Data Set Information
|
DATA_SET_NAME |
GALILEO NIMS SPECTRAL IMAGE TUBES: JUPITER OPERATIONS
|
DATA_SET_ID |
GO-J-NIMS-3-TUBE-V1.0
|
NSSDC_DATA_SET_ID |
NULL
|
DATA_SET_TERSE_DESCRIPTION |
GALILEO NIMS SPECTRAL IMAGE TUBES
|
DATA_SET_DESCRIPTION |
Data Set Overview
=================
The natural form of imaging spectrometer data is the spectral
image cube. It is normally in band sequential format, but has a
dual nature. It is a series of 'images' of the target, each in a
different wavelength, in ascending order. It is also a set of
spectra, each at a particular line and sample, over the target
area. Each spectrum describes a small portion of the area. When
transformed into cubes, the data may be analyzed spatially, an
image at a time, or spectrally, a spectrum at a time, or in more
complex spatial-spectral fashion.
NIMS Spectral Image Cubes are derived from NIMS Experiment Data
Records (EDRs), which contain raw data from the Galileo Orbiter's
Near Infrared Mapping Spectrometer [CARLSONETAL1992]. The
instrument covers the spectral range 0.7 to 5.2 micrometers,
measuring both reflected sunlight and emitted thermal radiation.
The cubes are of two kinds. TUBES contain data which have been
converted to science units (radiance or I/F) and rearranged into
band sequential form, but are unresampled and in NIMS instrument
space. Tubes are made for all NIMS observations during Jupiter
operations, and form this dataset. Data from most observations
are *also* binned and resampled while being projected onto the
target, forming MOSAICS (or G-CUBES) which comprise the separate
MOSAIC dataset. Geometric and other information is stored in
backplanes of both kinds of cubes.
This dataset includes radiance and I/F tubes of all Galileo NIMS
observations of Jupiter and its satellites, and associated flight
calibration data. Observations began in June, 1996 and will
continue until the (unknown) end of the mission. Spacecraft
clock (SCLK) times begin at 3449820:00. The tubes are archived
in volumes beginning with GO_1104, each of which contains both
tubes and mosaics from (usually) a single orbit of Jupiter, as
well as browse products (masks) which provide an overview of the
data. (Earlier volumes in this series contain data from Galileo
flybys of Venus, the Earth and the Moon.)
Parameters
==========
A band in a NIMS tube is generated for each of the 17 detectors
at each grating step. (The detectors cover the range 0.7 to 5.2
microns.) The motion of the grating is determined by the
commanded instrument mode:
Mode Grating Grating Bands Samples/RIM
steps increment
Fixed Map/Spectrometer 1 0 17 182
Short Map/Spectrometer 6 4 102 26
Full Map/Spectrometer 12 2 204 14
Long Map/Spectrometer 24 1 408 7
A secondary mirror moves through twenty cross-track positions in
the map modes, or is fixed near the center of its scan in the
spectrometer modes. The number of lines in each image of a tube
is always twenty, whether or not the mirror is moving. The
number of samples is determined by the mode and the duration of
the observation. (In the mosaic dataset, the image dimensions
are determined by the pattern created by the motions of the
secondary mirror and the scan platform.)
The approximate wavelengths of the bands are determined by the
mode, and by offset and start grating positions. The true
wavelengths are functions of the temperature of the grating and
parameters determined from the ground calibration and frequent
optical flight calibrations. Known absorptions on some targets
are also useful in determining these parameters.
The commanded gain state is one of four sets of gains for the 14
non-thermal detectors. The three thermal detectors have two
gains, automatically switching to the lower one near the
mid-point of their range. Raw data values of each detector and
grating step are functions of the gain state and the temperature
of the focal plane assembly (FPA). Radiances are determined from
raw data values using sensitivities based on the original ground
calibration corrected by frequent photometric and radiometric
flight calibrations. I/F values are simply radiances divided by
the solar absorption at the target's distance from the sun for
the wavelength in question.
The NIMS grating motion capability failed prior to the I24
encounter. Subsequently, NIMS returned only 12 useful
wavelengths instead of the original 408. Obviously, this reduced
the usefulness of the NIMS data in certain respects. However the
resulting data redundancy turned out to have noise reduction
advantages in the high radiation environment of Jupiter, and very
useful products were generated.
