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5 Advancing Global Ocean Color Remote Sensing into the Future O ENHANCEMENTS FOR THE FUTURE cean color observations from satellites are the princi- pal tool for the synoptic global monitoring of marine Advancing Ocean Biology and Biogeochemistry ecosystems. It is imperative that ocean color obser- Research vations are sustained and enhanced into the future. These observations serve the expanding needs of the scientific In 2007, the ocean biology and biogeochemistry research user community as it seeks to understand long-term trends community completed a consensus document that laid out in marine ecosystems and their interactions with the global four priority science questions for the NASA Ocean Biol- carbon cycle. In turn, managers apply this new knowledge to ogy and Biogeochemistry program (NASA, 2007). These value and manage marine resources. This means that future questions are: sensors and algorithms need to be enhanced to support an increasing diversity of ocean color products and applications. • How are ocean ecosystems and the biodiversity they However, creating long time-series remains a major chal- support influenced by climate and environmental variability lenge, one that is not unique to ocean color remote sensing and change, and how will these changes occur over time? (NRC, 2004b, 2008b). These challenges and approaches • How do carbon and other elements transition between for sustaining long-term ocean color remote sensing are various reservoirs in the ocean and Earth system, and how do explored in this chapter. The key requirements include plan- biogeochemical fluxes impact the ocean and Earth’s climate ning to ensure continuity and overlap among sensors; build- over time? ing and maintaining the human capital to process, reprocess, • How (and why) is the diversity and geographical and use ocean color products for research; and building distribution of coastal marine habitats changing, and what international coordination and cooperation. are the implications for the well-being of human society? Although the study task focuses on data continuity, it • How do hazards and pollutants impact the hydrogra- is important to note that continuity and advancements in the phy and biology of the coastal zone? How do they affect us, science require far more than simply sustaining SeaWiFS/ and can we mitigate their effects? MODIS-type measurements. Many applications listed in Chapter 2 require more advanced remote sensing capabili- The implementation strategy calls for a mix of new ties. This chapter, therefore, explores options for enhancing sensors including: ocean color research products. In addition, the U.S. academic research community, which is primarily funded by National 1. a global hyperspectral imager that would be an Aeronautics and Space Administration (NASA), needs ocean advanced Type 1/Type 2 sensor with capabilities as envi- color instruments such as Aerosol-Cloud-Ecosystems (ACE) sioned for ACE and PACE; and the Pre-Aerosol-Clouds-Ecosystems (PACE) for new 2. a Multi-Spectral High Spatial Resolution Imager and improved applications such as those described in a recent similar to Hyperspectral Infrared Imager (HyspIRI); NASA report and summarized below. 3. an ocean color sensor in geostationary orbit to focus on coastal and ocean processes that require multiple obser- vations during a single day to resolve changes on short time scales (like Geostationary Coastal and Air Pollution Events [GEOCAPE]); and 58
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59 ADVANCING GLOBAL OCEAN COLOR REMOTE SENSING INTO THE FUTURE Multi-Spectral High Spatial Resolution Imaging 4. a space-borne LIght Detection And Ranging (LIDAR) for improved atmospheric correction and oceanographic Many coastal applications—such as monitoring for measurements, as is also planned for ACE (NASA, 2007). Harmful Algal Blooms (HABs), ecosystem-based fisheries management, and research on benthic habitats including B oth the NASA Ocean Biology and Biochemistry coral reefs and coastal wetlands—require greater spatial (OBB) program and the National Research Council’s (NRC) resolution and additional spectral bands than are currently decadal survey plans call for a mix of ocean color satellite available from most satellites to resolve the complex optical mission types that would help ocean scientists answer the signals that coastal waters produce. These measurements high-level science questions they now face. historically have been made from airborne sensors, usu- ally flown by airplanes over a particular region. Airborne hyperspectral observations are well suited for routine stud- Global Hyperspectral Imaging Radiometer ies of localized areas (e.g., coral reefs, seagrass beds) and Answering the first two priority science questions above for episodic events (e.g., HABs, oil spills) that require high will require the development of advanced global remote spatial or spectral resolution, or on-demand repeat times. sensing capabilities. The planned NASA missions PACE/ The technology is well proven for mapping shallow-water ACE will provide some of these new capabilities, including: bathymetry and bottom type (e.g., Mobley et al., 2005; Dekker et al., in press), mapping and monitoring coral reefs • Ultraviolet (UV) bands to improve the separation (Hochberg and Atkinson, 2003; Lesser and Mobley, 2007), of chlorophyll and color dissolved organic matter (CDOM) and detection of oil spills (Lennon et al., 2006). For example, absorption and thus significantly improved accuracy of both the Airborne Visible and InfraRed Imaging Spectrometer1 products. This capability is especially important because of (AVIRIS), developed by the Jet Propulsion Laboratory (JPL), projected changes in the ocean due to rising temperatures made many flights over the Deepwater Horizon BP oil spill and ocean acidification. site.2 The hyperspectral information enabled researchers to • Short wave infrared (SWIR) bands (1,200-1,700 nm) map out the oil spill location and thickness. In addition, JPL demonstrated by Moderate Resolution Imaging Spectroradi- is supporting the construction of a portable hyperspectral ometer (MODIS), which in that case led to improved atmo- imager (i.e., the Portable Remote Imaging SpectroMeter spheric correction over turbid coastal waters, in comparison [PRISM]3). to what was achieved with Sea-viewing Wide Field-of-view Although the capability has been built and demon- Sensor (SeaWiFS). strated, past applications of this hyperspectral technology • Additional bands in the UV that would help correct have been limited to short surveys yielding single snapshots for absorbing aerosols, a major source of uncertainty for of a given coastal region. Routine and sustained surveys the present generation of ocean color sensors particularly of the U.S. coastal waters are not undertaken because it is in coastal waters, and a specific UV band at 317.5 nm that difficult to find the necessary funding to routinely fly these would provide simultaneous ozone corrections. airborne systems. Other countries, Australia and the People’s • Improved atmospheric correction by determining Republic of China in particular, have invested heavily in air- aerosol altitude and type using a profiling LIDAR, advanced borne hyperspectral imaging systems and routinely employ polarimeter, or both as envisioned for ACE. them in studies of their coastal and inland waters. The United States also would benefit greatly from dedicated and With these capabilities, it will be possible to separate adequate support for airborne hyperspectral imaging systems phytoplankton functional groups such as carbon exporters that could be used for routine observations of coastal waters (diatoms), nitrogen fixers (Trichodesmium sp.), calcium or to respond to episodic events as needed. carbonate producers (coccolithophores), and the microbial High spatial resolution, hyperspectral measurements loop organisms (Prochlorococcus sp.). It also will be pos- also can be made from satellite missions and were rec- sible to enable derivation and optimization of fluorescence ommended as part of the Decadal Survey (NRC, 2007). retrievals, which are particularly beneficial in quantifying Airborne missions that gather such measurements provide phytoplankton chlorophyll biomass during phytoplankton them on an intermittent basis. Satellite hyperspectral remote blooms and in coastal waters. sensing would make these observations routine and allow sustained application of the data for HAB detection, oil Conclusion: Advanced ocean color remote sensing capa- spill monitoring, shallow benthic habitat characterization, bilities are central to answering questions related to chang- and other research and research management applications. ing conditions in the marine ecosystem and biogeochemical cycles due to climate change. 1 See http://aviris.jpl.nasa.gov/. 2 See http://www.jpl.nasa.gov/news/news.cfm?release=2010-184; ac - cessed February 8, 2011. 3 See http://airbornescience.jpl.nasa.gov/prism/.
