| DATA_SET_DESCRIPTION |
Data Set Overview
=================
951 Gaspra radiance Near Infrared Mapping Spectrometer point spectra data
description
1 - INTRODUCTION
This data volume contains a radiometrically corrected point spectra of
asteroid 951 Gaspra as acquired by the Galileo spacecraft Near Infrared
Mapping Spectrometer (NIMS, Carlson et al., 1992) on October 29, 1991
(Carlson et al., 1992B). These spectra record the observations of the NIMS
sensor as the Galileo spacecraft approached the asteroid. These data are
products of the calibration of the raw data number files gap015tn.qub,
gap035tn.qub, gap036tn.qub, gap037tn.qub, and gap038tn.qub (DATA SET ID
='GO-A-NIMS-3-TUBE-V1.0') with calibration factors (DATA SET ID =
'GO-E-NIMS-4-MOSAIC-V1.0') acquired during the first Earth/Moon encounter
of the Galileo mission. These raw data .qub files are archived in the
Imaging Node of the NASA Planetary Data System (PDS). These calibrated
spectra consist of radiance measurements for wavelengths between 0.7 - 5.2
micrometers. The data files are named by combining the name of the raw
data source file and the data product type. For instance
gap015radiance.tab is a combination of the first six characters of the
source data file gap015tn.qub and the radiance data type. The .tab suffix
indicates that the file is a PDS formatted text table file. The radiance
data type refers to files that contain radiance spectra. The iof data
type refers to
files that with incidence over flux spectra.
2 - DATA OVERVIEW TABLE
File Bands Object Observation Target
Start(UT) Distance(km)
gap015radiance.tab 329 951 Gaspra 22:01:16 14820.1
gap015iof.tab 329 951 Gaspra 22:01:16 14820.1
gap035radiance.tab 17 951 Gaspra 21:38:18 26585.1
gap035iof.tab 17 951 Gaspra 21:38:18 26585.1
gap036radiance.tab 17 951 Gaspra 21:51:36 21024.7
gap036iof.tab 17 951 Gaspra 21:51:36 21024.7
gap037radiance.tab 100 951 Gaspra 21:54:29 17624.2
gap037iof.tab 100 951 Gaspra 21:54:29 17624.2
gap038radiance.tab 100 951 Gaspra 22:07:13 10391.9
gap038iof.tab 100 951 Gaspra 22:07:13 10391.9
3 - GALILEO MISSION DESCRIPTION
Galileo was a mission to Jupiter to perform long-term studies of the
Jovian atmosphere and detailed studies of the Galilean satellites (Wolf,
1992). It consisted of a two part spacecraft, an orbiter and an
atmospheric probe. Both components were launched from the space shuttle
Atlantis on October 18, 1989. The orbiter contained remote sensing,
particles, and wave sensors. The remote sensing instruments included the
Solid State Imager and the Near Infrared Mapping Spectrometer. The
atmospheric probe contained in-situ sensors to directly measure the
atmosphere of Jupiter. The Galileo spacecraft encountered Venus (February
10, 1990), the Earth Moon system (December 8, 1990 and December 8, 1992),
asteroid 951 Gaspra (October 29, 1991), asteroid 243 Ida (August 28, 1993)
before entering the orbit of Jupiter on December 8, 1995. This data set
is from the asteroid 243 Ida encounter. The Galileo mission investigated
the Jovian system by deploying its atmospheric probe on July 13, 1995 and
conducting observations until plunged into Jupiter on September 21, 2003.
4 - INSTRUMENT DESCRIPTION
The Near Infrared Mapping Spectrometer (Carlson et al., 1992) instrument
was an imaging spectrometer covering the wavelength region 0.7 to 5.2
micrometers. 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
antimonide) sample approximately uniformly across the spectrum. The
stepping of the grating provided NIMS with a variety of spectral data
collection modes that ranged from 17 channels (no grating step movements)
to 408 channels (24 grating step movements).
5 - 951 GASPRA ASTEROID ENCOUNTER NIMS OBSERVATIONS
The Galileo spacecraft performed a flyby encounter of asteroid 951 Gaspra
with its closest approach occurring at 22:37:00 on October 29, 1991 UTC.
The closest approach was at 951 Gaspra's dark
side, 3.9 degrees north ecliptic latitude, at a 1600 kilometer range.
