NASA Technical Memorandum 104566, Vol. 22

SeaWiFS Technical Report Series

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

Volume 22, Prelaunch Acceptance Report

for the SeaWiFS Radiometer

Robert A. Barnes, William L. Barnes,

Wayne E. Esaias, and Charles R. McClain

September 1994
NASA Technical Memorandum 104566, Vol. 22

SeaWiFS Technical Report Series

Stanford B. Hooker, Editor

NASA Goddard Space Flight Center, Greenbelt, Maryland

Elaine R. Firestone, Technical Editor

General Sciences Corporation, Laurel, Maryland

James G. Acker, Technical Editor

Hughes STX, Lanham, Maryland

Volume 22, Prelaunch Acceptance Report

for the SeaWiFS Radiometer

Robert A. Barnes

ManTech, Inc.

Wallops Island, Virginia

William L. Barnes

Wayne E. Esaias

Charles R. McClain

NASA Goddard Space Flight Center, Greenbelt, Maryland

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

1994
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Abstract

The final acceptance, or rejection, of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) will be determined

bythe instrument’s on-orbit operation. There is, however, an extensive set of laboratorymeasurements describing the operating characteristics of the radiometer. Manyof the requirements in the Ocean Color Data

Mission (OCDM) specifications can be checked onlybylaboratorymeasurements. Here, the calibration review

panel (composed of the authors of this technical memorandum) examines the laboratorycharacterization and

calibration of SeaWiFS in the light of the OCDM performance specifications. Overall, the performance of the

SeaWiFS instrument meets or exceeds the requirements of the OCDM Contract in all but a few unimportant

details. The detailed results of this examination are presented here byfollowing the outline of the specifications,

as found in the Contract. The results are presented in the form of requirement and compliance pairs. These

results give conclusions on many, but not all, of the performance specifications. The acceptance by this panel of

the performance of SeaWiFS must onlybe considered as an intermediate conclusion. The ultimate acceptance

(or rejection) of the SeaWiFS data set will relyon the measurements made bythe instrument on orbit.

1. INTRODUCTION

In addition to its role as an ocean color experiment, the

Sea-viewing Wide Field-of-view Sensor (SeaWiFS) serves

as a satellite procurement experiment for the National

Aeronautics and Space Administration (NASA). For the

SeaWiFS Project, NASA is procuring data, not an instrument designed bythe agency. NASA has entered into

a contractual agreement, the Ocean Color Data Mission

(OCDM) contract, (hereinafter referred to as the Contract,

unless otherwise stated) with Orbital Sciences Corporation (OSC) to obtain, at a fixed price, an ocean color data

set. OSC has, in turn, entered into an agreement with the

Hughes Santa Barbara Research Center (SBRC) for which

SBRC, as a subcontractor, has built the satellite sensor required to provide these data. In this arrangement, SBRC

has had the freedom to design an instrument which meets

the predetermined set of specifications. The design of the

testing procedures for the instrument has also been left to

SBRC.

Although not written in the specifications, it is the responsibilityof the Project to understand the design, the

operation, and the calibration of the satellite sensor. The

Project also has the responsibilityof transferring this understanding to the communityof scientists who will use the

ocean color data set. Without a specific requirement in the

Contract, SBRC, OSC, and the Project set up an unofficial

program of visits to the instrument builder bya Project

representative during the construction and calibration of

SeaWiFS. From the outset, these visits developed into a

collaboration between SBRC and the Project, rather than

strict supervision byNASA, over the instrument development.

This arrangement has been non-standard in another

way, since the Contract remains an agreement between

NASA and OSC. Care has been taken to ensure no interference from the Project in the contractual obligations between OSC and SBRC. It should be noted that adherence

to the OCDM performance specifications is the responsibilityof OSC. The contract between OSC and SBRC has

incorporated the OCDM performance specifications almost

completely. There have been a few instances in which the

deliverables in the subcontract between OSC and SBRC

have not provided the information needed bythe Project to

assure compliance with the OCDM specifications. In those

instances, the informal arrangement has given a mechanism for the Project to obtain the necessaryinformation.

The major events in the construction and testing of the

SeaWiFS instrument are given in Table 1. Throughout this

period, the Project has worked as an active partner with

SBRC. This partnership has been of great advantage to the

Project. The Project representative has been given access

to all of the technical information about SeaWiFS, plus

access to the engineers and technicians working on the instrument. Technical problems that arose during construction were openlydiscussed bySBRC and the Project. In

addition, test procedures were developed, in part, through

informal talks between SBRC engineers and the Project

representative. The representative was an active participant in several of the tests.

Again, this active participation has been of great advantage to the Project. In this review of the SeaWiFS

specifications, the review panel can base its conclusions on

a set of tests and calibrations with results and procedures

that the panel understands. More importantly, the active participation has provided a greater understanding of

the operation of SeaWiFS. This includes an understanding of some of the characteristics that are particular to

the instrument—characteristics that the SBRC engineers

have called its personality. These include the instrument’s

along-track modulation transfer function (MTF), discussed

below, and its straylight characteristics, which will be discussed at length in a future volume within the SeaWiFS

Technical Report Series. Such understanding will be crucial as the Project works to interpret the data that the

radiometer will transmit from orbit.

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Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 1. Major events in the construction and testing of the SeaWiFS instrument.

Date Noteworthy Event

16 May1991 Letter Contract Signed between OSC and SBRC

6 July1991 PreliminaryDesign Review

16 December 1991 Critical Design Review

16 September 1992 Engineering Design Unit Completed

27 October 1992 SBRC and GSFC† Integrating Sphere Comparison

8 December 1992 First Field Test (lunar and solar measurements)

15 January1993 Vibration and Thermal-vacuum Testing

7 March 1993 Second Field Test

27 April 1993 Initial Pre-Ship Review

27 May1993 StrayLight Paths Review (at GSFC)

3 August 1993 Performance Specification Modification (at OSC)

26 October 1993 Vibration Testing of Modification Workmanship

1 November 1993 Third Field Test

22 November 1993 Completion of Instrument

2 December 1993 Post-Modification Pre-Ship Review

†Goddard Space Flight Center

It is the opinion of the review group that in light of

the instrument design and testing program, the prelaunch

performance of the SeaWiFS radiometer meets, or exceeds,

the requirements of the Contract in all but a few small details, which are considered to be minor. In addition, it

is the opinion of the review group that the testing of the

instrument has also been adequate to allow these conclusions.

In Sections 2 through 22, individual parts of the performance specifications are addressed. Some of these sections include requirements that do not involve the radiometer’s performance. One such section is Section 20, which

concerns satellite pointing data. These additional requirements are discussed in the compliance section for those

specifications. Section 23 gives a short summaryof the

conclusions.

2. FIELD-OF-VIEW

2.1 Requirement

The instantaneous field-of-view (IFOV) at nadir and

0◦ tilt shall be between 1 and 1.21 km. Sampling shall be

done once per nominal (square) IFOV.

2.2 Compliance

The field-of-view of the instrument is determined from

data measured byscanning a narrow slit across the nadir

pixel, both along-scan and along-track. The narrow slit is

0.16 mrad wide (about 0.1 of the width of a pixel), and the

slit is scanned in 0.1 mrad increments. The slit overfills the

pixel in the direction perpendicular to its narrow opening.

The term for the results from this type of measurement is

a line spread function.

The along-scan and along-track line spread functions

for the eight SeaWiFS bands are shown in Fig. 1. The

along-scan values are given in the direction of scan, i.e., the

values are representative of the instrument scanning from

left to right. In the same manner, the along-track values

are given in the direction of flight, i.e, for an instrument

moving left to right. For the purposes of Fig. 1, the offsets

have been removed from the data, and the data have been

normalized to unity. As shown in Fig. 1, the right-most

data points in both scans have been set to zero. The scans

in Fig. 1 also provide the basis for the MTF calculations

and for the band-to-band registration calculations.

The basic method for determining the field-of-view is

to calculate the full-width at half-maximum (FWHM) for

the along-scan and along-track measurements. These results are presented in Table 2. In addition, the results are

presented in terms of the length of an arc that subtends

the angle at a distance of 705 km. Such an arc represents

the width of the footprint of the SeaWiFS measurement,

assuming that the instrument is 705 km above the Earth.

The values calculated here are in agreement with the calculations found in the SeaWiFS Calibration and Acceptance

Data Package (SCADP). The SCADP was generated byA.

Holmes of SBRC in the course of constructing, calibrating,

and testing the SeaWiFS instrument. Manyof the conclusions made bythe review panel are based on information

found in the SCADP.

However, these calculations do not adequatelyrepresent the two-dimensional nature of the field-of-view. The

shape of a SeaWiFS footprint is not a perfect rectangle,

since the line spread functions are not perfect square waves.

To better represent the SeaWiFS footprint, the along-scan

and along-track values for each band have been combined

into two-dimensional arrays. Each element in the array is

the product of the value at the ordinate (the along-scan

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R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Fig. 1. The along-scan and along-track line spread functions. The top figure shows the along-scan line

spread function. In this figure, the zero offsets have been removed from the data, and the results have

been scaled to unity. The bottom figure shows the along-track line spread function. As in the top figure,

the zero offsets have been removed from the data, and the results have been scaled to unity.

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Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 2. Field-of-view calculations. All measurements given are the FWHM of the line spread function.

Along-Scan Values Along-Track Values

Band These Results SCADP These Results SCADP

No. [mrad] [km] [mrad] [km] [mrad] [km] [mrad] [km]

1 1.84 1.30 1.70 1.20

2 1.78 1.26 1.69 1.19

3 1.82 1.28 1.68 1.19

4 1.78 1.25 1.70 1.20

5 1.82 1.28 1.69 1.19

6 1.80 1.27 1.67 1.18

7 1.76 1.24 1.66 1.17

8 1.77 1.2 1.65 1.16

Mean 1.80 1.27 1.8 1.27 1.68 1.18 1.7 1.20

Std.Dev. 0.03 0.02 0.02 0.01

relative response) and the value of the abscissa (the alongtrack relative response). This result can be represented as

a three-dimensional figure (Fig. 2), with the base given as

the ordinate and abscissa locations, and the height as the

product of the ordinate and abscissa values.

Figure 3 shows the 50% cross section of the instrument response for band 7. It gives the edge of the threedimensional figure at the half maximum of the response for

the band in Fig. 2. Figure 3 also shows the values for the

major axes, i.e., for the axis with an along-scan value of

unityand for the axis with an along-track value of unity.

Each of the two axes gives the widest possible distance

across the cross section in each direction. When these

widest possible distances are multiplied together, theygive

an area that is larger than the actual footprint in Fig. 3.

Figure 4 shows the 50% cross section without the added

axes. This footprint for band 7 is nominally, i.e., roughly,

square. For the purposes of this review, it seems preferable

to define the field-of-view of the SeaWiFS measurements in

terms of the area of the (nominally) square footprint. For

the eight SeaWiFS bands, the area within the 50% cross

sections was calculated, and the length for the side of a

square that would enclose those areas was subsequently

determined. Table 3 gives the results of the calculations.

The lengths of the sides in Table 3 then give the best representation of the fields-of-view of the eight bands in the

instrument, as determined bythis review.

The average side length for SeaWiFS is 1.60 mrad, or

1.13 km, at an altitude of 705 km. The actual values range

from 1.10–1.16 km. All values conform to the requirement

of the specifications for an IFOV between 1 and 1.21 km.

3. CROSS-TRACK SCAN

3.1 Requirement

The active portion of the cross-track scan shall not be

less than 90◦ (±45◦ about nadir) nor greater than 116.6◦

(±58.3◦ about nadir). The swath width shall not be less

than 1500 km for tilts of ±20◦ to enable two-dayglobal

coverage from the nominal altitude. All scan data shall be

transmitted in the local area coverage (LAC) broadcast.

Global area coverage (GAC) data subsampled from the

cross-track scan need not include data taken at greater

than ±45◦.

Table 3. Field-of-view, calculated from the footprint area.

