2-5 December 2003 International Workshop on Radiometric and Geometric Calibration 1 System Level Approach to Characterization and Radiometric Calibration

2-5 December 2003 International Workshop on Radiometric and Geometric Calibration

1

System Level Approach to Characterization

and Radiometric Calibration of Space Based

Electro-Optical Sensors

Joe Tansock, Alan Thurgood, Mark Larsen Space Dynamics [email protected]

435-797-4369


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Outline

• Philosophy

– What is meant by a complete calibration

• Planning

• Subsystem/Component Measurements

• Sensor-Level Engineering Calibration

• Sensor-Level Calibration

– Facilities

– Data Collection

• On-Orbit Calibration


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Calibration Philosophy – Complete Cal

• A complete sensor calibration:

– Provides a thorough understanding of sensor operation and performance

– Verifies a sensor’s readiness for flight

– Verifies requirements and quantifies radiometric and goniometric performance

– Converts sensor output to engineering units that are compatible with measurement objectives

– Provides traceability to appropriate standards

– Estimates measurement uncertainties


4

Calibration Philosophy – Cal Domains

• A complete calibration will address five responsivity domains:

– Radiometric responsivity• Radiance and irradiance traceable to NIST• Response linearity and uniformity corrections• Nominal/outlying pixel identification• Transfer calibration to internal calibration units

– Spectral responsivity• Sensor-level relative spectral response

– Spatial responsivity• Point response function, effective field of view, optical distortion, and

scatter– Temporal

• Short, medium, and long-term repeatability, frequency response– Polarization

• Polarization sensitivity


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Calibration Philosophy – Cal Domains

• The goal of calibration is to characterize each domain independently

– Together, these individually characterized domains comprise a complete calibration of a radiometric sensor

• Domains cannot always be characterized independently– Complicates and increases calibration effort– Example: Spectral spatial dependence caused by Stierwalt effect

• Calibration parameters are grouped into two convenient categories:

– Calibration equation

• Converts sensor output (counts, volts, etc.) to engineering units

– Radiometric model

• All parameters not included in calibration equation but required for complete calibration


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Measured Radiance [W/cm2sr]

Array-average peak-radiance responsivity [counts per W/cm 2sr]

Array-average peak-radiance responsivity change vs. time [unitless]

Corrected detector response [counts]

Bad detector mask function [unitless]

Gain or integration normalization factor [unitless]

Nonlinearity correction function [counts]

Raw detector response [counts]

Detector offset correction [counts]

Linearity corrected response due to telescope thermal emission [counts]

Flat-fielding or non-uniformity coefficient [unitless]

Flat-fielding or non-uniformity coefficient change vs. time [unitless]

Detector index - parameter is unique to each detector

Time - parameter varies as a function of time

, , , , , , ,, , ,

, , , , ,

( )1 1 k I NL k T k t O k t B k tM k t k t

L L t L L t FF k FF k t

B G F r r rL r

F F

, ,M k tL

L

,k tr

,FF kF

IG

kB

, ,O k tr

, ,T k tr

,L t

, ,B k tr

, ,FF k tF

k

t

,NL kF

Typical Radiance (Extended Source) Calibration Equation for Imaging Array Based Radiometer

Calibration Philosophy – Cal Equation


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Radiometric Model ParametersNoise equivalent radiance ICU Response settling timeSaturation equivalent radiance ICU Response RepeatabilityNoise equivalent irradiance ICU Response UniformityNoise equivalent temperature Dark offset repeatabilitySaturation equivalent irradiance Dark offset drift rateIlluminated Short-Term Repeatability vs. Radiance Point source repeatability (short, medium, and long)Saturation Response Extended source repeatability (short, medium, and long)Relative Spectral Responsivity Angular measurement accuracy and precisionEffective Field-of-View Polarization SensitivityPoint Response Function UncertaintyModulation Transfer Function Off-axis rejectionNear angle scatter Optical DistortionArray coalignment

Typical Radiometric Model Parameters for Imaging Array Based Radiometer

Calibration Philosophy – Rad Model


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Calibration Philosophy – SI Units

• Express calibration results in SI units

– Standards maintained by national measurement institutes

– Recommended Practice: Symbols, Terms, Units and Uncertainty Analysis for Radiometric Sensor Calibration, NIST Handbook 152, Clair Wyatt, et. al.

