PDS_VERSION_ID = PDS3 /* GO_1115: jupmds.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 (adapted from cruise equivalent of 1993 Sep 24 by R.W. Carlson, W.D. Smythe and R. Mehlman) 2001 Jan 15 - Modified by R. Mehlman to include GEM and to describe new calibration work. 2001 Aug 31 - Modified by R. Mehlman to discuss the stuck grating beginning with the I24 encounter, and its effects on the data." RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "GO-J-NIMS-4-MOSAIC-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = " GALILEO NIMS SPECTRAL IMAGE CUBES: JUPITER OPERATIONS" DATA_SET_COLLECTION_MEMBER_FLG = "Y" START_TIME = 1996-178T14:13:40Z /* start Jupiter ops */ STOP_TIME = 1999-354T06:53:12Z /* end Galileo */ DATA_SET_RELEASE_DATE = 1998-12-10 /* \ Europa mission */ 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 are made for all NIMS observations during Jupiter operations, and form a separate dataset. Data from most observations are *also* binned and resampled while being projected onto the target. These resampled products comprise this dataset. Geometric and other information is stored in backplanes of both kinds of cubes. This dataset includes radiance and I/F mosaics of most Galileo NIMS observations of Jupiter and its satellites, excluding calibrations, limb scans, ring observations, sparse ridealong observations designed for other instruments, spectrometer-mode observations and any otherwise unsuitable for projection onto the target. Observations began in June, 1996 and will continue until the (unknown) end of the mission. Spacecraft clock (SCLK) times begin at 3496645:00. The mosaics are archived in volumes beginning with GO_1104, each of which contains both tubes and mosaics from (usually) a single orbit of Jupiter, as well as browse products (masks) which provide an overview of the data. (Earlier volumes in this series contain data from Galileo flybys of Venus, the Earth and the Moon.) Parameters ========== A band in a NIMS mosaic 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. After the data is binned and resampled and projected, the resulting mosaic (or g-cube) has image dimensions determined by the motions of the secondary mirror, the scan platform and the spacecraft. 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. The NIMS grating motion capability failed prior to the I24 encounter. Subsequently, NIMS returned only 12 useful wavelengths instead of original 408. Obviously, this reduced the usefulness of the NIMS data in certain respects. However the resulting data redundancy turned out to have noise reduction advantages in the high radiation environment of Jupiter, and very useful products were generated. Further details may be found in VOLINFO.TXT in the DOCUMENT directory of the archive volume, and in the instrument paper [CARLSONETAL1992]. Processing ========== Mosaics 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, converted to spectral radiance and I/F units using the best calibration available at the time of processing, and projected onto the target according to spacecraft and target position and scan platform orientation, using a complex binning and resampling procedure. Wavelength values are based on wavelength calibrations described above. Radiance values are derived from 10-bit NIMS raw data numbers by first subtracting average dark values (determined for each detector and mirror position and for each gain state), then dividing by band-dependent sensitivities, determined as described above. I/F values are derived from radiances by dividing each NIMS spectrum by the solar spectrum at the target's distance from the sun. Backplanes of geometric information 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, based on the mosaic (if one was produced) or the tube (if not). In the former case, the mask includes an RGB summary image formed from three NIMS bands, or functions of bands, averaged spectra from one to six selected areas keyed to the image, a 2-D histogram and annotation. The masks serve as browse products for the cubes. Data structure ============== The mosaic 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 cube. A third object is a 'sample spectrum qube': a 'stack' of six spectral plots, each an average over a selected area of the g-cube. (These also appear on the hardcopy and digital 'masks'.) The fourth and principal object is the actual NIMS spectral image g-cube. Spectral image cube 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 radiance or I/F units. Attached labels describe the structure and units of the cube 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' (for off-limb data). (See comments in the 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 preprint of the NIMS instrument paper [CARLSONETAL1992] is also included. Calibration files, average dark value 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. (They will be published in a separate volume at a later time.) However calibration information is present in the cube labels as vectors of sensitivity (for each wavelength) and of average dark value (for each detector). Some of the geometric information is implicitly contained in the projected 'images'; the rest is explicitly present in backplanes. Software ======== NIMS cubes 