Seawifs Technical Report Series Volume 25, Ocean Optics Protocols

NASA Technical Memorandum 104566, Vol. 25

SeaWiFS Technical Report Series

Stanford B. Hooker, Elaine R. Firestone, and James G. Acker, Editors

Volume 25, Ocean Optics Protocols for SeaWiFS Validation, Revision 1

James L. Mueller and Roswell W. Austin

February 1995 NASA Technical Memorandum 104566, Vol. 25

SeaWiFS Technical Report Series

Stanford B. Hooker, Editor NASA Goddard Space Flight Center Greenbelt, Maryland

Elaine R. Firestone, Technical Editor General Sciences Corporation Laurel, Maryland

James G. Acker, Technical Editor Hughes STX Lanham, Maryland

Volume 25, Ocean Optics Protocols for SeaWiFS Validation, Revision 1

James L. Mueller and Roswell W. Austin Center for Hydro-Optics and Remote Sensing San Diego State University San Diego, California

National Aeronautics and Space Administration

Goddard Space Flight Center Greenbelt, Maryland 20771

1995 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Preface

This document stipulates protocols for measuring bio-optical and radiometric data for Sea-viewing Wide Field- of-viewSensor (SeaWiFS) algorithm development and system validation. The protocols begin in Chapter 2, with speciﬁcations of the measured variables which are to be included in data sets for validation of the Sea- WiFS system’s radiometric performance, algorithm development, and algorithm validation. The protocols in Chapter 3 specify the engineering performance characteristics required of radiometers, optical instruments, and other instrumentation used to measure these variables at sea. Chapter 4 details laboratory and ﬁeld protocols for characterizing and calibrating each type of instrument. Procedures for actually measuring the validation variables are given in Chapter 5, and protocols for both data processing and methods of analysis are described in Chapter 6.

This volume, Revision 1, supersedes the earlier version (Mueller and Austin 1992) published as Volume 5 in the SeaWiFS Technical Report Series. Instrument performance specifications have been modified slightly from the previous version (e.g., radiometric saturation levels have been increased, and there are wavelength changes for some channels). Pigment protocols have been strengthened [R. Bidigare]† primarily to discriminate divinyl and monovinyl chlorophyll a. Spectrophotometric absorption protocols have also been expanded and reorganized [C. Trees, B. Mitchell, J. Cleveland, C. Roesler, C. Yentsch, and D. Phinney]. Newprotocols have been added for in situ measurements of spectral absorption and backscattering [R. Zaneveld], for above-water measurements of water-leaving radiance from ships and low-flying aircraft [R. Doerffer, K. Carder, and C. Davis], and for sampling in Case-2 water [R. Doerffer, K. Carder, C. Davis, and R. Arnone].

In general, the specifications and protocols set forth here simply describe and adapt instrument specifications and procedures that are common practice in the ocean optics community. In several areas, however, protocols call for significant improvements over today’s instruments and practices; these very challenging protocols should, at least for the present, be regarded more as goals than as strict requirements. The motives for adopting these goals as protocols are that the improvements called for are necessary to meet the rigorous SeaWiFS uncertainty goals, and that the community feels these standards can be closely approached with a significant but affordable effort. Areas in which new research and development must be done to satisfy challenging protocols, and those areas in which there has been recent progress, are summarized below. 1. Model sensitivity studies and experimental verification are needed to develop methods for adjusting in situ radiometric measurements at a given wavelength to correspond to SeaWiFS measurements at a wavelength as much as 4 nm away, and with a different spectral response function (Sections 3.1.1 and 6.1.7). 2. Laboratory research is needed to improve absolute standards of irradiance and radiance, and associated absolute calibration procedures, to achieve or approach 1% internal consistency in the responsivity calibrations of radiometers to be used in SeaWiFS validation experiments (Section 4.2). The SeaWiFS Intercalibration Round-Robin Experiment (SIRREX) program has made significant progress in this area (Mueller 1993 and Mueller et al. 1994). 3. Radiometric linearity test procedures must be improved to extend linearity characterizations over the full operating dynamic ranges (Table 4) of the various irradiance and radiance sensors (Section 4.1.7). This is especially critical for downwelling irradiance measurements at the sea surface, where irradiances of laboratory irradiance standards are only 2–15% (depending on wavelength) of saturation irradiance. 4. Instrument self-shading effects are a significant, but probably correctable, source of error (Sec- tions 3.1.8, 5.1.6, and 6.1.7). The diameter of a radiometer determines whether or not self-shading error can be corrected to within less than 5%. The maximum diameter which enables such a correction varies with the absorption coefficient, and is therefore a function of wavelength, water mass, and the solar zenith angle. For oligotrophic to moderately turbid coastal water masses, wavelengths less

† Names in brackets are the principal contributors to the revised protocols.

ii Mueller and Austin

than 600 nm, and solar zenith angles greater than 30◦, upwelled irradiance and radiance data from many of the currently popular radiometers having diameters of 20–40 cm can be adequately corrected. In more general conditions, however, new instruments must be developed to minimize self-shading effects, particularly for near-infrared wavelengths and in Case-2 waters. Self-shading corrections have been partially verified in recent experiments and a provisional protocol is included in this revision. 5. Measurements following the stringent protocols for avoiding ship shadows and reflections will require exclusive use of profiling radiometer configurations which are not widely used today (Section 5.1.1). Tethered free-fall systems appear to offer the most economical approach to meeting these requirements. More sophisticated and expensive approaches include optical systems on either remotely operated vehicles (ROVs), or on small surface platforms with self-contained winches. 6. Quantitative characterization of polarization sensitivity is critical for any airborne radiometer to be used for SeaWiFS radiometric validation, or algorithm development and validation. Protocols and procedures for polarization sensitivity characterization must be developed in more specific detail than was accomplished here because of time constraints (Sections 3.3 and 4.3). 7. The uncertainties specified here for cosine responses of irradiance collectors are significantly better than is typically realized in commercially available radiometers (Section 3.1.5). Moreover, the specified uncertainties may challenge the precision of the laboratory procedures used to characterize an instrument’s cosine response (Section 4.1.5). Error in cosine response almost directly translates to an equivalent error in downwelling irradiance for the clear-skies case so critical to the SeaWiFS validation. Therefore, a significant effort to carefully characterize the effect, and to work with instrument manufacturers to approach or achieve the specific uncertainties, is an important factor in our strategy for reducing the overall error budget for water-leaving radiance measurements to less than 5%. 8. The use of a portable standard to trace a radiometer’s performance stability during the course of a field deployment is called for in the present protocols (Section 4.2.5). Several manufacturers offer reasonably portable radiometric sources, which may be suitable for this purpose, but laboratory and field evaluations must be carried out to prove their suitability and to develop newand detailed procedures for their use in the field. The SeaWiFS Project Office (SPO) has sponsored work at the National Institute of Standards and Technology (NIST) to develop a prototype system. 9. The present protocols for deploying and analyzing data from moored and free-drifting optical systems are tentative, preliminary, and incomplete. Although moored and drifting optical systems have been used successfully in several oceanographic experiments, there are no previous examples of their use for ocean color remote sensing algorithm development, or for radiometric validation of airborne or satellite radiometers. Newmoored and drifting optical systems are currently being developed and tested in preparation for applications to SeaWiFS validation; significant progress has been made under the Marine Optical Buoy (MOBY) program (D. Clark). As results from this effort become available, new and detailed protocols for making these measurements will be developed and distributed with the next revision.

The protocols and recommendations in this document attempt to represent and consolidate the contributions of both the workshop participants and of many other individuals who participated in the review process. The final document, however, has by necessity been interpreted and rewritten by the authors, who accept full responsibility for any remaining mistakes and misrepresentations. As can be readily deduced from the above list of critically needed improvements, this document is an unfinished work in progress to establish protocols for ocean optical measurements. It will be appropriate to develop and issue a further revised set of protocols reflecting our collective experience in post-launch algorithm development and validation activities after the first year of operation of the SeaWiFS instrument.

San Diego, California — J. L. Mueller December 1994

iii Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Table of Contents

Prologue ...... 1 1. Overviewof SeaWiFS Bio-Optical Requirements ...... 3 1.1 Objectives ...... 3 1.2 Sensor Calibration ...... 4 1.3 Bio-Optical Algorithms ...... 5 1.4 Community Participation ...... 5 1.5 Vicarious Calibration ...... 5 2. Data Requirements ...... 7 2.1 Definitions ...... 7 2.2 Biogeochemical Data ...... 9 2.3 Above-Water Techniques ...... 11 2.4 Ancillary Measurements ...... 11 2.5 Optical Moorings ...... 11 2.6 Drifting Optical Buoys ...... 12 3. Specifications ...... 13 3.1 In-Water Radiometers ...... 13 3.1.1 Spectral Characteristics ...... 13 3.1.2 Responsivity, SNR, and Resolution ...... 14 3.1.3 Linearity and Stability ...... 16 3.1.4 Sampling Resolution ...... 16 3.1.5 Angular Response Characteristics ...... 16 3.1.6 Operating Depth ...... 16 3.1.7 Instrument Attitude ...... 16 3.1.8 Red and Near-Infrared Wavelengths ...... 16 3.2 Surface Irradiance ...... 17 3.2.1 Surface Radiometer Characteristics ...... 17 3.3 Above-Water Radiometry ...... 17 3.4 IOP Instruments ...... 18 3.5 Atmospheric Aerosols ...... 19 3.6 Spectral Sky Radiance ...... 19 3.7 Phytoplankton Pigments ...... 19 3.8 Hydrographic Profiles ...... 19 4. Sensor Characterization ...... 20 4.1 Radiometry ...... 20 4.1.1 Absolute Radiometric Calibration ...... 20 4.1.2 Spectral Bandpass Characterization ...... 21 4.1.3 Temporal Response ...... 22 4.1.4 Radiance Field-of-View ...... 22 4.1.5 Collector Cosine Response ...... 22 4.1.6 Immersion Factors ...... 24 4.1.7 Linearity and Electronic Uncertainty ...... 25 4.1.8 Temperature Characterization ...... 25 4.1.9 Pressure Effects ...... 26 4.1.10 Pressure Transducer Calibration ...... 26 4.2 Radiometric Standards ...... 26 4.2.1 Lamp Irradiance Standards ...... 26 4.2.2 Radiance ...... 26 4.2.3 Radiance Standardization ...... 27 4.2.4 Traceability and Comparisons ...... 27

iv Mueller and Austin

Table of Contents cont.

4.2.5 Portable Standards ...... 27 4.3 Above-Water Radiometry ...... 28 4.4 Calibration of IOP Meters ...... 29 4.5 Sun Photometers ...... 29 4.6 Sky Radiance Cameras ...... 30 4.7 Pigment Calibrations ...... 30 4.8 CTD Calibrations ...... 30 5. Measurement Protocols ...... 31 5.1 Irradiance and Radiance ...... 31 5.1.1 Ship ShadowAvoidance ...... 31 5.1.2 Depth Resolution in Profiles ...... 32 5.1.3 Instrument Dark Readings ...... 32 5.1.4 Surface Incident Irradiance ...... 32 5.1.5 Instrument Attitude...... 33 5.1.6 Instrument Self-Shading ...... 33 5.1.7 Airborne Ocean Color Radiometry ...... 33 5.1.8 Shipboard Radiance Measurements ...... 34 5.2 Ancillary Profiles ...... 35 5.2.1 Beam Transmittance ...... 35 5.2.2 Chlorophyll a Fluorescence ...... 36 5.2.3 CTD Profiles ...... 36 5.2.4 Spectral Absorption Profiles ...... 36 5.2.5 Backscattering Profiles ...... 38 5.3 Atmospheric Radiometry ...... 38 5.3.1 Sun Photometry ...... 38 5.3.2 Sky Radiance Distribution ...... 39 5.4 Water Samples ...... 39 5.4.1 Pigment Analysis ...... 39 5.4.2 Spectrophotometric Absorption ...... 40 5.4.3 Total Suspended Matter ...... 42 5.5 Ancillary Observations ...... 42 5.6 Prototype Optical Buoy ...... 42 5.7 Drifting Optical Buoys ...... 43 5.8 Sampling and Validation ...... 43 5.8.1 Initialization and Validation ...... 45 5.8.2 Case-1 Water Protocols ...... 45 5.8.3 Case-2 Water Protocols ...... 46 5.8.4 Case-2 Sampling Strategy ...... 46 5.9 Vicarious Calibrations ...... 47 6. Analytical Methods ...... 48 6.1 In-Water Radiometry ...... 48 6.1.1 Instrument Calibration Analysis ...... 49 6.1.2 Raman Corrections ...... 49 6.1.3 Normalization by Surface Irradiance ...... 49 6.1.4 K-Analysis ...... 49 6.1.5 Finite Bandwidth Correction ...... 52 6.1.6 Extrapolation to the Sea Surface ...... 52 6.1.7 Instrument Self-Shading Corrections ...... 52 6.1.8 Spectral Adjustments ...... 53

v Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Table of Contents cont.

6.1.9 Normalized Water-Leaving Radiance ...... 53 6.2 Above-Water Radiance ...... 54 6.3 Moored Radiometry ...... 55 6.4 Aerosol Optical Depth ...... 55 6.5 Sky Radiance ...... 55 6.6 Phytoplankton Pigments ...... 56 6.6.1 HPLC Pigment Concentration ...... 56 6.6.2 Fluorometric Determination ...... 56 6.6.3 In Situ Chlorophyll a Fluorescence ...... 56 6.7 Beam Attenuation ...... 56 6.8 Spectral Absorption ...... 56 6.8.1 Reﬂective Tube Measurements ...... 56 6.8.2 Analysis of Absorption Spectra ...... 58 6.9 CTD Proﬁle Analyses ...... 59 Acknowledgments ...... 60 Glossary ...... 60 Symbols ...... 61 References ...... 63 The SeaWiFS Technical Report Series ...... 66

vi Mueller and Austin

Abstract

This report presents protocols for measuring optical properties, and other environmental variables, to validate the radiometric performance of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and to develop and validate bio-optical algorithms for use with SeaWiFS data. The protocols are intended to establish foundations for a measurement strategy to verify the challenging SeaWiFS uncertainty goals of 5% in water-leaving radiances and 35% in chlorophyll a concentration. The protocols first specify the variables which must be measured, and briefly reviewthe rationale for measuring each variable. Subsequent chapters cover detailed protocols for instrument performance specifications, characterizing and calibrating instruments, methods of making measurements in the field, and methods of data analysis. These protocols were developed at a workshop sponsored by the SeaWiFS Project Ofice (SPO) and held at the Naval Postgraduate School in Monterey, California (9–12 April 1991). This report began as the proceedings of the workshop, as interpreted and expanded by the authors and reviewed by workshop participants and other members of the bio-optical research community. The protocols are an evolving prescription to allowthe research community to approach the unprecedented measurement uncertainties implied by the SeaWiFS goals; research and development are needed to improve the state-of-the-art in specific areas. These protocols should be periodically revised to reflect technical advances during the SeaWiFS Project cycle. The present edition (Revision 1) incorporates newprotocols in several areas, including expanded protocol descriptions for Case-2 waters and other improvements, as contributed by several members of the SeaWiFS Science Team.

Prologue The primary SPSWG goals for product uncertainty are derived water-leaving radiances to within 5% and chloro- The NIMBUS-7 Coastal Zone Color Scanner (CZCS) phyll a concentration to within 35% in Case-1 waters, both introduced ocean color remote sensing as a powerful new globally and throughout the projected five-year mission. tool for observing ocean bio-optical properties. Through These goals have been endorsed by the SeaWiFS Science the early 1980s, CZCS data were exploited by a growing Team and accepted by NASA. NASA has the responsibility number of scientists studying marine phytoplankton, ocean to lead the product assurance, calibration, and validation productivity, and ocean optical properties. Practical ap- program. NASA is also required to determine the degree to plications to marine fisheries were also demonstrated. Un- which the commercially procured ocean color data fulfills fortunately, a successor ocean color imaging system was the contractually stated NASA requirements. not developed before the CZCS ceased operating in mid- The CZCS mission was unquestionably a scientific suc- 1986. At present, therefore, research in these areas is lim- cess, but it also taught the participants that the satisfac- ited to retrospective, albeit productive, investigations of tory performance of a remote sensing satellite system can- the CZCS historical database. In mid-1995, the launch of not be taken for granted. Immediately after launch, and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), the periodically throughout its five-year mission, the SeaWiFS next generation ocean color sensor, will bring a welcomed system performance—including algorithms—must be in- and improved renewal of ocean color time-series observa- dependently verified using in situ optical measurements of tions to the ocean research community. the ocean and atmosphere. It is imperative that these sup- The National Aeronautics and Space Administration porting optical measurements meet a uniform standard of (NASA) SeaWiFS Prelaunch Science Working Group quality and accuracy if the primary SeaWiFS goals of 5% (SPSWG) has recommended baseline satellite ocean color uncertainty in water-leaving radiance and 35% uncertainty products consisting of: in chlorophyll a concentration are to be met, or even closely 1) normalized water-leaving radiance (LWN )atfive approached. To that end, the NASA Goddard Space Flight wavelengths; Center (GSFC) SeaWiFS Project convened a workshop to 2) aerosol radiance at three wavelengths; draft protocols and define standards for optical measure- 3) chlorophyll a concentration; ments to be used in SeaWiFS radiometric validation, and algorithm development and validation. The original ver- 4) a chlorophyll-like pigment (chlorophyll a plus sion of this document (Mueller and Austin 1992) reported phaeopigment a) concentration; the protocols agreed to by the participants, as expanded 5) the diffuse attenuation coefficient (K) at 490 nm, by the authors, made in consultation with both the work- K(490); and shop participants and other members of the ocean bio- 6) calibrated radiances observed at the satellite. optics community. The present revision incorporates the

1 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 expanded protocols and revisions contributed by several validate the SeaWiFS calibration. Economics dictate that members of the SeaWiFS Science Team and other col- these observations and data will accrue over several years leagues. from a variety of sources, using diﬀerent instruments and This report speciﬁes the type and quality of support- approaches. These data must be internally consistent, of ing in situ optical measurements and analytical protocols known and documented uncertainty, and in a form readily that will be required to develop bio-optical algorithms and accessible for analysis by the SeaWiFS investigators.

2 Mueller and Austin

Chapter 1

Overview of SeaWiFS Bio-Optical Requirements

Introduction

The Ocean Optics Protocols for SeaWiFS Validation, Revision 1 are intended to provide standards, which if followed carefully and documented appropriately, will assure that any particular set of optical measurements will be acceptable for SeaWiFS validation and algorithm development. It is true that in the case of ship shadow avoidance, for example, there are some circumstances in which acceptable radiometric proﬁles may be acquired considerably closer to a ship than is speciﬁed here (Section 5.1.1). When the protocols are not followed in such cases, however, it is incumbent upon the investigator to explicitly demonstrate that the actual error levels are within tolerance. The most straightforward way for an investigator to establish a measurement that is both accurate enough to meet the SeaWiFS standards and uncontaminated by artifacts, such as ship shadow, will be to adhere closely to the protocols.

1.1 OBJECTIVES fined. This objective includes the following subjects: Immediate concerns have focused this document on spe- a) laboratory calibration and characterization cific preparations for the SeaWiFS mission. A longer-term measurements, uncertainties, and procedures intention is the development of bio-optical databases that to be applied to instruments used in Sea- are relevant to future needs, and therefore, this document WiFS validation and algorithm development also recognizes the capabilities of other planned or poten- activities; tial ocean color sensors, including: b) pre- and post-deployment measurements and a) the Japanese Ocean Color Temperature Sensor procedures to be followed with moored in- (OCTS); strumentation; and b) the Moderate Resolution Imaging Spectroradi- c) methods for instrument calibration and char- ometer (MODIS); acterization, and the requirements for record c) the Medium Resolution Imaging Spectrometer keeping and traceability, including intercali- (MERIS); and brations of radiometric and optical standards d) the German Reflecting Optics System Imaging between participating laboratories. Spectrometer (ROSIS). 4. The at-sea optical sampling strategy and protocols The key objective of the working group was to rec- will be standardized. This objective includes such ommend protocols and standards for supporting in situ considerations as: optical measurements. These objectives addressed the fol- a) the rationale and justifications for moored, lowing subject areas: underway, drifting, shipboard, and airborne 1. The required and useful optical parameters to be measurements; used for validation of SeaWiFS normalized water- b) ship shadowavoidance, depth resolution in leaving radiances and atmospheric correction algo- optical profiles, and total sampling depths; rithms, and for monitoring the satellite sensor’s cal- and ibration and stability, will be defined. c) time of day, sky conditions, season, and geo- 2. The instrumentation requirements, and standards graphic considerations. for measuring the parameters in item 1, includ- 5. The analysis approaches to be used shall be refined. ing definitions of measured quantities, wavelengths, This objective includes procedures and methodolo- field-of-view(FOV) and band specifications, sensi- gies recommended for generating variables from in tivity, uncertainty and stability, will be delineated. situ observations, e.g., LWN (z) from Lu(z), K(z), 3. The optical instrument characterization, intercali- remote sensing reflectance, etc., as well as error bration standards, and related protocols will be de- analysis.

3 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

6. Protocols for ancillary measurements, data archiv- Radiometer (AVHRR) (Brown and Evans 1985 and Wein- ing, database population, and access to data will be reb et al. 1990). Because of the large atmospheric contri- standardized. bution to the total observed radiances (Gordon 1981) and 7. The required atmospheric measurements will be de- the great sensitivity of the bio-optical algorithms to the fined, and the degree to which standard methodolo- estimated water-leaving radiances (Clark 1981), small er- gies are available will be evaluated. rors in the calibration can induce sizable errors in derived geophysical products, rendering them useless for many ap- The development and validation of bio-optical algo- plications. rithms for SeaWiFS will be addressed by a separate work- By processing large quantities of so-called “clear wa- ing group, and thus, these topics are briefly examined in ter” imagery, i.e., water with pigment concentrations less this report. Nonetheless, the SPSWG was charged with than 0.25 mg m−3 (Gordon and Clark 1981), Evans and identifying data requirements and sampling strategies for Gordon (1994) were able to develop a vicarious calibration bio-optical support measurements in the context of the op- that was used in the global processing of the entire CZCS tical and radiometric measurements. This topic includes data set (Esaias et al. 1986 and Feldman et al. 1989). The the following subjects: approach, however, requires assumptions that may limit 1. Discrete chlorophyll a and pigment concentra- the scientific utility of ocean color imagery. Specifically, tions will be measured using the US Joint Global the normalized clear water-leaving radiances, LWN (443), Ocean Flux Study (JGOFS) program’s proto- LWN (520), and LWN (550), were assumed to be 1.40, 0.48, cols and standards for high performance liquid and 0.30 mW cm−2 µm−1 sr−1, respectively. The Ang-˚ chromatography (HPLC) pigment sampling and ström exponents were assumed to be zero and certain geo- analysis, which are adopted by reference to the graphical regions such as the Sargasso Sea were assumed to JGOFS Core Measurement Protocols (JGOFS be clear water sites at all times. Under these assumptions, 1991), Chapter 9, “Pigments and Chlorophyll.” analyses of the derived (LWN ) values were used to calculate 2. An assessment will be made of the roles of un- calibration adjustment coefficients to bring CZCS derived derway, moored, and discrete chlorophyll a fluo- (LWN ) values into agreement for these regions. The vi- rescence measurements, howsuch measurements carious calibration of the 443 nm band is tenuous because are calibrated, and their usefulness for satellite of the great variability in LWN (443) even in clear water. data product validation. Additionally, certain command and engineering data from the NIMBUS-7 platform were not archived, so that a de- 3. The need for biogeochemical measurements of tailed analysis of possible effects related to the spacecraft colored dissolved organic material (CDOM), coc- environment and the effects of spacecraft operation on the coliths, suspended sediment, detritus, etc., will calibration could not be performed. be examined on the basis of baseline product Unlike CZCS, SeaWiFS will routinely produce geophys- requirements. This subject will also the ad- ical fields in a near-real time, operational mode for distri- dress the extent to which standards and proto- bution to the science community. This aspect of the mis- cols have been defined for such biogeochemical sion necessitates constant evaluation of the sensor perfor- measurements. mance and the derived products. Therefore, a multifaceted This report is complementary to anticipated reports of US approach to address the problem of sensitivity degradation and International JGOFS working groups, which are con- and sensor characterization is required during both the pre- currently evaluating bio-optical needs and sampling strate- and post-launch phases. The goal is to ensure that Sea- gies for their respective science programs. WiFS level-1 radiances are accurately known and meet the specifications of the SPSWG. 1.2 SENSOR CALIBRATION The plan includes both onboard and vicarious calibration approaches. SeaWiFS will have a solar measuring The SPO must make every effort to track the sensor’s diffuser plate to reference the response to the sun (Gordon performance throughout the duration of the mission. Since 1981) and also will be capable of periodically imaging the the instrument will be designed for a five-year mission, it moon by maneuvering the spacecraft. The vicarious cali- is certain that the sensor calibration at each wavelength bration program will incorporate measurements of water- will change in some unpredictable manner as a function leaving radiances and other related quantities, from ships, of time. Experience with CZCS has shown it is very dif- drifting buoys, and fixed moorings, to develop time series ficult to determine a sensor’s calibration once it has been and geographically diverse samples of oceanic and atmos- launched (Viollier 1982, Gordon et al. 1983, Hovis et al. pheric data sets. Each approach has advantages and disad- 1985, Mueller 1985, and Gordon 1987). Similar prob- vantages, but when combined, they should provide a com- lems have been encountered with other Earth observing plementary and comprehensive data set that will be suffi- systems, such as the National Oceanic and Atmospheric cient to monitor short-term changes and long-term trends Administration (NOAA) Advanced Very High Resolution in the sensor’s performance.

4 Mueller and Austin 1.3 BIO-OPTICAL ALGORITHMS 1.4 COMMUNITY PARTICIPATION The SPO will be responsible for producing a standard The SPO will rely on the oceanographic community to set of derived products and will produce both CZCS-type perform field research for atmospheric and bio-optical al- products and baseline products. The CZCS-type products gorithm development, and for all of the in situ data collec- will consist of pigment concentration, K(490), five normal- tion for the vicarious sensor calibration. A minimal subset ized water-leaving radiances, and three aerosol radiances of these observations will be sponsored by the SPO, but based on constant default wavelength dependence (epsilon) many projects sponsored by the NASA Research and Ap- coefficients in aerosol corrections. The proposed baseline plication Program and other government agencies are ex- products will include five normalized water-leaving radi- pected to make major contributions to the global five-year ances, K(490), chlorophyll a concentration, three aerosol effort. This requires close coordination of the various pro- radiances, and one or more error analysis products. grams involved and a clear definition of the observations, The basic quantities to be computed from the sensor accuracies, and data collection protocols required for each radiances are the water-leaving radiances, from which all type of activity. The purpose of this document is to clarify other derived products except the aerosol radiances are these requirements. computed. Every effort must be made to ensure these radiances meet the specifications of the SPSWG, i.e., ±5% 1.5 VICARIOUS CALIBRATION in Case-1 waters. This requires the atmospheric correction algorithms to be considerably more sophisticated than the For ocean observations, it is easy to show(Gordon current CZCS algorithms. 1987 and Gordon 1988) that satellite sensor calibration The baseline bio-optical products must meet the ac- requirements based on the quality of the existing CZCS curacy requirements established by the SPSWG over a pigment algorithms exceed currently available capabilities. variety of water masses. The current CZCS algorithms Furthermore, the sensor calibration is unlikely to remain were based on a data set consisting of fewer than 50 data unchanged through launch and five years of operation in points (only 14 observations were available for the band- orbit. The only foreseeable way of approaching the ocean 2-to-band-3 ratio algorithm) and performed poorly in recalibration needs is through vicarious calibration, i.e., fine gions of high chlorophyll a concentration, high suspended tuning the calibration in orbit. sediment concentration, high CDOM concentration, and The methodology used to achieve vicarious calibration coccolithophorid blooms (Groom and Holligan 1987). Ac- for CZCS was described in detail by Gordon (1987). First, curate estimates of the baseline products are essential if the calibration was initialized after launch by forcing agree- SeaWiFS is to be useful in programs such as the Global ment between the sensor-determined radiance and the ex- Ocean Flux program [National Academy of Science (NAS) pected radiance based on radiometric measurements made 1984]. at the surface under clear atmospheric conditions. Next, SeaWiFS will have the capability, due to improvements since the CZCS responsivity was observed to be time de- in the signal-to-noise ratio (SNR), digitization, dynamic pendent, the algorithms were applied to other scenes char- range, and wavelength selection, to increase the accuracy acterized by bio-optical surface measurements and more of these products and to flag areas where anomalies or typical atmospheres, and the calibration was adjusted un- lowconfidence conditions exist. Clearly, a much larger til the measured water-leaving radiances were reproduced. database will be needed for developing a broader variety Finally, the surface measurements of pigments were com- of bio-optical algorithms, some of which will be region spe- bined with satellite pigment estimates for a wide variety of cific. The radiometric, optical, and chemical field observa- atmospheric conditions, and the radiance calibration was tions used in deriving bio-optical algorithms and for vicar- fine tuned until the best agreement was obtained between ious calibration of the sensor must, therefore, conform to the retrieved and true pigments. stringent requirements with respect to instrument calibra- The CZCS vicarious calibration was not radiometric. tion and characterization, and must also conform to the It was a calibration of the entire system—sensor plus al- observation protocols which have been specified to take gorithms. To predict the radiance measured at the sat- advantage of SeaWiFS capabilities. ellite, Lt, the water-leaving radiance, the aerosol optical The SPO will manage a program to compare the vari- thickness, and the aerosol phase function are all required. ous atmospheric correction and bio-optical algorithms pro- Also needed are ancillary data such as the surface pres- posed by the science community. The purpose of this pro- sure, wind speed, and ozone optical thickness. These data gram is to independently evaluate suggested improvements for vicarious calibration and validation will be obtained or additions to the SeaWiFS products. This component of by measuring the upwelling radiance distribution just be- the calibration and algorithm development program will neath the surface, along with the aerosol optical thickness run in parallel with, but off-line from, operational process- and the sky radiance, at the time of the satellite overpass. ing and will provide an essential mechanism for incorpo- The sky radiance will be used to deduce the required infor- rating data and analyses from the community at large. mation about the aerosol phase function (Voss and Zibordi

5 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

1989). The data set will be used to deduce Lt, at the top of sential for calibrating the SeaWiFS system, i.e., sensor plus the atmosphere, coincident with a SeaWiFS overpass from algorithms, and that it cannot be eﬀected without a high which the calibration will be initialized. quality surface data set obtained simultaneously with the It must be stressed that this exercise is absolutely es- satellite imagery.

6 Mueller and Austin

Chapter 2

Data Requirements

Introduction

The prime objective of in-water optical measurements for SeaWiFS is to derive accurate normalized water- leaving radiances that will be used both for direct validation comparisons with those derived from SeaWiFS data, and to develop and validate in-water bio-optical algorithms. Therefore, a comprehensive ﬁeld program to measure optical and biogeochemical state variables will be required.

− 2.1 DEFINITIONS Lu(0 ,λ) measured under a given set of illumination conditions, e.g., overcast skies, to LWN (λ), which would be The required and useful variables to be measured for measured under the restricted illumination and viewing SeaWiFS validation are listed in Table 1, classified into conditions associated with SeaWiFS measurements. As two discrete sets. The first set comprises variables that − − with Lu(0 ,λ), Ed(0 ,λ) must be determined by extrapo- can be used for radiometric initialization and ongoing val- lation from a profile of Ed(z,λ) over the upper fewoptical idation. The second set encompasses those variables that depths and reconciled with the direct surface measurement will be used for bio-optical algorithm development and val- of Es(λ). [Optical depth, τ(z,λ), in the context of this idation. report is the integral of K(z,λ), for either radiance or ir- − Surface incident spectral irradiance, Ed(0 ,λ); down- radiance, depending on the context, from the surface to a welled spectral irradiance, Ed(z,λ); and upwelled spectral given depth z.] radiance, L (z,λ), are the fundamental measurable quan- − u Upwelled spectral radiance, Lu(0 ,λ), is the in-water tities needed to derive normalized water-leaving radiances variable which, when propagated upward through the sea in most circumstances. Other ambient properties, like sky surface, leads to the measured value of LW (λ). LW (λ) radiance, sea state, wind velocity, etc., are also useful ini- is, in turn, adjusted using Ed(λ) to derive the normalized tialization and calibration measurements and are discussed water-leaving radiance, LWN (λ), for a clear-sky zenith sun below. at the mean Earth-sun distance. Unfortunately, it is not − − Surface incident spectral irradiance, Ed(0 ,λ), is usu- practical to measure Lu(0 ,λ) precisely at an infinitesimal ally derived from surface irradiance, Es(λ), measured on depth belowthe surface. Therefore, the profile of Lu(z,λ) a ship well above the water, but the use of a radiometer must be measured over the upper fewoptical depths with − floated just beneath the surface (z =0 ) may provide a sufficient accuracy to determine KL(z,λ) for Lu(z,λ), and − better approach (Sections 3.2.1 and 5.1.4). Ed(0 ,λ) varies to propagate Lu(z,λ) to the surface. At near-infrared (IR) due to fluctuations in cloud cover and aerosols, and with wavelengths, the first optical depth is confined to the upper − time of day, i.e., solar zenith angle. Profiles of Ed(z,λ) fewtens of centimeters. Determination of Lu(0 ,λ) in this and Lu(z,λ) must be normalized to account for such vari- situation is more challenging and will require special in- abilities during a cast. struments and experiment designs to accommodate the ef- Downwelled spectral irradiance, Ed(z,λ), is required to fects of instrument self-shading, wave focusing, small-scale compute the diffuse attenuation coefficient, K(z,λ), which variability, possible fluorescence, Raman scattering, and in turn, is needed for diffuse attenuation coefficient algo- extremely small working volumes. Careful measurements rithm development (Austin and Petzold 1981), and for of inherent optical properties (IOPs), including a(z,λ), optically weighting the pigment concentrations to be es- c(z,λ), and bb(z,λ), and spectral fluorescence, may be timated from remotely sensed ocean color (Gordon and useful, in addition to Ed(z,λ) and Lu(z,λ) measurements Clark 1981). Ed(z,λ) is also required to compute the spec- made with specially designed radiometers. tral (remote sensing) reflectance, RL(z,λ), which is used Sky radiance is required to enable estimation of the to normalize Lu(z,λ) when developing and validating bio- aerosol phase function through inversion of the radiative optical algorithms. The need for this normalization arises transfer equation. It is also useful for estimating the mean because the spectrum of incident irradiance varies with cosine of the transmitted light field in the water. The sky changing solar zenith angle and atmospheric conditions. radiance should be measured directly; for the latter appli- − Ed(0 ,λ) can then be used, through RL(z,λ), to convert cation, however, it need only be estimated by occulting the

7 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Table 1. Required observations for initialization and system calibration for satellite product verification and radiative transfer (also ongoing calibration and atmospheric algorithm validation studies) and bio-optical algorithm development and validation. Product Radiative Bio-optical Verification Transfer Algorithms Primary Optical Measurements − Incident Spectral Irradiance, Ed(0 ,λ) × × × Downwelled Spectral Irradiance, Ed(z,λ) × × × Upwelled Spectral Radiance, Lu(z,λ) × × × Spectral Solar Atmospheric Transmission, τs(λ) × × × Submerged Upwelled Radiance Distribution, L(z,θ,φ) × × × Spectral Sky Radiance Distribution × × × Upwelled Spectral Irradiance, Eu(z,λ) × × Calculated or Derived Variables − Water-leaving Radiance, LW (0 ,λ) × × × Attenuation Coefficient Downwelled Irradiance, Kd(z,λ) × × × Attenuation Coefficient Upwelled Radiance, KL(z,λ) × × × Spectral Reflectance, RL(z,λ) × × × Ambient Properties Sea and Sky State Photographs × × × Wind Velocity × × × In Situ Fluorescence Profiles × × Aerosol Samples × × Temperature and Salinity Profiles × Secchi Depth × Primary Biogeochemical Measurements Phytoplankton Pigments (HPLC Technique) × × Phytoplankton Pigments (Fluorometric Technique) × × Total Suspended Material (TSM) Concentration × × Colored Dissolved Organic Material (CDOM) × × Inherent Optical Properties Spectral Beam Attenuation Coefficient, c(z,λ) × × Spectral Absorption Coefficient, a(z,λ) × × Spectral Backscattering Coefficient, bb(z,λ) × × Spectral Volume Scattering Function, β(z, λ, θ) × × Red Beam Attenuation, c(z,660 nm) × × Algorithm Specific Research Measurements Airborne Fluorescence and Radiances × × Coccolith Concentration × Detritus Absorption Coefficient × × Humic and Fulvic Acids × Inorganic Suspended Material × Organic Suspended Material × Particle Absorption Coefficient × × Particle Fluorescence × Particle Size Spectra × × Particulate Organic Carbon (POC) × Particulate Organic Nitrogen (PON) × Phycobilipigments Concentration × Phytoplankton Species Counts × Primary Productivity (14C) × Total Dissolved Organic Carbon (DOC) × × = Needed for the indicated effort.

