Calibration of SeaWiFS. II. Vicarious techniques - FTP Directory Listing

Calibration of SeaWiFS. II. Vicarious techniques Robert E. Eplee, Jr., Wayne D. Robinson, Sean W. Bailey, Dennis K. Clark, P. Jeremy Werdell, Menghua Wang, Robert A. Barnes, and Charles R. McClain

We present an overview of the vicarious calibration of the Sea-Viewing Wide Field-of-view Sensor 共SeaWiFS兲. This program has three components: the calibration of the near-infrared bands so that the atmospheric correction algorithm retrieves the optical properties of maritime aerosols in the open ocean; the calibration of the visible bands against in-water measurements from the Marine Optical Buoy 共MOBY兲; and a calibration-verification program that uses comparisons between SeaWiFS retrievals and globally distributed in situ measurements of water-leaving radiances. This paper describes the procedures as implemented for the third reprocessing of the SeaWiFS global mission data set. The uncertainty in the near-infrared vicarious gain is 0.9%. The uncertainties in the visible-band vicarious gains are 0.3%, corresponding to uncertainties in the water-leaving radiances of approximately 3%. The means of the SeaWiFS兾in situ matchup ratios for water-leaving radiances are typically within 5% of unity in Case 1 waters, while chlorophyll a ratios are within 1% of unity. SeaWiFS is the first ocean-color mission to use an extensive and ongoing prelaunch and postlaunch calibration program, and the matchup results demonstrate the benefits of a comprehensive approach. © 2001 Optical Society of America OCIS codes: 280.0280, 010.0010, 300.0300, 120.5630, 120.0120.

1. Introduction

The Sea-viewing Wide Field-of-view Sensor 共SeaWiFS兲 has been providing daily global imagery of the world’s oceans since September 1997. The goal of the SeaWiFS Project is to produce a five-year oceancolor data set with 5% absolute and 1% relative accuracies on the water-leaving radiances 共Lw兲 and 35% accuracy on the chlorophyll a concentrations in openocean regions.1,2 Because open-ocean reflectances are low, approximately 90% of the top-of-theatmosphere signal observed by SeaWiFS over the oceans is due to the scattering of sunlight by gases

R. E. Eplee, Jr. 共[email protected]兲, W. D. Robinson, and R. A. Barnes are with the Science Applications International Corporation, Beltsville, Maryland 20705. S. W. Bailey is with Futuretech Corporation, Greenbelt, Maryland 20770. D. K. Clark is with the National Environmental Satellite Data and Information Service, National Oceanic and Atmospheric Administration, Camp Springs, Maryland 20746. P. J. Werdell is with Science Systems and Applications Incorporated, Lanham, Maryland 21250. M. Wang is with the University of Maryland, Baltimore County, Baltimore, Maryland 21250. C. R. McClain is with the Laboratory for Hydrospheric Processes, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland 20771. Received 24 October 2000; revised manuscript received 18 June 2001. 0003-6935兾01兾366701-18$15.00兾0 © 2001 Optical Society of America

共Rayleigh scattering兲 and by aerosols within the atmosphere. The SeaWiFS atmospheric-correction algorithm must remove this atmospheric signal to yield the water-leaving radiances. Uncertainties in the sensor calibration and the atmospheric-correction algorithm require a mission-long vicarious calibration program to monitor the performance of the sensor system 共instrument plus the atmospheric-correction algorithm兲 to meet the radiometric constraints on the ocean-color data set.3,4 The Calibration and Validation Team 共CVT兲 of the SeaWiFS Project is using data from the NASA兾National Oceanic and Atmospheric Administration Marine Optical Buoy 共MOBY兲5 deployed 15 km west of Lanai, Hawaii, for the vicarious calibration of SeaWiFS. In addition, the CVT undertook an extensive prelaunch calibrationvalidation program to ensure a rigorous postlaunch validation capability.6,7 The vicarious calibration methods described in this paper are based on the underlying assumption that SeaWiFS provides stable top-of-the-atmosphere radiances over the course of the mission. The assurance of the temporal stability of SeaWiFS has both prelaunch and postlaunch components that are the subject of a companion paper.8 Prior to launch, the CVT performed a radiometric calibration of the instrument in the laboratory.9 Both before and after launch, the CVT performed a transfer-to-orbit experiment to initialize the on-orbit calibration of the instrument.10 Since launch, the CVT has used 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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monthly lunar calibrations to monitor the radiometric stability of the individual SeaWiFS bands.8,11 This paper describes all aspects of the vicarious calibration program. We will discuss the openocean aerosol analysis used to calibrate band 7, the MOBY measurements of Lw and the techniques used to calibrate bands 1 through 6 with use of the MOBY observations of normalized water-leaving radiances 共Lwn兲, the uncertainties in the vicarious gains, and the validation of the calibration by comparison with in situ measurements of Lwn, chlorophyll a, and Kd共490兲 and by analysis of the global clear-water time series of Lwn and chlorophyll a from the SeaWiFS eight-day binned products. 2. Theoretical and Conceptual Background

There are several reasons for a vicarious calibration of SeaWiFS. Since approximately 90% of the top-ofthe-atmosphere radiances arise from within the atmosphere, the sensor and the atmospheric-correction algorithm are a coupled system. The uncertainties in the prelaunch calibration of SeaWiFS 共3– 4% of the top-of-the-atmosphere signal兲 give rise to uncertainties of 20 – 40% in the Lwn. Finally, there may have been undetermined changes to the instrument during orbit raising. A vicarious calibration program has been undertaken to address these concerns. Gordon3 proposed the following guidelines for a good vicarious calibration of ocean-color sensors: The calibration should be made in a cloud-free air mass with a maritime aerosol that has an optical thickness of less than 0.1, and there must be uniform water-leaving radiances over the area. To meet these guidelines, the MOBY site, located off the coast of Lanai, Hawaii, was chosen because the aerosols in the vicinity are marine aerosols that are typical of the open ocean, there are proportionally fewer clouds in the area than in typical open-ocean regions, the waters are typically homogeneous, with low concentrations of chlorophyll, a sunphotometer could be located nearby 共on Lanai兲 to make in situ aerosol measurements, and logistical support facilities are in the vicinity 共in Honolulu兲. In practice, the vicarious calibration equates the top-of-the-atmosphere radiance over the ocean with the radiances as measured by SeaWiFS. The equation for the top-of-the-atmosphere radiance over the ocean is L t共␭兲 ⫽ L r共␭兲 ⫹ L a共␭兲 ⫹ L ra共␭兲 ⫹ T共␭兲 L g共␭兲 ⫹ t共␭兲 L wc共␭兲 ⫹ t共␭兲 L w共␭兲,

(1)

where ␭ is the wavelength of the measurement, Lr is the Rayleigh radiance, La is the radiance arising from aerosol scattering, Lra is the radiance arising from the interaction of molecular and aerosol scattering, Lg is the glint radiance arising from the specular reflection of the sun on the water surface, Lwc is the whitecap radiance, Lw is the water-leaving radiance, T is the direct transmittance of the atmosphere, and t is the diffuse transmittance of the atmosphere. The top-of-the-atmosphere radiance, as measured by 6702

