ince the development of the historical, proof-of-oncept Coastal Zone Color Scanner �CZCS�,1 whichuring 1978–1986 provided the first global ocean-olor data set from space, improved satellite ocean-olor sensors with global coverage capability haveeen designed and launched. They include the po-arization and directionality of the Earth’s reflectancePOLDER� instrument2 and the ocean color and tem-erature scanner �OCTS� onboard the Advancedarth Observing Satellite �ADEOS; November 1996–une 1997�; the Sea-viewing Wide Field-of-View Sen-or �SeaWiFS�3 onboard the OrbView-2 satellite
Pierre-Yves Deschamps, Pierre Lecomte, and Christian Ver-aerde are with the Laboratoire d’Optique Atmospherique, Uni-ersite des Sciences et Technologies de Lille, 59655 Villeneuve’Ascq, France. Bertrand Fougnie is with the Centre National’Etudes Spatiales, 18 Avenue Edouard Belin, 31401 Toulouse,rance. Robert Frouin �[email protected]� is with the Climateesearch Division, Scripps Institution of Oceanography, Univer-ity of California San Diego, 8810 La Jolla Shores Drive, La Jolla,alifornia 92037.Received 11 July 2003; revised manuscript received 10 March
August 1997 to the present�; and the Moderate-esolution Imaging Spectrometer �MODIS�4 onboard
he Terra �December 1999 to the present� and AquaMay 2002 to the present� satellites. Comparedith the CZCS, these new-generation sensors haveore adequate spectral bands �e.g., for atmospheric
orrection� and a higher signal-to-noise ratio. Otheride field-of-view sensors have been launched,amely, the Medium-Resolution Imaging Spectrom-ter �MERIS�5 onboard the ENVISAT satelliteMarch 2002� and POLDER-2 and the Global Imager6
nboard ADEOS-II �December 2002�, and they willontribute to a more complete, global, continuingime series of ocean-color observations.
Processing satellite ocean-color data into chloro-hyll a concentration, diffuse attenuation coefficient,r other variables characterizing the water body in-olves removing unwanted atmospheric effects. Atcean-color wavelengths, i.e., in the blue and green,nly a small fraction of the measured signal �typically0%� contains useful information in the form of pho-ons that have interacted with the water body. Con-equently, accurate atmospheric correction of theop-of-atmosphere signal is necessary to attain ac-eptable accuracy for the retrieved ocean variables.
The atmospheric contribution is due mainly to scat-ering by molecules and aerosols. Molecular scatter-
ng can be accurately computed from the surfaceressure7,8; however, aerosol scattering, highly vari-ble in space and time, is difficult to quantify. At-ospheric correction algorithms �e.g., Refs. 9 and 10�
enerally estimate aerosol scattering from the satel-ite radiance measured in two spectral bands in theed and near-infrared where the oceanic contributions very small or negligible �765 and 865 nm� or pre-ictable �670 nm for case 1 waters�. From the inten-ity and spectral dependence of the aerosol scatteringt these two wavelengths, aerosol optical thicknessnd type can be retrieved. This allows computation,herefore correction, of the aerosol scattering atcean-color wavelengths, yielding water-leaving ra-iance or, equivalently, marine reflectance. Even-ually, bio-optical algorithms are applied to theetrieved water-leaving radiance; see, for example,ustin and Petzold11 for the diffuse attenuation co-fficient at 490 nm and Morel12 or O’Reilly et al.13 forhlorophyll a concentration.
The best way to evaluate the performance of thetmospheric correction is to compare in situ measure-ents, over a wide range of conditions, with values of
he variables retrieved by the atmospheric correctioncheme. The necessary variables are the marine re-ectance, the aerosol optical thickness, and the aero-ol model.14–16 These variables also need to beeasured for the vicarious calibration of the sensorshile they operate in orbit. In the vicarious calibra-
ion procedure, the measurements are used either toompute the satellite signal by using a suitable radi-tive transfer code and then comparing the modeledignal with the measured signal17,18 or to evaluateatellite retrievals.19
The in situ measurements of water-leaving radi-nce or marine reflectance are usually made withnderwater instrumentation by measuring the depthrofile of upwelling radiance and downwelling irra-iance and by extrapolating the values to the surfacend across the air–sea interface. The logistics in-olved with this approach are not simple. It is nec-ssary to stop the ship and operate a winch, to driftway a free-fall system, or to deploy and maintainuoys.14,15 Furthermore, dedicated validation cam-aigns at sea are expensive and may not be costffective, because cloud cover may drastically limithe number of matchups between satellite estimatesnd in situ measurements.An alternative to the standard underwater tech-
iques are above-water techniques utilizing radiom-ters that measure upwelling radiance.14,20–29 Thehief difficulty with the above-water techniques isontamination by Sun glint and skylight reflected inhe instrument’s field of view, which is not easy toorrect. Fougnie et al.25 have shown that the glitteran be avoided and skylight reflection substantiallyeduced by using a vertical polarizer and viewing thecean surface in a specific geometry, i.e., 45° from theadir and 135° from the solar plane. Such water-
eaving radiance measurements are easier and fastero make than those with classical underwater instru-entation. A major advantage is that the ship does
ot need to stop, and the measurements can be ob-ained without interfering with ship operations.his offers the opportunity to collect data during aariety of research cruises, especially during voyagesy merchant ships traveling regular routes in theorld’s oceans.In the context of current and future ocean-colorissions, and in view of the stated requirements, weave developed a specific, dedicated hand-held radi-meter for evaluating satellite-derived ocean color.his radiometer, called SIMBAD, was optimized inerms of design and measurement protocol for easynd regular use onboard volunteer ships. It worksike a sunphotometer when aimed at the Sun, and it
easures water-leaving radiance when aimed at thecean surface. From these measurements aerosolptical thickness and marine reflectance can be de-uced. Thus the SIMBAD radiometer gives accesso the two basic atmospheric and oceanic variablesecessary to quantify atmospheric correction perfor-ance. In the following text, we present the instru-ent �including its concept and principle�, discuss
ata processing and accuracy, and compare measure-ents obtained by SIMBAD and other instruments
sunphotometer and underwater radiometer� duringhe Second Aerosol Characterization ExperimentACE-2�, the Aerosols-99 Experiment, and the Cali-ornia Cooperative Oceanic Fisheries InvestigationsCalCOFI� cruises.
