.0iL_76 51_b7 2973 6.3 .14421 5170 2.947 8.66,b I

510 ,'33_ 43 65.7 1.14_2 5£,_2 2636 8.4 .1470_ 5274 3.56 12.675

520 7823 59 70 _. 2224 b_15 2185 8.6 .14995 5379 4,686 23.66_

530 6779192.6 78.6 _.3717 6274 12#J_ 1.6 .15281 546_15.9"39 2:54 021

i

!

t
ORIGINAL PAGE IS

OF POORQUALITY

8

7- S

v

5--

• 4 _

' A

" !! !

1 2 3 4 5 6 7 8 9

(DEGREES)

9

, B

t -

i

;!

0 , I I i l I i i

0 10 2o 3o 4o 5o eo 70 so 90

ii FIGUREA.2 Dtstance Along Earth's Surface as Functton of Sensor Observation

! Angle (A) and Earth's Latttude (e).

, 90 C -

I
9

7 OF POOR QUALITY

6

; A

; 3 ;R

! 2

o k......l_ J........k__ _L .

0 1 2 3 4 5 6 7 8 9

_P(DEGREES)

m m m

AR = --_2o2

_1- / °"'_o/ =

2

I

0 10 20 30 40 50 60 70 80 90

0 (DEGREES)

FIGUREA.3 Lttmar and Area Variations (for equa] steps) Nomaltzed to the

Respective Dimensionat the Subsatelltte Potnt Relattve to Sensor

Observation Angle (A) and Earth's Latttude (B).

91
ORIGINALPAGE IS

f OF POOR QUALITY

80

70- gO- I

M,I

UJ

n,"

; W

o

i

10-

0 1 2 3 4 5 6 7 8 9

_o(DEGREES)

FIGURE A.4 Variationof Earth'sLatitude(fixedlongitude)with Observation

Angle.

illustrates the rapid increase in earth's latitude as the

observation angle increases.

The selection of the full FOV limits must be based on

several factors which include such consideration as: desired

surface coverage, SNR degradation and circular earth crosssection vs square sensor array. As seen in Figure A.IA, if the

full disc is covered , .-.latitude coverage to approximately 81

- degrees can be provided. For a nominal square sensor array, this

L._ choice would result in a significant number of plxcls which wlll

extend beyond the earths surface, hence not contributing to the

I primary objective of lightning detection. Although circular

-)

I arrays may not be out of the question, they would require

I complete new design with special processing circuits. Since, as

;i

seen in Figure A.3B, the high latitude areas will have rapid SNR

•i degration due to the observed area increase coupled wlth finite

source size, a smaller FOV coverage would be a more practical

._ choice.

.i

I 92
!

If a square inscribed within the full disc is chosen, the

coverage would be limited to about 44 N (the east-west can always

be covered by an increase in number of satellites). This

excludes a portion of the CONUS. Figure A.3B shows that the

constant longitude linear dimension (hence area in this case)

increases by a factor of 2 at about 52 degrees . The upper limit

of the CONUS is at about 49°N. The selection of 50°N would by a

reasonable compromise amoung the various factors mentioned

previously.

FOV Selected : (half angle) -- 7.37°(50°N latitude)

A.3 AREA PER PI×EL: AB

From Table 2.1 a nominal ground resolution goal of 5km is

. specified which for a 5C°N FOV would result in a linear pixel

count, N, of about N=S/ASN=5770km/ 5km = 1,114. The full disc

FOV would require an array with twice this dimension of 2228X

2228. As shown in the technology assessment section, single,

double or quad arrays satisying this criteria do not exist at

this time.

Special sensor developments for DoD applications are

directed at the 1250 square array catagory, some of which may be

amenable to LITMA? requirements (submlcron wavelengths). These

(e.g., RCA PtSi and Pd2Si technology) show promise (due to

- restrlced image spreading) for furture generations of LITMAP

(post 1990).

Within the present technology, the most practical single

" array size would be a 400 X 400 (maybe 500 X 400) active sensor

array with matching on chip frame storage. Chips with 390 X 292

active region with frame storage and moderate pixel size do exist

t which would require only the addition of parallel output ports to

handle the LITMAP framing time.

"- Based on the technology assessment (see Section 5), a square

arrangement of four 400 X 400 units was selected for the system

- trades. The average resolution for this array size is then given

: by the distance/number = S/N = 5570/400 = 13.9 km/plxel. Using

,._ ,'

• 93 ;

e ................
Figure 2.4 this nominal pixel size translates intu a _s

(subsatellite resolution) of 12.2 km and 10.3 km when the nomiral

13.9 km resolution element is located at the center of the upper

hemispheric FOV (25°N) or at the center of CONUS (37.5°N)

respectively.

N- ARRAY SIZE SELECTED: N = 400 (North) plus frame storage

Sub- Central 50°due north

Satellite CONUS Sub Satellite

Point

Scaling 1.00 1.35 1.82

_e_lution Selected (km) 11 14.9 26

Ab: Area/p_xel (km 2) 121 163 220

! A.4 SOURCE CHARACTERISTICS

The key source properties in the SNR relationship are:

source area, lightning pulse length, energy distribution,

detection wavelength, associated energy per photon and

source/pixel fill factor. Each of these will now be quantified.

A.4.1 Source Area: As

The source area specified in Table 2.1 is that of the source

illumination filling a representative cloud cell, given as 5km

radius. The ideal situation is that the source fill the full

pixel, that is keep the background area A b as small as possible

without dividing an lighting event between _ ,o or more pixels.

: This consideration is important when defining the true

statistical threshold detection capability. For larger lightning

events signifcantly above threshold, subdivided events can be

post-detection summed to give the original even energy. ,

4

Data on effective source area is very limited. One approach

is to assume that the source always fills the pixel FOV, but as

the FOV increases, this assumption could cause significant error

in assessing a design capability. Lopex (1977) has given some

data as shown in Figure A.5. Fifty precent points: range from

below 0.1 km to 25 km for various parts of the world with most

94
OF POOR QUA_.ITy

falling in the 2 to 10 km ravage. For baseline systems analysis,

m._

the filled pixel condition was assumed and a parametric analysis

made about this baseline. Due to pixel geometry and considering

the source size, a square geometry approximation was used.

As= nominal source area: 121 km 2 at sub-satellite point

A.4.2 Geometric Splitting Factor: v

In the ideal situation, the event is assumed to be centered

on the detection pixel. In the lightning mapper application,

the centroid of an event can fall randonly, anywhere with the

sensing pixel area. Even if the event's area is smaller than the

pixel's footprint at the source, there is stili the probability

that the event will straddle the boundary between two or more

pixels. Streuber and Baiiis (1980) has addressed this situation

_ with the quantitative results shown in Figure A.6A. When the b

L. pixel size B is equal to the source size 6s ( 6B/6 S = I where -

: JuL1r]977 RAUL ERLANDO LOPEZ

_' T,_ ] I u l l 1 I I I , I _ S I"_ , ,,_ g n,, u u , I n D , , !'T-__

m-

.: m: _--_,

J _ I ! I J l • Jk 1 I _

J N I ! S ID IO_40NIDIDID Illi Ell Ul liJ

k" Pt _7_"

I1_ '

Accumultted frequency distribu_iou_ of cloud _nd echo

It_- _or_out_ldimension.The ,tmi_bxlinescorrespondto the io8-

= / _ d_tnl:mbo,,th, atbat _t t_ d_eremt¢l_t_Jets. 1

I

I

FIGURE A.5 Distribution of Horizontal C_oudDimensions. I,

I

{

95
ORIGINALPAGE 19

OF POOR QUALITY

o I I

0 1 2 3 4

0.2

,,_. _ 0.1

-_, o I I I

' 0 1 2 3 4

6B/6S

FIGURE A.6 Average Ilium|hence Mlthtn a Ptxel as a

j Functton of Ptxel $tze 415 Normltzed

i Relative to Event Stze a8 P|xel Focal Plane.

96

L •
the source size has been choosen as the four sigma point) there

will be on the average, about 62% of the source energy falling

within a specific pixel bounds. The implication of Figure A.6A

is that the larger the background area, the better the results, a

condition which would be somewhat true for night time conditions.

But under daytime, solar background conditions, a larger

background FOV means more noise, hence a decrease in the SNR.

This overall factor is qualitatively illustrated in Figure A.6B

for the situation where solar background noise predominates over

all other noise sources. For background conditions where 6B/6 S

>> I, the SNR does decrease. As the background FOV area is

decreased however, there is a relative maximum in the SNR/6 S

factor, after which the SNR decreased slightly until SNR/6 S

becomes, in the limit, approximately I/2 • . This means, for a

fixed source size and as the background area becomes

significantly smaller than the source area, there is a point

where the pixel will always be filled with equal area of

background and the lightning source at which time the SNR will

converge to a fixed value. In practice, noise sources other than

background would result in a lower limit for the resolution size

Delow which no SNR improvement will occur. In addition, sensor

technology constrains the selection of the array plxel count.

