| DATA_SET_DESCRIPTION |
The UVIS Spectrum Dataset
==========================
The UVIS instrument is part of the remote sensing payload of
the Cassini orbiter spacecraft. UVIS has two spectrographic
channels that provide images and spectra covering the ranges
from 56 to 118 nm and 110 to 190 nm. A third optical path
with a solar blind CsI photocathode is used for a high
signal-to-noise ratio stellar occultation by rings and
atmospheres. A separate hydrogen deuterium absorption cell
measures the relative abundance of deuterium and hydrogen
from their lyman-alpha emission. These channels are referred
to as EUV, FUV, HSP, and HDAC in this document. The UVIS
science objectives include investigation of the chemistry,
aerosols, clouds, and energy balance of the Titan and Saturn
atmospheres; neutrals in the Saturn magnetosphere; the
deuterium-to-hydrogen ratio for Titan and Saturn; icy
satellite surface properties; and the structure and
evolution of Saturns rings. basic instrument design adapts
proven design concepts using a grating spectrometer followed
by a multi-element detector. We chose to use imaging, pulse-
counting microchannel plate detectors because of more than a
decade of experience using this kind of detector equipped
with a CODACON readout anode. The CODACON (Coded Anode
Array Converter) acts as a photon locator. The photon counts
are accumulated in an external memory to build a picture that
is periodically read out for transfer to the spacecraft memory
and eventually, transmission to the ground. The two dimensional
format for the CODACON detectors allows simultaneous spectral
and one-dimensional spatial coverage. The detector format is
1024 x 64 (spectral by spatial).
The Cassini HDAC consists of a channel electron Multiplier
photodetector equipped with 3 absorption cell filters: a
hydrogen cell, a deuterium cell and an oxygen cell. The
oxygen cell is not utilized in flight. The hydrogen and
deuterium cells function as adjustable absorption filters.
In each cell a hot tungsten filament disassociates the
hydrogen and deuterium molecules into atoms, producing an
atomic density determined by each of 16 different filament
temperatures. These atoms resonantly absorb the hydrogen
and deuterium Lyman-alpha lines passing through the cells.
Cycling the filaments on and off and comparing the
differences in signal gives a direct measurement of the
relative hydrogen and deuterium signals. Each cell has two
filaments controlled by separate filament current
regulators. Only one filament at a time per cell is used
during flight. A Pulse Amplifier Discriminator detects
photoelectrons from the CEM and sends pulses to the UVIS
instrument logic. S contains a high speed photometer with
an integration time as short as 2.0 ms to observe stars
occulted by the rings of Saturn. The photon counts
collected from the photocathode are passed as a time ordered
sequence to the instrument, then to spacecraft memory for
transfer to the ground. The data in a UVIS observation
are a copy of what was in the UVIS memory buffer. That is,
the observation consists of unprocessed experiment data
stored in binary format. An observation belongs to one of
four different types of data product: a spectrum, a time
series of spatial-spectral images, a time series of detector
counts, or an image at one wavelength. Each observation has
a unique identifier that associates it with a time range and
with the configuration of the instrument during that time.
Each data product will contain one observation and will be
completely defined by a PDS label. The objects will be
correct in the sense that they will conform to PDS formatting
requirements and will be consistent with data archived by the
UVIS team. They will be complete in that they will represent
all data taken by the UVIS instrument. In addition, CODMAC
level 3 data products will be derivable from the archived data
and an associated set of calibration data.
The EUV and FUV channels can be read out to produce
spectra. Each spectrum is generated by accumulating
detector counts over a fixed time interval. The time
interval is defined in the instrument configuration
associated with the observation. A spectrum consists of a
sequence of counts, each count being associated with a
detector column (or columns). In the simplest case, a
spectrum is a sequence of 1024 integers where each integer
is the number of counts taken during a fixed time interval
by the detector at a particular wavelength and summed over
the spatial dimension of the detector. In more complex
cases, each integer in the spectrum corresponds to a set of
columns and is derived by summing over both the spatial and
spectral dimensions. For example, if the binning defined on
the spectral dimension is two, a spectrum will consist of
512 integers, where each integer is derived by first summing
1024 columns in the spatial dimension, then summing
contiguous pairs of detector lines in the spectral
dimension. A still more complex case involves spectra
derived from a set of sub regions of the detector. In this
case, the detector is divided into a set of active rectangular
sub regions (windows). Each window can be binned in the manner
described above. A data product (also referred to as an
observation) is a sequence of spectra taken from a window
during the same instrument configuration.
All instrument configuration information including
window, bin, and integration time specifications will be
contained in the PDS object label.
For a more extensive description of the UVIS instrument
see the file ROOT/DOCUMENT/UVIS.TXT on this archive
volume.
Parameters
==========
The observation type for each observation is found in OBSERVATION_TYPE
column of INDEX.TAB. The specific purpose of each observation
may be found in the object DESCRIPTION field contained in the label.
