| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set has been generated by NASA's Galileo Project in
order to distribute the processed calibration files to be used
with the images acquired by the Solid State Imaging (SSI) camera
to the scientists and later to the Planetary Data System (PDS).
Included are all Dark Current files, Shutter Offset file, Blemish
files and Slope Offset/Calibration files, required to work with all
image files obtained from the SSI camera system from launch through
End of Mission. This disc also contains the technical documentation
necessary to understand how these files may be used, as well as the
source code for the computer program which the SSI scientists use
to apply these files.
Parameters
==========
The Galileo SSI camera acquires data in a digital raster-format
containing 800 scan lines and 800 samples per scan line. Each
picture element (pixel) in the two dimensional image array is
represented as an 8-bit value between 0 and 255, proportional to the
amount of light detected at that point (with 0 being the least
amount of light and 255 being the greatest amount of light). The
camera is equipped with seven color filters and a clear filter such
that images taken through complementary filters can be combined
during ground processing to produce color images. To make full
scientific use of the image collection, the radiometric and
geometric properties of the camera system should be understood.
This disc, GO_0001, provides calibration files and technical
documentation necessary for the understanding of the images produced
by the SSI camera. Also note that, calibration files and software
are available through the VICAR software system at MIPS, software
are available through the ISIS software system at USGS, Flagstaff
and SPICE files are available through the Galileo Project.
Processing
==========
The processing done to create each type of calibration file are
explained below. All calibration files were generated using the
VICAR software. The user should refer to reference 1 for more
details on the software programs.
Dark Current Files and Slope Offset/Radiometric Calibration Files
------------------------------------------------------------------
Dark Current files and Slope Offset (commonly called Radiometric
Calibration Files or cal files) are generated by the VICAR program
GALGEN. GALGEN generates radiometric files by fitting the data
from a light transfer sequence to a linear model of the
light-transfer curve.
GALGEN assumes a linear model of the light-transfer function.
For a given line-sample coordinate (i,j), the light-transfer
function is of the form:
d(i,j) = c(i,j)e(i) + dc(i,j) (1)
where e(i) is the exposure at image line i, d(i,j) is the output
camera signal (DN), and c(i,j) and dc(i,j) are the slope and
offset (dark-current)terms.
GALGEN assumes that the intensity of the light source is held
constant and that the exposure is varied by varying the shutter
speed of the camera. The exposure e(i) is computed from the light
of the light cannon, the commanded shutter time t, and the shutter
offset to(i):
e(i) = l*[t - to(i)] (2)
GALGEN operates in two modes. In the default mode (FIT=LINEAR),
GALGEN solves for both the slope and offset terms at each pixel
position in the image by fitting a straight line (via least
squares) through the input data points (extracted from input
files (D1,D2,D3,...). The inverse of the slope (1/c) is output to
the radiometric slope file (CAL) and the offset is output to DC.
If FIT=SLOPE is specified, then GALGEN solves for the slope-term
only by fitting the input points to the function:
d(i,j) = c(i,j)e(i) - do(i,j) (3)
where do(i,j) is extracted from the input dark-current file (DC).
The inverse of the slope (1/c) is output to CAL and no DC file is
output.
If the input frames have been pre-processed by PICSUM to sum
multiple frames, GALGEN will normalize the data by dividing by the
picture scale (extracted from the picture label) prior to
performing the fit. The picture scale may be optionally input via
the NUMB parameter.
For pixels with unsuccessful fits, the CAL file will receive a
0.0, the SAT, ERR, and RMS files will receive -1.0 and the DC file
will receive 0.
While processing each pixel, if a saturated level is detected,
that level and all higher levels are ignored. This saturation
value is 255.
Blemish Files
--------------
The VICAR program BLEMGEN is used to generate Blemish Files.
BLEMGEN uses files produced by the VICAR program GALGEN.
Blemish Files define each blemish as a vector of the form
(LINE,SAMP,CLASS,SATDN), where LINE and SAMP are the picture
coordinates where the blemish occurs, SATDN is the DN value
at which the pixel saturates at full-well, and CLASS describes
the interpolation algorithm to be used to remove the blemish.
The vectors are sorted first on LINE, then on SAMP.
Two types of blemishes are stored in the Blemish File:
1) permanent blemishes and 2) low-full-well pixels. Low-full-well
pixels are treated as blemishes only when the DN value exceeds
the full-well capacity of the pixel. Permanent blemishes are
identified by vectors with SAT=0. If SAT>0, then the vector
represents a low-full-well pixel.
BLEMISH IDENTIFICATION:
A permanent blemish is any pixel which has one of the following
characteristics:
o a light-transfer function whose slope or offset is
excessively high or low.
o a lower-than-average full-well.
o light-transfer function nonlinearities greater than
specified thresholds.
Blemishes are identified by applying user-specified thresholds to
the slope (CAL), offset (DC), saturation DN, maximum error, and
rms (RMS) associated with each pixel of the CCD and generated by
the VICAR program GALGEN. BLEMGEN parameters (MINSLOPE,MAXSLOPE,
MINDC,MAXDC, MINSAT, MAXERR, and MAXRMS) specified at the time the
files are created are used as follows:
1) Let z and dc represent the slope and offset terms of the
inverse light-transfer function
e = z(d-dc)
as extracted from input files CAL and DC, respectively.
