Comprehensive Approach to Space-Based Sensor Characterization and Calibration

1. System Level Approach to Characterization and Radiometric Calibration of Space Based Electro-Optical Sensors Joe Tansock, Alan Thurgood, Mark Larsen Space Dynamics LaboratoryJoe.Tansock@sdl.usu.edu435-797-4369

2. 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

3. 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

5. 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

6. Typical Radiance (Extended Source) Calibration Equation for Imaging Array Based Radiometer Calibration Philosophy – Cal Equation

7. Typical Radiometric Model Parameters for Imaging Array Based Radiometer Calibration Philosophy – Rad Model

8. 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

9. 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

10. Example On-Orbit Absolute Radiance Uncertainty Budget for Imaging IR Instrument Calibration Philosophy - Uncertainty

11. Calibration Philosophy – Phases of Cal • A complete and methodical approach to sensor calibration should address the following phases:

12. 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

13. 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 Instrument Requirements Calibration Measurement Parameters • Calibration Equation • Radiometric Model Sensor Design Cost & Schedule Calibration Planning GSE Hardware & Software Measurement Uncertainty Risk Calibration Planning

14. 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

15. 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

16. 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

17. 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

18. 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

19. MIC2 Interfaced with Sensor Under Test

20. Collimator Source Extended Source Jones Source Scatter Source MIC2 Source Configurations

21. SABER Calibration Facility Test Chamber and Work Area Collimator

22. SABER Calibration Facility

23. 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

24. 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

25. 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

26. Instrument Computers & Racks GSE Computer Data Collection Computer Quick Look Analysis Station Calibration Data Collection

27. Sensor Design/Fabrication Ground Calibration On-Orbit Operations Internal Calibration Unit (ICU) Response Trending On-Orbit Calibration/Verification 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

28. 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)

29. 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

30. 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!