NASA Technical Memorandum 104566, Vol. 1

SeaWiFS Technical Report Series

Stanford B. Hooker and

Elaine R. Firestone, Editors

Volume 1, An Overview of SeaWiFS:

and Ocean Color

Stanford B. Hooker, Wayne E. Esaias, Gene C. Feldman

Watson W. Gregg, and Charles R. McClain

July 1992
NASA Technical Memorandum 104566, Vol. 1

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

Volume 1, An Overview of SeaWiFS:

and Ocean Color

Stanford B. Hooker, Wayne E. Esaias, Gene C. Feldman

Watson W. Gregg, and Charles R. McClain

NASA Goddard Space Flight Center

Greenbelt, Maryland

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

1992
An Overview of SeaWiFS and Ocean Color

Preface

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project is very important to the ocean and Earth sciences

communities. SeaWiFS represents a crucial component in a continuing series of comprehensive observations of

the ocean in the visible and near infrared that are required for investigations of the marine biosphere, its

variability, marine ecosystem dynamics, oceanic biogeochemical cycles, and the role of the oceanic biosphere in

the global carbon system. With SeaWiFS data and correlative data from ships and moorings, scientists will, for

the first time, have a complete suite of measurements necessary to study seasonal and interannual variability of

oceanic phytoplankton on the ocean basin and on global scales.

The need for a more complete, continuing time series of observations was demonstrated by use of Coastal Zone

Color Scanner (CZCS) observations. That proof-of-concept mission, however, lacked the necessary rapid global

coverage capability. Research with the CZCS data also pointed out several basic improvements in the areas of

calibration and sensor characterization, spectral bands, and radiometric sensitivity, all of which are required for

the global applications; these improvements are included with the SeaWiFS system.

NASA has already proposed a follow-on mission, SeaWiFS II, to assure continuation of the time series until

data provided by dual Earth Observing Satellite (EOS) Moderate Resolution Imaging Spectrometer (MODIS)

sensors in complementary morning and afternoon platforms meets global coverage frequency requirements.

The European Space Agency (ESA) has proposed the Medium Resolution Imaging Spectrometer (MERIS)

sensor, and Japan is planning to fly their Ocean Color Temperature Sensor (OCTS) on the Advanced Earth

Observing Satellite (ADEOS) in 1997. Both of these sensors will make unique and important contributions

toward providing the highest possible coverage frequency. These missions will be accompanied by precision

altimeters and scatterometers, from which ocean currents and wind forcing of the marine biosphere can be

obtained.

The purpose of this series of technical reports is to provide current documentation of the SeaWiFS Project

activities, instrument performance, algorithms, and operations. This documentation is necessary to ensure

critical information related to the quality and calibration of the satellite data is available to the scientific

community. This technical series is not meant to substitute for the scientific literature. Instead, it will provide

a ready and responsive vehicle for the multitude of technical reports issued by an operational project.

Greenbelt, Maryland — W. E. Esaias

June 1992

ii
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

Abstract

SeaWiFS, the Sea-viewing Wide Field-of-view Sensor, will bring to the ocean community a welcomed and

improved renewal of the ocean color remote sensing capability lost when the Nimbus-7 Coastal Zone Color

Scanner (CZCS) ceased operating in 1986. The goal of SeaWiFS, scheduled to be launched in August 1993, is

to examine oceanic factors that affect global change. Because of the role of phytoplankton in the global carbon

cycle, data obtained from SeaWiFS will be used to assess the ocean’s role in the global carbon cycle, as well as

other biogeochemical cycles. SeaWiFS data will be used to help elucidate the magnitude and variability of the

annual cycle of primary production by marine phytoplankton and to determine the distribution and timing of

spring blooms. The observations will help to visualize the dynamics of ocean and coastal currents, the physics of

mixing, and the relationships between ocean physics and large-scale patterns of productivity. The data will help

fill the gap in ocean biological observations between those of the CZCS and the Moderate Resolution Imaging

Spectrometer (MODIS) on the Earth Observing Satellite-A (EOS-A).

1. INTRODUCTION

It has been apparent to the oceanographic community

that due to the dynamic nature of the world’s oceans and

climate and the importance of the ocean’s role in global

change, a follow-on sensor to the CZCS, which ceased operations in June 1986, after 7 1

2 years of operation, should

be flown. To this end, specifications for an improved ocean

color sensor were outlined by the Ocean Color Science

Working Group in the 1982 report entitled, The Marine

Resources Experiment Program (MAREX), and were further refined in the report, System Concept for Wide Fieldof-ViewObservation of Ocean Phenomena From Space,

written by the Joint Earth Observation Satellite Company

(EOSAT)/NASA SeaWiFS Working Group in 1987.

The SeaWiFS Project has been designated by the Office

of Space Science and Applications (OSSA) of the National

Aeronautics and Space Administration (NASA) Headquarters, and Goddard Space Flight Center (GSFC) to develop

and operate a research data system that will process, calibrate, validate, archive, and distribute data received from

an ocean color sensor. With support and concurrence

of the SeaWiFS Prelaunch Science Working Group (SPSWG), NASA established a set of criteria for ocean color

problems that are summarized in Table 1.

Selection of the SeaWiFS bands was based on historical considerations and the spectral absorption characteristics of some common in-water optical constituents, mean

extraterrestrial irradiances, and spectral transmittance of

the atmospheric constituents oxygen and water vapor. The

relation of these parameters as a function of wavelength is

shown in Fig. 1. The data required to accomplish such a

mission have been specified by the ocean science community, which will be procured as a data buy from a private

contractor, Orbital Sciences Corporation (OSC), who will

subcontract with Hughes/Santa Barbara Research Center

(SBRC) to build the sensor. OSC will then launch and

operate the SeaStar satellite carrying the SeaWiFS sensor.

Table 1. Major instrument parameters and characteristics of the SeaWiFS ocean color sensor.

Instrument Bands

Band Wavelength Saturation Input SNR2

FWHM [nm] Radiance1 Radiance1

1 402–422 13.63 9.10 499

2 433–453 13.25 8.41 674

3 480–500 10.50 6.56 667

4 500–520 9.08 5.64 640

5 545–565 7.44 4.57 596

6 660–680 4.20 2.46 442

7 745–785 3.00 1.61 455

8 845–885 2.13 1.09 467

Sensor Accuracy

Radiance Accuracy: <5% absolute each band

Relative Precision: <1% linearity

Between Band <5% relative band-to-band

Precision: (over 0.5–0.9 full scale)

Polarization: <2% sensitivity (all angles)

Nadir Resolution: 1.1 km LAC; 4.5 km GAC

Mission Characteristics

Orbit Type: Sun Synchronous at 705 km

Equator Crossing: Noon ±20 min., descending

Saturation Recovery: <10 samples

MTF3: ≥0.3 at Nyquist

Swath Width 2,801 km LAC (±58.3◦)

(at equator): 1,502 km GAC (±45.0◦)

Scan Plane Tilt: +20◦, 0◦, −20◦

Digitization: 10 bits

1. Units of mW cm−2

μm−1 sr−1.

2. Minimum values measured at input radiances.

3. Modulation Transfer Function.

The principal objective is to acquire data that is critical for the study of the role of ocean primary production

in global biogeochemistry, including the exchange of critical elements and gases between the atmosphere and ocean.

Data used for commercial and operational purposes must

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An Overview of SeaWiFS and Ocean Color

Fig. 1. Spectral absorption characteristics of some common in-water optical constituents, mean extraterrestrial irradiance, and spectral transmittance of the atmospheric constituents oxygen and water vapor, used to

select the SeaWiFS visible bands. The bandwidths of the SeaWiFS bands are highlighted (courtesy of W. Gregg).

be purchased directly from OSC. Data used for scientific

purposes may be obtained from NASA subject to prior

approval by the SeaWiFS Project, with a two-week embargo period. The data will become public domain after

five years.

1.1 Ocean Color Requirements

From a scientific perspective, acquisition of ocean color

data from space in the early 1990s is a high priority goal

that has been recognized in reports of the National Research Council of the National Academy of Science. For

example, satellite acquired ocean color measurements are

key to the success of the Global Ocean Flux Study (GOFS),

a major component of the National Science Foundation’s

Geosciences initiative. In addition to the U. S. scientific

community, scientists from Europe, Japan, and many other

countries are also involved through the international Joint

GOFS (JGOFS) program.

The scientific uses of ocean color data will be enhanced

if the next mission is concurrent with various NASA, European Space Agency (ESA), and Japanese National Space

Development Agency (NASDA) missions. For example,

the Japanese Ocean Color Temperature Sensor (OCTS),

due to be launched in 1996, on the Advanced Earth Observation Satellite (ADEOS) complements SeaWiFS in that

it utilizes the same visible spectral bands but also has two

thermal bands for sea surface temperature (SST). Additional sensors on ADEOS useful for ocean color objectives

are the NASA Scatterometer (NSCAT), which will provide surface winds for data processing and scientific analysis; NASA’s Total Ozone Mapping Spectrometer (TOMS);

and the French Polarization Detecting Environmental Radiometer (POLDER) for polarization measurements. Data

from TOMS and POLDER will be useful for atmospheric

correction. The joint U.S.–French TOPEX/Poseidon altimetry mission, due to be launched in July 1992, will provide information on large-scale ocean circulation patterns

useful for understanding the coupling between physical and

biological patterns of variability.