Processing
==========
The observation data products are generated by the Laboratory for
Atmospheric and Space Physics at the University of Colorado using
the Data Archiving and Processing System (DAPS) software. This
software receives telemetry packet SFDUs from the Telemetry Data
System, extracts science and engineering data, archives the data
in a database management system and produces PDS data and label
object files located on a CD-ROM or DVD physical storage medium.
All time information is generated from the spacecraft clock using
the Cassini SCLKSET files. The DAPS system uses NAIF software and
project generated SPICE Kernels to generate pointing information. All
of these values are contained in the PDS object label.
Data
====
The EUV and FUV spectra will be archived as a 1024x1xN
PDS Qube, that is, N spectra, each containing 1024 integers
representing detector counts. When windowing or binning is
defined data will be located in the upper left hand corner of
the window from which the data was derived and the Qube will
be padded with null values. For example, if a detector
window is defined with its upper left corner at (10, 10) and
its lower right corner at (20, 20) and its binning is defined
to be (2, 10) then the data for the Nth spectrum will be found
at (10, 1, N) through (15, 1, N) in the Qube. All the numbers
outside this range will be a null value.
***************************
0 *ddddddddddddddddddddddddd*
***************************
0 1024
Fig. 2.2.2.1 A single windowed sample with (1x64) binning where
d is data.
***************************
0 *nddddnnnnnnnnnnnnnnnnnnnn*
***************************
0 10 15 1024
Fig. 2.2.2.2 A window with an upper left hand corner at (10, 10),
and a lower right hand corner at (20,20) and 2x10 binning, where
d is data, n is null.
Calibration data are used to transform detector counts into
geophysical units. The EUV, FUV channels have an associated
calibration process. FUV and EUV data are converted to Rayleighs.
The UVIS team supplies calibration data files. The calibration
process is described below and in the PDS label file associated with
the calibration. data file.
Calibration data consist of an MxN matrix and a scalar value. Each
matrix and scalar should be used to scale the individual integrations
of a raw data product. The result is a calibrated data product which
is isomorphic to the original containing data in units of
kilorayleighs. In addition, each calibration data product contains a
mapping of detector lines to wavelengths which is stored in the BAND
BIN_CENTER keyword value.
Calibration data are archived as a PDS Qubes with the dimensions
1024x64x1 or 1024x1x1. All instrument configuration data for the
observation is contained in the associated PDS label; in addition the
label contains instructions for using the calibration data and a
detector column to wavelength mapping and the scalar multiplier
which is stored in the CORE_MULTIPLIER value).
In addition to this mechanism, non-standard calibration routines
developed by UVIS team members may be provided. These routines may
require user input and control. These procedures are not supported
nor are their validity guaranteed, however to the extent that they
are intended for general use by the UVIS team, we will submit
algorithms and associated data and documentation.
Ancillary Data
==============
The UVIS team supplies calibration data files and the algorithms used
to generate the FUV and EUV calibration data. The calibration
algorithms are archived as text files in the SOFTWARE/CALIB directory
of the PDS data volume. The file names contain channel and version
information. These algorithms generate calibration data which is
located in the CALIB/VERSION_n/... directories. The algorithms are
provided as a description of the process by which calibration data is
generated. They are not used to calibrate raw data.
Coordinate System
=================
In the UVIS data products, all time values that are represented as
strings are in UTC time. All time values are derived from the
spacecraft clock using the SCLKSCET translation table supplied by
the Cassini project. All pointing data are expressed in the J2000
coordinate system.
Software
========
LASP provides software for reading PDS data products. This software
is located in the ROOT/SOFTWARE/READERS directory. The software
requires Java 1.4 compatible class libraries and RSI/IDL version 6.
Instructions for running the routines are located in the file
READERS_README.TXT, located in the same directory.
These readers are provided as a convenience for users to access the
data. Users may choose another approach if desired. The readers
enable users to load PDS objects into an RSI/IDL process where they
are represented as 3 dimensional arrays of integers corresponding to
the PDS Qube object in which they are stored. PDS label data are
stored in an IDL structure variable.
DISC FORMAT
=================================================================
This disc has been formatted so that a variety of computer systems
(e.g. IBM PC, Macintosh, Sun, VAX) may access the data. Specifically,
the disc is formatted according to the UDF-Bridge DVD format
standard which provides ISO 9660 level 2 standard compatibility.
For further information, refer to the ISO 9660 Standard Document:
RF# ISO 9660-1988, 15 April 1988.
Specific to the ISO 9660 level 2 standard, filenames on this CD
conform to the 27.3 file naming convention i.e.,
1. The file name portion may be up to 29 characters long; or
2. The extension may be up to 29 characters long,
3. In no case, however, may the total file name length, including
the . exceed 31 characters.
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