Band Cross- Side Side

No. Section† Length ‡ Length §

1 2.67 1.64 1.15

2 2.51 1.58 1.12

3 2.55 1.60 1.12

4 2.44 1.56 1.10

5 2.73 1.65 1.16

6 2.48 1.57 1.11

7 2.53 1.59 1.12

8 2.50 1.58 1.11

Mean 1.60 1.13

Std.Dev. 0.03 0.02

†mrad2 ‡mrad §km

3.2 Compliance

The angular portion of the SeaWiFS measurements

is determined bythe rotation rate of the optics within

the instrument, the sampling frequencyof the instrument,

and the number of samples in a scan line. The optics

within the SeaWiFS instrument rotate 6 times per second 6×360◦ s−1). The time period between pixels is 42 μs.

There are 1,285 pixels per scan line. The calculation is

shown in (1):

6 × 360◦ s −1 ·

42 × 10−6 s

pixel ·

1,285 pixels

scan line = 116.58◦scan line . (1)

The specifications call for this angle to be between 90◦

and 116.6◦. It should be noted that the distance from the

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R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Fig. 2. The response (field-of-view) for SeaWiFS band 7 in three dimensions. The axes for the base are

in the along-scan and along-track directions. The height shows the response of the band normalized to

unity.

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Prelaunch Acceptance Report for the SeaWiFS Radiometer

Fig. 3. The cross section of the response of SeaWiFS band 7 at the 50% response level. The two axes

within the cross section correspond to the FWHM values in Table 1. The 1:1 point marks the maximum

response for the band. The other four points are located on the half maximum contour.

Fig. 4. The measurement footprint for SeaWiFS band 7. The best estimate of the field-of-view for this

band is calculated as the side of a square with an area equal to this footprint.

6
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

start to the end of the first pixel is one pixel. Thus, the

distance from the start of the first pixel to the end of the

1,285th pixel is 1,285 pixels. The distance from the center

of the first pixel to the center of the 1,285th pixel is 1,284

pixels. As a result, the angular distance from the center of

pixel 1 to the center of pixel 1,284 is 116.48◦.

A figure in the SCADP gives a clock rate of 1.905 MHz

±1%. It also shows that there are 80 bits per pixel, or

41.99 microseconds (±1%) per pixel. The rotation rate

and the number of pixels per scan line are also described

in the SCADP.

Subsampling of each scan line for GAC will be performed bythe SeaStar bus. This subsampling will give the

90◦ angular range (centered about nadir), which is required

in the specification.

4. FORE-AND-AFT POINTING

4.1 Requirement

The data shall be taken with a sensor capable of pointing the swath fore and aft (positive and negative with respect to the velocityvector, respectively) to avoid specular

solar reflectance from the ocean’s surface. The sensor scan

must be capable of tilts of 0.0◦, +20.0◦, and −20.0◦ from

nadir. Changes in the tilt angle from −20◦ to +20◦ shall

take less than 30 seconds. Data describing the tilt angle

shall be accurate to within 0.01◦.

4.2 Compliance

The nadir direction for SeaWiFS is given as the +x

axis. The instrument scans in the (x,z) plane, and the solar

diffuser points in the +y direction. The diffuser points at

the sun after the instrument has completed its Earth views

and is passing over the South Pole. The diffuser is at the

back of the instrument. SeaWiFS flies in the −y direction.

A forward tilt has a −y direction and a backward tilt has

a +y direction; the values here are from the SCADP. The

fore and aft angles have been measured relative to nadir.

The angles for nadir direction were measured with respect

to the alignment mirrors mounted on the instrument.

Here is a brief description of the procedure that was

used to measure the fore and aft tilt angles relative to

nadir. With the instrument at zero tilt, a collimated light

source was aligned with pixel 643 of the SeaWiFS scan.

The entire instrument was then rotated in a speciallydesigned cradle fore and aft 20◦, and the scanner was tilted

in the other direction. The cradle was designed to rotate

the instrument to these 20◦ locations with great accuracy.

The instrument tilt realigned the nadir pixel of the instrument with the collimated light source. The collimated

light source was moved in the y direction to determine the

difference in the tilt angle from 20◦.

Since the tilt angles used in this procedure were determined bySeaWiFS measurements of the source, these

angles include the optical paths through the instrument.

Thus, the values in Table 4 include measurements with

both sides of the half angle mirror. All of the values in this

table are given with respect to the alignment mirrors. The

SCADP gives the uncertaintyfor the aft tilt as ±0.003◦,

and for the nadir and forward tilt the error is given as

±0.006◦. The specifications call for tilt knowledge to be

within 0.01◦.

The SeaWiFS scanner tilts from −20 to +20◦ in about

13 seconds. The speed profile for the tilt is Gaussian, with

a maximum speed of about 6◦ s−1.

Table 4. SeaWiFS tilt angles.

Tilt Mirror Side Angle Uncertainty

Aft A 19.896◦ ±0.003◦

Aft B 19.888◦ ±0.003◦

Aft Average 19.892◦ ±0.003◦

Nadir A 0.075◦ ±0.006◦

Nadir B 0.068◦ ±0.006◦

Nadir Average 0.072◦ ±0.006◦

Fore A −19.850◦ ±0.006◦

Fore B −19.857◦ ±0.006◦

Fore Average −19.853◦ ±0.006◦

5. DARK LEVEL

5.1 Requirement

A portion of everyscan shall contain sensor output data

while the field-of-view is obscured and the input radiance

is less than the Noise Equivalent Differential Spectral Radiance (NEdL).

5.2 Compliance

The SeaWiFS instrument incorporates a zero offset, or

dark restore value, for each scan of the instrument. This

value is provided in accordance with the dark level measurements specification. For details on the design and positioning of the dark restore, the reader is referred to Fig. 1

of Woodward et al. (1993), which shows dark direct current

(DC) restore at the angular range between 140◦ and 220◦

from nadir for each SeaWiFS scan. SeaWiFS provides the

dark level measurements required bythe specifications.

6. BAND TOLERANCES

6.1 Requirement

The location of the band edges shall be ±2 nm (3 σ)

of the values in Table 1, and shall be stable to less than

±1 nm over the duration of the ground test program. The

edge range shall not exceed 50% of the bandwidth in any

spectral band.

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Prelaunch Acceptance Report for the SeaWiFS Radiometer

6.2 Compliance

6.2.1 Band Edges

The band edge calculations are described in Barnes et

al. (1994). The results presented here have been taken

from that manuscript, and are summarized in Tables 5–7.

These tables include the 5%, 50%, and 80% response levels

that are required to calculate the band edges and the edge

ranges. The specifications call for the band edges to be

within ±2 nm of the values in the specifications. The right

(upper) band edge for band 8 falls at the 2.0 nm limit,

and the remaining band edges are within the specification

limits. The band edge and edge range results in Barnes et

al. (1994) are in agreement with the results in found in the

SCADP.

6.2.2 Edge Ranges

The specifications require that the edge range for each

band shall not exceed 50% of the bandwidth. The edge

range is the wavelength interval between 5% of peak response and 80% of peak response. The edge range values

are given in Tables 6 and 7. These values were also taken

from the calculated results in Barnes et al. (1994). The

edge range for band 2, left edge, is 54% of the bandwidth.

The edge range for band 6, right edge, is 51%. The remaining ratios fall within the 50% limits.

7. OUT-OF-BAND RESPONSE

7.1 Requirement

The out-of-band response shall be less that 5% of the

within-band value. Each 1% point shall be within 1.5 times

the bandpass from the corresponding band edge. Compliance with this specification shall be determined for a

source with spectral shape equivalent to Lcloud (the spectral radiance of a Lambertian surface of 100% reflectance

illuminated bythe sun at 22.5◦ zenith angle). Note: The

Lcloud radiances can be found in Table 18, below. They

are also called the SeaWiFS maximum cloud radiances.

7.2 Compliance

7.2.1 Out-of-Band Response.

The in-band response is defined as the integrated response of each band between the 1% transmission points.

The out-of-band response is defined as the integrated response at all other wavelengths. The ratio of out-of-band

response to in-band response is used to give the percent

out-of-band response. These values have been calculated

byBarnes et al. (1994). Tables 8 and 9 have been adapted

from Table 12 of Barnes et al. (1994) and are based on

measurements using a 5,900 K blackbodysource, as required in the specifications. A 5,900 K blackbodyduplicates the spectral shape of the solar output over the wavelength range of the SeaWiFS measurements (Barnes et al.

1994). All of the calculated out-of-band values are well

within the 5% value in the specifications. Table 9 contains

a comparison of the results from Barnes et al. (1994) with

those from the digitized spectral data provided as part of

the SCADP. The two sets of calculations show reasonable

agreement.

7.2.2 1% Response Point

The specifications state that each 1% response point

shall be within 1.5 times the bandwidth from the corresponding band edge. The calculated results are given in

Tables 10 and 11. These results are also derived from calculations in Barnes et al. (1994). The calculations have been

made using a 5,900 K blackbodysource, in accordance with

the specifications. All of the 1% response points are well

within the specification limits.

In general, the shapes of the SeaWiFS bands are a significant improvement over the shapes required bythe specifications.

8. SPECTRAL DIFFERENCES

8.1 Requirement

If multiple detector elements are used within a band,

the spectral response of all detector elements in a band

shall be compared, as to location and shape, byuse of normalized spectral response curves. The central wavelength

of anyelement must be within ±0.5 nm of the average central wavelength for all elements of the band. The integrated spectral response between the 10% response points

shall not differ bymore than 10% for anytwo elements in

the band.

8.2 Compliance

8.2.1 Center Wavelength

No spectral measurements have been specificallydesigned for the SeaWiFS instrument to confirm this specification. However, there is evidence to indicate that the

central wavelength for each of the four channels in each

SeaWiFS band is well within 0.5 nm of the average central

wavelength for that band, as called for in the specifications.

The design of SeaWiFS has the four channels for each

band located below a single interference filter for that band.

As shown in Barnes et al. (1994), the shape of the spectral

response of the filter dominates the determination of the

central wavelength. Barnes et al. (1994) also shows that

the quantum efficiencyof the type of silicon photodiode

used in SeaWiFS is roughlyconstant over the 10–20 nm

half-widths of the interference filters. This spectral flatness implies a veryminor effect from the detectors on the

central wavelength of the band.

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R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Table 5. Reference wavelengths for band edge and edge range calculations. All measurements are in nanometers.

Band Left Right

No. 5% Point 50% Point 80% Point 80% Point 50% Point 5% Point

1 398.4 403.2 407.4 421.1 423.4 428.9

2 427.8 434.1 438.3 451.0 453.7 459.5

3 474.6 480.8 483.3 499.4 501.4 507.1

4 492.8 498.9 501.1 518.9 521.2 527.2

5 540.4 545.5 547.6 561.4 563.8 570.4

6 653.4 658.3 660.1 675.6 678.2 685.8

7 734.5 744.7 748.9 780.8 785.0 798.3

8 835.8 845.7 849.2 884.1 887.0 896.8

Table 6. Band edge calculations. All measurements and calculated quantities are in units of nanometers.

Band Left Band Edge Results Right Band Edge Results

No. Specified Measured Difference Specified Measured Difference

1 402 403.2 1.2 422 423.4 1.4

2 433 434.1 1.1 453 453.7 0.7

3 480 480.8 0.8 500 501.4 1.4

4 500 498.9 −1.1 520 521.3 1.3

5 545 545.5 0.5 565 563.8 −1.2

6 660 658.3 −1.7 680 678.2 −1.8

7 745 744.7 −0.3 785 785.0 0.0

8 845 845.7 0.7 885 887.0 2.0

Table 7. Edge range calculations. All measurements are in nanometers.

Band Bandwidth Left Edge Percent of Right Edge Percent of

No. Range Bandwidth Range Bandwidth

1 20.2 9.0 45 7.8 39

2 19.6 10.5 54 8.5 43

3 20.6 8.7 42 7.7 37

4 22.4 8.3 37 8.3 37

5 18.3 7.2 39 9.0 49

6 19.9 6.7 34 10.2 51

7 40.3 14.4 36 17.5 43

8 41.3 13.4 32 12.7 31

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Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 8. Calculated out-of-band responses for the eight SeaWiFS bands. The instrument responses are given

as the output of the photodiode in picoamperes (pA). The 5,900 K radiances in the calculations are normalized

to the expected saturation radiance for each band at the nominal center wavelength for each band. The upper

and lower extended band edges come from Tables 7 and 8 of Barnes et al. (1994). These results are calculated

over the wavelength range from 380 –1,150 nm.

