NASA Technical Memorandum 104566, Vol. 3

SeaWiFS Technical Report Series

Stanford B. Hooker and

Elaine R. Firestone, Editors

Volume 3, SeaWiFS Calibration and Validation Plan

Charles R. McClain, Wayne E. Esaias,

William Barnes, Bruce Guenther,

Daniel Endres, Stanford B. Hooker,

B. GregMitchell, and Robert Barnes

September 1992
NASA Technical Memorandum 104566, Vol. 3

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 3, SeaWiFS Calibration and Validation Plan

Charles R. McClain, Wayne E. Esaias,

William Barnes, Bruce Guenther,

Daniel Endres, and Stanford B. Hooker

NASA Goddard Space Flight Center

Greenbelt, Maryland

B. Greg Mitchell

National Aeronautics and Space Administration

Washington, D.C.

Robert Barnes

CHEMAL, Inc.

Wallops Island, Virginia

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

1992
SeaWiFS Calibration and Validation Plan

Preface

Experience with the Coastal Zone Color Scanner (CZCS) and other satellite sensors has underscored the importance of sustained and coordinated programs to verify sensor calibration and derived products, especially as

more rigorous specifications on measurement accuracies are required to address the geophysical and biological

problems that have been identified by the science community.

As a second generation ocean color instrument, SeaWiFS offers a variety of design improvements over the

CZCS, which should provide the capability to meet the mission objectives. This document outlines the calibration and validation program designed by members of the SeaWiFS Project Office (SPO) in consultation

with the SeaWiFS Prelaunch Science Working Group (SPSWG) and others. The program accelerates a number of activities already being conducted by some members of the Moderate Resolution Imaging Spectrometer

(MODIS) Instrument Team and incorporates a number of additional functions. The program is meant to initiate a long-term strategy for the calibration and verification of a sequence of ocean color missions scheduled for

launch during this decade and the next. Given the fiscal contraints within which the SPO must work, we feel it

is as comprehensive a program as can be implemented.

This plan was submitted to the SPO in early January 1992, but because of the rapid pace of activities

within the SPO, no attempt has been made to keep the document current. Therefore, some information is

dated, and some minor inconsistencies are present. The strategies, however, that are outlined remain the same

and significant progress has been made in executing each of the program elements. For example, all primary

sole source contracts, cooperative agreements, and memoranda of understanding required to execute the in

situ observation, algorithm development, and round-robin instrument calibration programs have been finalized.

As the calibration and validation program procedes, progress will be documented in subsequent volumes of

the SeaWiFS Technical Report Series. For instance, the proceedings of the Monterey Workshop on instrument

calibration and data collection protocols is Volume 5 of the series and has already been published out of sequence

due to the demand for it by the science community.

The progress of the calibration and validation program is reported to the SPO during quarterly reviews. To

date, reviews have been held in February, May, and August 1992, at the NASA/Goddard Space Flight Center

(GSFC). This review schedule will be maintained as long as the SPO deems necessary. These reviews are open

sessions and those interested in the program are encouraged to attend. The schedule and agenda for these

reviews are distributed via electronic mail. If anyone is not currently being notified, please let the SPO know

that you wish to be added to the mailing list.

Those of us in the SPO view the SeaWiFS mission as a collaboration with the ocean color community and

we are looking forward to working closely with them. The ocean color community has worked long and hard for

this mission and it is our expectation that together, we can make SeaWiFS a great success.

Greenbelt, Maryland — C. R. M.

August 1992

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McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Abstract

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) will be the first ocean color satellite since the Nimbus-7

Coastal Zone Color Scanner (CZCS), which ceased operation in 1986. Unlike the CZCS, which was designed

as a proof-of-concept experiment, SeaWiFS will provide routine global coverage every two days and is designed

to provide estimates of photosynthetic pigment concentrations of sufficient accuracy for use in quantitative

studies of the ocean’s primary productivity and biogeochemistry. A review of the CZCS mission is included that

describes the limitations of that data set and provides justification for a comprehensive SeaWiFS calibration

and validation program. To accomplish the scientific objectives of the mission, the sensor’s calibration must be

constantly monitored, and robust atmospheric correction and bio-optical algorithms must be developed. The

plan incorporates a multi-faceted approach to sensor calibration using a combination of vicarious (based on in

situ observations) and onboard calibration techniques. Because of budget constraints and the limited availability

of ship resources, the development of the operational algorithms (atmospheric and bio-optical) will rely heavily

on collaborations with the Earth Observing Satellite (EOS), the Moderate Resolution Imaging Spectrometer

(MODIS) oceans team, and projects sponsored by other agencies, e.g., the United States Navy and the National

Science Foundation (NSF). Other elements of the plan include the routine quality control of input ancillary data

(e.g., surface wind, surface pressure, ozone concentration, etc., used in the processing and the verification of the

level-0 (raw) data to level-1 (calibrated radiances), level-2 (derived products) and level-3 (gridded and averaged

derived data) products.

1. INTRODUCTION

The program for SeaWiFS calibration and validation

reflects the experience that too often, missions have focused on the engineering aspects of a flight program and

have not adequately addressed issues of sensor performance

after launch and algorithm development. The SeaWiFS

Project Office (SPO) will incorporate a variety of activities, both in the prelaunch and post-launch periods, designed to ensure that the performance of the instrument

is accurately quantified throughout the mission and the

products the SPO generates meets the requirements of the

scientific community. This document outlines the various

components of the calibration and validation effort. It is

important to note the length of time between the formation of the SPO and the launch of SeaWiFS is less than

two years. This is a relatively short amount of time to

prepare for a mission and will require a concerted effort on

the part of the National Aeronautics and Space Administration (NASA) and the science community in partnership.

1.1 Historical Perspective

The Nimbus-7 Coastal Zone Color Scanner (CZCS) was

launched in October 1978, and was the first satellite sensor designed specifically for the estimation of pigment concentrations in the ocean. The mission was designed as a

proof-of-concept experiment and had narrowly defined objectives, namely, a limit of 2 hours of coverage per day, a

1 year demonstration lifetime, and a 10% level-2 data processing goal (Hovis et al. 1980 and Hovis 1981). Table 1

provides the design characteristics of the CZCS.

Table 1. Major instrument parameters and characteristics of the CZCS ocean color sensor (from Ball

Aerospace, 1979).

Instrument Bands

Band Wavelength Saturation Input SNR2

FWHM [nm] Radiance1 Radiance1

1 433–453 11.4 8.41 350

2 510–520 8.0 5.44 342

3 540–560 6.4 4.45 280

4 660–680 2.9 2.60 209

5 700–800 24.0 1.61 50

6 10,500–12,500 N/A SST Applications Only

Sensor Accuracy

Radiance Accuracy: 5%

Band Registration: <0.1 pixel

Location Knowledge: ∼2 pixels

Saturation Recovery: ∼100 pixels

Polarization: <2%

Nadir Resolution: 0.825 km LAC

Mission Characteristics

Orbit Type: Sun Synchronous at 955 km

Equator Crossing: 1130 ±25 min., ascending

Duty Cycle: <10%

Swath Width: 1,800–1,600 km LAC (±20◦)

Scan Plane Tilt: +20◦ to −20◦ in 2◦ steps

Dynamic Range: 8 bits quantization; 4 gains

1. Units of mW cm−2

μm−1 sr−1; gain 1.

2. Measured at input radiances.

The bio-optical and atmospheric correction algorithms

for the CZCS were developed after launch (Clark 1981,

Austin and Petzold 1981, Gordon et al. 1983a, and Gor1
SeaWiFS Calibration and Validation Plan

Fig. 1. NET cruise station locations (Gordon et al. 1983a). Case 1 waters are those where the reflectance

is dominated by pigment absorption and all other water masses are classified as Case 2.

don et al. 1988). The observations for the bio-optical algorithms were collected during a series of Nimbus Experiment Team (NET) pre- and post-launch dedicated cruises

off southern California, Baja (Mexico), the Gulf of Mexico,

and the East Coast of the United States. (Clark et al. 1980,

Austin 1980, and Gordon et al. 1980) as shown in Fig. 1.

The observation data sets collected during the cruises included chlorophyll a and phaeophytin concentrations, total

suspended particulate matter concentration, atmospheric

solar transmission, subsurface upwelled spectral radiance

and subsurface downwelled spectral irradiance, and downwelled incident spectral irradiance. The radiance and irradiance measurements were from 400–700 nm at 5 nm increments with half power bandwidths of 4 nm.

Two water quality derived products were included in

the level-2 processing, pigment concentration and the diffuse attenuation coefficient (K) at 490 nm (K(490)). Both

were based on ratios of water-leaving radiances. The final

pigment concentration algorithm was based on a total of

49 data points for both Case 1 and Case 2 waters (Fig.

2). Case 1 water is defined to be that where reflectance is

determined solely by absorption (Morel and Prieur 1977)

while the reflectance of Case 2 water is significantly influenced by scattering. The final (K(490)) algorithm was

derived from 88 data points (Fig. 3). After the first year

of the mission, the NET field program was discontinued.

The removal of the atmospheric components (Rayleigh

and aerosol radiances) from the total observed radiance

is required to derive the estimates of water-leaving radiance as shown in Fig. 4 (NET CZCS algorithm). Successful accomplishment of this correction is essential, and,

in general, is not straightforward for any remote sensing

application. The problem lies in closure of the system

of radiance equations because there are many more unknowns than there are quantities measured or theoretically computed. In the case of the CZCS, an innovative

approach was developed based on two radiometric properties of the ocean, which allows the number of unknowns

to be reduced. First, the strong absorption of water in

the near-infrared (IR) results in very small water-leaving

radiances (LW ) in most open ocean regimes. Second, the

normalized water-leaving radiances (LW N ) at 520 nm and

550 nm (LWN (520) and LWN (550), respectively) assume

fairly constant values in Case 1 waters having concentrations less than about 0.25 mg m−3, so-called “clear water”

(Gordon and Clark 1981).

The normalized water-leaving radiances are water-leaving radiances corrected to correspond to a solar zenith angle of zero. By assuming the water-leaving radiance at

670 nm is zero and that portions of a scene are clear water regions, Gordon et al. (1983a) were able to estimate

the aerosol radiances at 443, 520, 550, and 670 nm. The

method assumes the three  ̊Angstr ̈om exponents needed to

relate the aerosol radiance at 670 nm to the aerosol radiances at the other visible bands have constant values over

the entire scene. Also, the approach assumes the aerosol

radiances are highly correlated and the aerosol radiance

value at 670 nm can be used to estimate the aerosol radiance at 443 nm.

The assumptions in the Gordon et al. (1983a) algorithm present some problems in Case 2 waters where the

water-leaving radiance at 670 nm is significantly different

from zero, e.g., sediment laden waters and coccolithophore

blooms. While the 750 nm band might have been better for

the aerosol correction, this band was designed for flagging

land and clouds and did not have the sensitivity required

for quantifying aerosol radiance. Also, in many situations,

the aerosols within a scene are not homogeneous but rather

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McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 2. NET pigment algorithm plots and algorithm switching criteria (Gordon et al. 1983a).

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SeaWiFS Calibration and Validation Plan

Fig. 3. NET K(490) algorithm plots, a) derived relationship between K(490) and the ratio of the

inherent upwelling radiance the ocean surface (solid line) together with the database (88 points) and b)

regression of calculated values of K(490) against the measured values (Austin and Petzold, 1981).

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McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 4. Schematic of the CZCS and SeaWiFS atmospheric correction algorithms. The elements used

in the global CZCS model are shown as solid lines. The dashed lines refer to the new elements to be

incorporated into the SeaWiFS model.

Fig. 5. Monthly total CZCS data collection.

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SeaWiFS Calibration and Validation Plan

are a mix of continental haze, marine haze, and dust, each

having different absorption and scattering properties and

different  ̊Angstr ̈om exponents. While the assumptions in

the correction algorithm are not strictly valid in such situations, the technique does produce reasonable results in

the majority of cases.

Despite the limited design lifetime of the CZCS mission, data were collected until June 1986. Fig. 5 shows

the volume of data collection by the CZCS. The original

volume of CZCS data (roughly 250,000 minutes of data

or 125,000 two minute scenes) was reduced substantially

by screening the level-0 data for cloud cover, so that only

reasonably clear scenes were converted to calibrated radiance tapes (CRT, level-1). The screening was performed

by the CZCS Project Scientist, Warren Hovis, shortly after the data were received. Even then, the data set filled

over 30,000 1600 bpi CRT’s with a final volume of approximately 700 Gbytes. The data were partitioned into scenes,

usually two minutes (970 scan lines) in length.

In 1985, the NASA/Goddard Space Flight Center

(GSFC) and the University of Miami undertook the task

of processing all of the CZCS data (Esaias et al. 1986 and

Feldman et al. 1989). A major component of the global processing was the duplication of data from tape to

write-once-read-many (WORM) optical disks which provide much greater data accessibility and media stability

than magnetic tape. The processing included the creation

of geophysical products in scanner coordinates (level-2)

and average geophysical fields on a uniform global grid

(level-3). There were three other major components to the

global processing effort: sensor calibration, quality control

of the level-2 products, and development of an archive and

distribution system. The archive is described in Feldman

et al. (1989).

The task of processing the entire data set was complicated by the time-dependent degradation of the sensor’s

sensitivity. It was determined early in the mission that

sensor sensitivity was degrading with time, but quantification of the degradation was difficult to estimate (Viollier 1982, Gordon et al. 1983b, Hovis et al. 1985, Mueller

1985, and Gordon 1987). Because of the relatively large

atmospheric contribution to the total observed radiances

(Gordon 1981) and the great sensitivity of the bio-optical

algorithms to the estimated water-leaving radiances (Clark

1981), small errors in the calibration can induce sizable errors in the derived geophysical products, thereby rendering

them useless for many applications. Thus, a comprehensive investigation of the calibration over the entire period

of sensor operation was required.

