Data Set Information
DATA_SET_NAME DAWN GRAND CALIBRATED MARS FLYBY COUNTS V1.0
DATA_SET_ID DAWN-M-GRAND-3-RDR-MARS-COUNTS-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Overview : The Dawn Mission's Gamma Ray and Neutron Detector(GRaND) is a nuclear spectrometer that will collect data needed to mapthe elemental composition of the surfaces of 4-Vesta and 1-Ceres[PRETTYMANETAL2003]. GRaND measures the spectrum of planetary gammarays and neutrons, which originate from cosmic ray interactions andradioactive decay within the surface, while the spacecraft (S/C) is inorbit around each body. The instrument, which is mounted on the +Z deckof the S/C, consists of 21 sensors designed to separately measureradiation originating from the surface of each asteroid and backgroundsources, including the space energetic particle environment and cosmicray interactions with the spacecraft. The nuclear spectroscopy dataprovided by GRaND will be analyzed to determine the abundance of majorrock forming elements, such as O, Fe, Ti, Si, Al, Mg, Ca, Cl andradioactive elements, including K and Th, as well as light-elements suchas H, C, and N, which are constituents of ices and the products ofaqueous alteration of silicate minerals and ices. The GRaND Reduced Data Records (RDR) are a time-orderedcollection of corrected gamma ray and neutron counting data andcalibrated pulse height spectra acquired by GRaND during Mars GravityAssist (MGA). Similar products will be developed for science mapping at4-Vesta and 1-Ceres and for data acquired during cruise. The RDR is acalibrated data product (Level 1B) that is derived from ExperimentData Records (EDR, Level 1A). The objective of the RDR is to providecounting rates and ancillary pointing and ephemeris data needed formapping. The ancillary data includes the subsatellite point, thedistance and direction to body center, and the spacecraft velocityvector relative to Mars, which is needed to account for the motion oflow energy (thermal and epithermal) neutrons relative to the spacecraft.In the current version of the data set, counting rates and fluxescorrected for altitude and spacecraft motion, which require knowledge ofthe instrument response, are not provided; however, future versions ofthe RDR, especially for Vesta and Ceres mapping will include countingrates corrected for solid angle and pointing as well as incident gamma ray and neutron fluxes. Processing steps for the RDR data set include:- Determination of the mid-point time of each science accumulation interval and the livetime, accounting for roll-over of the dead time counter; - Construction of pulse height spectra from the gamma ray and neutron event data;- Correction of pulse height spectra for ADC differential nonlinearity;- Calibration and gain/offset correction of pulse height spectra to provide a consistent energy scale for peak analysis;- Analysis of spectra to determine the net areas (counting rates) of selected spectral features, including: -- the 10B(n,alpha) peak area from the Category 1 (CAT1) spectrum, for the +Z phoswich, which is sensitive to epithermal neutrons from the target body; -- the 6Li(n,t) peak area from the +Z CAT1 phoswich, which is sensitive to thermal and epithermal neutrons from the target body; -- the net areas of the 478 keV gamma ray full energy peaks for each of the four CAT2 BGO pulse height spectra, resulting from coincidences between 93 keV deposited in the four boron-loaded-plastic (BLP) scintillators by the 10B(n,alpha)7Li* reaction and the associated gamma ray (7Li*->7Li+478keV): --- Because the +Z and -Z BLP scintillators are shielded by Gd and lithiated glass, their CAT2 counting rates are sensitive to epithermal neutrons; --- Because the side +Y and -Y scintillators are not shielded, their counting rates are sensitive to both thermal and epithermal neutrons. -- the fast neutron counting rates for each of the boron-loaded plastic scintillators; -- net counting rates for selected gamma rays. The data set consists of ASCII tables, divided byfunctionality into four categories: ephemeris and pointing data EPX(where X denotes the target: X:'X' for cruise, X:'M' for Mars, X:'V' forVesta, and X:'C' for Ceres); neutron counting rates (NCR); gamma raycounting rates (GCR), and the BGO pulse height spectrum (BGO). The fastneutron flux spectrum (with units of neutrons/cm2/s/MeV), which can beunfolded from the fast neutron pulse height spectrum given the responsefunction, and CZT composite spectrum will be included in futurereleases. The Level1b data are represented as a time series in which countingrates and spectra are averaged over a time-window, consisting of an oddnumber of science accumulation intervals (TELREADOUT) selected by theevaluator. Note that ephemeris