| DATA_SET_DESCRIPTION |
UVIS Cube Dataset Overview
================================================================
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 112 to 191 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. The 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. UVIS 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
contains one observation and is completely defined by a PDS
label. The objects are correct in the sense that they conform
to PDS formatting requirements and are consistent with data
archived by the UVIS team. They are complete in that they
represent all data taken by the UVIS instrument. In addition,
CODMAC level 3 data products are derivable from the archived
data and an associated set of calibration data.
A UVIS spatial spectral cube (which corresponds to a
PDS Qube data structure) is a time ordered sequence
of two-dimensional matrices of EUV or FUV detector counts.
In the simplest case, a cube is a time ordered sequence of
1024 x 64 matrices in which each element of the matrix is
the number of counts taken at an individual detector cell
during a fixed time interval. The time interval is defined
in the instrument configuration associated with the
observation. In more complex cases, each integer in the
cube corresponds to a sub region of the detector and is
derived by summing over both the spatial and spectral
dimensions. For example, if the binning defined on the
spectral and spectral dimensions is two, a cube consists
of 32x512 integers, where each integer is derived by summing
contiguous pairs of cells in the spatial and spectral
dimensions. The cube object is a sequence of 1024x64 in
which all data is located in a 32x512 subregion the
upper left hand corner of which will be located at 0,0. All
other locations in the cube contain null values. A still
more complex case involves cubes 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. When binning and windowing is applied
data is in the upper left hand corner of the window.
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, 2) then the data for the
Nth spectrum is found in the rectangle with a upper left
corner equal to (10, 10, N) and a lower right hand corner equal
to (15, 15, N) in the Qube.
A data product (also referred to as an observation) is a
cube generated during a particular instrument
configuration. All instrument configuration information
including window, bin, and integration time specifications are
contained in the PDS object label.
For a more extensive description of the UVIS instrument and
the structure and organization of data on this volume see the
file ROOT/DOCUMENTATION/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
====
A UVIS spatial spectral cube is a time ordered sequence of
1024 x 64 matrices in which each element of the matrix is the number
of counts taken at an individual detector pixel during a fixed time
interval. The time interval is set in the instrument configuration
associated with the observation.
UVIS is capable of windowing and binning the EUV and FUV detectors.
In these more complex cases, each integer in the cube corresponds
to a rectangular subregion of detector cells and is derived by summing
over the spatial and spectral dimensions. For example, if the binning
defined on the spectral and spatial dimensions is two, the cube consists
of 32x512 integers, where each integer is derived by summing the
counts from disjoint 2x2 subregions of the detector. The PDS Qube
object is a sequence of 1024x64 samples, in which all data are located
in a 32x512 sub-region in the upper left hand corner, located at (0,0).
All other locations in the PDS Qube contain null values. A still more
complex case involves cubes derived from a set of sub regions of the
detector called windows. In this case, the detector is divided into a
set of active rectangular sub regions (windows). Each window can also
be binned in the manner just described. The data stored in the PDS
Qube are located in the upper left hand corner of the window. 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 (2,2) then the data for the Nth sample is found in the
rectangle with a upper left corner equal to (10, 10, N) and a lower
right hand corner equal to (15, 15, N) in the 1024x64 sample of the
PDS Qube.
A data product (also referred to as an observation) is a cube
generated during a particular instrument configuration, including
pointing and instrument set up. All instrument configuration
information including window, bin and integration time specifications
are contained in the PDS object label.
The following diagrams illustrate these configurations as contained in
a sample of a PDS Qube.
0 ***************************
*ddddddddddddddddddddddddd*
*ddddddddddddddddddddddddd*
*ddddddddddddddddddddddddd*
*ddddddddddddddddddddddddd*
*ddddddddddddddddddddddddd*
64 ***************************
0 1024
Fig. 2.2.2.1 An unbinned, single windowed sample where d is data.
0 ***************************
*ddddddddddddnnnnnnnnnnnnn*
*ddddddddddddnnnnnnnnnnnnn*
32 *ddddddddddddnnnnnnnnnnnnn*
*nnnnnnnnnnnnnnnnnnnnnnnnn*
*nnnnnnnnnnnnnnnnnnnnnnnnn*
64 ***************************
0 512 1024
Fig. 2.2.2.2 An unbinned window with an upper left hand corner at 0, 0,
and a lower right hand corner at 512, 32. Where d is data, n is null.
0 ***************************
*dddddnnnnnnnnnnnnnnnnnnnn*
*nnnnnnnnnnnnnnnnnnnnnnnnn*
32 *nnnnnnnnnnnnnnnnnnnnnnnnn*
*nnnnnnnnnnnnnnnnnnnnnnnnn*
*nnnnnnnnnnnnnnnnnnnnnnnnn*
64 ***************************
0 0 256 1024
Fig. 2.2.2.3: A window binned by 32 in the spatial dimension and 2 in
the spectral dimension with an upper left hand corner at 0, 0, and a
lower right hand corner at 512, 32. Where d is data and n is null.
0 ***************************
*dddddnnnnnnnnnnnnnnnnnnnn*
*nnnnnnnnnnnnnnnnnnnnnnnnn*
32 *nnnnnnnnnnndddddddddddddd*
*nnnnnnnnnnndddddddddddddd*
*nnnnnnnnnnndddddddddddddd*
63 ***************************
0 0 256 512 1023
Fig. 2.2.2.4: Two windows where the first is binned by 32 in the
spatial dimension and 2 in the spectral dimension and where the first
has an upper left hand corner at 0, 0, and a lower right hand corner
at 512, 32. The second is unbinned with an upper left hand corner at
512, 32 and a lower right corner at 1023, 63. Where d is data and 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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