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DAWN GRAND CALIBRATED (RDR) CERES COUNTS V1.0 (PDS)

NSSDCA ID: PSSB-00677

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System.

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.

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_v1.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 (zenithfacing 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.

These data are available on-line from the Planetary Data System (PDS) at:

https://sbnarchive.psi.edu/pds3/dawn/grand/superseded/DWNCAGRD_1B/

Alternate Names

  • DAWN-A-GRAND-3-RDR-CERES-COUNTS-V1.0

Discipline

  • Planetary Science: Small Bodies

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Thomas H. PrettymanGeneral ContactPlanetary Science Instituteprettyman@psi.edu
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