As disussed previously the source dimension is likely to be

smaller than the background FOV for attainable array

configurations. For sub-satellite fill factors less than one, <

the _B/6S scales proportionally (for constant longitude). This

fill factor correction is an important parameter in the

parametric trades.

6B/3S v LOCATION

0.63 0._4 optimum

Selected I 0.62 at sub-satillite point

Values: 1.35 0.70 at 37.5 N

1.82 0.78 at 50 N

97
ORI_NAL PAQ£ W

OF POOR QUALM

A.4.3 Lightning Pulse Duration:T L

Lightning pulse duration contributes to the definition of

the shortest integration time that can be used if time splitting

of the pulse is to be avoided or at best optimized. Figure A.?

summarizes the effective pulse width of lightning for some recent

U-2 data (Glllapsy, 1/83) and for data from Turman (1976). Both

' imply an increase in pulse duration with an increase in total

energy. Care must be excersized here, because the peak energy

and peak power data were not necessarily from the same event.

Subsequent data from NASA/ MSFC (Gillaspy 6/83) reproduced in

Figure A.8A for maximum stroke per flash and Figure A.8B for all

strokes indicates an interesting alternative. Even though there

- is a variance of effective pulse duration for any energy level, a

constant average, effective pulse duration could be specified

m '---

10-3 -

.

0 -I '

2

0.01 0.1 I 2 5 10 20 50 80 90 95 99 99.999.99

PERCENT GREATER THAN ORDINATE

#

c FIGURE A.7 Effective Pulse Duration (peak energy/integrated power)

¢

- 98
;(axi_um pe]k power/event (,J(_/m'$r x 10 d)

Pe_k pow.lr (UWPmZSr t 104)

FIGUREA.8 MaximumRacltlnt Energy vs. NlxtmumPeak Power (A)

Ind all Strokes (B).
which would apply to all maximum strokes independent of total

energy envolved. Figure A.8B indicates that this conclusion

could be loosly applied to all strokes. An assessment of the

NASA provided data for the full width at half maximum indicates a

value of 0.54 millisconds at the 50% point which i_ relatively

close to previous data from Turman of 0.45 milliseconds.

T L- selected value: 0.54 milliseconds

A.q.4 Pulse Splitting:

Nagler (1981) addressed the fact that lighting events occure

randomly in time relative to the frame sampling time, hence

resulting in time splitting of strokes between two frames. The

nearer the integration time is to the pulse duration, the more

prominate this problem becomes. As mentioned for the spatial

splitti:Ig, the occurance of this phenomenon in most important

near threshold where the loss of part of a pulse could result in

a loss of an event wlth the concequence being an effectively

higher threshold. Sensor dead time (e.g. during frame transfer)

will also contribute to the loss of data. An assessment of some

of the pulse waveforms provided by NASA/MSFC indicates a quisitriangular waveform.

The time splitting factor for a triangular wave form is

given by :

{ " Ti / TL - (Ti / TL)2 /3 0 < TI l TL

• _ • I - ( TL / Ti ) /3 Ti _ TL

: In the SNR relationship, the full time dependency is given by or"

_/_4_, in normalized terms _(TL/Tt)t/(TL )1⁄2 . The factor TL is J

assumed constant and the normalized ((TL/T%) 1/2 is plotted in

Figure A.9. For maximum SN_, Ti=T L, but small variances in 1L

would result in significant variation in pulse detectivity. The

4

;i SNR falls off extremely rapidly for TL<TL. To minimize a large

;t variation in SNR as the event pulse width changes, a value of

!

e

"t

J

"i
; ORIGi.NAL PAGE m

OF poOR QUALr_

0.6

i.:.-.11__ SNR cr f ( Ti / TL )

"- 0.4

0.2

1 1 t I '

O0 2 4 6 8 10 12 14 16 18 20

Ti/T L

FIGURE A.9 Time Factor Effect on SM"

f

Ti>>T L should be chosen, a factor greater than 5 would be

recommended.

Other wave forms result in some variation of the constant

I/3 in _, but the peak will still occur in the same place(at

Ti=TL). Change in steepness of the curve and a change in the

absolute magnitude of this time splitting factor would occure,

-" however, with no significant change in the conclusion.

" _ - Stroke time splitting factor: (I - TL/3T i)

A.4.5 _ Strength: Es

:-. Table 2.1 indicates two candidate wavelengths , 8683A and

7??qA. A threshold of 107 watts (total optical power) is also

: given. NASA U-2 data (Gillaspy 1/83) indicates the following

d"

' values near the 8683A line:

uJ/m2-sr Probability Greater Than

q 905

13 505

2q mean

52 905

100 Maximum Observed

i

The total optical energy values were derived by assuming that a

101
ORIGINAL P._GE Ig

OF POOR QUALITY 10¢ _ TERRTURMAN --

TERRTURMAN'76-78

GUOANDKRIOER'1_2

I _ U-2§KMOIA

1o6 -0- u-2lOKMOIA

-_- SSL

T - 0.0O054SEC

I

I

,_ le4

E

f,4

2-

•- 103

102

I

I

10

• 0.1 I 5 20 40 60 I10 95 99.5

0.5 2 10 30 50 70 90 99 99.99

P_AOIANINTTENSITY• INTEGRATIONTIME(111FOR)k- 868.3_ M

FIGURE A.IO Energy D1strlbut_on for L1ghtnlng.

0

• nolatnal value of 151 of the optical energy ls contained within

-_'-" the 8683A line region. Figure A. IO compares ground based '

lighting data with that collected by NASA/U-2 and that from space

analyzed by Turman. Both above cloud collected data show a

narrower dynamic )'ange, a fact which may be real or due to a

limited data base. Some of the U-2 data sh_wed only a 12:1

dynamic range, but due to the limited data set, a _.ntmum dynamic i

range of 100:1 should be used for the system analyst._. A !

f

: threshold goal of 107 watts is given _n Table 2.1. This

/ transZates Into (15 percent of energy in t.;le line assumed) i

L

102

i,'-
ORIGINAL PAG._ I$

OF POOR QUALITY

TL Source Area Energy in Single Line

Milliseconds km 2 _ j/m2_s r

I 25 9.5

100 2.4

0,5 25 4.7

100 1.2

where a flat plate radiating from both surfaces has been taken as

a typical cloud deck with an internal lightning stroke. The 0.5

millisecond, 5km square source area give a integraLed source

strength comparable to the U-2 measurements. This result must be

reinforced by addition field data. Modelling by GUO and Krider

(1982) indicates that the source may not be Lambertian, but could

have preferential radiance in a cone about the vertical, a result

due to multiple internal scattering. The 1982 NASA/U-2 data w1?l

be used as the baseline value for the threshold integrated energy.

nEs - Radiant Energy: 4 uJlm2-sr NASA/U2 Data

k - Wavelength : 8683 A

EpHOT- hc/ k : 2.29 E-19 Energy/Photon

J/photon

A.5 SIGNAL NOISE RATION: SNR

The average false alarm rate FAR is defined as the average

number o£ times per seccnd the output noise electrons exceeds the

threshold setting of the detectors. This can also be _xpressed

in terms of the SNR as:

F A R = e'SNRT2/2,.

2'V3- TL

This FAR rate decreases rapidly as the threshold value (SNRT) is

raised. The probability of detection Pd is approximately the

103
ORiG|NRL pAGE |S

OF POOR QUALITY

probability *hat the signal plus noise exceeds the threshold at

the instand of signal peak, giving

Pd = (I/2) (I + erf ( ( SNRs - SNRT ) / _ ) )

where SNR T is the threshold SNR setting and SNR S is the SNR of a

given signal (note that when SNR S = SNRT, Pd = 0.5, i.e, 50%).

Figure A.11 graphically illustrates this relationship between FAR

, and SNR. For FAR <<I, the SNR requi, ement is not very sensitive

to the absolute value of the pulse duration between 0.54 and 1.0

milliseconds.

A FAR design goal of less than 10% is given in Table 2.1.

A.2 summarizes the SNR required on a pixel by pixel basis to

provide this FAR. A nominal value of the threshold SNR is around

_: 6.0. Note that the smaller the area which a pixel views, the more

stringent the SNR value due to the lower lightning activity in

8_ _ I--.._- e'SNR212

d

• °o_ I_ I_ lo-2 loo lo2 I@

f: FIGURE A.ll FAR vs. SNR.