APPLIED OPTICS 兾 Vol. 40, No. 36 兾 20 December 2001

SeaWiFS, is specified by the SeaWiFS level-1b calibration equation.11 A simplified version of that equation, with a number of calibration factors combined into a single parameter, is: L t共␭兲 ⫽ 关C out共␭兲 ⫺ C dark共␭兲兴 K 1共␭兲 K 2共␭兲 ⫻ ␣共␭兲关␤共␭兲 ⫹ ␥共␭兲共t ⫺ t 0兲 ⫹ ␦共␭兲共t ⫺ t 0兲 2兴, (2) where Cout are the counts from the sensor data; Cdark are the dark counts from the sensor data; K1 is the counts-to-radiance conversion factor; K2 represents a number of additional calibration factors; ␣ is the vicarious gain; ␤, ␥, and ␦ are the coefficients for a quadratic fit in the temporal correction; t is the timetag of sensor data; and t0 is the reference time for the temporal correction. For bands 1– 6, the CVT adjusts the vicarious gains, ␣共␭兲 in Eq. 共2兲, to minimize the differences between the water-leaving radiances retrieved by SeaWiFS, Lw共␭兲 in Eq. 共1兲, and the water-leaving radiances measured by MOBY.12 For the near-infrared bands, the CVT used the region around MOBY to characterize the open-ocean marine aerosols used in the atmospheric-correction algorithm. The vicarious gain of band 7 is adjusted so that the properties of the maritime aerosols that were thought to exist near the MOBY site are retrieved by the atmospheric-correction algorithm.13 The CVT has derived a set of vicarious gains that, when applied to the SeaWiFS calibration, yields waterleaving radiances from SeaWiFS retrievals that agree with MOBY measurements to better than 1%. The CVT makes several assumptions in performing the vicarious calibration of SeaWiFS. The single-point vicarious calibration of SeaWiFS to MOBY is applicable to the global ocean, where the atmospheric and oceanic conditions may differ from those near the Hawaiian Islands; the relationship between the aerosol radiances in bands 7 and 8 is specified by a specific aerosol model; the aerosol models used in the atmospheric-correction algorithm are accurate and representative; the laboratory calibration of band 8 is correct, and the vicarious gain for this band is unity; and the uncertainties in the MOBY radiances are small. The CVT is addressing the applicability of the single-point calibration to the global ocean through an extensive calibrationvalidation program that uses in situ measurements of water-leaving radiances from a number of investigators to evaluate the SeaWiFS retrievals of waterleaving radiances on a global basis.14 The additional assumptions are being addressed through the ongoing work of the CVT, particularly the question of the vicarious calibration of band 8 共Ref. 7兲. 3. Near-infrared Band Calibration

The vicarious calibration strategy for the SeaWiFS near-infrared bands takes advantage of the negligible water-leaving radiance in these bands. The atmospheric algorithm estimates the aerosol radiance 共La兲 for bands 7 and 8 and extrapolates to La in the other

SeaWiFS bands by use of the ratio of La in band 7 to that in band 8.15 Currently, the vicarious gain for band 8 is defined to be unity. The calibration of band 7 is accomplished through adjustment of the gain for that band so that the ratio of aerosol radiances has the expected value for a set of open-ocean scenes in the vicinity of MOBY.13 A.

Theory

Gordon and Wang15 derived an atmosphericcorrection parameter, ⑀, in terms of the singlescattering aerosol reflectance, ␳as共␭兲: ⑀共␭, 865兲 ⫽

L as共␭兲 F 0共865兲 ␳ as共␭兲 ⫽ , ␳ as共865兲 L as共865兲 F 0共␭兲

(4)

and Eq. 共3兲 becomes

⑀共765, 865兲 ⬇

throughout the mission and because it coincides with the area where the vicarious calibration of the other bands is done, as will be discussed later in this paper. The choice of the site near Hawaii allowed the use of 724 LAC scenes for the study. This large volume of data makes it possible to be more selective of the atmospheric conditions in each observation and still retain a large sample of observations. The site was chosen to have a square area of 3 ⫻ 3 LAC pixels where at least five pixels in the area pass the exclusion criteria listed in Table 1. Additional exclusion criteria are applied to the observations for this calibration. The effect of whitecap radiance is accounted for by a relationship between the wind speed and the whitecap radiance.18,22 The data used to derive this relationship show a wide variance as the wind speed increases and show a weak correlation between wind speed and the band 7 gain. Consequently, an upper limit on the wind speed of 8 m s⫺1 was imposed. The CVT has not observed additional correlations between the band 7 gain and time, number of valid pixels in the observation, aerosol radiance, aerosol optical depth, and atmospheric path length. To minimize the variation in aerosol type, a

关L t共765兲 ⫺ L r共765兲 ⫺ t共765兲 L wc共765兲兴F 0共865兲 . 关L t共865兲 ⫺ L r共865兲 ⫺ t共865兲 L wc8765兲兴F 0共765兲

For open-ocean areas, it can be assumed that a maritime aerosol is usually prevalent and that such sites have known ⑀ ⬇ 1.04. For sites like this, if ␣共865兲 is assumed to be unity, ␣共765兲 is then adjusted until Eq. 共5兲 produces a maritime ⑀. In practice, the single- to multiple-scattering aerosol-radiance conversion is applied during the computation of ⑀. B.

Land Clouds and ice Sun glint Stray light Total radiance above the knee of the bilinear gain Water-leaving radiance in band 5 less than 0.15 mW cm⫺2 sr⫺1 ␮m⫺1 Atmospheric correction algorithm failure Satellite zenith angle greater than 56° Solar zenith angle greater than 70° Turbid water Coccolithophore Aerosol optical depth in band 8 greater than 0.1

(3)

which is a constant for any one type of atmospheric aerosol and viewing geometry. Here, Las is the single-scattering aerosol radiance, and F0 is the solar constant. Computations of ⑀ have been made for a variety of aerosol types 共oceanic, maritime, coastal, and tropospheric兲 and at a variety of relative humidities 共50%, 70%, 90%, and 99%兲. Gordon and Wang15 derived a relationship between the term 共La ⫹ Lra兲 in Eq. 共1兲 and the singlescattering radiance Las for the above-mentioned aerosol models. Lr can be determined accurately with a knowledge of the surface pressure.16 Areas where the sun glint is significant can be predicted and avoided; residual glint outside the main region of glint contamination is removed.17 The whitecap radiance can be accurately estimated at low wind speeds.18 –21 Lw at the 765- and 865-nm bands of SeaWiFS can be considered to be zero in nonturbid, low-chlorophyll waters. Thus Eq. 共1兲 can be simplified for bands 7 and 8 to L t共␭兲 ⫺ L r共␭兲 ⫺ t共␭兲 L wc共␭兲 ⫽ L a共␭兲 ⫹ L ra共␭兲,

Table 1. Exclusion Criteria for SeaWiFS兾MOBY Matchups

Band 7 Calibration Method and Results

This calibration was performed through use of SeaWiFS data collected near the MOBY site located west of Lanai.5 The site is in the open ocean and has a consistent maritime aerosol type. This region was also chosen because it has good coverage by the full, 1-km resolution local-area coverage 共LAC兲 data

(5)

lower limit on the aerosol optical thickness of 0.03 was also imposed. Finally, to eliminate any statistical outliers, any determinations of the band 7 gain that deviated from the mean of those determinations by more than 2␴ were discarded. In each scene, the ⑀ retrieved by the atmosphericcorrection algorithm for the pixels that meet the exclusion criteria are averaged to produce the mean ⑀ for the observation. These values are matched against the expected maritime ⑀. Although the region for this study was selected to have a constant maritime aerosol, the atmospheric-correction algorithm incorporates four maritime aerosol models 共with relative humidities of 50%, 70%, 90%, and 99%兲. For the vicarious calibration, the ⑀ value used for the 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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A.