1.2 Program Goals

The Earth Science and Applications Division of OSSA

is committed to the U. S. Global Change Research initiative. Priority areas of research include climate and hydrologic systems, and biogeochemical dynamics. The purpose

of SeaWiFS is to examine oceanic biogeochemical factors

affecting global change. Because of the role of phytoplankton in the global carbon cycle, SeaWiFS data will be used

to assess the ocean’s role in the global carbon cycle as well

as other biogeochemical cycles. The NASA science goals

in support of this undertaking are as follows:

To determine the spatial and temporal distributions of phytoplankton blooms, along with the

magnitude and variability of primary production

by marine phytoplankton on a global scale;

To quantify the ocean’s role in the global carbon

cycle and other biogeochemical cycles;

To identify and quantify the relationships between ocean physics and large-scale patterns of

productivity;

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S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

To understand the fate of fluvial nutrients and

their possible effect on carbon budgets;

To identify the large-scale distribution and time

of spring blooms in the global oceans;

To acquire global data on marine optical properties, along with a better understanding of the

processes associated with mixing along the edge

of eddies and boundary currents; and

To advance the scientific applications of ocean

color data and the technical capabilities required

for data processing, management, and analysis

in preparation for future missions.

The scientific and technical experience gained in the

SeaWiFS mission will be invaluable in the preparation for

the MODIS mission.

1.3 Project Objectives

The purpose of the SeaWiFS Project is to obtain valid

ocean color data of the world’s oceans for a five-year period, to process that data in conjunction with ancillary

data to meaningful biological parameters, and to make

that data readily available to researchers. GSFC will develop a data processing and archiving system in conjunction with the Earth Observing Satellite Data and Information System (EOSDIS), which includes a ground receiving

system, and will oversee a calibration and validation effort

to ensure the integrity of the final products. The expected

launch of SeaStar is in late August 1993. The SeaWiFS

Project objectives in support of this undertaking are as

follows:

To serve as the NASA liaison to OSC for the

procurement of SeaWiFS data for the oceanographic research community;

To facilitate the operation and scheduling of the

SeaWiFS sensor system;

To develop and validate algorithms for bio-optical

properties and atmospheric correction;

To characterize and calibrate the SeaWiFS system and to assess on-orbit performance;

To achieve radiometric accuracy to within 5%

absolute and 1% relative, water-leaving radiances

to within 5% absolute, and chlorophyll a concentration to within 35% over the range of 0.05–50.0

mg m−3;

To collect, archive, and process recorded data,

as well as global maps of bio-optical properties

and chlorophyll a concentration, for the research

community;

To provide quality assurance monitoring of, and

coordinate collection of, direct broadcast data

by NASA’s selected research local area coverage

(LAC) receiving stations; and

To support the SeaWiFS Science Working Group

(SWG).

1.4 Project Overview

The success of the SeaWiFS mission will be determined

by the quality of the ocean color data set and its availability. Inherent in any space flight mission for scientific

applications are project functions to assure this success.

Figure 2 shows the SeaWiFS Project elements within an

end-to-end system overview. The SeaStar satellite broadcasts SeaWiFS data in real time to the GSFC High Resolution Picture Transmission (HRPT) station, as well as

to other stations, and recorded data—Global Area Coverage (GAC) and LAC—to GSFC, Wallops Flight Facility

(WFF), and OSC. These HRPT, GAC, and LAC data are

then transferred to the SeaWiFS Data Processing System

(SDPS). SDPS comprises a number of elements, namely:

SeaWiFS data processing; calibration, validation, and data

quality; data capture; and GSFC mission operations.

The data processing element receives raw spacecraft

data and generates standard global ocean color data products. The standard products go to the GSFC EOSDIS Version 0 Distributed Active Archive Center (DAAC), which

is responsible for archiving and distributing SeaWiFS data

to the scientific community.

The calibration and validation element is responsible

for establishing and updating calibration procedures for

SeaWiFS data, using correlative data for validating higherlevel products; analyzing trends and anomalies; and selecting ancillary data sets that are used in data processing,

such as winds, ozone, and atmospheric pressure. Initialization of the spaceborne sensor and continuing algorithm

refinement requires in situ radiometric measurements that

will be made from a marine optical buoy (MOBY) moored

off the coast of Hawaii, and dedicated cruises. For the latter, a full suite of supporting measurements will be made

including, but not limited to, particle and pigment analysis, sky radiance, and hydrographic and in situ spectrometry.

Mission operations serves as the link between OSC and

the SeaWiFS Project Office (SPO) for the purposes of reporting problems, monitoring telemetry, and scheduling

the scientific aspects of mission operations. It also provides

navigation information and orbital elements required for

processing and data collection. OSC is responsible for the

space segment, mission management, and the command

and data acquisition station.

All of the project elements are described in more detail

in later sections.

2. RESEARCH APPLICATIONS

Studying the variety of features and scales found within

basin circulations from research ships is difficult because of

competing temporal and spatial requirements. Small-scale

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An Overview of SeaWiFS and Ocean Color

Fig. 2. SeaWiFS Project schematic showing major elements of shore and at-sea data processing.

features (less than approximately 30 km) evolve faster than

research vessels can map them. Mesoscale features (approximately 30–150 km) change more slowly, but mapping

them from ships is still difficult because of the features’

large size. The larger basin-scale circulation is obviously

very difficult to map from ships, even with multiple vessels and international cooperation. The dimensions of both

small- and mesoscale features are clearly visible in satellite imagery. Frequently, cloudiness effects can be removed

using simple compositing techniques. Short-term averaged

basin-scale images can be assembled from the individual

overpass images and will be a continuing application for

satellite acquired ocean color imagery.

Measurements from aircraft and ships in the 1960s, had

proven that ocean color is a powerful tool for understanding oceanic, biological, and physical processes and demonstrated it would be possible to use satellites to measure the

spectra of sunlight reflected from ocean waters. For most of

the world’s oceans, the radiance reflected from the ocean

in the visible wavelength region (400–700 nm) is related

to the concentration of chlorophyll and other plant pigments. Since chlorophyll is a green pigment and the color

of sea water changes from blue to green as the concentration of chlorophyll increases, the amount of phytoplankton

in ocean waters can be calculated if the concentration of

chlorophyll is known. Thus, satellite acquired ocean color

data constitute a powerful tool for determining the abundance of ocean biota on a global scale.

The inversion of radiance to biomass is conceptually

straightforward, but not without difficulty. For example,

the amount of radiance reflected from the sea surface is

very small compared to the atmospheric radiance arriving

at the sensor due to Rayleigh scattering. In fact, 90% or

more of the signal measured over oceans is due to atmo4
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

spheric radiance. It is crucial, then, to correct the satellite

measured radiance for atmosphere effects if quantitatively

useful biomass estimates are to be derived.

The difficulty associated with calibrating and validating a spaceborne color sensor is summarized in Fig. 3 which

shows CZCS, SeaWiFS, and MODIS sensitivities compared

to water-leaving radiances and radiances measured at the

satellite over cloud-free oceans according to model results

provided by H. Gordon. The model simulates high latitudes (60◦ solar zenith angle) at nadir, for a typical atmosphere (0.35 cm ozone, 0.5 cm water vapor, 0.2 optical

depth at 670 nm with an  ̊Angstr ̈om coefficient of 0.5), and

for extremely low and high oceanic chlorophyll concentrations (Chl=0.01 and 10.0 mg m−3, respectively). Note that

virtually the entire range of oceanic chlorophyll concentration is contained within the two lines for at-satellite radiances, requiring a highly stable and sensitive instrument

with precise corrections for atmospheric radiances.

Due to a lack of in situ observations for comparison,

less confidence is ascribed to the model results at wavelengths greater than 700 nm than for the visible. Low

Noise Equivalent delta Radiance (NEδL), defined as the

quotient of input radiance to signal-to-noise ratio (SNR)

in the red and near infrared region, is especially important

for atmospheric correction on a pixel by pixel basis and

in turbid waters, and was beyond the capability of CZCS.

Instrument sensitivities shown are the NEδL for the instrument bandwidth, and represent approximately one digital

count for SeaWiFS. CZCS values are actual performance

measurements, while those for SeaWiFS and MODIS are

minimum design specifications. The minimum NEδL for

SeaWiFS was specified to equally apportion errors in derived chlorophyll concentrations between the sensor noise

and inherent variability of in-water bio-optical algorithms

available in 1983, using the Gordon model. MODIS specifications reflect the minimum expected improvement of

both in-water and atmospheric algorithms by 1998.

2.1 CZCS Review

The CZCS was the first instrument to acquire ocean

color data from space. It was one of eight experiments on

NASA’s Nimbus-7 satellite and collected visible radiance

data over most of the world’s oceans. These data are still

being used to study questions concerning the role of the

ocean in the global carbon budget and ocean circulation.