. SIMBAD Radiometer
. Concept
everal requirements were considered in the designf the SIMBAD instrument. First, the instrumentad to provide concomitant measurements of aerosolptical thickness and water-leaving radiance in typ-cal spectral bands of satellite ocean-color sensors.econd, quality data had to be collectable from anyoving platform at sea. Third, cost had to be kept
ow to allow the manufacture and deployment of aufficient number of instruments for sampling variedceanic and atmospheric regimes. In view of theseequirements, we opted for a compact, light, and por-able filter radiometer operating in two modes,amely Sun-viewing mode for measuring direct solarxtinction and sea-viewing mode for measuringater-leaving radiance.To reduce Sun-glint and skylight-reflection effects
n the sea-viewing mode, the instrument measuresertically polarized radiance. A preliminary studyheoretically and experimentally identified the opti-um viewing angles that minimize the perturbing
ffects.25 It was concluded that the radiometriceasurements should be made at a nadir angle of 45°
near the Brewster angle� and at a relative azimuthngle of 135° between the solar and the viewing di-ections. In this configuration the residual reflectedkylight in the measured signal is correctable to a few0�4 in reflectance units, and the resulting error onhe diffuse marine reflectance in the blue and greenavelengths is less than 1% for most water types.
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rotov and Vasilkov26 also recommended that angu-ar geometry for polarized measurements be justbove the sea surface.Compared with the viewing geometry for unpolar-
zed measurements suggested by Lee et al.22,23 andsed by Toole et al.30 and others �i.e., 30° from nadirnd 90° from the solar plane�, the viewing geometryor polarized measurements reduces skylight-eflection effects by a factor of 3–4.25 Another argu-ent in favor of polarized measurements near therewster angle is that the residual skylight is much
ess sensitive to uncertainties in viewing angles, windpeed, and aerosol amount.25 Measuring sky radi-nce is not necessary, which simplifies measurementrocedures. In addition, aiming the surface at 45°rom nadir instead of 30° makes it easier to avoidoam and bubbles generated by ship movement.
Water-leaving radiance depends strongly on themount of solar radiation reaching the surface �thereater the incident solar irradiance, the greater theater-leaving radiance�. It is important, therefore,
o normalize water-leaving radiance by downwellingolar irradiance. For example, if the surface mea-urements are not performed at exactly the time ofatellite overpass, the change in solar irradiance be-ween surface and satellite measurements may pre-ent water-leaving radiance comparisons. TheIMBAD radiometer was not designed to measureownwelling solar irradiance. This variable is com-uted by use of the aerosol optical thickness mea-ured by the instrument. This computation isccurate under clear-sky conditions �Sun not ob-cured by clouds�, which are the same conditions foratellite ocean-color evaluation—the purpose of thenstrument.
. Description
he main characteristics of the SIMBAD radiometerre summarized in Table 1. Polarized radiance iseasured in five spectral bands centered at 443, 490,
60, 670, and 870 nm and approximately 10 nm wideFig. 1�. These spectral bands are typical of satellite
cean-color sensors and contain the basic informationor atmospheric correction and bio-optical modeling.he measurements are made simultaneously in theve spectral bands through five aligned optical sub-ssemblies �Fig. 2�. Each subassembly includes aertical polarizer, interference filter, lens, and siliconetector. The incident signal is acquired at a fre-uency of 10 Hz and is amplified, digitally converted,nd stored internally. The instrument is equippedith a magnetometer and two inclinometers for mea-
uring viewing angles and a Global Positioning Sys-em for determining the time and the geographicocation. Four lights controlled by the magnetom-ter and inclinometers help the operator to aim pre-isely at the surface in the required geometry �45°rom nadir and 135° in azimuth with respect to theolar plane�. The total field of view is 3°, a goodompromise between a small field of view for mea-uring direct solar extinction �necessary to minimize
ig. 2. Diagram of the SIMBAD radiometer. The main parts areve optical subassemblies, including interference filter, lenses, po-
arizer, and detector �right-hand side�; electronic card �middle�;emory, GPS, inclinometer, and magnetometer card �left-hand
ide�; and power and GPS antenna input �back face�. Only one setf lenses, corresponding to one of the optical subassemblies, israwn.
ig. 1. Spectral response of the SIMBAD instrument. Band 1 isentered on 443 nm, band 2 on 490 nm, band 3 on 560 nm, band 4n 670 nm, and band 5 on 870 nm.
Table 1. Characteristics of the SIMBAD Radiometer
Parameter Value
Number of spectral bands 5Center wavelength �bandwidth� 443 �10�, 490 �10�, 560 �10�,
670 �10�, 870 �10� nmDetector Silicon photodiodesDynamic range 1–500,000 CNa
Sampling rate 10 HzNumber of gains 2Field of view 3°Weight �4 kgShape, size Cylindrical, �40 cm � 15 cmBatteries Ni–CdMemory 458 MbytesNoise-equivalent reflectanceb �2 � 10�5
aCN, number of counts for Sun and sea measurements.b
tmospheric scattering� and a larger field of view foreasuring water-leaving radiance. The polarizers
o not affect the measurement of direct solar extinc-ion because direct sunlight is not polarized. Thexternal shape of the instrument is cylindrical, theength is 40 cm, the diameter is 15 cm, and the totaleight is 4 kg.Two electronic gains are used, a low gain in Sun-
iewing mode and a high gain in sea-viewing mode,nd they can be selected by pressing a button locatedn the back of the instrument. A third mode, re-erred to as dark-viewing mode, allows measurementf the dark current for high and low gains. Thus theeasurements of direct solar extinction and water-
eaving radiance are made sequentially, not simulta-eously, but the small time difference between thewo types of measurements �see the measurementrotocol in Subsection 2.C� is not a significant factoror satellite ocean-color evaluation. The internal
emory �458 Mbytes� and the Ni–Cd batteries allowhe instrument to be operated for three months inormal mode �i.e., measurements at the time of sat-llite overpass� without saturating the memory oreeding to recharge the batteries. The parametersnd variables stored in the internal memory are lat-tude, longitude, date, time, air pressure and temper-ture, radiance counts, and magnetometer andnclinometer angles. The data stored in the memoryan be downloaded and the memory emptied at anyime during a campaign in which specific personalomputer software is used.
. Measurement Protocol
he measurement protocol consists of making consec-tive measurements in Sun-, sea-, and dark-viewingodes. In Sun- and sea-viewing modes data are ac-
uired in 10 s; in dark-viewing mode, 20 s �10 s forach gain�. A complete measurement sequence,herefore, can be made within 1 min. Repeating theeasurement sequence several times is recom-ended to improve data quality. Each type of datale has the same format, facilitating data processing.n Sun-viewing mode the instrument is pointed to-ard the Sun and properly aligned by use of a finder
Fig. 3, top�. Only the maximum value over 1 s istored in Sun-viewing mode �10 values total�, reduc-ng experimental errors due to ship motion. In Sea-iewing mode the instrument is positioned at 45°rom nadir and at 135° from the solar plane by use ofhe four light indicators �two for nadir angle, two forzimuth angle; see Fig. 3, bottom�. In addition, theurface must be lit by direct sunlight. The operatoras a choice of two azimuth positions at 135° from theun and selects the position that avoids shadows andhip wake. Onboard a ship the best measurementsre generally obtained from the bow, upwind fromxhaust fumes �the ship is usually in the wind whenot steaming�.In addition to the data stored in the SIMBAD in-
trument memory, ancillary information about cloudype and coverage, barometric air pressure, windpeed, and surface state �wave height and direction,
hitecaps� are logged by the operator for each mea-urement sequence. This information is necessaryo correct the data for various perturbing effects ando control data quality �see Section 4�.