Fig. 1. Band 7 vicarious gain over the MOBY site. a, plotted as a function of time; b, plotted as a histogram.

site is the average of the ⑀ values derived from the four models. The band 7 gain that produces this ⑀ is the gain determined for that observation. For the global ocean, approximately 88% of the aerosol models selected by the atmospheric-correction algorithm have optical properties that are consistent with these four maritime models. Figure 1 shows both a plot of the band 7 gain determined for the MOBY site as a function of days since SeaWiFS became operational 关Fig. 1共a兲兴 and a histogram of these gains 关Fig. 1共b兲兴. Of the 724 LAC data sets covering the Hawaii area, 89 satisfy the screening tests. The mean band 7 gain is 0.946 ⫾ 0.008. The subsequent vicarious calibration of the visible bands should compensate for any residual errors in the band 7 gain. 4. MOBY Measurements of Lw

MOBY provides data for the primary in situ vicarious calibration for SeaWiFS bands 1– 6. In this section, we discuss the MOBY measurements of waterleaving radiance and the computation of normalized water-leaving radiances 共Lwn兲 from these measurements of Lw 共Ref. 23兲. 6704


MOBY Data Selection

MOBY measures downwelling irradiances and upwelling radiances over the wavelength range of 340 – 900 nm with use of two spectrometers coupled by a dichroic beamsplitter. The beamsplitter gives the blue spectrometer a bandpass of 340 – 600 nm and the red spectrometer a bandpass of 630 –900 nm. The blue spectrometer has a resolution of 0.51 nm and the red spectrometer has a resolution of 0.81 nm. The potential for stray light is greatly reduced by splitting the visible spectrum at the beginning of the water absorption region, because most of the short wavelength energy is diverted from the entrance slit of the long wavelength spectrometer. This splitting also allows the spectrometers to be optimized, in terms of free spectral range and integration times, for the two distinctive spectral domains. The MOBY data are mean upwelling radiances 关Lu共␭兲兴 and downwelling irradiances 关Ed共␭兲兴 measured over 30-min intervals centered on local noon, the satellite overpass time. In-water measurements are made at depths of 2, 5, and 9 m. These measurements are used to compute diffuse attenuation coefficients for each depth, which in turn are used to derive Lw at the surface. Surface irradiance 共Es兲 measurements are also made. For the MOBY spectra to be considered valid, the diffuse attenuation coefficients computed at each depth must be consistent with each other. The calibrated MOBY spectra are convolved with the relative spectral-response functions of the SeaWiFS bands for use in the matchup analysis. The processed MOBY data for a given day includes Lw for bands 1– 6 and Es for bands 1– 8. Estimates of Es from the subsurface data are not currently provided. High-calibration accuracy for the MOBY data requires that the buoy to be swapped out for refurbishment and recalibration approximately every three months. In addition, monthly in-water calibrations are performed between buoy swap outs by divers using calibrated lamps to illuminate the MOBY detectors. To ensure continuous data, the MOBY Project maintains three buoys, one in the water and two undergoing refurbishment. The vicarious calibration used in the recently completed third reprocessing of the SeaWiFS global mission data set incorporates data from ten MOBY deployments. B.

Computation of Lwn for MOBY Data

In performing the vicarious calibration, the CVT evaluated satellite to in situ matchups of Lwn between SeaWiFS and MOBY. Since the MOBY measurements provide Lw for bands 1– 6 and Es for bands 1– 8, there are several methods that can be used to normalize the MOBY water-leaving radiances. In the first method, Lwn is computed from MOBY measurements of Lw and Es:

L wn共␭兲 ⫽

冋

册

L w共␭, ␪ 0兲 F 0共␭兲, E s共␭, ␪ 0兲

(6)

where F0 is the solar constant and ␪0 is the solar zenith angle. This method requires reliable surfaceirradiance measurements. Shortly after the launch of SeaWiFS, the above-water Es detector on MOBY failed, so the Es measurements from MOBY for September–November 1997 are invalid. In addition, the data from the above-water Es detectors on later MOBY deployments are highly variable, and the data quality toward the end of each deployment tends to decline. Because of these problems with the Es measurements, the CVT uses computed Es rather than measured Es for estimating MOBY Lwn values in the matchup comparisons of the vicarious calibration: E s共␭, ␪ 0兲 ⫽ F 0共␭兲cos共␪ 0兲t共␭, ␪ 0兲 f sol,

(7)

where t共␭, ␪0兲 is the computed atmospheric diffuse transmittance. fsol is the correction for the varying Earth–Sun distance: f sol ⫽

冉冊

2

R AU , R ES

(8)

where RAU is the Astronomical Unit in km and RES is the Earth–Sun distance in km. The normalized water-leaving radiance can be redefined as L wn共␭兲 ⫽

冋

册

L w共␭, ␪ 0兲 . cos共␪ 0兲t共␭, ␪ 0兲 f sol

(9)

The CVT has used two methods for computing the atmospheric diffuse transmittance. In the first approach, the transmittance is defined as24:

(

t共␭, ␪ 0兲 ⫽ exp

)

⫺兵␶ r共␭兲兾2 ⫹ ␶ oz共␭兲 ⫹ 关1 ⫺ ␻ a共␭兲 F a共␭兲兴␶ a共␭兲其 , cos共␪ 0兲

where ␶r is the computed Rayleigh optical thickness, ␶oz is the ozone optical thickness from the SeaWiFS ancillary data, ␻ is the aerosol single-scattering albedo, Fa is the forward-scattering probability for aerosols, and ␶a is the aerosol optical depth. The aerosol effects are computed for an aerosol optical thickness of 0.1 and the maritime aerosol model for 90% relative humidity.25 In this vicarious calibration method, the SeaWiFS retrievals of Lw are normalized using Eq. 共9兲 so that the same formula for normalized water-leaving radiance is applied to both SeaWiFS and MOBY data. In the second vicarious calibration method, the CVT uses the atmospheric transmittances retrieved by the SeaWiFS atmospheric-correction algorithm to compute the transmittance that is used to normalize the MOBY water-leaving radiances:

冋再

t共␭, ␪0M兲 ⫽ exp

⫺␶ e共␭, ␪ 0s 兲 cos共␪ 0M兲

册

where ␶e is the effective atmospheric optical thickness, t0 r is the top-of-the-atmosphere-to-surface Rayleigh and aerosol transmittance, t0 oz is the topof-the-atmosphere-to-surface ozone transmittance, ␪0S is the solar-zenith angle for SeaWiFS, and ␪0M is the solar-zenith angle for MOBY. The solar-zenith angles for the SeaWiFS observations and for the times of the MOBY measurements are used to correct for changes in the solar elevation between the times of the SeaWiFS overpass and the MOBY data collection. In this vicarious calibration method, the MOBY measurements of Lw are normalized through use of Eq. 共11兲 so that the same formula for normalized waterleaving radiance is applied to both SeaWiFS and MOBY data. The time series of MOBY Lwn for band 1, plotted for the SeaWiFS matchups in Fig. 2, indicates that the MOBY-measured radiances are stable from one buoy deployment to the next. The figure shows Lwn calculated by both approaches for computing the atmospheric diffuse transmittance and Lwn computed from the surface Es measurements where the Es measurements are valid. The scatter in the data is due to variations in the water mass and in the atmospheric aerosols with time. Examination of the plots shows that while the two approaches to the computation of diffuse transmittance yield similar Lwn, the Lwn computed from the Es measurements indicates that the Es varies both within and between MOBY deployments. Consequently, the CVT chose to use Lwn normalized by the computed diffuse transmittance in the vicarious calibration. The CVT used

冎

⫺ln关t 0r共␭, ␪ 0s 兲t 0oz共␭, ␪ 0s 兲兴cos共␪ 0s 兲 ⫽ exp , cos共␪ 0M兲

the first approach to computing the diffuse transmittance 关Eq. 共10兲兴 for the third reprocessing of the SeaWiFS data set 共completed in May 2000兲. Since the second approach 关Eq. 11兴 uses retrieved atmospheric transmittances rather than transmittances that were computed for an assumed aerosol model, the CVT will employ this method in the future, even though the differences in the two approaches are small.

5. Vicarious Calibration of Bands 1– 6

The CVT has performed the vicarious calibration of SeaWiFS by comparing Lwn measured by MOBY with Lwn retrieved by SeaWiFS from contemporaneous overflight images of the buoy site. In this section, we discuss the SeaWiFS retrievals and the SeaWiFS兾 MOBY matchup comparisons. A.

(11)

(10)

SeaWiFS Data Selection

The SeaWiFS measurements are water-leaving radiances 共Lw兲, which are retrieved from the 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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Fig. 2. Time series of normalized water-leaving radiances for MOBY band 1. Lwn that are computed with use of the approximate diffuse transmittance, the retrieved diffuse transmittance, and the measured Ew are plotted.

top-of-the-atmosphere radiance according to the relation L w共␭兲 ⫽

冋

1 L t共␭兲 ⫺ L r共␭兲 ⫺ t共␭兲 L wc共␭兲 t共␭兲 ⫺ ⑀共␭, 865兲 L as共865兲

册

F 0共␭兲 . F 0共865兲

(12)

This relation is obtained by combining Eqs. 共1兲 and 共3兲. The Lwn are computed from Lw by use of Eq. 共9兲. The observations used in the vicarious calibration are mean Lw and mean Lwn retrieved for 3 ⫻ 3 pixel regions that are centered on the pixel containing MOBY, where at least five pixels in the region pass the exclusion criteria listed in Table 1, the same criteria that are used in the vicarious calibration of band 7. These criteria are based on standard quality control masks and flags, computed on a pixel-by-pixel basis. It should be noted that some of these criteria are not directly applicable to observations obtained around MOBY, such as turbid water and coccolithophores, but are included to maintain consistency with the SeaWiFS in situ matchup calibration vali6706


dation analyses discussed later in this paper. Sun glint in the SeaWiFS scenes can be interpreted by the atmospheric-correction algorithm as aerosol radiance. To avoid sun glint contamination of the matchup data, an upper limit of 0.1 is set on the aerosol optical depth in band 8 for valid SeaWiFS retrievals. The application of this limit results in the loss of a considerable number of matchup scenes during the summer. Alternative statistical measures to the mean value were considered for determining the optimum value of Lwn in a scene. They include the median value of the pixels, the value of the central pixel, and the mode of the pixels. The CVT found that the median value can be affected by outliers; the central pixel does not provide a better value than the mean, as the standard deviation of the mean is typically small; and the mode may provide the best estimate of Lwn, but it is difficult to compute for only nine pixels. Consequently, the mean value is used in the vicarious calibration. The time series of mean SeaWiFS Lwn for the matchup scenes are plotted in Fig. 3. The figure shows Lwn computed from retrievals of Lw that have

Fig. 3. Time series of normalized water-leaving radiances for SeaWiFS band 1. Lwn that are computed with use of the approximate diffuse transmittance and directly retrieved by the atmospheric-correction algorithm are plotted.

been normalized by Eqs. 共9兲 and 共10兲 and Lwn retrieved by the atmospheric correction algorithm directly. The scatter in the data is due to variations in the water mass and in the atmospheric aerosols with time. As it did for the MOBY measurements, the CVT used the computed diffuse transmittance 关Eq. 共10兲兴 to normalize Lw in the vicarious calibration for the third reprocessing of the SeaWiFS data set. The CVT will use the direct retrieval of Lwn 关Eq. 共11兲兴 in future updates of the vicarious calibration. B.

Calibration Results

During performance of the vicarious calibration, the ratios of the SeaWiFS retrievals of Lwn to the MOBY measurements of Lwn are computed in each band for each scene. The CVT iteratively adjusts the vicarious gains 关␣共␭兲 in Eq. 共2兲兴 in the SeaWiFS top-of-theatmosphere radiances 关LT共␭兲兴 to optimize the agreement between the Lwn retrieved by SeaWiFS with the Lwn measured by MOBY in each band. Because the distribution of the ratios of Lwn 共SeaWiFS兾 MOBY兲 is more log normal than normal, the vicarious

gains are adjusted until the geometric mean of the ratios for each band is unity: f 共␭兲 ⫽

冓冔

s L wn 共␭兲 , M L wn共␭兲

(13)

where f共␭兲 is the geometric mean of the SeaWiFS兾 MOBY matchup ratios in each band and has a value that approaches unity. The results of the vicarious calibration are shown in Table 2. The SeaWiFS兾 MOBY matchup ratios for bands 1– 6 are plotted in Fig. 4 as functions of time and in Fig. 5 as functions of scan angle. Other estimators of the SeaWiFS兾 MOBY ratios over the matchup scenes were considered, 共the arithmetic mean, the median, and the mean of the center quartiles兲, but the use of these other estimators had negligible effect 共⬍0.1%兲 on the vicarious gains. The vicarious calibration of band 6 is difficult because of the low values of Lwn in this band, as is shown in Fig. 4. Several of the SeaWiFS scenes yield negative Lwn for band 6. Additionally, the calibration of MOBY over this bandpass is problematic because of the crossover between the two spectrom20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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Table 2. Results of SeaWiFS Vicarious Calibration