104
ORIGINAL Pi,C_ 13

TABLE A.2 SNR vs. FAR OF POOR QUALITY

EVENT RATE 5/_

FLkSH/km2see FLASH/SEC PIXEL FOR lOS FALSE EVENT3

SPIE 2m5 - WOLFE/NAGLER (10 Am) (20 km) (10 km) (5 km)

GLOBAL 2 10-7 2 10 -5 8 10-5 6.2 6.0

US 2.3 10 -7 2.3 10 "5 1.8 10-_ 6.2 5.9

STORM COMPLEX _ 10 -5 _ '0 -3 1,6 10-2 5.3 5.0

SEVERE 5TORN 1.7 10.3 1.7 10-1 6.8 10 -1 1.6 U.2

NESA/MSFC (Hugh Christian)

GLOBAL 2.9 10-7 2.9 10 -5 1.2 10"_ 6.2 5.9

_150/lee total)

that reduced area. The respective values of SNR for footprints

of 5,7,10,13, and 20km are 6.4,6.3,6.2, 6.1 and 6.0 for a nominal

global rate of 2 X 10-7 flashes/km 2 sec. The higher the local

activity, the lower the SNR can be set for the same FAR.

There is also a 90% probability of detection specification.

Figure A.12 relates the probability of detection relative to the

SNR threshold setting (i.e, SNR at Pd = 50%). From Figure A.12,

an event with a SNR greater than 45% above the thre hold setting

will have a-probability of detection greater than 90,

SNR - Baseling value: 6.0

A.6 BACKGROUND: IB, ALBEDO

: The solar spectral irradiance can be found from a number of

handbooks. Approximate values for IB _ (W/m 2 - um)

m=O m=2 (zenith = 60 °)

8683 qSO 800 W/m2-um

7776 1160 907 W/m2-um

Most of the cloud tops are above a significant part of the

atmospheric air mass, in particular, above much of the earths

water vapor (c.f. Figure A.13). Hence a worse case but realistic

• 1

zos

i: I
.L

j J L ! -

1.050 60 70 80 90 100

Pd (%)

FIGURE A.12 Probability of Detection vs. SNR.

. !

I

,.4 ,o__ _=., =i,. __-},." ,_ f<--

_'m.

._/_, r ---"_¢ _I T

i._ < _ _._

lpproxllllal¢ sizesOfilrlou_ Iiihlnln I produ¢lnI ,Ioud_

FIGURE A.13 Cloud Top Altitudes.

• 106

1
ORIGiN.EL PAGE IS

OF POOR QUALITY

so]_ " condition would be m : 0 giving for 8683,

IB : 302 W/m2-um-sr.

The albedo (solar reflectance) varies for different

bac,:grounds. Of specific interest are clouds, but a pixel may

view a broken cloud deck. Some albedo values are

OBJFCT k=7776 8683A

_in er snow/ice 0.85 0.8 worse care background

Clo ds 0.7 0.67 normal cloud cover

Summer ice 0.5 0.45_

Soil Vegetation /Rocks 0.4 0.3 0.5 0.33 Isurface background

A nominal baseline value of 0.7 was used for the system

2arametric analysis.

8683A 7774A Units

I% - Selected Values: 300 396 W/m 2 - um-sr

A bedo - Selected Values: 0.7 0.7

A.7 OPTICS: [/NO.,K

The two primary characteristics of the optics in the SNR

relationship are the F/No (relating focal length and aperture

diameter) and the optical throughput efficiency,K.

The preliminary analysis indicated that the F/No would need

to be _ushed t_ optimize the SNR. This is one of the few factors

which co_t: ibutes proportionaly to the SNR rather than by the

square root. Pushing the F/No. often means mot glass, higher

fabri,_ation precision and mechanical limits. A F/I is often

considered the lower limit with F/1.5 being more practical.

_eparate design activities have indicated that an F/0.78 system

might be doable for the LITMAP requirements. Refractive and

reflective alternatives are possible. The requirements of the

filte," must be considered when defining the optics.

107
®

OR:_NAL ?A_E IS

OF POORQUALITY

The optics system will probably have many surfaces, hence

appropriate coatings to maximize throughput must be used.

Coatings with K=0.98 to 0.99 transmittance per surface should be

attainable.

F/No-Baseline: 1.0

K-Sele ted value: 0.98

A.8 FILTER:

When considering the filter requirements, care must also be

taken that the throughput of the filter is also considered.

Although the optics and filter were not part of this study, their

significant impact on the overall system trades necessitated a

: look at their impact during a related IR&D activity.

Table A.3 compares the overall effects of two filter types

and several optical band passes. The interference filter must

have a bandpass sufficient to handle the angular effects (c.f.

TABLE A.3 Filter/OpticsFigureof Merit

FILTER TYPE _i. K K*e _i' Merit,

(l) PILTEll FZLT[I! • OPTICS

IIT[IIFEIIEIICE 25 O. 90 O.57 - O.72 0.15 - O. 17

5 (rietmn|ulmr) O.TO 0.56 (]) 0._13

'_ (5) o.31 - 0.33

2.5 (|,umstmn) O.iO 0.25 - 0.32 0.31 - 0.35

il El[Iti IIIG[IIC[

($OLC) 2 0.25 0.16 - 0.20 0.28 - 0.32

1 0.25 0.16 - 0.20 O.II . O.llll

J

• o1' optaoel line beth| leolmted l* aamumedto be one or two angstroms.

el Aeiualed optlo| with 11 elemente (22 Ilurfeeee): 10.99122 • 0.8; 10.98122 • 0.611

: Flgure A.14 while the SOLC filter can handle the larger FOV while

maintaining a narrow bandpass.

• 108

,i ') ...... _-_----_mm*_mmm maim real_ H. ; _ -
5)'

ORIGtNfL P#GE I_

OF POOR QUALITY

3.5

m

Z5-

I= 2-

x

! 1 --

0.5-

!

L O_ I 2 3 4 5 6 7 8 9

_0(DEGREES)

FIGURE A.14 Angular Shift for InterferenceFilters.

For wider FOV's (e.g., 18Cfull angle) the SOLC has a 2 to 3

advantange over the required 25A bandpass interference filter.

For the narrower FOV (as exists when the full disc is split four"

. wayswith separate optics for each, see Section 4) both filters

' provide comparable performance with the development risk being

larger with the SOLC filter. If more than 3 A bandpass is

required, interference filters are the most cost effective

: approach.

_;. For band passes of a few angstroms, a single line of the

lightning spectra can be isolated. An approximate value of 15%

of the total optical energy is associated with each of the

candidate lines of 3683 and 7774 angstroms.

i

A_- Selected: 5 A

- Selected: 0.15

"

] 109
:@ ®

A.9 SENSOR CHARACTERISTICS

A.9.1 Quantum Efficiency;

The quantum efficiency term includes the overall conversion

of the total incoming photon flux to charge carries within the

detector. This not only includes the absorption properties of

the active region but also accounts for the optically obscured

areas. This latter is very dependent on sensor fabrication

techniques and overall sensor topography design.

The photon-to-charge carrier conversion efficiency is also

wavelength dependent• One primary effect for silicon based

detectors is that for wavelengths greater than 0.7 um, the

silicon starts to become transparent• Also when the thickness of

the active region exceeds the pixel dimension, lateral charge

carrier migration can occur resulting in image spreading (c.f.

Figure A.15).

This spreading will divide the event amount two or more

pixels resulting in an effective increase in the detection

threshold. Shorter wavelength and/or special sensor material

selection (e.g., Pt Si) can minimize these effects. Relative

: values of the total energy within the 7774 and 8683 angstrom

• lines must be obtained before this spreading effect can be fully

, assessed. Larger pixel dimension is one of the geometric trades

which can be used to offset the image spreading factor.

: Commerical device quantum efficiencies range from 0.12 to

: 0.6 with shorter wavelengths in the region of interest having

higher values. Some isolated cases have claimed quantum

efficiencies up to 0.8 for back illuminated devices but this does

nct appear to be consistant with the bulk of the 8683 angstrom

wavelength devices.

}_ Typical Goal

Q- Selected range: 0.35 - 0.q5 (0.6)

i

110

-- .° p
®

ORIGINAL P,_GE IS

OF POOR QUALITY

LLL et _.. $OLII) STATE. IMAGE $k_NSOR IrOR IMAGE REX_OKDIN_

d • ,! . ! I • '! ! ! I ! | ! I I

' S

;J •, ;,,,,,4 ...,,, $ _ o O 0 Z $ a S 6 ,

Columr_ i_,,on

HorLr.onud Une-.spreadfuncuon mcasured8t vu.rk_s ws_lcr4ths.

I.inc of mumuuJUon u4th 6-,um h_-wDdch wu po_Uoned -t the

aumu of' _ntTaJ pixeL

\\\. • _.o_..