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60 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS Conclusion: Almost all coastal research and operational Satellite hyperspectral remote sensing mission was applications require ocean color remote sensing capabili- piloted with the Hyperion EO-1 mission.4 The EO-1 satel- ties that are not routinely available. To sustain and advance lite was launched in fall 2000 to demonstrate the technol- these coastal applications, a high spectral and high spatial ogy for the Landsat Data Continuity Mission. Because of resolution sensor is required. the satellite’s success, the research community successfully advocated to continue the image acquisition from EO-1. Data from the hyperspectral satellite SCanning Imaging Geostationary Radiometers and Geostationary Absorption spectroMeter for Atmospheric CartograpHY Hyperspectral Imaging Radiometers (Type 4 sensors) (SCIAMACHY), a European sensor designed to measure various trace gases, aerosols and clouds, also are used for All current sensors except for the South Korean sensor deriving ocean color measurements and have been applied Geostationary Ocean Color Imager (GOCI) are in polar orbit to distinguish phytoplankton groups (Bracher et al., 2009). (see Chapter 4). South Korea launched the first geostation- In addition, the Hyperspectral Imager for the Coastal ary ocean color sensor in June 2010 (Table 5.1). Although Ocean (HICO;5 Lewis et al., 2009; Davis et al., 2010) was the limited geographic scope of this satellite makes it less developed by the Office of Naval Research and installed relevant to the U.S. research and operational community, on the International Space Station in late 2009. Because of access to these data would be valuable for developing future mission constraints of the International Space Station, HICO geostationary missions in the United States. Initial Korean currently collects only one image per orbit of selected targets, plans called for open data access in 2011. but with good results. Although unable to provide global or Although the current systems in polar orbit can provide highly accurate data, HICO is well suited for certain appli- global data coverage once every day or two, they offer only cations that require high spatial or spectral data in coastal coarse spatial resolution. The resulting lack of data with high waters during a limited period. HICO collects hyperspectral spatial and temporal sampling frequency was recognized imagery (380-1,000 nm by 5.7 nm bandwidth) at ~100 m many years ago. Although near-daily data may be adequate spatial resolution. Its primary applications are retrieval of for climate data records from the open ocean (assuming coastal optical properties, bathymetry, bottom classification, data comparability from different sensing systems), daily and water inherent optical properties (IOP), along with ter- acquisition is inadequate for critical coastal operational rain and vegetation maps. Nevertheless, data availability to and research applications. Coastal waters and shorelines the community is minimal. exhibit significant diurnal variability. A single geostation- Plans also are under way for a global hyperspectral ary earth orbit (GEO) instrument could provide near-hourly mission, the HyspIRI,6 as outlined by the Decadal Survey data updates for the continental U.S. coasts, as required to (NRC, 2007). HyspIRI is a Tier 2 mission in the Decadal properly characterize important changes in coastal marine Survey, which NASA plans to launch after 2020 (NASA, environments. For example, water clarity, tidal variability 2010). The goal of the HyspIRI mission, which is currently of shoreline and estuaries, effluent discharge, diffusion and in the study stage, is to detect ecosystem changes due to absorption, and other parameters must be tracked frequently climate change and human impacts on land and in the ocean. (IOCCG, 2008). Because many space agencies are interested HyspIRI will make hyperspectral observations of radiance in and have plans for geostationary ocean color satellites, from 380 to 2,500 nm at 10-nm resolution with a 60-m pixel a new International Ocean Colour Coordinating Group at nadir. It will also be configured with a multispectral ther- (IOCCG) working group was formed to address require- mal IR imager. The temporal revisit times are ~3 weeks for ments, advocate for coordination, and foster collaboration. the visible/SWIR instrument and a one-week revisit for the Two U.S. options exist for increasing the supply of thermal IR sensor. Besides looking at ecosystem changes, ocean color data from sensors in geostationary orbit. First, HyspIRI will be able to map surface rock, soil, and snow a geostationary ocean color sensor hosted on a commercial composition. Because it samples a fixed location in the satellite could be a cost-effective choice to obtain coastal global ocean only once every three weeks, HyspIRI’s ability high-resolution or hyperspectral ocean color radiance (for to measure change in fast-moving planktonic communities details see Appendix D). A second option is the GEOCAPE is limited and not as useful for characterizing pelagic ocean mission. The Earth Science Decadal Survey (NRC, 2007) conditions. The high spatial and spectral features of the r ecommended GEOCAPE as a Tier 2 mission, which sensor will be used to assess coastal habitats on global and NASA plans to launch after 2020 (NASA, 2010). This mis- seasonal scales, particularly benthic features such as corals, sion would focus on retrievals of tropospheric trace gases seagrasses, and kelp. and aerosols and coastal ocean color from a geostationary spacecraft. Although an ocean color GEOCAPE would be optimized for coastal observation, its orbit also allows for observations of offshore waters. This capability could 4 See http://eo1.gsfc.nasa.gov/. support research cruises within the covered area. Because 5 See http://hico.coas.oregonstate.edu; accessed May19, 2011. GEOCAPE would be able to dwell over any area, it could 6 See http://hyspiri.jpl.nasa.gov.