8 Mueller and Austin sun’s image on a deck cell measuring the incident spectral bb(λ)/a(λ) is modeled statistically as a polynomial func- radiance from the sun and sky. The mean cosine at the tion of chlorophyll a concentration. This will introduce a surface can be used with profile measurements of Ed(λ), strong, empirical component to the algorithm, suppress- Eu(λ), and c(λ) to estimate bb(λ) (Gordon 1991). An abil- ing the physical links between RL(λ) and bb(λ)/a(λ) and ity to exploit this and similar relationships will greatly between bb(λ)/a(λ) and phytoplankton pigment concentra- enhance both development and verification of bio-optical tion. A more robust algorithm would be based on direct algorithms. The spectral sky radiance distribution over measurements of absorption, scattering, pigments, and re- zenith and azimuth angles is required to determine the flectance. Due to recent advances in instrumentation, it aerosol scattering phase functions at radiometric compar- is nowpractical to routinely measure profiles of a(z,λ) ison stations during the system initialization cruises and (Section 5.2.4) and backscattering variables (Section 5.2.5) will be very useful if measured at all validation stations from which bb(z,λ) may be approximated. Future develop- throughout the mission. ment and validation experiments involving this algorithm Upwelled radiance distribution measurements just be- must, therefore, include absorption, beam attenuation, and neath the sea surface will be required for quantifying the scattering measurements. angular distribution of water-leaving radiance at stations Red beam attenuation coefficient, c(660), and in situ used for system calibration initialization and long-term chlorophyll a fluorescence measurements are exceptionally system characterization. These measurements will also be useful in analyzing profiles of E (z), L (z), and E (z)to useful in relating radiance and irradiance reflectance, and d u u derive profiles of K (z), K (z), and K (z), respectively. K profiles, to IOPs and biogeochemical substances, e.g., d L u If these profiles are viewed in real time, they are also use- chlorophyll a and CDOM, during bio-optical algorithm de- ful guides for taking water samples at depths that allow velopment and validation. the vertical structure of chlorophyll a and suspended par- Atmospheric transmittance spectra should be measured ticles to be accurately resolved in the top optical depth. using a sun photometer in order to determine aerosol op- Finally, the chlorophyll a fluorescence profile is used to in- tical depths at each station. These data are particularly needed to verify the atmospheric corrections in direct com- terpolate HPLC and extracted fluorescence measurement of chlorophyll a concentrations from water samples at dis- parisons between SeaWiFS LW (λ) estimates and those de- − crete depths. It is desirable to make these measurements termined from in-water Lu(0 ,λ). Sea state photographs are required to document surface simultaneously with irradiance and radiance profiles, if it wave conditions during radiometric measurements. This can be done in a way to avoid self-shading of the instru- information is essential for identifying measurements made ment (Section 5.1.6). under questionable environmental conditions. Secchi depth measurements are required for real-time Wind velocity is required to generate, through mod- assessment of water transparency during a station and as els, estimates of the surface wave slope distribution, which a quality check during analysis of radiometric profiles. will be used to calculate reflected skylight and sun glint Aerosol concentration samples using high volume tech- in radiative transfer models (Cox and Munk 1954). Sur- niques will be useful, in conjunction with aerosol optical face wave models driven by wind velocity may also be used depth spectra determined from sun photometer measure- to provide quantitative estimates of surface wave induced ments, for chemical, size, and absorption characterization radiometric fluctuations. Qualitatively, wind velocity, and of aerosols, especially in studies of the effects of Saharan photographs or videotape recordings of sea state, will be and Asian dust clouds on atmospheric corrections. useful for assessing station data quality. Upwelled spectral irradiance, Eu(z,λ), is a useful mea- 2.2 BIOGEOCHEMICAL DATA surement, in addition to Ed and Lu, because there exist both empirical and theoretical relationships between Pigment concentrations will be determined using HPLC IOPs, phytoplankton pigments, TSM, and irradiance re- and fluorometric methods to develop and validate chlo- − − rophyll a algorithms, and to assess the effects of acces- flectance. Lu(0 ,λ) and Eu(0 ,λ) are related by the factor Q(λ), which is not well determined at present, and has sory pigment concentrations on water-leaving spectral ra- been shown to vary with solar zenith angle (Morel and diances. These data will also be used to calibrate con- − Gentili 1993). Combined measurements of Lu(0 ,λ) and tinuous profiles of chlorophyll a fluorescence (Section 2.1). − Eu(0 ,λ) will be extremely useful in determining Q(λ), Phytoplankton pigment concentration will also be deter- which will, in turn, allow traceability of SeaWiFS mea- mined using classical chlorophyll a and phaeopigment flu- surements to previously derived irradiance reflectance re- orescence techniques that were used for CZCS pigment lationships and algorithms. validation and algorithm development. The HPLC tech- IOPs must be measured for development and valida- nique provides more accurate and precise information for a tion of the SeaWiFS semi-analytic Case-2 chlorophyll a greater number of pigments, but gives different values than algorithm. This algorithm is based on an explicit theo- does the fluorescence method for various species composi- retical function of the ratio of backscattering to absorp- tions and chlorophyll a-to-phaeopigment ratios. While the tion, bb(λ)/a(λ). In the first implementation, however, HPLC method is the primary pigment technique required

9 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 for SeaWiFS, the classical technique is still required to al- Humic and fulvic acids comprise the bulk of CDOM lowthe CZCS and SeaWiFS data sets and algorithms to and have different specific spectral absorption coefficients. be compared. Their concentrations are useful for determining the correc- Phycobilipigments, present in cyanobacteria and cryp- tion used for phytoplankton pigment concentration algo- tophytes, are treated separately from the HPLC fat sol- rithms in Case-2 waters and for estimating CDOM from uble pigments. Phycoerythrin and phycocyanin are the ocean color observations. two major groups of phycobilipigments found in the ma- Coccolith concentration, which is the number density rine environment. The concentration of these water solu- of small plates (coccoliths) composed of calcium carbon- ble pigments is important due to the contribution of solar ate (CaCO3), is very important to light scattering. Coc- stimulated phycoerythrin fluorescence to the underwater coliths are produced in copious amounts by marine phy- light field, and also to characterize the phytoplankton pop- toplankton called coccolithophores. Scattering of light by ulation. At times, species which contain phycobilipigment coccoliths is highly apparent in visible wavelength satel- can account for a large fraction of the primary productivity lite imagery, because they perturb the usual relationships (especially in oligotrophic waters) and have been difficult between water-leaving radiances and chlorophylla concen- to quantify due to their small size. These measurements tration and adversely impact atmospheric corrections. Ad- are not required because SeaWiFS does not contain bands ditionally, coccolith formation, sinking, and dissolution are at their absorption or fluorescence peaks. The measure- significant factors in the ocean carbon flux budget. It ments are desirable, however, since several aircraft sensors, is, therefore, necessary to measure coccolith concentra- e.g., the Airborne Visible and Infrared Imaging Spectrom- tion, both as number density and CaCO3 concentration, eter (AVIRIS), Airborne Oceanographic Lidar (AOL), the to aid in 1) the correction of chlorophyll a concentration Multispectral Airborne Radiometer System (MARS), and algorithms, 2) coccolith algorithm development, and 3) at- future satellite sensors, e.g., MODIS and MERIS, do pos- mospheric correction development and validation. sess bands at the absorption or fluorescence peaks of phy- The particle absorption coefficient, which is comprised cobilipigments. If such pigment information is required for of absorption by living, dead, and inorganic particles, is a particular study, it is recommended that the glycerol- a useful variable for modeling the portion of solar energy uncoupling technique described by Wyman (1992) be used that is absorbed by phytoplankton and bacteria. to determine phycoerythrin concentrations. Detritus (or tripton) absorption coefficient, i.e., absorp- TSM measurements are required to assess the effect of tion of light by detritus, represents a major loss of light suspended sediment on the derived products. TSM is of which would otherwise be available to the phytoplankton primary importance in coastal waters, where simple radi- component of the marine hydrosol. In many cases, absorp- ance ratio algorithms for TSM have uncertainties equiva- tion by detritus is a significant term in the marine radiative lent to, or better than, those for estimating chlorophyll-like transfer processes, and its determination is useful for phy- pigment concentration. Organic suspended matter and inorganic suspended matter concentrations are subfractions toplankton production models and for modeling the light of TSM; this partitioning of TSM is particularly useful in field. process studies. Particle size spectra are very useful for in-water ra- Particulates, both POC and PON, are required for pro- diative transfer calculations, particularly if measurements cess studies to help characterize the adaptive state of phy- include particles smaller than 1 µm. toplankton and to inventory critical biogeochemical ele- Particle fluorescence, measured using laser sources on ments. single-cell flowsystems, may be used to calculate particle DOC has been shown to be a major pool of carbon scattering-to-fluorescence ratios for evaluating the popula- in the oceans. Quantification of the transformations of tion structure of the plankton (both phyto- and zooplank- this pool is crucial to understanding the marine carbon ton). cycle. The colored fraction, CDOM, of the DOC is highly Phytoplankton species counts are important because absorbent in the blue range, thus decreasing blue water- species-to-species variability in optical and physiological leaving radiances, and it must be taken into consideration properties represents a major source of variability in bio- for pigment concentration algorithms. DOC measurements optical algorithms and primary productivity models. This are needed to develop robust relationships between CDOM has been recognized, but it is generally ignored in remote and DOC, which are needed to evaluate the usefulness of sensing algorithms due to the tedious nature of species ocean color observations for estimating DOC concentra- enumeration, the small sizes of many species, and the large tions. number of species involved. This information, however, at CDOM concentrations are required to assess the ef- various levels of rigor, is useful in evaluating the population fect of Gelbstoff on blue water-leaving radiances and chlo- and pigment composition. This is especially important for rophyll concentration. This is of primary importance in some groups, such as coccolithophores. Case-2 waters, but is also relevant to phytoplankton degra- Primary productivity, using the radioactive isotope 14C dation products. estimation method, is not strictly required for validation of

10 Mueller and Austin water-leaving radiances or system initialization. Further- near ship and mooring stations and to provide independent more, primary productivity is not a standard derived Sea- assessments of bio-optical algorithms. WiFS product, owing to the complexity of relating ocean color to production. It will, however, be extremely useful 2.4 ANCILLARY MEASUREMENTS for process study applications of ocean color data if these measurements are made at the same time that the water Hydrographic data, water temperature (T), and salin- column optical properties are determined. These data will ity (S), derived from conductivity, temperature, and depth aid in the development of models of primary production (CTD) profiles, are useful for characterizing the physical using satellite ocean color observations, a goal which is cen- water mass regime in which an optical profile is measured. tral to the overall SeaWiFS science mission. Of special im- A T-S characterization is especially important near ocean portance are determinations of key photo-physiological pa- fronts and eddies where interleaving water masses of very rameters derived from production measurements as func- different biogeochemical composition, and therefore funda- tions of irradiance. If 14C productivity measurements are mentally different bio-optical properties, can produce com- made, they should conform to the JGOFS Core Measure- plex spatial and temporal patterns of near-surface optical ments Protocols (JGOFS 1991). properties. In these circumstances, T-S profiles can provide an indication of whether a station location is suitable 2.3 ABOVE-WATER TECHNIQUES for reliable remote sensing validation and algorithm development comparisons. Above-water ocean color radiance measurements from aircraft and ships can augment in-water measurements of 2.5 OPTICAL MOORINGS Lu(z,λ) made to compare directly with SeaWiFS measurements for validation of its radiometric performance and Optical moorings will be maintained in one or more re- algorithms. Above-water radiance measurements can, if gions of lowoptical variability to provide long-term time they are accurately made, contribute an additional use- series comparisons between in situ and SeaWiFS measure- ful constraint in defining internally consistent sun-ocean- ments of normalized water-leaving radiance. Moored op- atmosphere-sensor models that will comprise the essence tical systems will also be extremely useful in a variety of of SeaWiFS radiometric validation. For this application, oceanographic studies. For example, global satellite obser- above-water radiometers must meet the SeaWiFS specifi- vations of ocean pigment biomass and estimates of phyto- cations for radiometric uncertainty, SNR, and spectral re- plankton production are essential to achieve the objectives solution at a spatial resolution that will permit direct com- of the JGOFS program (NAS 1984); SeaWiFS will play a parisons with in-water measurements, and with SeaWiFS key role in this effort. measurements by means of spatial averaging. Conversely, The oceans exhibit physical and biological variability above-water radiance measurements made with less accu- over a wide range of space and time scales. This variabil- racy than the SeaWiFS prelaunch specifications would in- ity, and the need to synoptically measure distributions of troduce an unacceptable error source into the validation physical and biological properties over large areas and long models and cannot be used for this purpose. time periods, has motivated recent developments utilizing Airborne ocean color data may also be used to deter- contemporaneous buoy, ship, aircraft, and satellite sam- mine spatial variability in ocean optical properties during pling strategies (Smith et al. 1987). In addition, long-term shipboard algorithm development and validation experiments. In this context, airborne ocean color measurements mooring data are required to provide continuous observa- will be especially valuable in productive Case-1 and Case- tions and permit an optimization of the accuracy of the 2 waters, where variability in ocean optical properties can derived satellite products (Booth and Smith 1988). be large over mesoscale and smaller distances. Synoptic There are two sources of systematic error in estimates maps of ocean color distributions can be advantageously of phytoplankton pigment biomass derived from satellite utilized to guide sampling by ships. It can also be used ocean color data. First, errors in satellite estimation of to place in-water data from an individual station in con- pigment biomass arise because physical forcing, biologi- text with respect to nearby variability, and thus provide cal properties, and ocean optical properties all vary sys- a basis for spatial interpolation and averaging when com- tematically with depth, and the upper layer optical signal paring in-water bio-optical measurements with SeaWiFS observed by satellites may not adequately represent struc- image data. This application can be accomplished using ture deeper in the water column. In many circumstances, aircraft radiometers meeting somewhat less stringent per- subsurface changes may go undetected unless contempo- formance specifications than is demanded for direct vali- raneous water column profile data from either shipboard dation comparison between SeaWiFS and aircraft radiance or moored sensors is available. Second, visible wavelength measurements. sensor systems do not obtain data when the atmosphere Airborne measurements of fluorescence by chlorophyll, is cloudy. Air-sea interactions giving rise to cloudiness are CDOM, and phycoerythrin, both by laser and solar excita- often closely linked to biological processes. For example, tion, are useful to evaluate spatial and temporal variability Michaelsen et al. (1988) showed that episodic wind events,

11 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 which give rise to coastal upwelling and subsequent phy- over an ARGOS link. Storage procedures are designed toplankton production along the California coast, cause to make maximum use of the limited ARGOS bandwidth. cloudiness that will bias the statistics of pigment concen- (An interrogating store and forward satellite system with trations derived from ocean color imagery. Similar biases greater bandwidth would be beneficial for this purpose.) of a factor of 3–4, due to wind mixing during cloudy pe- The upwelling radiance data are obtained at a depth of riods, were observed by Muller-Karger et al. (1990). In approximately 0.5 m and must be propagated through the circumstances of this nature, visible and infrared satellite surface using KL(λ) estimated from the relative spectral observations of the ocean are not random samples. Moored shape. These water-leaving radiance estimates will, there- optical sensors can measure systematic temporal variabil- fore, be inherently less accurate than surface reflectance ity in the vertical distribution of pigment biomass and, observations made together with optical profiles and more at the same time, provide the continuous time series that complete ancillary observations. may be used to remove the sampling bias associated with Both of these systems are in the test and evaluation cloudiness. phases. High risk areas which are being examined in- The detection and verification of intra- and interan- clude long-term stability, identification of bio-fouling ef- nual fluctuations in productivity and associated bio-optical fects, operating lifetime, and validation of the techniques variables are key goals of programs focused on studying used for calculating the water-leaving radiance from the global change. These goals place stringent requirements simple drifting sensors. The accuracy of the ARGOS sys- for long-term accuracy and precision on the measurement tem for drifter location is sufficient for global area coverage systems to be employed. The monitoring of bio-optical pa- (GAC), but experience needs to be gained in analysis of rameters to resolve variability at global and decadal scales, the data to demonstrate the feasibility of using ARGOS as proposed by the SeaWiFS and MODIS missions, will positioning (150 m to 1 km uncertainty) for system cali- require that moored in situ optical instruments be main- bration and validation work in water mass regimes where tained to supplement and support the satellite data sets. mesoscale variability is significant. For example, the CZCS sensor degradation and the difficul- The cost of optical drifters will limit the number de- ties encountered in attempting to characterize that degradation, strongly point out howvaluable in-wateroptical ployed. Proponents envision 50 such buoys adrift at any measurements would have been to that program. one time throughout the world—a number sufficient to provide a large enough sample size to support viable global 2.6 DRIFTINGOPTICAL BUOYS validation. In such a scenario, typically 60% of the drifters would be obscured by clouds during a SeaWiFS pass Drifting optical buoys, which are expendable and anal- (those in some areas will have a much greater probability ogous to the ARGOS sea surface temperature (SST) drif- for clouds). Furthermore, current divergence areas will be ters, represent a viable, cost-effective way to obtain sig- systematically undersampled by drifters. nificant numbers of daily optical observations to validate The uncertainty of calculated LW derived from drifter global data sets of water-leaving radiance. Due to high data has been estimated to be of the order of 15%, even cost, there will probably never be more than a few per- though at the time they are deployed, the calibrated un- manent optical moorings. Shipboard observations provide certainty of the instruments will be less than 5%. This only a fewmeasurements on any given day, due to both the estimate may be pessimistic, based on: cost of at-sea research and investigator availability. Op- tical data from drifting buoys, while less complete than 1) the untestable possibility of drifts in radiomet- measurements from ships and fixed optical moorings, can ric responses during long-term deployment of an potentially surpass both in terms of global coverage and expendable instrument, and the number of near-real time comparisons with SeaWiFS 2) errors associated with propagating Lu(0.5 m,λ) observations. Judicious seeding of inexpensive drifters pro- through the water column and interface to esti- vides one means of sampling conditions and regions critical mate LW (λ) without benefit of measurements of to SeaWiFS product verification. For example, chronically K(z,λ), surface roughness, and other ancillary high levels of aerosols caused by desert dust storms con- measurements to be carried out at correspond- stitute a regional condition that will affect atmospheric ing ship stations. corrections. These instruments may also allowstudy of possible variations in SeaWiFS performance as a function If LWN from drifters is only good to 15%, then they can- of latitude, due to orbital variations in sensor performance not be used to verify SeaWiFS radiometry within 5%, no and sun angle dependency of algorithms. matter howmany drifter comparisons are made. Uncer- Optical drifter development activities occuring at Dal- tainties must be less than 5% if this technology is to be housie University intend to develop instruments to measure used for SeaWiFS radiometric validation. Uncertainties seven and three upwelling spectral radiances, respectively, of 15% may, however, be useful for validating SeaWiFS and a single downwelling irradiance. Along with temper- and derived products, and for interpolating SeaWiFS data ature and barometric pressure, these data are transmitted through periods of extensive cloud cover.

12 Mueller and Austin

Chapter 3

Speciﬁcations

Introduction

This report describes measurements of optical properties, and other variables, necessary for validating data obtained with the SeaWiFS instrument, and for the development of in-water and atmospheric algorithms. The specifications herein are those required of instruments used on ships, or other platforms, to acquire that optical data. In some cases, the specifications have been selected to allowuse of instruments that are affordable and that either currently exist, or that can be developed without major improvements in today’s state-of- the-art technology. In a fewcases, newor improved instruments must be developed to realize the specified performance characteristics. The data uncertainty requirements for this program are more severe than those for a general ocean survey. Here, various investigators will use a variety of instruments that will be calibrated independently at a number of facilities, and contribute data to a common database which will be used to validate SeaWiFS measurements. The resulting radiometric and bio-optical database will provide an essential means of detecting and quantifying on-orbit changes in the SeaWiFS instrument relative to its prelaunch calibration and characterization. This chapter specifies instrument characteristics and data uncertainties thought by the SPSWG to be necessary, as well as sufficient, for this task. The validation analysis would be significantly degraded should calibration errors or differences of even a fewpercent, or wavelength errors or differences of a fewnanometers, occur in (between)the instruments used to acquire the SeaWiFS bio-optical database.

3.1 IN-WATER RADIOMETERS Table 2. Recommended spectral bands for discrete wavelength filter radiometers using 10 nm full-width This section specifies radiometric characteristics for in- at half-maximum (FWHM) bandwidths. In addi- struments that are used to measure Ed(z,λ), Eu(z,λ), and tion, out-of-band blocking in the far tails of the in- −6 Lu(z,λ). The specifications are applicable to filter ra- strument response functions should be at least 10 . diometers and to spectroradiometers based on monochro- SeaWiFS Wavelengths E , E , L E mators. Minimum performance characteristics are spec- d u u s Band [nm] [nm] [nm] ified for spectral resolution, radiometric responsivity and resolution, SNRs, radiometric saturation and minimum de- 1 402–422 412 1 412 tectable values, angular response, temporal sampling reso- 2 433–453 443, 435 2 443 lution, linearity, and stability. 3 480–500 490 490 4 500–520 510 510 3.1.1 Spectral Characteristics 5 545–565 555 555 6 660–680 665, 683 665 3 In-water radiometers shall be capable, as a minimum, 7 745–785 4 780 of making measurements at the wavelengths shown in Ta- 8 845–885 4 875 5 ble 2. The table presumes the use of properly blocked in- terference filters to provide the required spectral bandpass 1 A preferred option is to replace two separate 10 nm and out-of-band rejection (10−6 or better). Care must also FWHM bands centered at 406and 416nm, with a single 412 nm channel. The two channels would al- be taken to avoid possible out-of-band leakage due to flu- low more accurate modeling of LWN (412) matching orescence by filter, or other optical component, materials. SeaWiFS characteristics. Filter radiometers should have channels with center wavelengths, as measured in the assembled instrument, match- 2 An optional extra band is used to improve modeling of L (λ) radiances to match the SeaWiFS 443 nm ing those given in Table 2 to within ±1 nm for 410 and WN channel. 443 nm, and within ±2 nm for all other spectral bands. Shifts of these magnitudes in center wavelengths will re- 3 Es deck, only one channel in this band is necessary. sult in changes in measured radiometric values of approx- 4 Due to the specialized nature of infrared in-water imately ±1% or less (Booth pers. comm.) and this speci- measurements, specialized sensors will be needed. fication should be met if possible. 5 Optional for Es.

13 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

± It is recognized, however, that enforcing a 1 nm hard- nominal wavelength (λn). In any event, the actual spec- and-fast specification could be prohibitively expensive, and tral response function of each instrument channel must be this tolerance should be regarded as a goal. With knowl- measured and known to be accurate to less than 0.2 nm. edge, to less than 0.2 nm, of the actual center wavelengths High resolution monochromator-based spectroradiome- and complete spectral response functions, corrections prob- ters, with adequate sensitivity and stray light rejection ably can be made to infer effective radiometric quantities characteristics, are also suitable instruments and are rec- for the SeaWiFS channels, when the spectral characteris- ommended for many algorithm development studies. Suit- tics of SeaWiFS channels have also been measured, shortly able specifications for such instruments are given in Ta- before launch. Bandwidths must be 10 nm ±2 nm FWHM. ble 3. (These instruments must also meet the specifications They are made narrower than the SeaWiFS channels to re- summarized in Tables 2 and 4.) duce the skewing of the parameters derived from underwater irradiance or radiance profiles in spectral regions where absorption by natural sea water may exhibit rapid varia- 3.1.2 Responsivity, SNR, and Resolution tion with wavelength. The expected operating limits for radiometric respon- To maintain the above tolerances, it is anticipated that sivities, SNR, and digital resolution are specified in Ta- filters will be ordered to a center wavelength with a toler- ble 4, the limits for which were derived as follows: ance of λ0 ±1 nm and a FWHM bandwidth of 8.5 ±1 nm. ◦ −2 −1 When the filter is installed in a radiometer with a 10 (half- 1. An Ed saturation value of 300 µWcm nm is angle) FOV, however, the spectral bandpass will broaden assumed at all wavelengths. by 2–3 nm, and the center wavelength will shift. Further- 2. Implicit, but not stated, in Table 4 is that the min- more, as a filter ages in use, its transmission curve may −2 −1 imum required Ed(0) is 20 µWcm nm ; it will undergo changes to further broaden the FWHM bandpass not be appropriate to occupy validation stations and shift the peak. The tolerances specified above include when illumination is less than this minimum. an allowance for some degradation before expensive filter and detector changes must be done. 3. The minimum Ed(0) implies a minimum detectable −2 −1 Ed(z) value of 1 µWcm nm at 3 optical depths Table 3. High resolution spectroradiometric spec- (3/K). ifications. 4. Digital resolution must be less than or equal to Optical Sensors 0.5% of the reading to maintain a 100:1 SNR. To Spectral Range: 380 to 750/900 nm permit a 1% uncertainty in absolute calibration, if Spectral Resolution: 5 nm (or less FWHM) that goal can be met in the calibration laboratory, Wavelength Accuracy: 10% FWHM of reso- the instrument must digitally resolve 0.1% of the lution (0.5 nm) irradiance (radiance) produced by the laboratory Wavelength Stability: 5% FWHM of reso- standards used; typical irradiance (radiance) values lution (0.25 nm) for calibration using 1,000 W FEL standard lamps Signal-to-Noise Ratio: 1,000:1 (at minimum) traceable to the National Institute of Standards and Stray Light Rejection: 10−6 Technology (NIST), and required digital resolutions Radiometric Accuracy: 3% at these signal levels, are given in Table 4 as “Cali- Radiometric Stability: 1% bration Irradiance” and “Digital Resolution (cal.),” ◦ FOV Maximum: 10 (for radiance) respectively. A SNR of 100:1 requires a resolution ◦ −2 Temperature Stability: Specified for 0–35 C in Ed(z) at three optical depths to 0.005 µWcm Linearity: Correctable to 0.1% nm−1 per count, i.e., 2.5 digit resolution. At the −2 Ancillary Sensors surface, Ed(0) should be resolved to 0.05 µWcm −1 ◦ nm per count. Temperature: 0.2 C Pressure: 0.1% (full scale) 5. The Case-1 saturation values of Eu(0) represent Horizontal Inclination 1◦ over 40◦ range the Instrument Specification Subgroup’s estimate of maximum reflectances to be expected in ordinary In a single instrument, all channels at a given nominal Case-1 waters: 12.5% at 410 nm, 7.5% at 488 nm wavelength should match within 1 nm, if possible. It is and 0.5% at 670 nm. These saturation values will be desirable, therefore, to obtain all of the filters used by an too lowfor measurements in Case-2 watersor cocco- investigator for measurements at any nominal wavelength lithophore blooms. In these situations, a maximum (λn) from a single manufacturing lot when possible. If this expected reflectance of 40% for λ<660 nm and ≥ is done, Es(λn), Ed(λn), Eu(λn), Lu(λn), and any atmos- 20% for λ 660 nm is assumed. This implies that pheric radiometric quantities measured with that investi- the expected maximum irradiance in Eu(0) should − − gator’s systems, would all have a greater likelihood of be- be 80 µWcm 2 nm 1 for λ<660 nm and 40 µW ing measured over the same range of wavelengths, for each cm−2 nm−1 for λ ≥ 660 nm.

14 Mueller and Austin

Table 4. Required instrument sensitivities for SeaWiFS validation and algorithm development as a function of radiometric measured variable and wavelength. Property Variable 410 nm 488 nm 665 nm Comment Ed(z,λ), Ed(0) max 300 300 300 Saturation Irradiance 3 Downwelled Ed 1 1 1 Minimum Expected Irradiance Kd dE × −3 × −3 × −3 Irradiance dN 5 10 5 10 5 10 Digital Resolution (proﬁles) dE × −2 × −2 × −2 dN 5 10 5 10 5 10 Digital Resolution (surface unit)

Eu(z,λ), Eu(0)max 120 120 60 Saturation Irradiance (Case-2/coccoliths) Upwelled 37 22 1.5 Saturation Irradiance (Case-1) 3 −2 −2 −3 Irradiance Eu 1 × 10 2 × 10 1.5 × 10 Minimum Expected Irradiance Kd dE × −4 × −4 × −5 dN 5 10 5 10 5 10 Digital Resolution (surface unit) dE × −5 × −5 × −6 dN 5 10 5 10 5 10 Digital Resolution (proﬁles)

Lu(z,λ), Lu(0)max 24 24 8 Saturation Radiance (Case-2/coccoliths) Upwelled 7.5 4.5 0.3 Saturation Radiance (Case-1) 3 −3 −3 −4 Radiance Lu 2 × 10 4 × 10 3 × 10 Minimum Expected Radiance Kd dL × −4 × −4 × −5 dN 5 10 5 10 5 10 Digital Resolution (surface unit) dL × −5 × −5 × −6 dN 5 10 5 10 5 10 Digital Resolution (proﬁles)

Ecal, Source Ecal 2 5 15 Calibration Irradiance dE × −3 × −3 × −2 Irradiance dN 2 10 5 10 1 10 Digital Resolution (Ed, Es, Eu cal.)

Lcal, Source Lcal 0.6 1.5 4.5 Calibration Radiance dL × −4 × −3 × −3 Radiance dN 6 10 1 10 4 10 Digital Resolution (Lu cal.) −2 −1 −2 −1 −1 Notes: 1. Eu and Ed are in units of µWcm nm and Lu is in units of µWcm nm sr . 2. Responsivity resolution in radiometric units per digital count at the minimum required signal level. 3. Speciﬁed ranges should maintain a 100:1 SNR.

6. The minimum required irradiances at three opti- c) The dynamic range of the instrument’s linear sen- cal depths (as given in Table 4) assumes minimum sitivity must extend to include the signal levels en- reflectances of 1% at 410 nm, 2% at 488 nm, and countered during laboratory calibrations, and the 0.15% at 670 nm. calibration signals must be recorded with a digital resolution of 0.1% of reading to permit 1% uncer- 7. The saturation and minimum radiances, and radi- tainty in calibration. ance responsivity resolutions, for Lu(0) and Lu(z) at three optical depths are 0.2 times the correspond- In general, the above performance specifications do not pose exceptionally difficult engineering challenges, with the ing specification for E (0) or E (z). This assumes u u possible exception of the full dynamic range implied by Eu/Lu = Q = 5 at all wavelengths and depths. Case-2 or coccolith saturation radiance Lu(665) to mini- The specifications in Table 4 are meant as guidance to mum expected Lu(665). In any event, this situation will interpret the following required performance requirements: require specially designed radiometers (Section 3.1.8). It a) The instrument must maintain a 100:1 SNR at every is not necessary that every radiometer used for SeaWiFS validation operate over the full dynamic ranges given in operating range encountered, during field measure- Table 4. A radiometer is merely required to maintain the ments. above performance specifications over the dynamic ranges b) The data for measurements obtained in the field of irradiance and radiance existing at locations and associ- must be recorded with a digital resolution less than ated illumination conditions where it is used for SeaWiFS or equal to 0.5% of reading. validation or algorithm development.

