The Cassini Imaging Science Subsystem (ISS) has created an archive of digital images acquired during ISS ground subsystem calibration of the Flight Model instruments. Compact Disc Write-Once (CD-WO) is the archive medium used for long-term storage and distribution.
Each CD-WO volume contains approximately 600 images stored in individual compressed files along with a Planetary Data System (PDS) label description of each image. In addition to the images, software for decompression of images is included. Also, documentation is provided about the Video Image Communication and Retrieval (VICAR) image labels, the PDS image labels, decompression software and the contents of the disc as well as any errors on previous discs. Finally, there is an Image Index File containing information about all the images stored in the archive. 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.
This document describes the organization of the archive. It
provides information on how to access files stored on a CD-WO, and
gives details on the format and contents of the image files and their
supporting supplemental files.
The ISS uses two separate camera designs to satisfy the scientific objectives of the Cassini mission. The first is a Narrow-angle Camera (NAC) design which obtains high-resolution images of the target of interest. The second is a Wide-angle Camera (WAC) design which provides less resolution but more complete coverage spatially. The spacecraft carries one NAC and one WAC. The NAC is also used to obtain optical navigation images for the mission with the WAC acting as a functionally redundant backup unit.
Each camera is a framing charge coupled device (CCD) imager. They differ primarily in the design of the optics: the NAC has a focal length of 2000 mm and the WAC has a focal length of 200 mm. Both cameras have a focal plane shutter of the Voyager/Galileo type, and a two-wheel filter changing mechanism derived from the Hubble Space Telescope WF/PC.
The CCD detector is cooled to suppress dark-current and is shielded from protons and electrons. The detector uses three phase, front side illuminated architecture, with a coating of Lumogen phosphor to enhance ultraviolet response. The layout is a square array of 1024x1024 pixels with a pixel size of 12x12 micrometers. The camera electronics is capable of summing 4 or 16 pixels before forming the image, making 512x512 and 256x256 images possible. In addition, the 12-bit electronics yields a dynamic range of 4096 data numbers, with options including conversion to 8-bits, Lossless compression and Lossy compression.
All images taken in compressed modes were decompressed in the image acquisition process and the decompressed version is stored in this archive. (Note: This instrument compression must not be confused with the compression used to pack images onto the discs of this archive. See image format description below.)
A small number of the Performance Test Plan images were taken
in Direct Access Mode. This mode produced normal image files and
files which bypassed the electronics for comparison with the normal
images. These Direct Access files are archived with the same filename
as the normal images except the extension is .DA instead of .IMG. The
.DA files do not contain binary prefix or telemetry labels.
Both ISS Flight Model instruments went through calibration in an environmental test chamber. The vacuum of space was simulated and the temperature was controlled to simulate various operating conditions.
A series of tests were performed to characterize the instrument performance. Each test consisted of taking image data of either flat-fields (using an integrating sphere source), targets (using collimated light sources) or of nothing (unshuttered frames).
The following types of tests were performed for calibration purposes and the resulting images are stored in this CD-WO archive:
Focus - to measure the focus quality
Shutter Offset - to derive the exposure time correction
System Gain - to measure the electrons/DN of each gain state
Point Response Function - to measure the response to point sources
Modulation Transfer Function - to measure the MTF of the system
PC Voltage - to characterize the system behavior at various voltages
Electrical Noise - to search for coherent noise
Light Leakage - to search for light leakage through the closed shutter
Light Transfer Function - to measure the radiometric response
Geometric Distortion - to measure the optical distortion
Lossy Compression - to measure the performance of the compressor
Navigation Target - to measure response to simulated planets
Antiblooming - to characterize the antiblooming mode of the camera
Polarization - to measure the axis of the polarizing filters
Bit Weighting - to analyze the Analog-to-digital converter performance
Dark-current - to measure the dark-current vs. modes and temperatures
FTP - "Functional Test Plan" - a basic functionality test
PTP - "Performance Test Plan" - a checkout of all operational modes
Gain Tests - characterization of electronics performance
Signal Analyzer Tests - characterization of electronics performance
Filter Exposure Tests - to check exposures in each filter combination
Bias - characterization of electronics performance at zero exposure
Cross Filter - to measure response with filters positioned to block all light
Blemish Analysis - identification of pixels with non-linear response
Blemish & Gain - to measure the electrons/DN of each gain
state and identify pixels with non-linear response
The image files in this CD-WO archive are raw; no processing has been performed other than compressing the image using the public domain software INFOZIP. They contain ASCII VICAR labels and pixel data and have associated PDS labels in separate files.
