DATA_SET_DESCRIPTION |
Data Set Overview
=================
NIMS Experiment Data Record (EDR) files contain raw data from
the Galileo Orbiter Near-Infrared Mapping Spectrometer
(CARLSONETAL1992). This raw data requires considerable
processing before it is readily amenable to science analysis.
The EDRs comprise the base dataset from which spectral image
cubes will be created by continually evolving software using
successively more accurate calibration and geometry data.
Parameters
==========
The files consist principally of 10-bit sensor values from the
17 NIMS detectors ordered by time of acquisition, but these are
interleaved with NIMS background and housekeeping values, and,
less frequently, various instrument temperatures, AACS (scan
platform and rotor pointing) data and engineering measurements.
Sensor values are measured simultaneously in all detectors; a
set of 17 values is acquired at each of 20 cross-track
positions (via a secondary scanning mirror) in a period of 1/3
second. A grating may step between mirror scans; as many as 24
steps may result in 20 spectra of as many as 408 wavelengths
each. A second spatial dimension is added by spacecraft and
scan platform motion.
Processing
==========
The EDRs are generated by the Multimission Image Processing
System (MIPS) at the Jet Propulsion Laboratory (JPL) from data
sources derived ultimately from decommutated Galileo telemetry.
They are divided into files containing up to 100 RIMs (60 2/3
second intervals) of data. Each file contains the best data
available from the several ground stations receiving Galileo
telemetry.
Data
====
The data are organized into 1024-byte records, one for each
minor frame (2/3 second) of spacecraft clock time. In that
time, the NIMS instrument has collected sensor values for all
17 detectors over two scans (20 positions down and 20 positions
up) of the secondary mirror.
A detailed description of the structure of the EDR may be found
in the Galileo Software Interface Specification (ZAMANI1991).
The same information is available in both human and
machine-readable form in the Planetary Data System (PDS)
structure files on the NIMS EDR CD-ROM volumes.
As received from MIPS, each file has a Vicar label preceding a
PDS/ISIS label. The Vicar labels have been removed from the
EDRs on the CD-ROMs but they are present in detached form. The
PDS/ISIS-labelled EDRs themselves may be readily displayed and
further processed by software in the ISIS system, and by PDS
utilities.
NOTE
====
The failure of the Galileo high gain antenna will result in
extensive modifications to NIMS operations and data structures
during Jupiter operations. Extensive editing and compression
of NIMS data will occur on the spacecraft, and the EDR
structure will necessarily be different during phase 2.
Ancillary Data
==============
A Postscript-format Guide to the planned observations,
including footprint plots on the target, instrument parameters,
etc. is included with the EDRs on the CD-ROM, as are tables of
parameters for each EDR file, each observation and each RIM of
data, files of observed dark values in different modes and gain
states, a digitized map of the Galileo booms in cone-clock
space and tables of NIMS internal geometry and timing.
Calibration files, spike files and SPICE files (spacecraft
positions, planetary positions and constants, processed
pointing geometry, spacecraft clock versus universal time,
etc.) may be included in later volumes of the CD-ROM set.
Presently, calibration files are packaged with cube generation
software in the ISIS and Vicar systems, and SPICE files may be
obtained from the Galileo Science Data Team.
Software
========
Simple software is available on the EDR CD-ROMs for displaying
the raw data in various ways, but use of this software requires
considerable familiarity with the operation of the NIMS
instrument. Most science analysis requires generation of
spectral image cubes using ancillary geometric data, and
decalibration of the raw data numbers. This may be done with
one of several similar sets of cube generation programs
available in the ISIS and Vicar systems. (Data cubes generated
in this way will be made available in a separate series of
CD-ROMs.) Programs for displaying validity and status
information and for re-attaching the Vicar label to the EDR are
also available.
Media/Format
============
The NIMS EDRs are archived on CD-ROM for delivery to the
Planetary Data System (PDS). Formats are based on standards
for such products established by PDS.
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CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
NIMS EDRs contain essentially raw data as received from Galileo
telemetry. These are generally processed into spectral image
cubes for science analysis. A fuller discussion of confidence
levels will be more appropriate for the spectral image cube
dataset.
Review
======
The NIMS EDRs are reviewed prior to archiving by the NIMS team.
Data received is compared with data expected, and replays of
missing data requested. The status and validity information on
the EDRs themselves is examined, and spikes (bad points) are
identified. Spectral image cubes of NIMS target data are
generated and reviewed for spatial data coverage and
consistency against observational plans. Format and
documentation of the CD-ROM archive volumes are reviewed by
PDS.
Data Coverage and Quality
=========================
The EDRs contain flags which indicate the validity of
individual subsets of NIMS sensor data. Zeros are used as fill
values when data is missing or invalid.
Limitations
===========
The raw data numbers on the NIMS EDRs are of limited use.
Proper science analysis requires the computation of radiances
according to the best understanding of the instrument
calibration, and proper projection of the data onto the target
in the form of spectral image cubes.
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