For instruments that carry onboard solar diffusers to orbit, such as the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), it is possible to convert the instrument’s reflectance measurements to radiance measurements by knowledge of the solar irradiance. This process, which generally requires the application of a solar irradiance model, is described. The application of the irradiance model is separate from the measurements by the instrument and from the instrument’s reflectance calibration. In addition, SeaWiFS was calibrated twice before launch for radiance response by use of radiance sources with calibrations traceable to the National Institute of Standards and Technology. With the inclusion of the at-launch diffuser-based radiance calibration, SeaWiFS has three possible radiance calibrations for the start of on-orbit operations. The combination of these three into a single calibration requires changes of 4% or less for the current at-launch radiance calibration of the instrument. Finally, this process requires changes of 4% or less for the reflectance calibration coefficients to provide consistency among the radiance calibration, the reflectance calibration, and the solar irradiance.
Robert A. Barnes, Robert E. Eplee, Stuart F. Biggar, Kurtis J. Thome, Edward F. Zalewski, Philip N. Slater, and Alan W. Holmes Appl. Opt. 39(30) 5620-5631 (2000)
Robert E. Eplee, Wayne D. Robinson, Sean W. Bailey, Dennis K. Clark, P. Jeremy Werdell, Menghua Wang, Robert A. Barnes, and Charles R. McClain Appl. Opt. 40(36) 6701-6718 (2001)
Robert E. Eplee, Gerhard Meister, Frederick S. Patt, Robert A. Barnes, Sean W. Bailey, Bryan A. Franz, and Charles R. McClain Appl. Opt. 51(36) 8702-8730 (2012)
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The irradiances are in units of mW cm-2 μm-1. The wavelengths are the nominal center wavelengths for each band.
Ref. 14.
Refs. 16 and 17.
Refs. 18 and 19.
Ref. 20.
Table 2
Percent Differences of the Irradiances of Wehrli, modtran, and Thuillier et al. from the Irradiances of Neckel and Labsa
The calibration coefficients are calculated with Eq. (8) and the constants in Tables
1 and 3. There is one coefficient for each SeaWiFS band and each solar irradiance model. The preferred calibration coefficients are those derived from the model of Thuillier et al.20
Ref. 14.
Refs. 16 and 17.
Refs. 18 and 19.
Ref. 20.
The diffuser BRDF values for the SeaWiFS bands at the time of the SRBC, FD(tS), are the same as those at the start of on-orbit operations FD(t0) from Table
3. This equality is based on the results of the transfer-to-orbit experiment.13 The net digital numbers DNC(tS) come from Table
3 of Barnes et al.13
Table 6
Derived Values of kS
(t
S
) from the Prelaunch SRBC of SeaWiFS (units of mW cm
-2
sr
-1
μm
-1
DN
-1
)
a
The calibration coefficients are calculated by Eq. (14) and the constants in Tables
1 and 5. There is one coefficient for each SeaWiFS band and each solar irradiance model. The preferred calibration coefficients are those derived from the model of Thuillier et al.20
Ref. 14.
Refs. 16 and 17.
Refs. 18 and 19.
Ref. 20.
Table 7
Calibration Coefficients from the Two Prelaunch Laboratory Radiance-Based Calibrations of SeaWiFS (units of mW cm-2 sr-1 μm-1 DN-1)a
SeaWiFS Band
1993 Laboratory Calibration k
1993
(t
0
)
1997 Laboratory Calibration k
1997
(t
0
)
1
0.014201
0.013845
2
0.013541
0.013423
3
0.010655
0.010698
4
0.009189
0.009213
5
0.007483
0.007615
6
0.004226
0.004360
7
0.003013
0.003110
8
0.002136
0.002223
Both calibrations were made with integrating spheres as radiance sources. The coefficients from the 1997 calibration, k1997(t0), are the official prelaunch calibration coefficients for the instrument.11
Table 8
Calculation of the Revised At-Launch Radiance Calibration Coefficients kL*
(t
0
)
a
SeaWiFS Band
kL(t
0
)
1993 Laboratory Calibration k
1993
(t
0
)
1997 Laboratory Calibration k
1997
(t
0
)
kL*(t
0
)
1
0.013969
0.014201
0.013845
0.014005
2
0.013332
0.013541
0.013423
0.013432
3
0.010325
0.010655
0.010698
0.010559
4
0.008898
0.009189
0.009213
0.009100
5
0.007239
0.007483
0.007615
0.007446
6
0.004067
0.004226
0.004360
0.004218
7
0.002884
0.003013
0.003110
0.003002
8
0.002094
0.002136
0.002223
0.002151
These values are the averages of the on-orbit [kL(t0)] and the two prelaunch [k1993(t0) and k1997(t0)] coefficients. The on-orbit coefficients come from Table
