Environmental Engineering Reference
In-Depth Information
8.2 Methodology for Consistent MSU/AMSU FCDR
Development
The development of MSU/AMSU Level-1c radiance FCDR mainly involves accurate
calibration of the satellite raw count data to remove instrument specific errors. This
requires an understanding of two fundamental processes: the calibration principles
and the origin of the calibration errors. The following subsections describe these two
processes, followed by an introduction of the SNO approaches for the removal of
calibration errors. The last subsection evaluates the performance of the SNO-
calibrated FCDR in modeling reanalysis applications.
8.2.1 MSU/AMSU Level-1c Calibration
The MSU and AMSU instruments use an onboard calibration method that includes
two calibration targets as its end point references: the cosmic space cold target and
an onboard blackbody warm target. Cold space has a temperature of 2.73 K, and the
warm target temperature is measured by the platinum resistance thermometers
(PRT) embedded in the blackbody target. In each scan cycle, the MSU/AMSU
looked at these targets as well as the Earth, and the signals in the form of electric
voltage were converted to digital counts through an analog-to-digital converter.
These digital counts were output as raw observations. The Level-1c calibration
equation, also known as the radiometer transfer function, for converting the raw
count observations to the radiances using the two calibration targets is written as
(Zou et al.
2006
,
2009
)
R ¼ R
L
δR þ μZ;
(8.1)
where
R
is the Earth scene radiance,
R
L
¼ R
c
þ SðC
e
C
c
Þ
, representing the domi-
nant linear response and
Z ¼ S
2
ðC
e
C
c
ÞðC
e
C
w
Þ
is a nonlinear response.
C
represents the raw count data of the satellite observations, and
S ¼ R
w
R
c
=C
w
C
c
is the slope determined by the two calibration targets. The subscripts e, w, and c refer
to the Earth view, onboard blackbody warm target view, and cold space view,
respectively;
is a nonlinear coefficient.
A schematic viewing of this calibration process is illustrated in Fig.
8.1
.
Equation (
8.1
) is a modified version of the calibration algorithm originally
suggested by Mo (
1995a
), where
δR
represents a radiance offset; and
μ
δR
was assumed to be zero and the nonlinear
coefficient
was obtained from prelaunch chamber test datasets (Mo
1995b
;Mo
et al.
2001
). The prelaunch calibration was used to generate NOAA operational
Level-1c radiance products and is thus also referred to as the NOAA operational
calibration. The prelaunch calibration was conducted for each MSU/AMSU
instruments independently and thus left residual biases between satellites. In the
postlaunch calibration as described in this chapter, the offset
μ
δR
was included in
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