Operational Calibration of the Imagers and Sounders
on the GOES-8 and -9 Satellites, Page 8
Michael Weinreb, Michael Jamieson, Nancy Fulton, Yen Chen, Joy Xie Johnson,
James Bremer, Carl Smith, and Jeanette Baucom
4.2 Derivation of emissivities
The scan-mirror emissivity values were determined soon after launch from in-orbit
observations of space between the extreme west and the extreme east. Data for a
number of scans were averaged to reduce random noise. For the sounders, the instrument
output varies not only with scan position, but with time, as the temperature of the
instrument changes over the minute or so required for each scan. (This is not a problem
for the imagers, which clamp on space every few seconds.) We removed the temperature-driven
drifts from the sounder data after estimating them by regression on the optics temperatures.
Emissivities were derived separately for each detector in each channel of each instrument.
In the following, we suppress reference to instrument, channel, and detector, since the procedure
was the same for all of them. Since measurements on space were available at all values of
q, not just at the east and west limits, the definition of the
calibration slope in Eqs. (7) and (8) could be simplified to
where Xsp(45) is the instrument output in counts when it views space at a scan
angle of 45o. The emissivity was given by
where RM is the radiance of the mirror computed from its temperature during the
scans, and Xsp(q) is the instrument's output in counts when it views space at
scan angle q. The emissivities at 45o in all imager and sounder channels were
provided from laboratory measurements on witness samples10.
The entire process was carried out for data collected approximately once an hour for 24 hours. A
daily-average profile of e vs q was then computed
as the average over all hours.
For each channel, we fitted a quadratic, i.e.,
to the profiles of emissivity vs q. (Also, the variable
q was transformed to become the east-west position of the scan mirror
expressed in inductosyn increment numbers, which is uniquely related to scan angle.) This produced the
coefficients a0, a1, and a2. They are stored in a database in the
ground-system computer and are used in the processing to generate the emissivities vs angle.
Approximately once every three months, new emissivity-vs-angle profiles are computed from east-west
scans of space, as previously described. Thus far, the profiles have changed slightly for some
instruments, and not at all for others, since the launches of the GOES-8 and -9 satellites.
Figure 2 attests to the success of the generalized calibration equations. It shows the radiances of
space vs scan angle derived from the original calibration equations ("before") and the generalized
equations ("after") in channels 4 (10.7 µm) and 5 (12 µm) of the GOES-8 imager. The generalized
equations removed apparent east-west radiance differences of approximately 1mW/(m2-sr-cm-1 ) and 2 mW/(m2-sr-cm-1) in channels 4 and 5, respectively.
Fig. 2. Radiances of space vs mechanical scan angle (degrees)
in channels 4 and 5 of GOES-8 Imager.
The label, "before", indicates data before correction for
scan-mirror emissivity variation; "after" indicates data after correction. The
uncorrected radiance values are negative because the calibration establishes
the zero-radiance level at the space clamp, which in this case was on the east.
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Contact Michael P. Weinreb at michael.weinreb@noaa.gov
Latest Revision: July 9, 1997
