Data Set Overview
=================
The Cassini Orbiter Imaging Science Subsystem (ISS) archive datasets
consist of the Cassini ISS raw, uncalibrated experiment data record
image files, attached and detached label files (VICAR and PDS),
helpful and required PDS files, including an index table containing a
host of parameters for each image on the volumes, and related ISS 
instrument documentation. The volumes containing these products are 
referred to as the 'DATA' volumes.

Additionally, the ISS datasets include pre-launch ground calibration 
images, assorted calibration data files, algorithms, and 
documentation, the ISS calibration processing software (CISSCAL), and 
the ISS Data User's Guide. These volumes are referred to as the 
'CALIBRATION' volumes. (NOTE: ISS in-flight calibration images are 
found on the DATA volumes, as sequenced in Spacecraft Clock order.)

Several hundred thousand ISS images were taken throughout the entire 
CASSINI mission, including images taken during flybys of Earth, 
Venus, and Jupiter, and images taken of Saturn and Saturn's rings and 
moons while in orbit around Saturn.

In addition to imaging these targets, instrument calibration images 
were taken prior to launch and also while in-flight, as well as 
support images for other Cassini instrument teams and images for 
optical navigation; all of which are contained within these datasets.

Three separate datasets are generated by the ISS team:

  1) Cassini Orbiter Earth/Venus/Jupiter ISSNA/ISSWA EDRs -- Contains 
  all cruise phase imaging, including Earth, Venus and Jupiter flyby 
  images, and in-flight calibration images.

  2) Cassini Orbiter Saturn ISSNA/ISSWA EDRs -- Contains all Saturn 
  Tour phase imaging, including Saturn, Saturn's Rings and Satellites,
  along with in-flight calibration images.

  3) Cassini Orbiter Calibration ISSNA/ISSWA EDRs - Contains EDR 
  calibration related files, including calibration data files (eg., 
  dark currents) sample calibrated images, Cassini ISS calibration 
  processing software, calibration documents, the ISS Data User's 
  Guide, and the collection of pre-launch ground calibration images.

The ISS Data User's Guide can be found in the document directory of 
the most recent CALIBRATION volume, or accessed from:

http://pds-imaging.jpl.nasa.gov/software/

More information on the details of this volume can be found in the
aareadme.txt file at the root level of this volume and in the document
directory.



Data
====

Image VICAR Files
-----------------
All ISS images are in JPL/MIPS VICAR (Video Image Communication And
Retrieval) image format.  VICAR is an entire system of software,
formats, and procedures for image storage and processing.

Each VICAR image file is accompanied by a detached ASCII PDS label
file. The label consists of ASCII 'keyword=value' lines describing the
important characteristics of the image.

A full explanation of VICAR, its standards, its software, and
reference information can be found at website:
http://www-mipl.jpl.nasa.gov/vicar.html. 


Image Index Table
-----------------
The image index table file, INDEX.TAB, contains keyword information
about each image on the volume.  Some of this information comes
directly from the EDR detached PDS image label; for example, keywords
such as FILE_NAME, DATA_CONVERSION_TYPE, IMAGE_MID_TIME, FILTER_NAME,
etc.  The remaining keywords come from the Autonav software (as
discussed above) which calculates many geometrical quantities and
target information TARGET_DISTANCE, PIXEL_SCALE, PHASE_ANGLE,
TWIST_ANGLE, etc. 

This file consists of fixed-length records in ASCII character format.
Each line is a record containing all the keywords for a particular
image on the volume.  Fields in a record are delimited by commas. 
Non-numeric fields are enclosed in quotes and left-justified, whereas
numeric fields are not enclosed by any characters and are
right-justified.  Multi-valued fields are enclosed in brackets and
each item in that field is separated by a comma.

The file INDEX.LBL details the keyword name, data type, start byte,
number of bytes, and format so that keywords can be easily referenced
and the file can be properly read into a database.



Ancillary Data
==============
The Cassini Project produces SPICE files (spacecraft positions,
planetary positions and constants, processed pointing geometry,
spacecraft clock versus universal time, etc.) for use in observation
planning and in calculating many of the image keywords populating the
INDEX.TAB file on this volume.  These Cassini SPICE files are not
included in this ISS data archive but can be obtained from the PDS
NAIF node. 

However, provided to support image searching and querying, the
INDEX.TAB file contains over 100 keywords related to each image,
including geometrically-oriented keywords.  Some of these keywords
are supplied by IO/MIPS as part of the EDR processing, others are
generated by the ISS auto-navigation software.

Other ancillary files include the collection of software interface
specifications related to the production of the EDR data files and the
archive volume DVDs, documents related to camera calibration and the
calibration processing software, and, also, a list of published
references that can provide a thorough discussion of the ISS science
goals and objectives and ISS camera instrument.



Coordinate System
=================
For proper interpretation of the image data, one should use a
Cartesian coordinate system referenced to the Earth mean equator of
J2000. 

There are two ISS coordinate systems in use: that officially used on
the Cassini Project to describe camera orientation (X_cm, Y_cm),
which is directly related to the readout directions of the CCD
samples and lines, and that in general use by imaging scientists,
(X_im, Y_im}, to describe images which are rotated from the target
being imaged. There is also the spacecraft coordinate system {X_s/c,
Y_s/c, Z_s/c}.  The cameras, and other instruments on the RSP, are
pointing in the Y_s/c direction. The positive Z_s/c axis points
towards the spacecraft s main engines; the -Z_s/c points towards the
High Gain Antenna; the +X_s/c axis is up. 