15 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

3.1.3 Linearity and Stability 3.1.6 Operating Depth Errors attributable to linearity or stability should be The instruments shall be capable of operating to depths less than 0.5% of the instrumental readings over the dy- of 200 m. Depths should be measured with an uncertainty namic ranges specified in Table 4. This is a challenging of 0.5 m and a repeatability of 0.2 m for profiles in bands goal, but one which must be met if the equally challenging 1–6. goal of achieving 1% uncertainty in absolute calibration is to be meaningful. 3.1.7 Instrument Attitude The orientations of the instrument with respect to the 3.1.4 Sampling Resolution vertical shall be within ±10◦, and the attitude shall be ◦ Sampling frequency should be compatible with the pro- measured with orthogonally oriented sensors from 0–30 ± ◦ filing technique being used. For the preferred multispectral with an uncertainty of 1 in a static mode; it is not in- filter radiometers and spectroradiometric (dispersion) intended that this uncertainty be maintained while an instru- struments using array sensors, the minimum sampling fre- ment is subject to large accelerations induced by surface quencies are determined by the profiling rate and the depth waves. These data shall be recorded with the radiometric resolution required. In general, five or more samples per data stream for use as a data quality flag. meter should be obtained at all wavelengths. All channels of Ed(z,λ), Eu(z,λ), and Lu(z,λ) at all wavelengths 3.1.8 Red and Near-Infrared Wavelengths −2 should be sampled within 10 s at each given depth. The fact that the SeaWiFS red and near-IR channels— The time response of the instrument to a full-scale (sat- bands 6, 7, and 8 at wavelengths of 665, 780, and 865 nm, uration to dark) step change in irradiance should be less respectively—have such short attenuation lengths in water than one second to arrive at a value within 0.1%, or one requires that special attention must be paid to these mea- digitizing step, whichever is greater, of steady state. In surements. Problems due to instrument self-shading (Gor- addition, the electronic e-folding time constant of the in- don and Ding 1992) and very rapid attenuation of Lu(z,λ) strument must be be consistent with the rate at which the must be considered at these wavelengths. Large instru- channels are sampled, i.e., if data are to be acquired at ments, such as the standard Marine Environmental Ra- 10 Hz, the e-folding time constant should be 0.2 s to avoid diometer (MER) packages from Biospherical Instruments, aliasing. Individual data scans may be averaged to improve Inc. (BSI), are not adaptable to these measurements. signal-to-noise performance, provided adequate depth re- Suggested procedures for making the measurements are solution is maintained. to use either fiber optic probes carrying light back to a remote instrument, or very small single-wavelength discrete 3.1.5 Angular Response Characteristics instruments. Each of these concepts is adaptable to deployment from a small floating platform. Care must be The response of a cosine collector to a collimated light taken to avoid direct shading by the supporting platform, source incident at an angle θ from the normal must be such but at these wavelengths, the large attenuation coefficients that: of water makes shadowing by objects more than a few me- 1) for Eu measurements, the integrated response ters away irrelevant. to a radiance distribution of the form L(θ) ∝ The minimum measurement scheme would be two dis- 1+4sinθ should vary as cos θ accurate to within crete (10 nm FWHM) channels at 780 and 875 nm. Ad- 2%; and ditional channels at 750 and 850 nm, or more elaborately, high resolution spectroradiometry, would be useful in de- 2) for E measurement, the response to a colli- d termining the spectral distribution of the upwelling light mated source should vary as cos θ accurate to ◦ ◦ field in these bands. less than 2% for angles 0 <θ≤ 65 and 10% ◦ ◦ These measurements should be performed as part of for angles 65 <θ≤ 85 . the standard validation data acquisition, because of their Departures from cos θ will translate directly to approxi- importance in the atmospheric correction algorithms. It is mately equal errors in Ed in the case of direct sunlight. anticipated that in the majority of cases, and particularly The in-water FOV for upwelled radiance bands should in most Case-1 waters, these measurements will show neg- ◦ be approximately 10 (half-angle). The resulting solid an- ligible upwelling light. In Case-2 waters, cases of extremely gle FOV (approximately 0.1 sr) is large enough to provide high productivity, or in coccolithophore blooms, LWN (λ) reasonable levels of flux, using silicon detectors, yet small at these wavelengths may be significant, and these mea- ◦ enough to resolve the slowly varying (with θ, for θ<30 ) surements will become very important. field of upwelled radiance. Smaller FOV sensors are ap- When in-water measurements are performed at these propriate, of course, if all of the other performance speci- wavelengths, the deck cell channels should be expanded to fications are satisfied. include bands at 750 and 875 nm (Table 2).

16 Mueller and Austin 3.2 SURFACE IRRADIANCE only a single wavelength. If only a single channel deck radiometer is available, its spectral characteristic should The spectral irradiance at the ocean surface shall be closely match one of channels 2–5 with a 10 nm FWHM measured at wavelengths which correspond to the Sea- bandwidth. A broad-band, or photosynthetically available WiFS spectral bands (Table 2), but with 10 nm FWHM radiation (PAR), radiometer should never be used for this bandwidth. A total radiation pyranometer may provide purpose. helpful ancillary information, but this is not a required instrument. 3.3 ABOVE-WATER RADIOMETRY Instruments mounted aboard ships must be positioned to viewthe sky withminimum obstruction or reflections The performance characteristics to be specified for an from the ship’s superstructure, antennas, etc. Particular above-water ocean color radiometer will vary, depending care must be taken to prevent sun shadows from antennas on howa particular instrument is to be employed in Sea- falling on the irradiance collecting surface. Gimbal mount- WiFS validation experiments. For radiometric comparing of the deck sensor may be helpful to keep the surface isons with SeaWiFS and in-water measurements, the fun- of the sensor horizontal. Improperly designed gimbal sys- damental criterion to be met is that estimates of spectral tems, however, can accentuate fluctuations caused by ship normalized water-leaving radiance derived from shipboard motion, and if there is obvious oscillation in the measured or airborne measurements must have the same uncertainty irradiance, the gimballing should be improved to eliminate specified for those derived from in-water measurements of L (z,λ) (Table 4). A less accurate radiometer may be used the problem. u to semi-quantitatively characterize spatial variability near An intuitively attractive technique is to measure irra- ship stations. diance with a sensor floated a fraction of a meter below In general, the spectral characteristics of above-water the sea surface, far enough away from the ship to avoid radiometers should match those specified for L (λ)inTa- ship shadows. The flotation assembly should be designed u ble 2. In some cases, however, it may be acceptable for to avoid shadowing the radiometric FOV and to damp a radiometer to match the SeaWiFS specifications, which wave-induced motions. This type of arrangement has an specify center wavelength within 2 nm and 20 nm FWHM additional potential for supporting a small sensor to also bandwidth. Recalling the sensitivity of solar radiometry to measure upwelling radiance, Lu(λ), just belowthe surface. the exact center wavelength and detailed spectral response Unfortunately, the oceanographic community has only had function (Sections 3.1.1 and 4.1.2), any use of airborne very limited experience with this approach for measuring radiometers must quantitatively account for the different Es(λ) (Waters et al. 1990) and its attendant difficulties spectral responsivity functions between measurements of with wave-induced fluctuations in near-surface Ed. Addi- radiance by SeaWiFS, in-water radiometers, and above- tional research should be performed to evaluate the use of water radiometers at each channel’s nominal center wave- a floating surface unit as the potentially preferred method length. for measuring Es(λ) in future revisions to these protocols. A high-altitude imaging radiometer must have a radiometric uncertainty and SNR in all channels equal to those 3.2.1 Surface Radiometer Characteristics of the SeaWiFS instrument if its imagery is to be used for direct radiometric verification of SeaWiFS radiometric The specified number of channels and spectral charac- performance. In some cases, the requisite SNR may be teristics of deck cells are the same as those for subsurface realized through pixel averaging to a 1 km spatial resolu- irradiance measurements as shown in Table 2. Saturation tion commensurate with that of SeaWiFS. Direct radio- irradiances are the same as for Ed(λ) (Table 4). The dy- metric comparisons between aircraft and SeaWiFS radi- namic operating range for these sensors needs to only be ances, however, also require that the different atmospheric 25 db, with a SNR of 100:1 but must include the nominal path effects be carefully modeled, and that the uncertainty calibration irradiance (Table 4). Linearity must be within in those modeled adjustments be independently estimated. ± 0.5%. Sampling frequency should match the frequency This can be done most effectively when the aircraft mea- of the underwater radiometer, which should be 1 Hz or surements are combined with the full suite of shipboard faster, and all wavelengths should be sampled within an in-water, atmospheric, and ancillary measurements (Ta- −2 interval less than or equal to 10 s. Cosine response char- ble 1). In this case, direct comparisons between aircraft acteristics should give relative responsivity to a collimated and ship radiometry may require that both the SNR and source (in air) which matches cos θ, accurate within 2% the uncertainties realized in combined analyses of the two ◦ ◦ ◦ ◦ for 0 ≤ θ<65 , and within 10% for 65 ≤ θ ≤ 90 .If data sets will represent a smaller spatial resolution than a floating surface radiometer is used, its cosine response the nominal 1 km instantaneous field-of-view(IFOV) for and immersion characteristics must meet the same specifi- SeaWiFS. cations as those for profiling irradiance meters. Performance characteristic specifications are similar for For some oceanographic process studies, it may be ac- ocean color radiometers used to measure water-leaving ra- ceptable to use a radiometer system measuring Es(λ)at diance from either the deck of a ship or an aircraft flown

17 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 at lowaltitude, i.e., 200 m altitude or lower. Radiomet- 3.4 IOP INSTRUMENTS ric characteristics should match the criterion set forth for The IOPs are: in-water Lu(λ) radiometers in Sections 3.1.1–3.1.4 and Ta- bles 2–4. The instrument FOV should be between 5◦ and 1) the beam attenuation coefficient, c(z,λ), in units −1 10◦ (full angle), and all wavelengths must be coregistered of m ; within 10% of the IFOV. All channels must be scanned 2) the absorption coefficient, a(z,λ), in units of −1 simultaneously, or within less than 10−2 s (depending on m ; and the digitizing design), to avoid aliasing due to varying wave 3) the volume scattering function, b(z,λ0,θ), in − − reflectance in shipboard measurements, and to avoid time- units of m 1 sr 1. space aliasing in airborne measurements. This constraint The integral of the volume scattering function over 4π precludes use of filter wheel radiometers and others which steradians is the total scattering coefficient, b(z,λ), with scan channels sequentially over a time interval greater than units of m−1. The integral of the volume scattering func- 10−2 s. Sampling over longer periods of time may be done tion over the back hemisphere is the backscattering coeffi- −1 by either electronic integration of all channels simultane- cient, bb(z,λ), with units of m . ously, or by averaging multiple scans. It will be possible to measure the spectral attenuation A radiometer’s sensitivity to the polarization of aper- and absorption coefficients in situ at the time of SeaWiFS ture radiance is critical for ocean color remote sensing ap- deployment. The instruments for the measurement of the plications. Polarization sensitivity is likely to be present spectral absorption and attenuation coefficients should, at in any radiometer having mirrors, prisms or gratings in a minimum, have the characteristics given in Table 5. its optical path. To measure accurate water-leaving radi- Spectral resolution at more than SeaWiFS wavelengths ances using instruments of these types, it is necessary to would be desirable to deduce pigment concentrations. In depolarize aperture radiance using either fiber-optics or a the case of beam attenuation coefficients, the requirements for uncertainty and precision correspond to changes in c(λ) pseudo-depolarizer. Shipboard and airborne ocean color resulting from changes in concentration of approximately radiometers must have a polarization sensitivity of less 5 and 2 µg l−1 of suspended mass, respectively. Stability than 2% in all channels. The sole exception to this rule should be tested with instruments connected to the data will occur in the case of instruments designed to actually acquisition system. Stability with time should be better measure the polarization components of aperture radiance, than 0.005 m−1 between calibrations. e.g., the polarization channels of the French Polarization and Directionality of the Earth’s Reflectances (POLDER) Table 5. Minimum instrument characteristics for instrument. the measurement of the spectral absorption and at- Each application of a particular above-water radiome- tenuation coefficients. ter system, if it is proposed for SeaWiFS validation, must Instrument Characteristics be evaluated on its own merits. The instrument’s respon- Spectral Resolution: 410, 443, 490, 510, sivity, uncertainty, stability, FOV, and spectral character- 555, and 670 nm istics must be evaluated in the context of the models to be Bandwidth: 10 nm used to compare its radiance measurements to in-water, or Uncertainty: 0.005 m−1 SeaWiFS, radiance measurements. The suitability of spa- Precision for λ<650 nm: 0.002 m−1 tial averaging to improve SNRs must be evaluated in terms Precision for λ ≥ 650 nm: 0.005 m−1 of the spatial variability prevailing in the experiment site, Stability with 0.005 m−1 over particularly when in-water and aircraft radiances are to Temperature: 0–25◦ C be directly compared. Finer resolution aircraft imagery, Sampling Interval: ≥ 4 samples m−1 or low-altitude trackline data, will often be essential for Source Collimation Angle: ≤ 5 mrad determining the validity of attempts to directly compare Detector Acceptance Angle: ≤ 20 mrad in-water and SeaWiFS radiances measured at a particular Depth Capability: 200 m site. In summary, airborne and shipboard above-water ra- The spectral total scattering coefficient cannot be mea- diometry can obviously contribute extremely valuable data sured directly. It can be obtained from b(λ)=c(λ − a(λ), for validating the radiometric performance of the SeaWiFS provided c(λ) and a(λ) are determined with the appropri- instrument and the algorithms employed with SeaWiFS ate uncertainty. The spectral backscattering coefficient, data. There is, however, a wide possible range of radiome- bb(λ), has the same requirements for spectral resolution, ter characteristics that can be applied to this program, and bandwidth, and linearity as a(λ) and c(λ). Since bb(λ)is detailed specification of required characteristics can only not a transmission-like measurement, however, the uncer- be done in the context of each particular experiment’s de- tainty of its determination will be approximately 10%. sign. Only the guiding principals and desired end-to-end The shape of the volume scattering function can, at performance are specified here. present, be determined in situ only crudely with devices

18 Mueller and Austin like the ALPHA and Scattering Meter (ALSCAT) and the tral characteristics of the sky radiance camera channels General Angle Scattering Meter (GASM), which were built are those specified for Es(λ) (Table 2). Data should be more than a decade ago at the Visibility Laboratory of in a format such that absolute radiance values can be ob- the Scripps Institution of Oceanography. These are single tained with an uncertainty of 5% and sky irradiance can angle measurement devices, which must be scanned as a be determined from integrals of the data to within 10%. If function of angle and wavelength. Because measuring scat- the dynamic range of the camera is insufficient to capture tering with these instruments is a slow process, they do not both the sun and sky distribution, neutral density filters lend themselves readily to incorporation into other instru- (or some other method) should be used so that radiance ment platforms. Since it will be possible to independently from both the sun and sky can be measured. determine b(λ) and bb(λ) when SeaWiFS is deployed, the determination of the shape of the volume scattering coeffi- 3.7 PHYTOPLANKTON PIGMENTS cient could possibly be determined with acceptable uncertainty by means of measurement of a fewmoments of the HPLC equipment and associated standards must con- scattering function. A newinstrument development effort form to protocols specified in Chapter 9 of the JGOFS would have to be initiated to pursue this approach. Core Measurement Protocols (JGOFS 1991). In situ chlorophyll fluorometers should have a resolution of at least 3 3.5 ATMOSPHERIC AEROSOLS 0.001 mg of chlorophyll a per m . Sun photometers should be used to measure atmos- 3.8 HYDROGRAPHIC PROFILES pheric aerosol optical thickness. These sun photometers should have specifications in agreement with the World A calibrated CTD system should be used to make pro- Meteorological Organization (WMO) sun photometer spec- files to maximum depths between 200 and 500 m. The ifications (Frohlich 1979). Specifically, the instruments instrument should meet the minimum specifications given should have a 2◦ FOV, temperature stabilization, and a in Table 6. ± precision of 0.01%. The specific wavelengths of channels Table 6. The minimum instrument characteristics should correspond to the recommended WMO wavelengths for the measurement of hydrographic profiles are of 380, 500, 675, 778, and 862 nm. For SeaWiFS valida- listed. tion, additional channels at 410, 440, 490, 510, and 555 nm Parameter Range Uncer- Resolu- should be added to the WMO set. tainty tion 3.6 SPECTRAL SKY RADIANCE Pressure [dbars] 0–500 0.3% 0.005% Temperature [◦C] −2–35 0.015◦ C 0.001◦ C Measurements of spectral sky radiance distribution Salinity [PSU] 1–45 0.03 PSU 0.001 PSU should be made using a photoelectric all-sky camera. Spec-

19 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Chapter 4

Sensor Characterization

Introduction

This chapter details procedures to characterize the sensor performance specifications prescribed in Chapter 3. Procedures for characterizing environmental radiometers are presented in Section 4.1, including special characteristics of underwater radiometers. Section 4.2 describes radiometric standards used as a basis for absolute responsivity characterization of radiometers. Special consideration for radiometers measuring LW (λ) above water, either from a ship or aircraft, are discussed in Section 4.3. The remaining Sections, 4.4–4.8, address calibrations of IOP instruments, sun photometers, radiance distribution cameras, HPLC and fluorometric systems, and CTD profiles.

4.1 RADIOMETRY 4.1.1 Absolute Radiometric Calibration The characterization of radiometric instruments used Determination of the absolute radiometric responses of for the acquisition of field data for SeaWiFS validation and the irradiance and radiance sensors requires the availability algorithm development shall include the determination of of a properly manned and equipped radiometric calibration those instrument characteristics that affect its calibration facility. Such a facility must be equipped with suitable as used in the field environment. In addition to the ob- stable sources and sensors, e.g., lamp standards of spec- vious radiometric calibration, it is therefore necessary to tral irradiance and flat response radiometers, respectively. determine: Either the sources or the sensors must have defined spectral radiometric characteristics that are traceable to NIST a) the spectral sensitivities of the various measure- (Section 4.2). The calibration facility must also have a va- ment channels; riety of specialized radiometric and electronic equipment, b) the angular sensitivities of an irradiance or ra- including: reflectance plaques, spectral filters, integrating diance sensor in the medium, i.e., air or water, spheres, and highly regulated power supplies for the opera- in which it is to be used; tion of the lamps. Precision electronic measurement capabilities are also required, both for setting and monitoring c) the temporal response of the system; lamp current and voltage and for measuring the output of d) the effects on responsivity caused by water im- the radiometer. mersion; and It is not expected that every investigator will be able e) the effects of temperature and pressure on the to perform his own radiometric calibrations; because of above characteristics. this, a fewcentrally located facilities willbe equipped and staffed to perform these calibrations as a routine service for The elements of radiometer characterization and calibra- the community. The facilities will perform frequent inter- tion are outlined schematically in Fig. 1. comparisons to assure the maintenance of the radiometric For any instrument to provide suitable data for SPO traceability to the NIST standard of spectral irradiance. use, the investigator must be certain that the instrument The goal shall be to provide reproducible calibrations from characterization has not changed beyond accepted limits 400–850 nm to within better than ±1%; the minimum re- and that the time history of the calibration is traceable. quirement for radiometric data to be used in SeaWiFS val- Certain attributes, e.g., angular response characteristics, idation is for repeatable calibrations within less than 5%. are sufficiently constant that they only need to be deter- Radiometric calibrations of irradiance sensors will be mined once, unless the instrument is modified. The ex- performed after it has been ascertained that: the confor- act nature of instrument modifications during maintenance mity of the sensor angular response to the required cosine will determine which characterization procedures must be function is satisfactory, the sensor linearity is satisfactory, repeated. On the other hand, radiometric calibrations and and the spectral sensitivity, including out-of-band block- the assessment of system spectral characteristics of filter ing, is known and satisfactory. radiometers, must be repeated before and after each major Radiometers shall be calibrated using a 1,000 W FEL field deployment. standard of spectral irradiance, with calibration traceable

20 Mueller and Austin

Recurring Initial System Instrument Class Characterizations Characterizations Characterizations

Spectral Radiometric Temperature Radiometric Collector Immersion Bandpass Sensitivity Sensitivity Linearity Pressure Factors Sensitivity Central λ Radiance Ancillary Geometric FWHM Calibration Sensors Orientation

Scan/Array Irradiance Calibration

Stray Signal-to- Light Noise Ratio

Electronic Pressure Interface Sensor Fig. 1. Elements of radiometer characterization and calibration. to NIST and lamp operation in accordance with Walker known Lambertian reflectance. The plaque is nor- et al. (1987). The irradiance collector is placed normal to, mal to, and centered on, the lamp calibration axis. and at the prescribed distance from, a working standard The radiance sensor is positioned to viewthe plaque lamp of spectral irradiance. The lamp should be of appro- at an angle of 45◦ from the plaque normal (any other priate size to provide an irradiance at the sensor that will angle at which the diffuse reflectance of the plaque be at least 30%, and preferably above 50%, of full-scale for is known is acceptable also). It must be established the sensor channel being calibrated, although this is not that the plaque fills the sensor’s FOV and that the always achievable in practice (Table 4). The lamp-sensor presence of the sensor case has not perturbed the space shall be appropriately baffled and draped so that oc- irradiance on the plaque. The instrument response culting the direct path between lamp and sensor will result and dark signal is recorded. It must be verified in a response of less then 0.1% of the response to the lamp that the plaque fills the FOV with uniform radi- flux. ance for each channel of a multichannel radiance For multispectral instruments, all channels may be cal- sensor. Separate calibration setups may be required ibrated simultaneously if sufficient flux is available at all for different channels and the lamps may have to wavelengths. The instrument response is recorded for all be moved as much as 3 m away from the plaque to channels together with associated dark responses. Am- assure uniform illumination. This procedure is dif- bient and photosensor temperatures are recorded, where ficult to apply to sensors with a large FOV. available. For characterization, the radiometric calibration 2. An integrating sphere with an exit port of sufficient − ◦ should be performed at temperature extremes of 2 C and size to fill the FOV of the radiance sensor may be ◦ − ◦ ◦ 40 C for in-water sensors, and at 10 C and 45 C for used if the radiance of the exit port, at the chan- irradiance sensors used above the surface. If responses dif- nel wavelengths, can be determined with sufficient fer significantly at temperature extremes, responses should uncertainty. also be determined at intermediate temperatures. These methods are discussed more fully in Section 4.2.2. The portable irradiance and radiance reference standard to be used to trace instrument stability during field deployments (Section 4.2.5) should be placed in position 4.1.2 Spectral Bandpass Characterization on the sensor immediately following the calibration to es- These instruments should be characterized to define the tablish the instrument response to this reference unit. nominal wavelengths and bandwidths, defined as the full Radiance calibration activities require a uniform source width of the passband as measured to the FWHM intensity of known radiance that will fill the angular field of view of points. The nominal, or center wavelength, will usually be the radiance sensor. The two procedures that may be used defined as the wavelength halfway between wavelengths at are given below. which the normalized response is 0.5, and the channel is Calibration Methods characterized by this wavelength and the FWHM band- 1. A working lamp standard of spectral irradiance is width. The determination of the spectral response func- placed at the prescribed distance from a plaque of tion, i.e., the passband, will be made for each channel with

21 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 a scanning monochromatic source, with a bandwidth less Consideration must also be given to unblocked fluores- than 0.2 nm; the source output must be normalized to a cence by the filters, or other optical elements, as a possi- detector of known spectral sensitivity. The response func- ble source of light leaks. Methods to test for fluorescence tion thus measured is then normalized to the maximum contamination specifically, are not well established at this (peak). time. Although the results of this characterization are usually While leakage of blue light into red channels is the most represented by only the nominal wavelength and FWHM significant oceanographic optical problem, the leakage of bandpass, the full normalized response function should red and IR light into blue channels can cause significant be recorded for use in detailed wavelength adjustments errors when the instrument is calibrated using a red-rich and comparisons with the SeaWiFS channel response func- source. A convenient way to measure this leakage is to tions, which will not be known until shortly before launch. place a long wavelength-pass, sharp-cut, absorbing glass It is further recommended that the internal instrument filter that does not exhibit fluorescence between a broad temperature be monitored during these tests, and that band (e.g., incandescent) source and the sensor. A non- the test be repeated at two temperatures at least 15◦ C zero response indicates unwanted out-of-band red response apart, e.g., 10◦and 25◦ C. If a significant shift, greater than and the need for improved red blocking. 1.0 nm, with temperature of either the center wavelength or bandwidth is detected, then additional temperature cal- 4.1.3 Temporal Response ibration points are recommended. Dark offsets must be recorded during each test. The temporal response of a spectrometer may be ex- For spectral characterizations of irradiance diffusers, amined by introducing a step function of near full-scale the entire surface of the diffuser should be illuminated by flux to the system using an electrically operated shutter the monochromator’s output. In the case of radiance de- and measuring the system’s transient response at 0.1 s, or tectors, a diffuser should be used to diffuse the monochro- shorter, intervals. The response should be stable within mator slit image and uniformly fill the instrument’s FOV. one digitizing step, or 0.1%, whichever is greater, of the The wavelength response of a monochromator-based steady state value in one second or less. radiometer is calibrated by scanning over line sources, with sharp peaks at well known wavelengths. Suitable spectral 4.1.4 Radiance Field-of-View calibration sources, such as, mercury, cadmium, and neon It is required that the radiance FOV of the instrument lamps, are provided by several vendors, together with tab- be known. The FOV should not normally enter into the ulations of the wavelengths of the emission lines generated absolute calibration, however, if the FOV is fully filled by by each source. a calibration source of uniform radiance. The width of the slit function of a monochromator may In this test, the instrument is placed on a rotational be estimated by scanning over a laser line, e.g., helium- stage with the entrance aperture of the radiometer over the neon, at a very small wavelength interval. The instrument rotation axis. A stable light source with a small filament FOV must be filled during the test. is placed several meters in front of the instrument, which It is anticipated that the monochromator-based spec- is then scanned from −30◦ to +30◦ in 2◦ increments. The tral characterization will not be able to adequately measure angle positioning should be within ±0.1◦. The on axis, 0◦, leakage of broadly distributed out-of-band radiation; there- mechanical alignment is made using the window surface fore, blocking of blue light in channels longer than 540 nm as reference, by adjusting to get the reflection of the lamp must be routinely tested. Where continuous wave (CW) filament to return on axis. The error in this alignment argon lasers are available, out-of-band response should be is approximately ±0.1◦. The in-air measurement angles, measured at 488 nm. One recommended test that can be θa, are converted to corresponding angles in seawater, θw, performed during the absolute calibrations at λ ≤ 640 nm using the relation θw = θa/nw, where nw is the index of is the sequenced measurement of three Schott BG-18 fil- refraction of seawater at the particular wavelength of each ters, each 1 mm thick, using an FEL-type light source. The channel. procedure is to measure the channel signal using each filter separately, then in combination, and comparing the com- 4.1.5 Collector Cosine Response puted and measured transmissions. If a significantly higher combined transmission of the three filters, when they are The directional response of cosine collectors must be used in combination, is measured relative to the calcu- characterized. The directional response of the deck cell is lated transmittance, then spectral leakage is present. At determined in air, and the in-water instruments are mea- wavelengths greater than 640 nm, other filters that atten- sured immersed in water. Full spectral determinations are uate the wavelength of interest, with a transmission value required. For instruments measuring upwelling irradiance of less than or equal to 0.1 and which pass shorter wave- Eu(z,λ), it is recommended that the cosine response of length light with significantly greater transmission, should each instrument be measured individually. For downwell- be substituted for the BG-18. ing irradiance Ed(z,λ) instruments, checking a production

22 Mueller and Austin run may be satisfactory if the vendor’s material and design the beginning, the middle, and the end of each run and are demonstrated to be uniform throughout the duration examined as a measure of lamp and instrument stability of the run. over the time involved. At least two runs should be made Absolute responsivity calibration of an irradiance me- about different axes through the surface of the diffuser. ter is done in air, using light incident normal to the collec- All readings are normalized to 1.000 at θ =0◦ and then tor. To properly measure irradiance incident on the plane compared with the value of the cosine of each angle. If at all angles θ (relative to the normal), the instrument’s V (θ) is the normalized measured value, relative local error response should followa cosine function. In other words, at angle θ is given as (V (θ)/cos θ) − 1. for an instrument response V (0) to a given collimated ir- Assuming the average response to the four measure- radiance incident at θ = 0, if the instrument is rotated ments made at each θ (four separate azimuth angles φ) to the angle θ away from the original normal axis, the re- adequately represent the overall mean cosine response of sponse should be V (θ)=V (0) cos θ. If this requirement is the collector, then the error, , in measuring irradiance met, then the on-axis calibration is sufficient and the de- over the interval θn <θ<θN for a uniform radiance dis- vice will correctly measure irradiance arriving at the plane tribution is approximately of the collector, regardless of the directional distribution at which the light arrives. N The preferred irradiance collector design has an im- V (θi) sin θi ∆θ proved cosine response over that of a simple flat plate dif- = i=n − 1, (1) N fuse collector (Boyd 1955 and Tyler and Smith 1979). This cos θi sin θi ∆θ improvement is mostly for near-grazing angles (θ approach- i=n ing 90◦ to the normal) and is particularly important when measurements of the upwelling underwater irradiance are using a simple trapezoidal quadrature. Similarly, for a made, i.e., with the collector facing downward. In that radiance distribution of the form 1 + 4 sin θ, to simulate case, most of the light is from the sides, in the region of upwelled irradiance these near-grazing angles. Since E (z,λ) measurements are to be made underwa- N d V (θ )(1+4sinθ ) sin θ ∆θ ter, the testing to determine the fidelity of the instrument i i i = i=0 − 1, (2) to the cosine function must be made with the instrument N submerged. A description of the suitable experimental pro- cos θi (1 + 4 sin θi) sin θi ∆θ cedure follows Petzold and Austin (1988). i=0 The instrument is suspended in a tank of water while where θ =0,θ = π and ∆θ = π . supported by a fixture designed to allowrotation about 0 N 2 2N The asymmetry of the cosine response, δ, is equivalent an axis through the surface and center of the collector. A to an effective tilt of an ideal cosine collector with respect tungsten-halogen lamp with a small filament is enclosed in to the instrument’s mechanical axis, which can be quanti- a housing with a small exit aperture and placed approximately 1 m from a large window in the tank. The collec- fied as θ2 tor is placed approximately 25 cm behind this window; an cos(θ + θt) sin θdθ equivalent lamp distance of 1.25 m or more is required. A θ1 δ = , (3) circular baffle should be placed immediately in front of the θ2 − window to reduce stray light. The water should be highly cos(θ θt) sin θdθ θ filtered to the extent that the effects of scattered light are 1 indiscernible. where θt is the tilt angle. The equivalent air path lamp distance should be ap- The measured asymmetry is computed as the ratio of proximately 1.25 m or greater. At this distance, the fall- sums of measurements at opposite φ (θ ≥ 0) and −π (θ< off at the outer edge of a 6 cm diameter diffuse collector 0) in the same plane, that is, would be 0.9994, or −0.06%, when the diffuser is at θ =0◦ with the normal. The net effect over the entire area of θN=π/2 ◦ the diffuser would be 0.9997 or −0.03%. When θ =90, V (θi, 0) sin θi ∆θ i=0 − with the diffuser edge-on to the lamp, the distance to the δ = − 1, (4) θN =π/2 lamp varies for different points on the surface. The net V (θi) sin θi ∆θ error over the entire surface for this condition is 0.99997 i=0 or −0.003%. All other angles fall between these limiting ± π cases. for ∆θ = 2N . The signals from the instrument are recorded for θ =0◦ Variations in asymmetry from channel to channel may and at 5◦ intervals to θ = ±75◦ and 2.5◦ intervals over be due to the placement of the individual detectors behind 75◦ <θ<90◦. The readings at θ =0◦ are recorded at the diffuser. Any offset of the average asymmetry with the

23 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 mechanical axis could be due to any one of a variety of each carefully measured depth. A final reading is taken causes: with the water level below the collector, i.e., with the col- 1) the alignment on the rotating test fixture not lector in the air. The amount of energy arriving at the being correct, collector varies with the water depth and is a function of several factors: 2) tilt of the diffuser, a) the attenuation at the air-water interface, which 3) the detector array not being centered, varies with wavelength; 4) nonuniformity of the reflectance of the internal b) the attenuation over the water pathlength, which surfaces of the instrument between the diffuser is a function of depth and wavelength; and and the sensor array, or 5) nonuniformity of the diffuser. c) the change in solid angle of the light leaving the source and arriving at the collector, caused by the light rays changing direction at the air-water 4.1.6 Immersion Factors interface, which varies with wavelength and wa- When a plastic, opal-glass, or Teflon diffuser is im- ter depth. mersed in water, its light transmissivity is less than it was Using Fresnel reflectance equations, the transmittance in air. Since an instrument’s irradiance responsivity is cali- through the surface is brated in air, a correction for this change in collector transmissivity must be applied to obtain irradiance responsivity 4n (λ) w coefficients for underwater measurements. Ts(λ)= 2 , (5) 1+n (λ) The change in a collector’s immersed transmissivity is w the net effect of two separate processes: a change in the where n (λ) is the index of refraction of the water at wave- reflection of light at the upper surface of the collector, and w length λ. internal scattering and reflections from the collector’s lower The transmittance through the water path, T (λ), will surface. A small part of the light flux falling on the col- w be lector is reflected at the air-plastic, or water-plastic, inter- −K(λ) z face, and the majority of the flux passes into the collector Tw(λ)=e , (6) body. The relative size of this reflectance, called Fresnel where K(λ) is an attenuation coefficient of the water and reflectance, depends on the relative difference in refractive z is the path length in corresponding units. indices between the diffuser material and the surrounding The change in solid angle with water depth z is given medium. by the factor The refractive index of the collector material is always larger than that of either water or air, and because the −2 z 1 refractive index of water is larger than that of air, Fresnel G(z,λ)= 1 − 1 − , (7) reflectance is smaller at a diffuser-water interface than at d nw(λ) a diffuser-air interface. Thus, the initial transmission of light through the upper surface of an irradiance collector where d is the distance of the lamp source from the collec- is larger in water than in air. The immersed upper sur- tor surface. face is, on the other hand, also less effective at reflecting The immersion correction factor Fi(λ) for irradiance is the upward flux of light backscattered within the diffuser then calculated for each depth z as body and light reflected at the lower diffuser-air interface in the instrument’s interior, processes that are not affected Ea(λ) Fi(λ)= Ts(λ) Tw(z,λ) G(z,λ), (8) by immersion. Therefore, a larger fraction of the internally Ew(z,λ) scattered and upwardly reflected light passes back into the water column than would be lost into air. Because the in- where Ea(λ) and Ew(z,λ) are the irradiance in air and the creased upward loss of internally reflected flux exceeds the irradiance underwater at depth z, respectively. gain in downward flux through the diffuser-water interface, There are two unknowns in (5)–(8): the attenuation co- the net effect of these competing processes is a decrease in efficient of the water K(λ) and the immersion factor Fi(λ). the collector’s immersed transmissivity. A minimum of three measurements must be made to solve To measure this effect, a suggested and acceptable pro- for Fi(λ): one in air to get Ea(λ), and two at different cedure is as follows: The instrument is placed in a tank of water depths for Ew(z,λ). The recommended method is water with the irradiance collector level and facing upward. to take readings of Ew(z,λ) at many depths. Then, using A tungsten-halogen lamp with a small filament, powered the exact form of (8), a least-squares regression is solved by a stable power supply, is placed at some distance above for the Fi(λ) and K(λ) terms giving the best fit. The com- the water surface. The depth of the water is lowered in plete derivation of (5)–(8) is given in Petzold and Austin steps and readings are recorded for all wavelengths from (1988).