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61 ADVANCING GLOBAL OCEAN COLOR REMOTE SENSING INTO THE FUTURE TABLE 5.1 Current and Planned Type 4 Sensors (geostationary) Spectral Coverage Sensor/Satellite Agency Launch Date Swath Resolution Bands (nm) GOCI/COMS KARI/KORDI 2010 2,500 500 8 400-865 (S. Korea) GOCI-II/KMGS-B KARI/KORDI 2018 1,200 × 1,500 TBD 250/1,000 13 412-1,240 TBD (S. Korea) GEOCAPE NASA (USA) After 2020 300 m Hyperspectral at 10 nm be used to cross-calibrate radiances measured by different information on vertical aerosol structure at a resolution ocean color satellites in polar orbit. well beyond what’s required for ocean applications (<0.5 NASA has initiated plans and science and engineering km). LIDAR aerosol profiling measurements, simultane- studies to equip GEOCAPE with the high spatial and tempo- ous with passive radiometric data, will enable unsurpassed ral resolution required for coastal research.7 The capabilities atmospheric corrections that will result in vastly improved of the geostationary ocean color sensor include: multiple ocean geophysical parameters. This capability is currently sampling per day of continental U.S. coastal waters and available from CALIPSO (Cloud-Aerosol LIDAR and Infra- Great Lakes, 300- to 375-m spatial resolution and hyperspec- red Pathfinder Satellite Observation) and is planned for the tral resolution (with capability of binning spectral bands), ACE mission. broad spectral coverage including UV-visible spectrum In addition to aerosol assessments, LIDAR remote (VIS), near-infrared (NIR), and SWIR bands, high signal- sensing can measure the ocean’s light-scattering proper- to-noise ratio (SNR) and dynamic range, cloud avoidance, ties independent of passive radiometer observations. Mea- minimal polarization sensitivity (<0.2 percent), minimal sures of LIDAR light scattering are related to particulate stray light, narrow field-of-view (FOV) optics, low scatter concentrations in the mixed layer (e.g., Churnside et al., gratings (<0.1 percent), no image striping or latency, and the 1998). Airborne LIDAR have been used for some time to capability of performing solar and lunar on-orbit calibration.8 demonstrate the ability to make profiles of light scattering at 532 nm and 355 nm into the water column (Wright et al., Conclusion: The availability of a geostationary satellite 2001). Most recently, this technique has also been applied to would give federal agencies concerned with the degrada- LIDAR measurements from space with data from CALIPSO tion of coastal habitats the necessary capabilities to monitor (Hu, 2009). Radiative transfer modeling and observational the near-shore environment. data indicate that with an 100-m eye-safe Nd:Yg laser at an altitude of 600 km, sufficient subsurface scattering values can be retrieved to >15 m in clear ocean waters and to 5 m Active Remote Sensing—LIDAR—to Measure the in turbid coastal waters. To cover this full range of optical Ocean’s Scattering Properties and Phytoplankton conditions, space-based measurements will require 1 to 2-m Variable Fluorescence vertical resolution and a LIDAR angle of incidence of 15 Vertical changes in light-scattering properties measured degrees relative to nadir (to avoid detector saturation at the through the atmosphere and into the ocean from a space- surface). However, LIDAR will not provide global coverage based LIDAR9 can provide important new information for and thus will benefit from simultaneous passive remote sens- solving major ocean carbon and biogeochemistry science ing to obtain the global coverage. questions. Future missions that use measurements of water- LIDAR also could enable the measurement of the leaving radiances to retrieve geophysical parameters related photosynthetic rate and the physiological state of phyto- to ocean elemental cycles depend on accurate atmospheric plankton. LIDAR-based fluorescence has advantages over corrections. This requires a strict accounting for the contribu- passive, solar-stimulated fluorescence (such as implemented tion of absorbing aerosols to top of the atmosphere (TOA) on MODIS and described previously). This is because the radiances, including information on their vertical distribu- laser source provides a consistent input of radiance and tions and total optical thickness. LIDAR measurements, the changes can be measured over very short time peri- both ground-based (e.g., Micropulse) and space-based (e.g., ods, enabling the determination of variable fluorescence. Geoscience Laser Altimeter System [GLAS]), can provide Changes in variable fluorescence appear to behave in a consistent manner under a variety of environmental condi- tions, although a few stresses create unique behaviors. One 7 See http://geo-cape.larc.nasa.gov/. of these conditions is iron-limited growth in the presence 8 See http://oceancolor.gsfc.nasa.gov/DOCS/. of high macronutrient concentrations. However, techniques 9 LIDAR refers to a technology that measures the property of a target by illuminating it with light and detecting the reflected light.