Lower Lower In-Band Upper Upper Out-of-Band

Band Out-of-Band Extended Response Extended Out-of-Band Response

No. Response [pA] Band Edge [nm] [pA] Band Edge [nm] Response [pA] [%]

1 3.38 395.2 2175.34 433.6 11.77 0.70

2 9.59 424.1 3418.80 463.7 1.56 0.33

3 6.48 470.7 4301.14 511.7 28.08 0.80

4 17.32 488.1 4586.23 530.7 8.96 0.58

5 39.14 536.3 3631.84 577.2 46.14 2.35

6 12.66 646.7 2071.19 692.5 7.84 0.99

7 10.17 727.3 2818.97 813.4 29.58 1.41

8 66.36 826.4 2191.97 907.5 15.43 3.73

Table 9. A comparison of the calculated instrument out-of-band response results found in Barnes

et al. (1994) with SCADP results for the same parameter. Both sets of calculations show the instrument’s out-of-band response to be within the 5%

value in the specifications.

Band Barnes et al. (1994) SCADP

No. [%] [%]

1 0.70 0.65

2 0.33 0.40

3 0.80 0.80

4 0.58 0.57

5 2.35 2.35

6 0.99 0.98

7 1.41 1.41

8 3.73 3.73

The detectors in each band are etched from a single

piece of silicon. The overall dimensions for the set of four

detectors are approximately0.05 in × 0.01 in. Assuming

reasonable uniformityin the manufacture of the silicon material, the four detectors in each band should have nearly

identical spectral response over the half-width of the interference filter. It is the response of the interference filter,

not the detectors, that dominates the determination of the

central wavelength for each SeaWiFS band. The estimate

of the within-band spectral differences is based primarily

on an assumption of uniformityin the manufacture of the

interference filter. This estimate of uniformitycovers an

area of 0.05 in × 0.01 in.

8.2.2 Integrated Spectral Response

The within-band spectral response specification also requires an integrated spectral response, between the 10%

response points for the individual elements, to be within

10%, i.e., the range from greatest to least integrated response for the elements can be no more than 10% of the

value of the average integrated response. Measurements of

individual SeaWiFS channels with the SBRC integrating

sphere have shown that the output from the individual detectors in each band corresponds at the 1% level. That is,

the integrated response of the detectors agrees at the 1%

level. Of course, the output from each channel has been

adjusted (or rather, the values of the resistors in the operational amplifiers have been adjusted) to give uniformity

at the 1% level.

In general, switching from detector to detector in a

SeaWiFS band will give a change in output from the instrument on the order of 1%—considerablybetter than the

10% limit in the specification. This 10% specification was

included in the SeaWiFS specification, in case the design

of the flight instrument included area arrays with many

individual detectors.

9. BAND CO-REGISTRATION

9.1 Requirement

The IFOVs from all spectral bands shall be co-registered

to within 0.3 pixel (1 σ).

9.2 Compliance

The specification requires that the IFOVs from all spectral bands be co-registered within 0.3 pixel. From the fieldof-view calculations in Section 2 above, it was determined

that the IFOV for the instrument is 1.6 mrad on a side.

The band-to-band registration results presented here are

based on the along-scan and along-track line spread functions in Section 2. The line spread measurements were

made bymoving a narrow slit across the field-of-view of

the nadir SeaWiFS pixel. The slit was part of a collimated light source, and the slit was moved in increments

10
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Table 10. Reference wavelengths for one percent response point calculations. All values are in nanometers.

Band Left Center Right

No. 1% Point 50% Point Wavelength 50% Point 1% Point

1 395.2 403.4 413.4 423.4 433.6

2 424.1 434.2 444.0 453.8 463.7

3 470.7 480.8 491.1 501.4 511.7

4 488.1 498.9 510.1 521.3 530.7

5 536.3 545.4 554.6 563.8 577.2

6 646.7 658.3 668.2 678.1 692.5

7 727.3 744.4 764.6 784.9 813.4

8 826.4 845.5 866.1 886.7 907.5

Table 11. One percent response point calculations. The specifications call for a ratio to the bandwidth that is

less than 150%. The bandwidths were previouslygiven in Table 7.

Band Bandwidth Left 1% from Percent of Right 1% from Percent of

No. [nm] Left 50% [nm] Bandwidth Right 50% [nm] Bandwidth

1 20.0 8.2 41 10.2 51

2 19.6 10.1 52 9.9 50

3 20.6 10.1 49 10.3 50

4 22.4 10.8 48 9.4 42

5 18.4 9.1 50 13.4 73

6 19.8 11.6 59 14.4 73

7 40.5 17.1 42 28.5 70

8 41.2 20.1 49 20.8 50

(ticks) equal to a 0.1 mrad angular displacement as seen

bythe instrument. The center for each pixel (in ticks) was

calculated as the average of the positions of the two half

maximum points for each band. The along-scan and alongtrack centers were calculated independently. The absolute

values for the central positions (in ticks) are not important

in these results—it is the relative locations of the central

points that are relevant here. Table 12 gives the alongscan results. Table 13 gives the along-track results. Figure 5 shows the results relative to the average for the set

of band centers, and also shows the locations of the eight

band centers in two dimensions. Figure 5 also includes a

square 0.1 pixel wide, centered on the average band center. The maximum distance between band centers in the

along-scan direction is 0.15 pixels, and the maximum distance along-track is 0.21 pixels. The instrument’s spectral

band-to-band registration is significantlybetter than the

requirements of the specifications.

10. SENSITIVITY

10.1 Requirement

Table 14 provides the signal-to-noise ratio (SNR) specifications for all bands at a gain value of unity. The required

SNR shall be achieved at the typical spectral radiance levels (Ltypical). NEdL maybe calculated from the expression:

NEdL= Ltypical/SNR.

10.2 Compliance

The noise in the SeaWiFS instrument was measured

byviewing the SBRC integrating sphere. The SNR was

calculated bydetermining the mean and the standard deviation in a 21 pixel-long section of the scan of the sphere’s

center. These measured results are listed in Table 14. The

measurements were made close to, but not at, the exact

typical radiance levels required by the specifications. The

results have been scaled to the typical levels by changing

the SNRs, assuming that the noise in the measurement

varies as the square root of the change in signal level over

this small range. These measured SNRs exceed the requirements of the specifications. However, the SNR measurements include the non-uniformityin the output of the

sphere. This additional variation makes the measured values lower than the actual SNRs.

SBRC has provided a calculation of the SNRs, based on

the noise in the dark output from the instrument. These

calculated values are also listed in Table 14. For purposes

of evaluation, it seems reasonable to assume that the actual SNRs for SeaWiFS fall between the measured and

calculated SNRs in Table 14. In all cases, however, the

measured values (the lower limits for the actual SNRs) are

better than the requirements in the specifications. Section 19 describes the model for instrument noise devised

bythe Project. The SNRs from this model fall between the

measured and calculated values in Table 14. For the review

11
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Fig. 5. Band-to-band registration of SeaWiFS. The locations of the individual band centers are given

relative to the average for the eight bands. The distances are given in “pixels,” where 1 pixel equals

1.6 mrad. The figure also includes a square that is 0.1 pixel on each side and which is centered on the

average center location for the eight bands.

panel, the results in Section 19 give the best prelaunch estimates of the SNRs for the instrument at the Ltypical radiances. However, those values remain prelaunch estimates.

An extensive set of on-orbit measurements (McClain et al.

1992 and Woodward et al. 1993) will be used to obtain an

improved set of SNRs.

Table 12. Along-scan band center measurements.

The measurements were made in ticks, with each

tick equal to 0.1 mrad. The results are also given

in mrad and in pixels, where one pixel is 1.6 mrad.

The range gives the distance between the two bands

that are farthest apart.

Center Distance Distance

Band Position from center from center

No. [ticks] [mrad] [pixel]

1 57.56 −0.003 −0.002

2 56.71 −0.088 −0.055

3 58.08 0.049 0.031

4 57.03 −0.056 −0.035

5 59.00 0.141 0.088

6 58.26 0.066 0.041

7 57.52 −0.008 −0.005

8 56.58 −0.101 −0.063

Range 2.42 0.242 0.151

Table 13. Along-track band center measurements.

The same procedure used to obtain the measured

values in Table 12 was employed here.

Center Distance Distance

Band Position from center from center

No. [ticks] [mrad] [pixel]

1 22.50 0.050 0.031

2 20.29 −0.170 −0.106

3 21.15 −0.085 −0.053

4 20.78 −0.121 −0.076

5 23.02 0.103 0.064

6 21.83 −0.016 −0.010

7 23.67 0.167 0.104

8 22.73 0.073 0.046

Range 3.37 0.337 0.210

11. POLARIZATION

11.1 Requirement

The radiometric data shall be nominallyinsensitive to

linear polarization. The polarization factor (P F) as defined below, shall be no greater than 2% over scan angles

from +45◦ to −45◦ for all bands and tilt angles between

−20◦ and +20◦. Imax and Imin are the recorded maximum

12
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

and minimum output when the plane of incoming 100%

linearlypolarized light is rotated through 180◦.

P F = Imax − Imin

Imax + Imin

< 0.020 (2)

Table 14. SNRs for the SeaWiFS Bands.

Band Specified Measured Calculated

No. SNR SNR SNR

1 499 940 1107

2 674 950 1269

3 667 1156 1402

4 640 1055 1373

5 596 690 1242

6 442 798 846

7 455 860 971

8 467 670 796

11.2 Compliance

Polarization is defined in this requirement in terms of

the difference from the average for the output range. This

definition then gives a polarization value that is half of

the difference between the maximum output and the minimum output. The polarization sensitivityof SeaWiFS was

checked using two linear sheet polarizers, at different times,

between the light source and the instrument. The first polarizer (Polaroid HR) checked bands 1–6, and the second

(Polaroid HN) checked the two near infrared (IR) bands—

bands 7 and 8. Changes in the output of the instrument

during these tests included variations in the light through

the polarizers as well as polarization dependent changes in

the output of the instrument itself. The variation in light

through the polarizer is an artifact of the measurement

procedure, since the rotation axis for the polarizer is not

located at the center of the input aperture of the instrument. Crossed polarizers showed good extinction for each

band, indicating that the polarizers were creating polarized

light.

Using knowledge of the expected angular dependence of

the polarization effect, an effort was made to separate the

instrument’s portion of the polarization pattern. The polarization measurements were made byrotating each polarizer through 360◦ in 22.5◦ increments. Since polarization

changes from a minimum to a maximum in 90◦, the response of the instrument should show two cycles, i.e., two

maxima and two minima, in the 360◦ rotation. Fourier

analysis was used to look for the expected two cycle sinusoidal function in the polarization results.

The data were taken at 16 angles, from an arbitrarily

determined zero, through 360◦, back to the original angular zero. The two measurements at 0◦ were averaged

to give a data set with 15 points from 0 to 337.5◦. From

the Fourier transform of these data, sinusoidal waveforms

with 1–9 cycles per 360◦ were extracted for analysis. The

data for band 1 are shown in Fig. 6. These data are representative of the results for bands 1–6. The input data

show a dominant signal of 1 cycle per 360◦ and show an

average value of 100. This allows a direct conversion of

the average-to-peak values from the Fourier analyses into

percentages.

The results of the Fourier analysis are summarized in

Table 15. Theyare given in terms of the average-to-peak

values for the input to the Fourier transforms, for the onecycle component, and for the two-cycle component. It

is the two-cycle component that contains the information

about the polarization of the instrument. Since the average value for the measurements is 100, the average-to-peak

values translate directlyinto percentages.

As shown in Table 15, the one-cycle Fourier component accounts for almost half of the average-to-peak signal

in the data from bands 1–6. In the judgement of the review panel, the majorityof the observed variation in the

measurements of these bands does not come from polarization sensitivityin the SeaWiFS instrument. Presumably,

the bulk of the variabilityderives from a non-uniformity

in the polarizer plate used in the measurements. The effects of the polarizer plate repeat for each of the 6 bands

for which the polarizer plate was used. The polarization

in the SeaWiFS instrument for these bands is less than

0.25%.

Table 15. Summaryof polarization results. The

values in columns 3–5 are average-to-peak values,

expressed as percentages.

