PDS_VERSION_ID = PDS3 /* GO_1104: asttds.cat */ LABEL_REVISION_NOTE = " 1999 Jul 23 - Original by R. Mehlman, UCLA/IGPP with calibration notes by R.W. Carlson, NIMS P.I., JPL and pointing notes by L.W. Kamp, MIPS/NIMS, JPL" RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "GO-A-NIMS-3-TUBE-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = " GALILEO NIMS SPECTRAL IMAGE TUBES: ASTEROIDS" DATA_SET_COLLECTION_MEMBER_FLG = "Y" START_TIME = 1991-302T14:33:45Z STOP_TIME = 1993-242T19:22:13Z DATA_SET_RELEASE_DATE = 1998-12-10 DATA_OBJECT_TYPE = QUBE PRODUCER_FULL_NAME = "DR. ROBERT W. CARLSON" DETAILED_CATALOG_FLAG = "N" DATA_SET_DESC = " 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 were made for all NIMS observations of Gaspra and Ida and they form this dataset. Data from many observations may be binned and resampled while being projected onto the target, forming MOSAICS (or G-CUBES) which comprise the separate MOSAIC dataset. Mosaics may be made in the future for one high spatial resolution observation of Gaspra, and two of Ida; the other asteroid observations are not extensive enough to make this treatment useful. Geometric and other information is stored in backplanes of both kinds of cubes. This dataset consists of raw data number (DN) tubes of all Galileo NIMS observations of Gaspra, and of those observations of Ida which were played back, including associated flight calibration data. Due to problems with the NIMS calibration at Gaspra and Ida, radiance and I/F tubes were NOT produced at this time, though they may be in the future. Observations of Gaspra occurred in late October of 1991, most of the data being recorded on tape. Portions of the tape were played back subsequently, but the entire tape was played back just before the Earth-2 encounter in December of 1992. Observations of Ida occurred in late August of 1993, with data also recorded on tape. But due to the failed deployment of the Galileo high gain antenna, only a limited amount of the data could be returned. Both the Gaspra and Ida tubes are archived on volume GO_1104, along with data from the G1 encounter of Jupiter. 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 ========== 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, but left as raw DNs. The tube form provides a friendlier way to view the data than the EDRs. Wavelength values are based on wavelength calibrations described above. 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. 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 tubes. 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 raw data numbers, ranging from 0 to 1023. 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. 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.) 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)." CONFIDENCE_LEVEL_NOTE = " Confidence Level Overview ========================= NIMS tubes in this dataset contain unresampled data from all observations of Gaspra and Ida 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, but left as raw DNs. Geometric information has been added as backplanes of the tubes. Users of NIMS tubes should be aware of the effect of pointing errors on their geometry. This is the principal source of uncertainty in the dataset. There are also gaps in the planned spatial and/or spectral coverage of some Ida observations due to limited playback time and unplanned interruptions of data receipt at downlink stations. 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. 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 ========================= 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, and wavelengths of the bands in the tube. In addition, the index table on each volume collects much of the geometric information from the tube 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. Data quality is best judged by actually viewing the data products themselves. 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, 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 uncertainty in the NIMS asteroid dataset are due to errors in the 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. Occasionally, the star scanner was disabled, either for 'bright-body avoidance', as at the E2 encounter, or due to an anomaly such as a guide star being incorrectly acquired. 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.) 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 encounter may be found in the NIMS Guide to that encounter. See section 6B of VOLINFO.TXT (in the DOCUMENT directory of any NIMS Cube volume) for a more detailed description of how geometry data is obtained and corrected, and on its limitations, and for a guide to the documentation within individual products." END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = GASPRA END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = IDA END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = SKY END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = CAL END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = GO INSTRUMENT_ID = NIMS END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "CARLSONETAL1992" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END