By processing large quantities of clear water imagery,

Evans (pers. comm.) was able to develop a vicarious calibration that was used in the global processing of the entire

CZCS data set. However, the approach required assumptions that may limit the science that can be performed with

SeaWiFS. Specifically, LWN (520) and LWN (550) were assumed to be 0.48 and 0.30 mW cm−2 μm−1 sr−1, respectively, the  ̊Angstr ̈om exponents were assumed to be zero,

and certain geographical regions such as the Sargasso Sea

were assumed be clear water sites (pigment concentrations

less than 0.25 mg m−3) at all times. Under these assumptions, analyses of the derived normalized water-leaving radiances indicated what calibration adjustments were required to produce the nominal clear water normalized radiance values. The vicarious calibration of the 443 nm band

is tenuous because of the great variability in LWN (443)

even in clear water. Additionally, certain command and

engineering data from the Nimbus-7 platform were not

archived so that a detailed analysis of possible effects related to the spacecraft environment and operation on the

sensor’s performance and calibration could not be performed. Fig. 6 shows some of the results of the vicarious

calibration for the CZCS (Evans unpub.).

The quality control of the level-2 products was performed at GSFC’s Laboratory for Oceans (presently named

the Laboratory for Hydrospheric Processes), though some

scenes were automatically rejected during tape ingest for

particular problems, e.g., missing bands. The level-2 processing also employed a land and cloud mask based on a

fixed value of band 5 counts and a sensor ringing mask.

Sensor ringing occurs on the downscan side of bright targets i.e., clouds and highly reflective land masses and results in invalid total radiances and derived products (Mueller 1988). Software was incorporated in the University of

Miami DSP system (a software package for satellite data

visualization and processing) which allowed an interactive

quality control by the processing team. The procedure simultaneously displayed the daily global composites of pigment, LWN (443), LWN (550), and LA(670) and allowed for

sequencing through each daily mosaic of scenes with the

cursor. Thus, each scene was either accepted or rejected

and, if rejected, one of nine rejection criteria was selected

with optional comments by the investigator.

The six most important categories of scene rejection,

in decreasing order, were: duplicate scenes, “dubious” normalized radiances, low sun elevation, no useful data, sun

glint contamination, and high LA(670). Of nearly 62,000

scenes reviewed, over 9,000 were rejected. Fig. 7 presents

the distribution of rejections by year and category. The duplicate scenes were due to the creation of multiple copies

of the CRTs, which were kept by the Nimbus Project in

the tape archive. Most scenes rejected for “dubious” normalized radiance had large areas where LWN (550) was less

than half the nominal value of 0.30 mW cm−2 μm−1 sr−1

that were not clearly the result of sun glint or atmospheric

dust. (Sun glint and dust both produce high LA(670) values.) Sun glint can be distinguished from dust because of

the consistent pattern it produces in the center of equatorial scenes. A sun glint mask was applied in the level-2

processing, but the mask did not always cover the entire

area of contamination because the sun glint pattern is wind

speed dependent and the mask algorithm used a constant

6 m s−1 wind.

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McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 6. Vicarious calibration results for the CZCS (unpublished data from R. Evans). On the top is the

long-term sensitivity degradation functions of the four visible bands. These functions were used in the

global CZCS processing. On the bottom are estimates of the high frequency variability in the calibration

of the 520 nm and 550 nm bands.

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SeaWiFS Calibration and Validation Plan

Fig. 7. CZCS quality control results.

The “no useful data” classification was used when a

scene was over land or completely cloud covered. Finally,

scenes where the solar zenith angle was greater than about

60◦ were rejected as “low sun elevation” because Fresnel

reflectivity for the direct solar radiation increases rapidly

as the zenith angle increases beyond 60◦. Thus, at large

solar zenith angles, the amount of light entering the water column is relatively small as are the upwelling water

radiances. Also, errors in the atmospheric correction are

expected to be greatest at high zenith angles. The net result was these scenes almost always had very high pigment

concentrations which were considered to be erroneous.

The global CZCS data processing was completed in

March 1990, and provided a glimpse of the global distribution of total pigment concentration (Fig. 8) and K(490)

even though its coverage was sparse (Fig. 9). Clearly, the

mission exceeded the original goals, but the data set does

not clearly resolve the seasonal cycle and interannual variability on global scales. Recent reviews of the science derived from the CZCS data set are found in Abbott and

Chelton (1991) and McClain et al. (1991a) with the latter

also providing an overview of the quality control procedures used in the CZCS global processing.

Much was learned from the CZCS experience in terms

of calibration requirements, sensor design, algorithm design, and scientific applications. SeaWiFS is designed to

compensate for a number of shortcomings in the CZCS

data set. The CZCS band selection did not allow the

separation of photosynthetically viable chlorophyll a from

degradation products. The inclusion of the 410 nm band

in SeaWiFS should help separate these pigments. Also,

it is expected that the bio-optical algorithms will be improved based on the development of more comprehensive

bio-optical data sets and that algorithm problems such as

those noted by Denman and Abbott (1988) and MullerKarger et al. (1990) will be eliminated. The design of the

near-IR band precluded its use for atmospheric corrections

in turbid waters. SeaWiFS will have bands at 765 nm and

865 nm.

The CZCS internal calibration via calibration lamps

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McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 8. Global pigment composite containing all the CZCS data processed and accepted during the

quality control.

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SeaWiFS Calibration and Validation Plan

Fig. 9. Global sampling from CZCS for total 92-month operation (11/78–6/86). Note that the global

processing subsampled the original level-1 data by one-sixteenth.

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McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

was unreliable. This limitation required the application of

a vicarious calibration technique based on nominal values

of LWN (443), LWN (520), and LWN (550) which proved to

be reasonably correct for some periods, but only marginally

correct for others (Balch et al. 1991 and Hay et al. 1991).

SeaWiFS will use imagery of the moon, data from a solar

illuminated diffuser plate, and a mission-long field program

to track the sensor performance. Finally, the instrument

signal-to-noise ratios (SNR) are vastly improved which,

coupled with 10-bit digitization, will allow more highly resolved quantification of radiances and derived products.

Tables 1 and 2 provide the CZCS and SeaWiFS performance specifications, respectively.

Table 2. Major instrument parameters and characteristics of the SeaWiFS ocean color instrument.

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 5.44 616

5 545–565 7.44 4.45 581

6 660–680 4.20 2.60 447

7 745–785 3.00 1.61 455

8 845–885 2.13 1.09 467

Sensor Accuracy

Radiance Accuracy: <5% absolute each band

Band Registration: <0.3 pixel

Location Knowledge: <1 pixel

Saturation Recovery: <10 pixels (7 estimated)

Polarization: <2% (<1% expected)

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

Duty Cycle: 100% daylight

Swath Width 2,800 km LAC (±58.3◦)

(at equator): 1,502 km GAC (±45.0◦)

Scan Plane Tilt: +20◦, 0◦, −20◦

Dynamic Range: 10 bits quantization; 4 gains

1. Units of mW cm−2

μm−1 sr−1; gain 1.

2. Measured at input radiances.

1.2Future Perspective

SeaWiFS will be the first in a sequence of ocean color

related missions (Fig. 10) that include Japan’s ADEOS

Ocean Color Temperature Sensor (OCTS) and the EOS

MODIS-N. Tables 3 and 4 provide the sensor specifications

for OCTS and MODIS-N, respectively. Other missions at

various stages of planning and development are Germany’s

Reflective Optics System Imaging Spectrometer (ROSIS),

the European Space Agency’s Medium Resolution Imaging

Spectrometer (MERIS), and Japan’s Global Imager (GLI).

Thus, SeaWiFS will initiate a continuous global time series

of ocean color data that should extend well into the next

century. Because SeaWiFS will have a design lifetime of

at least 5 years, it is likely to be operational when OCTS

and MODIS are launched.

Table 3. Major instrument parameters and characteristics of the OCTS satellite.

Instrument Parameters

Band Wave- Band- Radiance Gain Sensilength1 width1 Normal Low tivity2

1 0.412 0.020 145 97 0.095

2 0.443 0.020 150 100 0.098

3 0.490 0.020 130 87 0.085

4 0.520 0.020 120 80 0.078

5 0.565 0.020 90 60 0.0586

6 0.665 0.020 60 40 0.0391

7 0.765 0.040 40 27 0.0264

8 0.865 0.040 20 13 0.0127

9 3.70 0.30

10 8.50 0.50

11 11.0 1.0

12 12.0 1.0

Instrument Characteristics

SNR Performance: Bands 1 and 8, 450

Bands 2–7, 500

NE∆T (at 300◦ K): Bands 9–11, 0.15◦ K

Band 12, 0.20◦ K

Scan Angle: ±45◦

Swath Width: 1,400 km

Tilt Angle: ±20◦

Polarization Band 1, 5%

Sensitivity: Bands 2–8, 2%

Digitization: 10 bits

Design Life: 3 years

Instrument Calibration

VISNIR, Type: Deep Space (each scan)

Sunlight (once a day)

Lamp (once a week)

VISNIR, Accuracy: ±10% abs. (each band)

±3% rel. (band-to-band)

IR, Type: Deep Space and Black

Body (each scan)

IR, Accuracy: ±0.4◦ K at 300◦ K

1. Units of μm.

2. Ten bits, high gain, in units of rad./count.

In order for the data from these missions to be useful for quantifying long-term trends in oceanic biological

processes, comprehensive and consistent calibration and

algorithm validation programs are needed for each mission. These programs must include product and calibration comparisons when missions overlap in time, since the

instruments will not be identical, in terms of their radiometric characteristics, and the derived products from each

will be based on sensor specific algorithms. Therefore, care

must be taken to ensure that the algorithms, both atmos11
SeaWiFS Calibration and Validation Plan

Fig. 10. Ocean color mission launch and operation schedule.

pheric and bio-optical, produce reasonably similar derived

products. The techniques developed for calibration and atmospheric and bio-optical algorithm development for SeaWiFS will provide the baseline for subsequent ocean color

missions.

Table 4. Major instrument parameters and characteristics of the MODIS-N sensor.

Sensor Accuracy

Polarization: 2% max., <2.2 μm

IFOV (Number of 29 @ 1,000 m

bands @ IFOV): 5 @ 500 m

2 @ 250 m

Spectral Bands: 36, 19 @ 0.4– 3.0 μm

17 @ 3.0–15.0 μm

(10–500 nm width)

Radiometric Accuracy: 5% absolute, <3 μm

1% absolute, >3 μm

2% reflectance

NE∆T (at 300◦ K): <0.05

Mission Characteristics

Orbit Type: Sun Synch., 705 km

Swath Width: 110◦, 2,330 km

Average Data Rate: 11.0 Mbps (day)

1.8 Mbps (night)

Dynamic Range: 12 bits quantization

Duty Cycle: 100%

1.3 Science Mission Goals

The design of SeaWiFS was driven by science requirements as defined by the SeaWiFS Prelaunch Science Working Group (SPSWG). The SPSWG was an ad hoc committee selected by NASA Headquarters for the purpose of

providing to NASA guidance in the formulation of mission objectives, specifications, and goals. Table 5 is a

list of the membership. This group was dissolved prior to

the release of the NASA Research Announcement (NRA)

for SeaWiFS. A new Science Working Group (SWG) will

include those individuals selected for funding under the

NRA. The science requirements have been refined over the

past 10 years, as documented in The Marine Resources Experiment (MAREX) Program Report (NASA 1982), and a

NASA/Earth Observation Satellite Company (EOSAT) report written by the Joint EOSAT/NASA SeaWiFS Working Group in 1987. The SeaWiFS science mission goals are

the following:

1) Determine the magnitude and variability of the

annual cycle of global oceanic primary production.

2) Quantitatively assess the ocean’s role in the global carbon cycle and other biogeochemical cycles.

3) Quantify the relationships between ocean physics

and large scale patterns of productivity.

4) Determine the spatial and temporal distributions of phytoplankton blooms.

5) Advance the scientific applications of ocean color

data and the technical capabilities required for

data processing, management, and analysis in

preparation for future missions.

Given these scientific objectives, the specific set of obser12
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

vational specifications are the following:

1) Radiometric accuracy to within 5% absolute and

1% relative.

2) Derivation of water-leaving radiances to within

5% absolute.

3) Derivation of chlorophyll a concentration to within 35% over the range 0.05–50.0 mg m−3.

4) Derivation of global primary production to within 50% absolute with a precision to within 10%.

These are rather stringent requirements and necessitate

calibration verification and algorithm development programs that are far more comprehensive than were undertaken for the CZCS or even envisioned in earlier CZCS

follow-on studies.

1.4 Sensor Characteristics

SeaWiFS differs from the CZCS in a number of ways

(compare Tables 1 and 2). It has 6 (412, 443, 490, 510, 555,

and 670 nm) rather than 3 (443, 520, and 550 nm) bands for

pigment concentration algorithms. Two additional bands

in the near-IR (765 and 865 nm) are for quantification of

aerosol radiances. It employs a rotating telescope to reduce

polarization sensitivity inherent in the scan mirror design

of the CZCS. The digitization is 10 bits rather than 8.