24 Mueller and Austin

The absolute calibration for the spectral radiance chan- 4.1.7 Linearity and Electronic Uncertainty nels is found by viewing a surface of known radiance in air in the laboratory. When the instrument is submerged in The linearity of the radiometric channels must be de- water, a change in responsivity occurs and a correction termined over their expected range of use. The above- must be applied. This change in responsivity is caused by surface (deck cell) and underwater irradiance sensors in- the change in the indices of refraction of the different me- tended for the measurement of downwelling irradiance have dia in which the instrument is immersed—in this case air full-scale (saturation) values that are not readily obtained and water. Two optical changes occur, both of which are with the usual incandescent blackbody sources, such as caused by the change in refractive index. The two effects 1,000 W 3,200 K tungsten-halogen projection lamps. The to be corrected are: linearity at the high end of the calibrated range may be determined by using 900–2,000 W high pressure xenon arc 1) the change in transmission through the inter- lamps, which provide a small, stable source of high inten- face between the air and the window during cal- sity (approximately 6,000 K) radiation. With such lamps, ibration, and the same effect through the water- irradiance levels approximating full sunlight can be at- window interface during data measurement, and tained. Using such sources for the high end, and the more 2) the change in the solid angle included in the un- easily managed tungsten-halogen lamps over the range be- derwater FOV relative to that in air. low20–30% of full scale, the linearity of the response char- Since the refractive index of seawater, nw(λ), is a function acteristic of the radiometric channels can be assessed. The of wavelength λ, the correction factor Fi(λ) will also be a flux should be changed in 5 db (0.5 log), or less, steps using function of wavelength. a proven and accepted procedure for controlling irradiance If the refractive index of air is assumed to be 1.000 at such as inverse square law, or calibrated apertures. These all wavelengths, and if ng(λ) is the index of refraction for suggested procedures for testing linearity at the higher lev- the (glass) window and nw(λ) is the index of refraction for els are not well established in practice, and research is water, then, as shown in Austin (1976), the correction for needed to determine the precision which can be attained. the change in transmission through the window, Tg(λ), is If departures from linearity are found, they must be incorporated into the calibration function for the instrument 2 n (λ)+n (λ) and be properly applied to the rawlevel -1 data to obtain w g Tg(λ)= 2 , (9) calibrated level-2 irradiance and radiance data. Level-1 nw(λ) 1+ng(λ) and level-2 data are defined in Section 6.1. It is recommended that all instruments utilizing inputs and the correction for the change in the FOV, Fv,is from ancillary sensors, e.g., transmissometers, be characterized for the linearity and uncertainty of the voltage mea- 2 Fv(λ)= nw(λ) . (10) surement covering the full output range of the ancillary sensor. For instruments with range dependent gain chang- The index of refraction of a PlexiglasTM window, ng(λ), ing, either manual or automatic, the scale offset and linear- may be computed using an empirical fit to the Hartmann ity for each range should, at a minimum, be tested annu- formula, that is, ally. Uncertainties exceeding 0.1% of any reading within the normal working range must be investigated and cor- 7.5 rected. n (λ)=1.47384 + , (11) g λ − 174.71 Other characteristics of electronic sensor systems may adversely affect measurement uncertainty. During the de- where λ is the wavelength in nanometers (Austin 1976). sign and engineering prototype development of a radiome- The index of refraction for seawater nw(λ) may be similarly ter, the design and implementation must be analyzed to computed using an empirical fit of the data from Austin characterize, and correct as needed, possible effects of hys- and Halikas (1976), teresis, overload, recovery times, cross talk between either optical transducers or electronic channels, and sensitivity 6.6096 n (λ)=1.325147 + . (12) to orientation in the Earth’s magnetic field, which is par- w λ − 137.1924 ticularly likely with photomultiplier tubes.

The immersion factor Fi(λ) is then obtained as 4.1.8 Temperature Characterization Two major types of temperature-induced variation may Fi(λ)=Tg(λ) Fv(λ) 2 be seen in an optical radiometric instrument: 1) oﬀset or n (λ) n (λ)+n (λ) (13) w w g dark changes, and 2) scale responsivity changes. Each = 2 . 1+ng(λ) underwater instrument must be individually characterized over the range of −2–40◦ C. In the case of deck cells, the

25 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 temperature range for testing should be extended to −10– or both (Section 4.2.2). An ongoing series of SeaWiFS In- ◦ 45 C. Sensors exhibiting temperature coefficients greater tercalibration Round-Robin Experiments (SIRREXs) has ◦ than 0.01% per C over this temperature range, should be been initiated by the SPO to assure internal consistency fully characterized over their respective ranges to estab- between the laboratories which calibrate radiometers for lish the means and precision with which post-acquisition SeaWiFS validation (Mueller 1993 and Mueller et al. 1994). processing can be used to correct for temperature dependency. Although knowledge of the zero, or dark current, 4.2.1 Lamp Irradiance Standards drift is essential for working at the lowest radiances or irradiances, it should be emphasized that more significant The options available for radiometric calibration stan- near-surface errors may be induced by temperature varia- dards are limited to standard sources or standard detec- tions in responsivity. These possible responsivity changes tors. Lamp standards of spectral radiance and irradiance must be individually determined across the spectrum. are provided by NIST and various commercial standard- In the above discussion, the temperatures cited are en- izing laboratories and manufacturers who furnish NIST vironmental temperatures, but it should be emphasized traceable secondary standards. The uncertainty cited for that any correction must use the temperature of the af- these standards by NIST is, at best, 1% in the visible and fected element, which is normally in the interior of the 3% is a more realistic estimate of absolute uncertainty at- instrument. This is best accomplished by routinely using tainable using lamp standards alone. Over the calibration temperature sensors placed at critical locations within the range from 250–2,500 nm, the uncertainty is approximately instrument. For highest precision, dynamic temperature 6% at the endpoints. testing involving temporal transients, as well as possible The lamp standard of spectral irradiance is tradition- temperature gradients within an instrument, may be ap- ally used for radiometric calibration, mainly because of its propriate. ease of use compared to the spectral radiance lamp. NIST publishes guidelines for the setup, alignment, and use of 4.1.9 Pressure Effects these standards. The vendors that manufacture and calibrate these lamps also issue guidelines for their use. Pressure can cause radiometric measurement errors by deforming irradiance collectors. Pressure coefficients asso- 4.2.2 Radiance ciated with polytetrafluoroethylene (PTFE) based irradiance diffusers are known to exist, but they are not uniform Spectral radiance may be obtained by using an irradi- and there may be hysteresis effects. It is recommended ance standard lamp and a Lambertian reflecting plaque. that each type of irradiance detector be examined for vari- The standard lamp is positioned on axis and normal to ations in responsivity with pressure. If a significant effect the center of the plaque at the calibrated distance. The instrument or detector package to be calibrated is nomi- is observed, then pressure-dependent responsivity coeffi- ◦ cients should be determined separately for each instrument nally positioned to viewthe plaque at 45 measured from and collector. The pressure characterization should also the axis. The radiance, then, is given by test for, and quantify, hysteresis and temporal transients 1 in responsivity under a time varying pressure load. The L(λ)= ρ(λ)E(λ), (14) characterization of pressure effects has not previously been π common practice, and the requisite procedures are there- where ρ(λ) is the bidirectional reflectance of the plaque for fore poorly defined; newprotocols must be developed. 0◦ incidence and 45◦ viewing, E(λ) is the known spectral irradiance from the lamp during calibration and the total 4.1.10 Pressure Transducer Calibration FOV of the instrument being calibrated is filled by the The radiometer’s pressure transducer, which is used to illuminated plaque. measure instrument depth during profiles, should be tested The known radiance of the plaque provides an uncer- and calibrated before and after each major cruise. tainty comparable with that of the irradiance standard lamp, i.e., less than or equal to 3%, for calibrating a ra- ≈ ◦ 4.2 RADIOMETRIC STANDARDS diance detector with a very narrow FOV ( 1 ). Large plaques, e.g., 40 cm2, have been successfully used to cali- This section describes sources and methods by which brate radiance sensors having up to 25◦ full-angle FOVs. the NIST scale of spectral irradiance is transferred to cali- Intercomparisons of calibrations on underwater radiance brate irradiance and radiance sensors. The principal work- sensors (possessing in-air full-angle FOVs ranging from 20– ing standards used for spectral irradiance responsivity cal- 24◦), made using this technique at different laboratories, ibration are FEL lamp working standards (Section 4.2.1). have generally agreed within approximately 5%. The spectral irradiance scales of the FEL lamps are in A better approach to calibrating multispectral radiance turn transferred to spectral radiance scales using plaques sensors is to viewan integrating sphere that is uniformly il- of known bidirectional reflectance, or integrating spheres, luminated by stable, appropriately baffled lamps, and that

26 Mueller and Austin also has an exit port large enough to completely fill the sen- A self-calibrating radiometer may be used directly to sor’s FOV. The sphere and exit port must be large enough calibrate and map the radiance distribution of integrating to place the radiance sensor far enough away to prevent sig- sphere sources (Method 2 in Section 4.2.2 above). The self- nificant secondary illumination of the sphere walls due to calibrating radiometer standard of radiance may be trans- retro-reflection off the sensor’s entrance optics; if the sensor ferred to a stable lamp source of irradiance through the re- is too close, the retro-reflected light will both increase and versal of the reflectance plaque technique, described above, distort the uniformity of the radiance distribution within for calibrating radiance sensors with a standard lamp irra- the sphere. Traditionally, the calibration of an integrating diance source. sphere radiance source has been accomplished by appropri- These ideas have not yet been incorporated into a prac- ately transferring the known output from a standard lamp tical and widely accepted set of procedures for the calibra- irradiance source. tion of oceanographic or airborne radiometers using self- Sphere Calibration Methods calibrating radiometric standards. A significant level of 1. The approach used at NASA/GSFC is to view laboratory work must be done to establish the repeatability the irradiance output of the lamp, initially, and of results attainable through these techniques under a va- then the sphere, with a spectroradiometer equip- riety of conditions, and to codify that experience into cal- ped with integrating input optics (McLean and ibration protocols. The spectral responsivity of the QED- Guenther 1989 and Walker et al. 1991). The 200 type detector is known, for example, to vary systemat- spectral irradiance responsivity of the radiome- ically with temperature (Kohler et al. 1990), and the spec- ter is calibrated using the lamp data, and the tral transmission functions of the filters in a self-calibrating (assumed) Lambertian radiance of the sphere is radiometer must be re-characterized at a frequency that determined by dividing the measured spectral will guarantee the uncertainty of the calculated radiance. irradiance output of the sphere by π. This frequency must be established through experience, 2. An alternative method is to calibrate a stable, but a reasonable first approximation is that filter trans- narrowFOV radiometer by viewingthe stan- mission functions should be remeasured every fewmonths. dard lamp output reflected from a plaque, as de- One goal of the SeaWiFS mission is to base radiometric scribed above. The output from the sphere’s exit validation on shipboard, moored, and airborne radiometry port is then viewed within this radiometer. The with 1% uncertainty. If that goal is to be substantially radiometer should also be used, at this point, to achieved, then the work described above to establish new map the angular distribution of radiance in the calibration standards and protocols must be pursued vig- sphere as viewed through the exit port. This im- orously over the next two years. portant verification of a uniform radiance distribution is not possible when Method 1 is used to 4.2.4 Traceability and Comparisons calibrate sphere radiance. A promising variant The variety of instruments available for validation mea- of Method 2 is to calibrate the sphere using a self-calibrating radiometer (Palmer 1988). surements makes it imperative that some common calibration traceability exists. Recognizing that it would be 4.2.3 Radiance Standardization impractical to characterize and calibrate all oceanographic and airborne radiometers at GSFC, several remote calibra- Detectors of the type embodied in the United Detec- tion facilities should be established, and working standards tor Technology QED-200 (where QED stands for Quantum and protocols used at these facilities should all be traced Efficient Device) radiometer are 99.99% quantum efficient. directly to those at the GSFC calibration facility. This Palmer (1988) shows how such a detector may be com- organizational structure is shown schematically in Fig. 2. bined with precision apertures and well-characterized fil- Methods of standards intercomparison may include use of ters to measure self-calibrated spectral radiance with an NIST calibrated filter radiometers to track and document absolute uncertainty less than 1%. A calibration approach the operation of each facility (radiometer wavelengths for based on such radiometer standards is essential to achieve this intercomparison will be determined). Round-robin 1% internal consistency in the radiometric uncertainty of blind calibration comparisons of a standard instrument measurements made for SeaWiFS radiometric validation. would also be implemented to benchmark the internal con- It is worth emphasizing here that the essential objec- sistency of calibrations performed at the various facilities tive is to achieve internal consistency in the SeaWiFS op- involved. tical database through uniform application of calibration techniques based on a common radiometric standard with 4.2.5 Portable Standards precision approaching 1%, or less if possible. An important, but not essential, goal is to establish NIST traceable Between radiometric calibration activities, stable lamp absolute uncertainty of less than 1% with this standard. sources in rugged, fixed geometric configurations should be

27 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

National Institute of Standards and Technology

Radiometric Primary Standard Standards Protocols

Irradiance Sources and and Radiance Detectors

Marine Optical NASA Goddard Calibration Facilities Space Flight Center

Radiometric Secondary Radiometric Secondary Standard Standards Standard Standards Protocols Protocols

Radiometric Inter- System calibration Calibration Standards

Portable Portable Radiometric Radiometric Source Source

In Situ Optical Instruments Space Instruments

Fig. 2. Organizational structure for optical instrumentation characterization and calibration. used to track instrument performance. Irradiance chan- not a substitute for complete calibrations. The temporal nels can be monitored with irradiance sources at fixed dis- record they provide will, however, be invaluable in cases tances from the collectors, while radiance sources can be where the pre- and post-cruise calibrations disagree or if monitored by filling the FOV with diffuser plates placed the instrument is disturbed, e.g., opened between calibra- in front of the irradiance sources, or by using integrating tions or if the data quality are otherwise suspect. These cavity sources. In each case, careful attention must be portable standards are an important part of the recom- given to fixing specific geometries of source and detector mended instrument package. in each use. The stability of the lamp output and the Although several manufacturers offer somewhat port- repeatability of measurement must be sufficient to detect able irradiance and radiance sources, there has been very 2% variations in an instrument’s performance. An instru- little previous work to validate and use portable radiomet- ment should be connected to the portable standard and its ric standards to test oceanographic radiometers in the field. response recorded daily, keeping a record of instrument re- Therefore, detailed hardware specifications and procedural sponsivity throughout an experiment. Furthermore, these protocols must be developed through a series of laboratory sources would provide an essential warning of problems if and field tests using candidate equipment and standards. they appear. The portable field reference source must be available 4.3 ABOVE-WATER RADIOMETRY when the complete radiometric calibrations are performed so that a baseline may be established and maintained for In general, the protocols specified in Section 4.1 for each sensor channel (Section 4.1.1). These sources are in-water oceanographic radiance sensors are applicable for

28 Mueller and Austin characterizing and calibrating shipboard and airborne ra- extremes of instrument operating temperatures, then po- diometers used to measure water-leaving radiance. Obvi- larization sensitivity measurements should be made at sev- ous exceptions are that immersion and underwater FOV eral additional temperatures in that range. characterizations are not appropriate for above-water sensors. 4.4 CALIBRATION OF IOP METERS Polarization sensitivity is more critical in above-water radiometry than underwater radiometry. If a radiometer Calibration of beam transmissometers has traditionally been carried out by means of in-air measurements, with a measures polarization components of radiance, then its subsequent adjustment for changes in the Fresnel reflec- responsivity and rejection of cross-polarization radiance tions by the windows upon submergence into water. The must be characterized for each component channel. For 660 nm transmissometers produced by Sea Tech, Inc. are above-water scalar radiance instruments, as with the Sea- independently calibrated at the factory after which cali- WiFS radiometer, sensitivity to linear polarization must bration is maintained via frequent air calibrations (Sec- be less than 2%, and the actual degree of polarization sen- tions 5.4.1 and 6.7). This approach is adequate for this sitivity must be characterized for each channel. generation of 660 nm transmissometers. A generalized protocol for characterizing the polariza- The newer generation of combined spectral absorption tion sensitivity of a radiometer is given here. The in- and beam transmission meters require a more accurate cal- strument should viewa source of linearly polarized ra- ibration procedure. This class of instruments must be cal- diance, and its apparent radiance response L (λ) should 1 ibrated with optically pure water, which has been freshly be recorded. The instrument should then be rotated 90◦ prepared using multiple-pass reverse osmosis filtration and about its FOV axis, still viewing the linearly polarized ra- analyzed with special optical techniques to verify optical diance source, and the apparent radiance response L2(λ) purity. Ordinary laboratory grade pure water, prepared should be recorded. The polarization sensitivity of the in- using distillation, deionization, or ordinary reverse osmo- strument will be calculated as sis filtration, will not ordinarily be optically pure enough − to serve as a calibration standard. In general, it will not 2 L1(λ) L2(λ) P (λ)= . (15) be practical to perform this calibration procedure either L1(λ)+L2(λ) at sea or in the average oceanographic laboratory. An- nual recalibration, either by the manufacturer, or another As required for the SeaWiFS radiometer, airborne and qualified laboratory, together with air calibrations every shipboard radiometers must satisfy P (λ) < 0.02. three days during deployments, are recommended (Sec- A very simple, semi-quantitative test of a radiome- tion 5.2.4). The air calibration procedure for this class ter’s polarization sensitivity can be performed outdoors of instruments is also more elaborate than for the red on a cloud- and haze-free day. The instrument should (660 nm) transmissiometers (Section 5.2.4). be pointed at the sky in the zenith-sun plane at an an- ◦ Procedures for calibrating a scattering meter depend on gle of approximately 90 from the sun, and its response the detailed design of the instrument. For example, back- L1(λ) recorded. Since singly-scattered Rayleigh radiance ◦ scattering meters are calibrated with Spectralon plaques is 100% polarized at a scattering angle of 90 , if aerosol of known reflectance. Detailed protocols for calibrating scattering is small, the sky radiance viewed at this angle scattering meters remain to be developed in a future revi- will be strongly polarized. If the instrument is then rotated ◦ sion to this document. 90 about its FOV axis to measure L2(λ), an approximate estimate of P (λ) may be computed, as above. 4.5 SUN PHOTOMETERS Specification of detailed protocols for laboratory characterization of a radiometer’s polarization sensitivity will Sun photometer calibrations should be performed at require more attention than is available here. In particular, least annually, when used consistently, through a Langley protocols should be developed which describe in detail: calibration procedure. In this procedure, the solar sig- 1) laboratory setups for producing a stable, uni- nal transmitted through the atmosphere is measured over form, extended source of linearly polarized radi- different air masses, i.e., at different solar zenith angles, ance; and throughout the course of the day. The validity of using these measurements as calibration of a sun photometer 2) laboratory procedures for measuring the actual hinges strongly on the assumption that aerosols are uni- degree of polarization of the polarized radiance formly distributed and do not vary throughout the day. source and for determining the uncertainty of These Langley calibrations, therefore, should be performed the polarization sensitivity estimate achieved us- in areas of atmospheric stability with low aerosol loading. ing a particular experimental setup. Suitable locations include the astronomical observatories Temperature dependence of an airborne radiometer’s at Mauna Loa, Hawaii, and Kitt Peak, Arizona. In be- polarization sensitivity should initially be characterized at tween these calibrations, radiance calibrations with stan- 5◦and 30◦ C. If significant differences in P (λ) exist at these dard lamps may be used as a stability check (Shaw1976).

29 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Temperature stability should be characterized for each result in the underestimation of chlorophyll a concentra- instrument. Linearity and spectral calibrations should be tion because of errors associated with fluorescence con- performed with the same frequency as the absolute cali- tamination (Latasa et al. 1994). Bench fluorometers used bration; these characterizations must be performed in the to measure concentrations of extracted chlorophyll a and laboratory. phaeopigments should be calibrated using authenticated standard chlorophyll a adopted for HPLC. In situ fluorom- 4.6 SKY RADIANCE CAMERAS eters should be calibrated against extracted chlorophyll a from concurrent bottle samples. Absolute and spectral calibrations should be performed on the radiance distribution camera before and after each 4.8 CTD CALIBRATIONS cruise. A full characterization of the instrument should be performed initially, including camera lens roll-off char- The conductivity probe, temperature probe, and pres- acteristics for each camera (Voss and Zibordi 1989). If sure transducer of the CTD should be recalibrated be- attenuation devices are used to prevent solar saturation, fore and after each major cruise by a properly equipped these should be calibrated frequently to track drift. Lin- physical oceanographic laboratory, including those main- earity calibrations should also be performed with the same tained by many CTD manufacturers. In addition, the con- frequency as the absolute and spectral calibration. Pro- ductivity probe should be independently calibrated dur- cedures for characterizing this class of instruments are es- ing the course of each cruise by obtaining salinity water sentially the same as for other radiance detector systems. samples (Section 5.2.3) simultaneous with CTD readings. Each individual detector element in the detector array is These salinity samples are to be analyzed, either at sea essentially regarded as an independent radiometer. or ashore, with a laboratory salinometer calibrated with International Association for the Physical Sciences of the 4.7 PIGMENT CALIBRATIONS Ocean (IAPSO) Standard Seawater. If simultaneous deployment of the CTD with optical HPLC equipment used to measure phytoplankton pig- instruments having independent pressure transducers is ment concentrations is to be calibrated using standards practical, the two depths measured by the different instru- distributed under the auspices of the SeaWiFS Program.† ments should be compared over the range of the cast. If Concentrations of the pigment standards should be de- depth measurements disagree significantly, these compar- termined using a monochromator-based spectrophotome- isons may be used to correct whichever transducer is found ter and the extinction coefficients summarized in Bidigare to be in error through analysis of pre- and post-cruise pres- (1991). The use of a diode array spectrophotometer can sure transducer calibrations.

† The HPLC pigment standards are available from Robert R. Bidigare, Department of Oceanography, University of Hawaii, Honolulu, HI 96822.

30 Mueller and Austin

Chapter 5

Measurement Protocols

Introduction

This chapter describes the methods and procedures by which measurements are to be made during oceanographic deployments for SeaWiFS algorithm development and validation. Section 5.1 gives protocols for obtaining radiance and irradiance measurements from ships and aircraft. Section 5.2 gives protocols for measuring IOP, chlorophyll a ﬂuorescence, and CTD proﬁles. Section 5.3 gives protocols for sun photometry and sky radiance distribution measurements. Section 5.4 covers protocols for obtaining and processing water samples for phytoplankton pigment concentration measurements and spectrophotometric determination of absorption by particles and dissolved matter. Sections 5.5–5.7 describe ancillary observations to be recorded at each station, radiometric measurements from moorings, and radiometry on drifting buoys. Finally, Section 5.8 gives protocols addressing sampling strategy, station location, and coordination of ship and aircraft measurements in Case-1 and Case-2 water masses.

5.1 IRRADIANCE AND RADIANCE which can be minimized at a fixed depth by averaging over several wave periods, but which can pose severe problems Determinations of in-water spectral Ed, Eu, and Lu, in vertical profiles during which the instrument descends at both near the surface and as vertical profiles, are required − speeds of 0.5–1 m s 1. Raman scattering and fluorescence for calibration and validation of the water-leaving radi- result in second-order errors near 490 nm (CDOM fluores- ance as retrieved from the SeaWiFS satellite sensor. Near- cence), and at longer wavelengths, contributions from phy- surface measurements should profile through at least the coerythrin and chlorophyll a fluorescence and water Raman top three optical depths to reliably extrapolate to z =0; scattering are significant. it is essential to obtain a profile through at least the top optical depth. To better characterize the water column for remote sensing applications, e.g., primary productiv- 5.1.1 Ship Shadow Avoidance ity estimation, deeper vertical profiles should be made to The complete avoidance of ship shadow, or reflectance, 200 m, or seven optical depths whenever possible. Sea bed perturbations is a mandatory requirement for all radio- reflection influences on Lu and Eu should be avoided for metric measurements to be incorporated into the SeaWiFS SeaWiFS validation and algorithm development by collect- validation and algorithm database. The influence of ship ing data only from water deeper than six optical depths shadowis best characterized in terms of attenuation length for E (490); remote sensing applications for optically shal- d 1/K (λ) (Gordon 1985). Because L is required with an lowsituations wherebottom reflectance is present are not d W uncertainty of 5% or better, the protocol requires that ver- within the scope of these protocols. tical profiles be measured outside the effects of ship per- There are two primary sources of error in the determination of these optical parameters: the perturbation of turbation to the radiant energy field. To accomplish this, the in-water radiant energy field by the ship (Gordon 1985, the instrument must be deployed from the stern, with the Smith and Baker 1986, Voss et al. 1986, and Helliwell et al. sun’s relative bearing aft of the beam. 1990), and atmospherically induced variability in radiant Estimates of the minimum distance away from the ship, energy incident on the sea surface during in-water mea- under conditions of clear sunny skies, are given below. The surements (Smith and Baker 1984). The influence of ship distances are expressed in attenuation lengths to minimize shadowon the vertical profiles of Ed, Eu, and Lu is depen- error. For Ed(λ) measurements, the general equation for dent upon the following variables: solar zenith angle, the distance away, ξ, in meters is given as spectral attenuation properties of the water column, cloud sin(48.4◦) cover, ship size (length, beam, draft, and freeboard) and ξ = . (16) color, and the geometry of instrument deployment. At- Kd(λ) mospheric variability is primarily dependent upon sun elevation and variations in cloud cover. The near surface in- The distance from the ship is required to be 3/Ku(λ) m for water data also show variability caused by wave focusing, Eu(λ) and 1.5/KL(λ) m for Lu(λ) measurements. These

31 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 distances should be increased if the instrument is deployed 5.1.2 Depth Resolution in Profiles off the beam of a large vessel. A variety of methods have been used to deploy opti- The instrument sampling rate and the speed at which cal instruments beyond the influence of the ship. During the instrument is lowered or raised through the water col- CZCS algorithm development, floating plastic frames were umn should yield at least two, and preferably six to eight, equipped with small winches and instruments to obtain samples per meter. near surface optical profiles at some distance away from the ship. An umbilical cable provided power and data 5.1.3 Instrument Dark Readings transfer. These platforms, while being somewhat difficult The dark current of optical sensors is frequently tem- to deploy, worked well at avoiding ship shadow. Alterna- perature dependent. As a consequence, accurate radiomet- tively, extended booms can be used to deploy the instru- ric measurements require that careful attention be given to ment away from the ship and have the advantages of allow- dark current variability. It is recommended that each opti- ing relatively rapid deployment and simultaneous rosette cal measurement be accompanied by a measurement of the bottle sampling. As a point of caution, however, very long instrument dark current. When there is a large tempera- booms may accentuate unwanted vertical motions due to ture difference between the instrument on the deck and ship pitch and roll. the water temperature, the instrument should be allowed Waters et al. (1990) used an optical free-fall instrument to equilibrate with ambient water temperature at the be- (OFFI) that allows optical data to be obtained outside ginning of each cast. the influence of ship perturbation. In addition, the OFFI Deep casts, e.g., 500 m, may permit the determination approach allows optical data to be obtained independently of the dark current in each optical channel at the bottom of from violent ship motion, which may be transmitted to the each cast. Many instruments, however, are not designed to instrument via the hydrowire, especially on a long boom. be safely lowered to 500 m, and this approach is usually not Yet another method for the deployment of optical sensors feasible. Furthermore, there is some intrinsic uncertainty is via an ROV. Some groups, e.g., Smith (pers. comm.), over possible contamination by bioluminescence when dark have deployed a spectrometer on an ROV and obtained readings are obtained in this way. If the instrument is data completely free of ship influences. equipped with a shutter, dark currents can be measured at The above criteria for ship shadowavoidance are ad- any depth in the cast. If the dark current is not determined mittedly very conservative. Unfortunately, the above cited during the cast, it should be determined as soon as possible models and observations provide only approximate guid- after the instrument is returned to the deck. ance on minimum distances at which ship reflectance and Temperature effects on sensor responsivity can be sig- shadoweffects become insignificant under all circumstan- nificant and should not be ignored. Therefore, sensors ces. Therefore, the SPSWG has adopted relatively extreme should be equipped with thermistors on detector mounting distance criteria, recognizing that in many specific combi- surfaces to monitor temperatures for data correction. Oth- nations of lighting conditions, ships and optical properties, erwise, deck storage should be under thermally protected ship shadow, and reflection effects may become unimpor- conditions prior to deployment and on-deck determination tant much closer to the ship. of dark voltages. The essential requirement is that each investigator establish that any measurements of Ed, Eu, and Lu, submit- 5.1.4 Surface Incident Irradiance ted for SeaWiFS validation and algorithm development, are free from ship-induced errors. The simplest way to do Atmospheric variability, especially under cloud cover, this is to adhere to the above distance criterion, which is leads directly to variability of the in-water light field and not difficult when using either a tethered free-fall system must be corrected to obtain accurate estimations of optical or instruments mounted on an ROV. In other cases, it is in- properties from irradiance or radiance profiles. First order cumbent on the investigator to otherwise demonstrate the corrections for this variability can be made using above absence of ship effects, e.g., through analysis of a series of water (on deck) measurements of downwelling spectral ir- + profiles at increasing distance. radiance, Es(λ)=Ed(0 ,λ). Smith and Baker (1984) and At wavelengths where attenuation lengths are the same Baker and Smith (1990) theoretically computed the irra- − order of magnitude as, or less than, the size of the instru- diance just belowthe air-waterinterface, Ed(0 ,λ), from ment package, e.g., in the ultraviolet-B (UVB) or red and deck measurements to correct in-water profile data. near infrared spectral regions, care must be taken to con- The deck sensor must be properly gimballed to avoid sider possible perturbation of the radiant energy field by large errors in Es(λ) due to ship motion in a seaway. Im- the instrument package itself. Methods of accounting for proper gimballing can actually accentuate sensor motion self-shadowing by the instrument are not well established under some circumstances, however, and this aspect of and newmeasurement approaches must be developed (Sec- a shipboard radiometer system must be engineered with tion 5.1.6). some care.

32 Mueller and Austin

Recently, Waters et al. (1990) have demonstrated a with instruments having maximum diameters of 24 cm for − ≤ method to more directly determine Ed(0 ,λ) by deploy- λ 650 nm, and with instruments of maximum diame- ing an optical surface floating instrument (OSFI) to obtain ter 10 cm for 650 <λ≤ 700 nm at solar zenith angles ◦ continuous optical data just belowthe air-waterinterface. (θ0) greater than or equal to 20 , and maximum chloro- − −3 These E (0 ,λ) are used as a normalization factor to cor- phyll concentrations of 10 mg m . To measure Lu(λ) cor- d ◦ rect for variations in irradiance during a vertical profile, rectable to less than 5% error at θ0 =10 (with chlorophyll − or over the period of a day for a sequence of profiles. Re- concentrations less than or equal to 10 mg m 3), maximum search and development is needed to determine whether instrument diameters are 12 cm for λ ≤ 650 nm and 5 cm this should be the preferred approach for SeaWiFS valida- for 650 <λ≤ 700 nm. Even with these corrections, how- tion measurements. ever, instrument diameters of 1 cm or less must be used to assure self-shading Lu(λ) errors are 5% or less at 780 and 5.1.5 Instrument Attitude 875 nm. The Gordon and Ding (1992) model predictions have An instrument’s attitude with respect to the vertical is recently been compared to experimental measurements of a critical factor in measurements of Ed(z,λ) and Eu(z,λ), Lu(λ) just beneath the sea surface, using a fiber-optic ra- and is only slightly less critical for Lu(z,λ). Roll and pitch diometric probe (Zibordi and Ferrari 1994). The exper- sensors must, therefore, be installed in the underwater ra- iment was performed in a lake, with solar zenith angles ◦ ◦ diometers used for the SeaWiFS program. The data from ranging between 25 and 50 , on several days with cloud- these attitude sensors are to be recorded concurrently with free skies. Spectrophotometric methods (similar to those the data from the radiometric channels and are to be used in Sections 5.4.2 and 6.8.2) were used to measure absorp- as a data quality indicator. It is not deemed necessary to tion by particles and Gelbstoff. At wavelengths of 500, determine or control attitude determination errors result- 600, and 640 nm, a series of discs was employed to vary ing from surface wave-induced accelerations at very shal- instrument self-shading geometry in several steps over the lowdepths. range 0.001

33 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 are much smaller than SeaWiFS pixels; therefore, these the water-leaving radiances must be normalized by the ra- data are more comparable to shipboard measurements. At tio F 0(λ)/Es(λ), where F 0(λ) is the mean extraterrestrial a qualitative level, this information can indicate howwell solar flux, and Es(λ) is the measured incident spectral irra- shipboard radiometric and bio-optical measurements can diance propagated to the sea surface. This normalization be compared to SeaWiFS data at greater than 1 km pixel is also critical for ratios of water-leaving radiance, because resolution. For more quantitative work, if an airborne ra- the spectral quality of incident irradiance is modified sig- diometer’s characteristics conform to specifications of Sec- nificantly, in a generally unpredictable way, by atmospheric tion 3.3, and if accurate corrections are applied for at- transmittance. mospheric and surface reflection effects, the high resolu- Under either clear skies or a completely uniform cloud tion airborne data may be spatially integrated to quan- cover, i.e., overcast skies, the downwelling irradiance mea- titatively extrapolate shipboard bio-optical measurements surements may be used directly to model corrections for to SeaWiFS pixel scales. (Note that the original data must lowaltitude atmospheric transmission and path radiance, be corrected for sun and sky glitter, as well as atmospheric and for reflected sky radiance. On the other hand, ocean transmission and path radiance, before spatial integration color radiance data measured under variable cloud con- to SeaWiFS resolution.) Finally, in cases when airborne ditions are very difficult to correct and interpret. Under normalized water-leaving radiances estimated from aircraft these conditions, the area on the sea surface viewed by the measurements are independently validated as being in con- radiometer is often illuminated differently than is the in- formity with SeaWiFS uncertainty goals, they may be used cident irradiance sensor at the aircraft, and sky radiance for direct radiometric validation of SeaWiFS radiometric incident on the surface also varies unpredictably. performance. The requirement to directly measure incident spectral Methods for atmospheric correction and estimation of irradiance may sometimes be relaxed when the experimen- normalized water-leaving radiances from high altitude air- tal objective is simply to characterize spatial variability of borne ocean color imagery are nearly identical to, and as bio-optical properties qualitatively. Under cloud-free con- challenging as, those methods which must be applied to ditions, moreover, it may often be possible to model surface SeaWiFS data itself (Carder et al. 1992 and Hamilton et incident irradiance accurately enough for semi-quantitative al. 1992). These problems and their solutions lie beyond analyses of chlorophyll concentrations from lowaltitude the scope of the ocean optical protocols per se, at least in airborne spectral upwelled radiance measurements alone. this revision. The elements of ocean color radiometry from For atmospheric correction and analyses of aircraft ra- lowflying aircraft are more closely associated withship- diance data, the following ancillary parameters must be board ocean optical measurements and will be addressed recorded: date and time [Greenwich Mean Time (GMT)], here. azimuth and nadir angle of the radiometer, heading of the Under clear skies, or partial cloud cover, the radiometer aircraft (and all other aircraft attitude angles, if available), ◦ should be pointed approximately 15–20 from nadir in the the flight altitude, and air pressure at that altitude. The azimuth direction opposite from the sun, to avoid specular height of the inversion should also be recorded, as most sun glint from wave facets. If seas are rough and the solar of the aerosols will normally be concentrated under this zenith angle is small, an even larger nadir angle may be height. Furthermore, a precise navigation recording is re- necessary. Under completely uniform overcast conditions, quired for identification of the aircraft track within the the instrument should point to the nadir. If an imaging satellite image and for comparisons to shipboard measure- radiometer is fixed in the nadir plane, sampling should be ments. In addition to the radiometer, a video camera is scheduled to provide a solar zenith angle between 35◦ and ◦ very helpful to check for the effects of cloud shadows, foam, 45 , and all flight lines should be flown toward or away and ocean fronts—effects which are difficult to identify in from the sun. the horizontal radiance profile alone. For quantitative lowaltitude ocean color radiometry, solar downwelling spectral irradiance must be measured 5.1.8 Shipboard Radiance Measurements using a sensor mounted on top of the aircraft. Incident solar irradiance spectra are modified significantly by trans- Water-leaving radiances can, of course, be measured mission through the atmosphere above the aircraft, and di- from the deck of a ship using methods directly analogous to rect spectral irradiance measurements are absolutely essen- those of low-altitude airborne radiometry (Section 5.1.7). tial for computing normalized water-leaving radiances from Given a very close proximity to the sea surface, no correc- flight level radiances. The measured flight-level down- tions are needed for atmospheric transmission and path ra- welled irradiances should be used to model atmospheric diance. Shipboard water-leaving spectral radiances must, transmission and path radiance corrections, which must however, be corrected for light reflected at the sea surface, be applied at altitudes of 100 m and higher. Reflected and must also be normalized using direct measurements of skylight must also be removed from the measured radi- Es(λ). Performance characteristics for radiometers used to ances. Finally, for use in algorithms related to SeaWiFS, measure Es(λ) and Lu(λ) are specified in Sections 3.2–3.3.