. Radiometric Calibration
n the following equations the numerical count forun and sea measurements, CN, is considered to beorrected for dark current. The dark-current count,herefore, is omitted for clarity. These two measure-ents require different calibration methods, which
re described below.
. Sun-Viewing Mode
he instrument in Sun-viewing mode is calibrated byse of the Bouguer–Langley method. The calibra-ion procedure consists of measuring the Sun inten-ity through a stable atmosphere as a function of airass and extrapolating the measurements to zero airass. After passing through the atmosphere, the
ig. 3. R. Frouin making SIMBAD measurements of marine re-ectance �top� and aerosol optical thickness �bottom� from thecripps Institution of Oceanography pier in La Jolla, California.o measure marine reflectance, he points the instrument towardhe ocean surface at a nadir angle of �45° and an azimuth angle of35° with respect to the Sun’s principal plane. The instrument isecured with a cord. To measure aerosol optical thickness, heoints the instrument toward the Sun with the help of a finder.
Se
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un intensity �irradiance� in spectral band i can bexpressed as
Ii��s� � I0i�d0�d�2 exp��im��s�, (1)
here I0i is the extraterrestrial Sun irradiance inand i for the mean Earth–Sun distance d0, �s is theun zenith angle, i is the total optical thickness ofhe atmosphere in band i �assumed to be constanturing the calibration�, m is the air mass, and d is thearth–Sun distance during the calibration. The I0ialues are obtained from Neckel and Labs,31 m isomputed as a function of �s following Kasten andoung,32 and d is computed according to Paltridgend Platt.33 Equation �1� is valid only in the absencef absorption by water vapor and minor gases.During Sun-viewing mode, the measured numeri-
al count in band i, CNi, is proportional to Ii, allowingne to write
ln�CNi� � ln�CN0i� � 2 ln�d0�d� � im��s�, (2)
here CN0i is the calibration coefficient for spectraland i. Therefore CN0i is obtained by plotting theNi values as a function of air mass and by extrap-lating to zero air mass or, more accurately, by re-ressing ln�CNi� versus m. The measurements areerformed under the clearest conditions possiblee.g., at a high-altitude site� to minimize the effects ofvariable atmosphere.
. Sea-Viewing Mode
he instrument in sea-viewing mode is calibrated byse of an integrating sphere, whose output spectraladiance is calibrated with equipment and methodshat are traceable to the National Institute of Stan-ards and Technology and that are further controlledn radiometric intercomparison activities �e.g., seeefs. 34 and 35�. The equivalent radiance of thephere in band i, Lsi, is first computed as follows:
Lsi � ��i
L��� R���d�����i
R���d�� , (3)
here � is wavelength, L is the radiance delivered byhe sphere, R is the spectral response of the SIMBADnstrument, and the integral is over the spectralange of band i. Because the numerical counts thatre measured by the instrument placed in front of thephere are proportional to the sphere radiance, thealibration coefficient for band i, Ki, is then given by
Ki � Lsi�CNi. (4)
The integrating sphere is also used to measure theadiometric noise of the instrument at various radi-nce levels. For realistic radiance levels and the fivepectral bands, the noise is fairly constant at �2 �0�5 when expressed in reflectance. Note that com-lementary calibrations can be made for multi-emporal control during the campaign by using, forxample, a Spectralon plaque.
. Data Processing
. Aerosol Optical Thickness and Angstrom Coefficient
o deduce aerosol optical thickness from the mea-urements in Sun-viewing mode, one must first com-ute the total optical thickness of the atmosphere iy inverting Eq. �2�:
i � �ln�CN0i� � 2 ln�d0�d� � ln�CNi��m��s�. (5)
he total optical thickness, i, is then corrected forolecular scattering and gaseous absorption, dueostly to ozone. This gives the aerosol optical thick-ess in band i, ai as
ai � i � ri�P� � oi�Uo�, (6)
here ri and oi are the Rayleigh and ozone opticalhickness in band i, respectively. The Rayleigh op-ical thickness depends on surface air pressure, P.t is computed by use of a depolarization factor of.0279.7,8 The ozone contribution is computed fromhe vertically integrated ozone amount, Uo, obtainedrom climatology �see Ref. 36� or derived from Totalzone Mapping Scanner observations.The Angstrom coefficient, , defined by the law
a��� � �� , is determined by regressing on a log–logcale ai versus the equivalent wavelength of band i,i, by using the instrument’s five spectral bands.he determination of is more difficult at low-aerosolptical thickness simply because the uncertainty onai is rather constant in absolute value but becomesncreasingly large in relative value as ai decreases.
. Total Atmospheric Transmittance
he total �i.e., direct plus diffuse� atmospheric trans-ittance in spectral band i, Ti, needs to be estimated
o normalize the water-leaving radiance measure-ents into reflectance. It can be expressed �e.g.,ef. 37� as the product of the transmittance due toaseous absorption, mostly of ozone, Toi, and theransmittance due to molecular and aerosol scatter-ng �including aerosol absorption�, Trai. We have
Ti��s� � Toi��s, Uo�Trai��s, ri, ai�
� exp��koiUo m��s�Trai��s, ri, ai�, (7)
here Toi is explicited and koi is the ozone absorptionoefficient in band i. The transmittance Trai is com-uted by use of a radiative transfer model based onhe successive orders of scattering method38 with �s,ri, and ai as variable input. The effect of aerosolype on Trai is small39 and can be neglected. Alter-atively, the analytical approximation proposed byanre et al.39 can be used:
Trai��s� � exp���0.52ri � 0.16ai�m��s�. (8)
Using this formula to estimate photosyntheticallyvailable radiation at the ocean surface, Frouin etl.40 reported root-mean-squared differences of 4.7%n comparison with in situ measurements. Comput-ng Trai by means of the radiative transfer model ofeuze et al.38 or Eq. �8� is valid only under clear-sky
onditions �no clouds�. The extended use of the SIM-AD radiometer to any kind of cloudy conditionsould be interesting for developing and verifying bio-
ptical algorithms but would require a simultaneouseasurement of downwelling irradiance.