SeaWiFS has a strict requirement on sensor ringing that

requires the effect (bright object recovery) to be dampened

out within 10 pixels. Also, SeaWiFS will have a scan of

53.8◦ from nadir, as opposed to 39◦ for the CZCS. Only the

±45◦ portion of the scan will be used to produce the global

data set at GSFC. In order to utilize data at scan angles

greater than 39◦, more accurate atmospheric correction

algorithms will be needed than were used for the CZCS

processing.

Finally, SeaWiFS onboard calibration will employ a

solar diffuser plate and lunar imaging during scheduled

platform maneuvers rather than using internal calibration

lamps which proved inadequate on the CZCS. These onboard calibrations are possible because Gains 1 and 2 (Appendix A) have been designed specifically for these applications. Gains 3 and 4 will be used during normal Earth

viewing operations. Gains set the dynamic range of the

instrument and allow the range to be matched to the expected magnitude of the total radiance at each wavelength.

This provides a mechanism for optimizing the quantification of the radiance.

1.5 Program Structure

The calibration and validation program is made up of

three general components: the calibration of the SeaWiFS

instrument, the development and validation of the operational atmospheric correction algorithm, and the development and validation of the derived product algorithms,

e.g., chlorophyll a concentration. These activities will be

pursued through in-house (internal) tasks and contracts

with the outside community (external). The internal activities include all work to be conducted at GSFC by SPO

personnel and on-site contractors. The external activities

are performed elsewhere and include funding of contracts

to other government agencies and academic institutions for

specific services, such as in situ data collection, mooring

deployment, and maintenance. These external activities

are necessary because the SPO has neither the skill mix

nor the manpower to conduct certain functions.

Both internal and external activities will be discussed

in detail later and are separately listed in the budget to

clearly show how the work is being partitioned, what levels

of funding are being used at GSFC, and what constitutes

“pass through” monies from the SeaWiFS Research and

Technology Operation Plan (RTOP). Figures 11 and 12

provide overview schematics of the Calibration and Validation Program structure and milestones, respectively. Elements of these charts will be discussed in separate sections

below.

Figure 13 shows the overall organization of the SPO

(Code 970.2) at NASA/GSFC. The sensor calibration and

validation effort requires close coordination between the

Instrument Scientist (Dr. William Barnes, Code 970), the

Calibration Manager (Dr. Bruce Guenther, Code 920.1),

the SeaWiFS Project Scientist (Dr. Wayne Esaias, Code

971), the Algorithm and Validation Manager (Dr. Charles

McClain, Code 971), and Code 930 personnel will handle the routine data reception, processing, and operations

management. The composition of the SPO straddles organizational boundaries and reflects the fact that SeaWiFS

is an Earth Sciences Directorate (Code 900) activity.

The internal activities related to calibration can further

be divided into those associated with the onboard calibration (W. Barnes and B. Guenther) and those connected

with the vicarious calibration (C. McClain). The onboard

calibration is made up of analysis and documentation of

the initial prelaunch characterization and calibration data

provided by the Hughes/Santa Barbara Research Center

(SBRC) and the analysis of post launch data from the solar

diffuser plate and lunar imagery. The vicarious calibration

involves the collection, analysis, and comparison of optical surface truth with the derived satellite water-leaving

radiance fields. A separate calibration activity involving

aircraft will be coordinated with the MODIS calibration

program (Bruce Guenther and Peter Abel) as discussed

below.

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 SPSWG. Presently, the atmospheric correction algorithms needed for processing SeaWiFS data have not been

developed and a number of improvements over the CZCS

algorithms are necessary; however, to implement these improvements, some field studies are required to collect specific data sets. Similarly, because the existing bio-optical

13
SeaWiFS Calibration and Validation Plan

Fig. 11. SeaWiFS Calibration and Validation Program schematic.

Fig. 12. SeaWiFS Calibration and Validation Program milestones. (Key: * = some SWG and algorithm/calibration workshops may be combined, and ** = prototype development initiated prior to 1991

under separate funding.)

14
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 13. SeaWiFS Project Organization.

15
SeaWiFS Calibration and Validation Plan

Table 5. Members of the SeaWiFS Prelaunch Science Working Group.

Team Members

Mark Abbott23 Oregon State University Howard Gordon2 University of Miami

James Aiken PMEL, United Kingdom Graham Harris3 CSIRO, Australia

Robert Bidigare University of Hawaii Patrick Holligan United Kingdom

Peter Brewer3 MBARI Marlon Lewis Dalhousie, Canada

Otis Brown2 University of Miami Satsuki Matsumura FSFRL, Japan

Janet Campbell Bigelow Marine Laboratory Andre Morel IOS, France

Kenneth Carder2 University of South Florida James Mueller San Diego State University

Dennis Clark2 NOAA Mary Jane Perry University of Washington

Peter Cornillon University of Rhode Island Trevor Platt Bedford Institute, Canada

Curtiss Davis Jet Propulsion Laboratory James Simpson Scripps Inst. of Oceanography

Kenneth Denman IOS, Canada Raymond Smith Univ. of Calif. Santa Barbara

Hugh Ducklow University of Maryland Charles Trees San Diego State University

Wayne Esaias12 NASA/GSFC John Walsh University of South Florida

Richard Eppley Scripps Inst. of Oceanography James Yoder University of Rhode Island

Robert Evans2 University of Miami

Ex Officio Members

Greg Mitchell NASA Headquarters Rick Spinrad Office of Naval Research

Curt Mobley Office of Naval Research Stan Wilson NASA Headquarters

SeaWiFS Project Members

William Barnes Instrument Scientist Bruce Guenther Calibration Manager

Mary Cleave Deputy Project Manager Stanford Hooker Field Program Manager

Daniel Endres Asst. Validation Manager Robert Kirk Project Manager

Wayne Esaias Project Scientist Charles McClain Algorithm and Validation Mgr.

Gene Feldman Data Manager Charles Vermillion Ground System Manager

1. Chairman of SeaWiFS Prelaunch Science Working Group (SPSWG).

2. MODIS Science Team member (other MODIS Ocean Team members are: F. Hoge, GSFC; J. Parslow, Aus.; and I. Barton, Aus.)

3. EOS Interdisciplinary Investigator.

data sets applicable to SeaWiFS are either very limited

in scope or are nonexistent, a field program must be implemented to support dedicated bio-optical cruises. These

field studies are in the external activities category, however, some GSFC personnel may participate. Internal activities include the development of bio-optical databases

and the comparison of different atmospheric correction and

bio-optical algorithms. These databases will be accessible

to outside investigators, funded by the SPO, to assist in

algorithm development.

General accessibility to the databases by the research

community will not be supported because of system security and system resource utilization concerns. Appendix B

provides a statement of the data distribution policy as of

August 1991, when it was presented to, and accepted by,

the SPSWG. These data will be provided to an operational

archive such as the GSFC Distributed Active Archive Center (DAAC) at some point in time for distribution. Because the atmospheric correction algorithms utilize data

products from other sources, e.g., ozone concentrations,

surface wind and pressure fields, some quality control of

these fields must be undertaken before the data are incorporated into the processing. Finally, there must be a

quality control function for the final derived products that

compares them to simultaneous field observations submitted to the SPO by various field programs and individual

investigators.

All GSFC internal operations are 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

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, which 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.

2. CALIBRATION VERIFICATION

SeaWiFS calibration and characterization will consist

of three activites: pre-launch, on board, and post-launch.

16
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

2.1 Pre-launch Program

Pre-launch calibration and characterization of SeaWiFS

will be governed by four documents: 1) The Spacecraft

Product Assurance Plan, 2) The Space Segment Verification Plan, 3) The Sensor Product Assurance Plan, and

4) The Sensor Calibration and Stability Monitoring Plan.

The latter two will be subsets of the first two. A conceptual version of document 4 was a part of the contractor’s

proposal and included in Fig. 14, an overview of SeaWiFS

calibration and characterization activities. Table 6 is a

summary of tests and major hardware required to verify

SeaWiFS’ performance. The six major phases and

tasks leading to the delivery of SeaWiFS to the spacecraft

are described below:

1. Subsystem Assembly and Testing: This phase

consists of a) assembly and alignment of the scanner

rotating elements and focal beam optics and focus

adjustment of the off axis folded telescope; b) assembly, alignment, and focus adjustment of the relay and focal plane optics and static coregistration

of all spectral bands; and c) assembly and functional testing of the electronics module, including

validation of proper performance over the qualification temperature range and the expected output

power voltage variation.

2. System Assembly, Parameter Adjustment, and

Functional Operation Validation: This phase consists of the following: a) dynamic verification of

spectral band coregistration, IFOV size, and optical focus; b) measurement of spectral bandpass

parameters; c) adjustment of the transmitting polarization compensator to achive a minimum value

of polarization sensitivity over the 45◦ degree portion of the scan that is to be used for processing

to level-2 products; d) measurement and adjustment of the 20◦ tilt angle position and its operation;

e) verification of command execution and telemetry

output; f) adjustments of band offset and gain; g)

solar calibration throughput and functional operation; h) verification of immunity to the expected

spacecraft generated radio frequency environment

and conducted interference noise levels; i) power

levels and ground isolation measurements; and j)

bright target recovery measurements.

3. Radiometric Calibration: The calibration will be

accomplished using the 122 cm (48 inch) spherical

integrating source (SIS), designed by NASA, or its

equivalent. At this time, the final spectral band

gain trim adjustments will be made, if necessary, in

order to comply with the maximum full scale scene

radiance requirement.

4. Initial Baseline Testing and Performance Verification: In the initial phase, tests will be performed

that will provide a benchmark for comparison of

all subsequent testing. In particular, data comparisons will be made following each environmental test exposure to detect any change in parameter performance that would indicate a potential

problem area. The baseline test will include, as

a minimum, a measurement of the following performance parameters: a) polarization sensitivity as

a function of scan angle, b) radiometric sensitivity

(SNR), c) spectral band coregistration verification,

d) single point check of calibration, e) modulation

transfer function (MTF) measurements, and f) pixel

location characterization. In addition, power level

and ground isolation measurements will be made

and the functional operation of all command and

telemetry channels will be verified. Where feasible,

the baseline tests will be automated to provide test

uniformity for parameter performance change evaluation.

5. Environmental Testing: This phase will involve

subjecting the sensor to a) random vibration profile levels that will approximate the expected launch

environment and b) test under a thermal vacuum

environment. A baseline test will be performed after the vibration exposure and before the thermal

vacuum tests to verify that no performance or operational degradation occurred as a result of the

vibration exposure. The instrument will be configured in the thermal vacuum chamber so that it will

view and scan the SIS through a chamber window.

Prior to the start of thermal vacuum exposure, a

“calibration transfer” from the basic SIS to the window SIS arrangement will be made using the instrument as the transfer vehicle. The thermal vacuum

tests will be designed to test the instrument under a simulated on-orbit temperature and vacuum

profile. During the simulated 40% duty cycle, the

sensor will scan the SIS, and the recorded spectral

band data will be used to characterize the sensor’s

radiometric calibration as a function of the instrument’s temperature in a vacuum environment. In

addition, all functional operations will be verified

and the thermal response of the sensor will be evaluated as a means of validating the scanner and electronics module thermal nodal model.

6. Final Baseline Testing and Performance Verification: A final baseline, post environment test will

be performed prior to delivery of the instrument for

spacecraft integration. In addition, the SeaWiFS

spectral bandpass parameters will again be measured and compared with the initial measurements

for parameter stability validation.

GSFC’s role during integration and testing will be that

of an observer tasked with assuring system compliance

with NASA’s data requirements. The Calibration and Stability Monitoring Plan will require GSFC approval and

17
SeaWiFS Calibration and Validation Plan

Fig. 14. SeaWiFS instrument calibration and characterization sequence (provided by OSC).

18
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Table 6. SeaWiFS test plan summary.

Development Major Test

Requirement Phase for Demonstration Shift Equipment

Test/Demo. Required

2.2 Spatial Coverage Ambient With SeaWiFS scanning across an Collimator

(IFOV Size) Baseline illuminated knife edge within collimator, shift collimator to scan IFOV.

1 km IFOV ±0.15 km Post-amp outputs processed for all

bands.

Measurement made both AT and CT.

Geometric Coverage Ambient SeaWiFS mounted on a rotary table.

Scanning: ±58.3◦ Baseline Scanner operating scan across colliPointing: 0◦, mator knife edge.

+20◦, Measure one channel at a series

−20◦ ±0.01◦ of scan positions.

Dark Level Measurements Ambient Examine scan for response to built-in

Baseline DC.

Vacuum Restore target.

Spectral Coverage Ambient Scan mechanism turned off. Collimator

402–422 bandpass Baseline DC restoration from chopper in colli- with 0.25 m

433–453 edge ±2 nm mator. grating mon480–500 range < 0.5 FWHM Monochromator spectral slit < 0.3 ochromator

510–530 FWHM.

555–575 Calibrated reference detector.

655–675 Filter Measure filter transmittance at six Spectro745–785 Component month intervals. photometer

845–885 stable ±1 nm Measure over temperature range.

Out-of-band Response Filter Transmittance measurement of filters. Spectro-

< 5% integrated Component photometer

Within-band Spectral Differences Same as Spectral Coverage above.

Band-to-band Registration (BBR) Ambient BBR phased knife edge reticle in Collimator

(0.3 IFOV) Baseline collimator.

Measure both along-track and

cross-track.

All bands measured simultaneously.

Scanner operating.

Radiometric Sensitivity (NEδL) Ambient DC restore on bulkhead, scanner on. 48” Spherical

Baseline Integrating sphere source (minimum Integrating

Vacuum of 5 calibration levels), band SNR Source (SIS)

determined in this configuration.