␭ 共nm兲

MOBY Radiance

SeaWiFS Radiance

Mean Ratio 共SeaWiFS兾 MOBY兲

412 443 490 510 555 670 765 865

1.8263525 1.6133271 1.1106925 0.65016730 0.27024732 0.014412810

1.8263090 1.6133272 1.1106940 0.65016551 0.27024435 0.014432398

0.99997615 1.0000001 1.0000014 0.99999724 0.99998900 1.0057566

Vicarious Gain 1.00324 0.991554 0.962221 0.983602 0.991394 0.959477 0.946 1.000

a

MOBY and SeaWiFS normalized water-leaving radiances are in units of mW cm⫺2 sr⫺1 ␮m⫺1.

eters. As a result, the matchups where negative Lwn occurred for band 6 were excluded from the vicarious calibration. Figures 4 and 5 show a number of matchup ratios that differ from unity by more than 10%. The CVT has determined that for these outlier ratios, the SeaWiFS measurements are the source of the devi-

ation. This implies that the atmospheric conditions differ for the outliers. The CVT has attempted to determine the cause of this deviation from the mean of the matchup ratios by looking unsuccessfully for correlations between the matchups and aerosol optical depth, aerosol model, chlorophyll concentration, local wind speed, and ozone concentration. The matchup time series plotted in Fig. 4 do not show any trends with time, indicating that the time corrections applied to the SeaWiFS bands do not have any significant residual errors. 6. Effects of the Extraterrestiral Solar Irradiance

The particular values of the vicarious gains derived for SeaWiFS and listed in Table 2 depend on the extraterrestrial solar irradiance, or solar constant, F0共␭兲. This dependency implicitly enters Eq. 共1兲 in the Rayleigh, aerosol, Rayleigh–aerosol interaction, sun glint, and whitecap corrections. The SeaWiFS Project uses the Neckel and Labs solar irradiance,26 convolved with the band-specific relative spectral responses, to derive the solar constant for each band. These constants are listed in Table 3. To assess the effects of using a different set of

Fig. 4. SeaWiFS兾MOBY vicarious calibration matchups as a function of time. 6708


Fig. 5. SeaWiFS兾MOBY vicarious calibration matchups as a function of scan angle.

solar constants on the SeaWiFS-retrieved Lwn, the CVT derived a set of solar constants using the MODTRAN solar irradiance.27 A vicarious calibration was then performed with use of the newly derived solar constants. The resulting vicarious gains changed, but the retrieved Lwn were indistinguishable from those obtained with use of the original constants. Accordingly, the vicarious calTable 3. Solar Constants Used in SeaWiFS Data Processinga

a

␭ 共nm兲

Solar Constant

412 443 490 510 555 670 765 865

170.79 189.45 193.66 188.35 185.33 153.41 122.24 98.82

Constants are the Neckels and Labs solar irradiance26 convolved with the relative spectral response for each band. The units are mW cm⫺2 sr⫺1 ␮m⫺1.

ibration procedure makes the Lwn independent of the choice of the solar constants, F0共␭兲, as long as the same constants are used throughout the processing. Equation 共2兲 shows that because different solar constants result in different vicarious gains, the choice of solar constants affects SeaWiFS-derived top-of-the-atmosphere radiances. Particular analyses that use SeaWiFS top-of-the-atmosphere radiances 共such as land-application analyses兲 may preclude the use of the vicarious gains for specific cases.8 7. Uncertainties in the Vicarious Calibration

Uncertainties in the vicarious calibration of SeaWiFS can arise from variations in viewing geometry between SeaWiFS and MOBY, uncertainties in the laboratory calibration of the instrument, and uncertainties in the atmospheric-correction algorithm. These sources of uncertainty will be discussed in this section. A.

Uncertainties Due to Varying Observing Geometry

SeaWiFS data show variations in response as a function of the solar兾viewing geometry of the observa20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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Fig. 6. SeaWiFS兾MOBY matchups for bands 1– 6 as a function of airmass. The lines are linear fits to the data.

tions. The CVT examined the SeaWiFS兾MOBY matchups as a function of airmass to look for trends in the matchups with increasing path length through the atmosphere. For this analysis, the airmass is defined as m⫽

1 1 ⫹ , cos共␪ 0兲 cos共␪兲

(14)

where ␪0 is the solar zenith angle and ␪ is the sensor zenith angle of the SeaWiFS observations. The matchup ratios for bands 1–5 are plotted in Fig. 6, along with linear fits to the data. Figure 6 shows a decrease in the SeaWiFS radiances as the airmass, or the optical path through the atmosphere, increases. The CVT also looked for trends with viewing angle by examining the instrument response as a function of pixel number within the scan. The CVT averaged the scan lines in LAC scenes over MOBY on a pixelby-pixel basis for all of the cloud-free pixels in each of the scenes. The MOBY LAC data were selected to minimize areal variations in ocean reflectance while including coverage of a high-quality in situ time series. For this analysis, 149 scenes, spanning the 6710


mission from 19 September 1997 to 4 February 2000, were processed. The average chlorophyll for these data is approximately 0.08 mg m⫺3. The mean Lwn values are plotted in Fig. 7 as a function of pixel number and as a function of secant of the scan angle. The cutoff in the LAC data for the global-area coverage 共GAC兲 data occurs at a scan angle of 45°; the first and last 146 pixels of LAC data are excluded from the GAC data sampling. The plots in Fig. 7 show that the response has a slight east–west trend for bands 1 and 2; this trend may be due to variations in the water mass east and west of Hawaii. The plots also show a rolloff in the retrieved radiances of approximately 10% at the GAC data cutoff, which increases at larger scan angles. One explanation of these trends with the observing and viewing geometry shown in Figs. 6 and 7 is the effect of the bidirectional reflectance distribution function 共BRDF兲 of the ocean on the Lwn. Morel and Gentili28 have proposed a BRDF correction to the Lwn 共the f兾Q correction兲 to compensate for variations in the solar zenith angles, the instrument zenith angles, and the instrument viewing angles. Mueller29 calls for application of the f兾Q correction to in situ Lwn

Fig. 7. The mean Lwn values. a, plotted as a function of pixel number along the scan line; b, plotted as a function of the secant of the scan angle for each pixel. The two sides of the scan are denoted by the solid and dashed lines. The 45° GAC cutoff is indicated in the plot. 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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Table 4. Band 8 Vicarious Gain Bias Testa

␭ 共nm兲

High Bias Gain Ratios

Low Bias Gain Ratios

412 443 490 510 555 670 765 865

1.0014882 1.0027865 1.0048167 1.0064833 1.0103489 1.0341327 1.0361702 1.050

0.99616037 0.99493277 0.99246830 0.99093495 0.98710062 0.97451675 0.96276596 0.950

a Effect of changing the band 8 gain by 5% on the vicarious gains of the other bands is shown by the gain ratios.

measurements to compensate for variations in the solar zenith angles as well. BRDF effects in both the SeaWiFS retrievals and the MOBY in situ measurements could give rise to the trends shown in the SeaWiFS兾MOBY matchup ratios that are plotted in Fig. 6. The CVT has applied the most recent f兾Q algorithm to the SeaWiFS data and found that the f兾Q correction does not have a significant effect on the observed trends within the GAC cutoff and overcorrects the rolloff in radiances outside the GAC cutoff. Consequently, the CVT chose to not apply the f兾Q correction to the SeaWiFS data for the third reprocessing of the SeaWiFS data set. The CVT will reevaluate the f兾Q correction of the SeaWiFS data as the algorithm matures. A second explanation of the trends with observing and viewing geometry is that the atmosphericcorrection algorithm overcorrects the SeaWiFSretrieved Lwn as the optical path through the atmosphere increases. The CVT is evaluating possible improvements in the atmospheric-correction algorithm in preparation for the next reprocessing of the SeaWiFS global data set. B.