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62 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS have been developed to distinguish these iron-limited regions model. These spectrum matching approaches use both the from iron-replete areas (e.g., Behrenfeld and Kolber, 1999; radiance magnitude and shape to determine the environ- Behrenfeld et al., 2009). Therefore, remote sensing assess- mental conditions that generate the best-fit spectrum, i.e., to ments of the variable fluorescence can provide a means for determine quantities such as bottom depth and type, or water globally defining HNLC conditions, monitoring temporal absorption and backscatter coefficients. Spectrum-matching shifts in physiological province boundaries, and establishing makes full use of the available spectral information at all functional links between new iron inputs (e.g., dust deposi- wavelengths and has proved successful in the inversion tion events) and ecosystem responses. of airborne hyperspectral imagery (Mobley et al., 2005; Phytoplankton fluorescence kinetics has been measured Lesser and Mobley, 2007). These algorithms warrant further in the field for more than 20 years and, through NASA investigation by the broad ocean color community. Indeed, support, has been successfully measured from aircraft it is possible that spectrum matching to the TOA radiances (Chekalyuk et al., 2000). The “pump and probe” technique could be used to effect a simultaneous atmospheric correc- employed for these airborne tests will need to be modified for tion and bio-optical inversion, but this has been only briefly space-based applications to reduce LIDAR energy demands investigated for multispectral systems (Chomko and Gordon, and to meet eye safety requirements. Technological solutions 2001; Chomko et al., 2003; Stamnes, 2003). Furthermore, for these issues are under development. the potential for climate change and the complexity of the Globally defining and monitoring physiological prov- processes regulating the color of the ocean together raise inces through satellite variable fluorescence measurements questions about the suitability of empirical approaches in will require aggregation of multiple LIDAR returns to future oceans. Clearly, empirical band ratio algorithms are improve signal to noise ratios. Measurements are needed at locked into the era during which these datasets are collected midnight and at dawn to determine maximal variable fluores- (Dierssen, 2010). Better assessment of ocean color in the cence and the relative nocturnal percentage decrease in vari- future may come from creating advanced ocean color algo- able fluorescence (both critical diagnostics). This approach rithms that assess independently the different dissolved and would require two satellites and would also provide informa- suspended materials that absorb and scatter light in the sea. tion on the degree of midday light inhibition of photosyn- Similarly, improved atmospheric correction procedures thetic electron transport. The excitation laser wavelength may be required due to global change. Present atmospheric must penetrate a wide range of ocean waters (e.g., 532 nm correction approaches assume fixed relationships between from a Nd:Yg laser) and be effectively absorbed by chloro- near-infrared and visible aerosol optical properties. Changes phyll. Simultaneous Raman measurements at 651 nm are also in the chemistry and light absorption characteristics of required for baseline calibration. Additional capabilities for aerosols that are expected under future climate conditions detecting fluorescence at high spectral resolution (1-2 nm) would violate the assumption of constancy in aerosol optical would expand the utility of the LIDAR measurements by properties. Present plans for the ACE ocean ecology spec- allowing detection of specific phytoplankton groups through trometer (OES) include high-quality satellite observations taxon-specific fluorescence features. in the ultraviolet spectral region (as short as 345 nm). These planned observations will enable scientists to implement flexible atmospheric correction models that allow aerosol Improving Atmospheric Corrections, Ocean Color optical properties to vary, as would be anticipated in future Algorithms, and Products climates. The availability of new satellites and additional spec- tral bands will require improvements in algorithms and Validation atmospheric corrections. In addition, the creation of a more comprehensive field dataset and the use of standardized Even a limited number of comprehensive datasets for sampling protocols would significantly increase the quality selected water and atmospheric conditions would greatly of data products. advance ocean color remote sensing and environmental optics in general. A comprehensive dataset would have all the information needed to do a “round trip” radiative trans- Atmospheric Corrections and Algorithm Development fer (RT) calculation to propagate sunlight from the TOA, Advances in bio-optical algorithms and atmospheric through the atmosphere to the sea surface, through the sea corrections are required to make full use of hyperspectral surface into the water, from the water back to the atmosphere, data now becoming available from aircraft sensors. As with and finally through the atmosphere to the sensor (additional atmospheric correction, other types of bio-optical inverse details about this round-trip radiative transfer calculation are models have been developed by the hyperspectral airborne in Appendix C). This RT process is the physical basis for all imaging community. A broad class of such algorithms uses ocean color remote sensing and must be fully understood spectrum matching of the atmospherically corrected Lw either when evaluating the performance of any particular sensor to a pre-computed database of spectra or to a semi-analytic
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63 ADVANCING GLOBAL OCEAN COLOR REMOTE SENSING INTO THE FUTURE and the products it generates. To date, no entity has collected Atmospheric measurements: a truly comprehensive dataset that satisfies these RT needs. Comprehensive oceanic and atmospheric datasets are • Sea level pressure, temperature, humidity, and wind needed for: speed • Cloud condition and sea state • development and validation of remote sensing inverse • Sun photometer measurements models and algorithms (i.e., TOA ocean color radiances → • Direct and diffuse spectral irradiance incident onto atmospheric correction algorithm → water-leaving radi- the sea surface ance or remote sensing reflectance → bio-optical inversion • Above-surface upwelling spectral radiance in the algorithm → environmental products); direction needed to determine the water-leaving radiance • identification of weaknesses in existing algorithms and guidance for the development of improvements; and Oceanic measurements: • validation of coupled oceanic and atmospheric RT forward models, which underlie the development of both • Spectral absorption, beam attenuation, and backscat- sensors and algorithms, and of inverse models, which are the ter coefficients foundation of all remote sensing. • Measurements of CDOM and CDOM spectral and fluorescence characteristics • Concentrations of dissolved organic carbon (DOC), Field Data Standards and Standardized Sampling POC, and particulate inorganic carbon (PIC) Protocols • Phytoplankton carbon biomass and pigment The measurements from MOBY, the current U.S. vicari- concentrations ous calibration site off Lanai (Hawaiian Islands), meet strict • Net primary production, net community production, National Institute of Standards and Technology (NIST) stan- and carbon export rates dards. However, users of field instruments that generate data • Phytoplankton fluorescence and fluorescence quan- for bio-optical algorithm development or validation do not tum yields and taxonomic groups always meet recommended protocols or use defined calibra- • Particle size distribution tion standards. Future field observations need standardized, • Downwelling (and preferably also upwelling) spec- well-calibrated measurements that adhere to protocols to tral plane irradiance help ensure quantitative comparison of observations.10 For • Upwelling spectral radiance example, the chlorophyll concentration has long been the • Bottom depth and spectral reflectance in optically principal data product derived from satellite ocean color of shallow waters interest to oceanographers. There are several different tech- niques for measuring chlorophyll but no accepted national Extending Satellite Ocean Color Products to the or international standard protocol. To fill this need, NASA Vertical Dimension hosted a series of round-robin comparisons to evaluate dif- ferent methods and to promote guidelines and procedures Ocean color remote sensing provides information only for measuring chlorophyll.11 Despite this effort, few research about the surface of the ocean, at spatial scales ranging from labs to date adhere to these standard procedures developed kilometers to global. The vertical dimension is only partially for chlorophyll. In addition, sky radiance measurements are explored, from the surface to about 15-20 m in the clearest necessary to characterize, for example, aerosol properties. waters, and from the surface to only a few tens of centimeters Similar issues exist for the next-generation products, such as in turbid waters (i.e., the “penetration depth” from which CDOM, particulate organic carbon (POC), particle size dis- originate 90 percent of the photons exiting the water). tribution, primary productivity, net community production, However, phytoplankton grow throughout the lit upper carbon export flux, etc. Future satellite ocean color missions layer of the ocean to depths of 100 m or more. Therefore, need to take on the creation and community-wide adaptation ocean color sensors miss much of the information of interest; of field measurement standards and field sampling protocols this is particularly true for the Navy, which is interested in for future satellite ocean color data products. optical properties that extend from the surface to the bottom The minimum set of measurements that would need to of the ocean. be made, and for which standards and protocols need to be Determining the total amount of biomass or primary developed and followed (when appropriate), can be sum- productivity in the ocean currently requires assumptions marized as follows: about how phytoplankton cells are distributed with depth: either homogeneously when the upper ocean is well mixed or with a vertical structure. This vertical structure results 10 See http://www.ioccg.org/reports/simbios/simbios.html. from intermingled physical and biological effects and often 11 See http://oceancolor.gsfc.nasa.gov/DOCS/SH2_TM2005_212785. exhibits a deep maximum whose depth precisely depends pdf.