34 Mueller and Austin

The best position for measuring the water-leaving ra- The sequence of water-leaving radiance, diffuser plate, diance is usually near the bowof the ship. At this po- and sky radiance [Lsky(λ)] measurements can usually only sition, the ship will have the least influence with respect be completed under clear sky or uniform overcast condi- to shading or reflection. It is also easy to point in a ditions. When coupled with concurrent direct measurements rection away from the sun to reduce specular reflection of of Es(λ), however, LW (λ) and Lsky(λ) can probably be sunlight. While underway, ocean color radiance measure- measured under variable cloud conditions, provided a se- ments should always be made from the bow, because this is quence of several [LW (λ),Es(λ)] and [Lsky(λ),Es(λ)], sets the only position where the water is normally undisturbed of spectra can be measured in rapid succession without by the ship’s wake and without foam. significant variations. To measure LW (λ), the radiometer should point to the ◦ sea surface with an angle of about 20 from nadir and away 5.2 ANCILLARY PROFILES from the sun’s azimuth by at least 90◦. (A viewing angle that is 180◦ away from the sun’s azimuth may be contam- Beam transmittance, CTD profiles, and chlorophyll a inated by the glory phenomenon.) This is the point with fluorescence should be measured at the same stations as the the minimum in specularly reflected sun and sky radiance. irradiance and radiance measurements. Preferably, these Under overcast conditions, the instrument should point auxiliary profiles should be measured simultaneously with, closer to nadir, but not so close that ship shadowor hull or otherwise immediately before or after, the radiometer reflections affect the water-leaving radiance. (Indeed, some profiles. If possible, these profiles should be made in con- larger ships have a long bulbous bownose, which must be junction with bottle samples. For the verification of the avoided.) Under a high sun elevation (such as in tropical satellite sensor, these data will be used as a guide to the regions) and a rough sea, it may be very difficult, or even uniformity of the first optical depth and to determine water impossible, to avoid large amounts of specularly reflected bottle sampling depths. sunlight, since large nadir angles cause severe problems due The IOP, fluorescence, and CTD profiles will also be to variations in reflected skylight. In addition, foam and used as a guide for, and constraints on, the smoothing floating material must be avoided during measurements. of K(z,λ) from the radiometric profiles. The location of Because of temporal variability due to waves, it is im- maxima and other features in the structure of these profiles portant to record a number of spectra within a period of identify inflection points for segmenting the optical profiles a fewseconds, e.g., 30 spectra within15 s. Before calcu- into finite depth elements (layers) for the analytical meth- lating final mean and standard deviation spectra, outliers ods described by Mueller (1991) or Petzold (1988). Both of should be removed by computing initial estimates of these these techniques use multiple segments for the statistical statistics and rejecting radiance spectra containing values fit of analytic functions to the measured profiles. These falling more than 1.5 standard deviations from the esti- data will also be used to develop and validate pigment and mated mean. primary productivity algorithms. Incident spectral irradiance Es(λ) should be measured at a location free from shadows of, and reflections by, 5.2.1 Beam Transmittance the ship’s superstructure. As in low-altitude aircraft ra- The windows on the beam transmissometer must be diometry, Es(λ) must be used both to normalize measured water-leaving radiance and to calculate spectral re- cleaned with lens cleaner or a mild detergent solution and a flectance. Some investigators (Carder and Steward 1985 soft cloth or tissue, rinsed with distilled water, then rinsed and Doerffer pers. comm.) advocate calculating spectral with isopropyl alcohol and wiped dry. An approximate air reflectance using relative Es(λ) estimated by viewing a ref- calibration reading should be made before every cast to erence diffuse reflectance plate. Spectralon plates are used verify that the windows are clean. A transmissometer dark most often, but Kodak gray cards and plates painted with voltage should also be measured at this time. These on- NextelTM coating have good Lambertian reflectance char- deck air calibrations are not, however, very reliable mea- acteristics (Doerffer pers. comm.). sures of temporal drift or degradation in the instrument’s The contribution by specularly reflected sky light to source or detector. In the humid, or even wet, environ- a water-leaving radiance measurement can be calculated ment on the deck of a ship, the windows are often quickly from measurements of the sky radiance, made either by obscured by condensation, and the glass also tends to ab- looking at a horizontal first surface mirror (a mirror with sorb enough water to affect transmission slightly (Zaneveld no layers other than the reflective surface) at the same pers. comm.). A very careful air calibration should be per- nadir and azimuth angles, or by pointing the radiometer formed before and after each cruise under dry laboratory into the sky with a zenith angle equal to the nadir angle conditions. During an extended cruise, it is also recom- and with the same azimuth angle. The specular reflectance mended to remove the instrument to a dry location in a of the water surface can be calculated using the Fresnel shipboard laboratory, and after allowing several hours for equation (reflectance is approximately 0.02 within 20◦ of the windows to dehydrate, a careful air calibration should nadir). be performed. Only the laboratory air calibrations should

35 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 be used in the final processing of beam transmissometer inputs characterized. During processing, a correction of data. the form given in (19) must usually be applied to values Both the laboratory condition air calibration and dark recorded with the A/D circuits of CTDs and profiling ra- voltages, and the factory calibration voltages, assume the diometer systems. As in beam transmittance, the range data acquisition system measures instrument response as dependent A/D bias coefficients should be determined at true volts. It is imperative, therefore, to calibrate the approximately 5◦ C intervals over the range from 0–25◦ C end-to-end analog-to-digital (A/D) data acquisition sys- to characterize the temperature sensitivity of the data ac- tem and characterize its response to known input voltages. quisition system. Corrections are in the form of a linear function: Zero fluorescence offsets should be measured on deck before and after each cast; the optical windows should be V = g(T )V + f(T ), (19) shaded to avoid contamination of the zero offset value by ambient light. Before each cast, the fluorometer windows where T is temperature, must usually be applied to ex- should be cleaned following the manufacturer’s instruc- ternal voltage inputs recorded with the A/D circuits of tions. CTDs or profiling radiometer systems. The range depen- For chlorophyll a determinations, the fluorescence mea- dent A/D bias coefficients should be determined at ap- surements should be compared to HPLC and extracted pig- ◦ ◦ proximately 5 C intervals, over the range from 0–25 C, ment measurements from discrete water samples, to permit to characterize the temperature sensitivity of the data ac- comparison with JGOFS standard measurements and his- quisition system. torical databases. The in situ fluorescence measurements For the development of bio-optical algorithms describ- will be used to provide continuous vertical profiles of in- ing the inherent and apparent optical properties of the wa- terpolated pigment concentration using the bottle samples ter, and for algorithms estimating primary productivity, as tie points. more stringent requirements are recommended for transmissometer calibration and characteristics. Spectral mea- 5.2.3 CTD Profiles surements of beam transmittance should be made with absolute uncertainties of 0.1% transmittance per meter, or Vertical profiles of CTD should be measured to at least 0.001 m−1 beam attenuation coefficient c(λ). the depth of the deepest bio-optical profile. If the station Different and more stringent calibration protocols are schedule will permit it, sections of CTD casts extending specified in Section 5.2.4 for the newcombined spectral to 500 m, or deeper, will be useful for computing relative absorption and beam attenuation meters. Laboratory cal- quasi-geostrophic currents and shear, which may affect the ibrations of these instruments are based on measurements advection and mixing of bio-optical properties during a using optically pure water, produced using extraordinarily cruise. A real-time analysis and display of the CTD pro- careful reverse osmosis procedures. It is ordinarily imprac- file, together with displays of c(660) and in situ fluores- tical to produce and verify optically pure water either at cence profiles, should be available as a guide in choosing sea or in the average oceanographic laboratory. As with the depths at which water sampling bottles will be closed. the older red-wavelength beam transmissometers, air cal- If possible, a fewdeep (1,500 m depth or greater) CTD ibrations are prescribed as a basis for correcting instru- and bottle sample profiles should be made during each mental drift between calibrations at a qualified laboratory. cruise to obtain data for calibrating the CTD’s conductiv- The increased sensitivity required for the newinstruments ity probe. During these CTD calibration casts, water sam- makes it essential to carry out more frequent air calibra- ples should be taken at depths where the vertical gradient tions at sea. As discussed above, accurate air calibrations of salinity is very small. This practice will minimize errors must be carried out in a dry laboratory environment with in the conductivity calibration resulting from the spatial controlled relative humidity—conditions which may be dif- separation of the water bottle and CTD profile. The bot- ficult to implement on some ships. tled salinity samples may be stored for post-cruise analyses ashore at a laboratory equipped with an accurate salinome- 5.2.2 Chlorophyll a Fluorescence ter and IAPSO Standard Seawater, if suitable equipment and standard water are not available aboard the ship. An in situ fluorometer should be employed to measure a continuous profile of chlorophyll a fluorescence. The 5.2.4 Spectral Absorption Profiles fluorometer should be mounted on the same underwater package as the transmissometer, CTD, and water sampler, Currently, there are two methods for measuring spec- if one is employed. If possible, the radiometer should also tral absorption coefficients which use commercially avail- be on this package. able equipment. The first is conventional benchtop spec- The A/D channel used to acquire and record signal trophotometer measurements of spectral absorption by par- voltages from the in situ fluorometer must be calibrated, ticles extracted from water samples and concentrated on and its temperature-dependent response to known voltage filters, followed by measurements of the spectral absorption

36 Mueller and Austin by the dissolved component in the filtrate (Section 5.4.2). time all affect the particulate matter and distort its true More recently, in situ profile measurements of spectral ab- optical properties as they existed in the ocean, and as sorption have been made using instruments which capture they determine the remote sensing reflectance viewed by most scattered light using a reflecting tube. A combination SeaWiFS. The reflecting tube method has been used to of these two methods allows one to measure profiles of to- measure spectral absorption in the laboratory for many tal absorption and absorption by the dissolved component decades (James and Birge 1938). In recent years, this with the reflecting tube instrument, and to independently method has been adapted for use in the ocean (Zaneveld partition the absorption spectrum at discrete depths into 1992). Suitable instruments are nowcommercially avail- its components due to particulate and dissolved substances able and are coming into general use within the oceano- using filters and spectrophotometry. graphic community. In situ spectral absorption coefficient profiles can also The reflecting tube does not perfectly gather all scat- be measured with spectral radiometers conforming to the tered light and transmit it to the detector, and as a result, performance specifications listed in Section 3 above, if the there is a scattering error on the order of 13% of the scat- radiometric package is extended to measure Ed(λ) and tering coefficient. This error can be largely corrected if Eu(λ), as well as scalar irradiances E0d(λ) and E0u(λ). the beam attenuation coefficient is measured simultane- This combination may be approached either using hemi- ously. In that case, the scattering coefficient is obtained as spherical collectors to measure upwelling and downwell- b(λ)=c(λ)−a(λ). By assuming that the measured absorp- ing hemispherical irradiances, or by using cosine collection is due to water and scattering error at a wavelength in tors on one radiometer in tandem with spherical collec- the infrared, and by subsequent correction at other wave- tors on another radiometer. Given these irradiance com- lengths using a provisional b(λ), it is possible to correct the ponents, spectral absorption is then computed using Ger- spectral absorption to within a few percent of the scatter- shun’s equation as ing coefficient. Only in waters with very high scattering E(λ) and very lowabsorption wouldthis error pose a serious a(λ)=K(λ) , (20) absorption error. This correction method is described in E (λ) 0 more detail in Section 5.4.5 below. The in situ reflecting tube is normally used to mea- where E(λ)=Ed(λ)−Eu(λ) is vector irradiance, and K(λ) is the vertical attenuation coefficient for vector irradiance. sure profiles of total spectral absorption, but it has been In a recent experiment, comparisons between absorp- recently demonstrated that if the unit’s intake is fitted tion profiles measured using Gershun’s equation with E(λ) with a large area 0.2 µm filter, the spectral absorption and E0(λ) (scalar irradiance) data, and absorption profiles of the dissolved component can be measured (Zaneveld measured with a reflecting tube instrument, agreed within pers. comm.). A pair of reflecting tube absorption meters 8% (Pegau et al. 1994). This level of agreement is well can thus be used to determine the separate constituents of within the calibration uncertainties of the particular pro- absorption due to particulate and dissolved substances— totype instruments used for that experiment, which were a distinction of fundamental importance in relating ab- approximately 10% uncertainties in both the scalar irra- sorption to remote sensing reflectance. More traditionally, diance radiometer and in the reflecting tube instrument. the filtration and spectrophotometry techniques developed Less than 5% uncertainty in absorption is expected in fu- over the last decade also lend themselves well to this task. ture experiments. In very clear oligotrophic water, how- Using the methods described in Section 5.4.2, the spec- ever, uncertainty in water absorption values may make it tral absorption coefficient is partitioned into components impossible to realize this level of relative agreement. Un- associated with Gelbstoff, pigments, and non-pigmented fortunately, an adequate commercially available scalar, or particles (the latter sometimes referred to misleadingly as hemispherical, irradiance radiometer does not exist today, detritus). and development and characterization of suitable spheri- There are two primary weaknesses of the filtration spec- cal or hemispherical collectors is a prerequisite to applica- troscopic method: tion of this method. The present version of the protocols 1. Uncertainties associated with correcting the fil- will, therefore, emphasize the combination of filter spec- ter absorption values for scatter within the fil- trophotometry (Section 5.4.2) and in situ reflecting tube ter are not well characterized. The correction total absorption measurements. The present section covers field calibration and sampling procedures for using the factor for this phenomenon, called the β factor, reflective tube absorption and beam transmission meter. does not currently have a unanimous consensus Recommended methods of data analysis to derive vertical value. profiles of a(z,λ) and c(z,λ) are presented in Section 6.8.1. 2. The instrumental uncertainty in absorption due It is always best to determine optical properties in situ, to the dissolved component, which attenuates if possible. Sampling variability, changes of light inten- weakly over the 10 cm path of a typical spec- sities, filtration procedures, and sample degradation over trophotometer, is also not well characterized.

37 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Note that some longer path spectrophotometers are in use where θ is the centroid angle computed as experimentally, and may reduce this source of uncertainty following a suitable validation procedure. In any event, π θW(θ) sin θdθ when the filter spectroscopy absorption values are com- 0 bined with total absorption values measured with the re- θ = π , (22) flecting tube, the absolute values may be largely corrected W (θ) sin θdθ to remove these uncertainties, if two reflecting tube meters 0 are not available. Conversely, these comparisons serve to and ∆θ is the FWHM bandwidth of the weighting function validate and control the quality of the a(λ) profiles mea- multiplied by sin(θ), λ is the central wavelength of the color sured in situ. filter, and ∆λ is the FWHM bandpass of the filter. While the parameters do not give a complete description of the 5.2.5 Backscattering Profiles weighting function, they allow an informed comparison of While the spectral absorption can nowbe measured scattering meter results. Investigators making backscatter- with sufficient accuracy for the purpose of SeaWiFS, backing measurements are encouraged to report backscattering scattering is not routinely measured, yet it has a large meter results in the format βb(θ, ∆θ, λ, ∆λ). influence on the reflectance. The nowgenerally accepted Backscattering measurement should be made in close dependence of the remote sensing reflectance on the ratio proximity to absorption meters, if both are used, to avoid of the backscattering coefficient and the absorption coef- space-time aliasing. ficient has never led to a systematic investigation of the dependence of bb(λ) on the various particulate parameters. 5.3 ATMOSPHERIC RADIOMETRY Gordon (1989a) showed that the irradiance reflectance depends on the shape of the scattering function in the back- This section is concerned with two types of atmos- ward direction. Zaneveld (1994) derived the functional de- pheric radiometric measurements that are distinct from pendence of the remote sensing reflectance on backscatter- those described earlier in Section 5.1. The first of these ing. is the photometric measurement of the direct solar beam There is little historic data on the variation of the shape to determine the optical thickness of the atmosphere (Sec- of the volume scattering function, β(θ, λ), in the backward tion 5.3.1). The second is a measurement of the sky ra- direction. Petzold (1972) described β(λ) measured with diance distribution using a radiance distribution camera the GASM. This reference is the one most widely used to (Section 5.3.2). describe shapes of β(θ, λ). Since that time, only Balch et al. (1994) have published newdata regarding the shape of 5.3.1 Sun Photometry β(θ, λ). Several single wavelength integrated backscattering bb(θ, λ) meters have been constructed (Maffione et al. Measurements of the direct solar beam, using the sun 1991 and Smart 1992), all of which measure a weighted in- photometer, should be performed during the optical sta- tegral of the scattering function. Maffione’s meter has the tions. If sky radiance distribution measurements are per- centroid angle near 150◦, whereas Smart’s meter has the formed, it is important that these measurements are per- centroid angle near 170◦. The 150◦ centroid angle is more formed concurrently. While the preferred method of deter- appropriate for SeaWiFS. This observation also points out mining the optical thickness of the atmosphere is by mea- a problem in the reporting of results from scattering me- suring the solar transmission as a function of solar zenith ters, in that results are commonly reported as simple bb(λ). angle, the atmospheric conditions are rarely stable enough In the future, more detail should be provided to allowin- at sea for this method to work. Thus, a stable, well cal- tercomparisons between results reported by different inves- ibrated photometer can be used with measurements at a tigators. single zenith angle to obtain the solar transmission, and All scattering sensors measure a parameter that is a thus, the aerosol optical depth. weighted integral of the volume scattering function. The Atmospheric measurements should be performed only weighting function W (θ) depends on the geometry of the when clouds, including high cirrus, do not obstruct the device. Knowledge of the weighting function is critical in solar disc. Careful documentation of sky conditions is im- comparing scattering meters. For each scattering meter, portant, as are accurate recordings of the time of day and documentation should be available describing this weight- the geographic location of the station. The latter data ing function and howit wasderived. The weighting func- are important in determining the true solar zenith angle tion can generally best be derived by moving a Spectralon and, hence, the air mass in the solar path. It should also target through the scattering volume space. be obvious that care should be taken to avoid ship per- The scattering meter measurement may be defined as turbations (e.g., stack gas) from interfering with the mea- π surements. Ancillary measurements, such as barometric

βb(θ, ∆θ, λ, ∆λ)=2π β(θ, λ)W (θ) sin θdθ, (21) pressure, are important in separating Rayleigh scattering 0 from the aerosol scattering.

38 Mueller and Austin 5.3.2 Sky Radiance Distribution 5.4 WATER SAMPLES Complete sky radiance distributions should be mea- Duplicate samples should be taken at each of 12 depths, sured with a radiance distribution camera during the Sea- including at least three depths within the first attenuation WiFS radiometric initialization and validation optical sta- length, 1/K(490); however, in coastal areas with short tions. For this purpose, it is critically important that these attenuation lengths, this may not be possible. Samples measurements be obtained whenever totally clear sky con- should also be taken in the in vivo fluorescence and beam ditions persist. Coincident with these measurements, sun attenuation maxima. The remaining samples should be photometer measurements should be obtained. When lo- spaced throughout the water column using beam attenua- cating the camera system for these measurements, it is tion, in situ fluorescence, and CTD profiles as a guide. important that the FOV be as unobstructed as possible. While it would be optimum to have a completely unob- 5.4.1 Pigment Analysis structed FOV, this is often not practical. During measurements, therefore, at least one hemisphere (defined by the Water samples should be taken at the site of, and simul- sun-zenith plane) should be unobstructed; through sym- taneously with, the surface in-water upwelled radiance and metry, this should yield a complete radiance distribution. reflectance measurements, and at depth increments suffi- Ship perturbations, e.g., stack gas, must be avoided cient to resolve variability within the top optical depth. completely. It is important to document where the instru- The K(z,λ) profiles over this layer will be used to comment is located and what possible perturbations might ex- pute optically weighted, near-surface pigment concentra- ist, even though these effects may be obvious in the data. tion for bio-optical algorithm development (Gordon and It would also be highly desirable to add sky radiance Clark 1980). measurements to every SeaWiFS algorithm development When possible, samples should be acquired at several and validation cruise. Gordon (1989b) rigorously demon- depths distributed throughout the upper 200 m of the wa- strated the importance of determining µd, the mean co- ter column [or in turbid water, up to seven optical depths, sine for downwelling radiance (Morel and Smith 1982), for ln (E(0)/E(z) = 7], to provide a basis for relating chloro- the bio-optical interpretation of Kd(z,λ). Gordon (1989b) phyll a fluorescence signals to pigment mass concentration. also showed, for cloud-free skies, how to obtain a reason- For lowuncertainty determinations of chlorophylls a, + able estimate of µd(0 ) from spectral irradiance deck cell b, and c, as well as carotenoid pigments, HPLC tech- measurements with, and without, the sun blocked from the niques are recommended. It should be noted, however, viewof the irradiance collector. This procedure should be that the reverse-phase C18 HPLC method recommended done routinely whenever it is practical to do so. Unfortu- by the Scientific Committee on Oceanographic Research nately, the collective scientific experience is that cloud-free (SCOR) (Wright et al. 1991) is not capable of separat- skies rarely occur at ocean optical stations. ing monovinyl chlorophyll a from divinyl chlorophyll a nor If a calibrated spectral radiance distribution camera monovinyl chlorophyll b from divinyl chlorophyll b. This is available, then L (λ, θ, φ) should be measured several sky method, therefore, only provides estimates of total chloro- times during each spectral radiometer cast and used to phyll a and total chlorophyll b concentrations, respectively. compute µ (0+,λ). Radiance distribution cameras are ex- d Divinyl chlorophyll a, the major photosynthetic pig- pensive to build, however, and one is not likely to be avail- ment found in prochlorophytes, accounts for 10–60% of able aboard every SeaWiFS validation vessel. A recom- the total chlorophyll a in subtropical and tropical oceanic mended alternative approach is to acquire: waters (Goericke and Repeta 1993, Letelier et al. 1993, 1) all-sky photographs taken either with a conven- Bidigare et al. 1994, and Bidigare and Ondrusek 1994). tional camera or, preferably, a digitally recorded Divinyl chlorophyll a is spectrally different from normal camera system (some research must be done to (monovinyl) chlorophyll a, and its presence results in a sig- develop procedures for attenuating, or blocking, nificant overestimation of total chlorophyll a concentration the sun’s image and for using filters); as determined by the conventional HPLC methods (Goer- 2) measurements, with a narrow FOV spectral ra- icke and Repeta 1993, Letelier et al. 1993, and Latasa et diometer of Lsky(λ, θi,φi) at several discrete anal. 1994). To avoid these errors, it is recommended that gles θi and φi; and monovinyl and divinyl chlorophyll a be spectrally resolved 3) Es(λ) measurements of Esun(λ)+Esky(λ) with or chromatographically separated in order to obtain an un- the sun’s image blocked by shadowing the deck biased determination of total chlorophyll a (that is, total cell irradiance collector (Gordon 1989b). chlorophyll a equals divinyl chlorophyll a plus monovinyl + During the prelaunch experiments, µd(0 ,λ) values esti- chlorophyll a) for the purpose of ground-truthing SeaWiFS mated from measurements of these types should be com- imagery. These co-eluting chlorophyll species can be re- + pared with µd(0 ,λ) values determined from direct mea- solved spectrally following C18 HPLC chromatography surements of sky radiance using a spectral radiance distri- (Wright et al. 1991) and quantified using dichromatic equa- bution camera system. tions at 436 and 450 nm (Goericke and Repeta 1993 and

39 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Latasa et al. 1994). Alternatively, these two chlorophyll and filtrate collection because they protect the dissolved species can be separated chromatographically and individ- samples from ambient light. Caps lined with PTFE are ually quantified using the C8 HPLC technique described preferred to minimize contamination. Prior to each exper- by Goericke and Repeta (1993). (C18 and C8 designate iment, all filtration and storage bottles should be thor- column packing materials used in HPLC.) oughly cleaned with detergent and rinsed several times These protocols, to be employed in the SeaWiFS valida- with distilled water (DIW). tion program for HPLC pigment analyses, are prescribed in For measurement of absorption by dissolved organic the JGOFS Core Measurement Protocols (JGOFS 1991). matter (DOM), samples should be filtered through 0.2 µm These protocols include: membrane filters, e.g., NucleporeTM polyester 0.2 µmfil- a) use of Whatman GF/F glass fiber filters, ap- ters. Membrane filters minimize the contamination of the proximately 0.7 µm pore size, filtrate by filter fibers. It is recommended that separate water samples be filtered for DOM, particulate matter, and b) extraction in 90% acetone, and pigment analyses. Combining several analyses in one fil- c) calibration with authenticated standards. tration can introduce errors, because each analysis has a It is recommended that seawater samples not be pre- specific filtering protocol. filtered to remove large zooplankton and particles as this The procedure for preparing a sample for DOM mea- might result in the exclusion of pigment-containing colo- surements is as follows: nial and chain-forming phytoplankton, e.g., diatoms and 1. Before filtering a sample for the measurement, Trichodesmium sp. Large zooplankton can be removed filter at least 100 ml of DIW through the filter to following filtration using forceps. remove wetting agents or other organic material. When it is appropriate to measure phycoerythrin con- 2. Swirl and rinse the filter trap. centration, it is important to discriminate between Tri- chodesmium and Synechococcus phycoerythrins, as they 3. Filter 30–50 ml of the available volume, and cap- have different spectral absorption characteristics. The con- ture in a trap. centration of Trichodesmium phycoerythrin may be deter- 4. Rinse the filter and flask a second time with a mined using the glycerol uncoupling method described by second aliquot of sample, or, if sample is scarce, Wyman (1992). Separating Trichodesmium from Syne- refilter the first-pass filtrate. chococcus may require pre-filtering. 5. Swirl and rinse the trap after the second filtra- In addition to HPLC analyses, it is recommended that tion, and discard the filtrate. the standard fluorometric methodology used for measur- 6. Filter the remaining volume, capture in the trap, ing chlorophylls and phaeopigments (Yentsch and Menzel and refilter the sample. 1963, Holm-Hansen et al. 1965, and Strickland and Par- son 1972) also be applied to the same extracted pigment 7. Rinse a clean, amber glass, storage bottle three samples used for HPLC analysis (Section 6.6.1). This ad- times with filtrate, then fill, cap and store up- ditional analysis will enable a direct link to the historical right. bio-optical algorithms and database development during If it is necessary to delay the measurement of the Gelb- the CZCS validation experiments. stoff sample for more than a fewhours, freeze each filtrate sample in an amber bottle, and store upright and in the 5.4.2 Spectrophotometric Absorption dark. An alternate simple and rapid filtration method for This section describes laboratory protocols for separat- measuring absorption spectra of dissolved organic matter ing the total spectral absorption coefficient, a(z,λ), into in water samples has been proposed by Yentsch and Phin- its components due to absorption by pure water, in vivo ney (pers. comm.). A high-volume 0.22 µm SterivexTM fil- phytoplankton pigments [aφ(λ)], dissolved material [ag(λ), ter cartridge† is used to perform a one-step filtration with Gelbstoff], and other particulate material [at(λ), tripton] the sample introduced directly into the cuvette from the by spectrophotometric measurements of filtered discrete filter cartridge. The sample is pushed through the car- water samples. Associated data analysis protocols are pre- tridge using a 50 cc sterile plastic syringe attached by a sented in Section 6.8.2. Luer lock connection. Sample handling is minimized and All equipment utilized in water sample processing for preparation time, including rinses, is only a fewminutes Gelbstoff samples should minimize contamination by or- per sample. The cartridges cost approximately $2.00 each, ganic or colored material and should protect samples from which makes them expensive to use for only one sample. photodegradation. Glass provides the best material for fil- Yentsch and Phinney found, however, that rinsing between tration units and sample storage containers. Glass filtra- samples with distilled water permits the use of one car- tion units should have stainless steel filter supports and not tridge per station with no measurable contamination ef- ground glass frits, since these tend to clog over time and fects. They verified non-contamination by measuring the change particle retention efficiencies of the units. Amber- colored borosilicate glass bottles are preferred for samples † A membrane filter made by Millipore Corporation.

40 Mueller and Austin absorption spectrum of a final distilled water rinse after water (ROW) through a rinsed GF/F filter for reference all samples for a station were filtered, and then compar- as a blank, discarding the first 100 ml. Begin the analyses ing the absorption spectrum of this sample to the baseline when the sample and reference material are both at room spectrum measured prior to the sample analysis. temperature. Perform a newbaseline measurement range A potential disadvantage of the Yentsch and Phinney using a ROW blank each time the instrument has been method results from the need to detach and reattach the powered up or the configuration changes. cartridge to the syringe between sample and rinses. Back Using a dual beam spectrophotometer, scan the instru- pressure applied while removing the syringe plunger with ment’s baseline optical density spectrum ODb(λ) from ap- the cartridge attached can rupture the membrane. This proximately 300–700 nm, beginning at the lowest ultravio- problem can be avoided by using a peristaltic pump to in- let (UV) wavelength the instrument will measure without troduce the sample and rinse water directly from separate cuvettes in the light path. Place the reference blank ROW, bottles. filtered as described above, in both sample and reference The samples should be allowed to equilabrate to room cuvettes and scan the reference optical density spectrum temperature before spectrophotometer measurements are ODr(λ). Rinse the sample cuvette with approximately made, and they should be protected from ambient light. 5 ml of the sample three times. Rinse and fill the sample If the Yentsch and Phinney suggestion to inject the sam- cuvette with the DOM sample filtrate and scan ODg(λ). ple directly into a cuvette is followed, this implies use of Replace reference water blanks every few samples to pre- several cuvettes per station and some type of dark storage vent bubble formation in the reference cuvette. Between arrangement. Otherwise, the samples should be injected samples, block the reference light and open the sample into amber bottles which have been prerinsed in the sam- compartment door to minimize temperature changes in the ple filtrate. It may be possible to wait approximately 20 ROW reference. minutes before filtering, to allowthe samples to warmto The GF/F filter (which is binder-free and combustible, room temperature, provided an investigator demonstrates with a nominal pore size of 0.7 µm) is the best choice for that this delay does not allowmodification of the sample particle absorption sampling. This filter has a large scat- filtrate. tering coefficient at visible wavelengths, which increases To summarize, the following method can be employed the optical path length of the photons and improves the ac- on samples collected and stored using any currently estab- curacy of particulate absorption measurements. This type lished technique: of filter is also recommended by (JGOFS 1991) for carbon, 1. Prerinse a Sterivex cartridge using 100 ml of hydrogen, and nitrogen (CHN); pigment; and primary pro- Nanopure or similarly polished DIW. ductivity analyses. The optical transparency of the GF/F 2. For spectrophotometric baseline determination, filter decreases significantly below380 nm. rinse the reference and sample cuvettes 2 or 3 Some authors have reported that particulate material times with filtered DIW and fill the cuvette, less than 0.7 µm in size will not be retained by the GF/F measure the absorption spectrum, and store it filter, and that this fraction may contain up to 10–15% as the baseline. of the phytoplankton biomass as measured by chlorophyll concentration. Chavez et al. (1995), however, found no 3. For each successive sample, evacuate the car- statistical difference between GF/F and 0.2 µm filters for tridge, and rinse it with 50 ml DIW. Then, evac- chlorophyll and productivity measurements. uate the cartridge and rinse it with a 50 ml sam- The goals for filtration of particulate samples are to ple. Refill the syringe with a sample, rinse the minimize contamination and particle degradation, maxi- sample cuvette, and inject the sample from the mize retention, and concentrate an adequate amount of syringe through the filter cartridge into the cu- particles on the filters to permit accurate spectrophoto- vette. Measure the absorption spectrum in a metric measurements. Vacuum pressures below5 inches of spectrophotometer (see below). mercury, i.e., 117 mm Hg, are recommended to reduce the 4. After all samples have been analyzed, rinse the chances of particle breakage. It is currently recommended cartridge with 50 ml DIW, refill the syringe with to set the filtration volume for each sample to approxi- DIW, rinse the sample cuvette and fill it with mate an optical density value of 0.25 at 400 nm (Roesler filtered DIW, and measure the absorption spec- and Perry 1992). Small area (13 mm2) filtration units may trum. Compare this absorption spectrum to the be used to reduce water sample volumes. baseline. Optical density spectra of the filters should be mea- To measure the absorption of the dissolved fraction, sured as soon as possible, because pigment decomposition minimize the distance between the detector and the cu- may occur (Stramski 1990). If necessary to store filters, vettes in the spectrophotometer, and use 10 cm quartz cu- place the unfolded filters into polypropylene tissue capsule vettes to maximize the absorption signal. Warm the fil- containers and store in liquid nitrogen. Freezing in conven- trate samples to room temperature while protecting them tional freezers causes ice crystals to form on the particles from direct light. Filter reverse osmosis or Milli Q purified and leads to cell breakage and pigment degradation.