. Marine Reflectance
ecause the variable of interest is diffuse marineeflectance, defined as radiance times � and dividedy the extraterrestrial solar irradiance, the polarizedeflectance in spectral band i measured by viewinghe surface �sea-viewing mode�, �ui, is obtained fromhe recorded numerical count by using the followingormula:
�ui � �KiCNi�d�d0�2�I0i cos��s�. (9)
s indicated in Section 2, �ui should be measured inspecific viewing geometry, i.e., at a nadir angle of
5° and at an azimuth angle of 135° with respect tohe Sun, in order to minimize skylight reflection ef-ects. The inclinometer and magnetometer anglesre used to select the optimum viewing geometry.easurements made at a nadir angle outside the
ange 45° � 5° and at a relative azimuth angle out-ide the range 135° � 10° are not processed.The reflectance �ui is then corrected for residual
kylight and atmospheric transmittance to yield theertically polarized diffuse water reflectance, �wi�.e have
�wi� � ��ui � �0i��Ti, (10)
here �0i is the reflectance due to skylight reflectionn spectral band i. This reflectance is computed ac-urately from the molecular optical thickness, theerosol optical thickness and type �i.e., Angstrom co-fficient�, and the surface wind speed, according toougnie et al.25 Figure 4 shows �0 as a function ofadir angle for a Sun zenith angle of 47°, a relative
ig. 4. Parallel-polarized reflectance of the ocean at 443 nm for aolar zenith angle of 47°, a relative azimuth angle of 135°, a windpeed of 5 m s�1, and assuming a water-body reflectance of zero.25
he atmosphere contains molecules and coastal aerosols �C70odel from Ref. 9� of varied optical thicknesses.
zimuth angle of 135°, a wavelength of 443 nm, andwind speed of 5 m s�1. The atmosphere containsolecules and coastal aerosols of varied optical thick-esses �C70 aerosol model from Ref. 9�. Duringiewing at 45° from nadir, the skylight contribution isf the order of 0.002 �in reflectance units�. Notehat, because the surface is not flat, the minimumalue of �0 is not obtained at exactly the Brewsterngle �53°�, but at a smaller angle �45°�. The caseisplayed in Fig. 4, with a Sun zenith angle close tohe 45° view nadir angle, is not a special case. Re-ults for other wavelengths, solar zenith angles, anderosol types can be found in Fougnie et al.25
The radiometric measurements might be contami-ated by whitecaps caused by wind action on theurface or by foam and bubbles generated by the shipr by residual glitter. Sunlight scattered by cloudsay also be reflected by the surface in the instru-ent’s field of view. To remove these unwanted ef-
ects, we examine the reflectance measured in thepectral band centered at 870 nm �i � 5�, �u5. First,threshold, typically 0.001, is applied to �u5 to elim-
nate the most perturbed measurements. Second,ui�i � 1, . . . , 4� is corrected by using �u5 and byssuming that the extra reflectance due to whitecaps,louds, etc., does not depend on wavelength in thepectral range 443–870 nm. With this additionalorrection, �wi��i � 1, . . . , 4� becomes
�wi� � ��ui � �0i��Ti � ��u5 � �05��T5,
i � 1, . . . , 4. (11)
hus only �wi� in spectral bands 1–4, i.e., the bandsentered at 443, 490, 560, and 670 nm, respectively, isbtained after correction. Equation �11� can be ap-lied effectively because the radiometric measure-ents are acquired simultaneously in the
nstrument’s five spectral bands �cloud effectstrongly depend on surface-wave slope, and white-aps may be changing quickly with time�. On thether hand, treating whitecaps as gray bodies, evenhough they are not white spectrally41–43 is sufficientecause only the less-perturbed measurements areelected. Over turbid coastal waters, the diffuse re-ectance is not null at 870 nm; consequently, Eq. �11�
s not valid for those waters.Molecules and hydrosols polarize the light scat-
ered by the water body. Because the SIMBAD mea-urements are made through a vertical polarizer,olarization effects must be corrected to yield theotal water reflectance. If �i denotes the ratio ofertically polarized reflectance to total reflectance inpectral band i, we have
�wi � 2�i�wi�, (12)
here �wi is the water reflectance in spectral band i.he factor 2 in Eq. �12� is introduced because theadiometric calibration is based on the total radiancef the integrating sphere �see Eq. �5�, and this radi-nce is twice the radiance measured through the po-arizer. In other words, � � 0.5 for unpolarized
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ight. For the viewing geometry considered, and be-ause of refraction at the air–sea interface, the un-erwater scattering angle is generally large, i.e., inhe range 145°–160°. In a scattering geometry suchs this, polarization by the water body is small, typ-cally 10% �e.g., see Ref. 44�, and would be null if thecattering angle were 180°. A crude estimate of �ias obtained by Fougnie et al.25 under the assump-
ion that polarization is due only to water molecules,.e., �i varies between 0.41 and 0.47.
More accurate computations of �i were performedy use of the Monte Carlo method for a realisticcean–atmosphere system containing molecules,erosols, and hydrosols. The ocean surface wasavy, with the wave slope determined according toox and Munk.45 It was assumed that hydrosolsere spherical phytoplankton particles. Their char-cteristics �size distribution and refractive index�ere specified according to Morel et al. and Aas.46–48
ie theory was used to compute the scattering ma-rix and the scattering and absorption cross sections.o calibrate the theoretical results, we adjusted theolume number concentration so that the computedcattering coefficient matched the parameterizationf Morel.12 Varied volume number concentrations,hus pigment concentrations, were considered. Theolarization signature of downwelling radiance at theurface, due to air molecules and aerosols, was alsoaken into account.