and pointing data are not averaged.Rather, their instantaneous values at the mid-point of each time windowis reported. Two types of time series are possible: a central movingaverage (CMA), in which the averaging window is centered on each pointin the original time series, and a result is reported for each scienceaccumulation interval; and a decimated time series (DTS), in which theaveraging window is shifted forward in time by its width, resulting a data set that has fewer points than the original time series. The CMA is intended for mapping, for example, in cases where fullsampling of rapidly changing counting rates and solid angles is needed.The DTS is useful for averaging over long periods of time, for example,during cruise, when counting rates are not changing rapidly. Byaveraging over long time intervals, the DTS can be used to produce highprecision gamma ray spectra needed for accurate peak identification,analysis, and calibration. Parameters : The RDR data are derived from the EDR files, which coverarbitrary and irregular time periods, generally determined by thepattern of downlinks from the spacecraft. The RDR are evaluated for asingle instrument state found in the EDR files, generally correspondingto the instrument fully configured for science data acquisition.Consequently, the value of TELREADOUT is the same for all recordsextracted from a particular EDR file. The EDR science records need notbe contiguous; however, time-windows containing gaps are not included inthe RDR. Generally, science data records with elevated counting ratesassociated with solar energetic particle (SEP) events will be excludedfrom the RDR; however, throughout MGA, solar activity was minimal and norecords were lost. The RDR files are contained in a single directory. The name of thedirectory includes the type of time series (CMA or DTS): GRD-L1B-Y1M1D1-Y2M2D2_YCMCDC-CMA (directory name for a central moving average), or GRD-L1B-Y1M1D1-Y2M2D2_YCMCDC-DTS (directory name for a decimated time series). The top level directory name contains the SCET UTC dates for the firstand last science data records (Y1M1D1 and Y2M2D2, respectively), and thecreation date (YCMCDC) for the archive. For example, forGRD-L1B-090217-090218_090517, the first science data record was acquiredon 17-Feb-2009. The last science data record was acquired on18-Feb-2009. The archive was created by the processing code on 17-May-2009. The data files and formats are the same for both types of time series.All counting rates are reported as counts per second. Histograms haveunits of counts/second/channel. The processing parameters, including thewindow width and the value of TELREADOUT are given in the label files.The following files are included (note that 'TSI' is either 'CMA' or'DTS', depending on the type of time series): GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-EPM.TAB - Contains a time series of ephemeris and pointing data, including the total livetime, the distance and direction to body center, the velocity of the spacecraft rotated into the reference frame of the instrument, the subsatellite position and altitude, and the fractional solid angle subtended by the body at the spacecraft. In addition, the average counting rate for the 'coincidence of three or more sensors' scaler (SCALER_SCI[15]) is provided as an indicator of the interaction rate of galactic cosmic rays. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-NCR.TAB - Contains a time series of neutron counting rates determined from CAT1, CAT2, and CAT4 pulse height spectra. Counting rates sensitive to neutrons in the thermal, epithermal, and fast energy ranges are provided. Propagated uncertainties (1-sigma) are provided. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-GCR.TAB - Contains a time series of net areas for selected full energy interaction peaks from the pulse height spectrum acquired by the BGO sensor. Propagated uncertainties (1-sigma) are provided. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-TSI-BGO.TAB - Contains a time series of BGO 1024-channel pulse height spectra that have been adjusted so that the offset is 0 keV with a slope of 8.9 keV/channel. Note that the method for determining the uncertainty in the counting rates for each channel is provided in the format file. Processing : The RDR data from each EDR data file are processed in several steps:1. Specification of processing parameters, including the target instrument state, the time series type and window width, and parameters for spectrum processing and analysis;2. Extracting science data records from the EDR for the selected state;3. Generation of the time series structure;4. Construction of pulse height spectra from event data;5. Correction of pulse height spectra in