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MBCC Implementation in Calibration
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IMPLEMENTATION OF MIDNIGHT BLACKBODY
CALIBRATION CORRECTION (MBCC)
Michael
Weinreb,
and Dejiang Han
NOAA NESDIS Office of Satellite Operations
(Revised, August 7, 2003)
I INTRODUCTION
At certain times of the year, the magnitudes of the computed calibration
slopes for the GOES Imagers' infrared channels exhibit anomalous dips during
the approximately eight hours centered on satellite midnight. The amplitudes
of the dips usually decrease with increasing wavelength. For GOES-8, the
anomalous dips occurred during the months between April and October. However,
GOES-10 experiences this phenomenon all year round. GOES-12, which became
operational April 1, 2003, has so far exhibited the effect all the time,
but we do not know whether it will continue year-round. (Data collected
during post-launch checkout in the fall of 2001 also exhibited the effect.)
A complete discussion of this effect and an earlier version of the algorithm
to correct for it is contained in Johnson and Weinreb (1996).
We believe these dips are errors in the slope computations, for two
reasons. First, studies by the University of Wisconsin correlate the dip
with errors--of the order of 1 K-- in the measurement of ocean surface
temperatures. We also have seen other reports, both informal and in the
scientific literature, of anomalous observations around midnight. In addition,
such errors in computed slopes were predicted in 1988 by ITT (Annable,
1988), the Imagers' manufacturer. By analysis of the instrument's design,
Annable showed that radiation from the Image Navigation and Registration
(INR) sunshields in the imager's scan cavity, which can reach temperatures
up to 350K, could be reflected by the imager's internal blackbody to its
detectors during the blackbody sequence. The extraneous radiation from
the sunshields would cause slope errors with characteristics similar to
the dips we see in orbit.
The midnight blackbody calibration correction (MBCC) algorithm, a modification
of the algorithm presented in Johnson and Weinreb (1996), will be implemented
in processing in the Sensor Processing System (SPS) at the Wallops Command
and Data Acquisition Station (WCDAS) to correct the calibration slopes.
A description of the MBCC algorithm follows. Shown in Fig. 1 is an example
of how the MBCC algorithm improves the calibration by reducing the magnitudes
of the dips around midnight.
Fig. 1. Original slopes, and slopes corrected by MBCC
II MIDNIGHT ALGORITHM IN THE SPS AT WCDAS
A. Overview
The GOES ground processing system uses Imager observations of space
and its internal blackbody to determine the calibration slope, represented
by the coefficient m in the calibration equation,
R = qX2 + mX + b, (1)
where R is radiance, q the coefficient of the quadratic
term (determined in laboratory testing before launch), X the output
of the Imager channel in 10-bit digital counts, and b the calibration
intercept. The blackbody looks and their associated calibrations occur
once every 30 minutes. In actual operations, the calibration equation contains
additional terms to account for the polarization-induced change of the
scan mirror's reflectivity with east-west scan angle. Furthermore, the
slopes are usually filtered to reduced noise. The filtering is done via
a two-hour running average of slopes from the current day and several previous
days. The filtered slopes are called the "mode-3" slopes. (The original,
unfiltered slopes are "mode 1"). For details on the calibration of the
Imager, see Weinreb et al. (1997).
In the MBCC algorithm, most steps will involve the responsivity rather
than the calibration slope. The responsivity r is a simple function
of the slope m, i.e.,
r = (m + 2qXbb )-1 , (2)
where Xbb is the value of the instrument output (in
counts) at the blackbody view, which was originally used in the computation
of
m. Because of the relationship expressed in Eq. (2), the slope
and the responsivity are equivalent, i.e., whenever we know one, we know
the other.
At each blackbody sequence, the SPS will perform a quadratic regression
to generate the polynomial coefficients relating the responsivity to the
temperature of one of the Imager's optical components, using a dependent
sample of optics temperatures and slopes (and responsivities calculated
from them) collected over previous days. The dependent sample will exclude
the data from the hours around midnight, since they may be erroneous. The
SPS will then compare the responsivity associated with the slope calculated
by the normal techniques (the "original" responsivity) with a responsivity
estimated from the optics temperatures. If the original and estimated responsivities
are in agreement, the original slope will be deemed to be correct, and
the SPS will use it in further processing. If the two responsivities disagree,
it is assumed that radiation from the INR sunshields corrupted the original
slope computation, so the SPS will compute a slope from the estimated responsivity
and use that in further processing.
When the MBCC algorithm becomes operational, it will be used for all
the Imager IR detectors at each blackbody sequence, and it will be used
all the time, not only near midnight. As long as the responsivities associated
with the original slopes are reasonable, the original slopes will be accepted
and applied to transform the detector pixel outputs to radiances, as in
that case the original responsivities will agree with the estimates. On
the other hand, should the midnight effect, or a random noise spike at
any time of day, corrupt a computed slope, the SPS will replace it with
a slope computed from the estimated responsivity, which should be closer
to the truth. In that way, the compensating algorithm will be beneficial
even during the day, when the slope-dips are not expected to occur.