Other Uncertainties

One assumption of the current SeaWiFS vicarious calibration strategy is that the prelaunch calibration of band 8 共Ref. 9兲 is correct. The estimated uncertainty in the band 8 calibration is approximately 5%.8 The CVT preformed a bias test in which the band 8 gain was set to 1.05 and 0.95. For these two cases, the vicarious calibration of band 7 caused the gain to change by ⫹3.6% and ⫺3.7%, respectively, when calculating the desired ⑀ at the MOBY site. The effect of the changes in the band 7 and 8 gains on the vicarious gains of bands 1– 6 decreased with decreasing wavelength. The results of the bias test are summarized in Table 4. Again, the effect of the vicarious calibration is to minimize the impact of the changes in the band 7 and 8 gains on the global retrievals of Lwn and chlorophyll. The CVT performed a number of analyses in preparation for the third reprocessing of the SeaWiFS global mission data set in May 2000.7 As a result of these analyses, several modifications were made to the SeaWiFS atmospheric correction algorithm.30,31 6712


The CVT also updated the time-correction factors for bands 7 and 8 共Ref. 8兲. Each of the algorithm modifications and time-correction updates required recalculation of the band 7 gain to obtain the desired ⑀ at the MOBY site. Again, the vicarious calibration of bands 1– 6 minimized the impact of these changes on the global retrievals of Lwn and chlorophyll. The different iterations of the atmospheric-correction algorithm and the time-correction factors changed the vicarious gains of bands 1–7 by less than 0.3%. Accordingly, 0.3% is an estimate of the uncertainty in the vicarious gains for these bands. Since the vicarious gains are applied to the top-of-the-atmosphere radiance, this uncertainty in the gains corresponds to an uncertainty of approximately 3% in the waterleaving radiances. C.

Summary of Uncertainties

The departure of the vicarious gains for bands 1–7 from unity is the result of effects of varying observing and viewing geometries on the data collected, uncertainties in the atmospheric-correction algorithm, uncertainties in the laboratory calibration of SeaWiFS, uncertainties in the laboratory calibration of MOBY, and possible changes in SeaWiFS on orbit prior to the first lunar calibration. The uncertainty in the SeaWiFS calibration, as estimated by the vicarious gains, ranges 1– 4%, depending on the band. This result is consistent with the uncertainty derived from the prelaunch recalibration of SeaWiFS9 and with the uncertainty derived from the calibration transferto-orbit experiment.10 The CVT continues to investigate the uncertainties in the vicarious calibration in preparation for the next reprocessing of the SeaWiFS global data set. 8. Validation of Visible Band Calibrations

The calibration of band 7 to retrieve maritime ⑀ at the MOBY site and the calibration of bands 1– 6 against MOBY measurements of Lw yield a one-location vicarious calibration of SeaWiFS. The question arises about the applicability of this one-point calibration to the global ocean, since the vicarious calibrations against MOBY may be biased towards regions of the ocean with maritime aerosols or clear water. There is also a question of whether the time-correction factors yield stable water-leaving radiances over the course of the mission. To address these issues, the CVT has undertaken an extensive vicarious calibration-validation program. This program analyzes matchups between SeaWiFS data and in situ measurements from a number of investigators to evaluate the SeaWiFS retrievals of Lwn on a global basis. It also uses timeseries analyses of global clear-water radiances to monitor the temporal stability of the ocean data sets. A.

In Situ Matchup Procedure

In the matchup procedure, individual SeaWiFS scenes are compared with in situ data records of individual measurements, i.e., stations in time and space. The in situ data are contained within the

Fig. 8. Validation matchups between SeaWiFS and in situ data sets for bands 1–5 and chlorophyll.

SeaWiFS Bio-optical Archive and Storage System.32 For all valid matches, a level 2 product is generated from a 101 ⫻ 101-pixel extract file. A comprehensive suite of SeaWiFS level 2 products are generated, including all Lw共␭兲, Lwn共␭兲, Es共␭兲, chlorophyll a, Kd共490兲, and ancillary data 共ozone, windspeed, and atmospheric pressure兲. Only a small percentage of the candidate SeaWiFS files become final valid matches.14 A majority of matchups are excluded because one or more of the SeaWiFS data-quality flags

shown in Table 1 have been set; these are also the exclusion criteria used in the vicarious calibration. In this analysis, the algorithm for the SeaWiFS turbid water flag is applied to the in situ data to exclude turbid water. This approach was taken because the turbid water flag was being set erroneously in the SeaWiFS data near cloud edges or in high-chlorophyll waters. A number of additional exclusion criteria have been formulated to provide an objective set of points 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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for SeaWiFS validation that removes invalid or redundant data from consideration. The first exclusion criterion applied is a time difference between the in situ record and the satellite overpass. A time window of ⬃180 min from the satellite overpass time 共local noon兲 is used because this window is the time period of reasonable illumination in most situations and, presumably, constant atmospheric aerosol conditions. If a matchup passes the temporal exclusion criterion, the valid SeaWiFS pixels are averaged for a 5 ⫻ 5-pixel box centered on the matchup. Calculations currently used for the further exclusion of points and the matchup values include: 1. At least 13 of 25 pixels in the 5 ⫻ 5 box must be valid. 2. If there are GAC, LAC, and High-Resolution Picture Transmission 共HRPT兲 matches corresponding to the same in situ point, the order of preference is LAC, HRPT, and GAC. When multiple files of the same preferred type match a single in situ record, the closest temporal match is selected. 3. Cases where multiple in situ casts are performed at the same station are reduced to one representative record by selecting the cast with the highest Lwn共490兲. 4. Along-track in situ measurements are included in the analysis where adjoining measurements are greater than 5 km apart. 5. Theoretical Es values are calculated with use of the time and location for each satellite and in situ measurement and with use of Eq. 共9兲. 6. Satellite matchups with extreme variation between pixels in the 5 ⫻ 5-pixel box 共coefficient of variation, or the standard deviation divided by the mean value, ⬎0.2兲 are excluded. B.

