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 target.  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 Cube files are derived from NIMS Experiment
    Data Records (EDRs), which contain raw data from the Galileo Orbiter
    Near Infrared Mapping Spectrometer (CARLSONETAL1992).  The raw EDR data
    have been re-arranged into band sequential form, converted to spectral
    radiance units based on ground and flight calibration of the NIMS
    instrument, and (in most cases) projected onto the target based on
    the position of the spacecraft and target and the orientation of
    the spacecraft's scan platform.  This dataset contains files of
    projected data ('mosaics') only.  Another dataset contains files
    of unprojected data ('tubes').
 
    Calibration and geometric information used are the best available
    at the time of publication of these files, but they are subject to
    continual improvements as data analysis proceeds.  Thus better cubes
    may be generated in the future.
 
  Parameters
 
    A band in a NIMS cube is generated for each of the 17 detectors
    at each grating step.  The motion of the grating is determined by the
    commanded instrument mode:
 
      Mode                         Grating   Grating   Bands
                                    steps   increment
 
      Fixed Map/Spectrometer          1         0        17
      Bandedge Map/Spectrometer       2     variable     34
      Short Map/Spectrometer          6         4       102
      Full Map/Spectrometer          12         2       204
      Long Map/Spectrometer          24         1       408
 
    The wavelengths of the bands are determined by the commanded start and
    offset grating positions, and by wavelength calibrations conducted on
    the ground and occasionally during flight.  They are also weak functions
    of grating temperature.
 
    The pixels in a cube of a targeted observation are in (scaled)
    units of radiance, derived from the 10-bit raw data numbers by
    applying band-dependent sensitivities, which are in turn products
    of ground and flight calibrations, of the commanded gain state and
    chopper mode and of the focal-plane-assembly (FPA) temperature.  The
    radiance scaling is band-dependent, and is specified by vectors of
    offset and multiplier values.
 
  Processing
 
    Cube 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 CD-ROM
    series (GO_10xx).  For each planned observation, raw 10-bit data numbers
    have been re-arranged into band sequential form, converted to spectral
    radiance units according to ground and flight calibration of the NIMS
    instrument, and projected onto the target according to spacecraft
    and target position and scan platform orientation, using a complex
    binning procedure.  A secondary 'browse' product of this procedure is a
    hardcopy 'mask', a digital image of which accompanies the cube.
 
  Data structure
 
    The cube 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 [8] which describes the various steps of the
    generation process. The second object is a 2-D histogram of the cube.
    A third object is a 'sample spectrum qube': a 'stack' of six spectral
    plots, each an average over a selected area of the cube.  (These
    also appear on the hardcopy and digital 'masks'.)  The fourth and
    principal object is the actual NIMS spectral image cube.
 
    Spectral image cube structure follows PDS and ISIS 'qube' object
    standards.  The 'core' of the qube is a 3-dimensional array of 16-bit
    signed integers, arranged in band sequential order: sample, line and
    band.  (ISIS also supports 8-bit unsigned integers and 32-bit real
    values.)  The core of NIMS cubes in this dataset contains
    scaled 16-bit radiance values, the result of applying the NIMS
    calibration to 10-bit raw data numbers. The scaling parameters (offset
    and multiplier) for each band are stored in the label, as are the
    wavelengths and other band-dependent quantities.
 
    The core is followed by a set of backplanes, or 'extra' bands, made
    up of 32-bit VAX floating point pixels.  The backplanes contain seven
    geometric parameters, the standard deviation of one of them, the
    standard deviation of a selected data band, 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 latitude, longitude, incidence, emission and phase angle,
    slant distance and 'intercept altitude'. (See NOTEs in the cube 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 cube labels.  A
    late version of the NIMS instrument paper (CARLSONETAL1992) is also
    included.
 
    Calibration files, spike files and SPICE files (spacecraft positions,
    planetary positions and constants, processed pointing geometry,
    spacecraft clock versus universal time, etc.) were used in generating
    the cubes from EDRs but are not included in this dataset.  But much
    of the geometric information, transformed into 'images', is present
    as backplanes of the cube.
 
  Software
 
    NIMS cubes were designed to be accessed by the ISIS system,
    which includes extensive software for generating, manipulating,
    analyzing and displaying them.  ISIS exists in VMS and Unix versions,
    which must be obtained independently, as described in the documentation
    of this data set.
 
    But simple software for displaying bands, backplanes and spectra of
    cubes will be available by soon as an enhancement to the NASAview image
    display program, under development by PDS.  It will be included with
    later versions of this data set.
 
  Media/Format
 
    The NIMS cubes and 'mask' images 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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