preparation for peak analysis (including livetime, ADC differential nonlinearity and gain and offset corrections).6. Determining peak areas and uncertainties from time-averaged neutron and gamma ray spectra;7. Writing the data and labels. Details of the processing are described in Prettyman, T.H. and W.C.Feldman (2009), PDS data processing for the Dawn gamma ray and neutrondetector, GRD_L1B_Processing_V2.0.pdf, available in the documentdirectory of this volume. A summary of the processing algorithms andparameters is provided here. The RDR time series is constructed by averaging corrected counting ratesover a window using one of two methods: Central Moving Average (CMA) orDecimated Time Series (DTS). The CMA method provides a result for eachscience data record by averaging over records on either side of thecurrent point as illustrated here: Science records XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ... | 1 | | 2 | | 3 | | 4 | | 5 | ....where each of the time-ordered science records is indicated by an X andthe window boundaries are indicated by the vertical bars. The middle ofeach time series is labeled with an index (1, 2, ...). The DTS method marches the window through the science datarecords, resulting in a decimated time series as follows: Science records XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX ...| 1 || 2 || 3 || 4 || 5 |... The number of science data records averaged for each point in the timeseries is given by WINDOW_WIDTH, which is an odd integer. For bothmethods, all time, ephemeris, and pointing information is determined atthe midpoint of the window. A variety of spectrum analysis methods are used to extractpeak areas recorded in the neutron and gamma ray files. These aredescribed in detail in the data processing document. For Mars flyby,time-averaged background data were determined during a period of timefor which the instrument was far from Mars and sensitive only toneutrons produced on board the spacecraft by galactic cosmic rayinteractions. In some cases (e.g., analysis of the CAT1 spectrum),background continuum functions were determined using a coarse timewindow (CMA), under the assumption that the background shape does notchange as rapidly as the peak. The window width for Mars flybybackground subtraction was 11 science accumulation intervals. Ancillary Data : The Level 1B data include ancillary data in the form ofSCET UTC strings reported in each row of the time series tables. TheUTC strings are determined from the spacecraft clock ticks recorded ineach state-of-health packet and for the first packet in each sciencedata record using NAIF SPICE (leap seconds kernel). This information isused in Level 1B processing to accurately determine the mid-point ofeach science accumulation interval, which is used for mapping. Inaddition, pointing and ephemeris data are provided in the Level 1B fileat the center of each time interval: - the distance and direction of body center in the instrument coordinate system; - the velocity of the spacecraft rotated into the reference frame of the instrument - the subsatellite longitude, latitude, and altitude; - the fractional solid angle subtended by Mars at the spacecraft.The target was 'IAU_MARS'. Coordinate System : The instrument coordinate system (Fig. 1) determines thenaming convention of the sensors and orientation of the instrumentrelative to the spacecraft. The use of MZ indicates a sensor on the -Z(zenith-facing during mapping) side of GRaND; PZ indicates the sensoris on the +Z (spacecraft) side; MY indicates the sensor is on the -Yside (inboard) side of the instrument; and PY indicates the sensor is onthe +Y side (outboard, towards the +Y solar panel) side of theinstrument. The phototube assembly, marked 'P' on the diagram in Fig. 1 points along the +X axis (towards the high gain antenna). ................. . ooooooooooooo . . o o . . o o . . o +Z o . . o (PZ) o . . o o .---> +Y (PY) . ooo ooo . . P P . . P P . . PPPPPPPPP . . . ................. | v +X (PX) Figure 1. The coordinate system for GRaND is the same asthat of the S/C. For the diagram above, the observer is looking in the-Z (MZ) direction and can see the outline of the phoswich assembly (o)on the +Z side of GRaND. The phototubes are on the +X side and thescintillators are on the -X side. During mapping at Vesta and Ceres, theplanetary surface is in the +Z direction. Software : The RDR were generated using an IDL-based processingcode compatible with IDL Version 7.0, distributed by ITT VisualInformation Solutions, Boulder, CO (2008). Program name:GRD_L1B_Pipeline.pro. Version: 1.0. Ancillary data were generated using the NAIF/ICY IDL toolkit. Media/Format : The RDR label and data files are delivered by electronictransmission to the PDS.