.- \\\ .o.o,...,,,..

--..

Calculated and ¢xperimenUd llnNpnml fvnctJe_ at $00. 100.

8td tOO _m. The mlcvbted points m _ fzem • two-dlmen.

deed numedctJ mluuon of the diffusion quatkm by the medtod of'

• eOqellate Ir_dients. The ktepoud flux fu' _ piul Is

to the toud flux ovel the anu'ad t pi_els re IJob%mJ_lotod Iltd upe/MunUd vldues. The mk'Ulnlod pomtl (open c_cks) _ mlucmd

117| bM. which 88/q_ ImJy IS | Ivide. 1114,_ cnW ell lee

experimenudpoinu 0t low J_ht 18vebeomuq_ed0to 0 mbt_

JeqJonm of 0.002. Psak iiJvm_Jtion bvtt wM -$0 pmmt o(

mtvlition.

FZGURE A.15 Image Spread4ng tn Array Detectors.

111
1

ORIGnNALFAGR IS

OF POOR QUALITY

A.9.2 Pixel Dimensions: 6p

The selection of pixel size is dependent on a number of

pararmeters including resolution, aperture, effective F/No. and

orbit altitude as well as manufaeturabllity. When resolutuion is

the driver, diffraction limits must be considered. As shown in

Figure A. 16, the approximate diffraction limited plxel size (L -

2 AR/D) for a F/1 system is around 1.6 urn: over a deeade below

the pixel size required for the LITMAP application. Hence LITMAP

is not diffration limited for global coverage. This figure also

relates aperture diameter to object resolution for various pixel

sizes. These factors are related by the plxel angular FOV and is

given by:

L L - ( R 6p ) / ( (f/No) O )

Where all values are in meters and for Figure A.16

' R - 3.6 107 m

f/No - fit

Each L vs D curve applies to all conytant values of 6p/(f/No).

A

" 10 RCASCIENTIFIC '

CONUS TI EUROPEANTV

• 800 x 800

,HIGH RES,

)

% DIFF LIMITED

-. _ 1.6.m

i 0 "" i 1

t

t

0 10 2O 3O

: O (an)

i_ FIGURE A.16 Resolution vs. Aperture.

i 11z

J I Jl IIIIIrl i i
ORIGINAL PAGE IS

i OF POOR QUALITY

(

i Hence an f/l, 40 pm 6p gives the same curve as an f/2 and 80 um

I pixel. Note that for a specific pixel size, there is a limit on

the aperture size associated with a given resolution, e.g., 11 km

imposes an aperture limit of about 5cm for a 15 um pixel. Larger

pixels permit larger aperture. Figure A. 17 replots this latter

figure giving curves of constant resolution. The SNR

relatl.onship shows that a larger pixel size (hence larger

aperture) enhances the system performance. Solar background,

electronics processing bandwidth and signal dynamic range also

push for larger pixel size capability to enhance the threshold

performance of the sensor system.

In addition to shear bandwidth consideration for a given

[ focal plane array, there are also manufacturing considerations.

Although 800 X 800 arrays have been produced, primarily for the

astronomy community, they have 15 um pixels and are normally

,'eadout at sub-MHz rates. Discussions with TI indicates that

L (kin)

4

)

o 1 I I

[ 0 10 20 30 40

6p - PIXELSIZE (pm) f

, FIGURE A.17 Aperture vs. Ptxel Dtmnstons.

113

.i)
@ ®

they would probably not be interested in production of such an

array with larger pixels, say 20 to 30 um. The physical size

becomes so large that production yields drop Oramatically making

them prohibatively expensive to produce. Dividing the 800 X 800

in half, i.e., a qO0 X qO0 sensor with equal chip frame storage

makes their production more practical. A number of arrays are

now in production for commercial TV and scientific applications

which approach the 300 X qO0 and 400 X 500 imaging arrays (plus

frame storage) with pixel dimensions of 22 to 30 um.

_p selected dimension: > 25 um also dependent on other

system requirements.

A.9.3 Sensor Noise:

The primary intrensie sensor noise contributions are dark

current and amplifier noise. Analysis indicate that for daytime

operation, A leakage of 40 NA/em 2 would only contribute less than

10% more relative to the solar background noise. Existing data

indicates less than 26 NA/em 2 at 25 C maximum with 8 NA/em 2 being

typical. Hence, for the short integration times associated with

j LITMAP, there probably will be no cooling requirements due to

dark current.

Amplifier noise is a different problem. For su_ MHz readout

rates, as applied in astonomy for example, the preamplifier noise

: is on the order of a few tens of electrons rms, As the readout

frequency increases, in an attempt to minimize the off focal

' plane electronics required, the preamp noise increases. Special

4

on eglp preamp designs can be taylored to the desired operating

frequency regime and special techniques such as corrala¢tv_

do6ble sampling can be applied to minimize the associated readout

noise, Systems optimization often results in about equal noise

contributions for the most prominent sources, in this application

! two key contributions are background and readout (pr_amp) noise.

l

114

P
I ORIGINAL ['AGE IS

OF POOR QUALITY

Desired limits on readout noise will be a contributing factor on

defining the upper readout frequency for a given array

configuration.

The noise factor ( was defined as all other noise

contributions divided by the background contribution (all in

absolute values, not rms).

- Desired value: <I must be determined for each

configuration

A.9._ Integration Time: %i

The integration time is a deli'eate trade between (among

other) excessive noise and power dissipation at higher

frequencies versus increasing electronics and required full well

capacity to hat.de the solar background at lower bandwidths.

Figure A.18 illustrates the relationshlp between integration

time, readout rate and pixels per readout port. The partitioning

given on the right side is based on a total array of 800 X 800

..... AS

100-

• 80- - IOMHZ 8

, X MHZ " - o

6O- __ ;. _

§MHZ _ '

I

"J -32 :

X 20-

'l E 1 MHZ 64 '

0

0 1 2 3 4 § 6 7 8 9 10

;, IN'rERGRATION T;i_E (MS) i

;i FIGURE k.18 Focal Plane Partitioning ,'s. Sample Tim.

|

115

D
(640,000 pixels). A preliminary analysis indicated the desire to

hold the integration time to a nominal 4 mill!seconds. With an 8

to 10 MHz readout rate, the full array would have to be

partitioned and provided with 16 output ports. For the four

array configuration, each array would have four ports. Further

system optimization may result in 5 MHz readout rate per port,

hence 13nger integration tine (probably not acceptable) or

increase partitioning. Electronics weight, power, volume and

cost must be traded against array manufacturing constraints.

Four to eight ports per array ehlp appears to be achievable.

L Analysis discussed later shows that for SNR, _here is an

optimum integration time some where around a half to one

mllllsecond. From physlcal considerations, o longer tlme must be

utilized.

A consideration for the upper integration time limit is the

rate of change of the background. Assuming geosynchronous orbit:

o Day/Night terminator: approximately 103 mt/hr

or qqq m/see. Tlme to move:

Percent o__f2tixel FOV Seconds Frames

, 10 2.5 6_5

1 0.25 63

0.1 0.025 6

o Wind (Clouds): less than 102 ml/hr

or q4 M/see. Time to move:

, Percent of pixel FOV Seconds Frames

• 10 25 6250

1 2.5 625

o. 1 o. 25 63

l

; Note: vertical cloud growth can occasionally exceed the !

; _ M/see given above.

; For time averaging of the background, a tuther restriction

116

| O
may be necessary. For example, for a motion of 0.1% of a pixel

in ten frames (i.e., 0.1% in 40 milliseconds) the terminator

motion maybe a problem. Satellite jitter must also be assessed

carefully.

For LITMAP objectives, the upper integration time will

probably be more limited by sensor full well capacity rather tha_

background changes.

Ti - Selected baseline : 40 X 10-3 seconds

M - Partitians : 16

f - Frequency : 10 MHz

A.9.5 Full Well Capacity: FW

Th_s factor does not explicity appear in the SNR

relationship but is one of the limiting factors on the upper end

L

_ of the integration time. As background and other noise

. contributions fill the sensor storage area, the allowable signal

dynamic range decreases. For excessive integration times, the

ability to handle the signal will go to zero and the pixel

storage will saturate.

Some representative full well capacity data is illustrated

in Figure A.19 for several commerical ICCD's. The virtual phase

and some three phase devices fall along the lower solid curve

?

while the four phase devices fall along the upper curve.

- Although these charge capacities are concept dependent, there is

some latitude in well depth to adjust the full well capacity.