Further details may be found in VOLINFO.TXT in the DOCUMENT
directory of the archive volume, and in the instrument paper
[CARLSONETAL1992].
Processing
==========
Tube files in this dataset were generated by the Multimission
Image Processing System (MIPS) at the Jet Propulsion Laboratory
(JPL) from raw NIMS data on EDRs, which are available in a
separate volume series (GO_10xx). For each planned observation,
raw 10-bit data numbers have been re-arranged into band
sequential form and converted to spectral radiance and I/F units
using the best calibration available at the time of processing.
Wavelength values are based on wavelength calibrations described
above. Radiance values are derived from 10-bit NIMS raw data
numbers by first subtracting average dark values (determined for
each detector and mirror position and for each gain state), then
dividing by band-dependent sensitivities, determined as described
above. I/F values are derived from radiances by dividing each
NIMS spectrum by the solar spectrum at the target's distance from
the sun. Backplanes of geometric information and projection
co-ordinates are based on the position of the target and
spacecraft and the orientation of its scan platform, derived from
SPICE information from the Navigation Ancillary Information
Facility (NAIF) at JPL.
A secondary hardcopy mask and its digital image (for the volume)
were also generated, based on the mosaic (if one was produced) or
the tube (if not). In the latter case, the mask includes a
footprint plot of the observation on the target, averaged spectra
from one to six selected areas keyed to the plot, a 2-D histogram
and annotation. The masks serve as browse products for the
cubes.
Data structure
==============
The tube files follow PDS structure and labeling conventions. A
PDS/ISIS label begins each file, and describes all the 'objects'
within using ASCII keyword=value statements. The first object is
an ISIS history object which describes the various steps of the
generation process. The second object is a 2-D histogram of the
tube. A third object is a 'sample spectrum qube': a 'stack' of
six spectral plots, each an average over a selected area of the
tube. (These also appear on the hardcopy and digital 'masks'.)
The fourth and principal object is the actual NIMS spectral image
tube.
Spectral image tube structure follows PDS and ISIS 'qube' object
standards. The 'core' of the qube is a 3-dimensional array of
32-bit VAX floating-point pixels arranged in band sequential
order: sample, line and band. The pixels are in radiance or I/F
units. Attached labels describe the structure and units of the
tube and include vectors containing wavelengths and sensitivities
of the bands.
The core is followed by a set of backplanes, or 'extra' bands,
with 32-bit VAX floating-point pixels. The backplanes contain a
number of geometric parameters, native time, projected line and
sample and 0 to 10 'spectral index' bands, each a user-specified
function of the data bands. (The latter might be ratios of
bands, or band depths.) The geometric backplanes are of latitude,
longitude, incidence, emission and phase angles, slant distance
and 'intercept altitude'. Projected line and sample backplanes
describe the position of each pixel had the data been actually
projected on the target, which, in a tube, it has not. Due to
the way NIMS acquires spectra in modes with multiple grating
steps, there are multiple backplanes of each of latitude,
longitude and several projection co-ordinates, one backplane for
each grating position (up to 24) in the instrument mode. (See
comments in the tube label for details.)
Ancillary Data
==============
A Postscript-format Guide to the planned observations, including
footprint plots on the target, instrument parameters, etc. is
included in the data set, as are tables of parameters for each
observation. (Most of these parameters are also present in the
tube labels.) A preprint of the NIMS instrument paper
[CARLSONETAL1992] is also included.
Calibration files, average dark value files and SPICE files
(spacecraft positions, planetary positions and constants,
processed pointing geometry, spacecraft clock versus universal
time, etc.) were used in generating the tubes from EDRs but are
not included in this dataset. (They will be published in a
separate volume at a later time.) However calibration information
is present in the tube labels as vectors of sensitivity (for each
wavelength) and of average dark value (for each detector). Much
of the geometric information is present as backplanes of the
tube.
Software
========
NIMS tubes were designed to be accessed by the ISIS system, which
includes extensive software for generating, manipulating,
analyzing and displaying spectral image cubes. ISIS exists in
VMS and Unix versions, which must be obtained independently, as
described in the documentation of this data set.
Other software has since been developed for displaying spectral
image cubes, notably the ENVI system, written in the IDL language
and available from RSI, Inc. for Unix and Windows systems.
