CLARREO: The GSFC Role Solar Reflectance Calibration and Filter Radiometer Issues Related to Inter-calibration

1. CLARREO: The GSFC RoleSolar Reflectance Calibration andFilter Radiometer Issues Related to Inter-calibration GSFC Team Jack Xiong: Lead, MODIS Characterization Support Team; VIIRS calibration support Jim Butler: EOS Calibration Scientist, NPP Deputy Project Scientist for calibration Steve Platnick: MODIS, VIIRS atmosphere team science and algorithms CLARREO GSFC: Butler, Platnick, Xiong

2. Deliverables (~ Sept. 2008) • Documents/files including Relative Spectral Response w/wout out-of-band knowledge as available for MODIS and other radiometers of interest [lead: Xiong] • Assess new calibration/characterization measurement methodologies with potential applications to CLARREO for accuracy and precision, as well as feasibility for CLARREO testing [lead: Butler] CLARREO GSFC: Butler, Platnick, Xiong

3. Solar Inter-calibration You can’t inter-calibrate well … an instrument that isn’t well calibrated OR You can’t transfer a standard beyond the level to which an instrument is characterized CLARREO GSFC: Butler, Platnick, Xiong

4. Outline • Xiong: MODIS spectral characterization capability, history, and lessons learned • Butler: Filter radiometer characterization and stability (examples from SeaWIFS, etc.) • Platnick: Example MODIS reflectance sensitivity to filter position CLARREO GSFC: Butler, Platnick, Xiong

5. 1. MODIS Reflective Solar Band CalibrationJack Xiong • Outline • Instrument Background • Reflective Solar Bands (RSB) Calibration and Characterization • Radiometric (on-orbit) • Spectral (pre-launch) • Lessons and Challenges Page 5

6. MODIS Instrument • 20 Reflective solar bands (RSB): 0.41-2.2mm • 16 Thermal emissive bands (TEB): 3.7-14.4mm • 3 spatial resolutions at nadir: 250m, 500m and 1000m • 4 Focal Plane Assemblies (FPA): VIS, NIR, SMIR, LWIR • 5 On-Board Calibrators: SD, SDSM, SRCA, BB, and SV port • Two MODIS (Terra and Aqua): Complementary morning and afternoon observations • A broad range of applications: land, oceans, and atmosphere Terra Aqua Page 6

7. MODIS Key Specifications Page 7

8. MODIS RSB Calibration • Radiometric • Pre-launch • Calibration source: SIS-100 • Calibration activities: gain, nonlinearity, noise characterization, dynamic range (A/B electronics. 3 instrument plateaus for thermal vacuum test) • Solar diffuser BRDF calibration • On-orbit • Spectral • Pre-launch • On-orbit • On-board calibrator: SRCA • Calibration activities: center wavelengths and bandwidths Page 8

9. MODIS On-orbit Calibration BB for IR calibration SD/SDSM for VIS/NIR calibration SRCA for spatial and spectral Lunar observations SDSM Solar Diffuser SRCA SDSM Blackbody Scan Mirror Space View Moon Page 9

10. RSB SD/SDSM Calibration Reflectance Factor Sun Optional 7.8% Screen 1.44% Screen SDSM Scan Mirror (MODIS) DSD: SD degradation factor; GSD: SD screen vignetting function d: Earth-Sun distance dn*: Corrected digital number dc: Digital count of SDSM SD Page 10

11. RSB Lunar Stability Monitoring SD Calibration Lunar Calibration Geometric Factors Lunar Model Results: USGS (H. Kieffer and T. Stone) Page 11

12. Aqua MODIS Response Trending Wavelength and AOI dependent response change Little mirror side difference for Aqua MODIS; Large change for Terra mirror side 2 Page 12

13. Aqua MODIS SD Degradation SDSM detectors Center wavelength (nm) D1 411.97 D2 465.69 D3 529.74 D4 553.75 D5 646.14 D6 746.62 D7 856.49 D8 904.29 D9 936.23 Wavelength dependent SD degradation Similar for both Terra and Aqua MODIS when operated under the same conditions Page 13

14. RSB Calibration Issues • Calibration Requirements • 2% for reflectance and 5% for radiance for MODIS RSB • SD BRF calibration uncertainty (major contributor to RSB calibration uncertainty) • Pre-launch to on-orbit transfer • SD on-orbit degradation (for MODIS: VIS bands experienced more degradation) • Optics degradation (response versus scan angle) Page 14