Figures A.20 to A.22 relates allowable "noise" accumulation to

, signal dynamic range. The signal synamic range is defined as

maximum signal divided by minimum signal where the mlmlmum

._] detectable signal is defined as:

-_ Ns (peak) = SNR*N n (RMS).

iI Figure A.20 illustrates that larger full well capacities permit

larger allocations to background and noise. The large capacity

becomes more important as larger dynamic ranges are required

I coupled with larger backgrounds (e.g,, with longer integratiou i'

times). As seen in Figure A.21, this larger capacity allocation

117

.......
ORIGINAL PAGE 18

OF POOR QUALITY'

_-. 108 l

b,_.x FW = 1.1x 103 A_

> 6- / ?

I-=

MADAM

< / _ (HUGHES)

< WESTINGHOUS_

(.,1

•,J 4--

"J / FW= 0.37 x 103Ap (pro2)

l.IJ

_ ZX

",,J" 2 FAIRCH7 __...............-....- ARCA

....._......... ,,

;'.. / _ TI (800 x 8001

,• 0_" ! I I , I I I'

"- 0 200 40O 60O 800 1000

L I I I

1$x15 20×20 25x25 30x30

PIXEL AREA _m 2)

FIGURE A.19 Full Well Capacity.

• io4

• los 2x los ,: Ioe

_,o_- _-_,,. _ __

'-. _ I',_'°' "_..,_ __

u

_;, o ,e ........ _ .....

F

' l ',,'k\',\/I

'mkt° 'to2 I@ Io4 i@ I@

2;- ALLOWABLE BACKGROUND (e t's)

FIGURE A.20 Dynamic Range vs. Allowable Noise.

118
ORIGINAL PAGE IS

OF POOR QUALITY

104

' 105 2x 105 5x 105 106

_,=w =,_ _ _ _

10 2

+. If\

- I

- f I

- i I

t i I

" 1o0 I l l I I!II}+

.," 10,2 10"1 100 101 102

,+ _ AS PERCENT OF FULL WELL (_:/FW) ,

FIGURE A.21 DynamtcRangeas Percent of Full He]l. 

to noise is not only a result of an increase in the absolute fu?l

well capacity, but a larger percent o£ the full well can be

allocated to backgroundand noise as the full well capacity

11g
i *

ORIGINAL PAGE IS

increases. OF POOR QUALITY

Finally Figure A.22 provides curves for fixed dynamic ranges

and provides the relationship between absolute electron count for

background (and other noise) as a function of the full well

capacity. This latter figure illustrates that as the full

capacity goes up two decades, the allowable background goes up

2.5 to 4 decades for dynamic ranges of 10 to 103. The

conclusion: full well capacity is a very important factor when

dealing with large backgrounds. For a dynamic range of 100, full

capacities of at least 2 X 105 are almost required with values

above 5 X 105 being highly desirable. As shown in Figure A.19,

these larger capacities primarily comes from larger pixels with

some dependency on the CCD technology utilized. The upper curve

in Figure A.19 can be represented approximately by

FW = 1.1X 103 Ap (um 2)

}

where Ap is the pixel area. The allowable background can be

estimated by (c.f. broken curve):

": //

105

__ -

I

...I

,,J

u_ tn4 I IIlI

IV

I@ I@ i_ I@ I@

- BACKGROUND AND NOISE (# e'I)

FIGURE A.22 Absolute Noise Limits Relative to Full Well Capacity.

120

"'q -2 _ " .......
Nn 2 : _ FW: -7.86 X 104 + 629 Ap (um2)•

The lower solid curve in Figure A•19 is roughly one third of the

upper curve full well capacity, i.e. for the lower curve

FW ~ 0.37 X 10 Ap (um2)

Z FW _ -2.62 X 104 + 210 Ap (um2).

These sets of curves bound most presently existing commercial

ICCD devices.

FW - Selected baseline value: > 200,000 electrons

A. IO THRESHOLD VERSUS DYNAMIC RANGE

r

Whenever the technology is being pushed, th_ inevitable

question arises as to if performance can be traded versus

scientific objectives to relieve some technology _tressing

issues• LITMAP is no exception and one of the obvious trades is

that of threshold setting versus data set completness.

A previous section discussed observed source strengths.

This section addresses the relative impact of raising the

threshold on the loss of data. Figure A.23 illustrates several

log-normal distribution curves with the dynamic range being

defined between the 10% and 90% probability points. The value of

I on the ordinate is the nominal setting for threshold to provide

detection of 90% of the events.

As the threshold is raised, events are lost at the lower end

of the curve. A factor of three increase in the threshold for a

I0:I dynamic range would result in a loss of about 40% more of

the data. As the dynamic range increases, less sacrifice occurs

i for a factor of two or three increase in the threshold (hence

i SNR). For the LITMAP dynamic range design goal of I00:I, an

_I increase of two or three could be tolerated with less than 15% of

I

_ the data being lost. A word of caution is necessary. Present U2 data indicates that the dynamic range may be less than 100

between the 10% and 90% points, as low as 12:1. That data set is :t

T

very small at present, so care should be exercised when making :!

121

L .............. j
ORIGIKAL Pe.GE |,_

OF POOR QUALITY

I I

I I 1000-

I

I i

I OYNA.MAINCG_I

MJ I' 1_ I

" 11o I

,o .% 5 --,

" 3 J I

2 1

i

1

• 2 5 10 30 50 70 80 90 95 98

PROBABILITY

FIGURE A.Z3 Relative Dynamic Range vs. Probability.

k

t decisions on thresholds. Enough data is available to indicate

: that the above cloud observations are definitely lower than the

• 1000:1 dynamic range for below cloud observations. Based on the

-. NASA design goals of 100:1 dynamic range the following system

trade guideline has been assumed where Es is the lowest signal

: that is desired to be detected.

_, ET - Threshold: ET _ 3_E S

' i A.11 PAKAMETER SELECTION SUMMARY

A summary of the selected parameter values is given in Table

A q These data provided the input for the baseline

.I

122

_P

II II llll J
ON_NAL PAGE ILi

OF POOR QUALIFY

configuration as given in Table 2.2 of Section 2 and formed the

foundation of the parametric analysis of Section 3.

TABLE A.4 Parametric Analysis Selection Summary

Parameter Value Units Counts

FOV 7.37 Degrees Half Angle (50° latitude)

: Pixel lI km Sub-sate)7ire (nadir)

Resolution 15 km Centra] CONUS

20 km 5O°N US/CANADA ]ine

Geometric 0.6/0.44 Optimum

Split, v 1/0.62 SgbsatelIite

1.35/0.70 37.5°N

k 1.B2/0.78 SOON

TL 0.54 Mtl 1t seconds FWHM

v_

Pulse [Ti/TL - (Ti/TL)2/3] O<T• TL

Splitting,_. - "

[I- (TL/TI)/3] Ti _, TL

_}Es 4 /_41/m2°sr Single llne,NASA/U2

X 8683 Angstroms Alternate 7774

; SNR 6 From _ considerations

.

t

300 W/m2 - _ m-sr 8683A° IB

, 396 W/m2 -/_,n°sr 7774A°

AIbedo O,7 Both wavelengths

F/No. 1.0

K 0.4 Including filter

x 5 Angstroms

T) 0.15 Friction of optical energy

in single ;fne

Q 0.35 Range 0.35 to 0.6

123
I

ORIGINAL P'/,,_" i_

OFPOORQUAUTY

TABLEA.4 (COb/T)

Parameter Value Units Comments

6p • 25 microns Needs to be large

( <1 Device dependent (desire<0.5)

Ti 4 Milliseconds Integrationtime

M 16 Total number of outputs

F 10 MHz Rate per output

! FW •2xi05 Electrons Full well capacity

)

r'

ET _ 3n Es pJ/mZ-sr Thresholdrelative to NASA/U2 data

124

v ,__ 1,1
@ ® I

B. PARAMETRIC TRADES

B.I INTRODUCTION

As mentioned in Section 3, a good feeling for the parameter

dependencies can be obtained by simple inspection of the basic

SNR relationship, which in Section 3 was inverted to provide the

attainable spectral threshold energy for a given set of

parameters. This inspection approach can be used whenever all

terms of interest interact in a product/division manner. Hence,

once a given quantitative value is obtained for the threshold

energy for a given set of parameters, the adjustment of this

threshold can be made by inspection.

This straight forward approach is not as effective when

addative/subtractive factors are involved such as when more than

one noise term must be considered and for the time and spatial

terms which have interactive factors. The following sections

will provide some representative parametric trades which were

used to determine what parameter set would permit optimization

of the LITMAP _ensor system. The final selection of an

"optimized" parameter set is constrained by the practical limits

placed on each parameter as defined in Appendix A.