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64 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS SUSTAINING OCEAN COLOR REMOTE SENSING on the balance between physical and biological forcings. OVER THE LONG TERM The vertical structure is presently reconstructed using sta - tistical relationships between the satellite-derived values Long-Term Mission and Budget Planning and the vertical profile established from in situ information (e.g., Morel and Berthon, 1989; Uitz et al., 2006) or using Long-term planning is necessary to provide continuity the concept of biogeographical provinces to which a given between satellite missions and to ensure sensor overlap. In shape is assigned (Longhurst, 1998). These techniques use general, it takes six to eight years to move an ocean color a small number of measured vertical profiles. Consequently, mission from conception to launch. This time frame can be they perform poorly when applied to satellite pixels that much longer in a shrinking budget environment or when include by, definition, all possible cloud-free areas in the other problems arise. For example, the SeaWiFS concept ocean. was developed in 1986, but launch did not occur until 1997. New autonomous-profiling floats and gliders equipped Furthermore, mission and budget plans need to include w ith optical instrumentation are now available, which provisions for all mission requirements throughout the mis- provide vertical profiles of quantities such as chlorophyll sion’s life span and beyond. SeaWiFS was successful in part fluorescence or the particulate backscattering coefficient because mission planning and budget, through support from (Boss et al., 2008; Boss and Behrenfeld, 2010). In the case the EOS/MODIS program, included provisions not only for of gliders, radiance and irradiance sensors are now routinely the sensor and satellite, but also for many of the essential ele- integrated for subsurface profiles (Schofield et al., 2007). A ments such as vicarious calibration, stability monitoring, col- large deployment of these floats could enhance the satellite lection of in situ validation data, and a mission team with the data record by providing the missing vertical dimension. tools to handle data processing and reprocessing. In addition, Such arrays were initially designed and deployed for physical SeaWiFS experienced a long launch delay, which provided oceanography (the “Argo” array, e.g., Roemmich and Owens, sufficient time to make sure the necessary infrastructure 2000). These floats sample the water column between the was in place for the calibration and validation effort. Less surface and about 2,000 m, covering horizontal scales from than a year from the launch date, it remains unclear who ~1 m to ~1,000 km and temporal scales from one day to will fund and conduct the vicarious calibration for VIIRS several years. Interestingly, the intersection between the on NPP and who will be processing and reprocessing the spatio-temporal domains covered by both remote sensing data. In addition, planning for in situ data collection has and profiling floats encompasses the mesoscale oceanic pro- begun only recently. These missing elements of the VIIRS/ cesses as well as the seasonal cycle of mixed layer dynamics NPP mission contribute significantly to the uncertainty about and its impact on biomass cycles. These phenomena are the data quality of its ocean color radiance and ocean color fundamental to understanding the impact of physical forcing products, and if unresolved, might jeopardize the success of on ocean biology and biogeochemical cycles. the mission. Such Bio-Argo floats would be an ideal addition to an integrated observing system that includes ocean color satel- Conclusion: A satellite mission that does not include plan - lites, optically equipped gliders, and ship-board studies. ning and budgeting for all essential elements of a mission These floats would make vertical data available indepen- (e.g., vicarious calibration, stability monitoring, in situ dent of cloud coverage and with good temporal resolution data collection and archiving, algorithm development, data (Claustre et al., 2010). The optical Argo floats could be used processing and reprocessing; see Chapter 3 for additional to refine the satellite algorithms, improve global primary details), jeopardizes the success of the mission for many production estimates, estimate particulate organic carbon uses, especially for climate assessments. in the water, and even estimate the total sinking carbon flux (Bishop and Wood, 2009). Recommendation: To ensure success, a mission should include long-term planning and budgeting for all require- Recommendation: An array of “bio-geochemical floats” ments of the mission. should be implemented and progressively expanded. Long-Term Planning for Data Stewardship International collaboration is ongoing in order to build from the experience of the Argo network (e.g., Johnson et As discussed in previous chapters, producing high- al., 2009). The IOCCG has also set up a working group on quality ocean color data is complex and requires a concerted this topic and will issue a report in 2011.12 effort. As information becomes available about the sensor’s behavior, the continuous vicarious calibration effort and data reprocessing at regular intervals are vital to generate high- quality products. Therefore, plans for product development and data access and archiving need to be in place well in 12 See http://www.ioccg.org/groups/argo.html.