15. MODIS Spectral Calibration • Pre-launch Spectral Calibration Performed Using A Spectral Measurement Assembly (SpMA) • Relative spectral response (RSR), center wavelength (CW), bandwidth (BW) • Out-of-band (OOB) response • Optical leak characterization • On-orbit Spectral Performance Monitored Using An On-board Spectro-Radiometric Calibration Assembly (SRCA) • RSR (partial, covering FWHM), CW, BW (for VIS/NIR only) Page 15

16. MODIS VIS/NIR RSR Page 16

17. OOB Characterization Page 17

18. SWIR Optical Leak Terra MODIS Aqua MODIS Page 18

19. MODIS On-orbit Spectral Performance Band 2 not recoverable CLARREO GSFC: Butler, Platnick, Xiong

20. MODIS Spectral Calibration Issues • Stability monitoring during RSR characterization • In band and out-of-band stitching • Normalization, filter characterization • RSR characterization steps (Dl) • In band and OOB, especially overlapping region • Atmospheric impact (for ambient measurements) • “Smile effect” (for detectors aligned along-track) Page 20

21. Lessons and Challenges • RSB calibration stability can be effectively monitored with on-board calibrators (SD, lamp) and lunar observations • Absolute calibration accuracy can be improved, but still limited in the existing approaches • Calibration and observations should be made through the same optical configuration (path) • Effectively monitoring the detector response change over time, and time dependent response versus scan-angle (RVS) • High quality RSR characterization (component and system level) CLARREO GSFC: Butler, Platnick, Xiong

22. 2. Satellite Filter Instrument Radiometric Accuracy, Stability, and Intercomparisons: Status and Lessons LearnedJim Butler • Radiometric Performance: Accuracy • Radiometric Performance: Stability • On-orbit Intercomparisons of Filter Instruments • Current and future intercomparison developments Page 22

23. Radiometric Performance: Accuracy • Begins at the instrument system and subsystem specification/design stage • No subsystem is immune: filters, optics, electronics, static and dynamic structures, all coupled with anticipated on-orbit thermal influences • Initially established during pre-launch calibration and characterization • Directly limited by the ability to calibrate/characterize GSE sources, optics, detectors and data acquisition electronics • Source (e.g. MODIS integrating sphere) validated calibration at the 2 to 3% (k=1) accuracy level has resulted in pre-launch instrument-level absolute calibration accuracies of 4 to 5% (absolute) for radiance. Reflectance calibrations have been validated at the 2% (relative) level using piece-parts (e.g. EOS BRDF round-robins). There is little to no margin on both. Advances in GSE realized through the use of tunable intensity stabilized laser-fed spheres/collimators (e.g. NIST SIRCUS) calibrated for radiance using standard detectors should improve these numbers by at least a factor of 10.

24. Radiometric Performance: Accuracy • System level, end-to-end testing of on-board calibration systems should be required to properly validate piece-part approaches • Calibrating “like with like,” “as you fly, ” and “using multiple measurement methodologies” must be adopted as pre-launch testing mantras and applied where and when possible. • Calibration against tungsten lamp-based uniform sources does not represent scenes remotely sensed. LED-based spectrally tunable sphere sources (capable of producing output with spectral content reproducing remotely sensed scenes) will provide improved insight into on-orbit radiometric performance. • Development of sources such as the hyperspectral image generator will provide important information on instrument radiometric/spatial performance at the full and sub-scene level.

25. Radiometric Performance: Accuracy • Transfer to orbit calibrations utilizing the sun (e.g. SeaWiFS) provide the most detailed insight into calibration changes incurred through launch -Difference of initial on-orbit mmt from pre-launch predicted mmt. -Main uncertainty sources of technique: a. atm. transmittance mmt. 3% b. blue sky radiance mmt. 0.25% c. pre-launch to on-orbit alignment 0.3% d. extrapolation from pre- to post-launch 1 to 2% -Overall uncertainty of technique 3% (largest term) • For SeaWIFS, prelaunch courtyard-based • measurements of the Sun using the solar • diffuser were used to predict the first solar • Diffuser mmts on orbit. • -This is essentially a check of the on-board • reflected solar calibration system using the • Sun