Band Input One-Cycle Two-Cycle

No. to Transform Output Output

1 1.90 0.96 0.15

2 1.10 0.53 0.09

3 0.90 0.30 0.11

4 0.60 0.22 0.08

5 1.10 0.39 0.20

6 1.00 0.43 0.14

7 0.90 0.01 0.26

8 1.10 0.05 0.35

The test results for bands 7 and 8 show a pattern that

is similar for the two bands (see Fig. 7 for band 7). However, the pattern is noticeablydifferent from the pattern

for bands 1–6. Of course, bands 7 and 8 used the same polarizer plate, a different polarizer plate than the one used

for measurements on bands 1–6. The patterns for bands

7 and 8 show essentiallyno one-cycle component. In addition, the two-cycle component in the pattern is somewhat larger than for bands 1–6, falling between 0.25% and

0.35%. No instrument based explanation can explain why

the polarization in SeaWiFS for bands 7 and 8 should be

different from that for the other 6 bands. For this reason, a

large part of the two-cycle Fourier component for bands 7

and 8 appears to be an artifact caused byvariations in the

13
Prelaunch Acceptance Report for the SeaWiFS Radiometer

polarizer plate for these bands. This assumption explains

the similarityin the angular pattern for the measurements

on bands 7 and 8, in the same manner that it explains the

similarityin the patterns for bands 1–6.

Thus, for all eight SeaWiFS bands, the polarization in

the instrument is estimated to be less than 0.25%.

12. DYNAMIC RANGE

12.1 Requirement

The sensor shall be designed to operate over a dynamic

range that extends from the noise floor (NEdL) in each

spectral band to the maximum levels (Lcloud) given in Table 18. (There is an amendment to the Contract. With

this addition, there are 3 radiance levels in the specifications: saturation radiance, maximum ocean radiance, and

maximum cloud radiance. The specification radiances are

given in Tables 16, 17, and 18, respectively.)

12.2 Compliance

The specification, which gives the saturation levels for

the eight SeaWiFS band, was supplemented byan amendment to the contract in August 1993. The amendment

provides the maximum ocean radiances and the maximum

cloud radiances for the instrument. These changes to the

specifications were required due to the use of bilinear gains

in SeaWiFS. However, the saturation values from the original specification remain an important part of the SeaWiFS

measurements. The saturation values are the maximum radiances that the SeaWiFS bands would provide if the knees

the point where the radiance vs. counts slope changes) in

the bilinear gains are ignored. The saturation values define

the sensitivity of SeaWiFS, in counts per unit radiance, for

the instrument’s ocean measurements.

In summary, the saturation radiances define the sensitivities of the SeaWiFS bands for ocean measurements,

i.e., for measurements below the knees of the bilinear gains.

The maximum ocean radiances are the radiances at the

knees for the eight bands. The maximum cloud radiances

are the greatest values that the SeaWiFS bands will measure. With the addition of the “maximum ocean” and the

“maximum cloud” radiances to the specifications in the

contract amendment, the term “saturation” radiance has

become a misnomer. However, the term has been retained

here.

Table 16 gives the saturation radiances from the SeaWiFS calibration (ignoring the bilinear gain knees). Table 17 gives the maximum ocean radiances, and Table 18

gives the maximum cloud radiances. All radiances are calculated in terms of milliwatts per square centimeter per

micrometer per steradian (mW cm−2μ m−1sr−1). The differences from the specifications are also calculated. They

are small, and present no problem with regard to specification compliance.

13. QUANTIZATION

13.1 Requirement

Data shall be quantized at 10 bits. The differential

linearityof the quantizer(s) shall be better than one-half

a least significant bit.

13.2 Compliance

The data from SeaWiFS are quantized at 10 bits. The

data from each detector in each band are digitized with

12-bit analog-to-digital (A/D) converters. For 4-to-1 Time

Delayand Integration (TDI), the 12-bit values from each

detector are summed to give a 14-bit value. The bottom

four bits from this output are removed, with the upper

10 bits sent to SeaStar. Measurements bythe instrument

manufacturer (SBRC), using a voltage ramp, show linearityat better than one-half of the least significant bit of the

output 10 bits. This procedure was relativelyeasy, since

the least significant bit for each detector corresponds to

four times the least significant bit in the A/D converters,

themselves.

14. MTF

14.1 Requirement

The MTF of the data shall equal or exceed the values

tabulated in Table 19 below, in both the along-track and

cross-track directions for a sine wave input. The Nyquist

frequency† has a spatial period equal to two IFOVs on the

ground.

The MTF specifications shall be satisfied for modulations between dark and Ltypical and between dark and

Lmax, for everydetector element in each spectral band.

Data describing the MTF shall be provided from prelaunch

testing to verifythat the specification is met. Data from

lunar views and/or internal stabilized sources will also be

provided for analysis of the MTF on orbit.

14.2 Compliance

The SeaWiFS MTFs are calculated from the line spread

functions in Fig. 1. Theyconsist of the Fourier transforms

of the line spread functions. The results in Table 20, below,

use the MTF calculation program provided bySBRC. The

values in the specification give minimum amplitudes for

several low frequencysinusoidal waves from the Fourier

analysis. The waves are given in terms of their wavelengths

relative to the width (FWHM) of the field-of-view, i.e., in

cycles per pixel.

† The Nyquist frequency is the minimum sampling frequency

of a digital system sufficient to reconstruct the original information. For SeaWiFS, the original information is the input

sequence of radiances.

14
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Fig. 6. Polarization measurements for SeaWiFS band 1. a) The instrument response measured at

22.5◦ increments of polarizer rotation. These values are the input values for Fourier analysis. b) The

polarization values with and without the one-cycle component b1(k) equal the measured values, and h(k)

equal the measured values without the one-cycle component. c) The one-cycle polarization component

generated byFourier analysis. d) The polarization values with and without the two-cycle component

b1(k) equal the measured values and h(k) equal the measured values without the two-cycle component.

e) The two-cycle polarization component generated by Fourier analysis.

15
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Fig. 7. Polarization measurements for SeaWiFS band 7. a) The instrument response measured at

22.5◦ increments of polarizer rotation. These values are the input values for Fourier analysis. b) The

polarization values with and without the one-cycle component b7(k) equal the measured values, and h(k)

equal the measured values without the one-cycle component. c) The one-cycle polarization component

generated byFourier analysis. d) The polarization values with and without the two-cycle component

b7(k) equal the measured values and h(k) equal the measured values without the two-cycle component.

e) The two-cycle polarization component generated by Fourier analysis.

16
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Table 16. SeaWiFS saturation radiances (in mW cm−2μ m−1sr−1). The measured and specified saturation radiances

appear in columns 3 and 4, respectively.

Band Measured Specified Percent

No. Gain Radiance Radiance Difference

1 1 13.76 13.63 1.0

2 1 13.44 13.25 1.4

3 1 10.52 10.50 0.2

4 1 9.22 9.08 1.5

5 1 7.47 7.44 0.4

6 1 4.25 4.20 1.3

7 1 3.02 3.00 0.5

8 1 2.15 2.13 1.0

Average % Difference 0.9

Greatest % Difference 1.5

Least % Difference 0.2

Table 17. SeaWiFS maximum ocean radiances (in mW cm−2μ m−1sr−1). The measured and specified ocean radiances

appear in columns 3 and 4, respectively.

Band Measured Specified Percent

No. Gain Radiance Radiance Difference

1 1 10.90 10.84 0.5

2 1 10.56 10.46 0.9

3 1 8.18 8.19 −0.1

4 1 7.16 7.05 1.6

5 1 5.74 5.74 0.0

6 1 3.25 3.21 1.2

7 1 2.29 2.29 0.0

8 1 1.64 1.62 1.0

Average % Difference 0.6

Greatest % Difference 1.6

Least % Difference −0.1

Table 18. SeaWiFS maximum cloud radiances (in mW cm−2μ m−1sr−1). The measured and specified cloud radiances

appear in columns 3 and 4, respectively.

Band Measured Specified Percent

No. Gain Radiance Radiance Difference

1 1 60.16 60.02 0.2

2 1 67.91 66.24 2.5

3 1 68.21 68.17 0.1

4 1 66.47 65.62 1.3

5 1 64.97 65.16 −0.3

6 1 54.93 53.78 2.1

7 1 42.98 42.95 0.1

8 1 34.38 34.05 1.0

Average % Difference 0.9

Greatest % Difference 2.5

Least % Difference −0.3

17
Prelaunch Acceptance Report for the SeaWiFS Radiometer

The MTF gives an idea of the response of the instrument to variations in the input radiance, both along-scan

and along-track. For example, consider a case in which

the instrument scans across a scene with a variation of the

input radiance having a sinusoidal shape and a spatial period of two pixels per period. In this case, the scene varies

at the Nyquist frequency. According to the specification,

the output from the sensor should show a variation (from

pixel to pixel) across the scene that is greater than 30% of

the amplitude of input sinusoidal radiance variation.

Table 19. MTF requirements for spatial resolution.

Frequency/ MTF

Nyquist Frequency

0.00 1.0

0.25 0.9

0.50 0.7

0.75 0.5

1.00 0.3

15. GAINS

15.1 Requirement

Band independent gains shall be provided, which are

commandable band byband, and which will increase or

decrease sensitivityaccording to the following:

Sn = Gn × Si (3)

where Si and Sn are the initial detector signal and the

signal with gain, respectively, and Gn is the gain factor at

gain setting n. The nominal Gn values for gain settings

of n equal to 2, 3, and 4 shall be based on the values in

Table 21. These G values include those required for onboard solar and lunar capabilities. The values of G will be

within 5% of the specifications in Table 21, and shall be

known relative to G1 = 1 with an accuracyof greater than

99.5%. The nominal Gn value for n = 2 is 2.

15.2 Compliance

Gain values for gain settings 3 and 4 were re-evaluated

in the summer of 1993 during a meeting at OSC. Gains

3 and 4 are used for solar and lunar measurements. The

SCADP contains the predicted on-orbit lunar and solar

diffuser radiances. Theyhave been derived from ground

based solar and lunar measurements at SBRC (Biggar et

al. 1993). The values presented in Table 21, below, are

appropriate for those radiances. The gains are given for a

4:1 TDI.

These relative gain values can be measured on orbit.

Current plans will have these values checked twice a day

at the start of the mission.

16. TRANSIENT RESPONSE

16.1 Requirement

Radiometric data should be relativelyfree of effects of

overshoot and ringing when the IFOV scans across a steep

gradient in radiance, from a maximum radiance of Lcloud

to a minimum radiance of Ltypical. For this radiance step

change, the output signal shall settle to within 0.5% of its

final value within 10 IFOVs.

16.2 Compliance

16.2.1 Original Specification

The original SeaWiFS specification describes the required response of the instrument when it scans across a

steep gradient in radiance. The specification describes a

gradient that is expected be found on orbit, i.e., from the

radiance level for a cloud (Lcloud) to the typical radiance

level for ocean measurements (Ltypical). The original specification requires that the output from the instrument settles to within 0.5% of its final value (Ltypical) within 10

pixels. This limit, calculated in counts using values from

the specifications, is given in Table 22. Since the values

are small, the limit is given to 0.1 count relative to the

quantization of the data.

During the April 1993 SeaWiFS Pre-Ship Review, it

was determined that the instrument did not meet the original bright target recovery(BTR) specification. At that

time, it was decided that the instrument manufacturer

would rework SeaWiFS to improve its straylight characteristics. The series of modifications included the tilting of

filters, incorporation of bilinear responses and corresponding changes to gains, refiguring the polarization scrambler,

and testing. The modifications did not include changes

to the instrument’s focal planes. A description of the revised BTR specification is given in the following section.

The discussion presented here centers on the performance

of the reworked SeaWiFS radiometer onlywith respect to

the original specifications.

The results in Table 23 were derived from laboratory

measurements using the output of the SBRC integrating

sphere. The measurements were made in four sets—one

for each focal plane in the instrument. Color glass filters

were placed over the output from the sphere in order to

give a spectral shape which approximated the sun over the

wavelength range of the bands on each of the four focal

planes. Cross-talk between bands on a focal plane is a

significant factor in the recoveryof the instrument from

bright targets. The measurements were made for a three

pixel wide slit.