41 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 A newinstrument baseline scan should be measured 5.5 ANCILLARY OBSERVATIONS each time the spectrophotometer is powered up or any- time its configuration has been changed. To measure the Ancillary observations are often of key importance in particulate absorption spectrum, first prepare a blank fil- flagging and interpreting apparently aberrant data. The ter by soaking it in 50 ml DIW for 20 minutes (remember- minimal set of ancillary supporting observations must in- ing to select it from the same box as the sample filters). clude: An alternative approach is to use prefiltered seawater (pre- 1) date and time (both GMT and local); filtered using a 0.2 µm Millipore GS filter) to wet the GF/F 2) geographic location, using the Global Position- filter blank (Cleveland pers. comm.), for the reason that ing System (GPS) if possible, before and after the sample filters are also wetted with seawater. If using each cast and at times of satellite and aircraft a single beam spectrophotometer, scan the instrumental overpasses; baseline ODb(λ) without the filter. Then insert the wet- 3) solar azimuth and zenith angles, as calculated ted filter blank to obtain the reference spectrum ODr(λ). from position, date, and time; With a dual beam spectrophotometer, two wetted refer- 4) position of the optical cast in relation to the ship ence filter blanks must be used to measure the reference orientation and position of the ship relative to spectrum, and one is left in the reference beam during sam- the sun (a sketch in the field notes is recom- ple measurements. Some investigators recommend aligning mended); the filter fibers in a fixed orientation each time a filter is inserted. 5) sea state (photographed if possible) with ap- If the samples were frozen, remove the sample filter proximate swell height, direction, and notes on from the liquid nitrogen. Place a fewdrops of DIW or presence and density of whitecaps; filtered seawater on a glass (or Plexiglas) slide and then 6) quantitative measurements of surface wire an- place the sample filter, bottom side down, on the water. gles during deployments of the instrument pack- This moistens the filter and hastens thawing. Place the age; filter in the sample compartment with the filtered mate- 7) time of cast (begin and end), as well as time and rial facing the source. Scan the ODp(λ). Place the sample depth of water samples collected; filter back in the filter cup. Gently pour 30–50 ml of hot 8) percent cloud cover and cloud type, and solar spectrophotometric- or HPLC-grade methanol over the fil- occlusion conditions; and ter, taking care not to lift any particulates off the filter 9) wind direction and velocity. (Kishino et al. 1985). Let the filter sit 10–15 minutes under the methanol, filter through, and repeat the process. Desirable additional ancillary measurements include: Rinse with 10–20 ml of 0.2 µm DIW. Seawater is not used a) an all-sky photograph plus a photographed time here, because methanol may cause precipitates to form history of sea surface is advised for radiometric on the filter. Position the filter in the spectrophotome- stations; and ter and scan ODt(λ). This spectrum is associated with b) Secchi depth. raw, non-pigmented particulates, often referred to as detritus or tripton. The absorption spectrum should decrease 5.6 PROTOTYPE OPTICAL BUOY exponentially with wavelength. If there is a residual chlorophyll a absorption peak at 676 nm, repeat the extraction A prototype optical measurement system designed for process until the peak disappears. long-term buoy deployment with a satellite data telemetry Recommended methods for analysis to convert the mea- capability is presently under development and is focused sured optical density spectra to absorption spectra are dis- on satisfying the SeaWiFS optical data requirements. The cussed in Section 6.8.2. concept is constrained by the requirement that the instrument be capable of maintaining measurement integrity 5.4.3 Total Suspended Matter while being unattended for long periods of time. This constraint has lead to a design that minimizes the num- All suspended particulate material (SPM) dry weight ber of moving parts to one, and has resulted in the spec- − (mg l 1) will be determined gravimetrically as outlined in trographic application of concave holographic diffraction Strickland and Parsons (1972) and as specified in JGOFS gratings. These holographic gratings provide an approxi- (1991). In general, samples are filtered through 0.4 µm mate flat focal field to the degree that planar silicon pho- preweighed polycarbonate filters. The filters are washed todiode arrays may be used as detectors. Inherent within with three 2.5–5.0 ml aliquots of DIW and immediately this technology are the features of simplicity, compactness, dried, either in an oven at 75◦ C, or in a dessicator. The durability, and stability. filters are then reweighed in a laboratory back on shore The optical system utilizes two spectrographs, in tan- using an electrobalance with at least seven digits of preci- dem with a dichroic mirror specifically designed to avoid sion. measurements in the water absorption region, in order to

42 Mueller and Austin measure radiometric properties with high spectral resolu- water-leaving radiance ratios from clear sky and tion and stray light rejection. The dichroic mirror is de- overcast conditions comparable enough that the signed to transmit the red (630–900 nm) and reflect the drifter data can provide a basis for interpolating blue portions (380–600 nm) of the spectrum, making the SeaWiFS data through cloudy periods? transition from reflectance to transmittance between 600 When answers to these fundamental questions are in hand, and 640 nm. The potential for stray light is greatly re- it will be possible to draft and implement more detailed duced by splitting the visible spectrum at the beginning protocols for the use of optical drifters in SeaWiFS radio- of the water absorption region, since most of the short metric validation and algorithm development. wavelength energy is diverted from the entrance slit of the Many potential applications of optical drifters in ocean- long wavelength spectrograph. The splitting also allows ographic research using SeaWiFS data are more obvious, the spectrographs, i.e., gratings and sampling periods, to but from a radiometric standpoint are less stringently de- be optimized for the two distinctive spectral domains. A manding. Protocols for those applications are, however, further reduction of stray light for the long wave spectro- beyond the scope of this report. graph will be achieved by utilizing a minus blue filter. The optical system will be deployed on a slack-line moored wave rider buoy that has a 10–20 m optical bench 5.8 SAMPLINGAND VALIDATION attached. Apparent optical properties will be measured The following discussion of bio-optical sampling pro- by a series of remote collectors that are coupled to the tocols is organized into three subtopics: sampling for the instrument with fiber optics. Data will be compressed, initial and ongoing validation of the SeaWiFS radiomet- stored, and forwarded through a NOAA Geostationary Op- ric system performance (Section 5.8.1), algorithm develop- erational Environmental Satellite (GOES) and an ARGOS ment and validation in Case-1 waters (Section 5.8.2), and telemetry link. algorithm development and validation in Case-2 waters This type of optical mooring represents a newand chal- (Section 5.8.3). The distinction between the first subtopic lenging technology. Detailed protocols for deploying and and the second two is clear-cut, but what precisely is meant maintaining this type of mooring, and for evaluating its by Case-1 and Case-2 water masses? data quality, must be developed in light of the experience In its literature and reports, the ocean color research to be gained over the next 2–3 years. community has formally adopted definitions originally due The current practice is to apply marine antifouling com- to Morel and Prieur (1977), who stated: pounds (such as OMP-8) to prevent the growth of marine “Case-1 is that of a concentration of phytoplankton organisms on the windows of moored radiometer systems. [which is] high compared to that of other particles. This approach is less satisfactory for IOP instruments, be- The pigments (chlorophyll, [and] carotenoids) play cause for collimated light, transmission characteristics of a major role in actual absorption. In contrast, the the optical windows can be adversely affected by the layer inorganic particles are dominant in Case-2, and pig- of the anti-fouling material. ment absorption is of comparatively minor importance. In both cases, [the] dissolved yellowsub- 5.7 DRIFTINGOPTICAL BUOYS stance is present in variable amounts and also con- Drifting optical instruments are a recent development tributes to total absorption.” and there is almost no history of their quantitative appli- In practice, however, only those water masses where cation to problems in ocean color algorithm development the CZCS-type blue-green ratio algorithms for phytoplank- and remote sensing radiometric validation. It is probable ton pigment concentration (chlorophyll a + phaeopigment that significant experience in the uses and limitations of a) work reasonably well have been treated as Case-1. All such instruments will be gained in SeaWiFS related exper- other water masses have often been loosely lumped into the iments during the prelaunch period. The critical questions Case-2 definition, albeit with considerable confusion over about these instruments, which should be answered during howto categorize coccolithophorid blooms, and similar prelaunch work, include: phenomona normally classified as Case-1 waters, in which 1. Howaccurately can LW (λ) be estimated from strong concentrations of Gelbstoff vary independently from Lu(z,λ) at a single near surface depth, using chlorophyll a concentration. only an estimate of KL(λ) obtained from ocean In the present discussion of sampling protocols, Case-1 color ratios? Is 5% uncertainty feasible? will be considered to refer to what might be called ordinary 2. Howaccurately can a normalized water-leaving open ocean Case-1 waters, wherein scattering and absorp- radiance ratio LWN (λ1)/LWN (λ2) be estimated tion are dominated by phytoplankton, pigments, and Gelb- with these instruments using only a single chan- stoff concentrations, and where global blue-green color ra- nel Ed(λ) measurement for normalization, which tio algorithms for chlorophyll a concentration and K(490) is contemplated for instruments currently being work well. Most areas in the deep ocean belong to this developed (Section 2.6)? Are the normalized case. Water masses which do not satisfy this criteria will

43 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 be grouped under the heading Case-2. Within Case-2, by the remote sensing reflectance. Vertical stratification of this definition, water masses with a wide diversity of bio- the water column becomes especially important in many optical characteristics will be found. Prominent subcate- Case-2 waters, where the top attenuation depth may be as gories include: shallowas 1–2 m and the entire euphotic zone may be con- 1) coccolithophorid blooms, wherein the detached fined to less than 10 m depth. It is important, therefore, coccoliths dominate light scattering and remote to minimize ship-induced disruption of vertical stratifica- sensing reflectance independently from pigment tion in the water column. Whenever possible, the ship concentration; should be maneuvered as little as possible while on station with its propellers and bow thruster, and the prac- 2) coastal areas, wherein DOM of terrestrial ori- tice of backing down hard to stop quickly when on station gin contributes a strong absorption component should be strongly discouraged. If wind and sea conditions which does not co-vary with pigment concentra- permit, the preferred method of approaching a station is tion; to take enough speed off the ship to coast to a stop over 3) phytoplankton blooms with unusual accessory approximately the last 0.5 km of approach to the station. pigment concentrations, e.g., red tides, which re- The approach should be planned to allowthe ship to be quire the use of special regional or local ocean turned, preferably using only the rudder, to place the sun color algorithms; and abaft the beam or off the stern, depending on where the 4) classical extreme Morel and Prieur (1977) Case- radiometers will be deployed. It must be realized, how- 2 waters where optical properties are dominated ever, that depending on wind and sea conditions, and a by inorganic particles, with many possible varia- particular ship’s hull and superstructure configuration, it tions in chemical and geometric characteristics. may not be possible to maintain an acceptable orientation, It is important to recognize that some aspects of the with respect to the sun, while the ship is adrift. In these water mass distinctions given above are dependent on the situations, some use of the engines to maintain an accept- spectral regions in which measurements are to be made. able ship’s heading may be unavoidable. Strong absorption at UV, red, and near-IR wavelengths The chief scientist should also consult with the ship’s requires the use of radiometric techniques similar to those captain and chief engineer to avoid, or at least minimize, required for Case-2 waters. overboard discharges while the ship is on station. Material In addition to determining the bio-optical category and from a ship’s bilge or sewage treatment system can signifi- characteristics of a particular water mass, the validation cantly change near-surface chemical and optical properties sampling strategy must be concerned with spatial and tem- if discharged near the immediate site of a bio-optical profile poral variability. Spatial and temporal variability in bio- or water sample. optical properties will profoundly affect the validity of com- In some coastal areas, where a relatively transparent parisons between SeaWiFS and in-water optical measure- water mass overlies a highly reflective bottom, LW (λ) in- ments. A single SeaWiFS instantaneous FOV measure- cludes light reflected from the sea floor. These cases require ment will integrate LW (λ) over approximately a square special treatment of bottom reflectance effects whether the kilometer, or a larger area at viewing angles away from local water mass regime is Case-1, Case-2, or a combina- nadir. Furthermore, the location uncertainty for a single tion of both. Methods of measurement, experiment design, pixel may be several kilometers, except in near-shore areas and sampling strategies to study bottom reflectance effects where image navigation can be improved by using land- are beyond the scope of this revision to the ocean optics navigated anchor points. protocols. There is a significant current research effort fo- Bio-optical profiles measured at a single station are rep- cused in this area (Carder et al. 1992, Hamilton et al. 1992, resentative of a spatial scale that is only a small fraction of and Lee et al. 1994), and it is expected that experience will a kilometer. Data from a grid of several station locations evolve the basis for newprotocols in this topic area to be may be required to estimate the spatial averages of opti- included in the next revision of this document. cal properties represented by a SeaWiFS pixel, or a block The bottom reflection of areas with a water depth ex- of pixels. Because the ship measurements over the grid ceeding 30 m normally does not contribute to the water are not instantaneous, temporal variability in bio-optical leaving radiance, LW (λ). Areas with a depth shallower properties can add additional uncertainty to the compar- than 30 m will be flagged in the SeaWiFS level-2 data isons. Aircraft radiometric observations can, conceptually, product. Pixels covering very turbid waters may, however, be used both to locate comparison sites away from areas of even be usable even in shallower areas. As a general rule, strong spatial variability and to document changes in the the water depth should be deeper than 2.5 attenuation pattern of spatial variability over the period required for a lengths, 1/K(490), at all SeaWiFS algorithm development ship to occupy all stations in a comparison grid. and validation stations. The prime exception to this rule Vertical stratification of water temperature, salinity, is in developing local SeaWiFS algorithms where bottom and density often affect the vertical structure of variability reflectance contributions must be taken into account (Lee in bio-optical properties. This variability, in turn, affects et al. 1994).

44 Mueller and Austin

Variables to be measured at each validation station are 5.8.2 Case-1 Water Protocols summarized in Table 1, and methods for making each measurement are discussed in the previous subsections of Sec- In open-ocean oligotrophic water, it is usually practi- tion 5. Methods of data analysis and reporting are dis- cal to assume that a station is in a Case-1 water mass, cussed in Section 6. although some caution must be taken to detect coccolithophorid blooms and suspended coccoliths. In more tur- 5.8.1 Initialization and Validation bid coastal transition regimes, however, the classification of the local water mass as Case-1 or Case-2 may be less ob- Data intended for direct comparisons between observed vious. In this environment, moreover, Case-1 and Case-2 Lu(λ) and SeaWiFS LW (λ) estimates should usually be ac- water masses may both be present in the domain sampled quired in areas where bio-optical variability is known to be by a ship. One example of this situation would be Case- very small. This will ordinarily dictate that such data be 1 water within an eddy-like intrusion from offshore into acquired from optically clear and persistently oligotrophic coastal areas normally occupied by Case-2 water masses. Case-1 water masses. Potentially suitable sites include the Another would be Case-2 waters in a major river plume northeastern Pacific central gyre off Baja, California (to intruding into an ambient Case-1 water mass regime. the southwest), and the central Sargasso Sea. When plan- In general, a water mass may be categorized as Case-1 ning validation cruise locations and timing, seasonal and if: regional cloud cover statistics should also be considered in 1) Gelbstoff absorption at 380 nm, ag(380), is less order to maximize the likelihood of simultaneous SeaWiFS − than 0.1m 1 (Sections 5.2.4, 5.4.2, 6.8.1, and and shipboard observations. A semi-oligotrophic site in 6.8.2); the northeast Pacific, near Hawaii, is the prime candidate −1 for placing a moored radiometer for continuous time-series 2) total SPM concentration is less than 0.5mgl (dry weight) (Section 5.4.4); radiometric comparisons with SeaWiFS LW (λ) estimates. A series of radiometric comparison stations should be 3) measured LW (λ) values, used in the SeaWiFS made over a wide range of latitude in both the Northern Case-1 algorithm, predict measured fluorometric and Southern Hemispheres, to look for evidence of cyclic chlorophyll a concentration within 35%; and thermal sensitivity affecting SeaWiFS. The spacecraft and 4) measured LW (λ), used in the SeaWiFS algo- instrument will be heated by sunlight throughout the de- rithm, predicts measured remote sensing K(490) scending (daylight) data acquisition segment of each orbit within 20%. and will be cooled by thermal radiation while in the Earth’s shadowthroughout the remainder of the orbit. This cy- The determination of criteria 1 and 2 above (Doerffer cling is likely to induce transient thermal gradients in the pers. comm.) will ordinarily require retrospective analy- instrument, as well as a time varying cycle in the tempera- sis. On the other hand, radiometric profiles and pigment tures of its detectors and other components; these thermal samples can ordinarily be analyzed on board to allowde- variations could affect the spectral bandpass or responsiv- termination of criteria 3 and 4 shortly after the samples ity of one or more SeaWiFS channels. Unfortunately, a are acquired. set of stations covering the full range of latitudes cannot SeaWiFS Case-1 algorithm development and validation all be sited in regions where mesoscale variability in ocean requires measurements from Case-1 water masses span- optical properties can be neglected. As when acquiring ning a wide range of optical properties and phytoplank- data for developing and validating Case-1 bio-optical algo- ton pigment concentrations. In optically transparent low- rithms (Section 5.8.2), a significant effort must be exerted chlorophyll oligotrophic water masses, spatial variability is to quantify spatial variability in normalized water-leaving usually small and a station location and sampling strategy radiance. When possible, airborne radiometer data, in like that discussed in Section 5.8.1 is appropriate. combination with careful characterization of atmospheric In high-chlorophyll mesotrophic Case-1 water masses aerosol and cloud conditions, should be employed to aug- with increased turbidity, mesoscale and smaller scale vari- ment shipboard radiometry at the stations selected for this ability is often significant. In very productive Case-1 water aspect of the validation. If aircraft support is not available, masses, station placement and many other aspects of sam- semi-synoptic shipboard transects covering a 20×20 km2 pling schemes are similar to those appropriate for Case-2 grid should be used to characterize spatial bio-optical vari- water masses (Section 5.8.3). At algorithm development ability near a sampling station. stations, where measurements need neither be coincident The minimum set of variables to be measured for radio- with, nor matched to, SeaWiFS observations, it will be nec- metric validation are those from the Product Verification essary to characterize spatial and temporal variability only column of Table 1. Measurements used to calculate nor- over the relatively short scales distinguishing the separate malized water-leaving radiance for direct comparison to in-water radiometric, optical, and pigment measurements. SeaWiFS radiances must be made under cloud-free condi- Airborne ocean color, or lidar characterizations, of spatial tions and within five minutes of the satellite overpass. variability in the vicinity of these stations will not usually

45 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 be essential, although such additional information may be To achieve valid comparisons between the ship and sat- very helpful. ellite data, sharp horizontal gradients and sub-pixel patchi- At stations where data are acquired for algorithm val- ness must be avoided, and image navigation must have land idation, and where a match to concurrent SeaWiFS mea- anchor points near the study site. Suitable landmarks are surements is required, it will be necessary to determine usually available in near-shore coastal waters. The other the patterns of spatial variability over a domain extending conditions are difficult to meet in Case-2 water masses, approximately 20×20 km2 centered at the station, and to where mesoscale and sub-mesoscale variability is typically place the ship in a 2×2km2 domain over which K(490) very strong. Sub-pixel variations of no more than ± 35% of and chlorophyll concentrations vary less than 35% about the mean pixel chlorophyll will be tolerated, but variabil- the mean. Within a fewhours before and after a Sea- ity must be measured and taken into account statistically WiFS overpass, in-water measurements should be made at in the analysis (see below). several random locations to characterize variability within From the above generalities, it is clear that signifi- the 2×2km2 validation comparison site. In some cases, cant problems are encountered in near-shore coastal wait may be possible to determine spatial variability ade- ters characterized by small scale patches and dynamic vari- quately from ship station data and along-track measure- ability due to tidal currents. A particular problem occurs ments alone. In regions of strong mesoscale variability, in the shallowareas which are influenced by strong tidal however, concurrent aircraft ocean color or lidar measure- currents—areas that are normally well mixed during part ments should be used both as a guide for selecting the of the tidal cycle. In the slack water tidal phase, how- ship’s location, and as a basis for spatially extrapolat- ever, a vertical gradient of the suspended matter concen- ing the in-water measurements to match the much coarser tration may form, which may cause problems in relating resolution of the SeaWiFS measurements (Section 5.1.7). water-leaving radiance to the concentration of suspended These topics are developed in more detail, with respect to matter. During calm periods with strong insolation, even Case-2 waters, in Section 5.8.3. water that is normally well mixed can become stratified. In these cases, the formation of very dense phytoplankton 5.8.3 Case-2 Water Protocols blooms, such as red tides, can be observed. Such blooms will occur in coastal seas when nutrient concentrations are Although coastal and continental shelf areas comprise elevated by the influx of river water. In these circumstan- only 10% of the total ocean area, they provide roughly ces, it is especially critical to avoid disturbing the vertical half of the oceanic newproduction and most of the se- stratification of the water column with the ship’s propellers questerable DOC (Walsh et al. 1981). These areas are (Section 5.8 above). typically higher in phytoplankton pigment concentration, and may include colored terrigenous constituents such as 5.8.4 Case-2 Sampling Strategy DOM and suspended sediments. In these Case-2 waters, the global color ratio algorithms break down because two One approach to sampling in this environment has been or more substances with different optical properties are suggested by R. Doerffer (pers. comm.). In order to get a present which do not co-vary with chlorophyll a concen- good statistical base, water samples are first taken in a ran- tration (Section 5.8 above). These might be waters with dom order within the area under research. The concentra- exceptional plankton blooms (such as red tides), areas dis- tions derived from the SeaWiFS data are then compared colored by dust transported by the wind from deserts into with the ground truth data by statistical parameters, such the sea, or coastal areas influenced by river discharge of as the mean, median, standard deviation, and the shapes mineral and organic suspended materials, and DOM, i.e., of histograms (frequency distribution). For this type of Gelbstoff, such as humic acids. statistical comparison, only sections of SeaWiFS images It is not always easy to decide to which case a wa- which match the area covered by the ship should be ana- ter mass belongs. As a starting point, the water belongs lyzed. Water samples and satellite data should also be tem- to Case-2 if any of the four Case-1 criteria, set forth in porally concurrent within the same tidal phase in order to Section 5.8.2, are not satisfied. For Case-2 waters defined avoid biases due to temporal variability. In these regimes, by any one of these criteria, it remains a further problem analyses to validate algorithms cannot be based on Sea- to determine the specific bio-optical characteristics which WiFS data directly, but must instead be based on water- distinguish it from Case-1. Case-2 sampling must usually leaving radiance spectra measured in situ (Sections 5.1.1– include a more complete subset of the variables in Table 1, 5.1.6) or from a ship (Section 5.1.8). This approach has in addition to ag(λ) (Sections 5.2.4 and 5.4.2), SPM con- the advantage that water samples and radiance spectra are centration (Section 5.4.3), chlorophyll a concentration, and taken nearly simultaneously. radiometry. For example, it may be necessary to determine Using either flow-through pumping systems or systems complete pigment composition and other optically impor- towed outside the ship’s wake, fluorometry can be used tant characteristics of exceptional phytoplankton blooms to assess chlorophyll patchiness if frequent, i.e., every 10- for such planktonic groups as coccolithophorids, diatoms, 15 minutes, chlorophyll fluorescence-yield calibration mea- cyanobacteria, or microflagellates. surements are performed. Towed absorption, scattering,

46 Mueller and Austin reflectance, and beam transmission meters can also be used lead to instrument self-shading effects in Lu(λ) which are to characterize spatial variability. Within a fewhours of correctable only for instruments with diameters no larger the overpass, the ship should occupy several stations at than approximately 1 cm (Gordon and Ding 1992). Ra- random locations within a 2×2km2 area central to the diometers with such a small shadow cross section are con- area selected for comparison with SeaWiFS data. Sam- ceptually feasible, and a fewprototype instruments ex- pling stations placed across a tidal front during a SeaWiFS ist which may be suitable, but they are not commercially overpass may help to identify two different water masses available, and self-shadowsensitivities have not yet been even when the front has moved. Comparisons between in experimentally verified for these extreme conditions. In situ and SeaWiFS data in patchy coastal areas may be these extreme cases, direct in situ measurements of a(λ) enhanced by using horizontal radiance profiles measured and c(λ) (Sections 5.2.4 and 6.8.1) and βb(θ, ∆θ, λ, ∆λ) from an aircraft flying at lowaltitude (Section 5.1.7). Sub- (Section 5.2.5), together with above-water measurements sets of such airborne profiles allowdirect comparisons with of LW (λ)orRL(λ), may provide the only practical means shipboard data. A corresponding profile may then be ex- of validating semi-analytic Case-2 algorithms. This topic tracted from the SeaWiFS data for a direct comparison to is an important area for near-term research and develop- the aircraft track line profiles. In Case-2 situations, such ment. direct radiometric comparisons are valuable for validating and tuning local algorithms, but are not appropriate for 5.9 VICARIOUS CALIBRATIONS SeaWiFS system validation per se. To validate the SeaWiFS atmospheric correction, water- An important obligation of any flight project is the leaving radiances measured in situ from the ship should production of a high quality, calibrated, Earth located be compared with those derived from the SeaWiFS data. (level-1) data set. Consequently, the production of a cali- Sample matching problems aside, Case-2 waters are of- brated set of SeaWiFS radiances that have been verified ten characterized by strongly varying patchiness in optical through direct, or vicarious, calibration techniques was properties, pigment concentrations, and remote sensing re- recommended by the original workshop participants. flectance at spatial scales smaller than a SeaWiFS pixel. One potentially useful technique follows the approach Because of the nonlinear relationship between bb(λ)/a(λ) currently in use to verify the responsivity of the AVHRR and remote sensing reflectance, the pigment concentration Television and Infrared Observation Satellite (TIROS) sat- derived from spatially averaged SeaWiFS LWN (λ) values ellite instruments. Twice a year, an aircraft instrument, will systematically underestimate the true spatial average which has been recently calibrated directly to laboratory- concentration by as much as a factor of 2 when sub-pixel based NIST traceable standards, should be used to obtain variability is significant. It is, therefore, essential to de- simultaneous views of a particular ocean scene. The air- scribe sub-pixel scale variability in Case-2 waters both craft scene must be obtained from a high altitude aircraft, statistically and in terms of organized structure. Such a such as the NASA Earth Resources-2 (ER-2), and flown at description may be accomplished through rapid sampling an altitude above most of the terrestrial atmosphere. The at closely spaced ship stations in combination with air- existing data sets of the AVHRR-NASA Aircraft/Satellite borne ocean color measurements—for this purpose, track Instrument Calibration (NASIC) Project demonstrate a line data from lowaltitudes and high-resolution imagery capability to limit the uncorrected trend in the AVHRR from high altitudes are both acceptable. data sets to under 2% over two years. This concept may Absorption coefficients are large enough in all Case- allowan independent verification of the atmospheric ra- 2 waters to require instrument self-shadow corrections to diative transfer models used to compute ocean biological − Lu(0 ,λ), even though the correction model (Gordon and quantities, when the aircraft data are used in conjunc- Ding 1992) has been experimentally verified only for the tion with the surface truth campaign measurements of case where a(λ)r is less than 0.1 (Sections 5.1.6 and 6.1.7). those same ocean biological quantities. The SeaWiFS ab- In extreme Case-2 waters, large values of spectral absorp- solute uncertainty requirements, however, are more strin- tion may confine the first optical attenuation depth to the gent than those associated with the AVHRR, and a cor- top 1–2 m, where it is difficult to measure remote sens- respondingly more accurate airborne radiometer system ing reflectance in situ. Such short absorption scale lengths (Sections 3.3 and 4.3) must be used (Section 2.3).

47 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Chapter 6

Analytical Methods

Introduction

This chapter describes methods for processing, analyzing, and reporting SeaWiFS validation data sets acquired following the protocols of Chapter 5. Where a consensus from the community exists for a preferred method, it has been adopted and protocols are given accordingly, e.g., for phytoplankton pigments (Section 6.5). In other instances, different methods are in current use, and as yet, there is insufficient evidence to give protocols specify- ing a preferred approach, e.g., the different methods of K-analysis described in Section 6.1.4. Intercomparisons are in progress to evaluate this particular topic.

6.1 IN-WATER RADIOMETRY 6) calibration date and file identification; 7) instrument identification; Spectral surface irradiance, Es(λ); upwelling irradiance, Eu(z,λ); downwelling irradiance, Ed(z,λ); and upwelling 8) method for determining K (level-3); radiance, Lu(z,λ), measurements should all be recorded 9) normalization algorithm (level-4); and archived at four levels. 10) Secchi depth; a) level-0: rawinstrument digital output; 11) depths of associated water samples, if any; and b) level-1: instrument output in volts, or frequency 12) depth offsets (to nearest cm) between the pres- (if appropriate), and depth; sure transducer and all sensor probes, including c) level-2: calibrated irradiance and radiance, an- Lu window, Ed and Eu collectors, and all ancil- cillary measurements in appropriate geophysical lary probes on a package. or biological units, and depth, with all variables In addition to profile files, each data set should contain: corrected for dark or zero offsets; i) calibration files used to compute level-2 data; d) level-3: Smoothed profiles of K(z,λ) and as- ii) level-0 and level-1 dark files, and an average sociated Ed(z,λ)orLu(z,λ) with irradiance or radiance normalized by measured surface irradi- dark voltage file used for computing the corre- ance; and sponding level-2 files (in some cases a dark value may be extracted from the deep portion of a pro- e) level-4: level-3 data normalized to clear-sky, a file); zenith sun at the mean Earth-sun distance, and spectrally adjusted to match the actual reference iii) files with data from comparisons with a portable wavelengths and FWHM bandwidths. irradiance and radiance reference standard made The formats of these data sets will vary somewhat be- in the field and used to track the instrument’s tween individual instruments. It is anticipated that the stability during a deployment; and SPO will promulgate suitable standard format specifica- iv) anecdotal and environmental information about tions, or guidelines, to facilitate database management and each profile, either in the header, or in an ac- interchanges of level-1, level-2, level-3, and level-4 data. companying American Standard Code for Infor- These data files should each contain a header record iden- mation Interchange (ASCII) text file. tifying as a minimum: In general, the data should be retained at full reso- 1) date and time, i.e., GMT, of the station; lution as recorded, but with noise-contaminated records 2) geographic location (latitude and longitude in removed through level-2. If the data are binned prior decimal degrees to the nearest 0.001); to K-determination, as is sometimes done in the derivative method (Section 6.1.4), the binned representations 3) cloud cover and sky conditions; should be recorded as a level-2a file, in addition to the full 4) identification of each variable, including units resolution level-2 file. Depth offsets between the sensor and wavelengths, for radiometric channels; port (where depth is determined) and the sensor apertures 5) source of dark (zero-offset) data; [Ed(λ) and Eu(λ) collectors and Lu(λ) window] must be

48 Mueller and Austin recorded as header information in level-2 data files, and 6.1.3 Normalization by Surface Irradiance applied to the data prior to K-analysis (see Section 6.1.4 below). The dominant errors in measured K(z,λ) profiles result from changes in cloud cover. Cloud cover variabil- 6.1.1 Instrument Calibration Analysis ity causes strong variations in incident surface irradiance, Es(λ, t), measured at time t, during the time required to Instrument data from pre- and post-deployment cali- complete a radiometric cast. In present usage, Es(λ, t) brations should be compared with: refers to incident spectral irradiance measured with a deck 1) each other; cell aboard a ship. Smith and Baker (1984 and 1986) dis- cuss a method for propagating Es(λ) through the sea sur- 2) the long-term history of an instrument’s calibra- − face to estimate Ed(0 ,λ), and they also present a model tions; and − for adjusting Ed(0 ,λ) to compensate for solar zenith an- 3) the record of comparisons with a portable field gle. irradiance and radiance standard, to be made An alternative, and conceptually better, scheme for es- frequently during a cruise. − timating Ed(0 ,λ) is to measure Ed(zr,λ) using a radiome- Based on this analysis of the instrument’s history, a cali- ter which has been floated away from the ship and held at bration file will be generated and applied to transform the a shallowdepth, zr, during a cast (Waters et al. 1990). In - data from level 1 to level-2. This analysis, and the ratio- either case, the record of Es(λ, t)orEd(zr,t) is recorded nale for adopting a particular set of calibration coefficients, together with profiles of Ed(z, λ, t), Eu(z,λ), and Lu(z,λ). both for responsivity and wavelength, should be fully de- Assuming that transmission of Es(λ, t) through the surface scribed in the documentation accompanying the data set, does not vary with time, then a simple and effective nor- preferably in an ASCII file to be retained on line with each malization of the profiles is obtained as data set. − Ed(z,λ)Es(0 ,λ) Ed(z,λ)= , (25) 6.1.2 Raman Corrections Es(λ, t)

Marshall and Smith (1990), and the references cited where Es(λ, t) is the deck cell irradiance measured at the − therein, showtranspectral Raman scattering contributes time t when the radiometer was at depth z and Es(0 ,λ)is significantly to measured irradiance between 500–700 nm. the measurement when the radiometer was at the surface. At a particular wavelength, the Raman contribution is Some previous investigators have used Es(λ, t) at a sin- excited by ambient irradiance at a wavenumber shift of gle reference wavelength, e.g., 550 nm, to normalize pro- 3,400 cm−1. For example, Raman scattering at a wave- files, and have thus ignored the usually small spectral vari- length of 500 nm (20,000 cm−1), is excited by light at wave- ations in incident irradiance. For SeaWiFS validation and length 427 nm (23,400 cm−1), and at 700 nm (14,286 cm−1) algorithm development, however, the recommended proto- −1 by light at 565 nm (17,686 cm ). Marshall and Smith col is to use multispectral Es(λ, t), or possibly near-surface (1990) give a transverse Raman scattering cross section (at Ed(zr,λ,t), to determine Ed(z, λ, t) at each wavelength. 90◦)of8.2 × 10−30 cm−2 molecule−1 sr−1, a value within Because of spatial separation between the surface and the range of other published observations. By integration, underwater radiometers, cloud shadow variations are not they derive a total Raman scattering coefficient of: measured, either identically or in phase, by the two instruments. The E (λ, t)orE (z ,λ,t) profiles should, there- × −4 −1 s d r br(488) = 2.6 10 m . (23) fore, be smoothed to remove high frequency fluctuations The wavelength dependence of the Raman scattering cross while retaining variations with periods of 15 seconds or − section is theoretically about the same as that for Rayleigh greater. The smoothed Es(0 ,λ)/Es(λ, t) profiles should scattering then be applied as a normalizing function to the irradiance − λ 4 and radiance profiles. b (λ) ≡ b (488) , (24) r r 488 6.1.4 K-Analysis although this has not yet been experimentally confirmed. A method for applying Raman corrections to measured Normalized and Raman corrected profiles of Ed(z,λ), profiles of irradiance and radiance is suggested and applied Eu(z,λ), and Lu(z,λ) (with z corrected for pressure trans- to homogeneous clear-water profiles by Marshall and Smith ducer depth offset relative to each sensor) should be fit to (1990). Additional work is needed to develop a robust the equations

Raman scattering correction model for general application z − in more turbid and vertically stratiﬁed water masses. The Kd(z ,λ)dz − relative magnitude, and thus importance, of the Raman Ed(z,λ)=Ed(0 ,λ) e 0 , (26) signal at each wavelength in the upper three attenuation z lengths should also be investigated more thoroughly than − Ku(z ,λ)dz − has been done to date. Eu(z,λ)=Eu(0 ,λ) e 0 , (27)

49 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 and at least 5–8 spectral bands. By using a multispectral ra- z − KL(z ,λ)dz diometer (such as the MER class of instruments) during a − Lu(z,λ)=Lu(0 ,λ) e 0 , (28) profile, the data in all channels will be sampled contem- poraneously and recorded digitally 2–10 times per meter. respectively. The vertical profiles of attenuation coeffi- These are level -1 data and are stored in files for subsequent cients Kd(z,λ), Ku(z,λ), and KL(z,λ), together with the − − − processing and analysis. respective values of Ed(0 ,λ), Eu(0 ,λ), and Lu(0 ,λ) The level-2 through level-4 data give increasingly re- at the surface, provide the needed specifications for the fined information in each successive processing level, which smoothed irradiance and radiance profiles. requires various amounts of intervention from the analyst. If the natural logarithm of (26), (27), and (28) is taken, After appropriate editing to remove artifacts, such as the equations of the following form are obtained: effects of ship shadow, vertical profiles of K are computed z from the logarithmic decrement with depth of the radio- − K(z)dz =lnE(z) − ln E(0−) , (29) metric profiles. Direct derivative method calculations of K profiles using computer techniques (see above) require 0 the use of a depth interval so large, frequently 20 m, that information about the slope, and hence, about K near the so that d ln E(z) top and bottom of the profile, is lost. Averaging over such K(z)=− . (30) dz a large interval causes the slopes in sharply defined layers, z e.g., regions of high gradients, to be poorly represented. The traditional method of K-analysis, e.g., Smith and Attempts to reduce these effects by using a significantly Baker (1984 and 1986), is to estimate K(z) as the local smaller depth interval result in unacceptably noisy K pro- slope of measured ln E(z) in an interval of a fewmeters files. centered at depth zm, i.e., at depths near depth zm, An improved approach, suggested by Petzold (1988), is to fit a series of analytic functions to the radiometric data ∼ ln E(z) = ln E(zm) − (z − zm)K(zm). (31) using non-linear least-squares regression fitting techniques. The profiles are broken up into as many layers as required, The unknowns ln E(zm) and K(zm) are determined as and functions are fit to each layer, with the constraint that the intercept and (negative) slope of a least-squares re- the functions and the derivatives of the functions be every- gression fit to measured ln E(z) data within the depth where continuous and finite. It is found that the logarithm interval zm − ∆z ≤ z

50 Mueller and Austin irradiance (Eu) or upwelling radiance (Lu). The form of At depth z, K(z,λ) is expressed as (32) is a hyperbolic tangent superimposed upon a straight line. It has a point of inﬂection at X = P4,Y = P1 and ap- K(z,λ)=K0(λ)γ01(ξ)+∂zK0(λ)γ11(ξ) ± (38) proaches the asymptote Y = P1 + P2B P3 as X becomes + K1(λ)γ02(ξ)+∂zK1(λ)γ12(ξ), larger or smaller than P4.

