Data Set Information
DATA_SET_NAME DAWN GRAND CALIBRATED (RDR) CERES COUNTS V1.0
DATA_SET_ID DAWN-A-GRAND-3-RDR-CERES-COUNTS-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Acronyms and Abbreviations : BGO Bismuth Germanate EDR Experimental data records (Level 1A) EPG Spacecraft Ephemerides, Pointing, and measurement Geometry GCR Galactic Cosmic Ray HED Howardite, Eucrite, and Diogenite meteorites PDS Planetary Data System RDR Reduced data records (Level 1B) SBN Small Bodies Node of the Planetary Data System SCLK Spacecraft Clock Overview : This data set contains Reduced Data Records (RDR) from the Dawn mission's Gamma Ray and Neutron Detector (GRaND). The archive includes data from Dawn's encounter with Ceres within the dataset start and stop times. The processing steps and file formats are the same as for Vesta encounter. A description of GRaND can be found in the instrument catalog (included in the archive) and in the literature [PRETTYMANETAL2003, PRETTYMANETAL2011]. GRaND data were used to study Vesta's geochemistry [PRETTYMANETAL2012, LAWRENCEETAL2013, PEPLOWSKIETAL2013, PRETTYMANETAL2013, YAMASHITAETAL2013, PRETTYMANETAL2015]. Maps developed by these studies are available from the PDS Small Bodies Node (SBN) as Level 2 data products for hydrogen, iron, neutron counting products, and high energy gamma rays. Similar data products are planned for Ceres and will be archived after LAMO data are acquired. The RDR includes calibrated, time-series spectra and counting rates along with information needed for analysis and mapping. Improved reduction and analyses of gamma ray spectra has enabled the identification of additional elements [Yamashita et al., 2014, 45th Lunar and Planetary Science conference, Abstract 2674] and quantification of global concentrations of K and Th [PRETTYMANETAL2015]. Energy-calibrated and corrected gamma ray spectra included in this volume support studies of Ceres' geochemistry and the space energetic particle environment. The RDR data set includes position, pointing, and measurement geometry data that can be used with both the RDR and Experimental Data Records (EDR) for studies with neutron and gamma ray data. The PDS archive volumme structure was updated following LAMO to include all Ceres data on a single volume (DWNCGRD_1B) rather than spreading them over multiple volumes. Data previously archived on DWNCAGRD_1B are included here unchanged and that volume is superseded by this one. Parameters : The RDR are a time-ordered collection of corrected and calibrated pulse height spectra and counting rates acquired by GRaND at Ceres. Spacecraft ephemerides, pointing, and measurement geometry (EPG) accompanies the counting data. The RDR were derived from a subset of the EDR (Level 1A) data archived at the PDS SBN. Portions of the EDR data set have not been subjected to energy calibration and, therefore, are not included in the RDR. For example, spectral data for the boron-loaded plastic (BLP) and lithium-loaded glass (LiG) phoswiches do not vary appreciably in gain and are free of digitization artifacts. For this data set, peak areas can be reliably extracted from the EDR within selected epochs for which instrument settings are constant [PRETTYMANETAL2012]. In contrast, spectra acquired by the bismuth germanate (BGO) scintillator contain pronounced digitization artifacts and must be subjected to gain-offset corrections in order to achieve the highest possible resolution for gamma ray elemental analysis. EPG data and selected scaler counting products are included in a single file, which contains an entry for every record found in the EDR within the start and stop times: GRD-L1B-150313-150509_YCMCDC-EPG.TAB where YCMCDC is the creation date of the data file. Each record of the EPG file includes data for a single science accumulation interval. Entries include the time of acquisition, mission phase, instrument setting and live time, the spacecraft position at the midpoint of the data acquisition interval, the solid angle subtended by Ceres at the spacecraft, and the triple coincidence counting rate. The latter can be used as a proxy for the galactic cosmic ray flux. Example uses of the EPG data include binning counts in longitude and latitude (mapping), correcting counting rates for variations in geometry (solid angle) and fluctuations in the galactic cosmic ray flux [e.g. see PRETTYMANETAL2011, PRETTYMANETAL2012]. Corrected and energy-calibrated gamma ray spectra are provided in separate files for each phase of Ceres encounter. The files have the following naming convention: GRD-L1B-Y0M0D0-Y1M1D1_YCMCDC-PHS-BGOC.TAB where PHS is the three-letter mission phase identifier (e.g. CSA is Ceres Science Approach; see the mission catalog for a complete list), Y0M0D0 is the date on which the first science record was acquired during the mission phase and Y1M1D1 is the date for the last science data record. YCMCDC is the creation date of the data file. Each record contains the spacecraft ephemeris time (SCET) in UTC format and the spacecraft clock (SCLK) ticks (seconds) followed by counts/channel for each of 1024 channels. Each spectrum has been subjected to a linear energy calibration and rebinned to a common energy scale. The midpoint of each channel can be obtained by multiplying the channel index (0 