The results in Table 23 give the distance, in pixels,

required for the instrument to settle to less than 0.5% of

Ltypical, using the counts given in Table 22. For bands 1–

5, the results give the pixels required to settle to 3 counts

18
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Table 20. MTF calculations for SeaWiFS. The amplitude is shown for each band at four different wavelengths:

0.500, 0.375, 0.250, and 0.125 cycles per pixel.

Band Along-Scan MTF Amplitude Along-Track MTF Amplitude

No. 0.500 0.375 0.250 0.125 0.500 0.375 0.250 0.125

1 0.35 0.57 0.76 0.93 0.44 0.65 0.84 0.96

2 0.36 0.58 0.76 0.92 0.41 0.62 0.81 0.95

3 0.36 0.58 0.77 0.93 0.41 0.62 0.82 0.95

4 0.36 0.58 0.77 0.93 0.40 0.61 0.81 0.95

5 0.35 0.57 0.76 0.93 0.48 0.67 0.84 0.96

6 0.37 0.59 0.78 0.93 0.43 0.63 0.82 0.95

7 0.39 0.61 0.79 0.94 0.49 0.68 0.85 0.96

8 0.34 0.55 0.74 0.91 0.47 0.66 0.83 0.96

Minimum Amplitude 0.30 0.50 0.70 0.90 0.30 0.50 0.70 0.90

above background. For bands 6–8, the results give the

pixels required to settle to 2 counts. Table 23 also gives

the number of pixels required for the instrument to settle to

zero counts after illumination bythe slit. All eight bands

settle to less than the specification limit within 10 pixels.

Table 21. SeaWiFS gain values. These values are

given relative to gain 1. The nominal value for gain

setting 2 is 2.

Band No. G1 G2 G3 G4

1 1 1.931 1.302 1.642

2 1 1.940 1.303 1.648

3 1 1.951 0.900 1.655

4 1 1.955 0.796 1.658

5 1 1.961 0.652 1.579

6 1 1.969 0.376 0.671

7 1 1.969 0.323 0.583

8 1 1.975 0.272 0.507

16.2.2 Revised BTR Specification

In the summer of 1993, there was a meeting at OSC

in Chantilly, Virginia. At that meeting, a set of improvements to ameliorate straylight in SeaWiFS was discussed.

Each proposed improvement was presented, accompanied

bya corresponding estimate of the resulting improvement

to performance of the instrument. A final and accepted

set of instrument improvements from that meeting was incorporated into a Contract modification. As demonstrated

bySBRC testing, this set of modifications has created the

anticipated improvements to the performance of the sensor. However, as described above, the reworked SeaWiFS

radiometer also meets the original BTR specifications.

17. ABSOLUTE ACCURACY

17.1 Requirement

An absolute radiometric accuracyof 5% (1 σ) shall be

achieved at the typical spectral radiance levels. At spectral

radiances between 0.2 Ltypical and 0.9 Lmax, the absolute

radiometric accuracyshall be within ±6%. Measurements

of the accuracyshall be made, as a minimum, at scan angles centered at 0◦, −40◦, and +40◦; tilt angles of 0◦, −20◦,

and +20◦; special tilt angles, should theybe required, used

to view the moon; and at all gains.

17.2 Compliance

17.2.1 Accuracy at Nadir

This specification calls for an absolute radiometric calibration at the 5% level. The SeaWiFS instrument was calibrated radiometricallybythe manufacturer (SBRC) using

an integrating sphere that was calibrated with standards

that are traceable to the National Institute of Standards

and Technology(NIST). In addition, the SBRC sphere has

been compared with the GSFC sphere, which has also been

calibrated using standards traceable to NIST. The comparison of the GSFC and SBRC spheres showed agreement at

the 2% level.

Fundamentally, the accuracy of the radiometric calibration of SeaWiFS reduces to the accuracyof the calibration of the integrating sphere. The absolute uncertainty

in the radiances from the sphere is the largest of the set

of uncertainties in the instrument calibration. In addition, manyuncertainties in the radiometric calibration of

SeaWiFS, such as the alignment of the sphere and the instrument, duplicate uncertainties in the calibration of the

sphere, such as the alignment of the sphere, the radiance

standard, and the transfer instrument.

The current understanding of the uncertainties in the

calibration of the GSFC sphere is reported byWalker et

al. (1991). Two of the authors specialize in radiometric

calibrations at NIST, and the third author is the principal investigator for the GSFC sphere. In the abstract,

Walker et al. (1991) states: “Recent measurements performed at NIST and NASA Goddard Space Flight Center

have demonstrated that the uncertaintyof sphere-source

radiance measurements can be improved from the present

19
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 22. Constants used in the calculation of the specification count limit. The Lcloud and Ltypical values

come from the performance specifications. The slopes (or sensitivities for each band) come from the SBRC

calibration data.

Band Lcloud Ltypical Lcloud Radiance 0.5% of

No. [mW cm−2μ m−1sr−1] [mW cm−2μ m−1sr−1] Ltypical per Count Ltypical [counts]

1 60.0 9.10 6.59 0.0137 3.3

2 66.2 8.41 7.87 0.0134 3.1

3 68.2 6.56 10.4 0.0105 3.1

4 65.6 5.64 11.6 0.00920 3.1

5 65.2 4.57 14.3 0.00746 3.1

6 53.8 2.46 21.9 0.00425 2.9

7 43.0 1.61 26.7 0.00301 2.7

8 34.0 1.09 31.2 0.00215 2.5

5–10% level to a 1–2% level.” This is a general statement about the technique for calibrating sphere sources.

However, it does not describe the actual uncertaintyin

radiances from the GSFC sphere.

Table 23. This table shows the SeaWiFS BTR

characteristics. Column 2 gives the value of 0.5%

of Ltypical, in counts, for each band. The instrument response must settle below this level of residual counts after scanning a bright target. Column 3

shows the number of pixels after the slit required for

the instrument to settle to the level of the specified

residual counts. Column 4 provides the number of

pixels the instrument requires to settle to a level of

zero residual counts.

Band Residual Cnts. Pixels to Pixels to

No. (0.5% Ltypical) Spec. Zero Cnts.

1 3.3 6 10

2 3.1 5 9

3 3.1 7 10

4 3.1 5 15

5 3.1 9 15

6 2.9 7 9

7 2.7 9 11

8 2.5 7 10

At the conclusion of the report, Walker et al. (1991)

states: “The normal stated uncertaintyfor the NASA calibration of their large-area sphere source is presently5–

10%. The repeatabilityof the calibration from month

to month is about 1 percent. The major contributors to

NASA’s large overall uncertaintyare the uncertainties associated with the standard lamp, the dimensions of the

apertures, the distance measurement, the alignment, and

the lamp current of the sphere source lamps. The results of

our measurements confirmed the accuracyof the spectral

irradiance method employed by NASA, and in the future

it will permit them to state uncertainties in the range of

1–2%. The reason for the difference in NASA’s independent check method has not been determined. More work

will have to be done if this problem is to be resolved.”

Recent discussions indicate that error propagation in

NASA’s independent check method has not changed. The

discrepancybetween the NASA 5–10% error estimate and

the 1–2% estimate in Walker et al. (1991) remains unresolved. This assessment has led to the conclusion that the

uncertaintyin the radiances from the GSFC sphere is in

the range of 2–5%. This conclusion is nothing more than

an educated guess. If the 2–5% uncertaintyin the GSFC

sphere is correct, and if the GSFC and SBRC spheres agree

at the 2% level, then the estimate of the uncertaintyin the

SBRC sphere is 2–5%. This is also the review panel’s estimate for the uncertaintyin the calibration of SeaWiFS.

17.2.2 Accuracy Over a Scan Line

These measurements were made at nadir. The specifications also call for a 5% accuracyat scan angles of +40◦

and −40◦. SBRC has made measurements to check the

output of the instrument at scan angles 20◦, 40◦, and 58◦

on both sides of nadir. The results of the measurements

at these angles give the instrument output relative to the

output at nadir; this effect is called scan modulation. A

fitted curve to the scan modulation gives a correction that

is good to 0.5% at all scan angles. This correction has

been incorporated into the data reduction procedures for

SeaWiFS. The absolute radiometric calibration of the instrument remains within 5% over its entire scan range.

17.2.3 Accuracy Over Tilt Angles

The specification calls for the radiometric calibration

to be known at all tilt angles. For SeaWiFS, the entire

scanner tilts, including all components from the optical

inlet to the focal planes. The optical path through the

instrument does not change as a function of tilt angle. Tilt

angle is not considered a factor in the radiometric accuracy

of SeaWiFS.

20
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

18. RELATIVE ACCURACY

18.1 Requirement

The design shall be capable of achieving an accuracy

within 2% (1 σ) relative to the sun. The calibrated data

shall be linear to within ±1% over the full range of input

signals.

18.2 Compliance

18.2.1 Accuracy Relative to the Sun

The instrument shall be capable of achieving an accuracy, within 2%, relative to the sun. SBRC has made field

measurements of the solar flux in March 1993 and in October 1993. Atmospheric transmission measurements for

these two field measurements were made bythe Universityof Arizona (Biggar et al. 1993). The differences in the

predicted on-orbit solar measurements from these two field

tests averaged 1.5%. The greatest difference was 3.6% in

band 8. These field measurements include uncertainties

in the measurement of the atmospheric transmission that

mayamount to 2 or 3%. The field test results also show

a consistencyin the SeaWiFS solar measurements at the

2-3% level. However, with the error source of atmospheric

transmission, ground based measurements cannot confirm

an accuracyof 2% in the SeaWiFS solar measurements.

Theydo, however, give some confidence that the instrument is capable of that accuracy.

18.2.2 Linearity

18.2.2.1 February 1993 Measurements

The measurements presented here were made in February1993. Theywere taken during the radiometric calibration of the instrument. The radiance measurements were

made at eight light levels to cover the radiance ranges of

the SeaWiFS bands. The results from those measurements

are given as the averages of 25 individual measurements for

each band (see Table 24). Due to the spectral shape of the

light from the integrating sphere, onlySeaWiFS band 1,

at 412 nm, produced output that did not saturate at any

of the radiance levels. For SeaWiFS band 2, light level 1

caused the output to saturate. For that band and that

light level, the band produced onlyits maximum digital

output. For band 3, two of the radiance levels caused the

band to saturate. This sequence continued up to band 8,

which saturated for seven of the eight light levels. However, this final light level was sufficient for the radiometric

calibration of band 8.

Table 24 gives the sensitivityfor SeaWiFS bands 1–4

at each light level. It is the consistencyof the instrument’s

sensitivitywith light level that is required bythe specifications. To check this sensitivity, the difference (in percent)

of the sensitivityat each light level from the average sensitivityfor the band was calculated. These differences should

be less than 1%. Except for one outlier in each band, all

differences are observed to be less than 1%. For light level

7, each band shows the sensitivityto be between 1.4% and

1.9% higher than the other levels. The review panel feels

that this discrepancydoes not come from the instrument,

but from the calibration of the integrating sphere. It occurs at one light level, and it is present in all bands.

With the knowledge of this discrepancy, the average

values for the bands in Table 24 have been calculated without the sensitivityfor light level 7. The inclusion of this

light level skews the results for the remaining measurements. In addition, it is interesting to note that the linearityof the SeaWiFS measurements is sufficientlygood

to reveal a 1.5–2.0% inconsistencyin the calibration of the

integrating sphere. It should also be noted that the consistencyof the calibration of the sphere is an integral part

of the linearitymeasurement.

18.2.2.2 November 1993 Measurements

Although not reported in the SCADP, a set of linearity

measurements was made bySBRC for the eight SeaWiFS

bands in November 1993. The results of these measurements are given in Table 25. The sensitivityof each band

is presented in the table at three radiance levels, approximating Ltypical, two-thirds Ltypical, and one-third Ltypical.

The least linear of the bands in Table 25 is band 2, with

differences of 1.2% and 1.3% from the average sensitivity.

As discussed above, the consistencyin the calibration of

the sphere is a fundamental part of these results. The review panel feels that imprecision in the repeatabilityof

the SBRC integrating sphere output has created a substantial portion of these differences. The repeatabilityof

band 2 from the February1993 measurements in Table 24

is significantlybetter than that in Table 25. As a result,

the review panel concludes that the linearities of the eight

SeaWiFS bands meet specification.