4.1 Compressed Format
ISS ground calibration images are stored in a compressed format using INFOZIP. INFOZIP achieves a significant reduction in the disk space required to store an image while maintaining full data precision. Some images are compressed by a factor of more than two. Using this technique, more than 600 compressed images can be stored on a single CD-WO rather than 300 in an uncompressed format.
4.2 File Naming Conventions
Each digital image file has a unique name constructed from its
image number. The general form of a compressed image filename is
xxxxxx.ZIP, where the 'xxxxxx' is the six digit image number. The
extension ZIP is an indication that the image is compressed. Once the
image is decompressed to its original form, the extension ZIP will
become IMG (or DA for the direct-access images).
The volume and directory structure of the CD-WO conforms to the level-1standard specification of the International Standards Organization (ISO). The ISO standard was used so that the discs can be accessed on a wide variety of computer systems.
The compressed image files are divided into directories based on the type of calibration test. There is a directory associated with each test for which there are images on the disc. Within these directories are compressed images with VICAR labels and their associated PDS label files.
The following table shows the structure and contents of these directories. In the table, directory names are followed by a slash (/) and upper-case letters indicate an actual directory or filename.
Top-level directory
|
| |- AAREADME.TXT - Brief description of the contents of the
disc.
|
|- ERRATA.TXT - Describes any errors on previous discs.
|
|- VOLDESC.CAT - Contains the VOLUME Object which gives a
high-level description of the contents of the volume.
|
|
|- DOCUMENT/ - Directory containing text files which describe
label formats.
| |
| |- DOCINFO.TXT - Describes the content of DOCUMENT
directory.
| |
| |- VICAR2.TXT - Describes the VICAR image label.
| |
| |- VOLINFO.TXT - Contains a textual description of the contents
of the volume.
|
|- INDEX/ - Directory containing an index file for searching for
specific images.
| |
| |- INDXINFO.TXT - Describes the content of INDEX
directory.
| |
| |- IMGINDEX.TAB - A tabular file containing information about
each image.
| |
| |_ IMGINDEX.LBL - A detached PDS label describing the fields of
the index table.
|
|- LABEL/ - Directory containing image engineering
information
| |
| |- LABINFO.TXT - Describes the content of LABEL
directory.
| |
| |- TLMTAB.FMT - A PDS-format file describing the binary
telemetry record of the images.
| |
| |_ PREFIX8.FMT - A PDS-format file describing the binary prefix
of the records of 8-bit images.
| |
| |_ PREFIX16.FMT - A PDS-format file describing the binary prefix
of the records of 16-bit images.
|
|- SOFTWARE/ - Directory containing software used to decompress
images on variety of platforms.
| |
| |- SOFTINFO.TXT - A brief description of the decompression
software and from where it was downloaded.
| |
| |- INFOZIP.TXT - Information about INFOZIP compression
software.
| |
| |- MAC/ - Directory containing decompression software for
Macintosh computers.
| | |
| | |- INSTALL.TXT - A brief description on how to install on
MAC.
| | |
| | |- UNZ_MAC.HQX - Decompression software for Macintosh
computers.
| |
| |- PC/ - Directory containing decompression software for PC
computers.
| | |
| | |- INSTALL.TXT - A brief description on how to install on
PC.
| | |
| | |- UNZ31.EXE - Self-extracting executable software for
computers.
| | |
| | |_ UNZ31.ZIP - Decompression software for WINDOWS 3.x
computers.