The major parameters and characteristics of the CZCS

are given in Table 2. Although envisioned as a proof-ofconcept sensor with only four visible spectral bands, limited recorder capacity, and a designed lifetime of about

one year, the CZCS provided high quality imagery from

the fall of 1978, until the summer of 1986, when the instrument ceased operations. Over its lifetime, the CZCS

demonstrated the feasibility of determining phytoplankton

pigment concentrations from space. Since it was the only

instrument of its kind, its loss was of major consequence to

those interested in research, commercial, and operational

applications for ocean color data.

Table 2. Major parameters and characteristics of

the CZCS.

Instrument Parameters

Band Center Width Phenomenon

1 443 nm 20 nm Chlorophyll Absorption

2 520 nm 20 nm Chlorophyll Correlation

3 550 nm 20 nm Gelbstoff, Sediments

4 670 nm 20 nm Chlorophyll Absorption

5 750 nm 100 nm Surface Vegetation

6 11.5 μm 2.0 μm Surface Temperature

Sensor Accuracy

IFOV: 0.865 × 0.865 mrad

Footprint at Nadir: 825 × 825 m (sea level)

Co-registration: <0.15 mrad (at nadir)

Viewing Accuracy: <2.0 mrad (at nadir)

Mission Characteristics

Orbit Type: Sun Synch. at 955 km

Ascending Node: 11:52 a.m. (local solar)

Modulation Transfer 1 at 150 km target size,

Function: (0.35 min at 0.825 km)

Swath Width: 1,600 km

Scan Plane Tilt: ±20◦ (in 2◦ steps)

Digitization: 8 bits

Abbott and Chelton (1991) provide a comprehensive

review of the CZCS literature. The use of CZCS data in

oceanographic research can be categorized in a variety of

ways. The majority of studies published to date have focused on both the spatial and temporal distributions of

pigment. The length scales are mesoscale (including regional studies), basin scale, and global scale. In a few

cases, only temporal scales are discussed, as with time series at specific locations, and these have usually addressed

questions about seasonal and interannual variability, e.g.,

Dueser et al. (1988) and Dueser et al. (1990). In both papers by Dueser, surface pigment concentrations were correlated with subsurface particle fluxes at various locations

in the North Atlantic Ocean.

Another category of research is algorithm development

which can be broken down into two groups. The first

group focuses on the relationship between water-leaving

radiances and other biological or optical parameters such

as pigment concentration and diffuse attenuation. These

studies rely primarily on field observations and theoretical

models. Examples of this group are Austin and Petzold

(1981), and Gordon et al. (1983). The second group infers other quantities from the satellite-derived products obtained from the first group of algorithms. Examples of the

second group are Eppley (1984), Platt et al. (1988), Campbell and O’Reilly (1988), Kuring et al. (1990), Thompson

et al. (1990), and Balch et al. (1992). With the exception of

Thompson et al., these studies have focused on estimating

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An Overview of SeaWiFS and Ocean Color

Fig. 3. Spaceborne sensor sensitivities compared to water-leaving radiances, and radiances measured at the

satellite over cloud-free oceans according to model results. (Courtesy of H. Gordon.)

primary production from satellite pigment concentrations.

Thompson et al. related satellite pigment concentrations

to the production of dimethylsulfide in the eastern North

Atlantic.

2.2 Mesoscale Processes

With one kilometer resolution, the CZCS provided excellent information on mesoscale processes. Because the

CZCS coverage was very inhomogeneous in both space

and time (McClain et al. 1992), research using the data

has been largely limited to mesoscale phenomena. The

majority of studies published have focused on temporal

and spatial variability and the coupling between physical

and biological processes. The scope of mesoscale studies

has ranged from the analysis of a single image showing

a particular feature of interest, e.g., McClain and Atkinson (1985), to regional studies involving large numbers

of images, e.g., Feldman (1986), McClain et al. (1988),

Michaelsen et al. (1988), Mueller and Lang (1989), Strub

et al. (1990), and Muller-Karger et al. (1991). Some studies focused on specific processes or oceanic features including river plumes (Barale et al. 1986, Muller-Karger et al.

1989, Muller-Karger et al. 1990), coastal upwelling (McClain et al. 1986, Abbott and Zion 1987), island wake effects (Feldman et al. 1984), ocean eddies (Gordon et al.

1982, Yentsch and Phinney 1985, Huary et al. 1986), open

ocean upwelling (Brock et al. 1991, Brock and McClain

1992) and ice edge processes (Sullivan et al. 1988, Comiso

et al. 1990). Other investigations based on time sequences

of images provided detailed information on biological processes on bloom event time scales, e.g., Denman and Abbott (1988), McClain et al. (1990a) and Ishizaka (1990a,

b, c). Other studies focused on statistical analyses that

quantified the scales of spatial variability, e.g., Barale and

Wittenburg-Fay (1986), Yoder et al. (1987), and Smith et

al. (1988). Examples of mesoscale phenomena are shown

in Plates 1, 2, and 3 (see envelope on back of cover for

all Plates).

2.3 Basin- and Global-Scale Processes

The use of the CZCS data for large-scale studies has not

been possible until recently when the global data set was

released from NASA/GSFC (Esaias et al. 1986, Feldman et

al. 1989). Wroblewski et al. (1988) used a CZCS pigment

composite of the North Atlantic to qualitatively verify the

results of an analytical model of phytoplankton concentrations in the North Atlantic. Platt and Sathyendranath

(1988) used the CZCS pigment fields to estimate basin6
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

wide primary productivity. McClain et al. (1990b) used

the monthly CZCS pigment composites to examine the relationships between physical and biological processes in the

North Atlantic. Plates 4 and 5 show pigment composites

of the Arabian Sea (Smith et al. 1991) and the North Atlantic, respectively. Balch et al. (1992) analyzed primary

productivity algorithms with respect to a global primary

productivity database.

3. NON-RESEARCH USES

The diverse interests of commercial and operational

users of remotely sensed ocean data include commercial

fishing; marine transportation; offshore oil and gas exploration and extraction; hydrology; marine ecology; naval

operations; environmental monitoring; ocean mining; and

value-added products, such as fisheries-aids charts. Satellite acquired data on ocean phenomena from the experimental CZCS and the operational Advanced Very High

Resolution Radiometer (AVHRR), from which SST maps

are routinely derived, have provided the basis for developing a number of important applications serving these

interests. The remainder of this section consists of brief

reviews of some of the commercial and operational uses

of ocean color imagery, presented predominately from the

viewpoint of how CZCS imagery was used in this context.

3.1 Commercial Applications

Data on SST, surface currents, and ocean color is useful

to the fishing industry in offering the potential for locating

fish populations, thereby improving catch efficiency. Maps

of remotely sensed oceanic currents are useful to ships at

sea to optimize ship routes, thereby reducing costs.

As the world supply of hydrocarbon fuel is steadily

depleted, more and more exploration and production is

moving offshore into deeper and deeper water. Although

offshore moorings and oil platforms are usually designed

to withstand the expected currents from eddies, certain

operations and hardware are adversely affected that can

result in costly delays due to the suspension of various

drilling operations. For example, loads on the drill riser,

rig movement, remotely operated vehicle activities, and

supply vessel operations are all sensitive to mesoscale circulations. Tracking the boundary of eddies and jets is a

high priority since it allows these types of operations to

continue as long as safety requirements permit.

SST data derived from AVHRR measurements are currently the major sources of the information used in preparing eddy forecasts for offshore oil exploration and production. Temporal changes in eddy size and position and

frontal locations are used to produce routine summaries

of their positions; surface flow directions; and estimates of

their probability, duration, and magnitude.

Unfortunately, the utility of infrared SST measurements

significantly deteriorates from the summer through fall in

the Gulf of Mexico and in many other regions at latitudes

between 30◦ south and 30◦ north, e.g., the equatorial Atlantic and Pacific Oceans, as well as the Indian Ocean. The

loss of surface expression occurs because of two separate effects. First, as a result of strong solar heating, surface conditions are relatively isothermal during these months with

temperature ranges in the narrow band from about 29–

31◦ C. Second, humid atmospheric conditions cause a severe attenuation of much of the thermal infrared signature.

The limitations due to surface heating are greatest when

the surface is calm and solar intensity is high. In some

instances surface features are completely masked, significantly diminishing the usefulness of the data. Additionally,

information on cloudiness and atmospheric aerosols that

are computed during atmospheric correction of the data,

show promise for use in meteorological investigations and

commercial forecasting.

3.2 Operational Applications

Monitoring of ocean dumping, noxious algal blooms

and red tides, regional characterization, habitat assessment, flow visualization, and oil spill trajectory analysis are potential operational uses of ocean color identified

by the National Oceanic and Atmospheric Administration

(NOAA) in the coastal zone.

Within larger estuaries such as the Chesapeake Bay,

the frequent coverage will be useful in monitoring turbidity and algal biomass, two major criteria of water quality

that relate to the health of submerged aquatic vegetation

and the development of anoxia. The accuracy of regional

algorithms, based on the standard products but tuned to

local conditions using local validation observations, can be

much higher than that of the generalized standard NASA

algorithms for such applications. While the Instantaneous

Field-of-View (IFOV) of SeaWiFS is not ideal for estuaries,

the daily coverage frequency more closely matches the inherent time scales in the coastal zone than does Landsat-6

or the French instrument, Satellite Pour l’Observation de

la Terre (SPOT).