Figure 5�a� displays � versus the nadir angle forhree types of phytoplankton particles. The Sun ze-ith angle is 30°, the relative azimuth angle is 129.4°,he wavelength is 450 nm, the aerosols are of oceanicype,49 with an optical thickness of 0.2, and the windpeed is 5 m s�1. The phytoplankton particles haveradius between 0.2 and 50 �m that is distributed
ccording to a Junge law �exponent of �3.6�, andheir refractive index is 1.03 � 0.001i �Type 1�, 1.10 �.00001i �Type 2�, and 1.15 � 0.00001i �Type 3�.igment concentration is 0.1 mg m�3. At a nadirngle of 45°, � is �0.44 and varies little �to within0.01� with phytoplankton type. Figure 5�b� shows
hat, for phytoplankton Type 2, � increases with pig-ent concentration, from �0.43 at 0.01 mg m�3 to
.48 at 10 mg m�3. Other simulations �not pre-ented here� indicate an increase of � with Sun zenithngle to 0.48 at 60° as well as a small dependenceith wavelength �i.e., a few 0.01� in the spectral
ange of interest. The Monte Carlo results agreeith measurements made by a scanning polarization
adiometer.50 Thus polarization effects by the waterody can be corrected to �2% from the Sun zenithngle and pigment concentration, which can be esti-ated from �w1���w3� or �w2���w3�.The data processing is illustrated in Fig. 6, which
isplays the measured polarized reflectance �ui �Fig.�a� and the final water reflectance �wi �Fig. 6�b� forselected sea-viewing file. The data were collected
n 29 January 1999 at 4.04 °S and 18.18 °W duringhe Aerosols-99 campaign. The solar zenith angleas 41.8°, the wind speed was 6.3 m s�1, and the skyas completely free of clouds. After correction of the
erturbing effects, the variability in �u is reducedver the 10-s measurement period �i.e., 100 samples�,nd �wi is slightly lower than �ui �by less than 0.001�.he correction of residual skylight, which decreasesui, is partly compensated for by the correction ofolarization effects, which increases �ui. The totalorrection is relatively small, except in spectral bandsand 4, which exhibit a low signal �i.e., the radiom-
ter almost measures the desired variable�. This isdefinite advantage over underwater techniques,
hich require extrapolation of radiometric measure-ents to the surface. The average water reflectance
ver the 10-s measurement period is 0.0214 � 0.0006,.0195 � 0.0006, 0.0048 � 0.0002, and 0.00014 �.00010 in spectral bands 1–4, respectively. Be-ause of surface waves, the radiometer may not al-ays be positioned at a nadir angle of 45° with
espect to the direction normal to the surface, causinguctuations in the measured signal. Reflected light
ig. 5. Polarization factor � versus view nadir angle for �a� threeypes of phytoplankton particles and a pigment concentration of.1 mg m�3 versus �b� pigment concentration for particles of Type.
ill pass through the vertical polarizer when the in-trument is not properly oriented; i.e., the measure-ents will exhibit higher reflectance. This effect,
onspicuous in spectral bands 1 and 2, can be cor-ected by selection of the smallest values in the timeeries.
. Error Budget
. Atmospheric Variables
n Sun-viewing mode the SIMBAD radiometer worksike a classic sunphotometer, for which the inaccu-acy of the retrieved aerosol optical thickness hasreviously been evaluated �e.g., see Ref. 16�. Thisnaccuracy is due chiefly to errors on the calibrationoefficient, the Rayleigh optical thickness, and thezone optical thickness �see Eqs. �5� and �6�. Fromougley–Langley plots obtained during several yearst Stevenson Peak �1896-m altitude� in the Mount
ig. 6. �a� Measured polarized and �b� final water reflectance forsea-viewing file corresponding to data collected on 29 January
999 at 4.04 °S and 18.18 °W during the Aerosols-99 campaign.he solar zenith angle was 41.8°, the wind speed was 6.3 m s�1,nd the sky was completely free of clouds.
aguna Mountains, California, the error on the cali-ration coefficient, ��ln�CN0i�, is estimated at �0.02,0.02, �0.015, �0.01, and �0.01 in spectral bands–5, respectively. The error on the Rayleigh opticalhickness, �ri, is due to computational uncertaintyi.e., �1% at standard pressure� and to uncertaintyn pressure �i.e., �10 hPa, conservatively�, translat-ng into �0.005, �0.004, �0.002, �0.001, and negli-ible in spectral bands 1–5, respectively. The errorn the ozone optical thickness, �oi, significantly af-ects spectral bands 3 and 4 �centered at 560 and 670m� and is attributed to the uncertainty in the ver-ically integrated ozone amount, i.e., �10%, whichives errors of �0.010 and �0.004, respectively.ote that the error on the calibration coefficient af-
ects the inaccuracy of the aerosol optical thickness inmanner inversely proportional to air mass �Eq. �5�.stimates of aerosol optical thickness are more accu-ate when observations are made at larger Sun ze-ith angles.The various errors are summarized in Table 2,hich also gives the resulting quadratic inaccuracy of
ai at zero air mass, �ai, as �0.021, �0.020, �0.018,0.011, and �0.010 in spectral bands 1–5, respec-
ively. Most of the inaccuracy is due to error in thealibration coefficient. The absolute inaccuracy ofai does not depend on ai. Thus the relative inac-uracy varies as 1�ai and is large when ai is smalle.g., 40% in spectral band 1 when ai � 0.05�. Thebove inaccuracy values do not take into account therror due to the size of the instrument’s field of view3°�. Not only direct sunlight is measured, but alsoome light scattered by the atmosphere, resulting inai estimates that are biased low. The effect is smallor continental aerosols, with values of 0.002 and.001 in spectral bands 1 and 5, respectively, when ais equal to 0.1, but may be 5–10 times larger for
aritime aerosols whose phase function exhibits auch higher forward peak. A correction can beade by use of the Angstrom coefficient �i.e., the
erosol model� obtained from the measurements.The inaccuracy of the Angstrom coefficient re-
ects the inaccuracy of the aerosol optical thicknessai. It decreases with increasing ai and . Usingable 2 and obtaining by linearly fitting ai versusi on a log–log scale, the inaccuracy of is approxi-ately �0.19 and �0.33 when a5 � 0.1 and � 1.5
nd 0, respectively. The values decrease to �0.10nd �0.16 when � 0.2. Over the open ocean, aind are often small �i.e., � 0.05 and � 0�,
Table 2. Inaccuracy of the SIMBAD Aerosol Optical Thickness �m � 1�
aking it difficult to estimate accurately. In suchonditions, however, the atmospheric correction ofatellite ocean color is not a difficult problem.Taking into account the above uncertainty on Ray-
eigh optical thickness �approximately �2%�, aerosolptical thickness �Table 2�, and ozone amount�10%�, the inaccuracy of the atmospheric transmit-ance Ti can be estimated. It is �0.7% in spectralands 1–3 and 0.4% and 0.2% in spectral bands 4 and, respectively, when a5 � 0.1, � 1, and the Sun ist zenith. The values increase with aerosol opticalhickness and Sun zenith angle. They become 1.6%,.8%, and 0.4% when the Sun zenith angle is 60°.he coefficient 0.16 in Eq. �8� may vary with aerosol
ype, but the consequence on the inaccuracy of Ti isegligible.
