This program is used to radiometrically correct flight images using a
linear model of the light-transfer function. This program is specific to
the Galileo SSI camera.
References:
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D-4264 MIPL Software Structural Design for the Instrument Calibration
of GLL SSI Science Processing.
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K. Klaasen, "Reduction in Number of Unique SSI Calibration Files",
29 May 1987.
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625-210,"Galileo SSI Calibration Report Part 1", K. Klaasen, H.
Breneman, November 1, 1988.
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JPL VICAR GALSOS application help page.
This program will radiometrically correct the input image, converting the
input DN values to some specified radiometric unit (either I/F or
radiance). The program requires a radiometric file generated from
calibration data acquired at the same filter position as the input image,
and a dark-current file acquired at the same camera gain-state, frame-rate,
PNI, and BPM settings. Also, extended exposure mode frames require an
extended-exposure mode dark-current file.
This program extracts the filter position, exposure time, and frame-rate
from the label of the input file to determine the required gain file
to use. The file $ISIS3DATA/galileo/calibration/gll_gain.sav defines the
gain file to use for the given filter, gain state, and frame mode. The
file $ISIS3DATA/galileo/calibration/gll_dc.sav defines the DC file to use
for the given, gain state, frame mode, frame rate, blemish protection mode,
clock mode, and extended exposure mode.
This program performs the following processing steps on each pixel:
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The radiometric correction is applied:
e = z(d - dc)
where:
z is retrieved from the Radiometric File.
d is the DN.
dc is retrieved from the Dark-Current File.
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The output pixel is scaled to radiometric units R (units of reflectance
or units of radiance). If output units are I/F, the following equation
is used:
S1 K
r = e * -------- * --- (D/5.2)2
A1(t-to) Ko
where:
S1 = filter-dependent conversion factor from ft-Lamberts to
I/F units for a reference distance of 5.2 AU from the Sun.
A1 = output picture scale factor (see SCALE parameter for more info).
t = commanded exposure time of the input image (msec).
to = line-dependent shutter offset.
K = system gain constant for the gain-state of the image.
Ko = system gain constant for the calibration file gain-state.
D = target distance from the Sun (in AU).
If output units are in radiance, the following equation is used:
S2 K
r = e * -------- * ---
A2(t-to) Ko
where:
S2 = filter-dependent conversion factor from ft-Lamberts to
units of nanowatts/cm2/steradian/nanometer.
A2 = output picture scale factor (see SCALE parameter for more info).
t = commanded exposure time of the input image (msec).
to = line-dependent shutter offset.
K = system gain constant for the gain-state of the image.
Ko = system gain constant for the calibration file gain-state.
Note: The original version of this equation (found in GALSOS) was
multiplied by 10,000. The Isis 2 version of the program chose to
remove this scaling factor, so we do the same here.
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The output DN is usually in the range of 0.0 to 1.0.
If the uneven bit weighting correction is enabled, the input DN values (d)
will be corrected for uneven-bit-weighting due to the ADC. If the input
dark-current file is in byte format (i.e. an individual
dark-current frame), then the correction will be applied to the
dark-current as well. The current correction table was supplied by Herb
Breneman, 2 Mar 89, and is based on 1975 calibration data.
Please note that the original Isis 2 version of this app (ssical) included
negative I/F values in the output. We will not be supporting this behavior
as negative I/F are not useful and do not make sense. Thus, this app
outputs LRS for any negative I/F values that are encountered.