Source fills SeaWiFS solid angle.

Test all bands.

Polarization Insensitivity Ambient Linear polarizer located in collimator. Collimator

≤ 2% over ±45◦ Baseline SeaWiFS on rotary table to test SIS at Cal.

0.4 to 0.9 μ multiple zones of scan. Focus

Scanner operating, DC restore

Measurements to Represent Glan Thompson polarizer in collimator.

all Bands Test all bands.

19
SeaWiFS Calibration and Validation Plan

Table 6 cont. SeaWiFS test plan summary.

Development Major Test

Requirement Phase for Demonstration Shift Equipment

Test/Demo. Required

Dynamic Range Ambient Part of radiometric sensitivity test. SIS

Quantization Subassembly Test electrical response.

Modulation Transfer Function (MTF) Ambient Phased knife edge reticle for along- Collimator

MTF = 0.3 at Nyquist Baseline track and cross-track scanner used

to scan PIKE reticles.

DC restoration in normal manner.

Measure all bands.

Gains Ambient Part of radiometric sensitivity test. SIS

Baseline

Transient Response Ambient The instrument scans across the Collimator

Less than 1% overshoot Baseline illuminated reticle at the focus of Transient

Settle to 0.5% within 2 km the calibrator. Response

The transient response reticle will Reticle

have five phase knife edges.

Radiometric Accuracy Analysis.

Absolute 5% Test at angles.

Radiometric Accuracy Analysis of on-orbit performance.

Relative 2% Measure linearity.

System Noise Measurements Use radiometric sensitivity test

setup.

Pointing Knowledge A combination of geometric coverage together with alignment and

spacecraft data.

Alignment References Ambient Measure boresight angle with reKnowledge of each pixel to: Baseline spect to the alignment cube.

60 arc sec. (3.4 mrad) Collimated target with SeaWiFS

< 60 arc sec. (3.4 mrad) on the rotary table.

Change throughout testing. Measure several scan positions and

determine the interval from scan

start to target.

Mission stability by analysis.

Scanner operating.

Radiometric Stability and Subset of radiometric sensitivity.

Repeatability Long-term stability by analysis.

Short-term stabiliy ±1% 2 wks

Long-term stability ±2% 5 yrs

Band-to-band repeatability 0.5%

In-flight Calibration Data Analysis Analyze detailed test. Goniometer

Lunar Component Measure diffuser spectral BRDF.

Solar Diffuser N/A

copies of all test data and anomaly reports and their disposition will be forwarded to GSFC. Moreover, it is anticipated that personnel from the GSFC SPO will be in close

contact with their SBRC counterparts via telephone, electronic mail, fax, and site visits during all phases of system

development. Formal action requests will be transmitted

through the SeaWiFS Project Manager to Orbital Sciences

Corporation (OSC) for subsequent transmittal to SBRC,

if it is required.

It is anticipated that GSFC personnel will participate,

as a minimum, in the SeaWiFS Preliminary Design Review

(PDR), the Critial Design Review (CDR), radiometric calibration, initial baseline test, environmental tests and the

post environmental baseline test. The level and extent of

participation is to be determined (TBD). A summary of

the major SeaWiFS milestones is given in Table 7.

20
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Table 7. SeaWiFS major milestones.

Milestone Month Year

Preliminary Design Review July 1991

Performance Assurance Plan August

Draft Calibration Plan December

Ground Software Plan December

Critical Design Review December

Design Completion March 1992

Start Integration & Testing September

Final Calibration Plan April 1993

Ground Software Completion June

Calibration June

Calibration Data to GSFC July

Complete Integration July

Satellite Launch August

2.2 Onboard Calibration

Onboard calibration and characterization activities will

be governed mainly by 1) the acquisition and use of data

acquired in flight from the solar measurements off the diffuser plate, 2) the solar measurements in scattering off the

moon, and 3) the use of the spacecraft engineering data.

Two principles will guide the use of the diffuser in

space. The first is to obtain the response of the SeaWiFS sensor to sunlight scattered off the diffuser over time

frames that are short when compared to the time frames

over which we expect changes to occur with the SeaWiFS

sensor and the diffuser. The diffuser will be exposed at

all times and will degrade. The second principle is to obtain enough measurements to discriminate between possible changes in solar irradiance and instrument drift. The

present plan for solar calibration, given that SeaWiFS will

be in a descending orbit, is to collect data over the South

Pole (Fig. 15). The frequency of data collection has not

been decided, but can be variable and as often as once

per orbit. Most likely, there will be periods when frequent

solar calibration is desirable, e.g., during the post launch

verification period, and other times when the frequency

can be relaxed.

The solar constant is known to vary over a solar cycle by

an amount near 2 W m−2, and varies somewhat according

to the expression,

S = 1371.33 + 0.0707Rz W m−2, (1)

where S is the solar constant and Rz is the sunspot number. Of this variation, only about one third to one half

can be identified to occur in the ultraviolet at wavelengths

below 400 nm down to the Lyman alpha lines. The remaining variations must be at wavelengths longer than 400 nm,

because the total radiated solar power below Lyman alpha

wavelengths is an inconsequential fraction of the total solar constant and cannot contribute significantly to the remaining 1.3 W m−2 variation. Further, recently published

measurements of the visible wavelength solar spectral irradiance, obtained by ground measurements from the Soviet

Union, indicate the solar irradiance near 400 nm differs by

greater than 8% from the published Neckel and Labs (1984)

spectrum. (This data does not display any clear solar cycle variation over the three years for which the data was

acquired.)

The variation with solar cycle of the solar constant

is shown in Fig. 16 taken from Mecherikunnel and Kyle

(1991), and is consistent with data sets from the Earth Radiation Budget Experiment (ERBE) and the Active Cavity

Radiometer Irradiance Monitor (ACRIM) measurements

program. Ground measurements of solar irradiance were

shown to depart from the Neckel and Labs data (BurlovVasiljev et al. 1991), thus it is imprudent to devise a quantitative ocean color remote sensing program that is dependent on these Neckel and Labs data when SeaWiFS could

be self-sufficient with a prudent calibration and flight observations scenario.

The prelaunch characterization of the diffuser must be

based on the various usage scenarios that might develop

from the application of the above principles. This will require the development of new approaches for using large

aperture integration devices with large aperture instruments and solar diffuser plates. Laboratory and analysis

approaches are being explored at the GSFC Standards and

Calibration Office (Code 920.1) to develop such techniques.

The data analysis approach is the production of a solar

flux data set. In the past, this approach has demonstrated the best quality control on the interpretation of the

changes in the performance of the flight diffuser. (A similar approach to this will be employed on the SeaWiFS

Project.) This approach is in use with the Backscatter

Ultraviolet Spectrometer (BUV) instruments, which have

continuously employed solar diffusers in space beginning

in 1974. The schedule for the on-orbit use of the solar

diffuser remains in the discussion stage. As with many

other operational details, the SPO must reach an agreement with OSC on the details of plans for on-orbit activities. In addition, the long-term stability of the solar

diffuser plate will dictate the plans for its use. If the diffuser degrades rapidly, frequent measurements of the moon

will be necessary. Such a rapid degradation occurred on

previous Landsat missions. However, the SeaWiFS design

includes an attenuator plate that limits the exposure of

the diffuser to solar flux by an order of magnitude. This

new diffuser design should extend the operational lifetime

of the diffuser to a period of several years.

The solar calibration will be made over the South Pole

with the SeaStar platform oriented in the standard nadir

direction. The sun can be viewed through the diffuser on

each orbit of the satellite. These measurements will be

made without tilting the SeaStar platform and will not

require aiming the diffuser into the spacecraft’s velocity

vector, but will be made with the diffuser pointing opposite from SeaStar’s line of flight. This scenario minimizes

21
SeaWiFS Calibration and Validation Plan

Fig. 15. SeaWiFS lunar calibration scenario (courtesy of Orbital Sciences Corporation).

Fig. 16. Solar irradiance measurements from Nimbus-7, Solar Maximum Mission (SMM), and the Earth

Radiation Budget Sensor (ERBS). Data from Mecherikunnel and Kyle (1991) and updated in 1992.

22
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

the loss of science data, in that science data will not be

collected at solar zenith angles greater than 70◦ and much

of the coverage would be over Antarctica anyway. However, some loss will occur, primarily during austral summer, when the instrument is tilted looking backwards and

is commanded to a nadir tilt in preparation for a calibration.

The main principle to define the frequency of measurements of the moon is to track the degradation of the solar

diffuser plate. In this concept, the instrument response to

the sunlight scattered from the surface of the moon is compared to the instrument response to sunlight scattered off

the surface of the flight diffuser. The surface of the moon

is expected to be stable. The frequency of lunar observations is determined by the expected rate of change, with

time in orbit, of the solar diffuser plate, but is modified by

the noise in the lunar observations. The primary noise in

the lunar observations is due to the relative inhomogeneity

of the scattering from the moon due to changes in lunar

phase angle and the portion of the lunar surface exposed

to SeaWiFS. The moon will be available at a full illumination (full moon) approximately once a month. Analysis

of the lunar observations will require establishing a lunar

atlas and the tracking of equivalent albedo from locations

across the moon as a function of illumination and observation geometry. A database of several months of data is

expected to be needed at the beginning of the SeaWiFS

mission to develop enough information to provide for corrections to the degradation of the flight diffuser. At that

point, the instrument calibration can be corrected retrospectively. Later in the mission, the instrument calibrations can be verified in near real time based on the lunar observations, using the lunar measurements database.

This scenario may have some adverse effect on the quality

of the SeaWiFS calibration provided early in the mission

for data to be used for commercial purposes. A lunar viewing scenario is provided in Appendix C.

There must be an initial characterization of both the

lunar view and solar diffuser measurements at the start of

the mission. Several measurements on adjacent orbits will

be necessary to determine the 95% confidence limits for the

solar measurements. Such measurements will be necessary

on a daily basis until the degradation rate of the diffuser

is determined. After this, a long-term operational schedule for solar diffuser measurements can be established. A

baseline must also be set up for lunar measurements. The

long-term frequency of lunar measurements (perhaps 2–4

times a year) will be set by the long-term stability of the

solar diffuser. Initial steps to prepare for the lunar calibration have been initiated. SPO has funded Kenneth Voss

and Howard Gordon, at the University of Miami, to collect

and analyze an initial set of radiometric observations of the

moon. Voss collected the first set of data from Mauna Loa

in May 1991. Additional observations will be collected at

Miami. Also, EOS is supporting a lunar reflectance mapping study scheduled to begin in 1994.

Another aspect of the calibration program will be the

routine review and archiving of certain spacecraft and sensor engineering telemetry. It should be emphasized that

these data were not routinely scrutinized and archived by

the Nimbus-7 CZCS NET. This made the task of correlating the CZCS degradation with spacecraft environmental parameters and operation procedures impossible. The

telemetry parameters that will be monitored are given in

Table 8. First order data processing will include simple

time series and correlation analyses, but a detailed procedure has not been designed at this time.

Table 8. SeaWiFS telemetry parameters to be

monitored.

Analog Supply Number of

Voltages Sources

Power Supply Voltages 8

Motor Drive Currents 4

Scan Speed Error Signal 1

Scan Synch Error Signal 1

Focal Plane B1,2 Temperature 1

Focal Plane B3,4 Temperature 1

Focal Plane B5,6 Temperature 1

Focal Plane B7,8 Temperature 1

Total 18

2.3 Post-launch Calibration

The post-launch vicarious calibration of SeaWiFS requires accurate measurements of the water-leaving radiances which are then compared with the estimated values

derived from the satellite using a specific atmospheric correction model. Thus, the approach treats the calibration

as a combined sensor atmospheric correction system. In

order to verify a vicarious calibration, it is best to select

a data collection site that has minimal variability in its

optical properties, i.e., open ocean Case 1 water with low

pigment concentrations. Also, a large number of in situ

and satellite observation pairs are needed to provide statistical confidence in the comparisons. For most latitudes,

coverage will be every other day and cloud cover will interfere a significant percentage of the time, so the best

approach is a fixed optical mooring that can be serviced

periodically. The optical mooring provides the capability

of collecting data at a higher frequency than the satellite

coverage. Of course, additional data will be available from

ship data, but not at a regular frequency.

The sensor degradation of the CZCS was large with

significant high frequency variability (as shown in Fig. 6),

thus, it is essential that continuous accurate field observations be available. In order to specify what observations

are required for the vicarious calibration and both atmospheric correction and bio-optical algorithm development, a

workshop was held in Monterey, California in April 1991.

The proceedings of the workshop, Ocean Optics Protocols

for SeaWiFS Validation (Mueller and Austin 1992), has

23
SeaWiFS Calibration and Validation Plan

been published as Volume 5 of the SeaWiFS Technical Report Series. The list of required variables for the vicarious

calibration are shown in Table 9.

The philosophy of the SPO is to support field programs

whose data can also be applied to other planned missions

such as OCTS and MODIS, and to begin the development

of a comprehensive archive of high quality data that are

suitable for satellite calibration and bio-optical algorithm

development. Under EOS sponsorship, Dennis Clark of

NOAA, has begun the design, fabrication, and field testing

of mooring and ship deployed radiometers for this purpose.

For the vicarious calibration, the radiometer is coupled to

an optical mooring. Components of the optical mooring include the collectors, fiber optic links to the radiometer, the

power subsystem, and a telemetry subsystem. Schematics

of the optical buoy system and mooring configuration are

shown in Figs. 17 and 18, respectively.