The CCD readout proceeds as follows. The bottom line of the CCD is
shifted down (ie, toward the remote sensing palette, toward -X_s/c))
into a vacant 1-line serial register. This line is shifted then to the
left (in the +Z_s/c direction), pixel by pixel, to the signal chain
until the entire line is read out. The pixels are numbered by the
order in which they proceed to the signal chain. Thus, the first has
sample = X_cm = 1, the last has sample = X_cm = 1024. That is, the
readout proceeds in the  X_cm direction. After this line is
completely read out, the next line is shifted down into the serial
register and read out, and so on until all 1024 lines have been
shifted into the register and then along to the signal chain. This
results in the following relationship between the spacecraft and the
physical ISS/CCD coordinate systems: (sample, line) = {+X_cm, +Y_cm}
= {-Z_s/c, +X_s/c}. 

The images of celestial bodies taken by the ISS are inverted up/down
and flipped left/right (ie, rotated 180 degrees) by the optics in both
cameras. The relationships between targets and inertial space, as well
as the relationship between the target and the orientation of the
Cassini spacecraft, are all maintained through this rotation. Thus,
the image of a celestial target, as well as the `image  of the
spacecraft coordinate system in the focal plane, are rotated from
their physical orientations. A celestial target with its North pole
aligned with the spacecraft +X_s/c axis would appear inverted and
flipped on the CCD: that is, in the focal plane and display image
plane (Fig. 26), the North pole of the target and the +X_s/c axis
would point in the direction of decreasing line (-Y_cm and -Y_im);
the target s western limb (or, astronomical East) and the -Z_s/c axis
would point towards decreasing sample (-X_cm and -X_im). The Cassini
C-Kernel contains information that is used by the Navigational
Ancillary Information Facility (NAIF) SPICE toolkit to derive a
matrix which transforms a vector in inertial coordinates into the
spacecraft coordinate system (X_s/c, Y_s/c, Z_s/c). The Cassini
Frames kernel describes a transformation matrix that transforms a
vector from the camera coordinate system (X_cm, Y_cm, Z_cm) into the
spacecraft frame. The proper combination of the two describes the
orientation of the physical camera/CCD system relative to inertial
space. To compute the correct orientation of inertial space, and the
targets in it, in the image plane, which is where anyone handling an
image will work, one must apply an additional 180 degree rotation
about the center of the image.  



Software
========
The 'CALIBRATION' volume contains the calibrated image and data 
files, calibration software pipeline (CISSCAL), algorithms, ground 
calibration images and related calibration documentation. These files 
together will facilitate the processing of the raw ISS images into
higher-level calibrated image products. The contents of these volumes 
continue to evolve over the course of the mission as knowledge of the 
instrument improves. As a result these volumes are released 
periodically with the latest available calibration files with updates 
described in the ERRATA.TXT file.

The image processing software used to create the EDR image files is
called VICAR (Video Image Communication And Retrieval).  VICAR is an
entire system of software, formats, and procedures for image storage
and processing and was developed and is maintained by JPL's MIPS.

A full explanation of VICAR, its standards, its software and reference
information can be found at: http://www-mipl.jpl.nasa.gov/vicar.html. 
Information on tools for visualizing VICAR images can be found there 
as well.



Media Format
============
This volume is being delivered to the Planetary Data System (PDS)
using DVD media.  Formats are based on standards for such products
established by the PDS [PDSSR1992].

Confidence Level Overview
=========================
Calibration of the ISS cameras is on-going, and incomplete at the
time of this archive release. In addition to improvements in the
absolute calibration, further work needs to be done to model the dark
current, determine any changes in the flatfields over time, and
improve the calibration algorithms already implemented in the CISSCAL
software package. The calibration files provided here are therefore
simply the best available at the time of archiving, and should not be
considered final.

Review
======
Validation is considered to have 2 aspects: 1) quality scientific
usability and 2) technical compliance to PDS standards.  In order to
nsure PDS-compliant products, the archive volumes are validated by a
collaborative effort between the ISS/CICLOPS team, the Imaging and
Central Nodes of the PDS, and non-Cassini imaging scientists. The
ISS/CICLOPS team is responsible for producing PDS-compliant archive
volumes, while the PDS personnel are responsible for ensuring that the
archive volume(s) meet PDS standards. Validation is performed on each
volume by PDS using their validation tools. ISS/CICLOPS-developed
operational volume verification tools and procedures are also utilized
prior to delivery to PDS Imaging Node.  Together these verification
checks ensure PDS-compliant archive volumes.

Scientific usability is assessed through the ISS science team's
normal an routine use of the ISS data sets in their science analysis.
 Additionally imaging scientists not associated with the Cassini
project participate in the archive volume peer review process where
they verify the 'science' content of the data set, the completeness
of the documentation, and the scientific validity (i.e., the
integrity and usability) of the datasets. 

Several reviews on sample archive volumes and directory files are
being performed prior to the start of volume production.  The peer
reviews of sample volumes is conducted by PDS.  These reviews serve
to validate the volume for proper structure, format, completeness,
and science usability. Any deficiencies in the reviewed archive
volume found are corrected and resolved. When all correctable errors
have been resolved, production of the archive volumes proceeds and
further validation is performed on a spot check basis by the both the
PDS and the ISS/CICLOPS team. Non-correctable errors (eg., an error
in the downlink data file) is described in the evolving errata file,
ERRATA.TXT, included on each archive volume in the Root Directory.