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MOBY Uncertainties

Plan for calibration and maintenance of AHAB Uncertainty Budget: Laboratory Components Carol Johnson NIST. MOBY Uncertainties. MOBY Calibration Workshop Nov 2003 to address uncertainties in measured water-leaving radiance Radiometric components Uncertainty in the primary calibration sources

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MOBY Uncertainties

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  1. Plan for calibration and maintenance of AHABUncertainty Budget:Laboratory ComponentsCarol JohnsonNIST

  2. MOBY Uncertainties • MOBY Calibration Workshop Nov 2003 to address uncertainties in measured water-leaving radiance • Radiometric components • Uncertainty in the primary calibration sources • Transfer uncertainty • MOBY radiometric scale maintenance during deployment • Systematic effects • Temperature • Stray light • Wavelength error • Environmental components (Ken’s Talk) • Instrument self-shading • Wave focusing: environmental ‘noise’ • Polarization

  3. Satellite sensors uncertainty requirements: 5 % in water-leaving radiance Radiometric calibration uncertainties of 4 % to 8 % (6 % > 400 nm) D. K. Clark, et al., Proc. SPIE 4483, 64-76 (2002) MOBY Radiometric Uncertainties • Established rigorous measurement protocols ensuring direct traceability to primary national radiometric standards • Established radiometric uncertainty budget conforming with international recommendations • Goal: uncertainty budget to be dominated by environmental factors. • Uncertainty goal: ~ 3 % (k=1)

  4. MOBY Uncertainties – Why do they matter?To evaluate what uncertainty components are important, it is important to understand how the MOBY data are used. • SeaWiFS and MODIS: MOBY data are used to set the T=0 post-launch gains. • In using a large number of MOBY matchups, random uncertainty components will be reduced, but systematic effects will not. • For a large-enough data set, final uncertainty in gain coefficients will be dominated by systematic effects. SeaWiFS

  5. NIST Standards NIST Standards Correction for temperature and stray light Correction for temperature and stray light Uncertainty MOBY Radiometric Calibration Flow Diagram

  6. Primary Calibration Sources

  7. How well can you do?Uncertainties in Irradiance Standards from NIST

  8. How well can you do?Uncertainties in Radiance Standards from NIST

  9. Uncertainties from Transfer of Scales

  10. How well can you do?EOS Laboratory Intercomparison Experiments Results of measurements of Santa Barbara Remote Sensing SIS100 lamp-illuminated integrating sphere: sphere was used for MODIS and Landsat ETM+ pre-launch calibrations Transfer radiometers from NIST, NASA’s GSFC, and the University of Arizona measured the sphere radiance under different illumination conditions and compared their results with the SBRS-determined radiance. This +/- 2% agreement is good result, based on our experience. J. J. Butler, et al., J. Res. Natl. Inst. Stand. Technol. 108, 199-228 (2003).

  11. NIST-traceable calibration: 3-5 % uncertainties (secondary standards laboratory) NIST calibration unc (400 to 700 nm): < 1 % MOBY Calibration Sources & Uncertainties • Re-calibrated every 6 months or 50 H of use • - Calibrated first with original lamps (1 % to 2% agreement) • - Re-lamped and calibrated a second time • Monitored during operation using NIST- calibrated filter radiometers called Standard Lamp Monitors (SLMs) • Yearly NIST visits with transfer radiometers and sources to validate the MOBY radiance scales

  12. Calibration Stability & Repeatability OL420 and SLM Radiance At 412 nm (870 nm results similar) OL Calibration History Stability ~ 0.5 % (SLM) within the calibration uncertainty

  13. Uncertainties Associated with the Deployments

  14. Internal Reference Lamps - Stability QC Both + - 0.5% Blue < 0.5 % Red < 0.5%

  15. Diver Reference Lamp Calibrations The changes observed are well within the uncertainty of the method. Hence we cannot draw any useful conclusions.

  16. Pre to Post Deployment Calibrations For top arm input and all deployments Averaged over deployments by wavelength Assume a rectangular probability distribution, the associated uncertainty is about 0.6%

  17. In Situ Wavelength Calibration with Spectral Features Blue Spectrograph 2.5 years Approx. +/- 0.6 nm Red Spectrograph 2.5 years Approx. +/- 1 nm

  18. Uncertainties Associated with Systematic Effects

  19. Temperature Affects the System Responsivity We measured and then applied a temperature correction to the pre- and post- deployment calibrations (as well as all MOBY data during deployments).

  20. Impact of Stray Light or Spectral out-of-band No instrument is perfect: every instrument measures unwanted radiation Spectral out-of-band of representative MODIS bands Stray light causes systematic errors: that is, errors that don’t average to zero with repeat measurements. What is its magnitude? Does it impact the measurement requirements?

  21. Stray Light in MOBY Stray light correction to MODIS Bands Images of Laser Lines Single Deployment Correction to Responsivity Multiple Deployment Time Series

  22. Evaluation of the uncertainties: Monte Carlo There are a number of parameters that go into the model; each has an uncertainty We doubled the uncertainty in the fits to those parameters to account for drift, etc. Then ran a Monte Carlo simulation: for each component we used a Gaussian probability distribution. Simulations run a minimum of 100 times & uncertainties calculated. Uncertainty in SLC for in-water upwelling radiance Lu

  23. MOBY Radiometric UncertaintyMOBY WorkshopPreliminary Results

  24. MOBY Summary: Lessons Learned for AHAB • Traceability to primary national and international radiometric standards and the SI • Good experiment design • multiple measurements (pre- & post- calibrations) • verify and validate • strict protocols • methods to monitor detectors and monitor sources • Characterize instruments thoroughly

  25. AHAB Implementations • Primary Calibration Sources • Blue-rich, tailored for ocean color spectral distributions • Transfer of Scales • Calibrate sources using NIST facilities • Expand SLM concept to hyperspectral (more spectral information) • During deployment • Internal sources (LEDs) • System level stability monitoring • Improvements to diver calibration lamps • Systematic • 2-D stray light characterization at NIST SIRCUS facility

  26. Primary Calibration Sources Source Spectral Power Distributions

  27. LED Sources

  28. LED Source: Field Tests More flux in the blue; Better matched to the ocean’s spectral distribution

  29. Spectrally Tunable, Detector-based Source Under development using GOES-R funding

  30. System Level Stability Monitoring • Stable LED sources, as proven in MOBY (0.5% during deployments) • Fiber-coupled to external optical input • Allows Daily system level monitoring of AHAB responsivity • Eliminates major source of unknown behavior for MOBY

  31. AHAB Characterization • Thermal characterizations at NIST • design for good thermal control and stability • Detailed and thorough stray light characterization at NIST • spectral and spatial • smaller system means all tests can be done at NIST, resulting in full and dense wavelength coverage • Spatial effects have been dealt with under R&O work • Excellent wavelength stability • Monitor using solar lines as with MOBY

  32. Summary • AHAB’s radiometric uncertainties will be the lowest possible for this type of field activities.

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