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GOES Calibration
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Correction For GOES Imager Midnight Calibration Errors
In the fall of 2003, NOAA/NESDIS proposes to make a change
in the calibration of the GOES Imagers to correct for the deleterious
effects of solar heating on the calibration
during the eight hours surrounding local midnight (0500 UT
for GOES-12, 0900 UT for GOES-10). At certain times of the
year (see below), data users may notice changes in measured
radiances reaching a maximum of approximately 1K (for a scene
at 300K) within an hour of midnight and decreasing with increasing
time before and after midnight. A brief description of the
midnight calibration errors and the correction algorithm follows.
The midnight blackbody calibration error affects the calibration
slopes that are computed in real time on orbit. The slope
is defined as the coefficient of the linear term in the radiometric
calibration equation, the equation relating scene radiance
to sensor output counts. It is computed once every 30 minutes
from data collected when the Imager views space and its internal
blackbody. At certain times of the year, we find that the computed slopes
may be in error by as much as 3% during the period around
local satellite midnight. The effect is most pronounced in
channels 2
and 3 (3.9 and 6.7 um, resp.) of the Imager, but it is also
noticed to a lesser extent in the channels at longer wavelengths.
For GOES-8, which was operational at 75W longitude from 1994
until April 1, 2003, the effect occurred primarily in the months
from April through October. For GOES-10, operational at 135W
longitude since 1998, the effect occurs almost year-round.
For GOES-12, operational at 75W longitude since April 1, 2003, the effect
has been present continuously so far, but at the time of this
writing (July 2003), we do not yet know if it occurs year round
or only
during part of the year. An example of the effect is shown
as the upward spikes in the time series of calibration slopes
displayed in the linked figure.
Such slope errors artificially depress measurements of brightness
temperature of the Earth by as much as 1K. This effect is believed
to be caused by extraneous radiation, probably from hot components
in the imager's scan-mirror cavity, reaching the Imager's detectors
during its calibration cycle.
The correction algorithm is based on the observation that when this effect is absent, there is a
very high correlation between the calibration slope and the temperature of several optics components,
particularly the telescope's primary mirror. Accordingly, when the effect occurs, we replace the bad
slope values by estimates computed by regression on the primary-mirror temperature. The figure shows
the original slopes that have been replaced (x's inside squares) by the regression estimates (diamonds),
which we believe are better estimates of the instrument's true slopes.
An accompanying report presents details of
the correction algorithm.
For even more information, see Johnson, R.X. and M.P. Weinreb, GOES-8 Imager Midnight Effects and Slope
Correction, in GOES-8 and Beyond, Edward R. Washwell, editor, Proc. SPIE 2812, pp 596-607 (1994).
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