Data Set Overview
    =================
      This data volume contains a radiometrically corrected spectral image
      cube of the highest spatial resolution observation of asteroid
      951 Gaspra as acquired by the Galileo spacescraft Near Infrared
      Mapping Spectrometer (NIMS) on October 29, 1991.  It is the product
      of the calibration of the raw data number file gap016tn.qub with
      calibration factors contained in the file e1wanta2.qub and projected
      in a point perspective geometry. Both files are contained within the
      NASA pds archive of Galileo NIMS data. This spectral image cube,
      gaspra_nims_hires_radiance.fit, combines data acquired during the
      asteroid 951 Gaspra encounter and the Earth encounters to produce a
      radiometrically calibrated product.
 
      The Near-Infrared Mapping Spectrometer (NIMS) instrument is an imaging
      spectrometer covering the wavelength region 0.7 to 5.2 micrometers --
      a region not studied by the Pioneer and Voyager spacecraft.  Its
      spectral resolution is 0.025 micron beyond 1 micron, and 0.0125
      microns below 1 micron, yielding 204 spectral elements in nominal
      mode.  The nominal pixel size is a square 0.5 x 0.5 milliradians.
      The instrument acquires data in the order: (1) sampling of 17
      detectors, (2) stepping of the scan mirror (20 elements in cross-cone),
      (3) stepping of the grating (nominally 12 steps per cycle).  The
      nominal 204 wavelength cycle requires 4 1/3 seconds.  The detectors
      (2 Silicon, 15 Indium Antinomide) sample approximately uniformly
      across the spectrum.  A detailed description of the instrument may be
      found in [CARLSONETAL1992] and in the document directory of this
      data volume.
 
      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.  It is also a set of spectra, each at a particular line
      and sample, over the area observed.  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.
 
      The gaspra_nims_hires_radiance data product is a spectral image cube
      of the highest spatial resolution NIMS observations of asteroid 951
      Gaspra. It is derived from data archived in the NASA Planetary Data
      Systems file gap016tn.qub that consists of data in raw data number
      values stored as a spectral image cube in a band sequential integer
      array.  These raw data numbers were converted into little endian
      floating point values using sensor sensitivity values contained in the
      NIMS Earth observation e1wanta2.qub also archived in the NASA Planetary
      Data System.  The gaspra_nims_hires_radiance pixels were rectified by
      creating a point perspective instrument projection and limbfitting
      pixels to the corresponding 951 Gaspra Galileo spacecraft CCD (Solid
      State Imager).  The result is a rectified band sequential radiance
      spectral image cube of 951 Gaspra that is stored in the
      gaspra_nims_hires_radiance FITS file.
 
 
    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.
 
      Further details may be found in VOLINFO.TXT in the DOCUMENT
      directory of the archive volume, and in the instrument paper
      [CARLSONETAL1992].
 
 
    Processing
    ==========
      The gaspra_nims_hires_radiance data product is a spectral image cube of
      the highest spatial resolution NIMS observations of asteroid 951
      Gaspra.  It is derived from data archived in the NASA Planetary Data
      Systems file gap016tn.qub that consists of data in raw data number
      values stored as a spectral image cube in a band sequential integer
      array.  These raw data numbers were converted into little endian
      floating point values using sensor sensitivity values contained in the
      NIMS Earth observation e1wanta2.qub also archived in the NASA
      Planetary Data System.  The gaspra_nims_hires_radiance pixels were
      rectified by creating a point perspective instrument projection and
      limbfitting pixels to the corresponding 951 Gaspra Galileo spacecraft
      CCD (Solid State Imager). The result is a rectified band sequential
      radiance spectral image cube of 951 Gaspra that is stored in the
      gaspra_nims_hires_radiance FITS file.
 
    Data structure
    ==============
      The gaspra_nims_hires_radiance.fit file is a FITS formatted file
      that contains three little endian floating point arrays with
      associated header data.  The first array is a three dimensional
      that is a radiance spectral image cube of the asteroid 951 Gaspra
      in a band sequential format. The second array is another three
      dimensional array that contains the per pixel wavelength value
      of the pixels in the first array.  The third array is a three
      dimensional array that contains the radiance precision of each
      pixel of the radiance spectral image cube.
 
      This FITS file has a detached PDS label file that describes
      the FITS data file structure, spacecraft mission, and
      the observation geometry.
 
    Ancillary Data
    ==============
      Guides (in text and Adobe Acrobat files) to the planned observations,
      including footprint plots on the target, instrument parameters, etc.
      are included in the data set, as are tables of parameters for each
      observation. A copy of the NIMS instrument paper [CARLSONETAL1992]
      is also included in text and postscript file formats.
 
      The calibration factors and dark current values used to create the
      radiance spectral image cube are included in the data volume as an
      ASCII formatted table file.  These values were obtained from the
      E1WANTA2.QUB NIMS Earth observation that is archived in the NASA
      PDS Imaging node.
 
      Tagged Image File Format (TIFF) files containing sample images of the
      data in the gaspra_nims_hires_radiance can be found in the
      EXTRAS directory.
 
 
    Software
    ========
      NIMS tubes were designed to be accessed by the ISIS system.
    No software is provided with the gaspra_nims_hires_radiance data volume.
    The FITS formatted file can be read with astronomical image software
    packages that can ingest the FITS file format.  This includes IDL, a
    product of ITTVIS (http://www.ittvis.com) with the IDL Astronomy
    User's Library at NASA Goddard Space Flight Center at
    http://idlastro.gsfc.nasa.gov .  ISIS (Integrated Software for Imagers
    and Spectrometers) 3.0 may also be used to ingest the FITS file for
    analysis and is available from the USGS via
    http://isis.astrogeology.usgs.gov/ .

Confidence Level Overview
    =========================
      The radiance values contained in the gaspra_nims_hires_radiance
      data set were checked against corresponding values obtained by
      the Galileo Solid State Imager (SSI) multi-spectral imagery,
      NASA Infrared Telescope Facility spectra, and Eight Color
      Asteroid Survey data for asteroid 951 Gaspra.  This permitted
      the selection of the best calibration factors that
      represented the NIMS instrument at the time of the 951 Gaspra.
 
      The shape of the limb of asteroid 951 Gaspra as observed by SSI
      was also used as a constraint for the position of the NIMS
      pixels in the data set.  Each NIMS scan line was aligned into
      the SSI imaged asteroid limb during the time of the observation.
      The updated alignment was used to position the NIMS pixels
      in the gaspra_nims_hires_radiance data product.
 
    Data Coverage and Quality
    =========================
      The gaspra_nims_hires_radiance data product contains only the
      highest spatial resolution NIMS observations of asteroid
      951 Gaspra.  It contains data from the NIMS observation
      that is of a high signal to noise.  Lower signal to noise
      scans (near limb) of the asteroid were omitted from this
      data set.