Matchup Results

The current 共June 2001兲 in situ data records in the SeaWiFS Bio-optical Archive and Storage System were processed using the exclusion criteria outlined in Subsection 3.B 共Table 1兲. Of the 2015 in situ records that were considered 共73 data sets兲, 201, or 9.97% passed all exclusions to become final matches. One hundred forty-nine of these records represent Case 1 waters 共7.4%兲. Comparisons of Lwn between in situ sites and SeaWiFS show generally good agreement in the radiance values 共Fig. 8; Table 5兲, especially at the higher radiance values. Note the lower number of matchup points in the Lwn共412兲 and, to a lesser extent, Lwn共443兲 band. This reduction at lower wavelengths occurs in coastal areas where Lw共765兲 ⬎ 0 and Lw共865兲 ⬎ 0, conditions that cause the overestimation of aerosol radiances and low, often negative, Lw values in these bands. Negative Lw values can also result when absorbing aerosols are present, with the most severe effects occurring at the shortest wavelengths. In nearly all cases, this anomaly occurred in Case 2 waters or in a phytoplankton plume with relatively high chlorophyll concentrations. 6714


Table 5. Statistics for Lw for Bands 1–5, Chlorophyll a, and Kd共490兲a

Parameter

Means of Ratios

␴

Lw共412兲 Lw共443兲 Lw共490兲 Lw共510兲 Lw共555兲 Chlorophyll a Kd共490兲

0.8249 0.9679 1.0145 1.0213 1.0870 1.0056 1.1400

0.2176 0.2459 0.2278 0.2431 0.3009 0.4906 0.2723

Number of Range of In Situ Observations Values 130 144 149 139 149 94 135

0.1844–2.9280 0.2161–2.5107 0.2472–1.7331 0.2198–0.9132 0.1057–0.5385 0.0310–4.6500 0.0184–0.5040

a Means of the ratios are the SeaWiFS兾in situ values, and ␴ is the standard deviation. The number of points for each parameter is not the same because of differences in the specific bands measured in situ by different investigators.

Many of these values exhibited high coefficients of determination 共⬎0.2兲 in the 5 ⫻ 5-pixel set. Note that the lower number of points in the Lw共510兲 panel is not due to overestimation of the aerosol radiance but rather stems from the absence of this channel in some field radiometers. The comparison of Lwn共412兲 indicates that SeaWiFS retrievals have a slight negative bias that increases at lower values. The conditions reported above that result in negative Lwn retrievals may influence the retrievals at low values as well. The Lwn共443兲 comparison does not show the same bias seen in the Lwn共412兲 comparison, although it is possible that the effect is also present at this band, just to a much smaller extent. SeaWiFS compares quite well with the in situ measurements for Lwn共490兲, Lwn共510兲, and Lwn共555兲, although Lwn共510兲 and Lwn共555兲 may be slightly positively biased at higher values. The comparison of SeaWiFS to in situ chlorophyll demonstrates the reasonably high quality of the chlorophyll a data product for in situ values between approximately 0.03 and 5 mg m⫺3. These results are affected not only by inaccuracies in SeaWiFS imagery but also by variations in field chlorophyll measurement techniques. Work continues to improve the SeaWiFS-to-in situ comparisons from both the SeaWiFS imagery and the field measurement aspects of the effort. C.

Clear-Water Analysis

The CVT has computed global mean clear-water radiances for bands 1– 6 from the time series of eightday composite SeaWiFS images produced by the SeaWiFS Project, using the time-correction factors and vicarious gains implemented for the third SeaWiFS reprocessing. One hundred and thirty-one eight-day composites were used in the analysis, spanning a time range of 15 September 1997 through 18 July 2000. The time series of the clear-water radiances and chlorophyll concentrations are plotted in Fig. 9. The figure shows that the vicarious calibration and the time corrections for each band are stable over the course of the mission. Bands 1 and 2 show slight trends that may be a result of the interannual variability of global chlorophyll a concentrations 共e.g.,

Fig. 9. SeaWiFS clear-water analysis time series from eight-day timebin files. a, mean global clear-water radiances; b, mean global clear-water chlorophyll. 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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Fig. 10. SeaWiFS clear-water analysis time series atmospheric-correction parameters. a, mean Ångstro¨ m at 510 and 865 nm; b, mean ⑀共765,865兲; c, mean aerosol optical depth at 865 nm. 6716


El Nin˜ o and La Nin˜ a兲 and periodicities, due to the fact that the sampling of the ocean on a global basis shifts north and south seasonally. The same periodicities are apparent in the chlorophyll time series. The stability of the atmospheric correction over the course of the mission is shown in Fig. 10 by plots of the time series of the mean atmospheric-correction parameter epsilon 关⑀共765,865兲兴, the mean Ångstro¨ m exponent at 510 and 865 nm, and the mean aerosol optical depth at 865 nm 共Ref. 33兲. Slight downward trends are apparent over the first 600 days, presumably due to changes in the atmosphere during the El Nin˜ o–La Nin˜ a transition. 9. Summary

This paper documents the vicarious calibration of SeaWiFS sensor and atmospheric-correction algorithm system. Vicarious gains are applied to the top-of-the-atmosphere radiances that are measured by the instrument to yield the expected atmosphericcorrection parameters 共in the case of band 7兲 and the expected water-leaving radiances 共in the case of bands 1– 6兲. The band 7 gain is adjusted to retrieve an atmospheric-correction parameter ⑀共765,865兲 over the MOBY site for maritime aerosols. The gains for bands 1– 6 are adjusted to yield normalized waterleaving radiances that agree with measurements by MOBY. The uncertainty in the near-infrared vicarious gain is 0.9%. The uncertainties in the vicarious gains for the visible bands are 0.3%, which corresponds to uncertainties of approximately 3% in the water-leaving radiances. The applicability of the single-location calibration of SeaWiFS to data for the global ocean is demonstrated by the calibrationvalidation program. The means of the SeaWiFS兾 in situ matchup ratios for water-leaving radiances are typically within 5% of unity for Case 1 waters, while chlorophyll a ratios are within 1% of unity. The global clear-water time series show that the vicarious calibration is stable over the course of the mission. The CVT will periodically update the vicarious gains over the course of the SeaWiFS mission, e.g., at each reprocessing, as additional SeaWiFS兾MOBY matchups become available, as the time corrections to the SeaWiFS bands are updated, and as the atmospheric-correction algorithm is improved. Also, the CVT is studying alternative strategies for the vicarious calibration of SeaWiFS, including methods for the vicarious calibration of band 8. If an alternative approach provides improved SeaWiFS兾in situ matchups and reduces the occurrence frequency of negative water-leaving radiances in the global data sets, this method would be used in a subsequent reprocessing of the SeaWiFS data. SeaWiFS is the first ocean-color mission to use an extensive and ongoing prelaunch and postlaunch calibration program, and the matchup results demonstrate the benefits of a comprehensive approach. The authors thank Jim Mueller for his instructive comments, which helped to clarify several points in