DATA_SET_RELEASE_DATE 3000-01-01T00:00:00.000Z
START_TIME 2009-02-17T10:33:00.000Z
STOP_TIME 2009-02-18T12:45:00.000Z
MISSION_NAME DAWN MISSION TO VESTA AND CERES
MISSION_START_DATE 2007-09-27T12:00:00.000Z
MISSION_STOP_DATE 2018-10-31T12:00:00.000Z
TARGET_NAME
TARGET_TYPE
INSTRUMENT_HOST_ID DAWN
INSTRUMENT_NAME GAMMA-RAY AND NEUTRON DETECTOR
INSTRUMENT_ID GRAND
INSTRUMENT_TYPE NEUTRON SPECTROMETER
GAMMA RAY SPECTROMETER
NODE_NAME Small Bodies
ARCHIVE_STATUS LOCALLY ARCHIVED
CONFIDENCE_LEVEL_NOTE Review : The RDR will be reviewed internally by the Dawn ScienceTeam prior to submission to the PDS. The PDS will also conduct anexternal peer review of the RDR prior to releasing the data to thegeneral public. Data Coverage/Quality : The RDR does not include all of the science datarecords, only those corresponding to instrument state selected by theevaluator. Science data records acquired during instrument ramp up orpower on are typically excluded from the RDR. While Dawn successfully completed it's gravity assist at Mars, thespacecraft entered safe-mode shortly following closest approach. At thattime, all of the instruments were shut down. All planned activities whichhad not been executed did not occur. Any data from activities which hadtaken place, but which had not yet been dumped out of instrument memoryand into the spacecraft virtual recorder were lost. GRaND returned datathrough shortly after closest approach. FC2 returned 6 lit images ofMars. VIR returned no data for the Mars flyby. Limitations : The RDR is an intermediate data product. While it is a time series,sufficient information is provided to make maps of counting ratescorrected for altitude and pointing, suitable for scientific dataanalysis. Determination of spatially-resolved elemental abundancesrequires knowledge of the instrument response function, which will beprovided as a higher order data product, along with models of theradiation background and radiation output from the target body.
CITATION_DESCRIPTION Prettyman, T.H., DAWN GRAND CALIBRATED MARS FLYBYCOUNTS V1.0. DAWN-M-GRAND-2-RDR-MARS-COUNTS-V1.0. NASA Planetary DataSystem, 2009.
ABSTRACT_TEXT The GRaND RDR are a time-ordered collection ofcorrected gamma ray and neutron counting data and calibrated pulseheight spectra acquired by GRaND during all phases of the Dawn mission.This data set is specific to Mars Gravity Assist (MGA), and includesdata acquired during Mars approach and flyby. The RDR is a calibrateddata set derived from the Experiment Data Records (EDR), consisting oftime-series counting rates from which elemental abundances can bedetermined. Ancillary ephemeris and pointing data needed for analysisand mapping of the time series data are included in the data set.
PRODUCER_FULL_NAME THOMAS H. PRETTYMAN
SEARCH/ACCESS DATA
  • SBN PSI WEBSITE
    		•