The SPO will augment EOS support in order to expedite these development efforts so that these systems can

be in place before launch. These systems are designed to

transmit data daily to land based sites via satellite communication links. Thus, comparison of satellite and in situ

data can be achieved on a daily basis. The SPO will routinely schedule local area coverage (LAC) data collection

over the mooring site. The mooring site will be adjacent

to the Joint Global Ocean Flux Study (JGOFS) Hawaii

time series mooring. This simplifies logistical support and

ensures the routine collection of a variety of other useful

water quality parameters. The mooring and JGOFS data

sets will be kept on-line in a database at GSFC as part of

the calibration program. The first deployment of the optical mooring, without the radiometer, was in August 1991,

in Monterey Bay. The mooring fabrication is being done

at Moss Landing Marine Laboratory. The test deployment

of the mooring with the radiometer is scheduled for early

1992.

Under the terms of the contract between NASA and

OSC, NASA must determine if the SeaWiFS data meets

the specifications outlined in the contract. This decision

must be made within 120 days of launch. In order to certify the sensor performance and calibration, the SPO will

support at least one verification cruise. This cruise will

also provide for the initialization of the operational atmospheric correction algorithm and the validation of the

bio-optical derived products. For this purpose, a clear water region with relatively low cloud cover is required. The

present site selection is off Baja, Mexico, which is readily

accessible from San Diego. The cruise plan calls for a full

complement of in situ marine and atmospheric optical and

water quality observations, as outlined in the Monterey

Workshop proceedings. A vessel for the Baja validation

cruise has not been identified at this time.

Arrangements for other cruises during the 120 day period are being pursued by the SPO and by other agencies

and international groups. The strategy is to hold a workshop near the end of the 120 day period where those who

participated in validation cruises could present their preliminary findings. Based on these reports and the analyses

of the satellite data performed by the SPO, a recommendation will be drafted. To meet this deadline, all parties,

including the SPO, must be prepared to turn data around

in near real time. This requires all issues regarding satellite

data processing software (imagery, ephemeris, and engineering), ancillary data format and data exchange scenarios, data quality control, and data distribution be resolved

before launch. It also requires in situ data be processed

during the field programs.

As mentioned earlier, a joint effort with EOS MODIS

calibration development activities using the NASA Earth

Resources-2 (ER-2) aircraft is being considered as a fourth

calibration methodology. The technique is currently being

used for the calibration of the Advanced Very High Resolution Radiometer (AVHRR) on NOAA satellites. Congruent observations of radiance from the satellite sensor

and the ER-2 (Fig. 19) allow transfer of the radiance calibration of a radiometer on the ER-2 to the satellite sensor

(Hovis et al. 1985, Abel et al. 1988, and Abel et al. 1991).

Spectral radiance measured from the ER-2, accurately corrected to satellite altitude, is integrated across the spectral

response profile of the satellite sensor and compared with

the sensor measurement to derive sensor gain. The only

major assumption of the method (an assumption shared by

the buoy calibration) is that the spectral response profile

of the sensor bands does not deviate from pre-launch measurements. The absolute accuracy of the resulting gain

of the satellite sensor is estimated to be ±3%, including

calibration of the ER-2 radiometer (Guenther et al. 1991).

Precision is estimated at ±2%. Both uncertainties may be

reduced significantly through improvements that are now

in the planning stage. It is desirable to execute three underflights per year to adequately establish the long-term

satellite sensor performance. The cost incurred by the SPO

would be flight expenses only, which can be minimized by

conducting the missions out of NASA/Ames Research Center.

SeaWiFS performance will be monitored using solar,

lunar, and vicarious calibration techniques. Lunar calibration will be possible once per month (Appendix C). Solar

calibration using the diffuser plate will be at least as frequent. Because of the satellite’s swath, comparison with

the optical mooring data will, at best, be provided every

other day. Ultimately, the buoy’s vicarious calibration results will be used because they are tied to the primary

derived products, the normalized water-leaving radiances.

In the vicarious calibration, errors are attributed solely to

changes in the calibration. Differences between the onboard calibrations and the vicarious calibration will provide a better indication of the source and magnitude of

errors in the vicarious calibration, e.g., in the atmospheric

correction algorithm.

In the CZCS vicarious calibration, the mean LWN (520)

and LWN (550) for the clear water regions of the global

24
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 17. Schematic of the SeaWiFS optical buoy system (courtesy of D. Clark).

25
SeaWiFS Calibration and Validation Plan

Fig. 18. Drawing of the SeaWiFS optical mooring (courtesy of D. Clark).

26
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 19. Aircraft calibration technique: a) the aircraft radiometer observes the target along the same

view vector as the satellite sensor. The two instruments have different footprints, and the ground tracks

of the aircraft may be different from the satellite track, and b) the radiance for each wavelength in a

scan corresponds to a different footprint. Corrections are applied in the data analysis to optimize spatial

registration between the satellite and aircraft data.

ocean were assumed to be constant for pigment concentrations estimated to be less than 0.25 mg m−3. The basis of

this approach is the data set collected by the NET (Fig.

1), which does not account for regional and seasonal variations in the normalized clear water radiance values. While

this restriction may not have been serious for the CZCS,

it does undermine the objectives of the SeaWiFS mission,

which has very stringent accuracy requirements (Section

1.3).

Additional procedures will be executed to check the

consistency of the intergain calibrations. One method will

be to sweep through the gains on consecutive scans over

a clear water region to determine if each gain yields statistically similar total radiances. The instrument can be

commanded to change gains on a scan line-by-scan line

frequency.

3. ALGORITHM DEVELOPMENT

3.1 Derived Products

The SPO plans to produce three groups of level-2 derived products: CZCS-type, SeaWiFS baseline, and potential SeaWiFS products. A differentiation is made between

CZCS-type pigment and SeaWiFS baseline “chlorophylllike” pigment concentrations. The CZCS-type pigment is

the summation of chlorophyll a and phaeophytin concentrations. The 412 nm band was included for the purpose of

separating viable pigment from degradation by-products.

A more complete description of these products is given below.

CZCS-type Products:

Three-channel (Clark and Gordon) LWN (443),

LWN (520), and LWN (565) pigment algorithm.

Atmospheric correction with e = 1.0.

“Certified” and ready at launch.

Produced routinely: five LWN , K(490), pigment,

three La, and a confidence or error field.

SeaWiFS Baseline Products:

Developed by the SPSWG with prelaunch cruise

and other data.

27
SeaWiFS Calibration and Validation Plan

On-line at launch for evaluation and further development.

“Certified” after post-launch validation cruise.

Includes: wind-dependent glint correction mask,

aerosol and Rayleigh interaction, variable epsilon aerosol correction, initial Case 1 and 2

identification and corrections, and CZCS-type

products plus chlorophyll a concentration, nonchlorophyll absorbance correction value, and excess scattering correction value.

Potential Baseline Products:

Some other possible products that may be developed by the SWG or other principal investigators with pre- and post-launch data.

Routine project production following SWG certification and headquarters approval: accessory

pigment concentrations, primary productivity,

degradation product concentrations, coccolith

or CaCO3 concentration, sediment concentration, aerosol concentration and type, I0, and

other to-be-specified products.

The SPSWG has specifically expressed a requirement

for continuity between CZCS and SeaWiFS products. This

approach, if strictly adhered to, has several ramifications.

It would preclude incorporation of improvements in the

bio-optical algorithm, e.g., use of a 3-band pigment algorithm (Muller-Karger et al. 1990) and use of bio-optical algorithms based on normalized water-leaving radiances. It

would require the same atmospheric correction algorithm

be used as the one applied in the global CZCS processing.

It would require that only the NET data used in producing the CZCS bio-optical algorithms be used and these

data would need to be analyzed for the SeaWiFS bands.

It is probably better to reprocess the CZCS data before

the SeaWiFS launch than to produce SeaWiFS products

strictly similar to the existing global processing products.

Reprocessing would be advantageous because it would allow (1) the use of a 3-band normalized radiance pigment

algorithm, 2) the use of an improved atmospheric correction algorithm, 3) the development of an improved calibration (Hay et al. 1991) and 4) the development of a prototype operational SeaWiFS processing and calibration and

validation system. The CZCS-type products listed above

imply the use of advanced algorithms.

The level-3 products will be space-time average fields

binned into standard grid, presumably at 9 km resolution

so as to be comparable to the AVHRR sea surface temperature (SST) fields being produced by the AVHRR Pathfinder

project. Issues that must be addressed for the level-3 products are: what statistics should be computed, and what

error analyses should be incorporated as products.

Inevitably, changes in the operational derived product

algorithms and requirements for new derived products will

be suggested as more is learned about bio-optics and atmospheric corrections during the course of the mission.

Such additions and modifications are essential to providing the highest quality products possible and for producing

products that the research community routinely need. A

procedure for authorization of algorithm changes and new

products must be defined.

Recommended additions and alterations must first be

brought to the attention of the SeaWiFS Project Scientist

through a formal, written proposal. The proposal must

contain a detailed textual description of the algorithm, the

data and theory used to develop it, the limits of its applicability, the methodology for validation, and its potential

importance to scientific problems. The people involved in

reviewing the proposal are the NASA Headquarters Ocean

Color Program Scientist, the SeaWiFS Project Manager,

the SeaWiFS Project Scientist, the SeaWiFS Calibration

and Validation Manager, the SeaWiFS Data System Manager, and the SeaWiFS SWG. They will determine the

importance of the algorithm or product and feasibility of

its implementation based on the following criteria:

1) Publication in the open literature: While submitted or well tested algorithms must also obtain approval, publication in a peer reviewed

journal substantially increases the likelihood of

approval.

2) Significance: The ability of the algorithm to improve current observations or to extend the usefulness of SeaWiFS data into new areas of scientific importance is a major factor in its selection.

The potential demand by the science community

is critical.

3) Generality: Greater weighting is given to algorithms that apply universally, i.e., those not restricted to specific regions or seasons.

4) Accuracy: The ability of the algorithm to perform within the stated limits of its accuracy

based on independent validation data.

5) Resources: If the algorithm requires substantial

upgrading of the data system (hardware and/or

software), then the impact on the processing system in terms of manpower, cost, and system resources (central processing unit (cpu), storage,

etc.) must be evaluated. If it is determined that

the modification or addition is justified, then an

implementation plan will be developed.

3.2Database Development

The amount of historical data suitable for SeaWiFS

bio-optical algorithm development is very limited because

of the specific suite of simultaneous observations and the

radiometric accuracies required. Even for the CZCS, few

algorithms have been published and these hardly ever show

agreement. For example, Fig. 20 shows three different Case

1 algorithms (Morel 1980, Mitchell and Holm-Hansen 1991,

and Gordon per. comm.) and indicates a sizable range of

28
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 20. CZCS bio-optical pigment algorithm comparison.

values for a given radiance ratio (note that the axes are loglog). The differences are the result of data collected from

regimes having different bio-optical properties and probably from differences in instrumentation and sampling techniques. In order to minimize observational error, the SeaWiFS Monterey Workshop participants outlined the observations and sampling protocols required for bio-optical algorithm development (Table 9). Note that this suite of observations includes in-water and atmospheric optical, and

water quality measurements which will require 12–15 people on a cruise.

In order to begin the development of a high quality

bio-optical algorithm database, the SPO is funding Mr.

Dennis Clark to re-analyze the CZCS NET including the

computation of normalized water-leaving radiances. Normalization of water-leaving radiances removes the influence of solar zenith angle and should improve the correlations with water constituents. The SPO will solicit other

data sets of comparable quality from individual investigators. This database will be used to formulate and compare

advanced CZCS-type algorithms with the standard algorithms and to develop preliminary SeaWiFS chlorophylllike pigment algorithms. The acquisition of the historical

data sets will require the implementation of quality control, documentation, and cataloging procedures and the

design of a database structure suitable for bio-optical data

with a user friendly interface. This development will also

handle bio-optical data sets for SeaWiFS sponsored field

programs that will begin in 1992.

In situ near-infrared observations are needed for advanced atmospheric correction algorithm development in

turbid waters in order to avoid the assumption that waterleaving radiances at 665, 765, and 865 nm are zero. For the

CZCS data, attempts were made to work around the questionable assumption that water-leaving radiance at 670 nm

is zero (Smith and Wilson 1981). The Smith-Wilson iterative algorithm was based on data from the Southern California Bight and is not applicable to most coastal regimes,

which have differing spectral characteristics.

Historical data, unfortunately, in the near-infrared is

questionable because of instrument self-shading as indicated by recent theoretical studies by Howard Gordon (per.

comm.). An effort to correct the CZCS NET data is being

considered by Clark and Gordon, but no results have been

derived to date. To make reliable observations, the instrument must be removed from the light collection point, and

the collection be made very close to the surface. Because

of the very shallow optical depths in the near-infrared, the

surface must be placid. Clark is developing a collector

coupled to a radiometer via a fiber optic link. This sensor

system was field tested in mid-1991. Probable turbid water

data collection sites in 1992 include Chesapeake Bay, Moss

29
SeaWiFS Calibration and Validation Plan

Landing lagoon in Monterey Bay, and Lake Pend Oreille,

Idaho in collaboration with the Office of Naval Research

(ONR) sponsored Closure Experiment.

3.3 Research Cruises Program

Because of the number of observations and personnel

required for bio-optical algorithm development (Table 9),

dedicated cruises are necessary. Cruises of opportunity are

not usually amenable to the algorithm data collection requirements because of the limited number of berths available and conflicts over cruise planning, such as station locations, and ship resources, e.g., winches, wire time, and laboratory and deck space. The SPO’s approach is to support

a minimum of one dedicated cruise per year and to provide

supplementary funding to field programs whose objectives

can easily accommodate the requirements of bio-optical algorithm development. One primary consideration is ship

time, as NASA Headquarters will provide funding for a

dedicated cruise in FY94 only.