B.2 BASELINE PARAMETER SET

Appendix A defined a practial parameter set which was deemed

to be within the limits of the existing technologies and have a

reasonably low risk for implementation but did not necessarily

meet all of the requirements of Table 2.1. This parameter set is

. summarized in Table 2.2 as the Baseline. All parametric analysis

started from this baseline with the intent to determine how the

overall system (optics/sensor/elentronics) could be optimized

relative to cost, complexity, schedule and risk.

A graphical presentation of the spectral threshold energy NE

(j/m2-sr) as a function of integration time is shown in Figure

B.I Computer printout formats for the baseline data set is

illustrated in Figures B.2 (day) and B.3 (night). Note in Figure

B.I that the recent NASA/U2 90% threshold value of 4 uJ/m2-sr is

125

l
ORIGINAL FA-_E 15

OF POOR QUALITY

t_

C_J

E

v _' LI

-t

l-q

_- IONASA/U2

-- -- i i

NIGHT

I L ..... -3 /--

:' 4 E, :_;

I If-l" EGF',-,T [ F_tl T I HI:: , _1:,E': '

B.l Threshold Energy for 8683A as a Function of Integration

•" Time for the Baseline Set of Parameters Given in Table 2.2.

approached under night time conditions but a value more near 11

uJ/m2-sr (at 4 milsec) is attainable for woree case daytime

conditions. From the dynamic range discussion in Appendix A,

even a factor of 2 or 3 increase in threshold for an event

dynamic range of 100 would result in loosing less than 10% of the

data that normally would be attained within the 10 - 90% dynamic

range band. Previous analysis by others have shown more

optimistic threshold values due primarily to two factors. First,

much more optimistic assumptions were taken for parameters such

as sensor quantum efficiencies than this sturdy had deemed

, acceptable based on commerical devices (c.f. Appendix A for

discussions). Second, a major contribution to the lower values

attained in this study was the inclusion of the spatial and time

;, splitting factors which split lightning pulses, hence lowering

; their apparent energy. The dashed curves in Figure B.4 result

when the splitting factors are excluded. These curves are about

a factor of two below the more realistic curves (solid). A third

factor which was not included but will have some influence is the

-; 126

4
® (;

I

ORIGINAL PA'3E IS

OF POOR QUALITY

LIGHTNING MAF_E_ NOISE CALCULATIOrJS

F'I el slze on Earth redge of square_ II _M

Source slze 11 fM

PI ;e| ,ize of ECD _r, MIcron_

T_I eSCol_e f/number I

TelescoDe focal length 9.7(, CM

Telescope dlameter 9.70 CM

0

Sol ar Irradl ance 3 ",_ 14"M _ S_ uM

Clou_ a|bedo .7

ODtlc$ transm_sslon. Includlng f:Iter .4

Wavelength 868.-- NM

F_ iter bandwidth .5 NM

Geomet-lc image spllt1:ing factor .62

Llghtn=ng Pulsewidth .54 msec

ReQu_ red S/N 6

C,uantum efflclenc_ ._5

Satell;te altltude .'.5Or)O I M

Bac_ ground 48750C} el GEt ron

Ampl:fler Nolse (rms_ 150 Electrons

z:,

i

DAY

I

!

,,b

= IA

5

4 J _ i

£ 4 _ :B

* INTEGI_wTIOh; TIME , M'=',EC , ,

J

T T-FACTOR _AC_ GROUND S ZGNAL B_cr GROUND £ I8NAL TOTAL

(ELECTRONS) (ELECTRONS) NOISE NOISE

(RMS EL) uJ/M 2SR (RMS EL_

. .5 .64 22687 129_ 150. b2 7.54 215.5 _

, I .82 45374 1581 213.01 7. 19 2b_. 54

1.5 •88 b8062 182T 2bO. 88 7.7"_ 34:3.95

2 .91 q0749 20_7 301.24 8.35 _.'9.5-" ;

2.5 .92 11 _4_b 2230 _36.8 B. _ 3" 1 • 7

3 .9-_. 13b 124 2407 368.95 9.56 401.28 !

* _ 3.5 .94 158811 2572 398.51 IO. 12 428.81

; 4 .95 181498 2728 426.02 10. &6 454.67 '

i 4.5 .95 204186 2874 451.86 11.17 479.1: '

5 • _b 226873 3014 476.31 11.67 502.38

5,5 .96 2495bl _,147 499.56 1._, 14 5:4.b

6 .9b 272248 3275 521.77 12. b 545.91

:" b. 5 .@'/ _4935 33q;19 54_. 07 I_. 04 5bb •42

_ 7 .97 :317623 3517 563.5;19 |_.47 586.2

|

:i_ FIGUREB.2 ComputerPrtntout Format for Baseline/Day

127
ORK;;fN:'_LPAGE IS

LIGHTNING MAPF'E_ NOISE CALCULATION S OF POOR QUALITY

Pl -el slze on Earth (edge of square_ 11 _M

Source slze 11 _M

Pl::el slze of CCD _') Mlcron_

Telescope f/number i

Telescope focal length 9.7c: C_

Telescope dsameter 9.7_ CM

0

Solar Irradlance 0 W/M - SK uM

Cloud albedo .7

Optlcs t_ansmiss:on. Includlng fslter .4

Wavelength Bbe. T NM

F11ter _andwldth .5 NM

Geometrlc image spllttlng factor ._2

L1ghtnlng Pulsewldth .54 msec

Requlred S/N b

Quantum efflclency .o._

Satelllte altltude 35b00 kM

Background 487500 electrons

Ampllfler Nolse (rms) 150 Electrons

NIGHT |

i 25

!

2,D

z

• 1 0

• ,& , I , I _ ,

' 4 _ 8

INTEGRwTION TIME , MSEC ,

T T-FACTOR BACKGROUND SIGNAL BACKGROUND SIGNAL TOTAL

(ELECTRONS) (ELECTRONS) NOISE NOISE

(RMS EL) uJ/M 2SR (RMS EL)

.5 .64 0 918 0 5._5 15_.0_

1 .B2 0 918 0 4.17 15_.02

1.5 .B8 0 918 0 3.89 15_.02

2 .91 0 918 0 _.76 15_.02

2.5 .92 0 91B 0 3.69 15_.02

.9_ 0 918 0 _.64 15_.02

_.5 ._4 0 91G 0 _.61 15_.02

4 .95 0 _10 0 3.5B 15_.02

4.5 .95 0 _IG 0 _.56 15_.02

5 ._6 0 _18 0 3.55 15_.02

5.5 ._6 0 _lB 0 3.54 15_.02

6 ._6 0 _lB 0 3.5_ 153,02

6,5 .97 0 918 0 3.52 153.02

7 ._7 0 _I_ 0 3.51 153.02

FIGUREB.3 ComputerPrtntout for Baseline/Night

128
®'

"C

t_

-- _J

E ORIGINAL PAGE fS

.. I£I OF POOR QUALITY

,--'6

_f

_- l)-_ DAY

_._. .___ NIGHT

........... _J............. d _ i_..........

2 4 6, :-,

It:'IE)LF")-:T [)-_tl TItI[ ' FI'-,E)- '

FIGUREB.4 Baselinewith (--)and without (---)Spatial

and Time SplittingFactors

frame transfer dead time which will probably be between 10% and

30% depending on the selected integration time. Due to

eleetronie/sen-_o.- system constraints, the 10% level is being

sought. These _actors just emphasis that parameter evaluatlon

and selection must be done carefully. The values selected permit

a certain amount of latitude for improvement in some areas to

compensate for some inevitable degradation of other parameters.

o Lightning event splitting (time/spatial) is important.

)

B.3 OPTIMISTIC SET !

; Although many parameters can be adjusted slightly, three

parameters have been identified which, although will result it. !i

some increase in risk and possibly cost, would provide some

enhancement of the LITMAP performance. These parameter (K, Q, and l

pixel size) adjustments are provided in Table 2.2. Figure B.5 i

shows that these changes will reduce the attainable threshold

value to near 5 uJ/m2-sr Hence desired performance based on the

given assumptions, including source image size equal to pixel !, !:

!

129 !
E

_- ORIGINAL PP,GE It_

""-':' OF pOOR QUALITY

4,

i--g

_1=_,

2,

-- 10

--- DAY

r-:-_

LTT/'_ LI_14

: ._ .., -, ...... ...........

:' 4 E, :_::

I_ITEf_,P_,T[F;tl TIr,IE , r,l'i,E,_" ,

FIGURE B.5 Optimistic Performance

size, approaehes the desired detection threshold capability.

o Increased performance can be attained with some

increase in risk and possibly cost.

B.q LIGHTNING PULSE DURATION

The effect of dividing lightning events between two frames

• is illustrated if, Figure B.6 along with effects cf pulse

' duration. All of the curves in this appendix have th_s gene.al

shape. A finer step and larger range for the integration time

was used in Figure B.6 to emphasis the shape of this curve. From

: hardware contraints, times shorter than one millisecond would not

: be practical so mcst of the parameter trades were done with fewer

m

data points and longer times.

