2. INFRARED CALIBRATION DATA
In orbit, the imagers and sounders
periodically view space and their on-board warm blackbodies to acquire data
for calibrating their infrared channels. Each
instrument's blackbody is in front of its entire optical chain and fills its
optical aperture. (This is in contrast to the Visible Infrared Spin-Scan
Radiometer [VISSR] and VISSR Atmospheric Sounder [VAS] instruments, carried on
earlier geostationary satellites. In those instruments, the calibration target
was behind the fore-optics, necessitating modelling of the radiative
contribution of the fore-optics in the calibration4.) The data acquired at the
blackbody looks (and associated space looks) are used for inferring the
instruments' calibration slopes (radiance increment per output count), which are
the reciprocals of the responsivities. The data from the space looks allow us to
infer the calibration intercepts (radiance at zero counts).
To preserve the precision of the measurements, the instruments are calibrated
often, because temperatures on a three-axis stabilized satellite such as GOES
vary diurnally by tens of degrees Kelvin. The diurnal variation of the
intercepts, which measure the intensity of the radiation emitted by the
instrument itself, is considerably greater than that of the slopes, which depend
weakly on such quantities as the background flux on the detectors and the
temperature of the electronics. Therefore, space looks are executed more
frequently than blackbody looks. For the imagers, another reason to view space
as frequently as possible is provided by the presence of 1/f noise5 in
the channels that use photoconductive HgCdTe detectors. The 1/f noise manifests
itself as a drift in the imager's output in the time between the space clamps.
While viewing space, the imagers execute their DC signal restores ("space
clamps"), which reset the zero-radiance output to a predetermined value
(nominally 970 counts). The more frequent the clamps, then, the less severe the
drifts. The sounders chop the signal from the scene at a frequency of
approximately 50Hz against opaque "teeth" located between the filters on the
filter wheels, which largely suppresses the effects of 1/f noise.
The intervals between calibration measurements are listed in Table 3. For the
imager, the space looks for calibration all involve space clamps. Currently, the
2.2-sec space-look interval is used for imaging the full Earth, and the 36.6-sec
interval for imaging smaller sectors. The 9.2-sec interval is not used for
routine imaging. This selection of intervals represents a compromise between
radiometric precision and scheduling requirements imposed by the NOAA/National
Weather Service.
Table 3. Intervals between calibration measurements
| Measurement
type |
Imager |
Sounder |
| Space |
2.2,
9.2, or 36.6 sec |
2 min |
| Blackbody |
30
min |
20
min |
For the sounder, a space look may interrupt a scan line in progress. A space
look consists of the acquisition of 40 samples of data at a location at least
0.5 degrees away from the Earth. Typically, this requires a total of
approximately 8 sec, of which 4 sec is devoted to taking the data and the rest
to slewing and settling of the scan mirror. A blackbody sequence, which may
interrupt a frame (a pre-defined rectangular target area on the Earth) in
progress, consists of acquisition of 40 samples from a view of space (4 sec),
data for a check of the linearity of the electronics (1.6 sec), and 40 samples
during the view of the blackbody (4 sec). The entire sequence requires
approximately 55 sec, of which approximately 45 sec are devoted to slewing and
settling of the scan mirror. The view of the blackbody occurs approximately 23
sec after the view of space.
For
the imager, space looks occur only during scan reversals, i.e., when the
direction of the scan mirror's motion
reverses between two scans in opposite
directions. The location is at least 0.5 degrees from the edge of the Earth.
The
sequence of events at the reversal is acquisition of approximately 400 samples
from a view of space, the DC restore (clamp), and another acquisition of 400
samples of data from a view of space immediately following the clamp. These
two
views of space are called the "pre-clamp" and "post-clamp" views, respectively.
Acquisition of 400 samples requires approximately 73 msec, and an entire scan
reversal, including the clamp and the two space views requires 200 msec. The
purpose of acquiring two sets of data at each space look is to combat the effect
of drifts during the period of data-taking on the Earth that occurs between
any
two spacelooks. The calibration intercepts are interpolated (see below) in
time from the post-clamp of the first space look to the pre-clamp of the second. Imager blackbody sequences occur every half hour between frames. A sequence
consists of acquisition of 400 samples from the post-clamp phase of a space look
(73 msec), 1000 samples during the view of the blackbody (183 msec), and 400
samples (73 msec) from the pre-clamp phase of the space look following the
blackbody. The entire sequence requires approximately 44 sec, and almost all of
that is dedicated to the slewing and settling of the scan mirror. The blackbody
observations take place approximately 18 seconds after the first space look and
18 seconds before the second. To minimize the effect of drifts that will almost
certainly affect the instrument outputs in the intervals between the views of
the blackbody and space, we interpolate the outputs from the two space looks to
the time of the blackbody look. This accounts for the linear component of the
drifts but does nothing to correct for higher-order components5.
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