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HIRES NIMS GASPRA SPECTRAL IMAGE CUBE (PDS)

NSSDCA ID: PSSB-00536

Availability: Archived at NSSDC, accessible from elsewhere

Time span: 1991-10-29 to 1991-10-29

Description

This description was generated automatically using input from the Planetary Data System.

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/ .

These data are available on-line from the Planetary Data System (PDS) at:

http://sbn.psi.edu/pds/asteroid/GO_A_NIMS_4_GASPRACUBE_V1_0/

Alternate Names

  • GO-A-NIMS-4-GASPRACUBE-V1.0

Discipline

  • Planetary Science: Small Bodies

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Robert W. CarlsonData ProviderNASA Jet Propulsion Laboratoryrcarlson@issac.jpl.nasa.gov
Mr. James C. GranahanGeneral ContactUniversity of Hawaiigranahan@uhpgvax.pgd.hawaii.edu
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