The results in Table 24 also indicate that the response

of the SeaWiFS bands are linear at low radiance levels.

As shown for bands 1 and 2 in Table 24, the response of

the instrument is linear for radiance levels corresponding

to output from 15–50 counts.

Table 24 lists onlythe results for SeaWiFS bands 1–4.

The results for the other four bands are consistent with

those presented here, at least to the extend that theycan

be checked. For band 8 in particular, it is difficult to check

linearityusing one point only.

19.0 SYSTEM NOISE

19.1 Requirement

The SNR shall be determined for all bands at a sufficient number of spectral radiance levels between 0.2 Ltypical

21
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 24. SeaWiFS radiometric measurements given for four SeaWiFS bands at eight light levels. The

sensitivityfor each band is calculated for each light level. The differences are the percent differences for the

average sensitivityfor each band. See text for details.

Band Light Counts Offset Net Radiance SensitivityDifference

No. Level Counts [mW cm−2μ m−1sr−1] [radiance/count] [%]

1 1 842.77 21.76 821.01 11.80 0.014373 −0.1

2 435.54 21.76 413.78 5.97 0.014428 0.3

3 257.50 21.76 235.74 3.40 0.014423 0.3

4 194.59 21.76 172.83 2.49 0.014407 0.2

5 111.85 21.76 90.09 1.29 0.014319 −0.4

6 54.03 21.76 32.27 0.46 0.014255 −0.9

7 39.60 21.76 17.84 0.26 0.014574 1.4

8 36.98 21.76 15.22 0.22 0.014455 0.5

Average sensitivity 0.014380

2 1 Band output saturates

2 710.43 19.46 690.97 9.56 0.013836 0.2

3 413.26 19.46 393.80 5.45 0.013840 0.2

4 308.19 19.46 288.73 3.99 0.013819 0.0

5 170.34 19.46 150.88 2.08 0.013786 −0.2

6 73.63 19.46 54.17 0.75 0.013845 0.2

7 49.35 19.46 29.89 0.42 0.014052 1.7

8 44.90 19.46 25.44 0.35 0.013758 −0.4

Average sensitivity 0.013814

3 1 Band output saturates

2 Band output saturates

3 872.75 21.05 851.70 9.28 0.010896 0.2

4 646.69 21.05 625.64 6.80 0.010869 −0.0

5 349.98 21.05 328.93 3.56 0.010823 −0.5

6 139.07 21.05 118.02 1.28 0.010846 −0.3

7 86.97 21.05 65.92 0.73 0.011074 1.8

8 75.92 21.05 54.87 0.60 0.010935 0.6

Average sensitivity 0.010874

4 1 Band output saturates

2 Band output saturates

3 Band output saturates

4 897.99 20.80 877.19 8.19 0.009337 −0.3

5 482.88 20.80 462.08 4.30 0.009306 −0.6

6 186.47 20.80 165.67 1.55 0.009356 −0.1

7 113.04 20.80 92.24 0.88 0.009540 1.9

8 97.02 20.80 76.22 0.72 0.009446 0.9

Average sensitivity 0.009361

and 0.9 Lmax to characterize the signal dependence of the

system noise.

19.2 Compliance

The SNRs for the eight SeaWiFS bands have been measured near Ltypical. These measurements were made using

the SBRC integrating sphere. Uncertainties in the output

of the sphere, both over the area of the output aperture and

over short intervals of time, have caused a small (but significant) decrease in the SNRs from these measurements.

SBRC has created a model of their instrument noise, based

on dark noise measurements and on the electronic design

of the instrument. These model-based SNRs are slightly

higher than the measured values at Ltypical, which is in

agreement with the review panel’s understanding of the

measurements.

The SeaWiFS SNR model gives the calculated results

22
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Table 25. SeaWiFS radiometric measurements given for all bands at three light levels. The differences are the

percent differences from the average sensitivityfor each band. These measurements were made in November

1993.

Band Counts Offset Net Radiance SensitivityDifference

No. Counts [mW cm−2μ m−1sr−1] [radiance/count] [%]

1 695.60 20.84 674.76 9.246 0.013703 −0.5

516.70 20.84 495.86 6.817 0.013748 −0.1

268.60 20.84 247.76 3.432 0.013852 0.6

Average sensitivity 0.013768

2 701.30 18.00 683.30 9.122 0.013350 −1.3

491.60 18.00 473.60 6.485 0.013693 1.2

258.30 18.00 240.30 3.257 0.013554 0.2

Average sensitivity 0.013532

3 709.00 20.45 688.55 7.216 0.010480 −0.4

503.80 20.45 483.35 5.076 0.010502 −0.2

248.50 20.45 228.05 2.414 0.010585 0.6

Average sensitivity 0.010522

4 699.50 20.12 679.38 6.212 0.009144 −0.8

480.30 20.12 460.18 4.223 0.009177 −0.5

248.60 20.12 228.48 2.134 0.009340 1.3

Average sensitivity 0.009220

5 703.30 22.00 681.30 5.073 0.007446 −0.4

527.60 22.00 505.60 3.791 0.007498 0.3

281.40 22.00 259.40 1.940 0.007479 0.1

Average sensitivity 0.007474

6 590.60 23.10 567.50 2.427 0.004277 0.4

450.90 23.10 427.80 1.835 0.004289 0.7

195.70 23.10 172.60 0.727 0.004212 −1.1

Average sensitivity 0.004259

7 706.50 22.73 683.77 2.057 0.003008 0.8

347.10 22.73 324.37 0.966 0.002978 −0.2

209.60 22.73 186.87 0.555 0.002970 −0.5

Average sensitivity 0.002985

8 526.40 20.14 506.26 1.075 0.002123 −0.4

365.50 20.14 345.36 0.734 0.002125 −0.3

210.30 20.14 190.16 0.408 0.002146 0.7

Average sensitivity 0.002131

from the SBRC generated values for instrument noise. The

SBRC results give the SNRs for the eight SeaWiFS bands

at three radiance levels: Ltypical, one-half Ltypical, and onequarter Ltypical. The Ltypical radiance levels are listed in

Barnes and Holmes (1993). Theyare also given as part

of Table 26. The prelaunch calibration coefficients used to

convert the counts from the instrument into the measured

radiance and vice versa are found in Table 6 of Barnes et

al. (1994).

The model presented here gives the noise from each

band, in counts, as a function of the number of counts

in the measurement bythat band. The noise values that

form the basis for the model have been calculated from

the counts and from their associated SNRs in the SCADP

data. The counts of noise for one-quarter Ltypical in bands

3 and 4 in the SBRC data were smaller than the accepted

value for digitization noise (0.289 count). As a result, it

was concluded that digitization noise had not been incorporated into the SBRC data. Digitization noise is included

in the SeaWiFS noise model presented in Table 26. The

addition of digitization noise to the model lowers the SNRs

in the model’s results. However, the model SNRs remain

slightlyhigher than the measured results.

Noise values from the calibration data show a strong

linear dependence with the measured counts. Based on

this linear dependence, a noise value for each band has

23
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 26. SNRs calculated from the SeaWiFS noise model. The noise model is a linear function of the counts

from each SeaWiFS band. This model is given for the standard gain (gain 1) and the standard detector configuration (TDI 4) for the SeaWiFS ocean measurements. The noise values in the model incorporate digitization

noise.

Ltypical Ltypical Noise Intercept Noise Slope Noise† SNR

Band No. [mW cm−2μ m−1sr−1] [counts] [counts] [counts/count] [counts] at Ltypical

1 9.10 638.4 0.420 0.0003528 0.645 990

2 8.41 618.2 0.372 0.0003141 0.566 1,091

3 6.56 613.0 0.348 0.0002875 0.524 1,170

4 5.64 612.2 0.352 0.0002928 0.531 1,152

5 4.57 607.1 0.375 0.0003178 0.568 1,069

6 2.46 584.9 0.510 0.0004083 0.749 781

7 1.61 531.6 0.424 0.0003674 0.619 859

8 1.09 513.5 0.494 0.0004144 0.707 726

† Noise at Ltypical

been calculated for zero radiance. The noise model for SeaWiFS band 7 is shown in Fig. 8. The linear noise model

for the eight SeaWiFS bands (intercepts and slopes) are

given in Table 26. For the review panel, the SNRs in Table 26 give the best prelaunch estimates of the SNRs for

the instrument at the Ltypical levels.

As part of the SCADP data, there is a complete scan

line of data from the third outdoor field test on 1 November

1993. A description of the components of a solar scan

line are given in Woodward et al. (1993). For three of

the SeaWiFS bands, the solar diffuser measurements are

made using gain 1. The results for these bands from the

field test are given in Table 27. Theyare based on the

average counts from 25 consecutive measurements across

the diffuser. The noise values in Table 27 represent one

standard deviation about each average. The linear model

in Table 26 is used to convert those noise counts into the

noise values at Ltypical. The SNRs for bands 3, 4, and 5 in

Table 27 compare favorablywith those in Table 26.

Table 27. Measured SNRs from a SeaWiFS solar

scan. For bands 3, 4, and 5, the solar diffuser measurements are made using gain 1. The noise model

in Table 26 is used to convert the noise values from

the measured count levels to those at Ltypical. These

SNRs compare favorablywith those in Table 26.

Band Average Noise Noise† SNR†

No. Counts [counts] [counts]

3 282.0 0.400 0.495 1,238

4 331.2 0.431 0.513 1,193

5 421.5 0.571 0.630 963

† At Ltypical

For the review panel, the results from the solar scan

indicate that there has been a problem with SBRC SNR

measurements for band 5 (Section 10), the cause of which

is undetermined. The SBRC measurements for the calibration data book (SNR of 690 for band 5) show the instrument to meet the specifications for that band. However,

the measured results for band 5 in the SCADP data are

also substantiallylower than those from previous laboratorymeasurements bySBRC. The review panel feels that

the value of 1,069 from Table 26 gives a more proper value

for the SNR for band 5. This value, along with those for

the other seven bands, remains a prelaunch estimate. An

extensive series of on-orbit measurements (McClain et al.

1992 and Woodward et al. 1993) will be used to obtain

an improved set of SNRs. A summaryof the prelaunch

SNR model for SeaWiFS bands 1–4 is presented in Table 28. A similar summaryfor bands 5–8 is presented in

Table 29.

20. POINTINGKNOWLEDGE

Pointing knowledge is a system level requirement. It

includes knowledge of the nadir vector and scan plane of

the instrument relative to the spacecraft; of the location

of the spacecraft on orbit; and of the yaw, pitch, and roll

angles of the spacecraft relative to the Earth.

20.1 Requirement

The contractor shall provide in the downlinked data

stream, data describing the spacecraft attitude and location and sensor pointing angles required for calculation of

the location (in latitude and longitude) of each ocean IFOV

to within one IFOV at all scan and tilt angles.

20.2 Compliance

SBRC has provided OSC with the pointing coordinates

for the SeaWiFS radiometer, information also found in the

SCADP. These coordinates are given with respect to a set

of transfer mirrors mounted on the radiometer. These values, from the instrument, are onlypart of the information

required for pointing knowledge, since OSC must transfer

these coordinates into their own system for the spacecraft

24
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Fig. 8. Results of the noise model for SeaWiFS band 7. The input values for the model come from

the SBRC calibration data book. The top figure shows the noise model for SeaWiFS. The curve gives

the results of the noise model. The symbols give the input noise values at Ltypical, one-half Ltypical,

and one-quarter Ltypical. These three values include digitization noise. The bottom figure displays the

calculated SNRs for SeaWiFS. The curve gives the calculated results from the model. The symbols give

the input values from SBRC, with digitization noise added.

25
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 28. SNR model for SeaWiFS bands 1–4. The counts at each reference level are calculated using Table 6 of

Barnes et al. (1994). The noise values are calculated using Table 20.