to 1023) by 8.9 keV/channel. Digitization artifacts caused by the differential nonlinearity of the analog-digital converter have been removed. ***Cross-referencing with SCLK*** : Every GRaND EPG science data record and RDR record is labeled with spacecraft clock (SCLK) ticks, represented as a 9-digit integer. The value of SCLK is unique for each record. Thus, SCLK can be used as a serial code to identify the same science data record within other data files. Each record of the EPG file includes SCLK. This allows instrument settings, live time, position, pointing, solid angle, and GCR corrections to be reliably matched with data in any EDR or RDR file. Processing : Processing steps and validation of the ephemeris, pointing and geometry data are described in GRD_L1B_Ephemeris_Pointing_Geometry_v3.pdf. A description of the BGO data processing can be found in GRD_L1B_BGO_Data_Processing_v4.pdf. The data processing documents can be found in the DOCUMENTS directory. Ancillary Data : Spacecraft ephemerides and pointing data were determined using NASA's Navigation and Ancillary Information Facility (NAIF) SPICE Toolkit for IDL (version N65). SPICE kernels for Ceres encounter were downloaded from the NASA Planetary Data System (dawnsp_2000). Coordinate System : Longitudes are given in the International Astronomical Union (IAU) coordinate system for Ceres. The instrument coordinate system (Fig. 1) determines the naming convention of the sensors and orientation of the instrument relative to the spacecraft. The use of MZ indicates a sensor on the -Z (zenith- facing during mapping) side of GRaND; PZ indicates the sensor is on the +Z (spacecraft) side; MY indicates the sensor is on the -Y side (inboard) side of the instrument; and PY indicates the sensor is on the +Y side (outboard, towards the +Y solar panel) side of the instrument. The phototube assembly, marked 'P' on the diagram in Fig. 1 points along the +X axis (towards the high gain antenna). The RDR/EPG contains pointing information. Direction cosines found in each EPG record specify the direction of Ceres body center in the instrument coordinate system. The cosines DIR_U, DIR_V, DIR_W are measured relative to the X-, Y-, and Z-axes, respectively. ................. . 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 as that 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 the scintillators are on the -X side. During mapping at Vesta and Ceres, the planetary surface is in the +Z direction. Software : No software is provided with the archived data. Media/Format : The RDR files are delivered by electronic transmission to the PDS. The RDR consists of ASCII tables with separate labels containing the format specification.
DATA_SET_RELEASE_DATE 2017-09-01T00:00:00.000Z
START_TIME 2015-03-13T03:22:36.000Z
STOP_TIME 2018-10-26T06:09:05.000Z
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 1 CERES
TARGET_TYPE ASTEROID
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
CONFIDENCE_LEVEL_NOTE Review : The RDR were reviewed internally by the Dawn Science Team prior to submission to the PDS. The PDS will conduct an external peer review of the EDR prior to releasing the data to the general public. Data Coverage/Quality : The RDR were derived from the archived EDR data set, which includes all of the data acquired during flight. Nonetheless, the EDR contained gaps associated with instrument and spacecraft operations as well as corrupted data. A complete description of the EDR data coverage can be found in the CONFIDENCE_LEVEL_NOTE for the EDR data set. The EPG data file contains spacecraft position and pointing for each and every EDR record. Thus, the EPG data can be used in the analysis of any GRaND time-series data product (EDR or RDR). Position and geometry data are available for all records; however, for a small portion of the records triples counts, live time, and information regarding instrument configuration are unavailable and are flagged by MISSING_CONSTANT. In addition, the counting data may occasionally contain anomalies, such as spikes in live time or the triples rate. In some cases these may result from corrupt data (e.g. data for which the master frame contained an invalid checksum, which cannot be identified by the GRaND team). In others, the fluctuations may be due to natural, short-term variations in the energetic particle flux. Solid angles were calculated from a shape model using methods described in the RDR data processing document (GRD_L1B_Ephemeris_Pointing_Geometry_v1.pdf). Solid angles are accurate to better than 0.5% over the entire range of positions reported in the EPG file. Live time determination is described in detail in the EPG data reduction document (GRD_L1B_Ephemeris_Pointing_Geometry_v1.pdf). Live times reported in the EPG file are valid only for quiet Sun conditions. During solar energetic particle (SEP) events, the dead time counter can roll an indeterminate number of times, resulting in inaccurate determination of live time. SEP intervals can be identified by examining the triples counting rate. During SEP events, the triples rate is elevated. No attempt has been made to flag these intervals in the EPG file; however, records within periods of elevated SEP flux have been excluded from the BGO RDR data file. Additional exclusion rules