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Radiometric Calibration

Radiometric Calibration. Stuart F. Biggar, U of AZ Kurtis J. Thome, U of AZ Simon J. Hook, JPL. Outline. Preflight – Biggar Characterization Calibration On-board calibration systems Sensor artifacts In-flight VNIR and SWIR – Thome In-flight TIR - Hook. Calibration (absolute).

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Radiometric Calibration

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  1. Radiometric Calibration Stuart F. Biggar, U of AZ Kurtis J. Thome, U of AZ Simon J. Hook, JPL

  2. Outline • Preflight – Biggar • Characterization • Calibration • On-board calibration systems • Sensor artifacts • In-flight VNIR and SWIR – Thome • In-flight TIR - Hook

  3. Calibration (absolute) Why do we calibrate? • Understand sensor performance • L1 data in physical units (at sensor) • Radiance in Watts/m2-sr-micrometer (or similar) • Determine sensor linearity • Derived units • Reflectance • Geophysical units (temperature, etc) • Comparison to other sensors • Atmospheric correction • Reasonable looking imagery

  4. Calibration (2) What do we need to be able to calibrate? • Stable instrument • Over time • With changing environment • Characterized instrument • Spectral response • Spatial response • Noise • Systematic errors • Image artifacts • Stray light • Stable calibration source

  5. Characterization Piece part measurements • Filter transmittance • Lens and/or mirror transmittance and reflectance • Mirror reflectance and scatter (BRDF) • Lamp output, stability, longevity • Detector Quantum efficiency, spectral response, noise, temperature dependence

  6. Filter spectral characteristics

  7. Normalized Response

  8. Characterization (2) Assembly level • Alignment • Relative spectral response (of telescope) • Noise – amplifier assembly for example • Field-of-view • Stray light

  9. Characterization (3) Sensor level • Relative spectral response • Field-of-view • Modulation transfer function • Noise • Random • Coherent • Stray light

  10. ASTER design “features” (1) • Long linear array detectors with long strip filters in VNIR and SWIR – “pushbroom” • Good SNR due to longer integration time • Detector and filter uniformity issues • Many detectors (5000 in VNIR for example) • Gain • Offset • Spectral response • Large focal plane • Post detector electronic chains (amps, A/D, etc) • “Stripes” in images even of constant radiance scenes

  11. ASTER L1A,bands 1and 13

  12. ASTER design “features” (2) • Short linear arrays (10 elements) with a scan mirror in the TIR sensor – “whiskbroom” • Shorter integration time • Larger pixels to improve the SNR • Limited number of detectors • Individual amplifiers • “Stripes” in images but repeated along track

  13. ASTER design “features” (3) • Three separate sensors • 3 telescopes • Individually pointed • Alignment • Rotation of images • Focal plane distortions • 3 manufacturers • Different design practices • Different measurement practices • Different calibration methodologies and equipment

  14. Radiometric calibration Preflight in the laboratory • Well characterized and controlled source • Reasonable but not normal environment • May (or may not) approximate normal imaging operation mode • Scan mirror may not be operating • Source is usually smaller angular extent than the earth • “Uniform” source is not a normal image

  15. On-board calibrators • VNIR • Lamp based source with monitors • SWIR • Lamp based source with monitors • TIR • Blackbody that can be heated

  16. Philosophy Careful preflight calibration • Determine absolute response so we can convert from DN to radiance • At the same time, run the on-board systems to transfer a radiance value to them using the sensor (ASTER) • Determine noise (random and coherent) • Look for image artifacts • Use preflight values after launch Update after launch as sensors usually change • On-board • Vicarious (ground reference and moon)

  17. Preflight source for VNIR/SWIR Large aperture integrating sphere • 1 meter diameter sphere • Round output port • Multiple lamps • Internal surrounding the port • External through small ports • High reflectance coating on the sphere wall • Barium sulfate (BaSO4) • Spectralon (sintered PTFE) • Gold sandpaper (rough, diffuse surface)

  18. 40” BaSO4 integrating sphere

  19. Integrating sphere • Advantages • Uniform radiance across port (one % or so) • Lambertian (radiance independent of direction) • Multiple output levels • Change number of lamps (MELCO) • Change voltage/current of all lamps (voltage for NEC sphere) • Change aperture size between external lamp and sphere • Reasonably stable with good control of lamp (V or I) • Repeatable if temperature is controlled • Disadvantages • Low output in blue relative to NIR and SWIR