4, and the prelaunch coefficients come from Table
7. The units for the coefficients are mW cm-2 sr-1 μm-1 DN-1.
Table 9
Calculation of the Revised At-Launch Reflectance Calibration Coefficients kF*
(t
0
)
a
These coefficients are calculated as the revised at-launch radiance coefficients kL
*
(t0) divided by the solar irradiances EM,B. The irradiances come from Thuillier et al.20
Tables (9)
Table 1
Irradiances from Four Solar Models Band Averaged with the SeaWiFS Spectral Responses by Eq. (4)a
The irradiances are in units of mW cm-2 μm-1. The wavelengths are the nominal center wavelengths for each band.
Ref. 14.
Refs. 16 and 17.
Refs. 18 and 19.
Ref. 20.
Table 2
Percent Differences of the Irradiances of Wehrli, modtran, and Thuillier et al. from the Irradiances of Neckel and Labsa
The calibration coefficients are calculated with Eq. (8) and the constants in Tables
1 and 3. There is one coefficient for each SeaWiFS band and each solar irradiance model. The preferred calibration coefficients are those derived from the model of Thuillier et al.20
Ref. 14.
Refs. 16 and 17.
Refs. 18 and 19.
Ref. 20.
The diffuser BRDF values for the SeaWiFS bands at the time of the SRBC, FD(tS), are the same as those at the start of on-orbit operations FD(t0) from Table
3. This equality is based on the results of the transfer-to-orbit experiment.13 The net digital numbers DNC(tS) come from Table
3 of Barnes et al.13
Table 6
Derived Values of kS
(t
S
) from the Prelaunch SRBC of SeaWiFS (units of mW cm
-2
sr
-1
μm
-1
DN
-1
)
a
The calibration coefficients are calculated by Eq. (14) and the constants in Tables
1 and 5. There is one coefficient for each SeaWiFS band and each solar irradiance model. The preferred calibration coefficients are those derived from the model of Thuillier et al.20
Ref. 14.
Refs. 16 and 17.
Refs. 18 and 19.
Ref. 20.
Table 7
Calibration Coefficients from the Two Prelaunch Laboratory Radiance-Based Calibrations of SeaWiFS (units of mW cm-2 sr-1 μm-1 DN-1)a
SeaWiFS Band
1993 Laboratory Calibration k
1993
(t
0
)
1997 Laboratory Calibration k
1997
(t
0
)
1
0.014201
0.013845
2
0.013541
0.013423
3
0.010655
0.010698
4
0.009189
0.009213
5
0.007483
0.007615
6
0.004226
0.004360
7
0.003013
0.003110
8
0.002136
0.002223
Both calibrations were made with integrating spheres as radiance sources. The coefficients from the 1997 calibration, k1997(t0), are the official prelaunch calibration coefficients for the instrument.11
Table 8
Calculation of the Revised At-Launch Radiance Calibration Coefficients kL*
(t
0
)
a
SeaWiFS Band
kL(t
0
)
1993 Laboratory Calibration k
1993
(t
0
)
1997 Laboratory Calibration k
1997
(t
0
)
kL*(t
0
)
1
0.013969
0.014201
0.013845
0.014005
2
0.013332
0.013541
0.013423
0.013432
3
0.010325
0.010655
0.010698
0.010559
4
0.008898
0.009189
0.009213
0.009100
5
0.007239
0.007483
0.007615
0.007446
6
0.004067
0.004226
0.004360
0.004218
7
0.002884
0.003013
0.003110
0.003002
8
0.002094
0.002136
0.002223
0.002151
These values are the averages of the on-orbit [kL(t0)] and the two prelaunch [k1993(t0) and k1997(t0)] coefficients. The on-orbit coefficients come from Table
4, and the prelaunch coefficients come from Table
7. The units for the coefficients are mW cm-2 sr-1 μm-1 DN-1.
Table 9
Calculation of the Revised At-Launch Reflectance Calibration Coefficients kF*
(t
0
)
a
These coefficients are calculated as the revised at-launch radiance coefficients kL
*
(t0) divided by the solar irradiances EM,B. The irradiances come from Thuillier et al.20