In order to optimize the amount of data that is collected in FY92–FY94, the SeaWiFS Project must support

at least two teams of investigators. The first team includes

members of the MODIS instrument team: Dennis Clark,

Kenneth Carder, Howard Gordon, and Wayne Esaias. The

MODIS project will provide most of the support for this

group. Dr. Stanford Hooker of the SPO will work with

Dennis Clark in the development of a shipboard data collection system. The SPO views the collection, storage, and

preprocessing of the complete suite of in situ observations

as a major element of the bio-optical data set development

activity.

The architecture for the SeaWiFS field computing system is based on the unique requirements of collecting data

at sea. First, the media used to store, retrieve, and display

data should be removable so data can be archived daily;

thus, in the event of media corruption or damage, one day’s

worth of data is put at risk. Second, the peripherals, including the computers where possible, should support a

common architecture, so they can be freely reassigned in

the event of unexpected losses. Third, for distributed systems, that is, systems where data is collected on one computer and processed on another, the architecture should

provide an electronic network for fast and reliable data

transfer. For optical data collection there is the additional

requirement that many of the computing resources need to

be small and mobile so the instruments and the computers

that control them can be located where the measurements

are taken.

The SeaWiFS field computing network is divided into

separate acquisition and processing units based on the measurement or processing task each performs:

1) Satellite Imagery and Error Analysis (comprised

of a real-time error analysis workstation and an

image retrieval and display computer),

2) Hydrographic Data Reduction and Analysis (HYDRA) system, which is comprised of a CTD,

Global Positioning System (GPS), and profiling

transmissometer),

3) Submerged In Situ Spectral Radiometer (SISSR),

4) Radiance Distribution (comprised of sky and inwater, as well as optical thickness radiometers),

5) In Situ Station Measurements (comprised of particle and pigment analysis),

6) In Situ Along-track Measurements (composed of

fluorometer and transmissometer systems),

7) In Situ Data Quality and Control, and

8) Primary Productivity.

A schematic of the field computing system is shown in

Fig. 21. As can be seen in the figure, the system naturally divides in two: the larger multifunctional processors

and the smaller single instrument data collectors. The

apparent proliferation of the latter is a consequence of

the single point-of-control needed for the instrumentation.

That is, each instrument requires an individual operator in

near continuous interaction with the controlling computer.

Since all of the instruments will be making measurements

a few hours before and after noon, they cannot share a

duty cycle with a single computer. Despite the designed

separation of function and purpose, there is extensive media commonality and a certain amount of hardware and

software redundancy in the system to allow for unexpected

computer or peripheral losses, which is not explicitly shown

in the figure.

The computers communicate and exchange data along

an Ethernet thinwire spine. The backbone of the spine is

comprised of the larger computers requiring a central location and greater system stability because of their need for

regular system management. The smaller data collection

microcomputers, which are moved around with their attendant instruments, are connected to the spine through a

fan-out unit because they are most likely to be in difficult

places to access and run Ethernet cable to. In addition to

easing cabling concerns, the fan-out unit ensures a greater

degree of backbone stability or integrity by “insulating”

the backbone from computers that are likely to go on- and

off-line with some regularity. In the event of a complete

network failure, the Macintosh computers can communicate using Apple’s built in AppleTalk network, RS-232 serial lines, or media swapping (the so called “hand net”) .

For the other computers, only the latter two options are

alternatives.

The second group will be primarily composed of investigators supported by the Navy for SeaWiFS related field

studies. The Navy has indicated that it would support

dedicated cruises with Navy investigators for SeaWiFS algorithm development. James Mueller at the Center for

Hydro-Optics and Remote Sensing (CHORS) at San Diego

State University will be the primary SPO supported inves30
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 21. Shipboard computing network.

tigator in this group. He has been selected because of the

high level of technical expertise at CHORS and their existing calibration facilities which can support calibration

of other NASA supported investigators instrumentation.

Support to CHORS will augment the Navy’s sampling program to meet SeaWiFS requirements for algorithm development. The strategy assumed by the SPO is to fund

investigators who have the required experience and instrumentation for the in-water optics and then have both teams

subcontract to other investigators for specific observations,

e.g., High Performance Liquid Chromatography (HPLC)

pigment concentrations, which they are not equipped to

make. Fig. 22 shows field experiment opportunities in the

1991–94 period which have been identified to date. The

present plan with the Navy is to conduct a cruise in October 1992 (R/V U.S.N.S. DeSteiguer), with a probable

cruise during the post-launch verification period.

There will be some coordination with JGOFS activities.

David Siegel (of the University of California at Santa Barbara) has been funded by the NSF to conduct routine biooptical observations at the Bermuda Time Series Station.

The SPO will provide some support for radiometer calibration at Bermuda. Also, the JGOFS Steering Committee

has agreed to consider dedicated bio-optical cruises within

the Indian Ocean and Southern Ocean programs. Presumably, the SPO would support a team of investigators to

participate in those survey legs and NSF would provide the

ship time. Augmentations to approved field experiments

of opportunity, to include additional data collection for algorithm development, will be handled on a case-by-case

basis. In such cases, the field program would be supported

at whatever level the SeaWiFS budget allows, depending

on how adequately the program meets the SeaWiFS observational requirements, the location of the survey, and the

level of support requested.

The selection of sites for field studies will be determined

on the adequacy of the existing atmospheric correction and

bio-optical algorithms. First priorities will be to obtain

initial Case 1 data sets for chlorophyll-like pigments and

Case 2 data for the atmospheric correction because there is

31
SeaWiFS Calibration and Validation Plan

little or no data presently available for the SeaWiFS baseline product algorithms. These can be obtained from field

programs proximate to the U.S. However, as indicated in

Fig. 4, regional algorithms will be needed. For example,

high latitude data will be required from the polar oceans

where phytoplankton are photoadapted to low light levels and, therefore, have quite different absorption characteristics. These environments can also support very high

concentrations, e.g., the Bering Sea where surface pigment

concentrations can be greater than 40 mg m−3. Also, regions where atmospheric dust modifies the optical properties of the atmosphere and the ocean, e.g., the tropical

Atlantic Ocean and the Arabian Sea, have proven to be

troublesome. Much of the CZCS data off N.W. Africa was

discarded during the global processing quality control because of anomalous water radiances and aerosol patterns

evident in the water-leaving radiance and pigment fields.

Another topic of concern is the anomalous water-leaving

radiances produced in coccolithophores blooms. These

blooms are observed in most regions of the ocean and can

be quite expansive during the North Atlantic spring bloom.

In order to develop accurate estimates of the global derived product fields, algorithms designed to handle these

and other problems are needed and can only be derived

from an extensive database of high quality observations.

An adequate database does not exist at this time.

One of the contractual requirements imposed by the

SPO on supported investigators is the rapid delivery of

data to the SPO. The data will need to be calibrated, quality controlled, and well documented. Some pre-processing

will also be needed for the calculation of some parameters

such as the normalized water-leaving radiances. The Monterey Workshop proceedings outlines some recommended

procedures. Because of the rapid turnaround requirement,

especially for the post-launch verification, the observations

will need to be processed in near real time. Thus, it will

be necessary for the investigators to have the necessary

computing capabilities on the ship with appropriate software and mass storage. It will also require that data management, data format, and documentation specifications

be well developed. It would be preferable that some set

of standards for each data type be agreed upon by the

teams. The minimum requirement is that the SPO have

verified ingest routines in place prior to each cruise to handle each investigator’s data. These data will be accumulated and consolidated by the SPO at GSFC. Specifics on

documentation, ancillary data, e.g., instrument calibration

and characterization data, and format issues have not been

addressed to date and will need to be discussed at a cruise

planning workshop. Presently, two cruise planning meetings per year are being scheduled.

3.4 Algorithm Development

The data obtained from the field program will be ingested into SPO on-line databases at GSFC. These data

will be accessible by SPO approved investigators for the

development of advanced bio-optical and atmospheric correction algorithms. These investigators will include not

only members of the field program teams, but also others

who contribute data of comparable quality and investigators selected by the SPO to work on bio-optical algorithms.

The protocols regarding data rights, data access and distribution policy are outlined in Appendix B.

Having the historical and bio-optical cruise data in a

well documented form in a common database system designed for such data will allow much more flexibility for

investigators to review data and to try more innovative approaches than has been possible in the past. The SPO desires to promote new methodologies. One activity the SPO

will support in-house is the capability to independently review, implement, and compare algorithms proposed by the

research community. This will require a detailed knowledge of the data sets, data pre-processing procedures, and

relevant radiative transfer theory. The data processing required for these analyses will be performed separately from

the routine SeaWiFS data processing. Algorithm verification studies will require the collection of data not included

in the development of empirical algorithms. For example,

field observations of pigment concentrations from process

studies that do not include in-water optics will be available from many sources and can be used to test algorithm

performance. Given the location and times of field observations, satellite products can be derived from the online level-2 normalized radiance fields. These data sets

(field data, satellite radiances, algorithm products) can be

stored in on-line databases for quick access. This matchup data analysis will be updated as field data is received

and includes radiance data from the calibration mooring

and post-launch bio-optical and sensor calibration cruises.

Having the SPO independently review the performance

of the operational and proposed algorithms is essential.

Proposed changes or additions to the suite of operational

products will require approval of the SPO and the SWG

(after the NRA). The best approach would be to require a

formal written proposal from the investigator which outlines the algorithm, its limitations, the data used in deriving it, etc. This proposal would be reviewed by the

SPO and the SWG. The SPO would conduct its own evaluation of the algorithm’s performance and computational

requirements. Then, based on the results of these reviews,

the algorithm would be adopted or rejected.

3.5 Atmospheric Correction

Figure 4 shows the atmospheric correction algorithm

that was employed for the CZCS. The algorithm assumed

the following (Gordon et al. 1983a, Gordon and Casta ̃no

1987, and Gordon et al. 1988):

1) For a given scene, the aerosol type, as characterized by the  ̊Angstr ̈om exponents, are constant.

In the global CZCS processing, the  ̊Angstr ̈om

32
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. 22. Algorithm development field studies in 1991–94 as of October 1991.

33
SeaWiFS Calibration and Validation Plan

exponents for all scenes were 0.1, 0, and 0 for

443, 520, and 550 nm, respectively. These values imply almost no wavelength dependence in

aerosol scattering, which is approximately true

for marine atmospheres.

2) LW (670) was assumed to be zero everywhere.

3) The second order interaction between Rayleigh

and aerosol scattering was assumed to be zero.

4) The sun glint mask algorithm assumed constant

6 m s−1 wind speeds. No radiometric correction

was made for sun glint or sea foam.

5) The correction geometry assumed a flat Earth.

6) The Rayleigh optical thickness was assumed to

be constant. (In the global processing, the ozone

optical thicknesses have been derived from Total

Ozone Mapping Spectrometer (TOMS) Dobson

units.)

7) The water-leaving radiances were assumed to be

independent of scan angle.

8) The water-leaving radiances were assumed to be

independent of scan angle.

With the increased SNR and radiometric accuracy and

the additional bands in the ultraviolet and near-infrared,

the atmospheric correction algorithm will need to remove

most, if not all, of these assumptions, if the derived products are to meet the accuracy specifications outlined by

the SWG (1.3). Additionally, in order to allow LW (670)

to vary from zero, the  ̊Angstr ̈om exponents must be extrapolated from the 765 and 865 nm bands to even lower

wavelengths (412 nm) than before. How accurately this

can be done has not been established. Only assumption 8

can be considered secondary in all cases.

A variable Rayleigh optical thickness requires surface

pressure fields and is not a theoretical obstacle, but it does

present a problem in obtaining high temporal and spatial

resolution estimates of global fields having sufficient accuracy. Similarly, surface winds for the sun glint and foam

corrections will be needed. Data sets that have been received from the operational centers, i.e., National Meteorological Center (NMC), Fleet Numerical Oceanography

Center (FNOC), and the European Centre for Medium

Range Weather Forecasts (ECMWF), have had 12 hour

and 2.5◦ temporal and spatial resolutions. Presumably,

higher resolution data is produced and will be made available to the SPO and this possibility will be pursued. Thus,

space and time interpolation schemes will need to be developed which take the operational products and estimate

fields along the subsatellite track at the required space and

time resolutions. This interpolation will need to be performed for every orbit as the satellite progresses through

approximately seven orbits between operational product

fields. At NASA Headquarters’ direction, the SPO will

look to the EOS community for help in implementing an

operational interpolation algorithm. There are several individuals in the GSFC Earth Sciences Directorate who

could assist the SPO in this way.

Howard Gordon, funded under MODIS and the SPO,

will have the primary responsibility of providing the operational SeaWiFS atmospheric correction algorithm. Gordon’s initial development will focus on the  ̊Angstr ̈om extrapolation question (pertaining to assumptions 1–3), the

Rayleigh-aerosol interactions (assumption 4) and surface

roughness effects (assumption 5). Figure 4 illustrates the

various radiometric interactions he has proposed for the

initial algorithm. Robert Fraser at NASA/GSFC will also

be funded to investigate particular problems dealing with

topics such as sun glint, Saharan dust, and the  ̊Angstr ̈om

extrapolation problem.