to weed out invalid data from the BGO time series are described in GRD_L1B_BGO_Data_Processing_v4.pdf. These include out of range scaler values, records with abnormal spectral shape (due to anomaly or incomplete instrument configuration), and large accumulation times during which instrument settings were adjusted. Neither the EPG nor the BGO RDR contain replicate records. This is in contrast to the EDR, for which some records are repeated at the beginning and end of downlink intervals. Pointing Summary : The ingoing requirement for pointing during GRaND science data acquisition was to keep the instrument +Z axis within 5-deg of body center. When the spacecraft reached low altitude orbit pointing was off nadir due to ephemeris errors (during LAMO cycle C0). Requirements were met during cycles C1-4; however, for cycles C5-11, the pointing requirement was relaxed to 15-deg in order to accomodate targeting of surface features by the Framing Camera and VIR. In these cycles, the spacecraft was maintained at a constant attitude for long periods of time, which provided consistent geometry for GRaND measurements. Analysis of GRaND data show that within 15-deg, errors in mapped counting data are small and can be effectively managed when the deviation from body center is constant. The following table gives the deviation of the +Z axis from body center for all cycles of Ceres LAMO. Mapping Cycle Time period (UTC/SCET)* Pointing : : : C0 2015-350T01:00:00 to 2015-352T16:15:00 highly off-nadir because of ephemeris errors C1-C4 2015-352T17:05:00 2016-079T16:00:00 <<5-deg C5 2016-079T18:39:16 2016-102T09:59:00 5 deg C6-C7 2016-102T14:25:48 2016-148T16:29:00 10 deg C8-C9 2016-148T18:45:41 2016-195T14:59:00 11 deg C10 2016-198T15:05:19 2016-223T08:30:00 11 deg C11 2016-223T11:14:39 2016-246T11:29:00 5 deg *Date format: YYYY-DOY:HH:MM:SS In Dawn's second extended mission, the spacecraft was placed in an elliptical orbit with low periapsis, less than 50 km from Ceres' surface (C2E) to acquire elemental data with high spatial resolution. The elliptical orbit campaign commenced on 9-Jun, and the spacecraft completed 123 orbits before it ran out of hydrazine on 31-Oct-2018. For 10 of these orbits, the pacecraft was pointed away from Ceres to downlink data through the main antenna. These orbits have limited utility for elemental analyses. For the remaining 113 orbits, the +Z axis was well within 15 degrees of body center when the spacecraft was within 1000 km altitude. Limitations : The RDR gamma ray spectra can be used in the analysis of elemental composition and mapping studies. Standard methods for gamma ray peak fitting can be applied to extract counting rates. However, care must be taken when subtracting spectra accumulated during different mission phases. Background subtraction can introduce artifacts that can be misinterpreted as gamma ray peaks. A spectrum of gamma rays originating from Ceres can be obtained by subtracting a spectrum accumulated during approach (CSA) from one accumulated in low altitude mapping orbit (CSL). This approach to remove background contributions from the spacecraft will result in the introduction of artifacts due to variations in the energy resolution of the BGO sensor over large periods of time. To avoid introduction of artifacts, both spectra must be appropriately adjusted (by convolution) so that they have the same energy resolution prior to subtraction. The RDR time series is properly calibrated; however, no attempt has been made to adjust energy resolution on a spectrum-by-spectrum basis. The linear model used for energy calibration of BGO pulse height spectra produces relatively large absolute residuals in comparison to a quadratic model (see Fig. 5B, GRD_L1B_BGO_Data_Processing_v4.pdf). The observed deviations are likely due to a combination of the nonproportionality of BGO and the nonlinearity of the pulse processing electronics chain (from the photomultiplier tube through the analog-to-digital-converter). The linear calibration was selected for archive for the following reasons: - The linear model does a better job at predicting channel for the 7.6 MeV Fe neutron-capture peak than the quadratic model; - Users of the data set can re-calibrate the spectra without knowledge of the original calibration constants. As our understanding of nonlinear effects matures, the GRaND team will distribute improved versions of the calibrated spectra.
CITATION_DESCRIPTION Prettyman, T.H. AND N. YAMASHITA, DAWN GRAND CALIBRATED (RDR) CERES COUNTS V1.0, DAWN-A-GRAND-3-RDR-CERES-COUNTS-V1.0. NASA Planetary Data System, 2016.
ABSTRACT_TEXT The GRaND Reduced Data Records (RDR) contain a timeseries of calibrated spectra, counting data, and ephemeris, pointing,and geometry (EPG) data. The EPG data are contained in a single file,with an entry for every science data record acquired by GRaND within thedataset start-stop times. Each record has a unique identifier,spacecraft clock ticks, which is used as a serial number to identify datafrom the same record in other files containing spectra and counting data.The RDR data set includes a time series of corrected gamma ray spectraacquired by GRaND's bismuth-germanate (BGO) scintillator.
PRODUCER_FULL_NAME THOMAS H. PRETTYMAN
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