  20. TIR preflight cal source • Large area blackbody • Measure the temperature precisely and accurately • Measure (or compute) emissivity • Compute radiance output • Operated in a vacuum chamber with the sensor • Vary temperature of source to get multiple radiance values • Absolute calibration • Linearity correction (output usually fit to something other than a gain and offset – quadratic is common)

  21. Calibration chain • NRLM (Japanese equivalent of NIST) calibrated a set of fixed point blackbody simulators • Calibration transferred to portable, variable temperature blackbody simulators • Calibration transferred to the ASTER calibration sources (Spheres and TIR calibration blackbody) • VNIR and SWIR spheres were measured at NEC and MELCO with a set of transfer radiometers from NIST, NRLM, GSFC, and University of Arizona • NRLM (now AIST) and NIST collaborate to ensure that their scales are consistent

  22. Are preflight values usable? • Sometimes … • Launch may cause a shift in a sensor performance • Operating temperatures may be different • Something may have moved • On-board lamps may change output • Convection inside the lamp may be different • Aging of the lamp • Emissivity of the on-board blackbody may change • Mirror reflectance may change • Filters/detectors/amplifiers may change • There may have been unexpected “features”

  23. Unexpected “features” • VNIR • On-board calibrator monitor is “off-scale” • Change in output is more than expected so dynamic range of A/D is too small • SWIR • Unexpected stray light causes “crosstalk” • Present during preflight calibration and in all in-flight data

  24. VNIR OBC • VNIR OBC has two monitors for each of two redundant calibrators • One monitor diode at lamp • One monitor diode at output • They track but at different rates • We really want the output monitor to determine how well the on-board calibrator is working but we have only the lamp monitor “on-scale”

  25. VNIR OBC output monitor • NEC selected an expected output range based on preflight measurements, OPS experience with a similar calibrator, and desire to maximize resolution • Output has fallen to below the lowest expected monitor output • Telemetry value for monitor is now a flat line (signal – offset < zero)

  26. SWIR crosstalk • SWIR has multiple spectral bands • 6 linear PtSi detector arrays • Spectral selection filters over the arrays • Not all light hitting the detector is absorbed • Light hitting between the detectors is reflected • Some reflected light is reflected back down by the filters • Optical “crosstalk”

  27. Stretched RGB of a Japanese Island, SWIR bands 4,5, & 9, 400x400 pixels

  28. SWIR Crosstalk • Present in all images • Preflight calibration • In-flight calibration • “normal” images • Visible in images with strong contrast • Not “visible” but present in others • Currently NOT corrected for in normal processing of L1 data

  29. Crosstalk correction • If we know the amount of light leaking from one pixel to each other pixel in all the SWIR bands, we can correct for it • Preflight data for MTF determination would probably contain much of the needed information but it was recorded only for the band under test • Scan a line source across the array in both directions • Scan a point (pixel size or less) in both directions • It is possible to infer the correction from images with strong contrast (coast lines, islands, moon, and similar), however it is difficult and incomplete

  30. SWIR correction • Japanese team has developed a “beta” level, Windows (Win32) based, correction program. It does one scene at a time operating on a L1B HDF input file and writing a corrected L1B HDF file. • US team is developing a program that is run as part of L2 processing. It starts with L1A data. For example, you will be able to order atmospherically-corrected, crosstalk-corrected surface reflectance at L2.

  31. “Corrected” image, SWIR RGB with 4,5, & 9

  32. SWIR Crosstalk Correction • Qualitatively improves image • Largest effect is band 4 into 5 and 9 • However, any band should leak into all others (including itself) with the strongest effect on adjacent bands on the focal plane • Band 4 has higher typical radiance than the others • Japan correction has only band 4 into others • There is interplay between crosstalk and water vapor absorption, especially in band 9

  33. Conclusions (preflight) • ASTER was calibrated preflight • VNIR accuracy was probably within spec • SWIR accuracy is poorer due to crosstalk • TIR was probably within spec • Preflight calibration is probably not appropriate at this point • Change in sensor (VNIR and some TIR bands) • Crosstalk in SWIR • Calibration is being updated

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