– http://ts.nist.gov/ts/htdocs/230/233/calibration/uncert/index.htm

• Contains Links for – Guidelines for Evaluating and Expressing the

Uncertainty of NIST Measurement Results, 1994– Guide to the Expression of Uncertainty in

Measurement, International Standards Organization (ISO), 1993


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Calibration Philosophy - Uncertainty

• Components of standard uncertainty are identified by taking partial derivative of calibration equation with respect to each parameter

• Combined standard uncertainty

– Law of propagation of uncertainty

– Where ƒ is a function (typically the calibration equation) with N parameters

– If terms are independent, cross terms go to zero

– If uncertainties are expressed in percent

21

2 2 2,

1 1 1

2N N N

c i i ji i j ii i j

f f f

x x x

2

2 2

1

N

c ii i

f

x

2 2

1

N

c ii


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S h o rt-T erm Resp on se R epea tab ility(A)

M ed iu m -te rm R espo n se R epea tab ility(A)

In te rn a l Ca l M easu rem en ts(A)

O n -O rb it Ste llarM easu rem en ts

(A & B)

L o n g-te rm R esp on se R epeatab ility

R e flectan ce V ariab ilityas Fu nc tion of An g le

(A)R e flectan ce var iab ilityD u e to Co n tam ina tion

(A & B)

R espo n se V ariab ilityO ver F O R (fun c tion of

p oin tin g m irro r p os itio n)

R ad io m etr ic M easu rem ent p rec is ion (RM P)

S tand ard Error of R esp on s ivity C o effic ient

(A)L in earity C orrection

U n certa in ty(A)

R espo n se U n certa in ty Du e toT e lesco pe T hem al E m iss ion

(A & B)In teg ration M o de

N o rm a liza tion(A)

M easu rem en t Un certa in ties

B lackbo d y E m iss ivity U n certa in ty(A & B)

C a lib rato r Po intin g M irror R e flec tivity U n certa in ty

(A)B lackb od y Tem p eratu re

U n certa in ty(A)

R S R U ncerta in ty(A)

S o u rce Un certa in ties

P eak rad ian ce respo n s ivity un certa in tyd erived from gro un d ca lib ration

R espo n sivity T ren d Co rrec tion U ncerta in ty(A)

P eak Rad ian ceR esp on s ivity U n certa in ty

A rray R esp o nse Un iform ityto Ex tend ed S ou rce

(A)E x ten d ed S ou rce

R ad ian ce U n iform ityU n certa in ty

(A & B)

N U C U ncerta in tyfro m G ro un d C a lib ration

N U C trend correctio n U nc(A)

N U C U ncerta in ty

In teg ration M o deN o rm a liza tion U ncerta in ty

(A)

L in earity C orrectio n U ncerta in ty(A)

R espo n se U n certa in ty Du e toT e lesco pe Th erm a l Em iss ion

+ D ark O ffset(A & B)

R espo n se U n certain ty Du e toN o n -R e jected E arth R adian ce

(B )

C o rrec ted Respo n se U ncerta in ty

R ad io m etr ic M easu rem ent A ccu racy (RM A)

Example On-Orbit Absolute Radiance Uncertainty Budget for Imaging IR Instrument

Calibration Philosophy - Uncertainty


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Calibration Philosophy – Phases of Cal

• A complete and methodical approach to sensor calibration should address the following phases:

Calibration planning during sensor design

Ground measurements

Subsystem/component measurements

Sensor-level engineering tests and calibration

Sensor-level ground calibration

Integration and test

On-orbit measurements On-orbit calibration


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Calibration Planning

• Perform calibration planning during sensor design

– Sensor design should allow for efficient and complete calibration

– Sensor design and calibration approach can be optimized to achieve performance requirements

• Planning phase can help shake out problems

– Schedule and cost risk is minimized by understanding what is required to perform a successful calibration early in the design phase


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• Identify instrument requirements that drive calibration

• Identify calibration measurement parameters and group into:– Calibration equation– Radiometric model

• Flow calibration measurement parameters to trade study– Schedule – Sensor design feedback– GSE hardware & software– Measurement uncertainty– Risk

• Perform trade study to determine best calibration approach

Mission Requirements

Sensor Design

Cost & Schedule

Risk

GSE Hardware & Software


Measurement Uncertainty

Instrument Requirements

Calibration Measurement Parameters• Calibration Equation• Radiometric Model


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Subsystem/Component Measurements

• Subsystem and/or component level measurements

– Help verify, understand, and predict performance

– Minimize schedule risk during system assembly

• Identifies problems at lowest level of assembly• Minimizes schedule impact by minimizing

disassembly effort to fix a problem

• System/Sensor level measurements

– Allow for end-to-end measurements

– Account for interactions between subsystems and components that are difficult to predict


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Subsystem/Component Measurements

• Merging component-level measurements to predict sensor level calibration parameters may increase system-level uncertainties A,B

– SABER relative spectral responsivity (RSR)

• 9 of 10 channels < 5% difference• 1 channel 24% difference (reason unknown)

A.) Component Level Prediction versus System Level Measurement of SABER Relative Spectral response, Scott Hansen, et.al., Conference on Characterization and Radiometric Calibration for Remote Sensing, 1999

B.) System Level Vs. Piece Parts Calibration: NIST Traceability – When Do You Have It and What Does It Mean? Steven Lorentz, L-1 Standards and Technology, Inc, Joseph Rice, NIST, CALCON, 2003


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Sensor-Level Engineering Calibration

• Engineering calibration

– Performed before ground calibration (Lesson Learned)

– Perform abbreviated set of all calibration measurements

– Verifies GSE operation, test configurations, and test procedures

– Checks out the sensor

– Produces preliminary data to evaluate sensor performance

– Feedbacks info to flight unit, calibration equipment, procedures, etc.