Simple multi-platform software for displaying bands, backplanes
and spectra of cubes is being developed as an enhancement to the
NASAVIEW image display program by PDS. NASAVIEW supports Unix,
PC and Mac systems.
Media/Format
============
The NIMS tubes and 'mask' images are archived on CD-ROM for
distribution by the Planetary Data System (PDS). Formats are
based on standards for such products established by PDS.
Specifically, the discs are formatted according to the ISO 9660
level 1 Interchange Standard, and file attributes are specified
by Extended Attribute Records (XARs).
|
DATA_SET_RELEASE_DATE |
1998-12-10T00:00:00.000Z
|
START_TIME |
1996-05-24T05:08:00.000Z
|
STOP_TIME |
1999-12-20T06:53:00.000Z
|
MISSION_NAME |
GALILEO
|
MISSION_START_DATE |
1977-10-01T12:00:00.000Z
|
MISSION_STOP_DATE |
2003-09-21T12:00:00.000Z
|
TARGET_NAME |
SKY
JUPITER
J RINGS
CALLISTO
EUROPA
GANYMEDE
IO
UNKNOWN
|
TARGET_TYPE |
CALIBRATION
PLANET
RING
SATELLITE
SATELLITE
SATELLITE
SATELLITE
UNKNOWN
|
INSTRUMENT_HOST_ID |
GO
|
INSTRUMENT_NAME |
NEAR INFRARED MAPPING SPECTROMETER
|
INSTRUMENT_ID |
NIMS
|
INSTRUMENT_TYPE |
IMAGING SPECTROMETER
|
NODE_NAME |
Imaging
|
ARCHIVE_STATUS |
ARCHIVED
|
CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
NIMS tubes in this dataset contain unresampled data from all
observations of Jupiter and its satellites that were successfully
returned to earth. The raw data numbers from the EDR dataset
have been re-arranged into spectral image cube form in NIMS
instrument space, and converted to radiance and I/F by applying
the best available calibration. Geometric information has been
added as backplanes of the tubes.
Users of NIMS tubes should be aware of (1) the imperfect nature
of the calibration used to produce them, particularly the
wavelength calibration, (2) the effect of pointing errors on
their geometry and (3) the presence of radiation-induced noise in
the data. These factors are the principal sources of uncertainty
in the dataset. There are also many gaps in the planned spatial
and/or spectral coverage of many observations due to limited
playback time and unplanned interruptions of data receipt at
downlink stations. However it should be mentioned that the Rice
compression algorithm used in playback of NIMS recorded data is
lossless, and does *not* contribute any uncertainty to the data.
Review
======
NIMS data is reviewed by the NIMS team as it is being received,
and again before archiving. Since NIMS tubes are generated for
*all* observations, and since the data may be readily viewed as
images and as spectral plots, they are preferable to EDRs for
checking data quality and coverage. (Unlike data in mosaics
(g-cubes), tube data have not been resampled.) Spatial coverage
information is added to the pointer (footprint) plots of each
observation, and spectral coverage and timing information are
tabulated. The NIMS Guides are revised to include this
information, and the revised Guides included on the archival
volumes.
Several passes through the Galileo tape recorder are usually made
for each orbit, so that coverage information from the first pass
is used to fill gaps due to DSN outages, or to extend spatial
and/or spectral coverage if playback time is available.
Compression ratios are also checked, especially in the first
pass, and used to more accurately predict playback time during
the second pass, so as to maximize the amount of data that can be
returned. Knowledge of typical compression ratios is useful for
planning return of similar data in subsequent orbits.
Format and documentation of each archival volume is reviewed by
several NIMS team members, then by MIPS personnel, who format the
ancillary files according to ISO 9660 standards and write CDWOs,
and finally by PDS before mastering.
Data Coverage and Quality
=========================
Due to the failure of the Galileo high gain antenna to deploy,
the amount of data returned from Jupiter operations was severely
limited. Techniques were developed to maximize the scientific
value of the data that could be returned, including selective
editing of data in wavelength and mirror position, lossless
compression of the selected data, and an optional thresholding
capability. The resulting coverage is described in several
places on the volumes, to which readers are directed.
Information about planned observations is collected in NIMS
Guides to each encounter before that encounter takes place.
Coverage and quality information is added to each Guide after
data is received and before it is archived. The NIMS Guide for
an encounter is then included on the same volume(s) as the data
files for that encounter.