NIMS conducted four types of science observations during the 951 Gaspra
encounter. The highest priority observation was two scans of the asteroid
in fixed map mode (17 spectral channels) at a time near the closest
approach. The goal was to collect spatially resolved spectral data of 951
Gaspra. The second priority observation time was to collect short map
mode (102 spectral channels) at broken intervals with low phase angles.
The goal of these observations was to acquire the best spectral
observations during the 951 Gaspra encounter. The third priority
observation type was to collect full map mode (408 spectral channels)
during every 90 degrees of asteroid rotation. This was to provide the
highest spectral sampling of the asteroid as it rotated. The lowest
priority observations for NIMS was to collect short map mode data every 30
degrees and fixed map mode data at every 15 degrees of asteroid rotation.
Thirty two observations of all of these types were conducted by the NIMS
sensor during this encounter. Radiometric and photometric calibration
observations were conducted by NIMS after the completion of the science
observation sequence.
The products in this data volume are a subset of the NIMS observations of
asteroid 951 Gaspra. They do not include the closest approach fix map
mode data. That data has been calibrated and archived as the
GO-A-NIMS-4-GASPRACUBE-V1.0 in the PDS Small Bodies Node. They do include
five files containing two fixed map, two short map, and a full map mode
observation that occurred just prior to the closest approach observations.
Thirty seven files are archived the NIMS observations of 951 Gaspra in the
PDS Imaging Node. The data set was reduced to five observations by
calibrating only the data that had significant target signal and that did
not include the closest approach data.
6 - DATA FILE FORMAT DESCRIPTION
The data files for the 951 Gaspra radiance Near Infrared Mapping
Spectrometer point spectra are ASCII (American Standard Code for
Information Interchange) text files. Each data file contains columns of
floating point numeric values separated by spaces. The radiance files
consist of columns of wavelength values (micrometers), radiance
(microwatts*centimeters**-2*steradians** -1*micrometers**-1), and
instrument radiance precision (microwatts*centimeters**-2*steradians**
-1*micrometers**-1). The calibration files consist of columns of
wavelength values (micrometers), sensitivity
(micrometers*steradians*centimeters**2*microwatts**-1), and dark values
(data number). The sol (short for solar) files consist of columns of
wavelength values (micrometers) and radiance
(microwatts*centimeters**-2*steradians** -1*micrometers**-1). The iof
files consist of columns of wavelength values (micrometers), incidence
over flux ratio values, and instrument incidence over flux ratio precision
values.
7 - DATA PROCESSING HISTORY
The radiance data in this data volume are derived from archived NIMS
spectral images files available from the NASA PDS Imaging Node. The 951
Gaspra NIMS observations are archived in the Imaging Node as spectral
image files of data number or instrument count values (DATA SET ID
='GO-A-NIMS-3-TUBE-V1.0'). These data number values were converted into
radiance using the following equation: radiance value = (data number value
- dark value)/sensitivity value. The dark value was determined through an
average of background sky pixels for the observation. The sensitivity
value was obtained from recorded values used to process the NIMS
observations of the Earth and Moon (DATA SET ID =
'GO-E-NIMS-4-MOSAIC-V1.0') that employed the same instrument collection
modes and gains as the 951 Gaspra observations. Full map and short map
mode data were subjected to a running box filter average to compensate for
spacecraft/scan platform motion (jitter) while the NIMS sensor cycled
through multiple grating positions. This small (jitter) motion causes
unresolved observations to have small variations in their optical path
through the sensor as a multiple grating step spectrum is being collected.
If the motion is of sufficient scale, it can impart a periodic variation
in radiance in wavelength space. The box filter sample size was determined
when a repetition of saw teeth like radiance values were no longer present
in the spectral data. The solar radiance spectra were modeled from NIMS
solar radiance spectra archived in observations of the Earth and Moon DATA
SET ID = 'GO-E-NIMS-4-MOSAIC-V1.0') and the 951 Gaspra, Galileo
spacecraft, and Sun geometry data stored in the Galileo mission SPICE
kernels. The inverse square law was used to scale the values from the
Earth/Moon observation data set to the appropriate values for the 951
Gaspra observations. The incidence over flux spectra were created by
ratioing the radiance spectra with the modeled solar radiance spectra.
File specific processing information can be found in the following
paragraphs of this section.
The file gap015radiance.tab is derived from DN values from line 9 sample
11 in PDS file GAP015TN.QUB (DATA SET ID GO-A-NIMS-3-TUBE-V1.0) and
calibrated with values from the PDS file E1WANTA2CI.QUB (DATA SET ID
GO-E-NIMS-4-MOSAIC-V1.0). Radiance data was subjected to a running box
average filter of 21 samples. Channels with no signal are not included in
this data file.
