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![Page 1: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/1.jpg)
A processing package for atmospheric correction of compact
airborne spectrographic imager (casi) imagery over water including
a novel sunglint correction
Peter LandRemote Sensing Group, Plymouth Marine Lab, Prospect Place,
West Hoe, Plymouth PL1 3DH [email protected]
![Page 2: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/2.jpg)
Background
• Sunglint is a severe problem for aircraft imagery, often rendering the images unusable
• Methods which mask out areas of high sunglint require human intervention and are subjective
• Sunglint has a well defined spectral shape, and though its spatial distribution is highly variable and essentially random, its large scale distribution is quite well behaved and predictable given the wind speed
![Page 3: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/3.jpg)
Algorithm overview
• A processing package has been developed which atmospherically corrects casi images over water
• It is particularly suited to open ocean case 1 waters with uniform aerosols and wind, but will work anywhere that the standard SeaWiFS Arnone algorithm is acceptably accurate
• It includes a novel and effective sunglint correction
![Page 4: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/4.jpg)
Basis of the algorithm
• Ancillary data are read and interpolated in time and space, spectral data are interpolated to the casi wavebands used
• Tau and WS are adjusted to minimize the deviation from the theoretical NIR path radiance and sunglint for a ‘typical’ image line, with optional Arnone correction for NIR water leaving radiance. The optimized tau is assumed constant across the image.
• Actual (highly variable) sunglint is then calculated at each pixel with image-averaged geometry to give an estimate of surface albedo
• Finally, the path radiance is calculated at each pixel using the exact pixel geometry, and sunglint recalculated and subtracted to give water leaving radiance, chlorophyll etc.
• If the NIR aerosol optical depth (tau), wind speed (WS) and surface albedo are known, the path radiance and average sunglint can be calculated along a given image line using the Guzzi et al model
![Page 5: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/5.jpg)
North Atlantic raw images from 13 June 2001
![Page 6: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/6.jpg)
Stages of processing
Rayleigh corrected
aerosol corrected
sunglint corrected and interpolated
Rayleigh radiance
aerosol radiance
sunglint radiance
![Page 7: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/7.jpg)
Stages of processing
solid = measureddashed = Rayleigh correcteddash-dot = aerosol correcteddotted = sunglint corrected
![Page 8: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/8.jpg)
nLw comparison with SeaWiFS
![Page 9: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/9.jpg)
nLw comparison with SeaWiFS
![Page 10: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/10.jpg)
nLw comparison with SeaWiFS
solid = whole imagedotted = centre samplecrosses = SeaWiFS
![Page 11: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/11.jpg)
chlor-a comparison with SeaWiFS
![Page 12: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/12.jpg)
chlor-a comparison with SeaWiFS
![Page 13: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/13.jpg)
chlor-a comparison with SeaWiFS
•Each point is a SeaWiFS pixel•Error bars are relative
![Page 14: A processing package for atmospheric correction of compact airborne spectrographic imager (casi) imagery over water including a novel sunglint correction.](https://reader035.fdocuments.in/reader035/viewer/2022081515/5697bf9d1a28abf838c93cc6/html5/thumbnails/14.jpg)
Conclusions
• Visual image quality is generally good, with little noise, few artefacts except residual limb brightening, and very good discrimination of sunglint.
• Retrieved spectra are believable in shape, with good correction for atmospheric gas absorption.
• Comparison with SeaWiFS shows relative overestimate of blue nLw and consistent underestimate in chlor-a, suggesting an under-correction for Rayleigh scattering. The values are within the 35% nominal error limits for SeaWiFS.
• Decrease in nLw and increase in chlor-a with decreasing height is also seen in SeaWiFS, hence is not an artefact of processing.