• Engineering calibration data analysis

– Evaluates sensor performance, test procedures, calibration hardware performance and test procedures

• Based on results of engineering calibration, appropriate updates can be made to prepare for ground calibration data collection


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Sensor-Level Ground Calibration

• Provides complete calibration

• Is performed under conditions that simulate operational conditions for intended application/measurement

• Minimizes risk of not discovering a problem prior to launch

• Promotes mission success during on-orbit operations

• For many sensor applications

– Detailed calibration is most efficiently performed during ground calibration

– On-orbit calibration will not provide sufficient number of sources at needed flux levels

– Operational time required for calibration is minimized

• Best to perform ground calibration at highest level of assembly possible

– Sensor-level at a minimum is recommended


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Calibration Facilities

• Make sure calibration hardware has been tested and characterized (Lesson Learned)

– Problems with calibration hardware may cause schedule delays and degraded calibration

• If possible, integrate calibration measurements into single facility (Lesson Learned)

– Minimizes calibration time by reducing or preventing repeated cycle (i.e. pump, cool-down, warm-up) and configuration times

• Examples:

– The multi function infrared calibrator (MIC2) incorporates 4 source configurations in single package

– SABER calibration facility

• Test chamber interfaced with collimator provided calibration measurement configurations


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MIC2 Interfaced with Sensor Under Test


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Collimator Source Extended Source

Scatter Source Jones Source

MIC2 Source Configurations


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Test Chamber and Work Area Collimator

SABER Calibration Facility


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SABER Calibration Facility


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Calibration Data Collection

• Develop and write calibration data collection procedures

– Include:

• Test procedures • Time requirements• Preparation and data collection steps• Documentation of script files• Data collection log sheets


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• Data collection should be automated when possible and practical

– Automate with scripting language to make measurements efficient and repeatable

• Data collection procedures should be detailed and mature

• Sensor engineers and/or technicians may assist with data collection

– Requires familiarity with sensor under test

– Makes shift work possible to facilitate schedule

• Data quality should be verified for its intended use with quicklook analyses

• Contamination should be monitored using QCM and/or radiometric techniques

– Quantify contamination levels

– Determine when corrective action is required


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• Data collection environment includes:

– Test conductor and data collection station

– Ground support equipment (GSE) computer

• Controls and views status of GSE

– Instrument computer

• Controls and views status of instrument

– Data collection computer

• Initiates and executes data collection• Controls and monitors status

– GSE– Instrument

– Quick look analysis station


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Quick Look Analysis Station

Data Collection ComputerGSE ComputerInstrument Computers & Racks



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On-Orbit Calibration

• Calibration continues after sensor-level ground calibration

• Track, trend, and update calibration throughout a sensor’s operational life

– On-board internal calibration sources

– External sources

• Ground sources prior to launch• On-obit sources after launch

• Verifies calibration and quantifies uncertainty

Sensor Design/Fabrication

Ground Calibration On-Orbit Operations

Internal Calibration Unit (ICU) Response Trending

On-Orbit Calibration/Verification


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On-Orbit Calibration

• On-orbit sources

– Standard IR stars• StarsBoo, Lyra, Tau, CMa, Gem, Peg• Catalogs include IRC, AFGL, IRAS, MSX, 2MASS

– Planetary objects• Planets provide bright variable sources• Asteroids, moon, etc.• Sometimes you have to be creative:

– Off-axis scatter characterization using the moon

– Reference spheres

– Other techniques• View large area source located on surface of earth (remote

sensing applications)


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Summary

• What is meant by a complete calibration

• Calibration parameters are organized into two categories

– Calibration equation and radiometric model

• Overall calibration approach

– Perform calibration planning in parallel with sensor design

– Subsystem measurements are a good idea but don’t rely on these measurements to give system level calibration

– Perform engineering calibration to verify GSE, test procedures, and estimate sensor performance

– Obtain complete and thorough sensor level calibration

– Verify and/or update calibration throughout operational life


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Session Topics Include:

Concepts and Applications of Measurement Uncertainty

Solar, Lunar and Stellar Radiometric Measurements

Pre-launch to On-orbit Calibration Transfer: Approaches and On-orbit Monitoring Techniques

Developing National Calibration/Certification Standards for EO/IR Systems

Join us at Utah State University September 13-16, 2004!

The Annual Conference on Characterization & Radiometric Calibration for Remote Sensing addresses characterization, calibration, and radiometric issues within the IR, Visible and UV spectrums.

Documents

2-5 December 2003 International Workshop on Radiometric and Geometric Calibration 1 System Level Approach to Characterization and Radiometric Calibration