The file gap015calibration.tab contains the calibration values from the
PDS file E1WANTA2CI.QUB (DATA SET ID GO-E-NIMS-4-MOSAIC-V1.0) used to
create gap015radiance.tab. The calibration coefficients include the
sensitivity offset calculated by the running box average filter.
The gap015sol.tab contains the modeled radiance solar spectrum that
illuminated asteroid 951 Gaspra as observed by the Galileo spacecraft Near
Infrared Mapping Spectrometer at 22:01:16 UT on October 29, 1991. It was
modeled using solar radiance data archived in the NASA PDS Imaging Node
file E1WANTA2CI.QUB from the GO-E-NIMS-4-MOSAIC-V1.0 data set.
The gap015iof.tab contains the incidence over flux spectrum of asteroid
951 Gaspra as observed by the Galileo spacecraft Near Infrared Mapping
Spectrometer at 22:01:16 UT on October 29, 1991. It was created by a
ratio of gap015radiance.tab by gap015sol.tab with a 50% pixel fill factor.
The file gap035radiance.tab is derived from DN values from line 69 sample
4 in PDS file GAP035TN.QUB(DATA SET ID =GO-A-NIMS-3-TUBE-V1.0) and
calibrated with values from the PDS file E1WANTA2CI.QUB (DATA SET ID
GO-E-NIMS-4-MOSAIC-V1.0).
The file gap035calibration.tab contains the calibration values from the
PDS file E1WANTA2CI.QUB (DATA SET ID GO-E-NIMS-4-MOSAIC-V1.0) used to
create gap035radiance.tab.
The gap035sol.tab contains the modeled radiance solar spectrum that
illuminated asteroid 951 Gaspra as observed by the Galileo spacecraft Near
Infrared Mapping Spectrometer at 21:38:18 UT on October 29, 1991. It was
modeled using solar radiance data archived in the NASA PDS Imaging Node
file E1WANTA2CI.QUB from the GO-E-NIMS-4-MOSAIC-V1.0 data set.
The gap035iof.tab contains the incidence over flux spectrum of asteroid
951 Gaspra as observed by the Galileo spacecraft Near Infrared Mapping
Spectrometer at 21:38:18 UT on October 29, 1991. It was created by a
ratio of gap035radiance.tab by gap035sol.tab with a 33% pixel fill factor.
The file gap036radiance.tab is derived from DN values from line 252 sample
18 in PDS file GAP036TN.QUB (DATA SET ID GO-A-NIMS-3-TUBE-V1.0) and
calibrated with values from the PDS file E1WANTA2CI.QUB (DATA SET ID
GO-E-NIMS-4-MOSAIC-V1.0).
The file gap036calibration.tab contains the calibration values from the
PDS file E1WANTA2CI.QUB (DATA SET ID GO-J-NIMS-4-MOSAIC-V1.0) used to
create gap036radiance.tab.
The gap036sol.tab contains the modeled radiance solar spectrum that
illuminated asteroid 951 Gaspra as observed by the Galileo spacecraft Near
Infrared Mapping Spectrometer at 21:51:36 UT on October 29, 1991. It was
modeled using solar radiance data archived in the NASA PDS Imaging Node
file E1WANTA2CI.QUB from the GO-E-NIMS-4-MOSAIC-V1.0 data set.
The gap036iof.tab contains the incidence over flux spectrum of asteroid
951 Gaspra as observed by the Galileo spacecraft Near Infrared Mapping
Spectrometer at 21:51:26 UT on October 29, 1991. It was created by a
ratio of gap036radiance.tab by gap036sol.tab with a 33% pixel fill factor.
The file gap037radiance.tab is derived from DN values from line 34 sample
18 in PDS file GAP037TN.QUB (DATA SET ID GO-A-NIMS-3-TUBE-V1.0) and
calibrated with values from the PDS file E1WANTA2CI.QUB (DATA SET ID
GO-E-NIMS-4-MOSAIC-V1.0). Radiance data was subjected to a running box
average filter of 9 samples.
The file gap037calibration.tab contains the calibration values from the
PDS file E1WANTA2CI.QUB (DATA SET ID GO-E-NIMS-4-MOSAIC-V1.0) used to
create gap037radiance.tab. The calibration coefficients include the
sensitivity offset calculated by the running box average filter.