Radiance Band 1 Band 2 Band 3 Band 4

[mW cm−2μ m−1sr−1] Counts SNR Counts SNR Counts SNR Counts SNR

11.30 793 1,133

11.10 779 1,121

10.90 765 1,109

10.70 751 1,096 787 1,271

10.50 737 1,083 772 1,256

10.30 723 1,071 757 1,242

10.10 709 1,058 742 1,227

9.90 695 1,044 728 1,212

9.70 680 1,031 713 1,196

9.50 666 1,017 698 1,181

9.30 652 1,003 684 1,165

9.10 638 989 669 1,149

8.90 624 975 654 1,133

8.70 610 961 640 1,116

8.50 596 946 625 1,100

8.30 582 931 610 1,083 776 1,358

8.10 568 916 595 1,065 757 1,338

7.90 554 900 581 1,048 738 1,318

7.70 540 885 566 1,030 719 1,297

7.50 526 869 551 1,011 701 1,275

7.30 512 853 537 993 682 1,254

7.10 498 836 522 974 663 1,231 771 1,334

6.90 484 819 507 955 645 1,209 749 1,311

6.70 470 802 493 935 626 1,186 727 1,287

6.50 456 785 478 915 607 1,162 706 1,263

6.30 442 767 463 895 589 1,138 684 1,238

6.10 428 749 448 874 570 1,114 662 1,213

5.90 414 731 434 853 551 1,088 640 1,187

5.70 400 713 419 832 533 1,063 619 1,160

5.50 386 694 404 810 514 1,037 597 1,133

5.30 372 675 390 788 495 1,010 575 1,105

5.10 358 655 375 766 477 983 554 1,077

4.90 344 635 360 742 458 955 532 1,048

4.70 330 615 346 719 439 926 510 1,018

4.50 316 594 331 695 420 897 488 987

4.30 302 573 316 671 402 867 467 955

4.10 288 552 301 646 383 836 445 923

3.90 274 530 287 621 364 805 423 889

3.70 260 507 272 595 346 773 402 855

3.50 246 485 257 568 327 740 380 820

3.30 232 461 243 541 308 706 358 784

3.10 217 438 228 514 290 672 336 747

2.90 203 414 213 486 271 636 315 709

2.70 189 389 198 457 252 600 293 669

2.50 175 364 184 428 234 563 271 629

2.30 161 338 169 398 215 524 250 587

2.10 147 312 154 367 196 485 228 544

26
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Table 28. (cont.) SNR model for SeaWiFS bands 1–4. The counts at each reference level are calculated using Table

6 of Barnes et al. (1994). The noise values are calculated using Table 20.

Radiance Band 1 Band 2 Band 3 Band 4

[mW cm−2μ m−1sr−1] Counts SNR Counts SNR Counts SNR Counts SNR

1.90 133 285 140 336 178 445 206 500

1.70 119 258 125 304 159 403 185 454

1.50 105 230 110 271 140 361 163 407

1.30 91 202 96 238 121 317 141 359

1.10 77 173 81 203 103 272 119 309

0.90 63 143 66 168 84 226 98 257

0.70 49 112 51 133 65 178 76 203

0.50 35 81 37 96 47 129 54 148

0.30 21 49 22 58 28 79 33 90

0.10 7 17 7 20 9 27 11 31

Table 29. SNR model for SeaWiFS bands 5–8. The counts at each reference level are calculated using Table 6 of

Barnes et al. (1994). The noise values are calculated using Table 20.

Radiance Band 5 Band 6 Band 7 Band 8

[mW cm−2μ m−1sr−1] Counts SNR Counts SNR Counts SNR Counts SNR

5.75 764 1,237

5.65 751 1,223

5.55 737 1,210

5.45 724 1,197

5.35 711 1,183

5.25 697 1,169

5.15 684 1,155

5.05 671 1,141

4.95 658 1,126

4.85 644 1,111

4.75 631 1,096

4.65 618 1,081

4.55 604 1,066

4.45 591 1,050

4.35 578 1,034

4.25 565 1,018

4.15 551 1,002

4.05 538 985

3.95 525 969

3.85 511 951

3.75 498 934

3.65 485 916

3.55 472 899

3.45 458 880

3.35 445 862

3.25 432 843

3.15 418 824 749 918

3.05 405 804 725 900

2.95 392 785 701 881

2.85 379 764 678 861

2.75 365 744 654 842

2.65 352 723 630 821

27
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Table 29. (cont.) SNR model for SeaWiFS bands 5–8. The counts at each reference level are calculated using

Table 6 of Barnes et al. (1994). The noise values are calculated using Table 20.

Radiance Band 5 Band 6 Band 7 Band 8

[mW cm−2μ m−1sr−1] Counts SNR Counts SNR Counts SNR Counts SNR

2.55 339 702 606 800

2.45 325 680 582 779

2.35 312 658 559 757

2.25 299 636 535 734 743 1,066

2.15 286 613 511 711 710 1,037

2.05 272 590 487 687 677 1,006

1.95 259 566 464 663 644 975

1.85 246 542 440 638 611 942

1.75 232 518 416 612 578 908

1.65 219 493 392 585 545 873

1.55 206 468 369 558 512 836 730 917

1.45 193 442 345 530 479 798 683 879

1.35 179 415 321 501 446 758 636 840

1.25 166 388 297 471 413 717 589 798

1.15 153 361 273 440 380 674 542 754

1.05 139 333 250 408 347 629 495 708

0.95 126 304 226 375 314 582 448 659

0.85 113 275 202 341 281 532 400 607

0.75 100 245 178 306 248 481 353 552

0.65 86 215 155 270 215 427 306 493

0.55 73 183 131 232 182 370 259 431

0.45 60 152 107 193 149 310 212 364

0.35 46 119 83 153 116 248 165 293

0.25 33 86 59 111 83 182 118 217

0.15 20 52 36 68 50 112 71 135

0.05 7 18 12 23 17 38 24 47

to provide on-orbit pointing vectors for SeaWiFS and SeaStar. Pointing knowledge is a system level function. Verification of pointing knowledge will be made using measurements of land targets on orbit. Verification of a sufficient

set of downlinked data must wait for the Pre-Ship Review

of the SeaStar spacecraft.

21. STABILITY & REPEATABILITY

Bias errors will be removed from the data during ground

processing in order to improve radiometric accuracy. To

accomplish this, the sensor data must be stable over time,

as defined below.

21.1 Short-Term Stability Requirement

Short-term stabilityapplies to time intervals less than

two weeks. This stabilityalso applies to radiometric responses corrected on the ground using calibration data.

The mean radiometric response of each spectral band, shall

not differ bymore than ±1% from another response measurement made while viewing the same source operating

at equal radiance levels, but separated byanytime period

up to two weeks. This includes the effects of perturbations

at the orbital period. This stabilityrequirement shall also

be met for short-term temperature excursions that may

be expected to occur during sunlit portions of the orbit.

Data from lunar views, corrected for secular changes in

lunar radiance exitance, collected on several consecutive

orbits while the moon is near full phase, shall be provided

with sufficient frequencyto assess short term and long term

stability.

21.2 Compliance

Due to the short period between the completion of instrument modification and the Pre-Ship Review, it was

not possible to test this specification before launch. The

requirements will require examination of measurements on

orbit. However, the radiometric calibration equations for

the SeaWiFS bands (Barnes et al. 1994) contain factors,

such as the temperature dependences for their radiometric

sensitivities, which will be applied on orbit. Lunar measurements (Woodward et al. 1993) are also planned for

28
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

each measurement opportunity. These measurements are

planned for twice each month, as the moon reaches 96%

of full before and after each full phase. The correction

for temperature dependence will also be applied to lunar

measurements.

21.3 Long-Term Stability Requirement

Long-term stabilityapplies to time intervals between

two weeks and 5 years. The mean corrected and calibrated

radiometric response of each spectral band shall not change

bymore than ±2% over these time intervals. Compliance

will be demonstrated byan estimate based upon shortterm tests plus analysis.

21.4 Compliance

As shown in Table 1, the SeaWiFS instrument was completed (after straylight modifications) on 22 November

1993. The post modification Pre-Ship Review was held

ten days later on 2 December, which was an insufficient

interval of time to test long-term stability. However, measurements bythe instrument before its modification to

ameliorate the effects of straylight indicated radiometric stabilityfrom the instrument at the 1% level over a

few months. In addition, solar measurements during “field

tests” of the instrument (Section 18) have shown a consistencyin the instrument’s measurements at the 2-3% level

from March to November 1993. These tests suggest, but

do not guarantee, long term stabilityon orbit. Long-term

stabilityduring the SeaWiFS mission will be monitored,

and instrument changes will be corrected, through a series

of lunar, solar, and ground based measurements (McClain

et al. 1992 and Woodward et al. 1993)

21.5 Band-to-Band Stability Requirement

The relative amplitude stabilitybetween all pairs of

spectral bands shall be better than ±0.5% measured at

full-scale, and ±1% at half-scale. Each band shall be exposed to a source, and the mean calibrated responses determined. To compare outputs between bands, the ratio of

the means shall be calculated for each band with respect

to a common band. In addition, ratios shall be calculated

for selected pairs of bands, which will be used in common

retrieval algorithms. These ratios shall remain constant,

within ±0.5% at full-scale and ±1% at half scale, over

times separated byanyinterval up to two weeks.

21.6 Compliance

As discussed in Sections 21.2 and 21.4 above, the time

period between the completion of the SeaWiFS instrument

and the Pre-Ship Review was less than two weeks. This

short time period was instituted in an effort to help ensure

the launch of SeaWiFS and SeaStar at the earliest possible

date. The practical requirements for an earlylaunch have

eliminated the period of time for extended testing bythe

manufacturer. However, the anecdotal information in Section 21.4 indicates that the performance of the instrument

is at the level required bythis specification.

22. IN-FLIGHT DATA

22.1 Requirement

Data for calibration and stabilitymonitoring shall be

obtained from direct lunar views when the moon is greater

than 80% full phase, and either an onboard stabilized

source viewed byall optical elements or a solar diffuser.

The sources shall fill the optical aperture of the sensor.

These data shall be obtained for all channels with a SNR

no less than 10% of the SNR values specified in Section 10,

and shall measure changes in gain or throughput of the optical, focal plane, and electronic subsystems, using either

onboard, lunar, or solar sources. In-flight radiometric characterization, i.e., output digital value versus input spectral

radiance, shall be made with sufficient accuracyto assure

that the calibration and stabilityrequirements delineated

in this specification are achieved.

22.1.1 Lunar Calibration

Provision shall be made to use the moon at near full

phase as a target source for monitoring stability. The lowest (least sensitive) gain shall accommodate direct viewing

at near full lunar phase without saturation in anyband.

22.1.2 Solar Diffuser

If a solar illuminated diffuser is selected, data on the

diffuser characteristics shall be provided which, when combined with data from other calibration systems, will be adequate to maintain knowledge of the calibration and stabilityof the radiometric data to within stated specifications

throughout the five-year mission lifetime.

22.1.3 Internal Source

In-flight data on characteristics of onboard sources, to

show performance within the specifications, are required

(should that approach be taken).

22.2 Compliance

The SeaWiFS radiometer has been specificallydesigned

to make both lunar and solar diffuser measurements. There

are no internal sources within the instrument. As discussed

in Section 15.2, there are gains for each SeaWiFS band

specificallyset for the expected on-orbit solar and lunar

radiances. Also, as discussed extensivelyin the SeaWiFS

Technical Report Series (McClain et al. 1992, Woodward

et al. 1993, and Barnes et al. 1994) and Biggar et al. 1993,

lunar and solar diffuser measurements form a fundamental

part of SeaWiFS on-orbit calibrations.

29
Prelaunch Acceptance Report for the SeaWiFS Radiometer

23. SUMMARY

This technical memorandum contains onlya portion of

the information from the prelaunch characterization and

calibration of the SeaWiFS radiometer. The SCADP is

much more extensive. However, as discussed in this acceptance report, the requirements of the SeaWiFS specifications have determined the design of the instrument.

Compliance with these specifications has defined the fundamental operation of SeaWiFS.

This report summarizes prelaunch analyses that have

been made bythe review group, who are the co-authors of

this technical memorandum. The review group concludes

that, in the period between the deliveryand the launch of

the instrument, the SeaWiFS radiometer meets or exceeds

all applicable specifications. Within the restriction that

the instrument has not yet flown, SeaWiFS is found to

be acceptable. However, the complete set of the information necessaryfor the acceptance, or rejection, of SeaWiFS

and SeaStar is not yet available. There must be an extensive analysis of the on-orbit operational characteristics of

SeaWiFS before a final judgement about the acceptability

of the ocean color data set obtained bySeaWiFS can be

made. The completion of this work will require information from 60 days of on-orbit operation by the satellite and

its instrument.