B. Details
1. Regression
The regression will generate the coefficients ai,
from which an estimate of the responsivity r est will
be obtained as a quadratic in the temperature T of one of the instrument's
optics components:
r est = SiTi
, (3)
where i runs from 0 to 2.
The regression will also generate the standard errors of estimate, s.
There will be a different value of s for each detector. These will
be used as described in the next section.
The regression will be performed at every blackbody sequence and will
be done separately for each detector.
The dependent sample of data for the regression will be obtained from
the existing SPS 10-day history file. This file contains slopes and optics
temperatures sampled once every blackbody sequence (usually once every
30 minutes). The slopes in this file are those computed by the technique
described at the beginning of this section, i.e., a quadratic calibration
modified to account for the polarization-induced change of the scan mirror's
reflectivity with east-west scan angle. However, these slopes are not filtered
to reduce noise, i.e., they are computed in mode 1, not mode 3, and so
they are represented by m1. The optics temperatures in
this file are two-minute averages. An operator is able to select as the
predictor the temperature of any one of the available optics temperatures,
which include those of the scan mirror, primary mirror, secondary mirror
(2), baffle (2), aft (visible) optics, electronics, etc. Our development
work indicates that the primary temperature appears to be the best choice.
The predictand will be the mode-1 responsivity r1, which
in the dependent sample will be calculated from m1 for
the particular detector by Eq. (2).
The dependent sample will exclude data from the period from midnight
minus H1 hours to midnight plus H2 hours, because the slopes
in that period may be erroneous. H1 and H2 are operator selected
parameters.
The regression will be performed with data from the current day and
the previous ND days, where ND is an integer that can take
any value from 0 to10.
The optics temperature and responsivities in the regression will be
quality controlled. Optics temperatures outside a specified range of temperatures
will be eliminated. The upper and lower temperature thresholds for the
predictor optics temperature will be the limits provided in the factory
database. Before doing the regression, the SPS will compute the mean m
and standard deviation s of all the responsivities
in the dependent sample. The method for screening the responsivities will
be to reject from the dependent sample all responsivities whose values
are not between m - M s
and mm + M s,
where the factor M is an operator-selected integer.
2. Slope Computation
At each blackbody sequence, the slope will be computed as usual, including
correction for the polarization-induced change of the scan mirror's reflectivity
with east-west scan angle, and, if currently operational, the filtering
to reduce noise (mode 3). This is the original slope. At the same time,
however, the SPS will also use the unfiltered slope m1 (computed
in mode 1) to compute the original mode-1 responsivity r1
by Eq. (2).
At each blackbody sequence, after the regression coefficients have been
determined as described above, the estimated mode-1 responsivity r1est
will also be generated from Eq. (3).
For each detector, the difference r1est - r1
will then be formed. If this difference is less than or equal to Ns,
where s is the standard error of estimate of the responsivity (see discussion
following Eq. [3]), and N is an operator-selectable integer (a separate
value for each channel), then the original slope will be used in further
processing. If the difference is greater than Ns, then the slope
associated with the estimated mode-1 responsivity will be computed as
m1est = (1/r1est) - 2qXbb.
This slope will be used in all further processing until the next calibration.
If mode 1 is the operational calibration mode, then m1est
itself will become the operational slope. If mode 3 is operational, then
m1est
will be filtered before being used.
The midnight algorithm cannot be used until L days have elapsed
after an instrument is first turned on and after a patch-temperature change,
where L is an operator selected parameter. Otherwise, the dependent
data sample in the 10-day file will not be representative of current conditions.
For each detector, a flag indicating whether the midnight algorithm
is active or not at each blackbody sequence will be included in the SPS
10-day history file. However, these flags will not be transmitted in GVAR,
stored in the RPM short-term blackbody history file, or printed out with
the contents of that file in response to the PRINT SHORT BBCAL command.
III REFERENCES
Annable, R., Redesign of INR Sunshields (Imager) to Reduce In-Flight
Calibration Errors. Internal ITT Memorandum, April 6, 1988, 4pp.
Johnson, J. Outline of OGE Midnight Correction Procedure (Revised),
private communication, March 25, 1997, 5pp.
Johnson, R.X., and M.P. Weinreb, 1996: GOES-8 Imager Midnight Effects
and Slope Correction. In GOES-8 and Beyond, Edward R. Washwell,
Editor, Proc. Society of Photo-Optical Instrumentation Engineers (SPIE),
2812,
pp. 596-607.
Weinreb, M.P., M. Jamieson, N. Fulton, Y. Chen, J.X. Johnson, C. Smith,
Bremer, and J. Baucom, 1997: Operational Calibration of GOES-8 and -9
Imagers and Sounders.
Applied Optics, 36, 6895-6904.
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