Environmental Engineering Reference
In-Depth Information
Fig. 8.1
Schematic viewing
the calibration principle of the
MSU/AMSU instruments.
Symbols used in the plot are
defined in the main text
δR
Eq. (
8.1
) to allow for inter-satellite calibration, and the calibration coefficients,
μ
and
, were obtained using SNO matchups (Zou et al.
2006
,
2009
). There are other
detailed differences between the prelaunch and postlaunch calibration processes.
For example, the raw count values of the calibration targets in Eq. (
8.1
) were
averages over adjacent scan lines in prelaunch calibration (Mo
1995b
; Mo et al.
2001
). In postlaunch calibration, however, target count values of a single scan line
were used for radiance calculations of each footprint observation in that scan line.
The processing details described in this chapter are specifically applied to the SNO
postlaunch calibration.
In Eq. (
8.1
), the cold space radiance
R
c
is specified to be 9.6
10
5
mW (sr m
2
cm
1
)
1
for all scan lines of both the MSU and AMSU observations. This
corresponds to a brightness temperature of 4.78 K that includes the cold space
temperature of 2.73 K plus an increase of about 2 K owing to the antenna side-lobe
radiation. The algorithms described in Kidwell (
1998
) for MSU and in Goodrum
(
2000
) for AMSU-A are used to compute
R
w
. Once the offsets and the nonlinear
coefficient are known, radiance is computed through Eq. (
8.1
), and the brightness
temperature,
T
b
, is then computed using the Planck function.
The system parameters for MSU and AMSU are different; thus, the processing
details are slightly different for the two instruments. For instance, the MSU has 11
Earth views, one space view, and one warm target view in each scan line. There are
2 PRTs embedded on the blackbody target. In contrast, each scan line in the AMSU
observations contains 30 Earth views, two space views, and two warm target views.
The PRTs on each target ranges from 5 to 7, depending on instrument subunits.
For the MSU, the one space and blackbody views are used to calibrate the 11
Earth views in each scan line. However, the averages of the two space views or
blackbody views are used to calibrate the 30 Earth views for the AMSU-A instru-
ment. The warm target temperature is the average of the available good PRT
measurements for each instrument. Generally, there is a blackbody target for each
antenna system, but there are more than one antenna system for each instrument
unit. These differences between the MSU and AMSU-A units and antenna systems
are listed in Table
8.1
.
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