As outlined in the Monterey Workshop proceedings,

field observations of the angular distribution of downwelling sky radiance (sensor calibration studies) and spectral

atmospheric optical thickness (sensor calibration and biooptical algorithm studies) are required. The downwelling

sky radiance distribution observations require special hardware and will only be collected during certain SeaWiFS

sponsored dedicated cruises (at least those conducted by

the MODIS team). Also, as with bio-optical data, the

SPO will accumulate similar data sets from other sources

in order to develop an on-line database for atmospheric algorithm development. The SPO will also solicit from the

community independently developed atmospheric correction algorithms so that other techniques and methods can

be evaluated. Evaluations would require review of the algorithm’s theoretical basis, its computational requirements,

and comparison of the operational SeaWiFS derived products with those obtained using other algorithms. This capability will be maintained in-house and processing will be

done separately from the operational processing.

Finally, attention must be given to cloud detection. In

the CZCS global processing, the only flag used was a fixed

value in band 5. Such an algorithm will not be acceptable

for SeaWiFS as it does not account for changes in total

radiance as a function of solar elevation or for variability

in cloud albedo. This problem is being addressed as part

of the Pathfinder AVHRR reprocessing. Here the issue is

much more complicated in that it requires both daytime

and nighttime algorithms. The SPO will work with the

SWG in formulating an algorithm for SeaWiFS.

3.6 Field Instrumentation

As emphasized in the Monterey Workshop proceedings,

a community-wide program for the standardization of radiometer characterization and calibration must be initiated, especially as the number of investigators collecting

data for algorithm development and sensor calibration is

expected to grow rapidly during the SeaWiFS era. Some

laboratories in the academic community, e.g., CHORS, already have a basic capability under support from on-going

projects funded by a combination of NASA, Navy, and

34
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

NSF sources. In addition to standardized procedures, digital documentation of calibration results, and radiometer

maintenance logs for every instrument, at least those to

be used for SeaWiFS algorithm studies, should be filed

into a community database. This would provide traceability in data sets, something that is not commonly available

in historical data sets. As part of this strategy, the SPO

under Bruce Guenther’s direction, will support a roundrobin calibration of these laboratories’ secondary sources.

The frequency of routine visits to the laboratories would

be based on calibration source usage. Special trips will be

needed, for instance, when new equipment is installed that

requires certification by the GSFC Standards and Calibration Office (Bruce Guenther, Head).

4. QUALITY CONTROL

SeaWiFS quality control is based on the CZCS quality

control system and includes the review of level 1–3 products as well as the input fields used in the derivation of

level-2 products. One major difference is that the only

ancillary data used in the CZCS processing was historical

ozone concentrations derived from the TOMS while SeaWiFS requires other ancillary data received in near real

time. Meteorological data analyses will build on what has

already been developed within the SEAPAK software package (McClain et al. 1991b).

4.1 Level-1 Screening

Each day, both the recorded LAC and GAC and the

High Resolution Picture Transmission (HRPT) LAC received at GSFC will be checked for missing bands and other

problems that can be easily recognized by visual means.

More analytical tests will include statistical analyses of the

radiances, which will be saved in an on-line database. An

example would be mean values, after cloud screening, for

clear water regions in predefined latitude ranges. Such

analyses would provide an indication of changes in the

atmosphere and/or in the sensor calibration and will be

stored in an online database.

4.2Level-2Quality Control

The input fields, as presently identified, are surface

wind speed, surface pressure, and ozone concentration.

The surface wind and pressure fields will be obtained from

the operational meteorology centers. It is well known that

these products vary in their accuracies depending on location and on their source. There are a number of studies in

the recent literature that evaluate products from the operational centers (NMC, FNOC, and ECMWF), e.g., Toll

and Clune (1985) and Trenberth and Olson (1988). The

selection of product sources depends on how rapidly they

can be received as well as how accurate they are considered

to be. Reprocessing of the data using refined calibration

and ancillary data fields will be ongoing in parallel with

real-time processing and it is conceivable that the various

processings will use data from different sources. It is also

conceivable that in some processing scenarios, data from

different sources will be used in different parts of the world.

As with the space and time interpolation problem, the SPO

will rely on EOS investigators for direction on what products to use where. Coordination of data transfer from the

operational centers will be handled by the SeaWiFS Data

System group.

All data will be screened as it is received to identify

missing or bad data. In both cases, the interpolation algorithm will need to fill in values at the affected grid points.

Also, the data will be compared with climatological data to

identify large anomalies. The anomaly fields will be used

during the review of level-2 and level-3 products. Also,

the analysis software will be capable of extracting time series of wind, pressure, and ozone at any selected location

as an aid in determining if observed trends in the derived

products are correlated with environmental factors. Correlations can be the result of real processes or artifacts of

the processing algorithms. Analyses such as time series

at specific locations of interest will be stored on-line and

routinely updated.

4.3 Level-2Product Screening

All level-1 recorded LAC and HRPT LAC data captured at GSFC will be processed to level-2 and reviewed.

For scenes routinely sampled at LAC resolution, e.g., the

Hawaii optical mooring site and the U.S. East Coast, statistical analyses of the level-2 fields will be performed and

saved in an on-line database. Analysis products will include geometric and arithmetic mean values for clear water

regions. In cases where the LAC coverage was collected in

support of field projects, the level-2 data will be saved and,

when possible, transmitted to the research vessel(s). Also,

the extraction of the satellite data for comparison with

buoy data mentioned in Section 3.4 could be performed

during this procedure.

All level-2 GAC derived products will be reviewed in a

manner similar to the CZCS quality control scenario, but

with refined rejection criteria (see Fig. 7). In the CZCS

quality control procedure, a daily composite of four derived

products (pigment concentration, LA(670), LWN (443), and

LWN (550) was reviewed. The procedure allowed the user

to sequence through the day’s coverage scene by scene and

accept or reject the scene. If a scene was rejected, a list of

rejection criteria was presented and one selected with optional comments. This information was passed to the processing database and was used in identifying scenes to be

excluded from the level-3 products and was also included

in the CZCS browse system.

Several improvements in this system have been discussed including the incorporation of masks that use quantitative criteria to identify questionable data and the ability to flag portions of scenes for rejection without rejecting

35
SeaWiFS Calibration and Validation Plan

entire scenes. For example, the “Dubious Water Radiance”

and “Other” categories shown in Fig. 7 would be replaced

with more explicit radiometric tests. The definition of

these criteria will be based on analyses of field data in the

SPO databases and on the recommendations of the SWG

and other investigators. For example, much radiometric

data from coccolithophore blooms have been collected in

recent field studies that could be used in developing a coccolithophore flag. Also, with the additional near-infrared

bands, it may be possible to flag regions where dust is the

dominant aerosol. One difference in how quality control

will be executed lies in the fact that SeaWiFS data will

not be packaged into discrete two minute scenes, as were

the CZCS data, and there will not be duplicate copies of

the data.

4.4 Level-3 Product Screening

The level-3 products represent composites of fields averaged over a number of time scales (weekly, monthly,

annually). These products will be reviewed as they are

produced. Often problems missed during the daily review of level-2 products can be identified in the level-3

fields. The monthly composites of CZCS pigment were

reviewed as they were produced, and, in most cases, bad

data had been approved during the initial quality control.

In those cases, a second pass through the level-2 products was required in order to identify the specific problem

scenes. Analyses of the mean fields, such as mean pigment

concentration in clear water regimes, will be stored in an

on-line database. These time series will indicate long-term

trends in the products, which can be correlated with calibration data (solar, lunar, and vicarious), to determine if

trends in the derived products are artifacts of the calibration. These products could also be compared with other

diagnostic fields such as mean Ekman upwelling velocities

derived from the wind fields (McClain et al. 1990). Such

tests would provide additional insights as to how well the

system and algorithms were performing.

Appendices

A. Radiometric specifications for SeaWiFS. December

11, 1991 memo from Bob Barnes to Wayne Esaias.

B. SeaWiFS bio-optical data distribution policy.

C. Considerations on SeaWiFS Lunar Observations. July

9, 1991 memo from Bob Barnes to Wayne Esaias.

Appendix A

December 11, 1991

TO: Wayne Esaias

FROM: Bob Barnes

SUBJECT: Gains for the SeaWiFS Critical Design Review

REFERENCE:

1) Memo from Bob Barnes to Wayne Esaias, dated 6/28/91.

2) Fax from Hugh Kieffer to Bob Barnes, dated 7/31/91.

3) E-Mail message from Howard Gordon to SeaWiFS

Project Office, dated 11/13/91.

4) Memo from Bob Barnes to Wayne Esaias, dated 12/5/91.

5) Memo from Bob Barnes to Wayne Esaias, dated 8/20/91.

Since my zeroth order calculations of the lunar radiances for

SeaWiFS (Ref. 1), we have received improved estimates of these

radiances from Hugh Kieffer (Ref. 2) and from Howard Gordon

(Ref. 3). More recently, we have calculated corrections to the

science, lunar, and solar radiances at the new wavelengths for

channels 4, 5, and 6 (Ref. 4). In addition, we have looked

at the radiances from “super” blooms of coccolithophores. We

decided not to modify the science gains to insure that there will

be no saturation of the instrument by these blooms, since we

feel that coccolithophore blooms with reflectances up to about

20 percent can be handled by the existing science gains (Ref.

4). Based on this background information, we can now tabulate

the new sets of expected input radiances for SeaWiFS.

Table A-1. Revised radiometric specifications for SeaWiFS.

Wave- Saturation Input SNR Comlength Radiance Radiance ments

402–422 13.63 9.10 499

433–453 13.25 8.41 674

480–500 10.50 6.56 667

500–520 9.08 5.64 640 Rev.

545–565 7.44 4.57 596 Rev.

660–680 4.20 2.46 442 Rev.

745–785 3.00 1.61 455

845–885 2.13 1.09 467

Wavelength is in units of nanometers.

Radiance is in units of mW cm−2 μm−1 sr−1.

The revised radiances at 500–520, 545–565, and 660–680 nm

are 4% higher, 3% higher, and 1% lower than the original values,

respectively. The set of saturation radiances in Table 1 can

be compared with the expected saturation radiances from the

lunar and solar measurements. The lunar and solar radiances

were derived from Howard Gordon’s calculations in Ref. 3.

Table A-2. Expected saturation radiances for the lunar and solar measurements.

Wave- Saturation Gain Saturation Gain

length Rad. (Lunar) Rad. (Solar)

402–422 6.78 2.01 6.82 2.00

433–453 7.61 1.74 7.53 1.76

480–500 8.75 1.20 7.78 1.35

500–520 9.11 1.00 7.42 1.22

545–565 9.67 0.77 7.40 1.01

660–680 9.04 0.46 6.07 0.69

745–785 9.38 0.32 4.92 0.61

845–885 8.19 0.26 3.87 0.55

Gain is the gain required to change these saturation radiances

to the saturation radiances in Table A-1.

For SeaWiFS, there is a second science gain (gain of 2)

that will account for the predicted decrease in sensitivity of the

instrument. An examination of Table A-2 shows some duplications with gains from the science channels and some duplica36
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

tions between the lunar and solar gains. As a result, we have

devised gains that will account for the needs of the measurements and some new intermediate gains that fall between the

science gains of 1 and 2. The gains in this memorandum are

nearly identical with those recommended by Howard Gordon in

Ref. 3. These gains are also very similar to those in Table A-3

of Ref. 5, which was submitted to the Santa Barbara Research

Center last August, so that they could start the rough setting

of the instrument. The following tables have been set up for

easy comparison with the values previously sent to SBRC.

Table A-3. Multiplicative factors for the four SeaWiFS

gains relative to gain 1 saturation radiances.

Wave- Gain 1 Gain 2 Gain 3 Gain 4

length

402–422 1.00 2.00(1,2) 1.70 1.30

433–453 1.00 2.00 1.70(1,2) 1.30

480–500 1.00 2.00 1.25(1,2) 1.70

500–520 1.00(1) 2.00 1.70 1.25(2)

545–565 1.00(2) 2.00 0.77(1) 1.60

660–680 1.00 2.00 0.46(1) 0.70(2)

745–785 1.00 2.00 0.32(1) 0.61(2)

845–885 1.00 2.00 0.26(1) 0.55(2)

(1) = gain used for lunar measurements.

(2) = gain used for solar measurements.

Table A-4. Saturation radiances for the four SeaWiFS

gains.

Wave- Gain 1 Gain 2 Gain 3 Gain 4

length

402–422 13.63 6.82 8.02 10.48

433–453 13.25 6.63 7.79 10.19

480–500 10.50 5.25 8.40 6.18

500–520 9.08 4.54 5.34 7.26

545–565 7.44 3.72 9.66 4.65

660–680 4.20 2.10 9.13 6.00

745–785 3.00 1.50 9.38 4.92

845–885 2.13 1.07 8.19 3.87

Appendix B

SeaWiFS Bio-optical Data Distribution Policy

(Subject to Revision)

12/16/91

This policy covers data submitted to the National Aeronautics and Space Administration (NASA) SeaWiFS Project Office

(SPO) at Goddard Space Flight Center (GSFC) for inclusion in

the calibration and validation data collection. Its purpose is to

ensure that accurate in situ, shipboard and airborne bio-optical

measurements are made rapidly available to the Science Team

(ST) members (and other approved investigators) for advanced

algorithm development and data product validation purposes,

while ensuring that the observer or provider receives proper

credit and acknowledgement for the considerable expertise and

effort applied to obtaining and reducing the data.

SUBMISSION: Ocean color algorithm development is essentially observation limited, and rapid turnaround and access to

such data is crucial for progress. Data obtained under contract

should be submitted as soon as proper calibration information

can be applied, and not later than 6 months from collection.