. Marine Reflectance
able 3 displays, for a typical case, the various errorsontributing to the inaccuracy of the marine reflec-ance �wi and the resulting quadratic inaccuracy,�wi. The Sun zenith angle is 60°, the aerosol opti-al thickness is 0.1 in spectral band 5, the aerosolodel is C70,9 the phytoplankton pigment concentra-
ion is 0.1 mg m�3 �case 1 waters�, and the wind speeds 7.5 m s�1. The noise-equivalent reflectance,E��ui, is �4 � 10�5, which is twice the value given
n Table 1 because the air mass is equal to 2. Thencertainty in the radiometric calibration, in fact theum of the uncertainty on the calibration of the inte-rating sphere and the uncertainty on the spectralolar irradiance, is typically �5%. This introducesn error, �c�ui, of �12.1 � 10�4, �9.2 � 10�4, �2.3 �0�4, and �0.3 � 10�4 in spectral bands 1–4, respec-ively. The uncertainty on atmospheric transmit-ance, estimated at �1% for the same bands, yieldsn error �t�wi� of 2.7 � 10�4, 1.9 � 10�4, 0.4 � 10�4,nd 0.1 � 10�4, respectively. The error due to un-ertainty in the correction of residual skylight reflec-ion, �s�wi�, is computed with an assumedncertainty of �2 m s�1 on wind speed, �0.01 onerosol optical thickness in spectral band 5, �5° onhe viewing nadir angle, and �10° on the relativezimuth angle, giving values of �4.2 � 10�4, �3.5 �0�4, �2.8 � 10�4, and �1.4 � 10�4, respectively.he error due to uncertainty in the correction ofhitecap and cloud effects �Eq. �11� is neglected.inally, uncertainty in the polarization factor �
Table 3. Inaccuracy of the SIMBAD Diffuse Marine Reflectance
�2%; see Subsection 4.C� translates into an error,p�wi, of �4.8 � 10�4, �3.7 � 10�4, �0.9 � 10�4, and0.1 � 10�4, respectively. Summing the individual
rrors squared, the resulting ��wi is �13.9 � 10�4,10.7 � 10�4, �3.7 � 10�4, and �1.6 � 10�4 or5.7%, �5.7% �7.8% and �22.8% of the marine re-ectance in spectral bands 1–4, respectively. The
naccuracy of �w1 and �w2 results essentially from thencertainty in the radiometric calibration coeffi-ients. Imperfect correction of skylight reflectionalready minimized by the vertical polarizer� has amall effect on the inaccuracy in spectral bands 1 and, but it contributes significantly to the inaccuracy ofw3 and dominates the inaccuracy of �w4.Note that the error budget discussed above does
ot include the effect of the platform from which theeasurements are made and whose presence gener-
lly increases the amount of light reflected by theurface into the instrument’s field of view as well ashe downward solar irradiance. In a typical exper-mental configuration, the surface must have a slopef at least 45° for the instrument to measure the lighteflected from the platform. The probability of hav-ng such a slope is very small for wind speeds athich SIMBAD measurements are made ��15 m
�1�,45 and the resulting increase in measured reflec-ance is negligible. The effect on downward solarrradiance is not significant, even when the platformtructures are totally reflective �albedo of 1�. Wheniewing the ocean surface at an azimuth angle of 135°rom the Sun, the side of the platform from which theeasurements are made is either in the shade or lit
y the Sun at a very oblique angle. Consequently,he effective albedo of the structures is small, increas-ng downward solar irradiance typically by a fractionf 1%.
. Experimental Verification
. Aerosol Optical Thickness
o evaluate the aerosol optical thickness derivedrom SIMBAD measurements, we performed a com-arison by using another sunphotometer duringCE-2. This sunphotometer, known as AATS-6,
racks the Sun automatically and measures directolar extinction in six spectral bands.51 The field ofiew is 2.2° �half-angle�. During ACE-2 the spectralands were centered at 380, 451, 525, 863, 941, and021 nm. Data processing, including removal ofeasurements contaminated by clouds and ship-
oard structures, is described by Livingston et al.52
he aerosol optical thickness from AATS-6 was lin-arly interpolated to the SIMBAD spectral bands onlog–log scale. The comparison between SIMBAD
nd AATS-6 measurements was performed duringhree days �i.e., 24, 25, and 30 June 1997�. Variederosol conditions �i.e., optical thickness in the range.02–0.3 and Angstrom coefficient in the range 0.15–.30� were encountered. The agreement is good be-ween measurements made by the two instrumentsFig. 7 and Table 4�. For optical thickness, the meanifference is between �0.01 and 0.02, depending on
avelength, and the root-mean-squared difference is.01 or less. For the Angstrom coefficient, the val-es are �0.07 and 0.15, respectively. These corre-
ation statistics are consistent with the inaccuracyiscussed in Subsection 5.A. A useful index to iso-ate small particles �e.g., pollution-type aerosols� ishe product of the optical thickness in spectral band
and the Angstrom coefficient. Estimates of thisndex made by the two instruments agree to within0.01 �Table 4�. Other comparisons were madeith a Prede radiometer during a CalCOFI cruise53
nd with Microtops and shadow-band radiometersuring the Aerosols-99 Experiment.54 These com-arisons also showed agreement �i.e., average differ-nces of 0.02–0.03 at 500 nm� between the aerosolptical thickness measured by SIMBAD and thatade with the other instruments.
. Marine Reflectance
he marine reflectance derived from the SIMBADeasurements in sea-viewing mode was comparedith that derived from the underwater measure-ents of upward radiance and downward irradiance
uring four CalCOFI cruises, CalCOFI-9610 �October996�, CalCOFI-9710 �October 1997�, CalCOFI-9802
ig. 7. Comparison of aerosol optical thickness derived fromhree days of measurements made by the SIMBAD radiometer andy the AATS-6 sunphotometer51 during ACE-2 �23 matchups�.
