Assessing the Requirements for Sustained Ocean Color Research and Operations (2011) / Chapter Skim
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3 Lessons Learned from Ocean Color Satellite Missions and Essential Requirements for Future Success
3 Lessons Learned from Ocean Color Satellite Missions and Essential Requirements for Future Success
Pages 28-45
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From page 28... ...
This chapter surveys lessons mission to demonstrate that ocean color can be retrieved from from previous missions and outlines the requirements for space. Therefore, CZCS did not routinely or continuously obtaining useful ocean color data from a global remote collect global data because it had to share power and tape sensing mission.
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From page 29... ...
MODIS addresses atmosphere, features minimized polarization sensitivity and far-field stray land, and ocean research requirements; the Aqua sensor light and enabled the measurement of low signal ocean radi- continues SeaWiFS ocean color capability. ances, and land and cloud reflectance at very high signals, Unfortunately, the Terra MODIS sensor has major without saturation.
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From page 30... ...
After launch, two groups were setup: the "MERIS Conclusion: SeaWiFS/MODIS' success in producing high- Quality Working Group" and the "MERIS Validation Team," quality data is due to the commitment to all critical steps of which have continuously monitored the quality of the Level the mission, including pre-flight characterization, on-orbit 1 and Level 2 products and introduced significant improveassessment of sensor stability and gains, solar and lunar ments to the processing algorithms throughout the mission. calibration, vicarious calibration, atmospheric correction These groups recognized the importance of supporting users and bio-optical algorithms, product validation, reprocess- with dedicated and freely available tools (e.g., BEAM)
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From page 31... ...
Furthermore, sequential reprocessing of ocean color data requires both pre-launch characterization of the system, commitments to conduct on-orbit assessments of instrument performance throughout the mission, and support for the necessary research to improve the models used to derive successful ocean color data products. Hence, the requirements for an ocean color mission are multi-faceted and interconnected.
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From page 32... ...
Meet All Requirements, Not Just Sensor Requirements The committee's guidelines for achieving high-quality A key lesson from CZCS and SeaWiFS is that a suc- sensor performance fall into six general areas: Sensor stacessful ocean color mission requires that the team consider bility, waveband selection, scan geometry, sun glint, sensor from the beginning all aspects of what it takes to develop saturation, and polarization sensitivity. Explanations and high-quality ocean color products.
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From page 33... ...
Polarization <3 percent, Scrambler ~5 percent <2 percent <2 percent Sensitivity Scrambler SNR 100-150 360-1,000 1,250-2,700 >600-1,400 >1,000 Dynamic Range Ocean only Ocean, land, clouds Ocean only within Ocean color + land Ocean, land, clouds (Sensor Saturation) via bi-linear gain ocean color bands; and clouds via pixel instantaneous control other similar cloud/ automatic gain control land VNIR bands for bright scenes On-board No No 2 solar diffusers Yes Calibration plus an erbium doped diffuser for spectral calibration Monitoring of No Yes Yes Yes (2 solar Hopefully Stability diffusers)
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From page 34... ...
bitmap are much stronger than ocean color signals.2 With this new success from differentiating the colored dissolved organic matter (CDOM) absorption signal from the algal absorption information, future atmospheric correction/ocean color based on information from a single channel, 412 nm (Lee et algorithms likely will be coupled inversions similar to those al., 2002; Maritorena et al., 2002; Morel and Gentili, 2009)
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From page 35... ...
Different spectral bands on the same Sensor scan geometry impacts the reprocessing of ocean MODIS focal plane (e.g., ocean color bands) are read out at color data.
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From page 36... ...
However, use of a polarization descrambler precludes the simultaneous viewing of thermal infrared bands, as is done Recommendation: Future ocean color sensors should now in MODIS and VIIRS. Most recently, requirements for avoid sun glint by tilting the sensors' viewing away from polarization sensitivity are <1 percent.
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From page 37... ...
Some of these discrepancies spheric radiances, vicarious calibration of satellite ocean were due to differences in the test facilities used to test and color sensors also requires that the Lw signal be accurately characterize VIIRS (Turpie, 2010)
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From page 38... ...
(2007) conclude that it would take two to Although other approaches might produce acceptable three years of continuous in situ operations in order to estab vicarious calibration data, they have not been widely lish the calibration of an ocean color sensor with character implemented or deployed operationally.
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From page 39... ...
Note infrequent solar observations to monitor the degradation of that the level of uncertainty in the stability characterization the first. Multiple concurrent solar diffusers are important for is less than the uncertainty in the vicarious calibration (see assessing impacts of stability if the moon is not used as the Appendix B)
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From page 40... ...
Validation programs for SeaWiFS ances derived from SeaWiFS observations were employed as and MODIS included comparisons of satellite with in situ Lw, reference radiances to study the seasonal, latitudinal, cross- i.e., comparisons of derived ocean color data products such scan, and polarization behavior of the water-leaving radi- as chlorophyll, particulate organic carbon (POC) , particulate ances for MODIS on Aqua and Terra (Kwiatkowska et al., inorganic carbon (PIC)
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From page 41... ...
. Much is learned during the mission to the same vicarious calibration sources is likely the only about the sensor's behavior and the atmospheric correction, way to construct long-term ocean color data products.
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From page 42... ...
Users generating long time-series ity monitoring, vicarious calibration, an in situ calibration/ of science or climate-quality imagery across multiple satel- validation program, and a dedicated team for data processing, lite data streams want access to Level 0 data, or if not Level reprocessing, and distribution. As a result, SeaWiFS became 0 data, then a data level and ancillary information that allows "tweaking" of the calibration coefficients.
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From page 43... ...
scan angle (RVS) Better than for SeaWiFS Geometric Stability Aerosols NIR Coverage NIR band-set 765 and 865 nm with appropriate SWIR bands Sensitivity SNR Similar to MODIS for the NIR bands but >MODIS for the SWIR Clouds and Land Dynamic Range No saturation Auto gain control Global Daily Ocean Coverage Sun Glint Avoidance Off-nadir pointing Tilting from sun-glint pattern Stability Calibrated On-orbit reference Solar diffuser and monthly lunar view Lab calibration Pre-launch characterization ~0.1 percent stability compared with trend line NOTE: (‘>' signifies "better than")
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From page 44... ...
Because a Type 1 ocean color sensor will and mission operation standards are the minimum criteria for undergo a vicarious calibration, meeting pre-launch stansatisfying the current research and operational applications dards becomes less crucial to the success of the mission for ocean waters beyond the shallow (< 20 m depth) waters as long as the other aspects of pre-launch characterization near the coast.
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From page 45... ...
all raw, meta-, and processed ocean color data products, As discussed in greater detail in Chapter 5, meeting the algorithms, and processing codes that can distribute the diverse needs of the expanding ocean color user community data rapidly and efficiently; will require multiple sensors in both polar and geostationary 12. Detailed and comprehensive documentation of all orbit (Appendix D)
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