The PDS label of each tube contains information about data
coverage in the observation: time range, ranges of latitude and
longitude, and of other geometric parameters, wavelengths of the
bands in the tube, compression ratio during playback, etc. In
addition, the index table on each volume collects much of the
geometric information from the tube (and mosaic) labels of all
observations with products on that volume. Likewise, spectral
and spatial editing information may be found in the 'observation
table' on the volume. Cumulative versions of these two tables
are also present.
In addition, complex searches for data under various temporal,
geometric and other constraints may be conducted on the World
Wide Web using the Planetary Atlas at the PDS Imaging Node.
These make use of the index table and labels described above.
Data quality is best judged by actually viewing the data products
themselves. Radiation noise is the principal problem, roughly
inversely proportional to the spacecraft's distance from Jupiter
at the time the data is taken, and more pronounced in the higher
wavelengths than in the lower ones. Attempts to remove
significant radiation noise by a despiking technique have been
made for (resampled) mosaics of the icy satellites, but not of
Jupiter and Io data. Tubes, on the other hand, are never
despiked and the quality of the observation should be apparent on
viewing.
Tubes may contain special values in place of some data values,
indicating conditions such as missing data (lost during telemetry
or removed by editing, in wavelength or secondary mirror
position, on the spacecraft), instrument detector saturation,
data below threshold (when an observation was returned using the
thresholding capability), etc. These special values are assigned
according to ISIS conventions and documented in the label.
Sizeable spatial gaps (containing NULL data) in some or all bands
may be due to unplanned (and occasionally planned) interruptions
in the playback process.
Limitations
===========
As mentioned in the overview above, the principal uncertainties
in the NIMS dataset are due to errors in the calibration and in
the pointing geometry and to the presence of radiation noise.
Here, we discuss their effects on the NIMS Jupiter tube dataset.
Calibration
-----------
A set of calibration parameters called the 1998A Calibration,
derived early in that year, was used for NIMS archival data
products from the Galileo Primary Mission, orbits G1 to E11.
Subsequently, several errors and idiosyncrasies in the
calibration were discovered and are still being analyzed. They
are summarized here.
(1) The photometric calibration of Detectors 1 and 2 is
uncertain, due to temporal changes in their spectral
responsivities as well as in the onboard calibration target
used for correction. The spectral response for these two
detectors is therefore incorrect.
(2) The wavelength position behaviour of the instrument has
changed over the orbital mission, likely due to radiation
damage. A grating rotation parameter (PSHIFT), in units of
grating steps, reflects thermal changes in the instrument and
mechanical creep of the grating. A value of -1.3 was used
during orbits G1-E4, and -1.0 during E6-E11. However this was
discovered to be inadequate after E4, most likely because the
grating step size has gradually increased. This increase is
not reflected in the wavelengths listed in the labels of Prime
Mission products (G1-E11). However a new calibration was
developed for the Galileo Europa Mission (GEM) which accounts
for the changing grating step size. Labels of GEM products
(E12-I25) therefor contain more accurate wavelength values.
(3) The dark levels for some detectors and gain states have
changed with time, particularly for Detector 10 in the highest
gain state, where the dark level has dropped by about 10 data
numbers (DNs). We used constant values for the 1998A
Calibration.
(4) The dark level of Detector 9 is slightly increased by the
signal level of Detector 17, due to electronic crosstalk. This
occurs when Channel 17 is in the low gain state. For most
applications it is a small effect (a few DNs), occurring
generally when viewing Jovian 5-micron hot spots.
(5) Two detectors have become inoperative. Detector 3 failed
during orbit E4 and Detector 8 began producing erratic signals
in orbit E6.
(6) Three more detectors (1, 2 and 7) lost almost all of their
sensitivity (and usefulness) after the NIMS grating stuck prior
to the I24 encounter, and remained stuck.
See section 6A of VOLINFO.TXT (found in the DOCUMENT directory
of any NIMS Cube volume) for a description of how calibration
parameters are determined, and a more detailed discussion of
the limitations described above.
Methods to correct for the above-listed errors and
idiosyncrasies are being developed. Correction methods and
parameters (revised wavelengths, radiance corrections, revised
dark levels) will be placed on the NIMS website when available:
http://jumpy.igpp.ucla.edu/~nims/
or follow the link from the Galileo website:
http://www.jpl.nasa.gov/galileo/
to nimsinfo@issac.jpl.nasa.gov.
