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B. Carol Johnson, Ping-Shine Shaw, Stanford B. Hooker, and Don Lynch

Abstract

A portable and stable source of radiant flux, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Quality Monitor (SQM), was developed as a field instrument for use in experiments away from the calibration laboratory such as those encountered during oceanographic cruises. The SQM monitors the radiometric stability of radiance and irradiance sensors during these field experiments; a companion paper gives results acquired during the third Atlantic Meridional Transect cruise. In conjunction with laboratory calibration sources, the SQM can be used to transfer the calibration to the field experiment. Two independent lamp assemblies generate three flux levels, and the lamps are operated at constant current using active control. The exit aperture of the SQM is large and homogeneous in radiance. The SQM was designed to approximate a Lambertian radiator. An internal heater provides operational stability and decreased warmup intervals, which minimizes lamp hours. Temperature-controlled silicon photodiodes with colored-glass filters monitor the stability of the SQM, which is better than 1%. These independent monitors, which are integrated with the SQM, provide information on the flux from the SQM and can be used to normalize the output from the field radiometers during the experiment. Three reference devices, or fiducials, which are designed to mimic the optical surfaces of the field radiometers but are not functioning detector units, are used in place of the field radiometers to produce baseline monitor signals. The front surface of the fiducial is protected when not in use and kept clean during the field experiment. The monitor signals acquired using the fiducials provide additional information on the radiometric stability of the SQM. A kinematically designed mounting ring is used on both the field radiometers and the fiducials to ensure the devices being tested view the same part of the exit aperture each time they are used.

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Catherine Habauzit, Steven W. Brown, Keith R. Lykke, B. Carol Johnson, Michael E. Feinholz, Mark Yarbrough, and Dennis K. Clark

Abstract

The Marine Optical System (MOS) is a dual charge-coupled device (CCD)-based spectrograph system developed for in-water measurements of downwelling solar irradiance E d and upwelling radiance L u. These measurements are currently used in the calibration and validation of satellite ocean color measurement instruments such as the moderate resolution imaging spectroradiometer (MODIS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). MOS was designed to be deployed from a ship for single measurements and also integrated into the Marine Optical Buoy (MOBY) for longer time series datasets. Measurements with the two spectrographs in the MOS systems can be compared in the spectral interval from about 580 to 630 nm. In this spectral range, they give different values for L u or E d at a common wavelength. To better understand the origin of this observation and the sources of uncertainty in the calibration of MOBY, an MOS bench unit was developed for detailed radiometric characterization and calibration measurements in a laboratory setting. In the work reported here, a novel calibration approach is described that uses a tunable-laser-based, monochromatic, spatially uniform, Lambertian, large area integrating sphere source (ISS). Results are compared with those obtained by a conventional approach using a lamp-illuminated ISS. Differences in the MOS bench unit responsivity between the two calibration approaches were observed and attributed to stray light. A simple correction algorithm was developed for the lamp-illuminated ISS that greatly improves the agreement between the two techniques. Implications for water-leaving radiance measurements by MOS are discussed.

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Kenneth J. Voss, Howard R. Gordon, Stephanie Flora, B. Carol Johnson, Mark Yarbrough, Michael Feinholz, and Terrence Houlihan

Abstract

The upwelling radiance attenuation coefficient K Lu in the upper 10 m of the water column can be significantly influenced by inelastic scattering processes and thus will vary even with homogeneous water properties. The Marine Optical Buoy (MOBY), the primary vicarious calibration site for many ocean color sensors, makes measurements of the upwelling radiance L u at 1, 5, and 9 m, and uses these values to determine K Lu and to propagate the upwelling radiance directed toward the zenith, L u, at 1 m to and through the surface. Inelastic scattering causes the K Lu derived from the measurements to be an underestimate of the true K Lu from 1 m to the surface at wavelengths greater than 575 nm; thus, the derived water-leaving radiance is underestimated at wavelengths longer than 575 nm. A method to correct this K Lu, based on a model of the upwelling radiance including Raman scattering and chlorophyll fluorescence, has been developed that corrects this bias. The model has been experimentally validated, and this technique can be applied to the MOBY dataset to provide new, more accurate products at these wavelengths. When applied to a 4-month MOBY deployment, the corrected water-leaving radiance L w can increase by 5% (600 nm), 10% (650 nm), and 50% (700 nm). This method will be used to provide additional and more accurate products in the MOBY dataset.

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Michael E. Feinholz, Stephanie J. Flora, Mark A. Yarbrough, Keith R. Lykke, Steven W. Brown, B. Carol Johnson, and Dennis K. Clark

Abstract

The Marine Optical System is a spectrograph-based sensor used on the Marine Optical Buoy for the vicarious calibration of ocean color satellite sensors. It is also deployed from ships in instruments used to develop bio-optical algorithms that relate the optical properties of the ocean to its biological content. In this work, an algorithm is applied to correct the response of the Marine Optical System for scattered, or improperly imaged, light in the system. The algorithm, based on the measured response of the system to a series of monochromatic excitation sources, reduces the effects of scattered light on the measured source by one to two orders of magnitude. Implications for the vicarious calibration of satellite ocean color sensors and the development of bio-optical algorithms are described. The algorithm is a one-dimensional point spread correction algorithm, generally applicable to nonimaging sensors, but can in principle be extended to higher dimensions for imaging systems.

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