System Level Approach to Characterization and Radiometric Calibration of Space Based Electro-Optical Sensors

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System Level Approach to Characterization and
Radiometric Calibration of Space Based
Electro-Optical Sensors

• Joe Tansock, Alan Thurgood, Mark Larsen Space
  Dynamics LaboratoryJoe.Tansock_at_sdl.usu.edu435-79
  7-4369

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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 sensors 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

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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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Typical Radiance (Extended Source) Calibration
Equation for Imaging Array Based Radiometer
Calibration Philosophy Cal Equation
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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/u
  ncert/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

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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 Calibration planning during sensor design
Ground measurements Subsystem/component measurements
Ground measurements Sensor-level engineering tests and calibration
Ground measurements Sensor-level ground calibration
Ground measurements 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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Calibration Planning

• 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

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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 lt 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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MIC2 Source Configurations
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SABER Calibration Facility
Test Chamber and Work Area
Collimator
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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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Calibration Data Collection

• 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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Calibration Data Collection

• 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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Calibration Data Collection
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On-Orbit Calibration

• Calibration continues after sensor-level ground
  calibration
• Track, trend, and update calibration throughout a
  sensors operational life
• On-board internal calibration sources
• External sources
• Ground sources prior to launch
• On-obit sources after launch
• Verifies calibration and quantifies uncertainty

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On-Orbit Calibration

• On-orbit sources
• Standard IR stars
• Stars aBoo, aLyra, aTau, aCMa, bGem, bPeg
• 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 dont
  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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The Annual Conference on Characterization
Radiometric Calibration for Remote Sensing
addresses characterization, calibration, and
radiometric issues within the IR, Visible and UV
spectrums.

• 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!