The mimlmum threshold point is at the minimum o£ the curves

in Figure B.6. The left hand (small integration time) rise is

due to the time splitting factor and the gradual right hand rise

is due to the longer lntegretlon of the solar background. The

minimum occurs where the iptegratton time equals the pulse f

duration time. To avoid large _hanges in threshold as a

i

130

1

................. "- iii i rl I ....... 7
"_ 1 DAY

I -,r_

,_ OF POOR QIJALITY TL(MSEc

. ./.2

._ 15 1.0

0.8

LL_ 0._4

10

i

j............. .1...... -J. L

: 4 6 :---

II'TEII, F'HT.[F;t4 Title , r'l'-,E,- ,

[

FIGURE B.6 Pulse Duration Effects

function of pulse duration, integration times longer (rather than

smaller) than the pulse duration is desired. This integration

1

time requirement is also driven by hardware, hence the analysis

in Appendix A resulted in a baseline selection of 4 milliseconds

as the integration time. This selection somewhat minimizes the

t

. variations due to pulse durations from sub milliseconds to one or

" two milliseconds. Not that the longer pulses will have some

increase in their threshold, bdt these pulses tend to be the more

intense pulses, hence will be detected inspite of this increase.

. o Pulse duration and time splitting considerations

drive the integration time toward several milliseconds.

- This result is also compatible with some of the

hardware constraints (e.g. data rate).

B.5 FILL FACTOR

The source size relative to individual pixel FOV dimension

influences the threshold performance capability of the LITMAP

instrument. Figure B.7 illustrates the effects where all

: parameters were set at their baseline values (including fixed

131
I

L_R;GINAL PAGE 15

OF POOR QUALITY

_" --- J DAY

_ 20 ,-- 50°N LB (kin)

I"1

=1"'

-I-

'=' 1 =

_._ _ _0'o

w _ Ii (NADIRI

= J,-i

5 _7

I. i J l --.-J

.:2 4 d, :E:

I}ITEGF'_-_T [OH TIHE ' H"I, EL- ,

r

. FIGURE B.7 FOV Effects - Day

source size) and the FOV varied. The O°N (subsatellite point)

and 50°N (maximum viewing angle) limits are included. The

analysis was done such the maximum permissible aperture was

w

always used as constrained by the FOV and detector pixel size.

This gave the minimum threshold for a given set of conditions.

- As expected, the FOV to source size has strong effect on the

' threshold capability. It is thus emparative that the effective

source size be quantified. This will be discussed a little more

: in a subsequent section.

p

, For the present situation, with a fixed effective source

i

size, the background area viewed is larger as the viewing angle

, moves away from the sub-satellite point. A threshold degradation

of about 2,7 results at the maximum viewing angle.

Spatial splitting has been included and does have an effect

which is very apparent in the nighttime plots of Figure B.8

corresponding to the same conditions of Figure B.7. Without the

splitting factor the nighttime capability would be only amplifier

noise dependent, hence constant (without noon light etc.). But :-

132
k .........

NIGHT

' ':-":-' ORIGINAL F,A_.' ;;_

OF POOR QUALITY

,£-i

-_.

_1=,

'+ ___ LB(km )

tJ.J

: lO 20 - 50°N

_ _ , , -. _O°(NAI)IR)

" 7

i _.J J J

:2 4 6, ::;

IIII'F:CF-'_-,TrOH TIr,I-E , F'I'-,EC ,

_- FIGURE B.8 FOV Effects - Night

as seen in Figure B.8, the nighttime capability degrades

(threshold increases) as the FOV increases, primarily because of

the effect through the spatial splitting factor which requires

larger effective source strengths as the FOV increases relative

to source size.

The composite result for the FOV effect coupled with the

. spatial splitting falls between a linear and square dependancy as

illustrated in Figure B.9 where the solid line is for computed

values. All curves were made equal at the nominal value of 11 km

..

baseline sub-satellite resolution point.

o Off nadir viewing angles will have a threshold

degradation of up to a factor of about 3.

B 6 PIXEL SIZE

One of the key parameters that can be varied to enhance the

i

LITMAP instrument performance is that of sensor pixel size.

Larger pixels permits larger apertures and full well capacities, i

This latitude must then be traded with achievable array size i

i

133 i
ORIGINAL PAGE IS

OF POOR QUALITY

40-- _LB) 2E° (AREA) //

I

I

I

I

I

I

I

I

30-- /

" 2i"-1 Eo DAY

2o

!/ J ( LB/E° (LINEAR)

/.,,."

Eo /i 10 - - ' NIGHT

i

- /,/ .!

I , I

O_ 1 2

LB/LS

FIGURE B,9 Thresho]d Dependency on FOV,

.+ Basellne conditions and Ls= 11 km.

134

. . _--- II IU{I 11 Ir : lu|ill _ P_
which usually results in building a mosaic of smaller arrays.

This mosaic approach introduces its own complications including

optical/focal plane complexities.

Performance for pixel sizes from 15 to 40 microns are

illustrated in Figure B.IO for daytime operation. The 15 micron

pixel size as used for the large 800 X 800 NASA/JPL Galileo

project is not suitable for LITMAP, both from sensitivity

(limited aperture size) and full well (background handling)

requirements. Figure B.11 again illustrates that the larger

aperture permitted by a larger pixel results in a lower

detectable energy level.

o Large pixel size is necessary (> 20 microns)

B.7 READOUT NOISE

The process of extracting the photon produced electrons

from the sensor array will induced noise contributions in

addition to those associated with solar background and signal.

The primary contributor, as indicated in Appendix A, is the

readout amplifier associated noise. This noise contribution is

often specified as the number of rms noise equivalent electrons.

For the baseline conditions, the effect of varying the

amplifier noise is smmarized in Figures B. 12 (day) and B.13

(night). For normal solar background and cloud albedo, a

significant contribution from the amplifier noise does not occur

until somewhere above 200 e rms. From Figure B. 12, the present

baseline concept could accommodate up two or three hundred

. electron rms noise contribution without serious degradation of

: performance. But as noted in Figure B.13, as the amplifier noise

increases, the night performance degrades. As this contribution

approaches 400 e rms, the day/night performances will be about

equal for a 4 millisecond integration time. For larger noise |

contributions, the performance will become amplifier limited, not

solar background limited.

o Amplifier noise must be kept significantly below the

solar contribution if enhanced night performance is

desired (i.e., Nam p (rms) < Nso I (rms)/2).

:35

®
!

ORIGINAL PAGE" 18

OF POOR QUALITY

_' -. (MICRONS)

It.[

1 .L .J L

>' 4 6, :-:

f

_, ItlTEC, F'HT [Oil TIr'IE , M'-,EC '

FIGURE B.IO Pixel Size Effects - Day

NIGHT

I

o4 :'5

E

- "_ .:O 6 (MICRONS)

¢ .+

._ I% 15

; -- _0 ?0

22.5

_, 5 _ 25

'| 30

' ' 40

l £ J 1

_ :,.: 4 E, 8

: IIITEC, F'i_T I01_ T Ir,lE ,r,I'-I,EC.,

f

' FIGURE B.11 Pixel Size Effects - Night

, Of

-I

i

136
ORIGINAL PAGE IS

OF POOR QUALITY

DAY

I " _" I e Js

04 E .,

_""b -- 800

v 'L _

+ 400

i--J

,I=,

LLJ

-- 10

l ......... J_ J. ___I- ............

.:' 4 ,=, :--:

IIITEISP_!T[[Jtl TIHE , rII':-,E 12 '

r

L

FIGURE B.12 Amplifier Noise Effects - Day
B.8 WAVELENGTH OF POOR QUALITY

Primary focus has been on the 8683A wavelength. Other

candidates exist and should possibly be evaluated. The baseline

performance at 8683 and alternate 7774 angstroms were compared.

The following values were assumed:

Parameter Baseline Alternate Units

Wavelength 8683 7774 A

Q 0.35 0.5 -

IB 300 396 w/m2-sr-um

Figure B.14 illustrates that the threshold values are about the

same for the conditions considered. Other factors such as sensor

i

materials and/or larger absolute strength for one of the lines

r_ may result in a preference. At the present, the 8683 A line has

been baselined although it has a somewhat lower detection quantum

efficiency when silicon dioxide is used for the structural

; elements. If tin oxide is used, as for the Westinghouse devices,

both spatial line detection efficiencies are about equal and

fairly high (> 50%). Hence the tin oxide material would be a

good candiate for the "optimistic" system.

: o Relative optical magnitudes of the spectral lines are

: needed.

p

t

_. DAY

!