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 mosaics 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 mosaics in this dataset contain data from all observations of Jupiter and its satellites that were successfully returned to earth, and that were appropriate for projection onto the target. The raw data numbers from the EDR dataset have been binned, resampled and projected (as described above) and converted to radiance and I/F by applying the best available calibration and pointing information. The results have been written as spectral image cubes. Geometric information has been added as backplanes of the cubes. Users of NIMS mosaics should be aware of (1) the imperfect nature of the calibration used to produce them, particularly the wavelength calibration, (2) the effect of pointing errors on their geometry and (3) the presence of radiation-induced noise in the data. These factors are the principal sources of uncertainty in the dataset. There are also many gaps in the planned spatial and/or spectral coverage of many observations due to limited playback time and unplanned interruptions of data receipt at downlink stations. However it should be mentioned that the Rice compression algorithm used in playback of NIMS recorded data is lossless, and does *not* contribute any uncertainty to the data. 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. Several passes through the Galileo tape recorder are usually made for each orbit, so that coverage information from the first pass is used to fill gaps due to DSN outages, or to extend spatial and/or spectral coverage if playback time is available. Compression ratios are also checked, especially in the first pass, and used to more accurately predict playback time during the second pass, so as to maximize the amount of data that can be returned. Knowledge of typical compression ratios is useful for planning return of similar data in subsequent orbits. 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 ========================= Due to the failure of the Galileo high gain antenna to deploy, the amount of data returned from Jupiter operations was severely limited. Techniques were developed to maximize the scientific value of the data that could be returned, including selective editing of data in wavelength and mirror position, lossless compression of the selected data, and an optional thresholding capability. The resulting coverage is described in several places on the volumes, to which readers are directed. 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 cube contains information about data coverage in the observation: time range, ranges of latitude and longitude, and of other geometric parameters, wavelengths of the bands in the tube, compression ratio during playback, etc. In addition, the index table on each volume collects much of the geometric information from the mosaic (and 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. In addition, complex searches for data under various temporal, geometric and other constraints may be conducted on the World Wide Web using the Planetary Atlas at the PDS Imaging Node. These make use of the index table and labels described above. Data quality is best judged by actually viewing the data products themselves. Radiation noise is the principal problem, roughly inversely proportional to the spacecraft's distance from Jupiter at the time the data is taken, and more pronounced in the higher wavelengths than in the lower ones. Attempts to remove significant radiation noise by a despiking technique have been made for (resampled) mosaics of the icy satellites, but not of Jupiter and Io data. (Tubes, on the other hand, are never despiked and the quality of the observation should be apparent on viewing them.) Mosaics may contain special values in place of some data values, indicating conditions such as missing data (not played back, or lost during telemetry), instrument detector saturation, data below threshold (when an observation was returned using the thresholding capability), 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 uncertainties in the NIMS dataset are due to errors in the calibration and in the pointing geometry and to the presence of radiation noise. Here, we discuss their effects on the NIMS Jupiter mosaic dataset. Calibration ----------- A set of calibration parameters called the 1998A Calibration, derived early in that year, was used for NIMS archival data products from the Galileo Primary Mission, orbits G1 to E11. Subsequently, several errors and idiosyncrasies in the calibration were discovered and are still being analyzed. They are summarized here. (1) The photometric calibration of Detectors 1 and 2 is uncertain, due to temporal changes in their spectral responsivities as well as in the onboard calibration target used for correction. The spectral response for these two detectors is therefore incorrect. (2) The wavelength position behaviour of the instrument has changed over the orbital mission, likely due to radiation damage. A grating rotation parameter (PSHIFT), in units of grating steps, reflects thermal changes in the instrument and mechanical creep of the grating. A value of -1.3 was used during orbits G1-E4, and -1.0 during E6-E11. However this was discovered to be inadequate after E4, most likely because