| | |
| | |_ UNZ95NT.EXE - Decompression software for WINDOWS 95 and
WINDOWS NT.
| |
| |- UNIX/ - Directory containing decompression software for
UNIX-based machines.
| | |
| | |- INSTALL.TXT - A brief description on how to install on
UNIX.
| | |
| | |- INFOZIP.TAR - A tar file contains decompression software
for UNIX.
| |
| |- VMS/ - Directory containing decompression software for use
with VAX/VMS-based machines.
| |
| |- INSTALL.TXT - A brief description on how to install on
VAX-VMS.
| |
| |- UNZ_VMS.EXE - Executable decompression software for
VMS.
|
|- xACFM/ - Directories for the cameras being tested. x = N or
W
|
|- yyyyyyy/ - Subdirectories named by the calibration test type
(e.g., FOCUS).
|
|- nnnnnn.ZIP - Compressed VICAR image file
|
|- nnnnnn.LBL - Detached PDS label file for the image data file
with the same image number nnnnnn.
After decompression, an ISS image file is composed of fixed-length records. Image files begin with a VICAR label followed by the binary telemetry header and the image. Use the figure below as a guide to the descriptions which follow. Note: Direct-access images (extension DA, have no binary telemetry header or binary prefix).
|
2 records |
|
|
(Telemetry Header) 1 record |
|
|
PREFIX (Line Header) 24 bytes/line |
(Pixels ) 2048, 1024, 512 or 256 bytes/line 1024, 512 or 256 line records |
The VICAR label is a set of ASCII "keyword=value" sets of information describing the important characteristics of the image. The label is designed to be human-readable because it is often used to annotate products derived from the image, such as prints or plots. Because it is part of the image file, it is maintained through various processed versions of the image to allow traceability with the original data. Also, the label items may be extracted by software modules in order to guide automated processing procedures.
The VICAR label contains required System items (such as image size information), optional Property items (such as items describing gain states, etc.) and optional History items (such as processing steps). See the description in the file VICAR2.TXT in the LABEL directory.
This record holds machine-readable information which applies to the image as a whole. For ISS, the binary information is the Extended Science Header which accompanies each image in the telemtry stream (see ISS Flight Software Requirements Document, January 1, 1995, JPL D-10750). This record contains 53 items of information (including spare fields) and is subsequently padded out to the image record length with zeroes. The file TLMTAB.FMT located in the LABEL directory contains a description of the binary telemetry header in the Object Description Language (ODL) developed by the PDS project.
The format of this binary data and the Binary Prefix below is specified in the VICAR label by the BHOST keyword. All images in this archive have BHOST='SUN-4', indicating that the binary data was written by a Sun computer in its native binary format.
Each image line contains a Binary Prefix of 24 bytes followed by the 8- or 16-bit pixel data for the line. The Prefix contains 5 machine-readable fields (7 for 8-bit image files) about the image line derived from the telemetry. This is information which varies from line to line, such as the line number.
In the LABEL directory, the files PREFIX8.FMT and PREFIX16.FMT
describe the content of the Binary Prefix for the 8-bit and 16-bit
image files respectively. They are both in the Object Description
Language (ODL).
The detached PDS label files contain information pertaining to
each associated image file. The 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. The PDS label file has same name as
the data file it describes, except that it uses the extension
.LBL.
The file IMGINDEX.TAB located in the INDEX directory contains fields of information about the image files located on the volume. Included in the image index is information on the camera state, exposure time, target, filters, and other camera parameters. The file consists of fixed-length records of 370 bytes in ASCII character representation. Each record contains the information for one image.
This file is formatted so that it may be read directly into many database management systems on various computers. All fields are separated by a comma, 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 include the commas between fields and the quotation marks surrounding character fields.
The records are of fixed length and are followed by the ASCII carriage return and line feed characters. This allows the 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.
The file IMGINDEX.LBL, located in the INDEX directory, describes the contents of the index file in the Object Description Language (ODL).
Questions concerning this Cassini Ground Calibration product may be directed to:
Charles Avis
Mail Stop 168-514
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
818-354-6012