Pointing geometry
-----------------
The geometry of NIMS observations is determined by two classes of
datasets: (a) scan platform pointing data (either 'predict' or
provided by AACS downlink) and (b) data defining the relative
positions of spacecraft and target body (ephemerides). The latter
are highly reliable, with errors generally less than half a
milliradian. However, pointing data are subject to uncertainties
of various kinds.
In 'normal' operation (inertial mode, in which both gyros and star
sensor are functioning well) and with cone angles less than 150
degrees, the absolute pointing error is close to the nominal one
of 1.0 mrad in cone and clock. However the relative pointing
error within a given observation is better than this, about 0.2
mrad. The absolute error is 'reset' whenever the scan platform is
moved to a new 'aim point', which usually happens at the beginning
of an observation.
When the cone angle is greater than 150 degrees, wobble
compensation is disabled and the scan platform pointing shows an
oscillation with a period equal to that of the rotor spin (18 sec)
and an amplitude that varies but is at most about 1 mrad. In
principle this is not an additional error source, since it is
still tracked by AACS, but it degrades the NIMS data since they
are no longer Nyquist-sampled, and it is possible that the
relative pointing error is increased in this mode.
When the gyros are turned off (cruise mode), then the relative
error is considerably increased, but the absolute error remains
about the same. After orbit E11 one of the gyros started to
degrade, adding an uncertainty in one dimension immediately after
performing a clock or cone slew in the positive direction. The
errors depend on the size of the slew. A correction was developed
by AACS which gradually removes this error using the star scanner.
In E14, this procedure was able to remove 10% of the error every
spacecraft revolution (about 18 seconds); by E19 this fraction was
increased to 40%. Cruise mode was used increasingly as the GEM
mission progressed.
Occasionally, the star scanner was disabled, either for 'bright-
body avoidance' (E2 encounter, Jupiter orbit G1 and others) or due
to an anomaly such as a guide star being incorrectly acquired (G7
orbit). In this case, the pointing error is unpredictable and can
be very large.
When a significant portion of the target body's limb is in the
NIMS field of view, then the pointing can be corrected on the
basis of our knowledge of the relative geometry, which is always
much better than that of the scan platform pointing. This can
reduce the error to about 0.5 NIMS FOV, or about 0.25 mrad,
although it is usually somewhat greater since the limb is not
always sharply distinguishable due to the shape of the NIMS
response function and the scan-platform wobble. (If the limb is
visible in an SSI image during the same observation, the pointing
error can be determined to essentially zero error for that one
point in time, but this rarely occurs in Jupiter observations.)
Pointing corrections of this sort are documented to some extent in
the labels and history objects of the cubes to which they were
applied. A discussion of pointing anomalies in a particular orbit
may be found in the NIMS Guide to that orbit.
See section 6B of VOLINFO.TXT (in the DOCUMENT directory of any
NIMS Cube volume) for a detailed description of how geometry data
is obtained and corrected, and on its limitations, and for a guide
to the documentation within individual products.
Noise
-----
Attempts to correct for radiation noise by despiking were made on
mosaics (g-cubes, resampled products) of the icy satellites, but
the (unresampled) data in NIMS tubes were NOT corrected to remove
noise. For a discussion of this topic, see the Jupiter mosaic
dataset description (JUPMDS.CAT).
|
CITATION_DESCRIPTION |
Citation TBD
|
ABSTRACT_TEXT |
The natural form of imaging spectrometer data is the spectral
image cube. It is normally in band sequential format, but has a
dual nature. It is a series of 'images' of the target,
each in a different wavelength, in ascending order. It is also a
set of spectra, each at a particular line and sample, over the
target area. Each spectrum describes a small portion of the area.
When transformed into cubes, the data may be analyzed spatially,
an image at a time, or spectrally, a spectrum at a time, or in
more complex spatial-spectral fashion.
|
PRODUCER_FULL_NAME |
DR. ROBERT W. CARLSON
|
SEARCH/ACCESS DATA |
Imaging Planetary Image Atlas
Imaging Online Data Volumes (JPL)
Imaging Online Data Volumes (USGS)
|
|