On larger scales, frequent fields of diffuse attenuation

coefficient, of interest to studies of upper ocean heat budgets, will likely find use in large scale physical and biological models. The use of ocean color data for flow visualization is analogous to the current use of AVHRR-derived

SST for this use. The ocean color data will be especially

useful, even mandatory, under conditions when there are

weak gradients in SST. These conditions commonly exist at

temperate and low latitudes in the summer, and at higher

latitudes in the winter. A classic case is the identification

of the Loop Current in the summer. The observed ocean

color signal integrates over the upper few to tens of meters of the water column. This allows for slight differences

in the bio-optical characteristics in underlying waters of

the loop current and coastal waters to be readily distinguishable from each other with satellite color observations

7
An Overview of SeaWiFS and Ocean Color

at times when the upper centimeters of surface waters are

thermally homogenous. This capability is useful for identifying positions of current boundaries such as in routine

Gulf Stream frontal analysis, ship routing, and drill rig

operations. It also finds applications in oil spill trajectory

analysis.

Certain types of ocean dumping activities are also very

visible in ocean color data due to slight or profound changes

in water bio-optical characteristics. These operational uses

generally require near real-time access and dissemination.

For many cases, use of climatological or forecast fields of

ancillary data will be adequate for immediate use. Retrospective analysis, if required, can make use of the same

fields used in retrospective global data processing.

3.3 Joint Commercial Aspects

As part of a contractual agreement between NASA and

OSC, NASA retains all rights to SeaWiFS data for research

purposes and OSC retains all rights for commercial and

operational purposes. The sensor and mission operations

are designed to meet the research requirements of NASA.

There is a general embargo period of two weeks for distribution of research data by NASA, and a five-year period

before the data becomes public domain. There are three

exceptions to the embargo period: 1) field experiments requiring data for ship positioning, 2) instrument calibration

and validation activities, and 3) demonstrations to prove

feasibility for operational use, all of which require prior

approval from NASA. During the five-year period, access

through NASA will be permitted for research purposes by

authorized users only. Access to users for commercial or

operational purposes will be through OSC or its licensees.

After five years, the data may be used without restriction.

The NASA–OSC contract defines commercial use as

those involving the sale or resale of data, as well as data

derived therefrom for more than the cost of reproduction.

Operational use is defined as real-time or near real-time

use of the data, as well as data derived therefrom during

the two week embargo period provided above, other than

research use expressly authorized by NASA.

The selection of authorized research users will be based

on a submission of a brief proposal to NASA. Following

approval, users will sign an agreement with NASA stating

that the data will be used for research purposes only, and

that their distribution will be limited to other authorized

users only. The statement will also require signature by

users that they will not provide access to data by commercial or operational users.

NASA approved HRPT stations are authorized to receive broadcast LAC data by NASA. These stations will

also be committed to distributions of data only to authorized users. Normally, this authorization will be for retrospective use, meaning decryption codes will be provided

at least 14 days after reception. LAC data will have to be

recorded for later decryption.

In addition to the GSFC LAC facility, up to 12 stations at any given time may be licensed by NASA for near

real-time support for specific time periods and purposes,

and may receive the decryption key in advance. Criteria

for designating stations for rapid response include station

coverage of scientific cruises or experiment areas, technical capability, and performance. It is expected that this

designation and scheduling of assignment periods, will be

reviewed at least annually by a panel convened by NASA

in order to assure optimal coverage of experimental sites

and times. As in the case for research users, approval by

NASA can be revoked at any time if the non-disclosure

agreement is violated. This will guarantee that the SeaWiFS data will be distributed by NASA only to authorized

users for research.

NASA will also notify users of updates to calibration

and revisions in the processing algorithms or ancillary data

used for producing the SeaWiFS products. This will occur

through an electronic bulletin board.

4. SPACECRAFT DESCRIPTION

The SeaStar spacecraft, developed by OSC, will carry

the SeaWiFS instrument and will be launched to low Earth

orbit on board an extended Pegasus launch vehicle. The

SeaWiFS instrument will be the only scientific payload on

the SeaStar spacecraft. OSC has the sole responsibility

for the development, launch, and command and control of

the satellite. The development of the SeaWiFS instrument

was subcontracted to Hughes/SBRC, but OSC maintains

ultimate responsibility for the instrument.

Currently, the Pegasus is flown aloft under the wing

of a modified Lockheed L-1011 aircraft and released at an

altitude of about 39,000 ft, whereupon the launch vehicle

engages and lifts the spacecraft to a low Earth, circular,

parking orbit of 278 km with an inclination of 98.20◦. The

launch sequence is shown in Fig. 4. The solar panels are

deployed at this time (see Fig. 5), which along with the

NiH2 batteries, are the sensor’s power source. The solar

panels provide an end-of-life power production of 170.27 W

averaged over the orbit.

The SeaStar spacecraft has an onboard hydrazine propulsion system that is then used to raise the satellite to its

final 705 km circular, noon, sun-synchronous orbit. The final orbit is reached approximately 20 days following launch.

The launch is presently planned to occur from the U.S.

West Coast during daylight hours, although launch from

the East Coast is under consideration if an L-1011 is used.

At 25 days after launch, the SeaWiFS instrument is powered up and checked out. At launch plus 30 days, data

collection operations commence. The overall sequence of

launch to operational system is shown in Fig. 6.

The attitude control system (ACS) must be capable of

maintaining the 705 km noon, sun-synchronous orbit, performing lunar and solar calibration maneuvers, and providing attitude knowledge within one SeaWiFS pixel. The

8
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

Fig. 4. SeaStar launch sequence.

Fig. 5. SeaStar satellite with solar panels deployed.

9
An Overview of SeaWiFS and Ocean Color

Fig. 6. SeaStar operational system.

three-axis stabilized system consists of orthogonal magnetic torque rods for roll and yaw control and two momentum wheels for pitch stabilization. Sensors include redundant sun sensors, horizon sensors, and magnetometers.

The propulsion system consists of two subsystems, a

reaction control system, and a hydrazine propulsion system. The reaction control system uses nitrogen and provides third stage stabilization during the launch. The

hydrazine propulsion system is used for raising the orbit

from the nominal 278 km parking orbit to the 705 km sunsynchronous operational orbit. In addition, it is used for

orbit trim requirements over the life of the mission. Four

Hamilton Standard one pound thrusters are being used.

Two telemetry streams are transmitted. The first is

real-time LAC data merged with spacecraft health and instrument telemetry at 665.4 kbps. This is transmitted at

L-band with a frequency of 1702.56 MHz. The other telemetry stream consists of stored GAC and selected LAC,

along with spacecraft health and instrument telemetry, at

2.0 Mbps. This is transmitted at S-band with a frequency

of 2272.5 MHz. The command system uses S-band with an

uplink of 19.2 kbaud at 2092.59 MHz.

Redundant global positioning system (GPS) receivers

will be used for orbit determination. The orbit state derived from this is included in the spacecraft health telemetry. There are no plans to use the Tracking and Data

Relay Satellite System (TDRSS) for this Project.

5. SENSOR DESCRIPTION

The original thought was that the sensor would be built

by Hughes/SBRC and launched by the EOSAT on board

Landsat-6. However, the contract negotiations were not

completed. Subsequently, a competitive request for proposal (RFP) was issued in November 1990. After review of

the submitted proposals, OSC was selected to provide the

ocean color data, and a contract was awarded on March

29, 1991. OSC will thus build, launch, and operate the

SeaStar spacecraft, which will carry the SeaWiFS instrument built by Hughes/SBRC. Much was learned from the

CZCS experience in terms of calibration requirements, sensor design, algorithm design, data processing, and scientific

10
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

applications. SeaWiFS will be able to take full advantage

of this experience.

The SeaWiFS instrument consists of an optical scanner and an electronics module. A cut-away drawing of the

scanner is shown in Fig. 7. The instrument has scanning

mechanisms to drive an off-axis folded telescope and a rotating half-angle mirror that is phase-synchronized with,

and rotating at half the speed of, the folded telescope.

The rotating scanning telescope, coupled with the halfangle scan mirror arrangement, provides a design configuration that permits a minimum level of polarization to

be achieved, without field-of-view rotation, over the maximum scan angle requirement of ±58.3◦. The absence of

field-of-view rotation permits the use of a multichannel,

time-delay and integration (TDI) processing in each of the

eight spectral bands to achieve the required SNR. This capability, in turn, allows a relatively small sensor collecting

aperture and, therefore, results in a smaller and lighter

instrument than would otherwise be required.

Incoming scene radiation is collected by the folded telescope and reflected onto the rotating half-angle mirror.

The collected radiation is then relayed through dichroic

beam splitters to separate the radiation into four wavelength intervals—each wavelength interval encompassing

two each of the eight SeaWiFS spectral bands. The radiation in the four separate wavelength intervals is directed by

four corresponding aft-optics assemblies through two separate spectral bandpass filters that further separate the

radiation into the eight required SeaWiFS spectral bands.

The aft-optics assemblies also image each of the resultant

defined bands of radiation onto four detectors that are

aligned in the scan direction. The detected radiation signals are then amplified by preamplifiers for TDI processing

in the electronics module.