Glossary

A/D Analog-to-Digital

BTR Bright Target Recovery

DC Direct Current

FWHM Full-Width at Half-Maximum

GAC Global Area Coverage

GSFC Goddard Space Flight Center

HN (Polaroid) Not an acronym; a linear sheet polarizer

used to check the polarization sensitivity of bands

7 and 8.

HR (Polaroid) Not an acronym; a linear sheet polarizer

used to check the polarization sensitivity of bands

1–6.

IFOV Instantaneous Field-Of-View

IR Infrared.

LAC Local Area Coverage

MTF Modulation Transfer Function

NASA National Aeronautics and Space Administration

NEdL Noise Equivalent Differential Spectral Radiance

NIST National Institute of Standards and Technology

OCDM Ocean Color Data Mission

OSC Orbital Sciences Corporation

SBRC Santa Barbara Research Center

SCADP SeaWiFS Calibration and Acceptance Data Package

SeaWiFS Sea-viewing Wide Field-of-view Sensor

SNR Signal-to-Noise Ratio

TDI Time Delay and Integration

Symbols

b1(k) Input data for polarization calculations for SeaWiFS

band 1.

b7(k) Input data for polarization calculations for SeaWiFS

band 7.

G Gain factor.

Gn Gain factor at gain setting n.

h(k) Residual values without the calculated sinusoidal response.

Imax Recorded maximum instrument output in response

to linearly polarized light.

Imin Recorded minimum instrument output in response

to linearly polarized light.

Lcloud Maximum radiance from reflected light off of clouds.

Lmax Maximum saturation radiance.

Ltypical Expected radiance from the ocean measured on orbit.

n Gain setting.

P F Polarization factor.

Si Initial detector signal.

Sn Detector signal with gain.

x Abscissa or longitudinal coordinate, or the pixel

number within a scan line depending on usage.

y Ordinate or meridional coordinate.

z Mantissa coordinate.

σ One standard deviation.

References

Barnes, R.A., and A. Holmes, 1993: Overview of the SeaWiFS

Ocean Sensor. Proc. SPIE, 1,939, 224–232.

Barnes, R.A., A. Holmes, W.L. Barnes, W.E. Esaias, and C.R.

McClain, 1994: The SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization, NASA Tech. Memo.

104566, Vol. 23 S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, (in press).

Biggar, S.F., P. Slater, K. Thome, A. Holmes, and R. Barnes,

1993: Preflight solar-based calibration of SeaWiFS, Proc.

SPIE, 1,939, 233–242.

McClain, C.R., W.E. Esaias, W.L. Barnes, B. Guenther, D.

Endres, S.B. Hooker, B. Mitchell, and R.A. Barnes, 1992:

SeaWiFS Calibration and Validation Plan, NASA Tech.

Memo. 104566, Vol. 3, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 41 pp.

30
R.A. Barnes, W.L. Barnes, W.E. Esaias, and C.R. McClain

Walker, J.H., C. Cromer, and J. McLean, 1991: A technique

for improving the calibration of large-area sphere sources.

Proc. SPIE, 1,493, 224–230.

Woodward, R.H., R.A. Barnes, C.R. McClain, W.E. Esaias,

W.L. Barnes, and A.T. Mecherikunnel, 1993: Modeling

of the SeaWiFS Solar and Lunar Observations. NASA

Tech. Memo. 104566, Vol. 10, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 26 pp.

The SeaWiFS Technical Report Series

Vol. 1

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

C.R. McClain, 1992: An Overview of SeaWiFS and Ocean

Color. NASA Tech. Memo. 104566, Vol. 1, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, 24 pp., plus color plates.

Vol. 2

Gregg, W.W., 1992: Analysis of Orbit Selection for SeaWiFS:

Ascending vs. Descending Node. NASA Tech. Memo.

104566, Vol. 2, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 16 pp.

Vol. 3

McClain, C.R., W.E. Esaias, W. Barnes, B. Guenther, D. Endres, S. Hooker, G. Mitchell, and R. Barnes, 1992: Calibration and Validation Plan for SeaWiFS. NASA Tech.

Memo. 104566, Vol. 3, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 41 pp.

Vol. 4

McClain, C.R., E. Yeh, and G. Fu, 1992: An Analysis of GAC

Sampling Algorithms: A Case Study. NASA Tech. Memo.

104566, Vol. 4, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 22 pp., plus color plates.

Vol. 5

Mueller, J.L., and R.W. Austin, 1992: Ocean Optics Protocols

for SeaWiFS Validation. NASA Tech. Memo. 104566, Vol.

5, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 43 pp.

Vol. 6

Firestone, E.R., and S.B. Hooker, 1992: SeaWiFS Technical

Report Series Summary Index: Volumes 1–5. NASA Tech.

Memo. 104566, Vol. 6, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 9 pp.

Vol. 7

Darzi, M., 1992: Cloud Screening for Polar Orbiting Visible

and IR Satellite Sensors. NASA Tech. Memo. 104566, Vol.

7, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 7 pp.

Vol. 8

Hooker, S.B., W.E. Esaias, and L.A. Rexrode, 1993: Proceedings of the First SeaWiFS Science Team Meeting. NASA

Tech. Memo. 104566, Vol. 8, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 61 pp.

Vol. 9

Gregg, W.W., F.C. Chen, A.L. Mezaache, J.D. Chen, J.A.

Whiting, 1993: The Simulated SeaWiFS Data Set, Version 1. NASA Tech. Memo. 104566, Vol. 9, S.B. Hooker,

E.R. Firestone, and A.W. Indest, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 17 pp.

Vol. 10

Woodward, R.H., R.A. Barnes, C.R. McClain, W.E. Esaias,

W.L. Barnes, and A.T. Mecherikunnel, 1993: Modeling

of the SeaWiFS Solar and Lunar Observations. NASA

Tech. Memo. 104566, Vol. 10, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 26 pp.

Vol. 11

Patt, F.S., C.M. Hoisington, W.W. Gregg, and P.L. Coronado,

1993: Analysis of Selected Orbit Propagation Models for

the SeaWiFS Mission. NASA Tech. Memo. 104566, Vol.

11, S.B. Hooker, E.R. Firestone, and A.W. Indest, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 16 pp.

Vol. 12

Firestone, E.R., and S.B. Hooker, 1993: SeaWiFS Technical Report Series Summary Index: Volumes 1–11. NASA Tech.

Memo. 104566, Vol. 12, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 28 pp.

Vol. 13

McClain, C.R., K.R. Arrigo, J. Comiso, R. Fraser, M. Darzi,

J.K. Firestone, B. Schieber, E-n. Yeh, and C.W. Sullivan, 1994: Case Studies for SeaWiFS Calibration and Validation, Part 1. NASA Tech. Memo. 104566, Vol. 13,

S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 52 pp., plus

color plates.

Vol. 14

Mueller, J.L., 1993: The First SeaWiFS Intercalibration RoundRobin Experiment, SIRREX-1, July 1992. NASA Tech.

Memo. 104566, Vol. 14, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 60 pp.

Vol. 15

Gregg, W.W., F.S. Patt, and R.H. Woodward, 1994: The Simulated SeaWiFS Data Set, Version 2. NASA Tech. Memo.

104566, Vol. 15, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 42 pp., plus color plates.

Vol. 16

Mueller, J.L., B.C. Johnson, C.L. Cromer, J.W. Cooper, J.T.

McLean, S.B. Hooker, and T.L. Westphal, 1994: The Second SeaWiFS Intercalibration Round-Robin Experiment,

SIRREX-2, June 1993. NASA Tech. Memo. 104566, Vol.

16, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 121 pp.

Vol. 17

Abbott, M.R., O.B. Brown, H.R. Gordon, K.L. Carder, R.E.

Evans, F.E. Muller-Karger, and W.E. Esaias, 1994: Ocean

Color in the 21st Century: A Strategy for a 20 -Year Time

Series. NASA Tech. Memo. 104566, Vol. 17, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, 20 pp.

31
Prelaunch Acceptance Report for the SeaWiFS Radiometer

Vol. 18

Firestone, E.R., and S.B. Hooker, 1994: SeaWiFS Technical Report Series Summary Index: Volumes 1–17. NASA Tech.

Memo. 104566, Vol. 18, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, (in press).

Vol. 19

McClain, C.R., R.S. Fraser, J.T. McLean, M. Darzi, J.K. Firestone, F.S. Patt, B.D. Schieber, R.H. Woodward, E-n. Yeh,

S. Mattoo, S.F. Biggar, P.N. Slater, K.J. Thome, A.W.

Holmes, R.A. Barnes, and K.J. Voss, 1994: Case Studies

for SeaWiFS Calibration and Validation, Part 2. NASA

Tech. Memo. 104566, Vol. 19, S.B. Hooker, E.R. Firestone,

and J.G. Acker, Eds., NASA Goddard Space Flight Center,

Greenbelt, Maryland, 73 pp.

Vol. 20

Hooker, S.B., C.R. McClain, J.K. Firestone, T.L. Westphal,

E-n. Yeh, and Y. Ge, 1994: The SeaWiFS Bio-Optical

Archive and Storage System (SeaBASS), Part 1. NASA

Tech. Memo. 104566, Vol. 20, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 40 pp.

Vol. 21

Acker, J.G., 1994: The Heritage of SeaWiFS: A Retrospective on the CZCS NIMBUS Experiment Team (NET) Program. NASA Tech. Memo. 104566, Vol. 21, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, (in press).

Vol. 22

Barnes, R.A., W.L. Barnes, W.E. Esaias, and C.R. McClain,

1994: Prelaunch Acceptance Report for the SeaWiFS Radiometer. NASA Tech. Memo. 104566, Vol. 22, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 32 pp.

32
 Form Approved

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 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

 August 1994 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

6. AUTHOR(S)

 Robert A. Barnes, William L. Barnes, Wayne E. Esaias, and Charles R. McClain

Laboratory for Hydrospheric Processes

Goddard Space Flight Center 94B00126

Greenbelt, Maryland 20771

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

National Aeronautics and Space Administration

Washington, D.C. 20546–0001 TM–104566, Vol. 22

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

 Robart A. Barnes: ManTech, Inc., Wallops Island, Virginia; Elaine R. Firestone: General Sciences Corporation, Laurel,

 Maryland; and James G. Acker: Hughes STX, Lanham, Maryland

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS 15. NUMBER OF PAGES

 16. PRICE CODE

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

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

 REPORT NUMBER

Unclassified–Unlimited

Subject Category 48

 -

SeaWiFS, Oceanography, Acceptance, Radiometer, Dark Level, Fore-and-Aft Pointing 32

Requirement, Compliance, Polarization, Pointing Knowledge

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

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

The final acceptance, or rejection, of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) will be determined by the

instrument's on-orbit operation. There is, however, an extensive set of laboratory measurements describing the operating

characteristics of the radiometer. Many of the requirements in the Ocean Color Data Mission (OCDM) specifications can

be checked only by laboratory measurements. Here, the calibration review panel (composed of the authors of this

technical memorandum) examines the laboratory characterization and calibration of SeaWiFS in the light of the OCDM

performance specifications. Overall, the performance of the SeaWiFS instrument meets or exceeds the requirements of

the OCDM Contract in all but a few unimportant details. The detailed results of this examination are presented here by

following the outline of the specifications, as found in the Contract. The results are presented in the form of requirement

and compliance pairs. These results give conclusions on many, but not all, of the performance specifications. The

acceptance by this panel of the performance of SeaWiFS must only be considered as an intermediate conclusion. The

ultimate acceptance (or rejection) of the SeaWiFS data set will rely on the measurements made by the instrument on

orbit.

 Unclassified Unclassified Unclassified Unlimited

 REPORT DOCUMENTATION PAGE OMB No. 0704-0188

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI Std. 239-18, 298-102

 SeaWiFS Technical Report Series

Volume 22–Prelaunch Acceptance Report for the SeaWiFS Radiometer Code 970.2

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

 Technical Editor: James G. Acker