our presentation of the material. This work was conducted as part of the SeaWiFS calibration and validation program under NASA contract NAS500141. References 1. C. R. McClain, W. E. Esaias, W. Barnes, B. Guenther, D. Endres, S. B. Hooker, G. Mitchell, and R. Barnes, Calibration and Validation Plan for SeaWiFS, NASA Tech. Memo. 104566 3, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 1992兲. 2. C. R. McClain, M. L. Cleave, G. C. Feldman, W. W. Gregg, S. B. Hooker, and N. Kuring, “Science quality SeaWiFS data for global biospheric research,” Sea Technol. 39, 10 –16 共1998兲. 3. H. R. Gordon, “In-orbit calibration strategy for ocean color sensors,” Remote Sens. Environ. 63, 265–278 共1998兲. 4. R. H. Evans and H. R. Gordon, “Coastal zone color scanner system calibration: a retrospective examination,” J. Geophys. Res. 99, 7293–7307 共1994兲. 5. D. K. Clark, H. R. Gordon, K. J. Voss, Y. Ge, W. Broenkow, and C. Trees, “Validation of atmospheric correction over the oceans,” J. Geophys. Res. 102, 17209 –17217 共1997兲. 6. S. B. Hooker and C. R. McClain, “The calibration and validation of SeaWiFS data,” Prog. Oceanogr. 45, 427– 465 共2000兲. 7. C. R. McClain, “SeaWiFS postlaunch calibration and validation overview,” in SeaWiFS postlaunch calibration and validation analyses, Part 1, NASA Tech. Memo. 1999-206892 9, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 8. R. A. Barnes, R. E. Eplee, Jr., G. M. Schmidt, F. S. Patt, and C. R. McClain, “The calibration of SeaWiFS. I: Direct techniques,” Appl. Opt. 40, 6682– 6700 共2001兲. 9. B. C. Johnson, E. E. Early, R. E. Eplee, Jr., R. A. Barnes, and R. T. Caffrey, in The 1997 Prelaunch Radiometric Calibration of SeaWiFS, NASA Tech. Memo. 1999-206892 4, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 1999兲. 10. R. A. Barnes, R. E. Eplee, Jr., S. F. Biggar, K. J. Thome, E. F. Zalewski, P. N. Slater, and A. W. Holmes, “The SeaWiFS transfer-to-orbit experiment,” Appl. Opt. 39, 5620 –5631 共2000兲. 11. R. E. Eplee, Jr. and R. A. Barnes, “Lunar data analysis for SeaWiFS calibration,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 1, NASA Tech. Memo. 1999206892 9, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 12. R. E. Eplee, Jr. and C. R. McClain, “MOBY data analysis for the vicarious calibration of SeaWiFS bands 1– 6,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 1, NASA Tech. Memo. 1999-206892 9, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 13. W. D. Robinson and M. Wang, “Vicarious calibration of SeaWiFS band 7,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 1, NASA Tech. Memo. 1999-206892 9, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 14. S. W. Bailey, C. R. McClain, P. J. Werdell, and B. D. Schieber, “Normalized water-leaving radiance and chlorophyll a match-up analysis,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 2, NASA Tech. Memo. 1999-206892 10, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 15. H. R. Gordon and M. Wang, “Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm,” Appl. Opt. 33, 443– 452 共1994兲. 20 December 2001 兾 Vol. 40, No. 36 兾 APPLIED OPTICS

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16. H. R. Gordon, J. W. Brown, and R. H. Evans, “Exact Rayleigh scattering calculations for use with the Nimbus 7 Coastal Zone Color Scanner,” Appl. Opt. 27, 862– 871 共1988兲. 17. M. Wang and S. Bailey, “Correction of the sunglint contamination of the SeaWiFS aerosol optical thickness retrievals,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 1, NASA Tech. Memo. 1999-206892 9, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 18. H. R. Gordon and M. Wang, “Influence of oceanic whitecaps on atmospheric correction of ocean-color sensors,” Appl. Opt. 33, 7754 –7763 共1994兲. 19. R. Frouin, M. Schwindling, and P. Y. Descahmps, “Spectral reflectance of sea foam in the visible and near infrared: in situ measurements and remote sensing applications,” J. Geophys. Res. 101, 14361–14371 共1996兲. 20. K. D. Moore, K. J. Voss, and H. R. Gordon, “Spectral reflectance of whitecaps: instrumentation, calibration, and performance in coastal waters,” J. Atmos. Ocean. Tech. 15, 496 –509 共1998兲. 21. K. D. Moore, K. J. Voss, and H. R. Gordon, “Spectral reflectance of whitecaps: their contribution to water-leaving radiance,” J. Geophys. Res. 105, 6493– 6499 共2000兲. 22. W. D. Robinson, G. M. Schmidt, C. R. McClain, and P. J. Werdell, “Changes made in the operational SeaWiFS processing,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 2, NASA Tech. Memo. 1999-206892 10, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. 23. H. R. Gordon and D. K. Clark, “Clear water radiances for atmospheric correction of coastal zone color scanner imagery,” Appl. Opt. 20, 4175– 4180 共1981兲. 24. H. R. Gordon, D. K. Clark, J. W. Brown, O. B. Brown, R. H. Evans, and W. W. Broenkow, “Phytoplankton pigment concentrations in the Middle Atlantic Bight: comparison of ship

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25.

26. 27.

28.

29.

30.

31.

32.

33.

determinations and CZCS estimates,” Appl. Opt. 22, 20 –36 共1983兲. M. Wang, “A sensitivity study of the SeaWiFS atmospheric correction algorithm: effects of spectral band variations,” Remote Sens. Environ. 67, 348 –359 共1999兲. H. Neckel and D. Labs, “The solar radiation between 3300 and 12,500 Å,” Solar Phys. 90, 205–258 共1984兲. A. Berk, L. S. Bernstein, and D. C. Robertson, MODTRAN: a Moderate Resolution Model for LOWTRAN7, Tech. Report GLTR-90-0122 共Geophysical Directorate Phillips Laboratory, Hanscom Air Force Base, Mass., 1990兲. A. Morel and B. Gentili, “Diffuse reflectance of ocean waters. III. Implication of bidirectionality for the remote-sensing problem,” Appl. Opt. 35, 4850 – 4862 共1996兲. J. L. Mueller, “Overview of measurement and data analysis protocol,” in Ocean Optics Protocols for Satellite Ocean Color Sensor Validation, Revision 2, NASA Tech. Memo. 2000209966, G. S. Fargion and J. L. Mueller, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. M. Wang, “SeaWiFS atmospheric correction algorithm updates,” in SeaWiFS Postlaunch Calibration and Validation Analyses, Part 1, NASA Tech. Memo. 1999-206892 9, S. B. Hooker and E. R. Firestone, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. D. A. Siegel, M. Wang, S. Maritorena, and W. D. Robinson, “Atmospheric correction of satellite color color imagery: the black pixel assumption,” Appl. Opt. 39, 3582–3591 共2000兲. P. J. Werdell, S. Bailey, and G. S. Fargion, “SeaBASS data protocols and policy,” in Ocean Optics Protocols for Satellite Ocean Color Sensor Validation, Revision 2, NASA Tech. Memo. 2000-209966, G. S. Fargion and J. L. Mueller, eds. 共NASA Goddard Space Flight Center, Greenbelt, Md., 2000兲. M. Wang, S. Bailey, and C. R. McClain, “SeaWiFS provides unique global aerosol optical property data,” Eos Trans. Am. Geophys. Union 81, 197, 202 共2000兲.