The off-axis scanning telescope rotates at six revolutions per second in the cross-track direction, for HRPT

format compatibility, to provide contiguous scan coverage at nadir from a 705 km (380 nmi) orbital altitude with

the SeaWiFS spatial resolution of 1.6 mrads (1.13 km or

0.6 nmi at nadir). A scanner tilt mechanism enables the

instrument to be oriented in the along-track direction to

−20◦, 0◦, and +20◦ to avoid sun glint from the sea surface. Tilting the entire scanner, rather than only a section

of the optical train, assures that the SeaWiFS calibration,

polarization, and angular scanning characteristics will be

identical for all tilt positions and, thereby, simplifies the

ground processing of in-flight data.

Monitoring of sensor calibration over periods of a few

orbits, to several months or years, is accomplished using

solar calibration for the former and lunar calibration for

the latter. Solar calibration uses a solar radiation diffuser

and an input port located in a fixed position outside of the

±58.3 degrees SeaWiFS scene-scan interval. The diffuser

is located on the inside of a baffle pointed in the +y (minus velocity vector) direction. The diffuser will be covered

with an aperture plate with numerous small holes that will

adjust the diffuser system output to the required level and

minimize diffuser surface degradation from contamination

or ultraviolet exposure. The diffuser is located so calibration will take place near the southern terminator.

Lunar calibration is accomplished by a spacecraft maneuver to view the moon when the spacecraft is in the

nighttime part of the orbit. The spacecraft is oriented

such that the SeaWiFS scene-scan interval is 180◦ from

the normal Earth oriented position, i.e., looking outward.

The lunar observation can, therefore, be accomplished under nearly full moon conditions through the identical SeaWiFS optical path as that for Earth scenes.

The detected and amplified signals are routed from the

scanner to the electronics module where they are further

amplified and then filtered to limit the noise bandwidth.

The filtered signals are digitized by a 12 bit analog-todigital converter and the digitized signals directed to a

commandable processor where the TDI operation is performed in real time as data are generated. The resultant

summed signals are divided by four and rounded to 10 bit

numbers, and then sent from the processor to the spacecraft data system at 1.885 Mbps during the data acquisition period of each scan line.

The instrument angular momentum will be compensated by the angular momentum wheel. This is necessary

to avoid nutation coupling when the instrument is tilted.

Implementation will consist of a brushless DC motor driven

synchronously at approximately 2,000 rpm. The accurately

controlled frequency derived from the instrument clock will

ensure compliance to the 1 oz-in-sec uncompensated angular momentum requirement for the spacecraft attitude

control system.

The key sensor performance parameters, required performance, and predicted performance, are listed in Table 1.

The SeaWiFS design will meet or exceed the required performance values.

6. DATA MANAGEMENT

Figures 8 and 9 show the SeaWiFS system elements

within a detailed overview. Conceptually, the system divides into two parts: the Space Segment including the

Ground Support System (Fig. 8) and the SeaWiFS Data

Processing System or SDPS (Fig. 9). OSC is responsible for the Space Segment, the Mission Management, and

their Command and Data Acquisition Station. Figure 8

shows the SeaStar satellite downlinking SeaWiFS real-time

HRPT data to the GSFC HRPT station, and other approved stations, as well as GAC and LAC data to GSFC,

WFF, and OSC. The HRPT, GAC, and LAC data are then

transferred to the SDPS.

SeaWiFS Data Processing System: This element

receives raw science data and generates the global

ocean color data products. Within this element are

separate development, test, and production environments. Standard products will be provided to

11
An Overview of SeaWiFS and Ocean Color

Fig. 8. SeaWiFS Project schematic: ground and space elements.

12
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

Fig. 9. SeaWiFS Project schematic: system and processing elements.

13
An Overview of SeaWiFS and Ocean Color

Fig. 7. Schematic of the SeaWiFS scanner assembly which scans from west to east.

the GSFC DAAC for archive and distribution.

Calibration and Validation: This element oversees

the quality of the ancillary data and SeaWiFS products, and generates analysis reports. It is responsible for establishing and updating calibration procedures for the SeaWiFS data, using correlative data

for the validation of the higher level products, performing trend and anomaly analysis, and collecting

ancillary data sets such as winds, atmospheric pressure, ozone, etc., from various sources. A detailed

description of this element is given in Section 7.

Software Support: This element is responsible for

programming assistance and database support, software updates, software integration and testing, and

software configuration management.

Mission Operations: This element serves as the single point-of-contact for OSC and the GSFC HRPT

station to communicate, coordinate scheduling, and

report any problems. It also monitors telemetry,

performs scheduling activities for scientific aspects

of mission operations, and provides navigation information and orbital elements required for processing and data collection. Current calibration tables

and navigational information will be provided to

OSC to be uplinked to SeaStar. These will be included in the HRPT downlink.

6.1 Ground Systems and Support

The SeaStar Spacecraft Operations Center (SOC) will

be an OSC facility and will be located in the Washington, DC area. The SOC has a fully steerable five meter

antenna, PC-based tracking computer, and redundant receivers and demodulators for both the 2.0 Mbps S-band

GAC and LAC stored data transmission, and 665 kbps Lband direct broadcast HRPT LAC/telemetry data stream.

A digital front end includes redundant bit and frame synchronizers, command encoders, timing subsystems, and a

simulator for system data flow testing. Redundant telem14
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

etry/command consoles provide telemetry processing and

command capability. The SOC also contains commercial

science data processing and product generation facilities.

Although OSC will operate the SOC, the SPO will define recording times, sensor tilt angle, recorded data transmission times, and calibration events, within limits of safe

operation, and will forward these parameters to the SOC.

The SOC will be manned during the noon pass, but will

function automatically during the midnight pass.

The ground segment serves three functions: spacecraft

control, data delivery to GSFC, and data delivery to commercial users. The first and third functions are the sole

responsibility of OSC. The second function will be supported by the two ground receiving stations, one for the

acquisition of recorded data and one for the acquisition of

direct broadcast data. The ground station for the acquisition of the direct broadcast data will be at GSFC in Greenbelt, Maryland. This HRPT ground station will consist of

a prime computer system with two backup systems. The

Search and Rescue Satellite (SARSAT) Ground Station,

located in Building 22 at GSFC, will serve as its backup.

The primary antenna is an eight foot perforated parabolic

program track antenna. It will be dedicated to the capture of SeaWiFS HRPT data. For a downlink frequency of

1702.5 MHz, the antenna gain is about 30 db and the beam

angle is 5.14◦. The backup antenna is located on the roof

of Building 22 and will be available to SeaWiFS on a time

available basis. The orbital Acquisition of Signal/Loss of

Signal (AOS/LOS) Tables for the Greenbelt stations for

nominal conditions have been determined.

The ground station for the acquisition of the recorded

data is at WFF. The primary antenna is a nine meter autotrack that is currently being used as a backup for the

International Ultraviolet Explorer (IUE). The backup antenna is a 7.5 meter receive-only autotrack that is available on a non-interference basis with the Wallops Launch

Range. The orbital AOS/LOS Tables for the WFF Ground

Station for nominal conditions have also been calculated.

The recorded data will be transmitted to the SDPS facility in Building 28 at GSFC where the data will be stored

in a computer-addressable file on disk. The time required

to transfer 1.0 Gbits of data from WFF to the VAX at

792 kbps is about 22 minutes. The dumps will normally

occur during normal working hours at a mutually agreeable schedule. The raw data will be stored at WFF on

magnetic tape for 14 days.

6.2 Data Processing System

SeaWiFS will produce two types of science data. LAC,

with a nominal ground resolution of 1.13 km, will be directly broadcast and selectively recorded, while GAC, subsampled every fourth line and every fourth pixel, with a

nominal ground resolution of 4.5 km, will be recorded on

board. Direct broadcast LAC will be routinely received

at GSFC for the U. S. East Coast, and other real time

LAC will be received at various HRPT stations around

the world. Recorded LAC data for preselected areas and

all GAC data will be received at the WFF and forwarded

to the SDPS at GSFC.

The document, SeaWiFS Science Data and Information

System Architecture Report (1991), describes the basic

functions, interfaces, and requirements of the SDPS in detail. Briefly, SDPS will receive data; manage data, i.e., catalog and archive; generate standard products; generate calibration products; ensure data distribution; schedule and

monitor the science mission; calibrate and validate data

products; support software updates; and provide systems

support (see Fig. 9). These functions are performed by

three distinct elements. The SDPS receives and manages

science and ancillary data and generates standard products; the Mission Operations element performs planning,

scheduling, and health and safety monitoring; and the Calibration and Validation element develops procedures that

are then implemented by the SDPS. SDPS personnel will

provide software and systems support. The Data Capture

Facility (DCF) acquires data and transfers it to the SDPS.

Key external interfaces with the SDPS are fully described in the aforementioned report. These external interfaces are the Wallops Ground Station, OSC mission

management, GSFC HRPT station, non-SeaWiFS data

sources, SeaWiFS Science Team, Science Research Community, the GSFC V0 DAAC, and the NASA Sponsored

HRPT Stations.

