NOAA Office of Satellite and Product Operations

Operational Calibration of the Imagers and Sounders
on the GOES-8 and -9 Satellites

Michael Weinreb, Michael Jamieson, Nancy Fulton, Yen Chen, Joy Xie Johnson,
James Bremer, Carl Smith, and Jeanette Baucom



6.3 Calibration

Although the visible detector-channels are not calibrated in orbit, calibration coefficients measured by ITT before launch6 are transmitted to users in the GVAR data stream. A factor that converts radiance to reflectance factor, or effective albedo, is included as well. Since detector responsivities can vary unpredictably, the pre-launch calibration may not be valid after launch.

The calibration equation is either

Equation 10
mathematical formula

or

Equation 11
mathematical formula

where X is the instrument output in counts, the subscript sp refers to the view of space, and m and b are the calibration coefficients measured before launch6. For each visible-channel detector, the radiance R is the average of the spectral (monochromatic) radiance over the spectral response function for that detector, i.e.,

mathematical formula

where l is wavelength in µm, F the spectral response function, and R(l) the spectral radiance of the target. Units of R are W/(m2-sr-µm).

The value of b in Eq. (10) depends on the electronic zero level. As was discussed in the preceding section, this level varies with a standard deviation of approximately one count for the imagers and tens of counts for the sounder. Therefore, when the satellite is in orbit, the value of b determined in the laboratory (or at any other earlier time) may not be valid. Equation (11) is preferred.

Furthermore, relativization and normalization affect the calibration. Currently, visible-channel data from the imagers, but not the sounders, are being normalized. Since normalization makes the responses from all eight imager detectors the same as that of the reference detector, users of the pre-launch calibration of the imagers should apply the calibration coefficients for the reference detector (identified in Table 5) to the data from all detectors.

With relativization enabled, an instrument's output at the view of space is modified slightly, and the value of b in the calibration equation (Eq. [10]) needs to be modified accordingly. For this reason also, the best approach is to use Eq. (11). If Eq. (10) must be used, then the value of b should be determined from the equation:  b = -mM0

mathematical formula

in which, for the imagers, m is the slope for the reference detector. For the sounders, it is the slope for an individual detector. Values of b determined in this way, as well as the values of m (from Ref. 6) and X0, appear in Tables 5 and 6.

The reflectance factor (or effective albedo) is obtained6 from the radiance by

mathematical formula

where

mathematical formula

and where H is the solar spectral irradiance H(l) averaged over the spectral response function of the visible detector, i.e.,

mathematical formula

Values of H were computed6 by ITT from tables of solar irradiance vs wavelength provided by Rossow et al.13, whose values are based on measurements by Neckel and Labs14. The values of A lie between 0 and 1. When A has the value of 1, it corresponds to the radiance of a perfectly reflecting diffuse surface illuminated at normal incidence when the sun is at its annual-average distance from the Earth. Values of k appear in Tables 5 and 6.



Table 5. Visible-channel calibration coefficients for imagers

Satellite GOES-8 GOES-9
Identity of reference detector
(number in physical array)
2 3
m (reference detector)
(W/(m2-sr-m-count))
0.5501873 0.5492361
X0 29 29
b
(W/(m2-sr-µm))
-15.955 -15.928
k
((m2-sr-µm)/W)
1.92979 x 10-3 1.94180 x 10-3


Table 6. Visible-channel calibration coefficients for sounders

Satellite Detector
No.
m
(W/(m2-sr-µm-count))
X0 b
(W/(m2-sr-µm))
k
((m2-sr-µm)/W)


GOES-8
1 6.482527 x 10-2

920
-59.64

2.2008 x 10-3
2 6.522216 x 10-2 -60.00
3 6.560241 x 10-2 -60.35
4 6.642020 x 10-2 -61.11


GOES-9
1 6.416324 x 10-2

920
-59.03

2.2919 x 10-3
2 6.427129 x 10-2 -59.13
3 6.523361 x 10-2 -60.01
4 6.489786 x 10-2 -59.71


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Contact Michael P. Weinreb at michael.weinreb@noaa.gov
Latest Revision: July 9, 1997