The gap037sol.tab contains the modeled radiance solar spectrum that
illuminated asteroid 951 Gaspra as observed by the Galileo spacecraft Near
Infrared Mapping Spectrometer at 21:54:29 UT on October 29, 1991. It was
modeled using solar radiance data archived in the NASA PDS Imaging Node
file E1WANTA2CI.QUB from the GO-E-NIMS-4-MOSAIC-V1.0 data set.
The gap037iof.tab contains the incidence over flux spectrum of asteroid
951 Gaspra as observed by the Galileo spacecraft Near Infrared Mapping
Spectrometer at 21:54:29 UT on October 29, 1991. It was created by a
ratio of gap037radiance.tab by gap037sol.tab with a 33% pixel fill factor.
The file gap038radiance.tab is derived from DN values from line 49 sample
17 in PDS file GAP038TN.QUB (DATA SET ID GO-A-NIMS-3-TUBE-V1.0) and
calibrated with values from the PDS file E1WANTA2CI.QUB (DATA SET ID
GO-E-NIMS-4-MOSAIC-V1.0). Radiance data was subjected to a running box
average filter of 7 samples.
The file gap038calibration.tab contains the calibration values from the
PDS file E1WANTA2CI.QUB (DATA SET ID GO-E-NIMS-4-MOSAIC-V1.0) used to
create gap038radiance.tab. The calibration coefficients include the
sensitivity offset calculated by the running box average filter.
The gap038sol.tab contains the modeled radiance solar spectrum that
illuminated asteroid 951 Gaspra as observed by the Galileo spacecraft Near
Infrared Mapping Spectrometer at 22:07:13 UT on October 29, 1991. It was
modeled using solar radiance data archived in the NASA PDS Imaging Node
file E1WANTA2CI.QUB from the GO-E-NIMS-4-MOSAIC-V1.0 data set.
The gap038iof.tab contains the incidence over flux spectrum of asteroid
951 Gaspra as observed by the Galileo spacecraft Near Infrared Mapping
Spectrometer at 22:01:13 UT on October 29, 1991. It was created by a
ratio of gap038radiance.tab by gap038sol.tab with a 66% pixel fill factor.
8 - POINT SPECTRA SUPPORT INFORMATION
The point spectra support data information can be found in the table file
science_index.tab. This information includes the name of the data file,
the name of the source file archived in the Imaging Node of the PDS, the
instrument spacecraft mission name, the instrument name, the spacecraft
mission phase name, the target of the observation, the instrument
observation type/mode, the observation identifier, the observation start
and stop time (Universal Time), the observation start and stop time in
Julian date, the observation start and stop time as recorded by the
spacecraft clock, the instrument to target center distance (kilometers),
the target to Sun distance (kilometers), the spacecraft to Sun position
vector in the J2000 reference frame (kilometers), the target to Sun
position vector in the J2000 reference frame (kilometers), the target to
Earth distance (kilometers), the spacecraft to Earth position vector in
the J2000 reference frame (kilometers), the Earth to target position
vector in the J2000 reference frame (kilometers), the horizontal pixel
scale (kilometers), the vertical pixel scale (kilometers), the observation
phase angle (degrees), the Earth to spacecraft state vector in the J2000
reference frame (position in kilometers, velocity in kilometers/second),
and the Earth to target state vector in the J2000 reference frame
(position in kilometers, velocity in kilometers/second). These geometry
values were calculated using the SPICE Tool kit and the following SPICE
data files: s970311a.bsp, naif0008.tls, mips010314.tpc, and mk00062a.tsc.
9 - REFERENCES
Carlson, R. W., P. R. Weissman, W. D. Smythe, J. C. Mahoney,
NIMS-Science-Team, and NIMS-Engineering-Team (1992), Near-Infrared Mapping
Spectrometer Experiment on Galileo, Space Science Reviews, 60, 457-502.
Carlson, R. W., P. R. P.R. Weissman, W. D. Smythe, L. C. Byrne, A. C.
Ocampo, L. A. Soderblom, H. H. Kieffer, F. Fanale, J. Granahan, and T. B.
McCord (1992B), Gaspra: Spatially resolved infrared spectra from Galileo,
Bulletin of the American Astronomical Society, 24, 932.
Wolf, A. A. (1992), Jet Propulsion Laboratory Interoffice Memorandum
GLL-NAV-92-90, Satellite Tour 92-14A Data Package, May 12, 1992.
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