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65 ADVANCING GLOBAL OCEAN COLOR REMOTE SENSING INTO THE FUTURE 0 data and metadata is required to allow subsequent advance of the sensor launch. Plans also need to specify how reprocessing and merging of individual mission data into efforts carry over from one mission to the next to preserve sustained climate data records. data continuity. Currently, NASA’s Ocean Color Biology Processing Group (OBPG) at Goddard Space Flight Center (GSFC) is Both NASA and NOAA support ocean color applica- internationally recognized as a leader in producing well-cali- tions, with NASA focused primarily on research and devel- brated, high-quality ocean color data products from multiple opment and NOAA focused on operational uses. Because satellite sensors. For example, processing and reprocessing both agencies have a strong interest in climate and climate from Level 0 to Level 2 imagery require the most skill and impacts, they share a common interest in climate data records resources, including access to pre- and post-launch calibra- (CDRs). tion data, models, ancillary data and significant computa- As previously discussed, NOAA currently lacks the tional resources for production, archiving, and distribution. demonstrated capacity to readily produce high-quality ocean In effect, these production steps determine the quality of color products. Moreover, the committee anticipates major the final data products. Access to all raw, processed, and challenges to generating high-quality products from the metadata are critical if long-term time-series of ocean color VIIRS/NPP data. However, a recent NOAA report (NOAA, products are to be constructed across different mission data- 2010) makes a recommendation to build the in-house capac- sets (à la Antoine et al., 2005). ity for end-to-end data processing/reprocessing. If NOAA The Ocean Color Biology Processing Group at NASA builds its own data processing/reprocessing group, two GSFC currently provides this service for CZCS, SeaWiFS, independent federal groups will be developing ocean color and MODIS data. As new algorithms are developed, the products. Having two groups independently process ocean OBPG has repeatedly reprocessed all ocean color data from color data would provide the benefit of products that could SeaWiFS and MODIS to ensure a continuous, intercalibrated be tailored to the respective user-group. However, it results dataset of climate-quality data. To be able to routinely in some redundancy and potential questions about merg- reprocess the data, the raw data, radiance at the TOA, and ing datasets for building long-term climate records. Given water-leaving radiance needs to be archived and available for NASA’s experience with end-to-end data processing, NOAA the long term. In particular, the most recent version of the can draw on that agency’s expertise to build its own capacity. water-leaving radiance needs to be readily available to ensure Conclusion: NOAA would greatly benefit from initiating users can generate their own derivative products. and pursuing discussions with NASA for an ocean color T he OBPG currently provides broad access to mitigation partnership that would build on lessons learned ocean color data by creating and deploying SeaWiFS Data from SeaWiFS and MODIS, in particular. Analysis System (SEADAS), an image-processing software that can be installed on many different computer platforms. The OBPG has developed the necessary modules to make A near-term option for the partnership could be a real data easily accessible and has built the necessary structure or virtual “center” involving NOAA and NASA personnel, to archive the data, including the radiance at TOA and the with contributions from the academic research community. water-leaving radiance. NOAA is currently building capacity An important step in any research-to-operations transition is but does not have the know-how to provide these compre- for researchers to work directly with the people developing hensive services. Although NOAA’s National Climate Data operational capabilities. Thus, such a virtual center’s activi- Center (NCDC) plans to archive a climate-level13 radiance ties could include: research and development related to ocean data record, it is unclear how accessible the data will be. color products that serve research and operational users; and Easy access will be an important factor in contributing to processing/reprocessing of data from U.S. and foreign ocean the successful application of the ocean color data. color missions to ensure a sustained time-series of calibrated imagery to identify long-term trends and calibration and Conclusion: Because of the potential need to reprocess the validation activities. These involve for example, a NOAA- raw data years after collection, the committee concludes operated MOBY-like site, among other activities. that the water-leaving radiance and the radiance at the TOA need to be archived together with the metadata for the Recommendation: To move toward a partnership, NASA long term. In addition, the most recent version of the water- and NOAA should form a working group14 to determine leaving radiance needs to be readily accessible to all users. the most effective way to satisfy each agency’s need for ocean color products from VIIRS and to consider how to Conclusion: A permanent archive for repackaged Level produce, archive, and distribute products of shared interest, 13 Climate-level means repackaged data so they look like a MODIS 14 The committee was informed in March 2011 that NOAA and NASA granule and metadata repackaged accordingly to ease the reprocessing of have formed a new ocean color working group with a composition and the Level 0 data. charge that encompasses many of the recommendations listed above.
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66 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS such as CDRs, that are based on data from all U.S. ocean cessing, reprocessing, calibration, validation, model imple- color missions. This group should be composed of agency mentation, and distribution for satellite ocean color data. representatives and also include outside experts from the Ideally, such a group will be flexible and able to respond ocean color research and applications community. to and resolve a wide range of issues; the OBPG at GSFC responsible for SeaWiFS and MODIS ocean color data products provides a good model. The team has all required This working group could be the focal point for U.S. scientific, engineering, and technical skills to interact, for contributions to the Ocean Colour Radiometry Virtual Con- example, with competitively selected NASA science teams stellation (OCR-VC), for articulating U.S. needs for specific for individual missions. data from international missions, and for helping negotiate The committee judges it to be most efficient to establish how those needs would be met. As is currently done by the an interagency team to be involved with all ocean color mis- Ocean Color Group at GSFC, the new working group would sions over the long term and to interact with competitively be the ideal mechanism to routinely interact with ocean selected science teams for individual missions. This would color experts in the national and international academic minimize the loss of institutional memory. community. Subcommittees of existing federal advisory committees—the Earth Science Subcommittee of the NASA Conclusion: Long-term planning is needed that focuses Advisory Committee and NOAA’s Scientific Advisory Board on building a long-term data record, instead of focusing (SAB)—could provide oversight. To the extent possible, the on building individual satellite missions. A working group working group could develop and maintain centralized teams that is maintained across missions is the best choice to with all necessary skills to evaluate product quality and to coordinate this planning. take appropriate measures to improve quality for the long term. These teams would provide avenues for stakeholders Specialized technical and scientific expertise in various to engage and would enable the research community to con- aspects of ocean color radiometry exists within the civil tinuously demonstrate its need for, and ensure the existence service at NASA, NOAA, NIST, Office of Naval Research of, overlapping programs that can maintain the essential (ONR), and the Naval Research Laboratory (NRL), and is requirements (i.e., missions and accompanying infrastruc- concentrated at NASA’s GSFC. This expertise includes sen- ture) to generate multi-decadal climate-quality data records. sor design and calibration, aerospace engineering, systems To achieve a multi-decadal climate-quality data record will e ngineering, hydrological optics, physics, atmospheric necessitate a permanent planning function at the international physics, biogeochemistry, etc. Maintaining strong technical level (see discussion below). expertise within the civil service allows the agencies to tackle Recommendation: This working group should engage with technical questions quickly, to provide technical oversight the scientific community, develop a unified and coordinated of major instrument contracts, and to support the vigorous voice, provide long-term vision and oversight, and engage exchange of scientific ideas with the academic community with the international community. and other agencies. Recommendation: NASA and NOAA should ensure suffi- Building and Maintaining the Ocean Color Workforce cient levels of staffing in areas critical to the continuation of ocean color research and climate data collection. Developing and using high-quality ocean color research products requires a highly specialized and trained The committee also encourages NASA and NOAA to workforce with a diverse set of technical skills. Maintaining expand the use of academic researchers in Intergovernmental the viability of the field requires experts who know how to Personnel Act15 positions both in managerial and technical design and build a sensor; test and calibrate it; design and roles. This will expand the influx of ideas from academia operate vicarious calibration sites; conduct validation and into the federal agencies and will increase the number of calibration efforts; process, reprocess, and archive ocean academic researchers familiar with government procedures color data; and make these products easily available to and policies. Similarly, NASA and NOAA could enter into users. In addition, data users need to be trained in satellite agreements to facilitate temporary exchange of scientists and oceanography. engineers to encourage sharing of ideas and understanding of each agency’s missions, strengths, and weaknesses. Building and Maintaining the Expertise in Government Agencies As we learned from the SeaWiFS/MODIS experience, a team with the right mix of skills and knowledge is essential 15 Through the IPA program, NASA or NOAA can temporarily bring to advancing the quality of ocean color products. A group individuals from academia and state and local governments to the agency of experts needs to take responsibility for acquisition, pro- to provide scientific, administrative, and managerial expertise.
