Stephanie J. Flora

Stray-Light Correction Algorithm for Spectrographs

In this paper, we describe an algorithm to correct a spectrographs response for stray light. Two recursion relations are developed:?one to correct the system response when measuring broad-band calibration sources, and a second to correct the response when measuring sources of unknown radiance. The algorithm requires a detailed understanding of the effect of stray light in the spectrograph on the instruments response. Using tunable laser sources, a dual spectrograph instrument designed to measure the up-welling radiance in the ocean was characterized for stray light. A?stray-light correction algorithm was developed, based on the results of these measurements. The instruments response was corrected for stray light, and the effects on measured up-welling in-water radiance were evaluated.

An Example Crossover Experiment for Testing New Vicarious Calibration Techniques for Satellite Ocean Color Radiometry

Abstract Vicarious calibration of ocean color satellites involves the use of accurate surface measurements of water-leaving radiance to update and improve the system calibration of ocean color satellite sensors. An experiment was performed to compare a free-fall technique with the established Marine Optical Buoy (MOBY) measurement. It was found in the laboratory that the radiance and irradiance instruments compared well within their estimated uncertainties for various spectral sources. The spectrally averaged differences between the National Institute of Standards and Technology (NIST) values for the sources and the instruments were <2.5% for the radiance sensors and <1.5% for the irradiance sensors. In the field, the sensors measuring the above-surface downwelling irradiance performed nearly as well as they had in the laboratory, with an average difference of <2%. While the water-leaving radiance Lw calculated from each instrument agreed in almost all cases within the combined instrument uncertainties (a...

Assessment of MERIS reflectance data as processed with SeaDAS over the European seas

The uncertainties associated with MERIS remote sensing reflectance (RRS) data derived from the SeaWiFS Data Analysis System (SeaDAS) are assessed with field observations. In agreement with the strategy applied for other sensors, a vicarious calibration is conducted using in situ data from the Marine Optical BuoY offshore Hawaii, and leads to vicarious adjustment factors departing from 1 by 0.2% to 1.6%. The three field data sets used for validation have been collected at fixed stations in the northern Adriatic Sea and the Baltic Sea, and in a variety of European waters in the Baltic, Black, Mediterranean and North Seas. Excluding Baltic waters, the mean absolute relative difference |ψ| between satellite and field data is 10-14% for the spectral interval 490-560 nm, 16-18% at 443 nm, and 24-26% at 413 nm. In the Baltic Sea, the |ψ| values are much higher for the blue bands characterized by low RRS amplitudes, but similar or lower at 560 and 665 nm. For the three validation sets, the root-mean-square differences decrease from approximately 0.0013 sr-1 at 413 nm to 0.0002 sr-1 at 665 nm, and are found similar or lower than those obtained for SeaWiFS or MODIS-Aqua. As derived from SeaDAS, the RRS records associated with these three missions thus provide a multi-mission data stream of consistent accuracy.

The Marine Optical BuoY (MOBY) Radiometric Calibration and Uncertainty Budget for Ocean Color Satellite Sensor Vicarious Calibration

For the past decade, the Marine Optical Buoy (MOBY), a radiometric buoy stationed in the waters off Lanai, Hawaii, has been the primary in-water oceanic observatory for the vicarious calibration of U. S. satellite ocean color sensors, including the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectrometers (MODIS) instruments on the National Aeronautics and Space Administrations (NASAs) Terra and Aqua satellites. The MOBY vicarious calibration of these sensors supports international effort to develop a global, multi-year time series of consistently calibrated ocean color data products. A critical component of the MOBY program is establishing radiometric traceability to the International System of Units (SI) through standards provided by the U. S. National Institute of Standards and Technology (NIST). A detailed uncertainty budget is a core component of traceable metrology. We present the MOBY uncertainty budget for up-welling radiance and discuss additional considerations related to the water-leaving radiance uncertainty budget. Finally, we discuss approaches in new instrumentation to reduce the uncertainties in in situ water-leaving radiance measurements.