The first derivative is where K0 and K1 are values of K, and ∂zK0 and ∂zK1 are its vertical derivatives at the two nodes of the depth 2 dY − − P3 2 element containing z. With this representation of K(z,λ), = P2 1 . (33) dX P5 A + A it is possible to write (37) for each measured depth zm as the weighted sum When X = P4, B = 0, and Y = P1, then N τ(zm,λ)= hm,nKn + hm,n+N ∂zKn (39) dY P3 = −P2 − . (34) n=0 dX P5 for the n =0, 1,...,N nodes dividing the water column It is also possible to define K as the slope of the plot of into N depth elements. The coefficients hij are obtained as the natural logarithm of the measured radiometric variable analytic integrals over the Hermitian polynominals γij for against depth, or the finite elements above and including depth z; hij =0 for elements belowthe one containing z. Since such an dY K =2.3026 . (35) equation may be written for every measured optical depth, dX the profile may be represented in matrix form as At the point of inflection (X = P and Y = P ), 4 1 3τ = H K,3 (40) P3 where 3τ is the vector of measured optical depths, H is the K =2.3026 P2 + . (36) P5 3 matrix of coefficients hij, and K is the vector of Kn and ∂ K at the N nodes. The least-squares solution for the → z n In the limit K 2.3026 P2, and K will not exceed some 3 finite value. unknown vector K is obtained as The method described above was applied to radiomet- −1 3 HT H HT ric profile data from six cruises covering a wide variety of K = 3τ, (41) ocean regimes and latitudes from 24.0–77.4◦ N. Approxi- mately 2,100 profiles were fitted, and typically, the stan- which with (38) yields the complete profile K(z). dard deviation of the ratio between the function derived The surface boundary condition assumed by Mueller from the regression method and the original radiance and (1991) is that K(z) is constant between the sea surface irradiance data was 6% or less. For a large fraction of the (node 0) and the first subsurface node (node 1). If obvious fitted profiles, the standard deviations were between 1–3%. or suspected ship shadoweffects are present in the upper An alternative method of determining K-profiles was profile, the depth of node 1 is set immediately belowthe recently developed by Mueller (1991). Radiometric profiles affected area and the data in that top element are excluded are represented in terms of optical depth, τ, which from from the fit; the solution to (41) at nodes 0 and 1 is, in this case, determined entirely by the data from depths below (26), (27), and (28) is node 1. z The solution at the deepest node is not constrained and depends only on the observations in the depth ele- τ(z,λ)= K(z ,λ)dz ment immediately above it. The one-sided solution to (41) 0 (37) is often unstable at this node. If two nodes are placed close E(0−,λ) =ln . together at the bottom of the cast, then the unstable so- E(z,λ) lution is confined to only the bottom node, which may be discarded after (41) is solved. − The K-profile is represented analytically by Hermitian cu- In order to solve (41), the surface values of Ed(0 ,λ), − − bic polynomials, γij(ξ), over finite depth elements. The Eu(0 ,λ), and Lu(0 ,λ) must be independently deter- argument ξ is a local coordinate such that ξ = 0 at the mined or specified. At present, this is done iteratively center of a finite depth element, ξ = −1 at the shallowend by requiring the solution to closely approximate the mean point (node) of the element and ξ = 1 at the deep node. value of measurements in the top 1–2 m of the profile. Data [Hermitian cubic polynomials are defined in any text on from this near-surface layer are usually not significantly af- finite element modeling, e.g., Pinder and Gray (1977).] fected by ship shadow, but they may be severely affected

51 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

− Ku(z0,λ)z0 by irradiance fluctuations associated with light focusing by Eu(0 ,λ)=Eu(z0,λ)e , (44) surface waves. Additional research is needed to develop a more objective method of determining these surface values. and − KL(z0,λ)z0 In the present implementation (Mueller 1991), place- Lu(0 ,λ)=Lu(z0,λ)e . (45) ment of nodes is largely subjective, even when guided by If ship shadowis present, z0 may be 20 m or more, and the structure in accompanying c(660) and chlorophyll fluores- extrapolation becomes somewhat tenuous. cence profiles. Qualitatively, the integral solutions mimic If K(z) profiles are determined by means of the inte- − − − the structure in the c(660) and fluorescence profiles more gral method, then Ed(0 ,λ), Eu(0 ,λ), and Lu(0 ,λ) are faithfully than do the derivative solutions; they also do automatically determined as part of the fitting procedure. a better job of filtering irregularities that are apparently The surface values thus obtained are not necessarily su- associated with large fluctuations in deck cell irradiance. perior to those obtained by extrapolating the derivative Quantitative evaluation of sensitivity to exact node place- method solutions, but they do have the advantage of rep- ment is in progress. Development of objective criteria for resenting an internally consistent fit to the entire profile node placement will require further research. beneath the surface boundary layer. By either method, extrapolation of measured Ed(z,λ), − 6.1.5 Finite Bandwidth Correction Eu(z,λ), and Lu(z,λ)toz =0 becomes very difficult at λ ≥ 670 nm. At these wavelengths, the rapid decrease in Siegel et al. (1986) and Marshall and Smith (1990) dis- daylight over an extremely shallowfirst attenuation length cuss the effects of finite spectral FWHM bandwidth, and may compete with an increase in flux with depth due to the normalized response function, on determination of the chlorophyll fluorescence. Additional research is needed attenuation coefficient, K(λ), for a vertically homogeneous − to address measurement and estimation of E (0 ,λ) and water column. Given a channel’s nominal wavelength, λ , d L (0−,λ) at these wavelengths, especially in chlorophyll- and normalized response function, h(λ), the apparent at- u rich Case-2 waters. tenuation coefficient measured in a homogeneous water column is approximately 6.1.7 Instrument Self-Shading Corrections ∞ − K(λ)h(λ)e K(λ)zdλ A provisional protocol is given here for radiometer self- 0 − − Ks(z,λ )= ∞ . (42) shading corrections to Lu(0 ,λ) and Eu(0 ,λ) measure- h(λ)e−K(λ)zdλ ments (see also Section 5.1.6). The protocol is based on the 0 model of Gordon and Ding (1992) and limited experimental confirmation by Zibordi and Ferrari (1994). Although Marshall and Smith (1990) applied a correction for this additional research is necessary to extend and verify these effect to clear-water profiles of Ed(z,589). In general, cor- correction algorithms, the results published to date show rection of K (z,λ ) for finite bandwidth effects associated s clearly that even a provisional correction will significantly with K for pure water is straightforward. Additional re- improve L (0−,λ) and E (0−,λ) estimated from underwa- search will be needed to model, from the spectral irradi- u u ter measurements. ance data itself, additional bandwidth effects associated It is first necessary to estimate the spectral absorption with attenuation by phytoplankton and other particles, coefficient a(λ), preferably using measurements following and to correct K (z,λ) accordingly. s the protocols of Sections 5.2.4, 5.4.2, 6.8.1, and 6.8.2. It is also possible to estimate a(λ) using other approximations 6.1.6 Extrapolation to the Sea Surface suggested by Gordon and Ding (1992), based either on Because of surface waves, it is rarely possible to mea- measurements of phytoplankton pigment concentrations or sure Ed, Eu,orLu at depths that closely approximate of irradiance attenuation coefficients. ∼ − z = 0 . The shallowest reliable readings typically oc- It will also be necessary to measure, or estimate, the cur at depths ranging from 0.5–2 m. The data from this direct solar, Esun(λ), and skylight, Esky(λ), components of zone usually exhibit strong fluctuations associated with incident spectral irradiance, Es(λ), where Es(λ)=Esun(λ) surface waves, and thus require some form of smoothing +Esky(λ). Zibordi and Ferrari (1994) describe one method or averaging. It is almost always necessary to apply some of estimating the ratio Esky(λ)/Esun(λ), and Gordon and means of extrapolating the data upward to the sea surface. Ding (1992) suggest other alternatives. Whatever method is used should reconcile extrapolated Following Zibordi and Ferrari (1994), the coefficients, − Ed(0 ,λ) with deck measurements of Es(λ). κ , given in Table 2 of Gordon and Ding (1992), are fit If K(z) profiles are determined using the derivative to linear regression models as functions of the solar zenith ◦ ◦ method, the shallowest smoothed estimates will occur at angle θ0 in the range 30 ≤ θ0 ≤ 70 . The results given − depth z0 =∆z, if there are no ship shadoweffects. The for Lu(0 ,λ), with sun only, for a point sensor may be usual procedure is to extrapolate values to z =0− as computed as − − Kd(z0,λ)z0 × 3 Ed(0 ,λ)=Ed(z0,λ)e , (43) κsun,0 tan θ0w =2.07 + 5.6 10 θ0, (46)

52 Mueller and Austin and for a finite sensor occupying the full diameter of the so that − instrument, κsun =(1 f)κsun,0 + fκsun,l, (57) × −3 where f is the ratio of the diameter of the irradiance col- κsun,l tan θ0w =1.59 + 6.3 10 θ0, (47) lector to that of the instrument. where θ0 and θ0w are the solar zenith angles in air and For the sky component, κsky is defined as water, respectively, measured in degrees. In practice, the − diameter of the radiance sensor aperture is usually a small κsky =2.70 0.48f. (58) fraction of the instrument diameter. In the results reported by Zibordi and Ferrari (1994), the point sensor model al- Values of κsun and κsky from (57) and (58) are then sub- ways overestimated , and use of the finite sensor model stituted in equations (50) and (51) to obtain sun(λ) and (47) will always yield a lower estimate of . Pending new sky(λ), which are then used in (52) to solve for (λ). Fi- − insights from future theoretical and experimental work, it nally, corrected upwelled spectral irradiance Eu(0 ,λ)is is suggested to estimate estimated as

κ tan θ =(1−f)κ tan θ + fκ tan θ , (48) E (0−,λ) sun 0w sun,0 0 sun,l 0 E (0−,λ)= u , (59) u 1 − (λ) where f is the ratio of sensor-to-instrument diameters. The − − coefficient, κ , for the self-shading effect on Lu(0 ,λ) sky where Eu(0 ,λ) is determined from the upwelled spectral caused by incident diffuse skylight is similarly estimated irradiance profile (Section 6.1.6). as It is recommended that this correction algorithm be κ =4.61 − 0.87f, (49) − − sky applied to all Lu(0 ,λ) and Eu(0 ,λ) measurements used where the coefficients are derived from values given in for SeaWiFS validation and algorithm development. Rec- Table 3 of Gordon and Ding (1992). Self-shading errors ognizing the provisional nature of the correction, how- sun(λ) and sky(λ) for Esun(λ) and Esky(λ) components, ever, the uncorrected measured values must also be re- respectively, are then computed as ported. Moreover, the method and data used to estimate a(λ),E (λ), and E (λ) must be documented and re- sun sky κ a(λ)r sun(λ)=1− e sun , (50) ported with all data sets corrected using this protocol.

and κ a(λ)r 6.1.8 Spectral Adjustments sky(λ)=1− e sky , (51) Newmethods must be developed to reconcile in-water where r is the instrument radius in meters, and the ab- − measurements Lu(0 ,λ0 +∆λ1) integrated over a sensor sorption coefficient a(λ) is in units of m−1. − response function h1(λ) with SeaWiFS measurements of The self-shading error in Lu(0 ,λ) is then calculated Lt(λ0 +∆λ2) integrated over a wider sensor response func- as tion h2(λ). The challenge is to account for differing ra- sun(λ)+sky(λ)h (λ)= , (52) diometric sensitivities to fine-scale Fraunhofer structure 1+h in extraterrestrial solar spectral flux, F 0(λ), as modified where by atmospheric spectral transmittance, t(λ), and oceanic E (λ) h = sky . (53) spectral reflectance, R (λ). By assuming F (λ) is exactly E (λ) L 0 sun known, and that over the wavelength range defined by − Finally, the corrected radiance Lu(0 ,λ) is estimated h2(λ) and h1(λ), both t(λ) and RL(λ) vary slowly with as wavelength, it should be possible to adjust the LW (λ0 + L (0−,λ) − u ∆λ1) derived directly from the in-water instrument, to es- Lu(0 ,λ)= − , (54) 1 (λ) timate the water-leaving radiance LW (λ0 +∆λ2). This will − be transmitted through the atmosphere and contribute to where Lu(0 ,λ) is determined by analysis of the measured upwelled radiance profiles (Section 6.1.6). Lt(λ0 +∆λ2) measured by SeaWiFS. At the very least, − this type of correction should be practical for a given at- Similarly, for Eu(0 ,λ), the values given in Tables 2 and 3 of Gordon and Ding (1992) determine that for a mosphere. Prelaunch radiative transfer model sensitivity point irradiance sensor, studies and experimental verifications should be done to determine the magnitudes and uncertainties of such cor- − × −2 κsun,0 =3.41 1.55 10 θ0. (55) rections for the various SeaWiFS bands.

For an irradiance collector with a diameter equal to that 6.1.9 Normalized Water-Leaving Radiance of the instrument, To standardize the in-water SeaWiFS algorithms, it is − × −2 κsun,l =2.76 1.21 10 θ0, (56) necessary to normalize measured LW (λ) to those values

53 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 that would be measured were the sun at the zenith, at the where ρ(λ, θ) is Fresnel reflectance of the water surface. mean Earth-sun distance, and with the effects of the at- If Lsky(λ, θ, φ + π) is measured by viewing a horizontally mosphere removed. Following Gordon (1988), normalized oriented mirror of reflectance ρm(θ), then (64) must be water-leaving radiance as measured by the satellite sensor modified to may be defined as ρ(λ, θ) 2 LW (λ)=L(λ, θ, φ)+ Lsky(λ, θ, φ + π). (65) LW r ρm(λ, θ) LWN (λ)= ¯ , (60) t(λ, θ0) 1 − ρ(θ0) cos θ0 R + The remote sensing reflectance, RL(0 ,λ), may be cal- where θ0 is the solar zenith angle, ρ(θ0) is the air-water culated from LW (λ)as Fresnel reflectance for incident angle θ0, t(λ, θ0) is the at- L (λ) mospheric transmittance and R¯ is the mean Earth-sun dis- + W RL(0 ,λ)= , (66) tance. The Earth-sun distance on the day of the measure- Es(λ) ment, r, is given by where Es(λ) is incident spectral irradiance, preferably mea- R¯ sured concurrently with a calibrated irradiance meter, or r = , (61) 1+0.0167 cos D−3 estimated with greater uncertainty by viewing a horizon- 365 tally oriented gray reflectance plaque (Section 5.1.8). Nor- where D is the sequential day of the year. malized water-leaving radiance is similarly calculated using The (r/R¯)2 adjustment was not employed by Gordon (63). and Clark (1981) or Gordon (1988) because it cancels in An alternative approach may be used to calculate re- ratio algorithms, and the measurements they used were all flectance from an uncalibrated radiometer as taken within the span of a few months, so this source of S (λ) − ρ(λ, θ)S (λ) variation was very small in their data. The range of varia- R (0+,λ)= w sky , (67) L πS (λ)ρ (λ, θ) tion in (r/R¯)2 is approximately 6% over a full annual cycle. G g This adjustment should be made, nevertheless, for it be- where S (λ), S (λ), and S (λ) are the radiometer sig- comes important in algorithms predicting absolute values w sky G nals measured viewing the water, sky, and a reflectance of L (λ), as in the clear-water radiance model of Gordon W plaque of approximately 10% reflectance (e.g., Spectralon), and Clark (1981), and in algorithms for either estimating respectively; ρ(θ) is the Fresnel reflectance of the surface; or detecting anomalously high water reflectances in, for and ρ (λ) is the reflectance of the reference plaque. To example, a coccolithophore bloom. g remove residual surface reflectance due to wave facets, it To obtain L (λ) it is necessary to propagate L (λ) W u is assumed that the residual signal at 750 nm is entirely upward through the sea surface as due to surface reflection, so that the corrected reflectance − is calculated as + − 1 ρ(λ, θ) LW (0 ,λ)=Lu(0 ,λ) 2 , (62) nw(λ) − + RL(λ)=RL(λ) RL(0 , 750). (68) where ρ(λ, θ and n (λ) are the Fresnel reflectance and re- w When using this technique, it is critical to measure all fractive index of sea water, respectively. Normalized water radiances well clear of the ship’s superstructure and to hold leaving radiance is then computed as the reference plaque horizontally. It is more difficult to make verifiable corrections to F 0(λ) LWN (λ)=LW (λ) , (63) L(λ, θ, φ) when measured from low flying aircraft (Sec- E (λ) s tion 5.1.7). In this case, atmospheric transmittance and path radiance are not entirely negligible, and water-leaving where F (λ) denotes the mean extraterrestrial solar irra- 0 radiance must be calculated as diance (Neckel and Labs 1984). −1 ∗ LW (λ, θ, φ)=t L(λ, θ, φ) − L (λ, θ, φ) 6.2 ABOVE-WATER RADIANCE (69) − ρ(λ, θ)Lsky(λ, θ, φ + π), When upwelled radiance above the water surface is measured from a ship (Section 5.1.8), the measured ra- where t(λ, θ) is the atmospheric transmittance from the ∗ diance, L(λ, θ, φ), must be corrected for the reflected sky surface to the aircraft altitude, L (λ, θ, φ) is the atmos- radiance, Lsky(λ, θ, φ). If Lsky(λ, θ, φ + π) is measured by pheric path radiance at flight altitude, and Lsky(λ, θ, φ+π) directly viewing the sky, water-leaving radiance should be is the sky radiance incident at the surface. At altitudes calculated as up to 300 m, t(λ, θ) is close to 1, and may usually be estimated with relatively low uncertainty. Unfortunately, LW (λ)=L(λ, θ, φ)+ρ(λ, θ)Lsky(λ, θ, φ + π), (64) there is no straightforward way to measure Lsky(λ, θ, φ +

54 Mueller and Austin

π)orL∗(λ, θ, φ) from an aircraft. These variables must, Smith et al. (1991) and Dickey et al. (1991) together therefore, normally be estimated using an atmospheric ra- illustrate methods that can be used to specify protocols diative transfer model. Verification of model estimates for oceanographic analyses of bio-optical time series mea- ∗ of Lsky(λ, θ, φ + π) and L (λ, θ, φ) is difficult under ideal sured using moored optical systems. Such protocols would cloud-free sky conditions, and is wholly impractical under be very valuable for planning and executing oceanographic variable cloud cover. studies using data from moored systems together with Sea- Incident irradiance Ed(z,λ) measured at flight altitude WiFS time series data; they are not, however, directly rel- must also be propagated to the surface to compute RL(λ) evant to SeaWiFS validation. It is anticipated that optical and LWN (λ) from airborne measurements, i.e., protocols for US and International JGOFS will be published by working groups convened by these programs. LW (λ) RL(λ)= , (70) Ed(z,λ)td(z,λ) 6.4 AEROSOL OPTICAL DEPTH and If multiple measurements of the solar beam are ob- F 0(λ) tained during stable atmospheric conditions, then the Lan- LWN (λ)=LWN (λ) , (71) Ed(z,λ)td(z,λ) gley method can be used to obtain the atmospheric transmittance. This method consists of plotting the natural where td(z,λ) is the downward spectral irradiance transmittance from flight altitude z to the surface. logarithm of the voltage from the sun photometer versus the inverse of the cosine of the solar zenith angle. The The authors defer any recommendation for specific pro- slope of this straight line is the total optical depth of one tocols for applying atmospheric radiative transfer models atmosphere. If only a single measurement is obtained, the to estimate L (λ), R (λ), and L (λ) with (69)–(71), re- W L WN instrument calibration is applied to determine radiance, spectively. It is incumbent on each investigator to report which can be combined with the extraterrestrial solar ir- in detail the methods used to analyze (through measure- radiance to calculate the atmospheric optical depth. ments, models, or both) the necessary correction terms, To obtain the aerosol optical depth, total optical depth and to include a detailed error analysis of the results which must be used with computed optical depths due to molec- are obtained. Aircraft radiometric data sets that do not ular scattering (Rayleigh optical depth), and absorption include this information should not be used for SeaWiFS by ozone and other important gases (NO for some spec- algorithm development or validation. 2 tral bands). By subtracting the optical depths of these well-mixed gases from the total measurements, the aerosol 6.3 MOORED RADIOMETRY optical depth can be determined. Methods are not highly developed for analyzing data from moored radiometers to calculate LWN (λ). The prin- 6.5 SKY RADIANCE ciples of this analysis are well understood, but the commu- Sky radiance distributions, Lsky, measured with a cal- nity has had little experience with moored measurements ibrated radiance distribution camera, perhaps augmented of Lu(z,λ), determination of KL(z,λ), and extrapolation − by sun photometry or narrowFOV Lsky discrete measure- to Lu(0 ,λ). The moored optical system being developed ments in the zenith-sun plane, will be used to estimate by D. Clark of the NOAA National Environmental Satel- the aerosol phase function (Voss and Zibordi 1989). De- lite Data Information Service (NESDIS) for SeaWiFS and velopment of detailed protocols and methods of analysis, MODIS is the first system to be specifically engineered to including newinverse modeling techniques, for estimating address this problem. aerosol optical depths and phase functions will require new Smith et al. (1991) successfully acquired a nine-month + research. The spectral mean cosine µd(0 ,λ) for downwell- time series of spectral Ed(z, λ, t) and Lu(z, λ, t) at three ing radiance at the sea surface will be calculated directly depths (32, 52, and 72 m). They placed an additional from radiance distribution camera data, when available. above-water radiometer on a surface float, but this unit + Under cloud-free conditions, µd(0 ,λ) can also be esti- failed and provided no data. Smith et al. (1991) analyzed mated by measuring Esky(λ)+Esun(λ) with an irradiance the Kd(441) time series using a broad-band irradiance mea- deck cell. The algorithm for these computations is given − surement to estimate Ed(0 , 441) for the 0–32 m depth in- by Gordon (1989b). terval, and using measured Ed(z,441) over the 32–52 and When a spectral radiance distribution camera system 52–72 m depth intervals. They did not, however, estimate is not available and skies are not cloud free, it may be + KL(λ) for Lu(λ). They also developed and evaluated al- possible to estimate µd(0 ,λ) from some combination of gorithms for estimating phytoplankton pigment concentra- deck cell unshaded Esky(λ)+Esun(λ) and shaded Esky(λ) tions from spectral reflectance and from naturally stim- measurements, all-sky photographs, and measurements of ulated (by incident daylight) chlorophyll fluorescence at Lsky(λ, θi,φi) made at discrete angles with a hand-held Lu(683). Smith et al. (1991) demonstrated that continu- radiometer. Additional research will be required to de- + ous time series of Kd(λ, t) and pigment concentration may velop and test viable protocols for µd(0 ,λ) estimation be measured using this type of moored system. from these types of measurements.

55 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

where Vdark is the instrument’s current dark response with 6.6 PHYTOPLANKTON PIGMENTS the light path blocked, and Vair and Vair are, respectively, This section specifies methods of analysis for determin- the current air calibration voltage (Section 5.2.1) and the ing pigment concentrations by HPLC and fluorometry (fol- air calibration voltage recorded when the instrument was lowing acetone extraction) from filtered water samples ob- calibrated at the factory. V (z) is then converted to trans- tained at discrete depths, and for using in situ profiles of mittance, T (z,λ), over the transmissometer’s path length, chlorophyll a fluorescence to interpolate pigment concen- r, following the manufacturer’s instructions for the partic- tration over depth. ular instrument. The beam attenuation coefficient c(z,λ) is then com- 6.6.1 HPLC Pigment Concentration puted as 1 The JGOFS protocols and standards for HPLC pig- c(z,λ)=− ln T (z,λ) , (73) ment concentration analysis (JGOFS 1991) will be the pri- r mary method of determining pigment concentrations for all which has units of m−1. The apparent values of c(z,λ) SeaWiFS algorithm development and validation activities. should be further corrected, again following the manufacturer’s instructions, for the finite acceptance angle of the 6.6.2 Fluorometric Determination instrument’s receiver; this is usually a small, but significant, correction. Finally, the beam attenuation coefficient Protocols for fluorometric determination of the concen- due to particles is computed as trations of chlorophyll and phaeopigments were developed initially by Yentsch and Menzel (1963) and Holm-Hansen cp(z,λ)=c(z,λ) − cw(λ), (74) et al. (1965), and are described in detail by Strickland and

Parsons (1972). Although these measurements have been where cw(λ) is the beam attenuation coefficient, i.e., the shown to contain errors as compared to HPLC determina- sum of absorption, aw(λ), and scattering, bw(λ), for pure tions, e.g., Trees et al. (1985), the CZCS phytoplankton water. Smith and Baker (1981) tabulate aw(λ) and bw(λ) pigment concentration algorithms were based on them en- over the spectral range of interest here. tirely. The SeaWiFS protocols for this analysis will be those given in Strickland and Parsons (1972) as updated 6.8 SPECTRAL ABSORPTION by Smith et al. (1981). Protocols are given above in Section 5.2.4 for measuring 6.6.3 In Situ Chlorophyll a Fluorescence absorption profiles in situ using reflective tube meters, and in Section 5.4.2 for laboratory spectrophotometric analyses In situ fluorometers produce nearly continuous profiles of water samples captured at discrete depths. of artificially stimulated chlorophyll a fluorescence. Level -1 Data from a reflective tube absorption and beam atten- fluorometer data (in volts) should be converted to level-2 uation meter may be analyzed to obtain vertical profiles of simply by subtracting an offset, determined by shading the a(z,λ), ag(z,λ), and c(z,λ), and by difference b(z,λ)= instrument on deck. For qualitative guidance in K-profile c(z,λ) − a(z,λ) and a (z,λ)=a(z,λ) − a (λ, t), using the - p g analysis, level-2 (or even level 1) fluorometer profiles are protocols of Section 6.8.1. Optical density spectra for fil- adequate. trate and filtered water samples (Section 5.4.2) may be an- To produce vertical profiles of pigment concentration, alyzed to obtain independent measures of ag(z,λ), ap(z,λ), HPLC-derived pigment concentrations from water samples and at(λ, t), and by difference af (z,λ)=ap(z,λ)−at(z,λ), taken at discrete depths should be interpolated, with the using the protocols of Section 6.8.2. Methods for merging aid of in situ fluorescence profiles, for SeaWiFS bio-optical and comparing the two independent types of absorption algorithm development. These fluorescence interpolated measurements, and for interpreting the results in terms of profiles should then be used with K (z,λ) profiles to com- d remote sensing reflectance, are the subject of currently ac- pute optically weighted pigment concentration over the top tive research by several investigators. The next revision attenuation length (Gordon and Clark 1980). to this document may be expected to contain extensive modifications and extensions of these protocols. 6.7 BEAM ATTENUATION Rawbeam transmissometer voltage profiles, V(z), are 6.8.1 Reflective Tube Measurements first corrected for any range-dependent bias of the A/D At the time of this writing, there is only one commer- data acquisition system (Section 5.2.1). The corrected cially available reflective tube absorption and beam atten- voltages, V (z), are then further adjusted for instrument uation meter, the AC-9, manufactured by Western Envi- drift (occurring subsequent to the factory calibration) with ronmental Technology, Inc., of Philomath, Oregon. The the equation following data analysis protocol is, therefore, based on the Vair characteristics of that instrument (Zaneveld and Moore V (z)= V (z) − Vdark , (72) Vair pers. comm.).

56 Mueller and Austin

The AC-9 output in each a(λ)orc(λ) channel consists 1) the uncertainty in ci(λ) due to forward scattered of the digital responses of a reference detector and a wa- light, which is viewed by the detector (Section 6.7), ter transmission detector, Cr(λ) and Ct(λ). During the is a wavelength-independent fraction kc of total par- instrument’s factory calibration using optically pure water ticle scattering bp(λ); (Sections 4.4 and 5.2.4), the responses C(λ) and C (λ) r 2) the fraction k of bp(λ) included in ai(λ) is also in- are recorded and entered on the instrument’s calibration dependent of wavelength; and sheet, along with the instrument temperature T at which 3) at a near-IR wavelength λ , absorption is entirely the calibration was performed. Calibration factors for each r due to pure water, and the apparent absorption channel are then determined as ai(λr,T) is completely due to undetected scattering kbp(λr) and temperature dependent variations Cr(λa) −aw(λa)x A(λa)= e , (75) in water absorption (Pegau and Zaneveld 1993) rel- C (λa) t ative to absorption at T (the calibration tempera- for absorption at wavelength λa, and ture).

From assumptions 1) and 2), it follows that Cr(λc) −cw(λc)x A(λc)= e , (76) Ct(λc) b (λ ) b (λ ) p 1 = i 1 . (79) bp(λ2) bi(λ2) for beam attenuation at wavelength λc, where x is the optical pathlength in meters, and a (λ ) and c (λ ) are w a w c From assumption 3), it follows that the absorption and beam attenuation coefficients of optically pure reference water, respectively (Smith and Baker a (λ ,T)+S(λ )(T − T ) 1981). The instrument’s factory calibration also includes k = i r r , (80) bi(λr) determination of a temperature coefficient Bt(λn) for each channel. where S(λ ) represents the coefficient of water tempera- In the subsequent discussion in this section, the sub- r ture variation in aw(λ, T ). At λr = 712 nm, for example, script i designates initial values of the absorption beam at- − ◦ − S(712 = 0.0035 m 1 C 1, and S(λ) is negligible at λ< tenuation coefficients derived directly from measurements. 700 nm (Pegau and Zaneveld 1993). It is then useful to The subscript e denotes the corrected values of the same combine (79) and (80) to determine the absorption coeffi- coefficients due to (extra) substances other than water. cient a (λ) due to substances other than water (Zaneveld When data are recorded in the field, apparent absorp- e pers. comm.) as tion coefficients ai(λa) due to substances other than pure water are calculated as a (λ)=a (λ, T ) e i − − − Ct(λa,T) ai(λr,T) S(λr)(T T ) bi(λ) (81) ai(λa,T)= ln TA(λa) − . Cr(λa,T) bi(λr) (77) +(T − T )B (λ ) t a When equations (75) through (81) are applied to unfiltered + a (λ ) − a (λ ) , air a air 0 total absorption measurements, it follows that where T and T denote the instrument temperature, and aair(λ0) and aair(λ0) are air calibration values at the times ae(λ)=ap(λ)+ag(λ), (82) of the instrument factory calibration and field data recording, respectively (Section 5.2.4). A similar equation is where ap(λ) and ag(λ) are respectively the particulate and applied to obtain apparent beam attenuation coefficients Gelbstoff (dissolved substances) components of absorption. ci(λc) for substances other than water at each wavelength. If a 0.2 µm intake filter canister is used to exclude the par- Apparent absorption coefficients ai(λ, T ) must be cor- ticulate fraction from the water, then ae(λ) is due to ag(λ) rected for the fraction of scattering that is undetected by alone. If profiles are made with two AC-9 meters (one fil- the reflective tube and detector assembly (Section 5.2.4). tered and one unfiltered), the total absorption coefficient An initial estimate of particle scattering at each wave- may, therefore, be partitioned as length bi(λ) is obtained as a(λ)=aw(λ)+ap(λ)+ag(λ). (83) bi(λ)=ci(λ) − ai(λ, T ), (78) To further partition ap(λ)intoaφ(λ)(phytoplankton pig- recalling that attenuation by pure water already has been ments) and at(λ)(tripton) absorption coefficient compo- removed from ci(λ) and ai(λ, T ) using the instrument’s nents, it is necessary to obtain discrete water samples at calibration coefficients. It is assumed that: several depths distributed over the profiles, and to apply

57 Ocean Optics Protocols for SeaWiFS Validation, Revision 1 the filtration and spectrophotometry laboratory analyses some investigators have used apparent optical densities at described in Sections 5.4.2 and 6.8.2. 750 nm for this correction, but recent work has shown this The fraction of total particle scattering, kcbp(λ), in- wavelength to be inappropriate in some oceanic waters. cluded in the beam transmission measurement may be esti- The prevalent current practice is to estimate β empiri- mated from the detector acceptance angle and path length cally through a function that may be expressed in the form geometry of a particular instrument. Using this coefficient, −1 the corrected non-water beam attenuation coefficient ce(λ) β = C1 + C2 ODp(λ) − ODr(λ) , (90) may be calculated as

where C1 and C2 are coefficients of a polynomial regression c (λ)=c (λ)+k c (λ) − a (λ) , (84) e i c e e fit. For example, Mitchell (1990) gives C1 =0.392 and C2 =0.665, while Cleveland and Weidemann (1993) give or C1 =0.378 and C2 =0.523. ci(λ) − kcae(λ) ce(λ)= . (85) Spectrophotometers that disperse white light into its 1 − k c spectrum using a monochromator prior to illuminating the Therefore, the corrected particle scattering coefficient sample are termed pre-sample monochromators, while in- may be estimated as struments that pass the light through a monochromator after it has passed through the sample are termed post- − bp(λ)=ce(λ) ae(λ). (86) sample monochromators. The value of the pathlength amplification factor has been found to be independent of 6.8.2 Analysis of Absorption Spectra post-sample optical geometry for a variety of standard

Spectral absorption coefficients ag(λ) (Gelbstoff), ap laboratory instruments with pre-sample monochromators −1 (total particulates), and at (tripton) in units of m are (Mitchell 1990, Stramski pers. comm., and Cleveland pers. calculated from optical density spectra measured using the comm.). Clark (pers. comm.) and Cleveland (pers. comm.) protocols of Section 5.4.2. For Gelbstoff, have found, however, that diode array spectrophotometers with a post-sample grating require the use of an integrating 2.303 sphere attachment which may affect the β factor. Lee et al. ag(λ)= ODg(λ) − ODr(λ) , (87) l (1994) use a diode array system, which collects and focuses the post-sample light and gives rawOD values similar where l is the cuvette pathlength (0.1 m), and the other p to standard instruments with pre-sample monochromators terms are introduced in Section 5.4.2. (Cleveland pers. comm. and Carder pers. comm.). An equation similar to (87) is used to calculate ab- Substituting (90) in (89) directly yields the spectral sorption coefficients a (λ) and a (λ), but in these cases, p t absorption coefficient for particles in suspension as a correction must be applied to account for the effective increase in pathlength due to scattering in the GF/F fil- 2.303A ters (Mitchell and Kiefer 1984). The nominal absorption ap(λ)= C1 ODp(λ) − ODr(λ) pathlength l of the filtered material in suspension is given V s (91) 2 by + C OD (λ) − OD (λ) , V 2 p r l = , (88) s A where V is the volume of water filtered and A is the clear- and for tripton ance area of the filter. Scattering of light within the GF/F 2.303A filter lengthens the absorption pathlength. This factor is ap(λ)= C1 ODt(λ) − ODr(λ) termed the β factor by Mitchell and Kiefer (1988) after V (92) Butler (1962), which is not to be confused with the volume 2 + C OD (λ) − OD (λ) . scattering coefficient β(θ) used elsewhere throughout this 2 t r document. The absorption coefficient of filtered particles must, therefore, be corrected for pathlength amplification Finally, the spectral absorption coefficient for in vivo phy- and the equivalent absorption in suspension is computed toplankton pigments is computed as as 2.303A − a (λ)= OD (λ) − OD (λ) . (89) aφ(λ)=ap(λ) at(λ). (93) p βV p r To correct for residual offsets in the filter spectrophoto- The best overall performance of this algorithm will be metric measurement, it is assumed that at 850 nm the true achieved when filtered sample density yields ODp(675) be- optical densities are zero. The apparent measured values tween 0.05 and 0.1. The volume of water filtered for parti- of ODp(850) and ODr(850) are, therefore, subtracted from cle absorption measurements should therefore be adjusted ODp(λ) and ODr(λ) prior to applying (89). Previously, accordingly (Section 5.4.2).