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67 ADVANCING GLOBAL OCEAN COLOR REMOTE SENSING INTO THE FUTURE Building and Maintaining the Expertise in Academia other courses. While the course at the University of Maine focuses on hands-on training with a strong laboratory and Experience from all NASA ocean satellite missions field component, the IOCCG course would focus on current demonstrates the importance of supporting a competitive critical issues and emerging topics of optical oceanography research program associated with each mission. Researchers and ocean color remote sensing research. help improve data products, advance ocean color applica- The large number of applications for ocean color prod- tions, and train the new workforce. ucts demonstrates the demand for these intensive summer Scientists have contributed to significant, and in some courses. The limitation in both cases is funding to cover cases, immediate improvements to data products by con- faculty salaries, travel, laboratory and field costs, and student ducting research on topics such as in-water and atmospheric per diem expenses. The committee considers such costs to algorithms, sensor performance, and regional validation be small compared to the benefits of maintaining a highly of in-water and atmospheric correction algorithms. This trained and skilled workforce. research can provide critical feedback to the mission if there is clear communication with the project team, via open meet- Collaborating and Coordinating Internationally to ings and workshops where results are discussed and their Sustain Ocean Color Observations meaning debated. For example, application scientists were among the Two international committees—the IOCCG and the first to note that initial SeaWiFS processing had yielded OCR-VC16 organized under the Committee on Earth Observ- anomalous water-leaving radiance spectra with unusually ing Satellites (CEOS)—support international cooperation low radiance in the blue bands (in fact, negative radiances in for ocean color research and operations. The IOCCG is a many coastal waters). It required several years of interaction committee of users and space agency representatives. Among between applications scientists and the SeaWiFS project, and the primary IOCCG products are monographs on a wide several reprocessings, to fix the problem. range of topics that provide essential consensus advice to In addition, NASA, NSF, and NOAA have supported those planning and operating satellite ocean color missions. investigators who use ocean color imagery for basic research. The OCR-VC is a consortium of representatives from agen- The applications described in Chapter 2 lead to better under- cies that operate satellite ocean color missions. It seeks to standing of ocean processes and make important societal coordinate activities related to post-launch calibration and contributions. validation, data merging, and sharing of essential pre-launch Further, competitive research programs often result in characterization and calibration information. OCR-VC also new applications for or new approaches to using ocean color promotes training programs and outreach. data; these new applications add value to the mission. Lastly, The activities of the IOCCG and OCR-VC are increas- research projects naturally integrate graduate students into ingly important to U.S. users of satellite ocean color products the ocean color community, which is critical to maintain because neither NASA nor NOAA will provide all required a capable workforce in the private sector, at NASA and at data products for at least a decade. For example, the United NOAA, with continuity through missions. States currently has no geostationary mission in orbit and limited availability of high spatial resolution imagery (250- Training and Recruitment Through Summer Courses to 300-m pixels). Second, non-U.S. missions have the poten- tial to provide essential backup for global imagery in the Many scientists in academia and government, and sev- event of a failure during the launch or in the early stages of eral federal agency program managers, are graduates of sum- a U.S. mission. Finally, merging data from multiple sensors mer courses such as those at the University of Maine or Cor- significantly enhances global coverage. For these reasons, it nell University. For example, the intensive summer course in is essential that NASA and NOAA continue to support the optical oceanography and remote sensing at the University activities of the IOCCG and the OCR-VC. of Maine was first taught in the mid-1980s by experts from The production of a climate-quality long-term record around the world. It is held every two or three years and has of ocean color requires international collaboration. As dis- achieved an international reputation as a career-molding cussed above, establishing a climate-quality long-term data course. Applications always far outnumber the 12 to 15 record exceeds the capacity and mandate of a single U.S. seats available. The course has evolved with the science to agency. In general, it is difficult to maintain such long-term include lectures and extensive hands-on laboratory and field commitments due to budget uncertainties. International work, using many instruments of optical oceanography and collaboration to establish a long-term record can be a good vicarious calibration. NASA and other federal agencies have hedge against the uncertain funding from any single space provided substantial funding for the course. agency or nation. The IOCCG is currently organizing a recurring sum- mer lecture series, “Frontiers in Ocean Optics and Ocean Colour Science.” This class would build on and complement 16 See http://www.ioccg.org/ groups/OCR-VC.htm.