Then z and dc are acceptable if:
MINSLOPE < z < MAXSLOPE
and
MINDC < dc < MAXDC
Otherwise, the pixel is identified as a permanent blemish.
2) Let SATDN be the DN value at which full-well is reached, as
extracted from the input file SAT. Then if
SATDN < MINSAT
the pixel is identified as a permanent blemish. Else if
SATDN < 32767
the pixel is identified as a low-full-well pixel.
3) Let EMAX represent the maximum absolute difference between
the light-transfer data points and the fitted curve, as
extracted from the input file ERR. Then if
EMAX > MAXERR
the pixel is identified as a permanent blemish.
4) Let ERMS represent the rms error resulting from the fit, as
extracted from the input file RMS. Then if
ERMS > MAXRMS
the pixel is identified as a permanent blemish.
Shutter Offset File
-------------------
The Shutter Offset File was generated by Ken Klaasen in Jan. 1993
from Earth 2 in-flight calibration PCT files. These images had
the advantage of having high signal levels using the shortest
exposure time (which is the most sensitive to shutter offset
effects) as well as covering the entire image format allowing the
shutter offset dependence on line number to be determined. The
approach used was to determine the line-dependent shutter offset
by comparing the SSI response in two PCT frames taken in the same
filter at nearly the same spacecraft rotor clock angle - one frame
having a short exposure (# 25 msec) and the other having a long
(3 25 msec) exposure. The disadvantage of this approach is that
the two different exposures were typically acquired in different
camera gain states, and the changing rotor clock angle caused both
the absolute brightness level of the PCT and the brightness
nonuniformities across the PCT to vary somewhat between images.
It turns out, however, that corrections for these effects can be
made sufficiently accurately to yield useful shutter offset
determinations. In-flight calibrations never indicated any drift
in the pre-launch gain ratios (see below), so these were assumed
in this analysis.
The variations in PCT brightness as a function of rotor clock
angle were modeled based on the geometry of the PCT system on the
spacecraft. The variations are expected to follow a sine function
dependence on clock angle. The observed brightness variations in
flight as determined using the pre-launch SSI radiometric
calibration parameters closely approximated this sine dependence.
However, some obvious departures from a smooth sine dependence are
apparent; these departures are correlated with images taken with
the shortest exposure time indicating that the pre-launch shutter
offset was likely to be in error. An improved line-dependent
shutter offset was determined using a pair of green-filter PCT
images having commanded exposure times of 4 1/6 msec and 25 msec.
The rotor clock angles of these two frames differed by only
17 deg; an adjustment based on a sine fit to the clock-dependent
brightness was made, but the PCT shading differences were assumed
to be negligibly small over this small rotor clock difference. The
pre-launch zero-exposure calibration was used. The value at line
400 is 1.21 msec, about 1 msec longer than the prelaunch value and
consistent with the star-based in-flight values. The uncertainty
is estimated to be about 0.1 msec.
Ancillary Data
==============
All document files and detached label files contain a carriage
return character (ASCII 13) and a line feed character (ASCII 10) at
the end of each record. This allows the files to be read by the
MacOS, DOS, Unix, and VMS operating systems. Tabular files are also
described by a detached PDS label. The PDS label file has the same
name as the data file it describes, with the extension .LBL; for
example, the file IMGINDEX.TAB is accompanied by the detached label
file IMGINDEX.LBL in the same directory. The detached PDS labels for
Calibration files contain information pertaining to the image.
Tabular files are formatted so that they may be read directly into
many database management systems on various computers. All fields
are separated by commas, and character fields are enclosed in double
quotation marks. Character fields are left justified, and numeric
fields are right justified. The start byte and bytes values listed
in the labels do not include the commas between fields or the
quotation marks surrounding character fields. The records are of
fixed length, and the last two bytes of each record contain the
ASCII carriage return and line feed characters. This allows a table
to be treated as a fixed length record file on computers that
support this file type and as a normal text file on other computers.
Software
========
The following CD-ROM copy, display and processing software has been
successfully tested using the Galileo SSI data:
Copy Software:
--DCL Exchange/network (VMS) - for ASCII files - available with VMS.
--DCL COPY (VMS) - available with VMS.
--VICAR Copy (VMS and UNIX) - Available with VICAR package.
Display Software:
--NASAVIEW (PC, Apple Macintosh, UNIX-SUN OS) - Available from PDS.
--VICAR XVD (VMS and UNIX) - Available with VICAR package
--NIH IMAGE (Apple Macintosh) - A public domain program available
from the National Institute of Health.
--ISIS QVIEW or CV (UNIX) - Available with ISIS package
Processing Software:
--VICAR (VMS and UNIX) - Available through the Multimission Image
Processing Lab at JPL - http://rushmore.jpl.nasa.gov/
--ISIS (UNIX) - Available through the USGS in Flagstaff
http://wwwflag.wr.usgs.gov/
Media/Format
============
Each CD-ROM disc has been formatted such that a variety of computer
systems may access the data. Specifically, the discs are formatted
according to the ISO 9660 level 1 Interchange Standard, and file
attributes are specified by Extended Attribute Records (XARs).
Formats are based on standards for archive CD-ROM products
established by PDS.
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