Table 4. Comparison Statistic of Aerosol Optical Thickness, �a, at 44�a�870��, Derived from Measurements by the SIMBAD
February 1998�, and CalCOFI-9804 �April 1998�,nd during the Aerosols-99 Experiment �January–ebruary 1999�. The CalCOFI cruises were con-ucted off the California coast in clear and turbidaters. The Aerosols-99 Experiment occurred alongroute from Norfolk, Virginia, to Cape Town, Southfrica, and Port Louis, Mauritius. Waters with var-
ed optical properties were sampled in regions of theulf Stream, in subtropical gyres, in South Equato-
ial currents, and in the Aghulas Current.The underwater measurements were made by an
ntegrated profiling system, including a Multiwave-ength Environmental Radiometer Model 2040MER-2040� manufactured by Biospherical Instru-ents, Incorporated. The radiometer acquired ver-
ical profiles of downward irradiance, Ed�Z�, andpward radiance, Lu�Z�, in spectral bands centered at80, 395, 443, 455, 490, 510, 532, 555, 570, and 665m. The MER-2040 unit was deployed from the-frame on the ship’s stern at each station according
o SeaWiFS protocols.14 The Ed and Lu values werextrapolated to the zero depth �Z � 0�� according toitchell and Kahru.55 Shelf-shadowing effects were
orrected with the scheme recommended by Gordonnd Ding and by Mueller and Austin.14,56 Normal-zed radiance just below the surface, Lu�0���Ed�0��,as transformed into normalized radiance just above
he surface, Lu�0���Ed�0��, using the factor 0.529e.g., see Morel and Mueller57�. Marine reflectancew was computed as �Lu�0���Ed�0��. In addition tohe Lu and Ed measurements, water was sampled atelected depths with a separate conductivity, temper-ture, and depth rosette system. The chlorophyll and phaeopigment concentrations of the water sam-les were determined by the fluorometric method.58
Figure 8 presents a scatter plot of the marine re-ectance obtained from SIMBAD and MER measure-ents for the SIMBAD spectral bands centered at
43, 490, 560, and 670 nm �bands 1–4�. To estimatehe MER values in SIMBAD spectral band 3, a linearnterpolation was made between the MER measure-
ents in the spectral bands centered at 555 and 570m. Effects due to different viewing geometryMER measures at nadir and SIMBAD at 45° fromadir� were not corrected. A total of 36 matchupsre used for spectral bands 1 and 3, but only 21atchups for spectral bands 2 and 4. During thealCOFI-9610 cruise the SIMBAD radiometer didot measure in band 2 and look-up tables that areecessary for the data processing of band 4 were not
0, 560, 670, and 870 nm, Angstrom Coefficient, �, and Aerosol Index,meter and the AATS-6 Sunphotometer during ACE-2a
cient, and N is the number of matchups. A positive bias signifies
3, 49Radio
coeffi
gttuctbbtpSAb9leS0tm
Mp
FaAmatchups for 490 and 670 nm.
Ftm
enerated �see Subsection 4.C�. For these matchupshe Sun zenith angle was mostly less than 60° andhe fractional cloud coverage was less than 0.2. Fig-re 9 displays marine reflectance versus pigmentoncentration �chlorophyll a � phaeophytin a� for thewo types of instrument and each of the four spectralands indicated above. In the figure the relationetween marine reflectance and pigment concentra-ion predicted by the bio-optical model of Morel12 islotted for reference. The marine reflectance fromIMBAD and MER are in general agreement �Fig. 8�.t 443 nm the SIMBAD values are higher by 5.0%,ut at 490 and 560 nm they are biased low by 0.8 and.1%, respectively �Table 5�. For these three wave-engths, the normalized root-mean-squared differ-nce is in the range 13%–23%. At 670 nm theIMBAD values are, on average, higher by a factor of.76, and the root-mean-squared difference is rela-ively large �i.e., 90%; Table 5�; however, the radio-etric signal, thus the absolute difference, is small.The comparison statistics between SIMBAD- andER-derived marine reflectances are difficult to ex-
lain quantitatively because of the limited number of
ents at �a� 443 nm, �b� 490 nm, �c� 560 nm, and �d� 670 nm versusrometric method. The solid line represents the relation betweenbio-optical model of Morel.12
ig. 8. Comparison of marine reflectance derived from SIMBADnd MER measurements obtained during the CalCOFI anderosols-99 cruises: 36 matchups for 443 and 560 nm, and 21
ig. 9. Marine reflectance derived from SIMBAD and MER measuremhe pigment concentration estimated from water samples with the fluoarine reflectance and pigment concentration that is predicted by the
atchups. They may be attributed to several fac-ors: �1� inaccuracy of the SIMBAD measurements;2� environmental variability �e.g., MER measure-
ents are made from the ship’s stern and SIMBADeasurements from the bow�; �3� bidirectional char-
cteristics of the marine reflectance, which, since ig-ored, may affect differences by �10%59; �4� differentpectral responses of the SIMBAD and MER instru-ents, a critical issue in the blue wavelength, where
olar irradiance is variable spectrally; and �5� inac-uracy of the MER measurements, typically of therder of �12–24%.30 Note that applying the aver-ge bidirectional factors of Morel and Mueller57 toccount for the different viewing geometry of theIMBAD and MER measurements does not substan-ially modify the comparison statistics. The totalormalized root-mean-squared difference �i.e., in-luding the bias� changes from 20.5% to 21.3% at 443m, 13.2% to 15.5% at 490 nm, 24.6% to 17.5% at 560m, and 118% to 115% at 670 nm �i.e., only decreasesignificantly at 560 nm�. Improvement is probablyasked by more important effects, but uncertainty in
he bidirectional factors, which assume certain opti-al properties for the hydrosols �e.g., scattering phaseunction�, could have introduced additional noise.n view of these sources of uncertainty, the compar-son statistics are consistent with the SIMBAD the-retical inaccuracy, except at 670 nm �Section 4 andable 5�. On the other hand, the marine reflectancebtained by both types of instruments is in reason-ble agreement with predictions of the bio-optical
Table 5. Comparison Statistic of Marine Reflectance Derived fromAerosols
Statistic �w�443� �
Mean 0.01781 0Bias 0.00089 �0
�5.0%� ��RMSD 0.00354 0
�19.8%� �1R 0.9083 0N 36
aRMSD is the root-mean-squared difference, R is the correlationhigher SIMBAD value.
Table 6. Comparison Statistic of Three Bio-optical Indices Computedith Marine Reflectance Derived from SIMBAD and MER Measurements
aRMSD is the root-mean-squared difference, R is the correlationoefficient, and N is the number of matchups. A positive biasignifies a higher SIMBAD value. NDPI60 is defined as the nor-alized difference �� �443� � � �560��� �490�.
odel over the entire range of measured pigmentoncentration, except below 0.1 mg m�3 �Fig. 9�. Inuch oligotrophic waters, the inaccuracy of the pig-ent concentration �typically 0.02–0.03 mg m�3� is
elatively large. The SIMBAD-derived marine re-ectance at 670 nm increases with pigment concen-ration, as suggested by the bio-optical model,hereas the MER values exhibit much less sensitiv-
ty �Fig. 9�d�, giving confidence in the SIMBAD datarocessing.Satellite-derived ocean color generally provides
urface chlorophyll a or pigment concentration infor-ation. This variable is estimated by using simple
atios of marine reflectance in the blue and greene.g., Ref. 13 or more complex indices, such as theormalized Difference Phytoplankton Index
NDPI�.60 Table 6 displays comparison statistics forw1��w3, �w2��w3, and ��w1 � �w3���w2 �NDPI� ob-ained from the SIMBAD and MER data sets. Theias is similar for �w1��w3 and �w2��w3 �11.7% and.4%, respectively�, but the root-mean-squared differ-nce is substantially lower for �w2��w3 �13.7% com-ared with 21.6%�. A small bias of 0.2% is computedor ��w1 � �w3���w2, but the root-mean-squared dif-erence is relatively large �22.0%�. Thus errors inarine reflectance �see Table 5� partly compensate
or the ratios and NDPI. The pigment concentra-ion, however, should be measured independently forvaluation purposes.Obviously the comparisons presented in Tables 5
nd 6 and in Figs. 8 and 9 are not sufficient forrawing conclusions about the actual accuracy of theIMBAD measurements of marine reflectance.ore matchups acquired over a wide range of oceanic
egimes need to be analyzed. Yet, even numerousatchups may provide only a relative answer. The
rror budget presented in Subsection 5.B should beonsidered as a good indicator of the actual accuracy.