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68 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS Examples of successful international collaboration effort. Essential components would include revisiting cali- already exist in several domains. The Centre National bration factors and how they change over time for multiple d’Etudes Spatiales (CNES) and NASA have established a sensors; updating in situ databases for product validation; fruitful partnership to prepare, implement, and exploit altim- evaluating quality of in situ data; generating match-up data- eter missions (Jason series and Topex/Poseidon), leading to sets (satellite and in situ) to confirm accuracy; incorporating long-term, global sea-level data records that demonstrate a VIIRS/NPP data into the time-series; and other activities. gradual increase in sea level (IPCC, 2007). The Group for Conclusion: The CCI initiative presents an opportunity for High-Resolution Sea Surface Temperature (GHRSST) has real progress toward establishing a seamless time-series brought together many space agencies involved in measur- for a climate-quality ocean color data record. Engaging ing sea surface temperature from space in order to establish experts at NASA and NOAA could significantly contribute standards, perform comparisons of products and algorithms, to the success of this initiative with mutual benefits for the and produce global datasets by merging data from different United States and the international community. satellites. These efforts involved not only space agencies but also Conclusion: The IOCCG is the logical entity to lead the science community, which has to be organized in order to the planning and to build international support for the have a strong and single voice. The ocean color community establishment of a global climate-quality ocean color data still lacks this mechanism, but the creation of the OCR-VC record. NASA’s and NOAA’s support for and engagement represents a step forward. The OCR-VC provides a frame- with this group will be essential to the success of these work for agencies to strengthen collaboration and leverage efforts. individual efforts. The IOCCG is considering options for how to make such a virtual constellation a reality for the ocean color community. CONCLUSION Another promising development is ESA’s Climate Change Initiative (CCI) to produce Essential Climate Vari- The expanding user community and the diversity of ables (ECV; CDRs in NOAA/NASA terminology). ESA ocean color applications that fuel research and benefit ocean selected ocean color as one of the first 10 ECV projects that ecosystem health demonstrate the critical need to sustain and began in summer 2010. The goals for the ocean color ECV advance ocean color observations from satellites well into are: generate the most complete multi-sensor global satellite the future. As discussed in detail in Chapter 2, ocean color data products for climate research and modeling that meet remote sensing is the only way to obtain a global view of the Global Climate Observing System (GCOS) ocean color ocean biology. Ocean color data are essential to improving ECV requirements; quantify pixel uncertainties for different the understanding of the climate system, including global regions; and assess the applicability and impact of ocean color carbon fluxes. Ocean color satellite observations also are ECV products on ecosystem and climate models. Another key used to assess the health of the marine ecosystem and its goal is to form teams of observational scientists and climate ability to sustain important fisheries. Any interruption in modelers to ensure that ECVs are correctly incorporated in the ocean color record would severely hamper the work of climate models and models of climate impacts. climate scientists, fisheries and marine resource managers, ESA recognizes the importance of international coop- and an expanding array of other users, from the military to eration for this project and lists NASA, NOAA, JAXA, and oil spill responders. the IOCCG as external partners. Further, a closer NASA/ Increasing the spatial or temporal resolution, especially NOAA-led partnership with ESA would be a major contri- in coastal waters, would enable further advances in research bution to the OCR-VC and could stimulate participation by and resource management. In particular, an ocean color other agencies (e.g., JAXA, ISRO), potentially bringing other satellite in geostationary orbit would fill an important gap in expertise and satellite datasets into the project. From users’ observational capability (Appendix D). High spectral imag- perspective, this international partnership could go a long ing and a satellite in geostationary orbit would significantly way toward providing high-quality ocean color data from improve the ability to monitor for example HABs and coral many different missions and for many applications. reef health. Similarly, the addition of Bio-Argo floats would Prototype products of the first 10 ECVs will be available turn the two-dimensional satellite ocean color imagery in 2011-2012. Complete time-series (from multiple sensors) into dynamic three-dimensional depictions of the ocean will be available beginning in mid-2013. The objective for biosphere. These and other enhancements described above the ocean color time-series is to reduce bias among sensors would add tremendous value to oceanography, but they will (MERIS, SeaWiFS, and MODIS) to less than 1 percent, an be difficult to balance with the requirements to simply main- ambitious goal that will require reprocessing of all satellite tain the current capabilities, especially in the current budget datasets. environment. Thus, it will require careful and strategic long- A NASA/NOAA-led project, similar to NASA’s SIMBIOS term planning, in addition to international collaboration and program, could help meet this goal and complement the ESA coordination, to meet these diverse needs.
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69 ADVANCING GLOBAL OCEAN COLOR REMOTE SENSING INTO THE FUTURE Mission planning proved the concept (CZCS) and led to vision for the development of U.S. ocean color missions and the successful demonstration that water-leaving radiance can the delivery of ocean color products to users. The long-term be of such high accuracy that climate trends can be quantified vision also needs to ensure the next generation of satellite (SeaWiFS and MODIS). However, building a climate-quality oceanographers is sufficiently trained in order to maintain the time-series comes with stringent requirements—most nota- required expertise at every level, from sensor engineering to bly the need for continuity and sensor inter-calibration. Now, data processing and application. planning will need to extend beyond the next mission and In the long term, simply sustaining the current capabili- establish a strategy to sustain the climate-quality data record. ties of ocean color remote sensing will fall short of support- ing the array of ocean color applications described in Chapter Conclusion: A data-oriented long-term planning approach 2. Many ocean color applications require a commitment to will need to replace a mission-oriented approach for the advancing current capabilities. Foremost, these advances global climate-quality ocean color record. need to include hyperspectral and active imaging capabilities and a sensor in geostationary orbit for coastal applications. To develop this climate-quality time-series and to Therefore, the committee recommends above that advance ocean remote sensing, strategic long-term plan - NASA and NOAA form a working group to determine the ning and budgeting is required for all aspects of follow-on most effective way to satisfy each agency’s need for ocean missions, including how data are reprocessed, accessed, color products from VIIRS and future ocean color sensors. and stored across the individual missions. In addition, the This working group could be the focal point for U.S. collabo- institutional memory and workforce need to be maintained rations with the international community and for articulating and transitioned across individual missions to ensure some U.S. needs for specific data from international missions, for measure of consistency and to avoid inefficiencies. NOAA helping to negotiate how those needs will be met and for and NASA will continue to have mutual interests in the ocean advocating for advanced capabilities to support future ocean color climate data record as well as in advances in remote color applications. sensing. Therefore, they would benefit from sharing in the Moreover, because the community will require distinct development of these long-term plans. types of satellite sensors to meet all data product needs, no Going forward, a national working group similar to single nation will be able to develop or even maintain capa- the international IOCCG working group, with strong gov- bilities on its own. NASA and NOAA will need to continue ernance, clear mandate, and financial resources, is needed to actively engage and contribute to the development of the to guide the direction of ocean color remote sensing in the international OCR-VC. Ocean color remote sensing needs to United States and to implement changes at the national level. be an internationally shared effort. This committee also would provide oversight and long-term