| DATA_SET_DESCRIPTION |
The UVIS Solar Stellar Brightness 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. 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 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 UVIS instrument can generate time series from the HSP and
the HDAC channels. A time series consists of a sequence of
photometer counts each taken during a fixed time interval.
An observation consists of a time series taken during a
particular instrument configuration. The time series
generated by the HDAC channel may have additional
complexity. If the instrument dwell time configuration
parameter is equal to one, the HDAC is in photometer mode and
its output is a time series of detector counts. If the
dwell time is greater than one, the time series can be mapped
into a table of 32 columns, each column corresponding to an
HDACfilament voltage level in the order: d1..d16, h1..h16
where d1..d16 correspond to the 16 voltage levels of the d
cell and h1..h16 the same for the h cell. The time series
can be mapped to the table by mapping contiguous subsequences
into the successive columns of the table. The length of the
subsequence is determined by the dwell time parameter of the
instrument configuration. A data product (also referred to
as an observation) consists of a set of data taken during a
single instrument configuration.
The UVIS time series data product will be archived as PDS time
series objects. All instrument configuration data for an
observation will be contained in the associated PDS label.
For a detailed 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 UVIS instrument can generate time series from the HSP and the HDAC
channels. A time series consists of a sequence of photometer counts
each taken during a fixed time interval. An observation consists of a
time series taken during a particular instrument configuration. The
time series generated by the HDAC channel may have additional
complexity. If all the filament levels are 0 then the HDAC is in
photometer mode and its output is a time series of detector counts. If
there is a non-zero filament level the detector is in modulation mode
and the time series can be mapped into a table of 32 columns, each
column corresponding to an HDAC filament voltage level in the order:
d1...d16, h1...h16 where d1..d16 correspond to the 16 voltage levels
of the d cell and h1..h16 the same for the h cell. The time series
can be mapped to the table by mapping contiguous subsequences into
the successive columns of the table. The length of the subsequence is
determined by the dwell time parameter of the instrument configuration.
A data product consists of a set of data taken during a single
instrument configuration.
The UVIS solar and stellar brightness time history data product is
archived as a PDS time series object. All instrument configuration
data for an observation is contained in the associated PDS label.
Ancillary Data
==============
HSP and HDAC calibrations do not have a software implementation,
however a description of HSP calibration is located in
SOFTWARE/CALIB/HSP_CALIBRATION.TXT.
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
=================================================================
(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.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The UVIS data objects are organized into separate observations.
Each observation contains data taken from one configuration of
the instrument. There may be more than one observation generated
from one instrument configuration. This may occur because science
data generated by the UVIS instrument is dropped when corrupted
data is detected or transmission failures occur. The UVIS ground
system detects this and divides the data into two observations, one
terminated prior to the data drop the next beginning immediately
afterword. The start time and duration of an observation correspond
to the end times of the first and last records in the observation.
Incomplete observations are filled with zeroes. Over 95% of data
taken by the instrument is contained in the UVIS archive.
Only one hardware feature affects UVIS data. A light leak in the
instrument casing causes an increase in the background counts in
the lower half of the EUV channel wavelength range. This effect is
detectable by visual inspection of a graph of the data. No tools
for detection or correction of these background counts exists.
One anomaly in the flight software caused data errors to appear
when multiple windows which do not cover the entire EUV or FUV
detector are defined. In an observation, the data errors appear
as randomly located spikes in detector count values, and one
completely incorrect spatial line located at a random spatial
index. These errors are detectable by visual inspection of a
plot of the data. The anomaly was fixed by a revison to the
software and effects data between launch and 2001-071 09:00:00 UTC
Review
======
This volume has completed a peer review by the PDS. The peer review
panel consisted of Lyle Huber, Mitch Gordon, Steven Adams, Ron Joyner
and Mark Vincent representing PDS, David Judd and Wayne Pryor from the,
UVIS team Diane Connor from the Cassini Project and Kurt Retherford
and John Clarke as outside users.
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