26. Radiometric Performance: Stability • Satellite instrument calibration requires instrument system and subsystem level stability to be determined and monitored on-orbit • Stability is an instrument characteristic which influences all areas of instrument design • Instrument stability is often insufficiently determined and demonstrated pre-launch • Inadequate time for proper testing (time=$) • Testing must cover all possible on-orbit operating scenarios with respect to operating voltage, instrument and focal plane temperatures, and electronic cross-strapping configurations • The stability of vis/nir/swir on-board calibrators (e.g. solar diffusers and monitors) are often not tested • A well designed t/v test plan (i.e. one with T plateau repeats and transitions) can be used to demonstrate instrument stability. • Plans for adequate stability testing should be incorporated into the design of instrument test equipment and facilities (e.g. HIRDLS t/v chamber)

27. Radiometric Performance: Stability • On-orbit stability is either monitored using dedicated on-board hardware or using specific remotely sensed targets. • For the vis/nir/swir wavelength region, monthly lunar views have been used to monitor instrument degradation to better than 0.1%/yr (e.g. SeaWiFS) Requires and up-front commitment by projects to produce fully maneuverable s/c and realization that data reprocessing is a reality of climate research This level of precision achieved using repeat views of the Moon at nearly identical phase meets the stability requirement for climate change (NISTIR 7047)

28. On-orbit Intercomparison of Filter Instruments • Involve views (preferably simultaneous or near simultaneous) views of common targets. • Require knowledge of target spectral BRDF and spectral responsivities, spectral and spatial stray light responses of instruments • In the absence of a benchmark (i.e. truth) measurement, comparison techniques provide measures of the relative on-orbit calibration scales of instruments with the “goodness” of comparisons limited to the combined on-orbit measurement uncertainties of the instruments.

29. On-orbit Intercomparison of Filter Instruments

30. Current and future intercomparison developments • The WMO’s Global Space-based Intercalibration System (GSICS) was formed in November 2005 to achieve operational intercalibration of satellite instruments within the space-based component of WMO’s World Weather Watch/Global Observing System (WWW/GOS). • Members include CMA, CNES, EUMETSAT, JMA, KMA, NOAA, WMO, and NASA • The Calibration Support Segment (CSS) of GSICS plans the following activities: • Highly accurate, SI-standards traceable tests on satellite instruments and their on-board calibrations references using special satellite and ground-based instruments • Collecting high quality in-situ data from earth-based reference sites • Independent montoring of extra-terrestrial calibration sources (e.g. Sun, Moon, and stars) for on-orbit monitoring of instrument calibration • Simulations of satellite radiances, computed from NWP analyses of atmospheric conditions, compared with satellite instrument observations. • GSICS work to date has concentrated on the comparison of thermal infrared data from on-orbit operational and research satellite instruments (i.e. LEO and GEO) • AIRS and IASI have been designated as the benchmark instruments for these comparisons • Future work will include comparisons of satellite instruments working in the vis/nir/ir/swir and protocols/reviews of pre-launch instrument calibration/characterization. • CLARREO should be “one stop shopping” for this (and other) groups?

31. Summary • With respect to pre-launch calibration, the same measurement methodologies and calibration/characterization instrumentation used over the past 30 years have largely been recycled. • This has contributed to a “stagnation” of remote sensing radiometric accuracies in the vis/nir/swir at the 4 to 5% absolute radiance levels. • We have been slow to develop (and hopefully adopt) new technology to decrease these accuracies to levels required by climate science. • With respect to on-orbit calibration systems, system level pre-launch testing must be required to validate on-orbit radiometric models. • On-orbit calibration systems must be tested as if they are instruments in themselves. • To realize a 10x improvement in remote sensing accuracy/precision, a commitment for the incorporation of new measurement methodologies and technologies in instrument calibration and characterization both pre-launch and on-orebit must be made. • The uncertainties of these new approaches must be completely validated and cost and schedule impacts to programs accurately assessed.

32. 3. Example MODIS Reflectance Spectral Sensitivities to filter position: Simulations with MODTRANSteve Platnick • Sensitivity to filter position only (no change in spectral shape) • Does not include out-of-band response • Does not include other effects (spectral crosstalk, scattered light, etc.) • Assume a perfect transfer calibrator/calibration Page 32

33. MODTRAN Spectrum CLARREO GSFC: Butler, Platnick, Xiong

34. MODTRAN Spectrum CLARREO GSFC: Butler, Platnick, Xiong

35. MODTRAN Spectrum CLARREO GSFC: Butler, Platnick, Xiong

36. MODTRAN Spectrum CLARREO GSFC: Butler, Platnick, Xiong

37. Sensitivity Results CLARREO GSFC: Butler, Platnick, Xiong

38. Sensitivity Results CLARREO GSFC: Butler, Platnick, Xiong

39. Sensitivity Results CLARREO GSFC: Butler, Platnick, Xiong