the grating step size has gradually increased. This increase is not reflected in the wavelengths listed in the labels of Prime Mission products (G1-E11). However a new calibration was developed for the Galileo Europa Mission (GEM) which accounts for the changing grating step size. Labels of GEM products (E12-I25) therefor contain more accurate wavelength values. (3) The dark levels for some detectors and gain states have changed with time, particularly for Detector 10 in the highest gain state, where the dark level has dropped by about 10 data numbers (DNs). We used constant values for the 1998A Calibration. (4) The dark level of Detector 9 is slightly increased by the signal level of Detector 17, due to electronic crosstalk. This occurs when Channel 17 is in the low gain state. For most applications it is a small effect (a few DNs), occurring generally when viewing Jovian 5-micron hot spots. (5) Two detectors have become inoperative. Detector 3 failed during orbit E4 and Detector 8 began producing erratic signals in orbit E6. (6) Three more detectors (1, 2 and 7) lost almost all of their sensitivity (and usefulness) after the NIMS grating stuck prior to the I24 encounter, and remained stuck. See section 6A of VOLINFO.TXT (found in the DOCUMENT directory of any NIMS Cube volume) for a description of how calibration parameters are determined, and a more detailed discussion of the limitations described above. Methods to correct for the above-listed errors and idiosyncrasies are being developed. Correction methods and parameters (revised wavelengths, radiance corrections, revised dark levels) will be placed on the NIMS website when available: http://jumpy.igpp.ucla.edu/~nims/ or follow the link from the Galileo website: http://www.jpl.nasa.gov/galileo/ Questions and comments are welcome and encouraged. Send them to nimsinfo@issac.jpl.nasa.gov. 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. After orbit E11 one of the gyros started to degrade, adding an uncertainty in one dimension immediately after performing a clock or cone slew in the positive direction. The errors depend on the size of the slew. A correction was developed by AACS which gradually removes this error using the star scanner. In E14, this procedure was able to remove 10% of the error every spacecraft revolution (about 18 seconds); by E19 this fraction was increased to 40%. Cruise mode was used increasingly as the GEM mission progressed. Occasionally, the star scanner was disabled, either for 'bright-body avoidance' (E2 encounter, Jupiter orbits G1 and others) or due to an anomaly such as a guide star being incorrectly acquired (G7 orbit). 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, but this rarely occurs in Jupiter observations.) 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 orbit may be found in the NIMS Guide to that orbit. See section 6B of VOLINFO.TXT (in the DOCUMENT directory of any NIMS Cube volume) for a fuller description of how geometry data is obtained and corrected, and on its limitations, and for a guide to the documentation within individual products. Noise ----- Within the radiation belts of the Jupiter system, data collected by the NIMS instrument often contain spikes due to charged particles impacting on the instrument's detectors. NIMS (resampled) g-cube products of the icy satellites -- Europa, Ganymede and Callisto -- have been processed to remove the worst of the radiation-induced spikes in the data. Jupiter and Io observations were *not* despiked, since both of these bodies have localized hot spots with very different spectral intensities from adjacent pixels. (For all targets, the (unresampled) tube products were NOT despiked.) In the despiking procedure, spikes above a user-defined threshold are identified in (unresampled) NIMS tubes by a complex algorithm that convolves a 3-dimensional brick through the tube, looking for correlations among bands sampled simultaneously. A list of spikes is produced, and used to remove the spikes from raw EDR data at the beginning of the mosaic (g-cube) generation process. See section 6C of VOLINFO.TXT (in the DOCUMENT directory of any NIMS Cube volume) for a detailed discussion of the despiking procedure. This procedure, admittedly experimental, alters the original data. However, those data are still available in the tube products. If there is a discrepancy, for example, between the depth of an absorption band in a despiked g-cube and in the corresponding tube, the depth derived from the tube may be more reliable. (Of course the same feature may not be apparent in the tube, because its spectra aren't registered.) The low signal to noise ratio of Europa data at wavelengths greater than 2.5 microns makes it difficult for the despiking program to discriminate small spikes from non-noisy data. Therefore, despiked products of Europa may be unreliable at these longer wavelengths." END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = JUPITER END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = IO END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = EUROPA END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = GANYMEDE END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_TARGET TARGET_NAME = CALLISTO 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