The SPO will produce a limited set of standard data

products for archiving and distribution:

Level-1 GAC and LAC (∼4.5 km and ∼1.1 km at

nadir, respectively) resolutions.

Level-2 Processed data at GAC and LAC resolutions:

pigment and chlorophyll a concentration,

diffuse attenuation coefficient at 490 nm,

normalized water-leaving radiances (5),

aerosol radiances (3), and

an error field.

Level-3 Global gridded products (at approximately

9 km resolution) of the above listed parameters and statistics at time averages ranging

from daily to annual periods.

To accommodate expected schedules in receiving required

ancillary data, operational scenarios for three types of processing to level-3 products have been developed. These are:

Performance Assessment processing within 24 hours, Refined Processing within 30 days, and subsequent annual

Global Reprocessing (if needed). A three-tiered archival

strategy was developed that conforms to these scenarios,

with 30 day, 180 day, and permanent retention periods,

respectively.

The SDPS will ingest approximately 381 Mbytes per

day of science data and generate products totaling up to

3 Gbytes per day. The SDPS is sized to perform valida15
An Overview of SeaWiFS and Ocean Color

tion processing and global reprocessing at ten times the

speed of routine processing. The preliminary total central

processing unit (CPU) requirements for the processing and

reprocessing are estimated to be equivalent to a 250 million instructions per second (mips) scalable processor. The

permanent archive is sized to 6 Tbytes over the lifetime of

the mission, which assumes archiving only data received at

GSFC and WFF. The temporary archive is estimated to

be 300 Gbytes. Ongoing prototyping activities will refine

these estimates. Central archiving of HRPT level-1a data

will require approximately 100 Mbytes per station per day.

A high speed network, such as the Fiber Data Distribution

Interface (FDDI), is required to support the projected input/output (I/O) and network workloads.

The storage media and processor options have been examined. Based on trade-off studies, disk arrays are recommended for data staging, a magneto-optical drive jukebox

is recommended for the 30 day temporary archive, an 8 mm

library for the 180 day temporary archive, and a write once

read many (WORM) times optical library for the permanent archive.

Product generation CPU platforms have been surveyed

to select the most viable candidates. The primary evaluation criteria are scalability, high performance, and open

portability. Given their multiprocessing capability, highend workstations from Silicon Graphics were selected for

the product generation platform.

Two potential architecture implementations have been

assessed: a hybrid VMS and UNIX architecture based on

the current GSFC Ocean Color Processing System, and

an all UNIX architecture. The adoption of the hybrid architecture and development of an implementation plan to

move to an all UNIX environment is the baseline plan.

The justifications for this conclusion follow. In the long

run, the all UNIX implementation will achieve an open

system architecture with a potentially lower lifecycle cost

and greater flexibility and portability. However, the current process control and data management software, which

is VMS dependent, is a proven and operational system.

The use of these proven components will reduce schedule

and development risks.

6.3 Distribution System

The SeaWiFS Project has the responsibility to ensure

the distribution of SeaWiFS data to all NASA approved

SeaWiFS data users. Some of the Science Team members,

or NASA sponsored HRPT stations, may require near realtime SeaWiFS data. The Project will support the near

real-time distribution to this group of users. SeaWiFS data

will be permanently archived at the GSFC DAAC. Users

will get the SeaWiFS data through the GSFC V0 DAAC,

at the nominal cost of duplication.

The data distribution system will be in place prior to

launch. The distribution system will utilize the experience gained during the development and operation of the

current CZCS data distribution system. Authorized users

may also request HRPT LAC data archived at NASA sponsored HRPT stations. Some organizations may request distribution of certain types of products as they are produced.

This routine data distribution will first be approved by the

SPO, and implemented as a routine daily operation.

Access to the archive system will be provided to all

users who will initiate their data search through a user

interface, such as a terminal. Users will be asked to give

their name, address, phone number, institution, etc. All

users can access the Metadata or data catalog, but only

authorized users may access the Browse Archive. As the

user examines the Metadata or Browse Archive, parameter

attributes are defined. The user then makes a request to

the DAAC, which verifies the authenticity and authorization of the request. After the request has been approved,

data in the archive will be retrieved, copied onto appropriate media, and sent to the user. The user’s account will be

updated to include the data distributed for this request.

The cost of distribution has not been defined.

If the requested data reside in one of the NASA sponsored HRPT archive stations, a request will be sent to the

archival system of the HRPT station. The HRPT station

will acknowledge receipt of the request, search for the data,

and then retrieve, duplicate, and send the requested data

to the user. In this scenario, the user makes a request for

HRPT LAC data directly to the collecting HRPT station.

The HRPT station must verify the user’s authorization

before processing the request.

It is anticipated there will be on the order of 200 users

per year of SeaWiFS data. (There are about 100 users for

the current CZCS data.) Out of the 200 users, there will be

about 20 SeaWiFS Science Team members and 12 NASA

sponsored HRPT stations that will have the capability to

use data in near real-time, and an estimated 75 research

HRPT stations that will use data after the embargo period.

Of the Team Members and HRPT stations, about 20 users

are estimated to require all of the SeaWiFS data.

Depending on the frequency of data requests for the

level of data, the following two types of data granules,

or scenes, are envisioned for level-1 and level-2 data: 1)

10 minutes of data in spacecraft coordinates along-track

and cross-track for the whole swath, and 2) fixed area,

1500×1500 km for fixed tilt. Level-3 data will be available

for fixed regions for daily, eight-day, monthly, and annual

time periods.

6.4 Mission Operations

The primary objectives of the GSFC Mission Operations element are, in order of priority:

Acquire a complete set of scientifically useful

GAC data on board the spacecraft on full global

scales,

Acquire all GAC data on the ground at WFF,

16
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

Acquire LAC data over all calibration targets,

Develop navigation algorithms to provide pointing knowledge to 1 pixel accuracy, root mean

squared (rms, 1σ) on a global scale, and

Provide support for HRPT stations and the user

community.

These objectives define the primary functions of Mission Operations:

a) Provide sensor operations schedule; single pointof-contact on spacecraft operations with OSC’s

SeaStar Operations Ground Subsystem (SOGS);

b) Provide elements and vectors to WFF for stored

data acquisition;

c) Navigate observations;

d) Serve as single point-of-contact with the HRPT

station and the user community on pointing vectors and support; and

e) Assist in calibration by providing spacecraft-sunlunar geometrical information.

Central to the Mission Operations primary functions is

the operation of an orbital prediction model. This model

provides the information required to generate schedules

of sensor operations, ensure timely transmission of stored

data to the ground, and allow accurate pointing information for HRPT stations, as well as assist in navigation and

calibration.

Fulfillment of the primary functions of Mission Operations requires the establishment and maintenance of seven

interfaces:

1) OSC SOGS,

2) WFF,

3) Project Scientist,

4) Calibration and Validation Team (CVT),

5) SDPS,

6) DCF, and

7) NASA approved research HRPT stations.

The OSC SOGS interface is the element for the passing

of schedules. The SOGS performs the actual commanding

of the sensor and spacecraft based on the GSFC provided

schedules, as well as for OSC determined commercial purposes. Also, the SOGS will perform orbit maintenance

maneuvers to keep the spacecraft in an optimum orbit for

scientific observations 705 km, noon equator crossing time,

and perform attitude control necessary to meet the one

pixel navigation knowledge requirement. The SOGS will

assess the power and thermal environment to ensure the

five-year mission lifetime.

WFF is the primary downlink station for stored data

acquisition. Information of the spacecraft positions and

times for downlink are provided by GSFC Mission Operations.

The Project Scientist provides approval for all scientific

operations and oversight of the mission to ensure that the

scientific objectives are met. The Project Scientist will be

kept informed on mission status and scheduling conflicts

on a regular basis.

The CVT determines the locations, times, priorities,

and frequencies of coverage of calibration targets and communicates this information to Mission Operations. Mission

Operations then schedules recorded LAC coverage to meet

these observational requirements within the constraints of

power, thermal, and data recorder budgets. Mission Operations will also assist the CVT Manager by determining

spacecraft-sun-lunar geometries required for proper utilization of the calibration data.

The SDPS is the repository of SeaWiFS data. Telemetry necessary for navigation and for keeping the orbital

predictions models current, will be supplied to Mission Operations by the SDPS. Mission Operations will supply copies of the sensor schedules to the SDPS to assist in data

processing and to monitor the execution of commands.

The DCF operates an HRPT station at GSFC, Building 26, to receive real-time LAC data for health and safety

monitoring. The DCF also serves as an intermediary in

the pathway of stored data from WFF to the SDPS, reformatting data from NASA communication (NASCOM)

blocks, which is the transmission medium. Mission Operations will assist these efforts by providing updated orbital elements to the DCF for antenna pointing by making downlink schedules available, and by keeping DCF informed of mission status. The DCF also provides support

to NASA approved HRPT stations. Requests for support

are routed through Mission Operations.

Finally, Mission Operations serves as a single pointof-contact for the user community and NASA approved

HRPT stations. Mission Operations will operate and maintain an electronic bulletin board containing mission status;

orbital elements; a LAC requestor by which authorized

users may request recorded LAC time; an HRPT station

locator through which users may find the locations, names,

and addresses of HRPT stations containing data of interest; and other project information, such as the status of

data processing. Mission Operations will maintain and distribute the Authorized User List to HRPT stations and the

Data Processing element.