. Conclusion
field radiometer, called SIMBAD, has been de-igned and built for the specific purpose of verifyingatellite-derived ocean color. It measures aerosolptical thickness and diffuse marine reflectance inypical spectral bands of satellite ocean-color sensors.
preliminary study16 had determined that measur-ng these variables concomitantly at the time of sat-
AD and MER-2040 Radiometer Measurements during CalCOFI andruisesa
cient, and N is the number of matchups. A positive bias signifies
SIMB-99 C
w�490
.0153
.00010.8%
.00203.2%
.835421
coeffi
efTAmqimw
aaohfologTn5sddbct
tttrrct5pTmosMfbMSgrfd
goaiatntef
ssMs
SnAFogoaewdDiasllt
fifTwctrtrfuo
LUFdcRaBMofFdfa
R
llite overpass is necessary and generally sufficientor evaluating atmospheric correction algorithms.he radiometer is light, portable, and easy to operate.ny ordinary crew can quickly learn how to makeeasurements. Furthermore, the ship is not re-
uired to stop, making it possible to collect data dur-ng various research cruises and, in particular, on
erchant ship voyages along regular routes in theorld’s oceans.For diffuse marine reflectance, the radiometer is
imed at the ocean surface at a nadir angle of 45° andrelative azimuth angle of 135°. This viewing ge-
metry allows one to reduce substantially, with theelp of a polarizer, the skylight reflected by the sur-ace in the instrument’s field of view.25 For aerosolptical thickness, the radiometer is aimed at the Sunike a classic sunphotometer. The same optics �fieldf view of 3°� and detectors, but different electronicains, are used in both Sun- and sea-viewing modes.he radiometric measurements are made simulta-eously in five spectral bands centered at 443, 490,60, 670, and 870 nm, but sequentially in Sun- andea-viewing modes. To transform water-leaving ra-iance into marine reflectance, we calculated theownward solar irradiance at the surface, which cane achieved accurately when the sky is clear or partlyloudy �i.e., the fractional coverage less than 0.2� andhe Sun not obscured by clouds.
The inaccuracy of the SIMBAD aerosol opticalhickness and diffuse marine reflectance, estimatedheoretically, is adequate in view of the sensitivity ofhese variables to atmospheric correction. For ma-ine reflectance, in particular, the theoretical accu-acy is within the requirements for atmosphericorrection errors, namely �0.002 in the blue and 3–4imes smaller in the green for clear waters.10 The% inaccuracy objective specified by the SeaWiFSroject,61 however, will not be met for all waters.his is also the case with other instruments, evenodern in-water instruments.30 The SIMBAD radi-
meter has been evaluated at sea by comparing mea-urements by SIMBAD and other instruments �i.e.,ER-2040 for diffuse marine reflectance and AATS-6
or aerosol optical thickness�. There is agreementetween the marine reflectance from SIMBAD andER-2040 and between the optical thickness from
IMBAD and AATS-6, with comparison statisticsenerally consistent within expected levels of inaccu-acy. Unlike MER, however, SIMBAD provides dif-use marine reflectance measurements almostirectly.The SIMBAD instrument measures in an angular
eometry that may be different from that of satellitebservation. When evaluating satellite retrievalsgainst SIMBAD measurements, one should takento account the effects of angular differences, whichre within �10% for most remote-sensing condi-ions.56 Making the proper corrections, however, isot simple. Bidirectional effects depend on the scat-ering phase function of the hydrosols, which is gen-rally not known, and using average bidirectionalactors57 may introduce noise, as mentioned in Sub-
ection 6.B. Furthermore, atmospheric correctionchemes �e.g., those of Gordon10 for SeaWiFS andODIS� use an atmospheric transmittance that as-
umes a Lambertian water body.Adequate sampling was achieved by building 10
IMBAD radiometers for use in two complementaryetworks, one operated by the Laboratoire d’Optiquetmospherique �LOA� at the University of Lille,rance, the other by the Scripps Institution of Ocean-graphy �SIO�, California. The measurement pro-ram is based on ships of opportunity that participaten a volunteer basis or at a very little cost. Thepproach is complementary to dedicated evaluationxperiments, which are expensive, cannot sample theide range of expected atmospheric and oceanic con-itions, and generally provide only a few matchups.etailed information about the SIMBAD radiometer,
ncluding characteristics, measurement proceduresnd protocols, calibration history, and processingoftware, is available at http:��www.loa.univ-ille1.fr��simbad� �LOA network� and at http:��po-aris.ucsd.edu��simbad� �SIO network�, as well as inhe data sets acquired since November 1996.
The SIMBAD radiometer has already proved use-ul for vicariously checking the radiometric sensitiv-ty of the POLDER instrument onboard ADEOS andor evaluating POLDER ocean-color products.19,62
he accuracy of the vicarious calibration coefficientsas estimated to be better than 3%. A large de-
rease in the POLDER instrument response was de-ected in the blue spectral region, confirming theesults previously obtained with alternative calibra-ion techniques. The SIMBAD radiometer is cur-ently being used in the same type of activities, butor SeaWiFS, MODIS, and MERIS, and will contrib-te, in a cost-effective way, to the evaluation of futurecean-color missions.
The SIMBAD radiometer was developed at theaboratoire d’Optique Atmospherique �LOA� of theniversite des Sciences et Technologies de Lille,rance, with funding from the Centre National’Etudes Spatiales, the Centre National de la Recher-he Scientifique, and the Region Nord–Pas-de-Calais.. Frouin was supported by the National Aeronauticsnd Space Administration �NASA� and by the SIM-IOS Project. The authors wish to thank B. G.itchell and M. Kahru from the Scripps Institution
f Oceanography �SIO� in La Jolla and P. B. Russellrom the NASA Ames Research Center in Moffettield for making available MER-2040 and AATS-6ata. The programming support of J.-M. Nicolasrom LOA and of J. McPherson from SIO is gratefullycknowledged.
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