58 Mueller and Austin

For purposes of data reporting and archiving, the ab- ity spiking, an artifact which occurs when water tempera- sorption coefficients ag(λ),ap(λ), and at(λ) will be reported ture changes at a rate faster than the conductivity probe in m−1 as computed using equations (87), (91), and (92), can follow. The CTD data should then be processed to respectively. Uncorrected optical density spectra for the profiles of potential temperature (◦C), salinity (Practical blank filters and distilled water blank must be recorded Salinity Units [PSU] based on the Practical Salinity Scale and provided with absorption coefficients to allow reversal of 1978, PSS78), and density (kg m−3) using the algo- of the correction algorithms. The values of ODp(850) and rithms which have been endorsed by the United Nations ODr(850), which have been subtracted to remove resid- Educational, Scientific, and Cultural Organization (UN- ual instrument offset, must also be reported with each ab- ESCO)/SCOR/International Council of Exploration of the sorption spectrum. The pathlength amplification factor, Seas (ICES)/IAPSO Joint Panel on Oceanographic Tables together with a description of the method and date of its and Standards, and also by SCOR Working Group 51 (Fo- determination, must also be reported with all particle ab- fonoff and Millard 1983). sorption coefficients based on filter spectrophotometry. At this stage, each set of CTD profiles should be care- Roesler and Perry (pers. comm.) have proposed a theo- fully examined to detect any significant static instability retically derived expression for β, with a formulation differ- artifacts resulting from salinity spiking. After any such ent from (90). At this writing, however, this approach has major artifacts are removed by editing, the data should be not gained widespread acceptance in the community. Fur- further smoothed by averaging temperature and conduc- ther reviewof this protocol is warranted for possible future tivity data into 2 m depth bins, and the final profiles of revision. Sosik and Mitchell (1994) have proposed a spec- salinity, density, and other derived parameters should be trofluorescence excitation method to make direct estimates recomputed using the smoothed CTD profile. of absorption by the photosynthetically active pigments For any hydrographic station, descriptive hydrographic aps(λ) . They found aps(λ) ≤ aφ(λ), where the difference analyses should include T-S profile characterizations of wa- is attributable to photo-protective pigments in the phyto- ter masses. Features in the density profile which appear plankton. Carder and some of his associates (pers. comm.) to be related to physical mixing and stability should be M recommend the determination of aφ (666) in the methanol compared with features in the corresponding bio-optical extract using a spectrophotometric method at 666 nm with profiles. CTD profiles from horizontal transects (i.e., two- 850 nm zero. This method sets an upper limit to aφ(675) dimensional grids) should be used in the computation of for the unpackaged state, and allows an alternate estimate two-dimensional sections, or three-dimensional gridded ar- of chlorophyll a. rays, for such variables as geostrophic currents, temperature, salinity, and the density anomaly σt. These analy- 6.9 CTD PROFILE ANALYSES sis products, together with corresponding two- or three- dimensional representations of bio-optical variability, can Each CTD profile should be prefiltered to remove any be used to estimate the relative importance of advection depth reversal segments resulting from violent ship or hy- and isopycnal mixing in redistributing or modifying upper drowire motions. This will remove many instances of salin- ocean optical properties during a cruise.

59 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

Acknowledgments Glossary

These protocols originated from discussions and draft material A/D Analog-to-Digital contributed by the workshop participants in April 1991. The ALSCAT ALPHA and Scattering Meter (Note: the symbol authors thank all of them for their help. The participants’ α corresponds to c(λ), the beam attenuation coeffi- names and affiliations (as of the time the workshop was held in cient, in present usage.) April 1991) are given in Table 7. AOL Airborne Oceanographic Lidar ARGOS Not an acronym: the name given to the data collection and location system on NOAA Operational Table 7. Workshop Participants. Satellites Name Affiliation ASCII American Standard Code for Information Inter- Workshop Co-chairs change AVHRR Advanced Very High Resolution Radiometer Roswell Austin SDSU/CHORS AVIRIS Advanced Visible and Infrared Imaging Spectrome- James Mueller SDSU/CHORS ter Workshop Participants BSI Biospherical Instruments, Inc. Robert Arnone NOARL/Stennis Space Center CDOM Colored Dissolved Organic Material Rocky Booth BSI CHN Carbon, Hydrogen, and Nitrogen Kendall Carder Univ. of South Florida CTD Conductivity, Temperature, and Depth Dennis Clark NOAA/NESDIS CW Continuous Wave Donald Collins Jet Propulsion Laboratory CZCS Coastal Zone Color Scanner Wayne Esaias NASA/GSFC DIW Distilled Water Howard Gordon Univ. of Miami DOC Dissolved Organic Carbon Bruce Guenther NASA/GSFC DOM Dissolved Organic Matter Ronald Holyer NOARL/Stennis Space Center Robert Kirk NASA/GSFC ER-2 Earth Resources-2, a research aircraft Charles McClain NASA/GSFC FEL Not an acronym; a type of standard lamp for irra- James McLean NASA/GSFC diance and radiance calibration B. Gregory Mitchell NASA Headquarters FOV Field-of-View Donald Montgomery Ofc. Oceanogr. of the Navy FWHM Full-Width at Half-Maximum Raymond Smith Univ. of Calif. Santa Barbara GAC Global Area Coverage Charles Trees SDSU/CHORS GASM General Angle Scattering Meter Kenneth Voss Univ. of Miami GF/F Not an acronym; a specific type of glass fiber filter Alan Weidemann NOARL/Stennis Space Center manufactured by Whatman Ronald Zaneveld Oregon State University GMT Greenwich Mean Time Administrative Support GOES Geostationary Operational Environmental Satellite Meta Frost Westover Consultants GPS Global Positioning System GSFC Goddard Space Flight Center HPLC High Performance Liquid Chromatography Over the past two years, many of the above scientists, and others from the SeaWiFS Science Team and oceanographic com- IAPSO International Association for the Physical Sciences munity at large, have participated in the ongoing evolution of of the Ocean these ocean optics protocols, culminating in the present revi- ICES International Council on Exploration of the Seas sion. Several people deserve special recognition and thanks IFOV Instantaneous field-of-view for their time and effort. Robert Bidigare (Univ. of Hawaii) IOP Inherent Optical Properties drafted the improved protocols for pigment analyses. Ronald IR Infrared Zaneveld contributed draft versions of new protocols for cali- JGOFS Joint Global Ocean Flux Study brating and using instruments for in situ spectral absorption and beam attenuation profiles, and for backscattering measure- MARS Multispectral Airborne Radiometer System ments. Expanded protocols for spectrophotometric determi- MER Marine Environmental Radiometer nation of absorption by particles and dissolved matter were MERIS Marine Environment Research Imaging Spectrora- contributed through the combined efforts of Charles Trees, B. diometer (French) Gregory Mitchell, Joan Cleveland (SDSU/CHORS), Charles MODIS Moderate Resolution Imaging Spectrometer Yentsch (Bigelow Marine Laboratory), Colin Roesler (Univ. of NAS National Academy of Science Connecticut), Kendall Carder, and David Phinney (Bigelow NASA National Aeronautics and Space Administration Marine Laboratory). New protocols for Case-2 waters and NASIC NASA Aircraft/Satellite Instrument Calibration above-water radiometry are based on material contributed by NESDIS National Environmental Satellite Data Information Kendall Carder, Roland Doerffer (GKSS Forschungszenstrum, Service Germany), Curtiss Davis (Naval Research Laboratory), and NIST National Institute of Standards and Technology Robert Arnone. Thanks are due to all of these people, and NOAA National Oceanic and Atmospheric Administration other colleagues who continue to participate in the critical re- NOARL Naval Oceanographic and Atmospheric Research view, discussion, and improvement of this body of protocols. Laboratory

60 Mueller and Austin

OCTS Ocean Color and Temperature Sensor (Japanese) B Fitting coefficient for eB/P5 . OFFI Optical Free-Fall Instrument b(z,λ) Total scattering coefficient. OMP-8 Not an acronym; a type of marine anti-biofouling b(z,λ0,θ) Volume scattering coefficient. compound bb(z,λ) Spectral backscattering coefficient. OSFI Optical Surface Floating Instrument bi(λ) Initial estimate of the particle scattering coefficient; PAR Photosynthetically Available Radiation used for determining the apparent particle scatter- POC Particulate Organic Carbon ing coefficient for substances other than water. POLDER Polarization and Directionality of the Earth Re- bp(λ) Total particle scattering. flectance (French) br(λ) Total Raman scattering coefficient. PON Particulate Organic Nitrogen bw(λ) Scattering coefficient for pure water. PSU Practical Salinity Units c(z,λ) Spectral beam attenuation coefficient. PTFE Polytetrafluoroethylene, commonly known by the c(z,660) Red beam attenuation (at 660 nm). trade name Teflon ce(λ) Corrected non-water beam attenuation coefficient. QED Quantum Efficient Device ci(λ) Initial estimate of the beam attenuation coefficient; used for determining the apparent beam attenua- ROSIS Remote Ocean Sensing Imaging Spectrometer, also tion coefficient for substances other than water. known as the Reflecting Optics System Imaging cp(λ) Beam attenuation coefficient due to particles. Spectrometer (German) cw(λ) Beam attenuation coefficient for pure water equal ROV Remotely Operated Vehicle to a (λ)+b (λ). ROW Reverse Osmosis Water w w C1 Polynomial regression factor. SCOR Scientific Committee on Oceanographic Research C2 Additional polynomial regression factor. SeaWiFS Sea-viewing Wide Field-of-view Sensor C (λ) AC-9 factory calibration coefficient. SIRREX SeaWiFS Intercalibration Round-Robin Experiment Cr(λ) Additional AC-9 factory calibration coefficient. SNR Signal-to-Noise Ratio Cr(λ) Digital response of reference detector. SPM Suspended Particulate Material Ct(λ) Digital response of water transmission detector. SPO SeaWiFS Project Office D Sequential day of the year. SPSWG SeaWiFS Prelaunch Science Working Group d Distance of lamp source from collector surface. SST Sea Surface Temperature T-S Temperature-Salinity E(λ) Irradiance. ˆ TIROS Television Infrared Observation Satellite E(z,m Smoothed estimate of irradiance obtained by least- TSM Total Suspended Material squares regression fit in the center of a depth interval. UNESCO United Nations Educational, Scientific, and Cul- Ea(λ) Irradiance in air. tural Organizations Ecal Calibration source irradiance. UV Ultraviolet − Ed(0 ,λ) Incident spectral irradiance. UVB Ultraviolet-B Ed(z,λ) Normalized downwelled spectral irradiance WMO World Meteorological Organization Ed(z,λ) Downwelled spectral irradiance Es(λ) Surface irradiance. Symbols Esky(λ) Spectral sky irradiance. Esun(λ) Spectral sun irradiance. − A Fitting coefficient for P4 X, or clearance area of Eu(z,λ) Upwelled spectral irradiance. a filter, depending on usage. Ew(z,λ) Irradiance in water. A(λa) AC-9 instrument calibration factor for absorption. F (λ) Mean extraterrestrial solar flux. A(λc) AC-9 instrument calibration factor for beam atten- 0 uation. Fi(λ) Immersion correction factor. a(λ) Total absorption coefficient. Fv(λ) Field-of-view coefficient. a(z,λ) Spectral absorption coefficient. f Ratio of sensor-to-instrument diameters. ae(λ) Absorption coefficient due to substances other than f(T ) Offset voltage correction from the linear function water. characterizing temperature response. − af (z,λ) ap(λ) at(z,λ). G(z,λ) Solid angle dependence with water depth. ag(λ) Gelbstoff spectral absorption coefficient. g(T ) Coefficient of a linear function characterizing tem- ai(λa,T) Initial estimate of the apparent absorption coeffi- perature response. cient; used for determining the apparent absorption coefficient for substances other than water. H Matrix of coefficients hij . M aφ (λ) Phytoplankton pigment spectral absorption coeffi- hij Analytic integral coefficients over the Hermitian pol- cient determined in methanol extract. ynomials γij . ap(λ) Particulate spectral absorption coefficient. h(λ) Normalized response function. a (λ) Photosynthetically active pigment spectral absorp- ps tion coefficient. K Vector of Kn. at(λ) Tripton spectral absorption coefficient. K(λ) Generic irradiance attenuation coefficient. aw(λ) Absorption coefficient for pure water. K(z,λ) Diffuse attenuation coefficient. aφ(λ) Phytoplankton pigment spectral absorption coeffi- Kd(z,λ) Vertical attenuation coefficient for downwelled irra- cient. diance.

61 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

KL(z,λ) Attenuation coefficient upwelled radiance. V Volume of water filtered. Kn K at node depth zn determined, with its vertical Vˆ True voltage. derivative by least-squares fit to radiometric pro- V˜ Measured voltage. files. (See equations 37–41 and surrounding text.) V (z) Transmissometer voltage. Ks(z,λ ) Apparent attenuation coefficient measured in a ho- Vair Current transmissometer air calibration voltage. mogenous water column. V (θ) Normalized measured value for a cosine collector. Ku(z,λ) Vertical attenuation coefficient for upwelled irradi- V (θi) Mean normalized measured value of instrument re- ance. sponse. k Fractional factor of total particle scattering. Vair Factory transmissometer air calibration voltage. k y/ tan θ0w. Vdark Transmissometer dark response. kc Wavelength independent fraction. W Weighting function. l Cuvette pathlength. θ ls Nominal absorption pathlength. X Depth in meters. L(z,θ,φ) Submerged upwelled radiance distribution. L∗(λ, θ, φ) Atmospheric path radiance at flight altitude. y Empirical factor. Lcal Calibration source radiance. Y Base 10 logarithm of the radiometric measurement Lsky(λ, θ, φ) Spectral sky radiance distribution. Ed, Eu,orLu. L Radiance at top of atmosphere. t z Vertical coordinate. Lu(z,λ) Upwelled spectral radiance. ˆ z Corrected depth for pressure transducer depth offset Lu(λ) True upwelled spectral radiance. relative to a sensor. L˜u(λ) Measured upwelled spectral radiance. zm Centered depth. LW (λ) Water-leaving radiance. zr Shallow depth. LWN (λ) Normalized water-leaving radiance. zs Exclusion depth due to data contamination. L1(λ) Apparent radiance response to a linearly polarized source. β Filter absorption correction factor for scattering L2(λ) Orthogonal apparent radiance response to a linearly within the filter. polarized source. β(z,θ,λ) Spectral volume scattering function. n Starting index in measurement for angular measure- βb The measured integral of the volume scattering func- ments, or node index for the integral K analysis. tion in the backward direction. The symbol in the text—β (θ, ∆θ, λ, ∆λ)—is for the measurement in ng(λ) Index of refraction of Plexiglas. b nw(λ) Index of refraction of water. the backward direction over the angle θ at a given N Ending index in measurement sequence for angular wavelength λ. measurements. γij (ξ) Hermitian cubic polynomial. ODb(λ) Baseline optical density spectrum. ∆z Half-interval depth increment. ODg(λ) Optical density of soluble material (Gelbstoff). ∆θ Angular increment. ODp(λ) Optical density spectra of filtered particles. δ Cosine response asymmetry. ODr(λ) Optical density reference for filter or distilled water. ODt(λ) Optical density of non-pigmented particulates (trip- 2 Cosine collector response error. ton). 2sun Self-shading error for Esun. P (λ) Polarization sensitivity. 2sky Self-shading error for Esky. −k a(λ)r Pi Fitting coefficient for i =1to5. ε(λ)1− e . − − Q(λ) Lu(0 ,λ)toEu(0 ,λ) relation factor (equal to π θ Centroid angle of the scattering measurement. for a Lambertian surface). θa In-air measurement angle. R Mean Earth-sun distance. θi Any nominal angle. RL Reflectance from an uncalibrated radiometer. θN Angular terminus. RL(z,λ) Spectral reflectance. θn Angular origin. r Radius, or Earth-sun distance depending on usage. θt Tilt angle. θw In-water measurement angle. S(λr) A coefficient of water temperature variation in θ0w Refracted solar zenith angle. aw(λ, T ). θ1 Lower integration limit. SG(λ) Radiometer signal (uncalibrated) measured viewing a reflectance plaque. θ2 Upper integration limit. Ssky Radiometer signal (uncalibrated) measured viewing κ Self-shading coefficients. the sky. SW (λ) Radiometer signal (uncalibrated) measured viewing λ Channel of nominal wavelength. the water. λ0 Center wavelength. T Instrument temperature during calibration. λn Any nominal wavelength. λr Near-IR wavelength. Tg(λ) Transmittance through a glass window. Ts(λ) Transmittance through the surface. µd(z,λ) Spectral mean cosine for downwelling radiance at Tw(λ) Transmittance through a water path. depth z. td(z,λ) Downward spectral irradiance transmittance from flight altitude z to the surface. ξ(λ) Minimum ship-shadow avoidance distance.

62 Mueller and Austin

ρ(λ) Bidirectional reﬂectance. Butler, W.L., 1962: Absorption of light by turbid materials. J. ρg(λ) Gray card or plaque reﬂectance. Opt. Soc. Amer., 52, 292–299.

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65 Ocean Optics Protocols for SeaWiFS Validation, Revision 1

, K.J. and G. Zibordi, 1989: Radiometric and geometric Vol. 2 calibration of a spectral electro-optic “ﬁsheye” camera ra- Gregg, W.W., 1992: Analysis of Orbit Selection for SeaWiFS: diance distribution system. J. Atmos. Ocean. Technol., 6, Ascending vs. Descending Node. NASA Tech. Memo. 652–662. 104566, Vol. 2, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Mary- Walker, J.H., R.D. Saunders, J.K. Jackson, and D.A. McSpar- land, 16pp. ron, 1987: Spectral Irradiance Calibrations. NBS Special Publication 250–20, U.S. Dept. of Commerce, National Bu- Vol. 3 reau of Standards, Washington, DC, 37 pp. plus appen- McClain, C.R., W.E. Esaias, W. Barnes, B. Guenther, D. En- dices. dres, S. Hooker, G. Mitchell, and R. Barnes, 1992: Cal- ibration and Validation Plan for SeaWiFS. NASA Tech. , C.L. Cromer, and J.T. McLean, 1991: Technique for im- Memo. 104566, Vol. 3, S.B. Hooker and E.R. Firestone, proving the calibration of large-area sphere sources. Ocean Eds., NASA Goddard Space Flight Center, Greenbelt, Optics, B.W. Guenther, Ed., SPIE, 1,493, 224–230. Maryland, 41 pp.

Walsh, J.J., G.T. Rowe, R.L. Iverson, and C.P. McRoy, 1981: Vol. 4 Biological export of shelf carbon is a sink of the global CO2 McClain, C.R., E. Yeh, and G. Fu, 1992: An Analysis of GAC cycle. Nature, 291, 196–201. Sampling Algorithms: A Case Study. NASA Tech. Memo. 104566, Vol. 4, S.B. Hooker and E.R. Firestone, Eds., Waters, K.J., R.C. Smith, and M.R. Lewis, 1990: Avoiding NASA Goddard Space Flight Center, Greenbelt, Mary- ship induced light field perturbation in the determination land, 22 pp., plus color plates. of oceanic optical properties. Oceanogr., 3, 18–21. Vol. 5 Weinreb, M.P., G. Hamilton, S. Brown, and R.J. Koczor, 1990: Mueller, J.L., and R.W. Austin, 1992: Ocean Optics Protocols Nonlinear corrections in calibration of Advanced Very High Resolution Radiometer infrared channels. J. Geophys. Res., for SeaWiFS Validation. NASA Tech. Memo. 104566, Vol. 95, 7,381–7,388. 5, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 43 pp. Wright, S.W., S.W. Jeffrey, R.F.C. Mantoura, C.A. Llewellyn, Vol. 6 T. Bjornland, D. Repeta, and N. Welschmeyer, 1991: Im- proved HPLC method for the analysis of chlorophylls and Firestone, E.R., and S.B. Hooker, 1992: SeaWiFS Technical carotenoids from marine phytoplankton. Mar. Ecol. Prog. Report Series Summary Index: Volumes 1–5. NASA Tech. Ser., 77, 183–196. Memo. 104566, Vol. 6, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Wyman, M., 1992: An in vivo method for the estimation of phy- Maryland, 9 pp. coerythrin concentrations in marine cyanobacteria. Lim- Vol. 7 nol. Oceanogr., 37, 1,300–1,306. Darzi, M., 1992: Cloud Screening for Polar Orbiting Visible Yentsch, C.S., and D.W. Menzel, 1963: A method for the deter- and IR Satellite Sensors. NASA Tech. Memo. 104566, Vol. mination of phytoplankton, chlorophyll, and phaeophytin 7, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard by fluorescence. Deep-Sea Res., 10, 221–231. Space Flight Center, Greenbelt, Maryland, 7 pp.

Zaneveld, J.R.V., J.C. Kitchen, A. Bricaud, and C. Moore, Vol. 8 1992: Analysis in in situ spectral absorption meter data. Hooker, S.B., W.E. Esaias, and L.A. Rexrode, 1993: Proceed- Ocean Optics XI, G.D. Gilbert, Ed., SPIE, 1750, 187– ings of the First SeaWiFS Science Team Meeting. NASA 200. Tech. Memo. 104566, Vol. 8, S.B. Hooker and E.R. Fire- stone, Eds., NASA Goddard Space Flight Center, Green- , 1995: A theoretical deviation of the dependence of the belt, Maryland, 61 pp. remotely sensed reﬂectance on the IOP. J. Geophys. Res., (in press). Vol. 9 Gregg, W.W., F.C. Chen, A.L. Mezaache, J.D. Chen, J.A. Zibordi, G., and G.M. Ferrari, 1995: Instrument self-shading Whiting, 1993: The Simulated SeaWiFS Data Set, Ver- in underwater optical measurements: experimental data. sion 1. NASA Tech. Memo. 104566, Vol. 9, S.B. Hooker, Appl. Opt., (submitted). E.R. Firestone, and A.W. Indest, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 17 pp. The SeaWiFS Technical Report Series Vol. 10 Vol. 1 Woodward, R.H., R.A. Barnes, C.R. McClain, W.E. Esaias, Hooker, S.B., W.E. Esaias, G.C. Feldman, W.W. Gregg, and W.L. Barnes, and A.T. Mecherikunnel, 1993: Modeling C.R. McClain, 1992: An Overview of SeaWiFS and Ocean of the SeaWiFS Solar and Lunar Observations. NASA Color. NASA Tech. Memo. 104566, Vol. 1, S.B. Hooker Tech. Memo. 104566, Vol. 10, S.B. Hooker and E.R. Fire- and E.R. Firestone, Eds., NASA Goddard Space Flight stone, Eds., NASA Goddard Space Flight Center, Green- Center, Greenbelt, Maryland, 24 pp., plus color plates. belt, Maryland, 26pp.

66 Mueller and Austin

Vol. 11 Vol. 19 Patt, F.S., C.M. Hoisington, W.W. Gregg, and P.L. Coronado, McClain, C.R., R.S. Fraser, J.T. McLean, M. Darzi, J.K. Fire- 1993: Analysis of Selected Orbit Propagation Models for stone, F.S. Patt, B.D. Schieber, R.H. Woodward, E-n. Yeh, the SeaWiFS Mission. NASA Tech. Memo. 104566, Vol. S. Mattoo, S.F. Biggar, P.N. Slater, K.J. Thome, A.W. 11, S.B. Hooker, E.R. Firestone, and A.W. Indest, Eds., Holmes, R.A. Barnes, and K.J. Voss, 1994: Case Studies NASA Goddard Space Flight Center, Greenbelt, Mary- for SeaWiFS Calibration and Validation, Part 2. NASA land, 16pp. Tech. Memo. 104566, Vol. 19, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard Space Flight Center, Vol. 12 Greenbelt, Maryland, 73 pp. Firestone, E.R., and S.B. Hooker, 1993: SeaWiFS Technical Re- port Series Summary Index: Volumes 1–11. NASA Tech. Memo. 104566, Vol. 12, S.B. Hooker and E.R. Firestone, Vol. 20 Eds., NASA Goddard Space Flight Center, Greenbelt, Hooker, S.B., C.R. McClain, J.K. Firestone, T.L. Westphal, Maryland, 28 pp. E-n. Yeh, and Y. Ge, 1994: The SeaWiFS Bio-Optical Vol. 13 Archive and Storage System (SeaBASS), Part 1. NASA Tech. Memo. 104566, Vol. 20, S.B. Hooker and E.R. Fire- McClain, C.R., K.R. Arrigo, J. Comiso, R. Fraser, M. Darzi, stone, Eds., NASA Goddard Space Flight Center, Green- J.K. Firestone, B. Schieber, E-n. Yeh, and C.W. Sulli- belt, Maryland, 40 pp. van, 1994: Case Studies for SeaWiFS Calibration and Val- idation, Part 1. NASA Tech. Memo. 104566, Vol. 13, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Vol. 21 Space Flight Center, Greenbelt, Maryland, 52 pp., plus Acker, J.G., 1994: The Heritage of SeaWiFS: A Retrospec- color plates. tive on the CZCS NIMBUS Experiment Team (NET) Pro- Vol. 14 gram. NASA Tech. Memo. 104566, Vol. 21, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Mueller, J.L., 1993: The First SeaWiFS Intercalibration Round- Center, Greenbelt, Maryland, 43 pp. Robin Experiment, SIRREX-1, July 1992. NASA Tech. Memo. 104566, Vol. 14, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Vol. 22 Maryland, 60 pp. Barnes, R.A., W.L. Barnes, W.E. Esaias, and C.R. McClain, Vol. 15 1994: Prelaunch Acceptance Report for the SeaWiFS Ra- Gregg, W.W., F.S. Patt, and R.H. Woodward, 1994: The Sim- diometer. NASA Tech. Memo. 104566, Vol. 22, S.B. Hook- ulated SeaWiFS Data Set, Version 2. NASA Tech. Memo. er, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard 104566, Vol. 15, S.B. Hooker and E.R. Firestone, Eds., Space Flight Center, Greenbelt, Maryland, 32 pp. NASA Goddard Space Flight Center, Greenbelt, Mary- land, 42 pp., plus color plates. Vol. 23 Vol. 16 Barnes, R.A., A.W. Holmes, W.L. Barnes, W.E. Esaias, C.R. Mueller, J.L., B.C. Johnson, C.L. Cromer, J.W. Cooper, J.T. McClain, and T. Svitek, 1994: SeaWiFS Prelaunch Radio- McLean, S.B. Hooker, and T.L. Westphal, 1994: The Sec- metric Calibration and Spectral Characterization. NASA ond SeaWiFS Intercalibration Round-Robin Experiment, Tech. Memo. 104566, Vol. 23, S.B. Hooker, E.R. Firestone, SIRREX-2, June 1993. NASA Tech. Memo. 104566, Vol. and J.G. Acker, Eds., NASA Goddard Space Flight Center, 16, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Greenbelt, Maryland, 55 pp. Space Flight Center, Greenbelt, Maryland, 121 pp. Vol. 17 Vol. 24 Abbott, M.R., O.B. Brown, H.R. Gordon, K.L. Carder, R.E. Firestone, E.R., and S.B. Hooker, 1995: SeaWiFS Technical Re- Evans, F.E. Muller-Karger, and W.E. Esaias, 1994: Ocean port Series Summary Index: Volumes 1–23. NASA Tech. Color in the 21st Century: A Strategy for a 20 -Year Time Memo. 104566, Vol. 24, S.B. Hooker and E.R. Firestone, Series. NASA Tech. Memo. 104566, Vol. 17, S.B. Hooker Eds., NASA Goddard Space Flight Center, Greenbelt, and E.R. Firestone, Eds., NASA Goddard Space Flight Maryland, (in production). Center, Greenbelt, Maryland, 20 pp. Vol. 18 Vol. 25 Firestone, E.R., and S.B. Hooker, 1994: SeaWiFS Technical Re- Mueller, J.L., and R.W. Austin, 1995: Ocean Optics Protocols port Series Summary Index: Volumes 1–17. NASA Tech. for SeaWiFS Validation, Revision 1. NASA Tech. Memo. Memo. 104566, Vol. 18, S.B. Hooker and E.R. Firestone, 104566, Vol. 25, S.B. Hooker, E.R. Firestone, and J.G. Eds., NASA Goddard Space Flight Center, Greenbelt, Acker, Eds., NASA Goddard Space Flight Center, Green- Maryland, 47 pp. belt, Maryland, 67 pp.

67 Form Approved REPORT DOCUMENTATION PAGE OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503. 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED February 1995 Technical Memorandum 4. TITLE AND SUBTITLE 5. FUNDING NUMBERS SeaWiFS Technical Report Series Volume 25–Ocean Optics Protocols for SeaWiFS Validation, Revision 1 Code 970.2

6. AUTHOR(S) James L. Mueller and Roswell W. Austin

Series Editors: Stanford B. Hooker and Elaine R. Firestone Technical Editor: James G. Acker 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Laboratory for Hydrospheric Processes REPORT NUMBER Goddard Space Flight Center 95B00043 Greenbelt, Maryland 20771

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING AGENCY REPORT NUMBER National Aeronautics and Space Administration Washington, D.C. 20546–0001 TM–104566, Vol. 25

11. SUPPLEMENTARY NOTES James L. Mueller and Roswell W. Austin: San Diego State University, San Diego, California; Elaine R. Firestone: General Sciences Corporation, Laurel, Maryland

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE Unclassified–Unlimited Subject Category 48 Report is available from the Center for AeroSpace Information (CASI), 7121 Standard Drive, Hanover, MD 21076–1320; (301)621-0390 13. ABSTRACT (Maximum 200 words) This report presents protocols for measuring optical properties, and other environmental variables, to validate the radiometric performance of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and to develop and validate bio-optical algorithms for use with SeaWiFS data. The protocols are intended to establish foundations for a measurement strategy to verify the challenging SeaWiFS uncertainty goals of 5% in water-leaving radiances and 35% in chlorophylla concentration. The protocols first specify the variables which must be measured, and briefly review the rationale for measuring each variable. Subsequent chapters cover detailed protocols for instrument performance specifications, characterizing and calibrating instruments, methods of making measurements in the field, and methods of data analysis. These protocols were developed at a workshop sponsored by the SeaWiFS Project Ofice (SPO) and held at the Naval Postgraduate School in Monterey, Califor- nia (9--12 April 1991). This report began as the proceedings of the workshop, as interpreted and expanded by the authors and reviewed by workshop participants and other members of the bio-optical research community. The protocols are an evolving prescription to allow the research community to approach the unprecedented measurement uncertainties implied by the SeaWiFS goals; research and development are needed to improve the state-of-the-art in specific areas. These protocols should be periodically revised to reflect technical advances during the SeaWiFS Project cycle. The present edition (Revision 1) incorporates new protocols in several areas, including expanded protocol descriptions for Case-2 waters and other improvements, as contributed by several members of the SeaWiFS Science Team.

14. SUBJECT - TERMS 15. NUMBER OF PAGES SeaWiFS, Oceanography, Ocean Optics, Bio-optics, Protocols, Standards, Data 67 Requirements, Specification, Sensor Characterization, Measurement Protocols, 16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT OF REPORT OF THIS PAGE OF ABSTRACT Unclassified Unclassified Unclassified Unlimited

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