N_=25

: ,'_

"" 2 0 -

4-

i i

..-, 15 - 8683X

L.LJ

: -- 1,-i .-

; 5-

t

i := J ,,, I I I

;-' 4 E, 8

t

j I t ITE ="F' . _4T I f.'=t¢ T I r,1E ,:r,1,--E, r: -,

FIGURE B.14 Wavelength Sel_tlon

138
I

OR:G_,_IALPAGE IS

B.9 FILTER BANDPASS OF POOR QUALITY

The filter bandpass must be as narrow as possible to reject

the solar background. Birefringent filters have been developed

that can provide large FOV coverage while maintaining narrow

passbands but suffer from lower transmission, high cost and high

risk. Interference filters, although more technologically

available, exhibit a FOV angular dependency which does not

accommodate a large FOV (18 ° ) and narrow bandwidth (<5 A)

simultaneously.

The effect of optical bandpass on the daytime threshold

performance is illustrated in Figure B. 15. A 25A bandpass would

degrade the system performance by a factor of 2 relative to a 5A

filter. This degredation is in addition to the factor of 2 which

: occurs if optical splitting factors are not included.

• As a result of these considerations, the FOV was divided

into smaller purtions which permitted the use of interference

filters possessing around 5A passband to be used. If the" need

is sufficient, some improvement could be realized through the use

or' a birefringent type of filter.

o Narrow bandpas_ is necessary (<SA).

w

L / 25 DAY

! ., r.:

. %

"+ i

.-15

LL!

] I-t

------ ] i

i

".' 4 6 8

TI_TEJSF'/tT [Ot_ TII'!E , M';',EL" ,

FIGUREB.15 Optical Bandrass Effects iI

13g
!

OF POOR qUALITY

B, 10 EVENT SOURCE SIZE _/I;_'O_'_ALI'nf

Physical, complexity and cost constraints place restrictions

on the allowable number of array elements that can be utilized.

This limited array size coupled with full disc coverage places a

severe restriction on the source energy that can be resol#ed

during day operations. As discussed earlier, this detection is

also complicated by the geometric splitting.

The baseline concept assumes that the source and pixel FOV

are of equal size, 11km. Figure B.16 illustrates the increase in

; the required threshold energy as the source moves from II km

dimensions (lower curve) to 5 km. The night performance is not

as bad as illustrated in Figure B. 17. If the 4 uJ/m2-sr is a

t true indication of the source strength, and if the effective

,. source size is considerably less than 11 km square, then the

threshold will be much larger. This condition would result in

pushing the LITMAP instrument design hard toward the technology

limits tc provide the performance desired from a geosynchronous

orbit. An alternative would be to decrease the pixel FOV to

match source size. If the full disc coverage then requires too

many array elements_ a move to a very LITMAP compatible

step/stare concept eou]d be made.

?

o Event source size must be quantified.

• _ DAY

L 5 km 7

I _ c,

-

_ 2 t:t

I._t

- It

• _ __11

. -- 10

5 ,

J , t 1

2 4 E. 8

Tt IT EGE'HT ] Ot_ T I r'lE ,'r'lSEC ,

FIGURE B.16 Source Stze Effects - Day

140

[
NIGHT Z"

m OJ,iO;N/4- P¢._ [_i

OF POOR QUALITY

.

---- 't-t

kkt -" 5 km

r.- !

.......... ..J .............. I................ J.............. _1 .............

Z 4 _, :-',

IIITIL'_L,F'_,I" [C;I; T Ir,11-, rl,i,E:,

FIGURE B.17 SourceSize Effects- Night

B.11 EXISTING DEVICES

The capability of existing imaging array devices were

assessed to put the overall system trade evaluations into

perspective with the real world. A 22.4 micron TI (TV: IC-202)

device is compared with a RCA (SID 501D) 30 micron device. As

ca:_ be seen in Figure B.18, the RCA device comes close to the

LITMAP baseline limit. The smaller pixel device has a higher

threshold capability. Performance of other devices are

illustrat__d in Figure B.19 (8683A) and Figure B.20 (7774A), For

ease of wavelength dependency comparison, Figure 21, 22 and 23

illustrates the 8683A and 7774A results for RCA, Westinghouse and

Hughes. Where the detection quantum efficiencies are nearly

equal for the two wavelengths (as in the Westinghouse device) the

8683A line comes out ahead, otherwise the 7774A llne has a small

edge. The device pixel sizes and quantum efflciencles are given

" in a device characteristics table located in S_ction 5.

o Existing commercial devices approach the LITMAP

baseline performance speelflcatlons (array

partitioning required to handle data rate).

141

®
- F+y

ORIGINALPAOE r_

OF POOR QUALrr'¢

DAY

' -,=, _ TI-TC202

C,,El -

+_p

-,+_ I __, _itCA SID 501D

f

--" I c: -

.......... -J ........... I........... -J. i

,:; 4 E, :_::

flI'[FfJ/,g',-,T[F+P_ Ylr'11-- , FI",EC ,

: FIGUREB.18 ActualDevices- RCA, TI

_- : DAY ........................................... ]

• J ' _'1

'_'1 I _ WESTINGHOUSE 5040

c, _RCA

SID 501D ,

; +" ] 0

i

t,. b L

P

l ............. ...J. ............ I................ i .................. k

.-' .+ r ; :-'

' I tlTEGI;'#+T I OH T II'I["." , r'I':;KL"., [

FIGURE B.19 Actual Devices - 8683A i

i

!

I

t

[

++ 142

!!_1 _ ....... . .. +_,,.
@. a)

ORIGI.t,;AL. PAI3E 13

OF POOR QUAL!TY

_- DAY

t_

I

c,jE TI TC20 2 1

- _ HUGH WESTINGHOUSE ES H4008 5040

TI TC201 (US-TV)

,_! RCA SID 501D

c. i

L.......... .1................ i .......... L............. I .....

I tlr F i-,[;'_-,T. [ ,i_ r I *,'I_. ' t1'-.E-;

FIGURE B.20 ActualDevices- 7774A

"_ !................................................... DAY

E i

"- _ RCA SID 501D

p

,Lt 4

8683_

2, _ n7.R

4 t*f

: !

J

!

I

Ii_rV,,I-'Hl'l,qN I I_"IE , r,1'?.E,., _

: I T

'_. TABLE B.21 RCA:8683A and 7774A (SID 5010)

143
® ®

]

o_!_;_qAL PAGE IS

OF POOR QUALITY

--_ , DAY

J _ i WESTINGHOUSE 5040

i 7774X

+ 8683_

ktI

c+

........ _L ............. I........... L ......... L ......

-, ._ _ :-'

I t_ I EGF'H I i :-_t.4 r I I,I_. ,. r,l'-,Ea_ ,

FIGUREB.22 Westingtause: 8683A and 7774A (5040)

' _ I .................................................... -'H_S _ 4068 DAfi w, q_,ot_ i

, % 7774_ I

'-:+t 1

" ' I

_I "

1 t

, L............ J................ ..1. .... .L. -_----

k 4 _" :::

FIGUREB.23 Hughes: 8683A and 7774A (H4Of_)

• 144

k D
®- I

G_:a,.,AL PAaE I$

.OF.POOR QUALITY

B.12 SUMMARY

This section has illustrated the threshold dependency on the

various sensor design parameters. These analyses were also

weighed against electronic and physical constraints as well as

cost and schedule. Table B,I summarized the results of this

appendix.

TABLE B.l Parametric Trade Results

FACTOR COMMENT

,- o Baseline Lightningevent sp]itting (ti_/spatia|) is important.

o Optimistic Increasedperformancecan be attained with some increase

in risk and possiblecost.

o Pulse l>Jration Pulse durationand time splittingconsic._rationdsrive

the integrationtime toward several mil]iseconds. This

resultis also compatiblewith some of the hardware

constraints(e.g, data rate).

o Fill Factor Off nadir viewing angleswilt have a thresholddegradation

of up to a factor of about 3.

,_ o PixelSize Large Pixel size Is necessary(> 20 microns)

o ReadoutNoise Aanpliflernoise must be kept significantlybelow the

solarcontributionif enhanced night performanceis

-- desired (i.e., Namps (ms) <NsoI (rms)/2).

o Wavelength Relatlveoptical magnitudesof the spectral 11nesare

needed.

o Filter Bandpass Narrow bandpass is necessary (< 5A).

o Event Source Stze Event source stze must be quantified.

_'i o ExistingDevice Existingcommerclaldevices approach the LITMAP

baseline perfcrmnce specifications(array

",. partitioningrequiredto handle data rite).

!|

",_. 'I_'U S. GOVERNMENT PRINTING OFFICE 1983-746.070/81

145