Stray light correction algorithm for multichannel hyperspectral spectrographs

An algorithm is presented that corrects a multichannel fiber-coupled spectrograph for stray or scattered light within the system. The efficacy of the algorithm is evaluated based on a series of validation measurements of sources with different spectral distributions. This is the first application of a scattered-light correction algorithm to a multichannel hyperspectral spectrograph. The algorithm, based on characterization measurements using a tunable laser system, can be extended to correct for finite point-spread response in imaging systems.

Stray Light Correction of the Marine Optical System

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.

A Method to Extrapolate the Diffuse Upwelling Radiance Attenuation Coefficient to the Surface as Applied to the Marine Optical Buoy (MOBY)

The upwelling radiance attenuation coefficient (KLu) 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 (Lu) at 1 m, 5 m, and 9 m and uses these values to determine KLu and propagate the upwelling radiance directed toward the zenith, Lu, at 1 m to and through the surface. Inelastic scattering causes the KLu derived from the arm measurements to be an underestimate of the true KLu 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 KLu, based on a model of the upwelling radiance including Raman scattering and chlorophyll fluorescence has been developed which corrects this bias. The model has been experimentally validated, and this technique can be applied to the MOBY data set to provide new, more accurate products at these wavelengths. When applied to a 4 month MOBY deployment, the corrected water leaving radiance, Lw, can increase by 5 % (600 nm), 10 % (650 nm) and 50 % (700 nm). This method will be used to provide additional more accurate products in the MOBY data set.

Results in coastal waters with high resolution in situ spectral radiometry: The Marine Optical System ROV

The water-leaving spectral radiance is a basic ocean color remote sensing parameters required for the vicarious calibration. Determination of water-leaving spectral radiance using in-water radiometry requires measurements of the upwelling spectral radiance at several depths. The Marine Optical System (MOS) Remotely Operated Vehicle (ROV) is a portable, fiber-coupled, high-resolution spectroradiometer system with spectral coverage from 340 nm to 960 nm. MOS was developed at the same time as the Marine Optical Buoy (MOBY) spectrometer system and is optically identical except that it is configured as a profiling instrument. Concerns with instrument self-shadowing because of the large exterior dimensions of the MOS underwater housing led to adapting MOS and ROV technology. This system provides for measurement of the near-surface upwelled spectral radiance while minimizing the effects of shadowing. A major advantage of this configuration is that the ROV provides the capability to acquire measurements 5 cm to 10 cm below the water surface and is capable of very accurate depth control (1 cm) allowing for high vertical resolution observations within the very near-surface. We describe the integrated system and its characterization and calibration. Initial measurements and results from observations of coral reefs in Kaneohe Bay, Oahu, extremely turbid waters in the Chesapeake Bay, Maryland, and in Case 1 waters off Southern Oahu, Hawaii are presented.

Immersion coefficient for the Marine Optical BuoY (MOBY) radiance collectors

The immersion coefficient accounts for the difference in responsivity for a radiometer placed in the air versus water or another medium. In this study, the immersion coefficients for the radiance collectors on the Marine Optical Buoy (MOBY) were modeled and measured. The experiment showed that the immersion coefficient for the MOBY radiance collectors agreed with a simple model using only the index of refraction for water and fused silica. With the results of this experiment, we estimate that the uncertainty in the current value of the immersion coefficient used in the MOBY project is 0.05 % (k = 1).

Spectral Dependence of the Seawater–Air Radiance Transmission Coefficient

The transmission coefficient, TL, commonly used to propagate the upwelling nadir radiance, just below the ocean surface, to above the surface has been assumed to be a constant value of 0.543 in seawater. Because the index of refraction of seawater varies with wavelength, salinity, and temperature, the variation of TL with these parameters should be taken into account, especially if low uncertainty is required for the quantities derived using TL. In particular the wavelength dependence of this factor is important. For example at a salinity of 35 g/kg and a temperature of 26° C, TL will be 1.3% lower at 380 nm and 1.1 % higher at 700 nm than the constant value (0.543) and should be taken into account when calculating the water leaving radiance and normalized water leaving radiance from in-water measurements.