Science Team members and other investigators making suitable observations are encouraged to provide their data as soon

as possible. Data is expected to be submitted no later than one

year following collection, or at the time of submission of any

paper using the data by the provider. Investigators who make

observations of bio-optical parameters are expected to submit

their observations prior to accessing data from others.

DATA ACCESS: Access to the digital data will be limited

initially to approved users as determined by the ST and the

providers, for a period of one year following collection. Other

investigators interested in obtaining such data will be referred

to the provider for permission. Following an agreed upon period data will be deemed public, and access will be unlimited.

Records of distribution will be maintained and forwarded to the

provider, and citation requirements set forth below still apply.

Only information about the digital data (parameters, locations,

dates, investigators, etc.) will be available for unlimited downloading or distribution.

CONDITIONS ON DATA UTILIZATION: Users of data will

be required to provide proper credit and acknowledgement of

the provider. At minimum, this should be acknowledgement

by name and citation of any works describing the data or its

use. Citation should also be made of the data archive. Users

of data are encouraged to discuss relevant findings with the

provider early in the research. The user is required to give

to all providers of the data of which he has made use, a copy

of any manuscript resulting from use of the data, prior to or

coincident with initial submission for publication. Within one

year of data collection, the provider(s) shall be offered the right

to be a named co-author.

UPDATES AND CORRECTIONS: A major purpose of the

data base is to facilitate comparisons of absolute calibrations

and protocols between in situ observations (regionally, temporally, by technique, by investigator, etc.), as well as between in

situ and remotely sensed observations. Updates and corrections

to submitted data sets by the provider are encouraged. Records

will be maintained of updates and corrections, summaries of updates will be posted on a database board, and users shall be

notified of the updates. The methodology for doing this efficiently will probably have to be developed on a case by case

basis. The current data in the archive should be identical with

the data used in the provider’s most recent publications or current research.

FORMATS: Data should be provided in an agreed upon format, along with relevant information describing collection conditions, instrument performance and calibration, and statements of its accuracy. In general, parameters and units shall be

as described in the Monterey Calibration and Validation Workshop report, and recommended format is to be determined.

Data values shall be in their final form (e.g., providing volts together with conversion coefficients and drifts is unacceptable).

High level data sets are encouraged, e.g., normalized water37
SeaWiFS Calibration and Validation Plan

leaving radiance spectra, together with descriptions and/or citations of procedures used to derive the values. Data should

be segmented into rational sets, by station, date, parameter,

etc. Data quality, calibration traceability and history, drift,

and sampling protocols may be in text format. Listing of what

criteria need to be treated should be developed by the ST.

RECORD KEEPING: The SPO will maintain an accurate data

base of these data. All data will retain the investigator’s identification, and any necessary quality control information, at the

level of distribution. This necessitates that combined data sets

from several investigators retain their individual identifications,

if any such combining is done by the SPO. The SPO will maintain accurate records of distribution, and will inform the original provider of investigators requesting their data. The data

will not be released for inclusion in other data bases which do

not agree to honor conditions set forth above.

Appendix C

July 9, 1991

TO: Wayne Esaias

FROM: Bob Barnes

SUBJECT: Thoughts on SeaWiFS Calibrations

1. LUNAR DIAMETER. In terms of the SeaWiFS angular

resolution, the moon is just under 6 pixels wide. Figure C1 shows the moon in terms of MODIS-T pixels, which are

1.56 mrad wide (compared with the SeaWiFS 1.60 mrad).

For a swath across the center of the moon there will be one

partially illuminated pixel, followed by 4 fully illuminated

ones. Overall, the width of the moon is just over 1/2 of the

10 pixel response time (bright to dark) for the instrument

as per the SeaWiFS specifications. It remains an open question whether SeaWiFS has a sufficiently fast response time

to see the moon well enough for calibration purposes.

2. DIFFUSER DEGRADATION RATE. It is my understanding that the Landsat diffuser degraded to the point of uselessness in a few days. This was also true of the diffuser on

the Nimbus 4 BUV, which was useless after a week. However, the diffuser on the Nimbus 7 SBUV was stored in a

protected position, and it has lasted for several years. The

same is true of the diffuser on the NOAA 9 SBUV/2. The

degradation rate for the SeaWiFS diffuser will determine

its deployment scheme without regard to the plans that we

make.

3. A STRAW-MAN PLAN FOR DIFFUSER AND LUNAR

CALIBRATIONS. This plan assumes diffuser degradation

of about 10measurements over a 5-year lifetime. Semiannual lunar observations will give 10 measurements over

the same period. More frequent observations will improve

the statistics of these long term measurements. However,

an increased number of diffuser deployments can increase

the diffuser’s degradation, and more frequent lunar measurements increase the risks associated with changes in the

rotation rate of the spacecraft. Were the diffuser to become useless (due to rapid degradation) then quarterly lunar observations might become necessary to check instrument changes. Initial diffuser and lunar measurements must

be made soon after the completion of spacecraft outgassing.

These initial measurements will give the baseline for the

long-term sets of calibrations.

4. STANDARD SeaWiFS PLATFORM ROTATION RATE.

Preliminary calculations of mine show an orbital period of

98.7 min (5920 sec) for SeaWiFS. This gives the platform a

rotation rate of 1.06 mrad/sec to maintain pointing toward

the Earth’s center. During viewing the Earth, of course,

the surface scans along track under the nadir-pointing instrument. For the moon the along track scan comes from

the rotation rate of the SeaWiFS platform, itself.

5. A STRAW-MAN PLATFORM ROTATION PLAN FOR

VIEWING OF THE MOON. At an equatorial crossing on

the dark side of the Earth cut the rotation rate of the platform to 1/2 of the standard rate. On the second dark side

equatorial crossing that follows return the rotation rate to

normal. At the intermediate dark side crossing SeaWiFS

will point toward the moon.

a. The instrument will point 45 degrees or more from

the Sun on the illuminated side of the Earth. The

use of 20 degree fore and aft pointing can increase

this angular difference. The danger of exposing the

instrument optics to the sun should be eliminated in

this manner.

b. The rotation rate of the platform would be 0.53 mrad/

sec. For the 4 pixel (6.4 mrad) central area of the

moon (see Fig. C-1) and this along track rotation

rate, the scan time would be 12 sec. This would

include 72 across track scans of the instrument telescope.

c. This technique will remove two consecutive measurement orbits by the instrument. However, if this is

done twice a year, the loss of data is minimal. I

assume that these considerations will have to be addresssed in meetings with OSC.

Regards,

Glossary

ACRIM Active Cavity Radiometer Irradiance Monitor

A/D Analog-to-Digital

ADEOS Advanced Earth Observation Satellite

AT Along-Track

AVHRR Advanced Very High Resolution Radiometer

BBR Band-to-Band Registration

bpi bits per inch

BRDF Bidirectional Reflectance Distribution Function

BUV Backscatter Ultraviolet Spectrometer

38
McClain, Esaias, Barnes, Guenther, Endres, Hooker, Mitchell, and Barnes

Fig. C-1. A View of the Full Moon in Terms of Pixels. For SeaWiFS the pixel size is 1.60×1.60 mrad, equivalent

to the pixel size in this figure.

CalCOFI California Cooperative Fisheries Institute

Cal/Val Calibration and Validation

Case 1 Water whose reflectance is determined solely

by absorption.

Case 2 Water whose reflectance is significantly influenced by scattering.

cpuCentral Processing Unit

CDR Critial Design Review

CHORS Center for Hydro-Optics and Remote Sensing (San Diego State University)

CRM Contrast Reduction Meter

CRT Calibrated Radiance Tapes

CRT Cathode Ray Tube Display

CT Cross-Track

CTD Conductivity, Temperature, and Depth

CZCS Coastal Zone Color Scanner

DAAC Distributed Active Archive Center

DC Direct Current

DCP Data Collection Platform

ECMWF European Centre for Medium Range Weather

Forecasts

Ed(z, λ) Downwelled spectral irradiance

EOS Earth Observing Satellite

EOSAT Earth Observation Satellite Company

ERBE Earth Radiation Budget Experiment

ERBS Earth Radiation Budget Sensor

ER-2 Earth Resources-2

FWHM Full-Width Half-Maximum

FNOC Fleet Numerical Oceanography Center

GAC Global Area Coverage, coarse resolution satellite data with a nominal ground resolution of

approximately 4 km.

GLI Global Imager

GOES Geosynchronous Orbital Environmental Satellite

GPS Global Positioning System

GSFC Goddard Space Flight Center

HPLC High Performance Liquid Chromatography

HRPT High Resolution Picture Transmission

HYDRA Hydrographic Data Reduction and Analysis

IFOV Instantaneous Field of View

I/O Input/Output

IR Infrared

JGOFS Joint Global Ocean Flux Study

K Diffuse attenuation coefficient (at 490 nm:

K(490)

LAC Local Area Coverage, fine resolution satellite

data with a nominal ground resolution of approximately 1 km.

Level-0 Raw data.

Level-1 Calibrated radiances.

Level-2 Derived products.

Level-3 Gridded and averaged derived products.

Lu(z, λ) Upwelled spectral radiance

LW Water-leaving radiance (at 520 nm:

LW (520)).

LW N Normalized water-leaving radiance (at 550

nm: LWN (550)).

39
SeaWiFS Calibration and Validation Plan

MAREX Marine Resources Experiment Program

MERIS Medium Resolution Imaging Spectrometer

MODIS Moderate Resolution Image Spectrometer

MODIS-N Moderate Resolution Image Spectrometer—

Nadir

MODIS-T Moderate Resolution Image Spectrometer—

Tilt

MTF Modulation Transfer Function

NASA National Aeronautics and Space Administration

NE∆T Noise Equivalent Delta Temperature

NEδL Noise Equivalent delta Radiance

NET Nimbus Experiment Team

NIST National Institute of Standards of Technology

NMC National Meteorological Center

NOAA National Oceanic and Atmospheric Administration

NRA NASA Research Announcement

NSF National Science Foundation

OCTS Ocean Color Temperature Sensor (Japan)

ONR Office of Naval Research

OSC Orbital Sciences Corporation

PDR Preliminary Design Review

PIKE Phased Illuminated Knife Edge

QC Quality Control

RDF Radio Direction Finder

ROSIS Remote Sensing Imaging Spectrometer, also

known as the Reflective Optics System Imaging Spectrometer (Germany)

RTOP Research and Technology Operation Plan

Rz Sunspot Number

S Solar Constant

SBRC Santa Barbara Research Center (Hughes)

SBUV Solar Backscatter Ultraviolet Radiometer

SBUV–2 Solar Backscatter Ultraviolet Radiometer–2

SeaWiFS Sea-viewing Wide Field-of-view Sensor

SIS Spherical Integrating Source

SISSR Submerged In Situ Spectral Radiometer

SMM Solar Maximum Mission

SNR Signal-to-Noise Ratio

SPO SeaWiFS Project Office

SPSWG SeaWiFS Prelaunch Science Working Group

SST Sea Surface Temperature

ST Science Team

SWG Science Working Group

TBD To Be Determined

TOMS Total Ozone Mapping Spectrometer

VHF Very High Frequency

VISNIR Visible and Near Infrared

WORM Write Once Read Many

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41
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 September 1992 Technical Memorandum

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 SeaWiFS Technical Report Series

 Volume 3–SeaWiFS Calibration and Validation Plan Code 970.2

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 Unclassified Unclassified Unclassified Unlimited

 Oceanography, SeaWiFS, CZCS, Coastal Zone Color Scanner, Calibration/ 43

 Validation, Sensor, Algorithm Development, Quality Control

 B.G. Mitchell: NASA Headquarters, Washington, DC; R. Barnes: CHEMAL, Inc, Wallops Island, VA;

 E.R. Firestone: General Sciences Corporation, Laurel, Maryland.

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

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13. ABSTRACT (Maximum 200 words)

National Aeronautics and Space Administration

Washington, D.C. 20546–0001 TM–104566, Vol. 3

Laboratory for Hydrospheric Processes

Goddard Space Flight Center 92B00117

Greenbelt, Maryland 20771

6. AUTHOR(S)

 Charles R. McClain, Wayne E. Esasias, William Barnes, Bruce Guenther,

 Daniel Endres, Stanford B. Hooker, B. Greg Mitchell, and Robert Barnes

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

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) will be the first ocean color satellite since the Nimbus-7 Coastal

Zone Color Scanner (CZCS), which ceased operation in 1986. Unlike the CZCS, which was designed as a proof-of-concept

experiment, SeaWiFS will provide routine global coverage every two days and is designed to provide estimates of photosynthetic pigment concentrations of sufficient accuracy for use in quantitative studies of the ocean's primary productivity

and biogeochemistry. A review of the CZCS mission is included that describes the limitations of that data set and provides

justification for a comprehensive SeaWiFS calibration and validation program. To accomplish the scientific objectives of

the mission, the sensor's calibration must be constantly monitored, and robust atmospheric correction and bio-optical

algorithms must be developed. The plan incorporates a multi-faceted approach to sensor calibration using a combination of

vicarious (based on in situ observations) and onboard calibration techniques. Because of budget constraints and the limited

availability of ship resources, the development of the operational algorithms (atmospheric and bio-optical) will rely heavily

on collaborations with the Earth Observing Satellite (EOS), the Moderate Resolution Imaging Spectrometer (MODIS)

oceans team, and projects sponsored by other agencies, e.g., the United States Navy and the National Science Foundation

(NSF). Other elements of the plan include the routine quality control of input ancillary data (e.g., surface wind, surface

pressure, ozone concentration, etc., used in the processing and the verification of the level-0 (raw) data to level-1 (calibrated

radiances), level-2 (derived products) and level-3 (gridded and averaged derived data) products.