6.5 Sensor Operations Schedules

The primary responsibility of the GSFC Mission Operations element is to provide OSC with schedules of command sequences to maximize the coverage and scientific

usefulness of SeaWiFS data and to ensure data acquisition. The commands under the responsibility of Mission

Operations are:

1) Stored data downlink times at WFF;

2) Gain changes;

17
An Overview of SeaWiFS and Ocean Color

3) GAC record, tilt change, calibration, and LAC

record times;

4) Broadcast times for research HRPT stations;

5) Updated calibration tables; and

6) TDI sequence.

The passing of downlink times at WFF to OSC and

WFF ensures the timely transmission and acquisition of

stored data. This function is second in importance because the mission is a failure unless data are received on

the ground. SeaWiFS will pass over WFF near noon and

near midnight each day. Mission Operations will provide

the overpass schedules to WFF and to OSC so OSC can

command data transmission and WFF can receive it.

The tilt capability of SeaWiFS provides sun glint avoidance. Improper tilts will result in data of poor or useless

scientific quality. SeaWiFS can tilt +20◦ (aft of the velocity vector; toward the South Pole), −20◦ (forward along

the velocity vector; toward the North Pole) or 0◦ (nadir

pointing) in three steps. Provision of tilt times by Mission

Operations to OSC will enable minimization of sun glint

contamination.

Calibration times are necessary to provide quantitative

radiances of the highest scientific quality. There are three

types of calibration activities proposed for SeaWiFS: lunar, solar, and in situ. Lunar calibration will occur when

the moon is near phase (6◦), and is accomplished by pitching the spacecraft toward the moon on the dark side of the

orbit. Solar calibrations will occur as the spacecraft passes

near the South Pole as needed, perhaps as much as every

orbit. In situ calibration includes ship, moored buoy, and

drifting buoy observations, and perhaps pre-determined

calibration areas. Ship observations are sporadic events

requiring frequent coordination with the CVT. However,

some moorings may be in place for long periods of time

and may require routine spacecraft observations. Calibration activities take secondary priority in data recording

time to GAC observations. All calibration observations

will be placed in the LAC recording area on the onboard

computer.

Selection of gain enables maximum sensitivity of the

sensor under conditions of changing signal magnitude. The

proper gains must be chosen for calibration activities, i.e.,

solar and lunar calibration. Mission Operations will relay

these gain change schedules to OSC.

LAC record times provide the high resolution data required for certain scientific activities, such as event monitoring. Any space left on the onboard data recorder after GAC and calibration recording, may be used for LAC

recording. Assuming a nominal 40 minutes per day of GAC

recording, there will be 17–19 minutes of data per day

available for LAC recording and calibration. Requests for

LAC data may come from the SeaWiFS Project Scientist,

the SeaWiFS Science Team, or research users previously

authorized by the SPO.

Slots are available in the telemetry data stream on

board the spacecraft for calibration tables, by agreement

with OSC. The existence of these tables in the data stream

enables users, especially those with HRPT stations, to calibrate SeaWiFS data using the most updated coefficients

without requiring access to a different data source. Coefficients must be uplinked by the SOGS on a routine basis,

and changes must be made as necessary to ensure timeliness. Mission Operations will submit calibration tables to

the SOGS as part of the schedules. Maintenance of the

tables and decisions to update rest with the CVT. Mission Operations will add the most recent orbital elements

to the tables to enable HRPT stations to point antennas

using data in the data stream.

TDI is a technique using multiple detector elements to

improve SNR. The SeaWiFS sensor employs four detectors per band, providing four looks at each ground pixel.

The four readings are integrated to reduce noise by about a

factor of 2. Should an individual detector element lose sensitivity or become excessively noisy, the capability to eliminate it from the integration sequence and count another

element twice, is under command control. Periodically, the

various combinations of detectors in the TDI sequence will

be exercised to determine their performance, and changes

to the sequence will be implemented if necessary.

6.6 Sensor Monitoring

Both NASA/GSFC and OSC will monitor sensor performance, health, and safety. The Mission Operations element will be in a central position to perform monitoring

of incoming telemetry. Any anomalies detected anywhere

within the system must be rapidly communicated to all

participants, and the Mission Operations element provides

the primary interface with OSC for operational functions

and will be central in the dissemination of such information. The monitoring will be performed by graphical presentation of key telemetry parameters for each orbit. Multiple displays will be used to monitor the telemetry quality

and flag out-of-limit values. Also, subsystems information

such as alarm determination, power condition, command

and event mode, and configuration displays are provided.

Selected key parameters will be extracted from the telemetry data files and maintained in a dynamically configurable database to minimize database transaction processing and storage requirements. This database will be used

to perform long-term trend analyses and generate selected

and periodic performance assessment reports.

Orbit parameters are determined using GPS, yielding

position and velocity to within 70 m and 1 m s−1, respectively. Position information will be downlinked, along with

telemetry, in the data stream. Thus the need for sophisticated satellite tracking is eliminated.

7. CALIBRATION/VALIDATION

GSFC has the responsibility to lead a product assurance, calibration, and validation program, and to deter18
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

mine the degree to which the commercially procured ocean

color data fulfills the contractually stated NASA requirements. Background experience for this effort was obtained

from the CZCS sensor which had limited objectives, namely,

a limit of 2 hours of coverage per day, a one-year demonstration lifetime, and a 10% level-2 data processing goal.

Despite the limited design lifetime of the CZCS mission,

data was collected from October 1978, until June 1986. In

1985, GSFC and the University of Miami undertook the

task of processing all of the CZCS data. The global CZCS

data processing was completed in March 1990. Much was

learned from the CZCS experience in terms of calibration

requirements, sensor design, algorithm design, and scientific applications. The SeaWiFS Project is designed to

compensate for a number of shortcomings in the CZCS

data set both in terms of the instrument, and in the calibration and validation areas.

The calibration and validation program has three main

tasks: calibration of the SeaWiFS instrument, development and validation of the operational atmospheric correction algorithm, and development and validation of the

derived product algorithms, such as chlorophyll a concentration. Some of this work will be done internally at GSFC

while the remainder will be done externally at other institutions. NASA and the SPO are placing the highest priority on assuring the accuracy of derived water-leaving radiances globally, and over the entire mission. If these criteria

are met, development of global and regional biogeochemical algorithms can proceed on many fronts. These activities are discussed in detail in, The SeaWiFS Calibration

and Validation Plan (McClain et al. 1992). Figure 10 and

Table 3 provide overview schematics of the SeaWiFS calibration and validation program structure and milestones,

respectively.

Included in the validation program is the development

of atmospheric correction and bio-optical algorithms required to produce the derived products. The atmospheric

correction algorithms for processing SeaWiFS data are under development and include a number of improvements

over the CZCS algorithms. Some field studies are needed,

however, to collect specific data sets to implement these

improvements. External activities are largely being pursued by the MODIS Oceans Team and by other investigators via contracts and grants issued by the SPO. Most internal activities incorporate close collaborations with these

groups. The program also needs a field effort that includes

bio-optical cruises. The few existing bio-optical data sets

applicable to SeaWiFS are very limited in scope, and system algorithm initialization requires coincident space and

ocean measurements.

The field studies mentioned above fall into the external activities category. Internal activities include the development of bio-optical databases and the intercomparison of various atmospheric correction and bio-optical algorithms. Investigators funded by NASA to assist with algorithm development will have access to these databases.

Although general access will not be available immediately,

it is expected that these data will be provided to a suitable

archive facility within two years for unlimited distribution.

Atmospheric correction algorithms use data from other

sources, such as ozone concentrations and surface pressure

fields, and the quality of these fields must be assessed before the data are incorporated into the processing. Finally,

quality assessment and control functions for the final derived products will be performed prior to release of the

data by comparison with historical and simultaneous field

observations submitted to the SPO by various field programs and individual investigators.

The Internal activities related to calibration can further be divided into those associated with the onboard

calibration and those connected with the vicarious calibration. The Instrument Scientist and the Calibration Manager bear primary responsibility for the former and the

Algorithm and Validation Manager is responsible for the

latter. The onboard calibration includes analysis and documentation of the initial prelaunch characterization and

calibration data provided by Hughes/SBRC and the analysis of post-launch data from the solar diffuser plate and lunar imagery. The vicarious calibration involves the collection, analysis, and intercomparison of optical surface truth

with the derived satellite water-leaving radiance fields.

Table 3 provides a list of required observations for the

vicarious calibration, validation, algorithm development,

and support science, which were approved by the SPSWG

in August 1991. They are derived from the findings of

a workshop that defined the requirements and protocols

for in situ optical validation measurements (Mueller and

Austin 1992). The SeaWiFS Project has adopted the recommendations of the JGOFS on protocols for biogeochemical validation measurements, such as chlorophyll a.

Also included in the validation program is the development of the atmospheric correction and bio-optical algorithms required to produce the derived products as defined

by the Science Working Group. Presently, the atmospheric

correction algorithms needed for processing SeaWiFS data

have not been developed and a number of improvements

over the CZCS-class algorithm are necessary. To implement these improvements, however, some field studies are

required to collect specific data sets. Similarly, because

the existing bio-optical data sets applicable to SeaWiFS

are either very limited in scope or are non-existent, a field

program must be implemented to support dedicated biooptical cruises. These field studies fall in the External

activities category.

All GSFC Internal operations are co-located with the

SeaWiFS processing system and utilize certain common

subsystems and databases. This is different from the CZCS

processing scenario where calibration functions were undertaken at the University of Miami and the quality control of the derived products was conducted at what is now

the Laboratory for Hydrospheric Processes’ Oceans Computer Facility. The integrated SeaWiFS arrangement will

19
An Overview of SeaWiFS and Ocean Color

Fig. 10. SeaWiFS calibration and validation program structure.

20
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

Table 3. 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 (Mueller and Austin 1992).

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, KE(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.

21
An Overview of SeaWiFS and Ocean Color

simplify communication between the calibration and validation group and the operations personnel. Also, it more

readily accommodates the demands of near real-time data

processing that was not a consideration in the CZCS processing. The processing system is designed to accommodate the calibration and validation computational and data

storage requirements, so that these activities do not impede the operational data processing.

8. SEAWIFS PROJECT OFFICE

SPO personnel and their areas of responsibilities at

GSFC are given in Table 4. The sensor calibration and

validation effort requires close coordination between the

Instrument Scientist, the Calibration Manager, the Algorithm and Validation Manager, and the Project Scientist. The composition of the SPO straddles organizational

boundaries and reflects the fact that the Project is an

Earth Sciences Directorate (Code 900) activity.

Table 4. Key personnel and elements of the SPO.

SPO Address

NASA/Goddard Space Flight Center

SeaWiFS Project, Code 970.2

Greenbelt, Maryland 20771

Telephone: (301) 286–9676

FAX: (301) 286–3221

E-mail: SeaWiFS.Project/Omnet

SPO Personnel

Dr. William Barnes Instrument Scientist

Dr. Mary Cleave Deputy Project Manager

Dr. Wayne Esaias Project Scientist

Dr. Gene Feldman Data System Manager

Dr. Watson Gregg Mission Operations

Dr. Bruce Guenther Calibration Manager

Dr. Stanford Hooker Field Program Manager

Mr. Robert Kirk Project Manager

Dr. Charles McClain Algorithm and Validation Manager

Mr. Charles Vermillion Ground System Manager

Glossary

ACS Attitude Control System

ADEOS Advanced Earth Observation Satellite (Japanese)

AOS/LOS Acquisition ofSignal/Loss ofSignal

AVHRR Advanced Very High Resolution Radiometer

CDOM Colored Dissolved Organic Material

CPU Central Processing Unit

CVT Calibration/Validation Team

CZCS Coastal Zone Color Scanner

DAAC Distributed Active Archive Center

DC Direct Current

DCF Data Capture Facility

DOC Dissolved Organic Carbon

EOS Earth Observing Satellite

EOSAT Earth Observation Satellite Company

EOSDIS Earth Observing Satellite Data Information System

ESA European Space Agency

FDDI Fiber Data Distribution Interface

FWHM Full-Width Half-Maximum

GAC Global Area Coverage

GOFS Global Ocean Flux Study

GPS Global Positioning System

GSFC Goddard Space Flight Center

HPLC High Performance Liquid Chromatography

HRPT High Resolution Picture Transmission

IFOV Instantaneous Field-of-View

I/O Input/Output

IUE International Ultraviolet Explorer

JGOFS Joint Global Ocean Flux Study

LAC Local Area Coverage

MERIS Medium Resolution Imaging Spectrometer

MIPS Millions ofInstructions Per Second

MOBY Marine Optical Buoy

MODIS Moderate Resolution Imaging Spectrometer

MTF Modulation Transfer Function

NASA National Aeronautics and Space Administration

NASCOM NASA Communications

NASDA National Space Development Agency (Japanese)

NCDS National Climate Data System

NEδL Noise Equivalent delta Radiance

NOAA National Oceanic and Atmospheric Administration

NSCAT NASA Scatterometer

OCTS Ocean Color Temperature Sensor (Japanese)

OSC Orbital Sciences Corporation

OSSA Office ofSpace Science and Applications

POC Particulate Organic Carbon

POLDER Polarization Detecting Environmental Radiometer (French)

PON Particulate Organic Nitrogen

RFP Request for Proposal

rms root mean squared

SARSAT Search and Rescue Satellite

SBRC Santa Barbara Research Center

SDPS SeaWiFS Data Processing System

SeaWiFS Sea-viewing Wide Field-of-view Sensor

SNR Signal-to-Noise Ratio

SOC Spacecraft Operations Center

SOGS SeaStar Operations Ground Subsystem

SPO SeaWiFS Project Office

SPOT Satellite Pour l’Observation de la Terre (French)

SPSWG SeaWiFS Prelaunch Science Working Group

SST Sea Surface Temperature

SWG Science Working Group

TDI Time-Delay and Integration

TDRSS Tracking and Data Relay Satellite System

TOMS Total Ozone Mapping Spectrometer

TOPEX Topography Experiment

TSM Total Suspended Material

WFF Wallops Flight Facility

WORM Write Once Read Many

22
S.B. Hooker, W.E. Esaias, G.C. Feldman, W.W. Gregg and C.R. McClain

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An Overview of SeaWiFS and Ocean Color

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24
An Overview of SeaWiFS and Ocean Color

Color Plate Captions

Plate 1. Pigment concentration image from the southeastern U.S. continental shelf showing the Charleston

Gyre (McClain and Atkinson 1986). Ship transects and bathymetry contours are overlaid on the image.

Plate 2. Two images from McClain et al. (1986) showing the rapid development of a phytoplankton bloom

off northwest Spain. Plate 2 shows conditions before the onset of the bloom. Color coding is the same as in

Plate 1.

Plate 3. Continuation of the bloom for the area shown in Plate 2. The bloom concentrations increased by

more than a factor of 10, from Plate 2 to Plate 3, and was the result of episodic coastal upwelling. Color

coding is the same as in Plate 1.

Plate 4. Arabian Sea monthly mean pigment concentration images produced from the CZCS global processing

effort (Smith et al. 1991).

Plate 5. Seasonal composites of pigment concentrations in the North Atlantic for 1979 (McClain et al. 1990b).
Plate 1

An Overview of SeaWiFS and Ocean Color
Plate 2

An Overview of SeaWiFS and Ocean Color
Plate 3

An Overview of SeaWiFS and Ocean Color
Plate 4

An Overview of SeaWiFS and Ocean Color
Plate 5

An Overview of SeaWiFS and Ocean Color
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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

 REPORT NUMBER

 REPORT DOCUMENTATION PAGE OMB No. 0704-0188

 OF REPORT OF THIS PAGE OF ABSTRACT

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

 Unclassified Unclassified Unclassified Unlimited

SeaWiFS, Oceanography, Ocean Color, CZCS, Coastal Zone

Color Scanner, Sensor, Bio-Optics, Biogeochemical, SeaStar, Data Processing, 78

Calibration/Validation

 Stanford B. Hooker, Wayne E. Esaias, Gene Feldman, Watson W. Gregg, and Charles R. McClain: NASA-GSFC,

 Greenbelt, Maryland; Elaine R. Firestone: General Sciences Corporation, Laurel, Maryland.

Report is available from the Center for AeroSpace Information (CASI),

7121 Standard Drive, Hanover, MD 21076–1320; (301)621-0390

Unclassified–Unlimited

Subject Category 48

13. ABSTRACT (Maximum 200 words)

SeaWiFS, the Sea-viewing Wide Field-of-view Sensor, will bring to the ocean community a welcomed and improved

renewal of the ocean color remote sensing capability lost when the Nimbus-7 Coastal Zone Color Scanner (CZCS) ceased

operating in 1986. The goal of SeaWiFS, scheduled to be launched in August 1993, is to examine oceanic factors that

affect global change. Because of the role of phytoplankton in the global carbon cycle, data obtained from SeaWiFS will be

used to assess the ocean's role in the global carbon cycle, as well as other biogeochemical cycles. SeaWiFS data will be

used to help elucidate the magnitude and variability of the annual cycle of primary production by marine phytoplankton

and to determine the distribution and timing of spring blooms. The observations will help to visualize the dynamics of

ocean and coastal currents, the physics of mixing, and the relationships between ocean physics and large-scale patterns of

productivity. The data will help fill the gap in ocean biological observations between those of the CZCS and the Moderate

Resolution Imaging Spectrometer (MODIS) on the Earth Observing Satellite-A (EOS-A).

National Aeronautics and Space Administration

Washington, D.C. 20546–0001 TM–104566, Vol. 1

Laboratory for Hydrospheric Processes

Goddard Space Flight Center 92B00089

Greenbelt, Maryland 20771

6. AUTHOR(S)

 Stanford B. Hooker, Wayne E. Esaias, Gene Feldman, Watson W. Gregg,

 and Charles R. McClain

 Series Editors: Stanford B. Hooker and Elaine R. Firestone