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NASA Space Science Data Coordinated Archive Header

DAWN GRAND RAW (EDR) CRUISE CHECKOUT/BACKGROUND COUNTS V1.0 (PDS)

NSSDCA ID: PSSB-00538

Availability: Archived at NSSDC, accessible from elsewhere

Time span: 2007-10-16 to 2010-07-25

Description

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

Overview

The Dawn Mission's Gamma Ray and Neutron Detector (GRaND) is a nuclear spectrometer that will collect data needed to map the elemental composition of the surfaces of 4-Vesta and 1-Ceres [PRETTYMANETAL2003B]. GRaND measures the spectrum of planetary gamma rays and neutrons, which originate from cosmic ray interactions and radioactive decay within the surface, while the spacecraft (S/C) is in orbit around each body. The instrument, which is mounted on the +Z deck of the S/C, consists of 21 sensors designed to separately measure radiation originating from the surface of each asteroid and background sources, including the space energetic particle environment and cosmic ray interactions with the spacecraft. The nuclear spectroscopy data provided by GRaND will be analyzed to determine the abundance of major rock forming elements, such as O, Fe, Ti, Si, Al, Mg, Ca, Cl and radioactive elements, including K and Th, as well as light-elements such as H, C, and N, which are constituents of ices and the products of aqueous alteration of silicate minerals and ices. The GRaND Experimental Data Records (EDR) are a time-ordered collection of gamma ray and neutron counting data and histograms acquired by GRaND during different phases of the Dawn Mission, including assembly-test-and-launchoperations (ATLO), cruise, Mars Gravity Assist (MGA), and science mapping at 4-Vesta and 1-Ceres. The dataset also includes state-of-health data (instrument settings, temperature and voltage readings) needed for scientific analysis of the neutron and gamma ray data. The EDR is an intermediate data product (Level 1A) that is derived from Raw Data Records (Level 0) using reversible operations. The Level 1A are the lowest level of GRaND data archived in the PDS, from which all higher order data sets are derived. To support timely delivery of higher order products, the Level 1A data are processed using an automated pipleline, which operates on Level 0 data when it is queried by the DSC. The data set consists primarily of ASCII tables, divided into three functional categories: auxilliary information (AUX); gamma ray spectra and event data (GAMMA); and neutron spectra and event data (NEUTRON). Gamma ray and neutron event data are recorded in binary files. Some of the data in the ASCII files, which are human-readable, are repeated in the binary files to aid in the verification of user-written routines. The telemetry for GRaND consists of science and state-of-health data, accumulated over time intervals, which are commandable. Each science data record includes scalers, histograms, and event data accumulated over an interval specified by the commandable parameter TELREADOUT (s). The state of health data include average temperatures, voltages, and instrument state data acquired during time intervals specified by the commandable parameter TELSOH (s). Both intervals are adjustable, depending on the measurement conditions and objectives for each mission phase. During mapping, TELREADOUT will be set to sub-sample spatial pixels defined on the surface of Vesta or Ceres. During cruise, TELREADOUT was generally set to large values (e.g., 210s) to minimize data volume. TELSOH is generally set to subsample the science accumulation interval, providing information needed to determine whether and how many times the science scalers have rolled over and information needed to precisely determine the start time of the science acquisition interval. The data are downloaded regularly from the spacecraft by the Ground Data System. The UCLA Dawn Science Center (DSC) captures all of the payload instrument telemetry frames as binary files after the data have been cleaned up in post-pass processing to produce reconstructed Level-0 data. The files are inventoried within the Dawn Science Database (DSDb) and are retrieved by the GRaND team, which unscrambles, decompresses, decodes, and formats the raw telemetry data into scientifically useful data files. The decompressed and decoded data, along with their required PDS documentation, form the Level-1A EDR data sets. The Level-1a EDR data are determined by performing reversible operations on the Level-1a data set, to produce counting data and spectral products useful for mapping.

Parameters

The EDR data are derived from Level 0 raw data queried by the DSC over irregular time periods, generally determined by the pattern of downlinks from the spacecraft. The DSC divides the Level 0 data into separate files containing state of health and science data packets. The Level 1a pipeline operates on these files to produce the Level 1a archive. The directory structure for the Level 1a data is given by GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC (top level directory) LEVEL1A_AUX (directory containing auxiliary data) LEVEL1A_GAMMA (directory containing gamma ray counting data) LEVEL1A_NEUTRON (directory containing neutron counting data) The top level directory name contains the SCET UTC dates for the first and last science data records (Y1M1D1 and Y2M2D2, respectively), and the creation date (YCMCDC) for the archive. For example, for GRD-L1A-090217-090218_090517, the first science data record was acquired on 17-Feb-2009. The last science data record was acquired on 18-Feb-2009. The archive was created by the pipeline on 17-May-2009. The LEVEL1A_AUX directory contains the following files derived from the Level 0 state-of-health and science data: GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-STA.TAB - Instrument state file. The instrument state file contains the instrument settings, including the mode, power supply states, high voltage settings, the data accumulation interval, and coincidence windows. The first record of the state-of-health file is recorded in the state file, stamped with SCET UTC. Thereafter, rows are added only when the instrument settings change. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-RDG.TAB - Instrument readings file. This file contains a time-ordered list of temperature and voltage readings averaged over each state-of-health accumulation interval (TELSOH), converted to physical units. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-SOH-SCL.TAB - State of health scaler data. This file contains a time-ordered list of the scaler data recorded in the state-of-health telemetry. The accumulation time for the scaler data is TELSOH. Note that the scalers are reset at the end of each science accumulation interval (TELREADOUT). If the state-of-health accumulation interval is selected to subsample the science interval, then the state-of-health scalers can be used to detect and correct for rollover of the science scalers, such as the dead time counter. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-SCI-SCL.TAB - Science scaler data. This file contains a time-ordered list of the scaler data recorded in the science telemetry. The accumulation interval for the scalers is TELREADOUT. For each science and state of health record, values for 23 scalers are recorded in the -SCI-SCL.TAB and -SOH-SCL.TAB files, respectively. The scalers provide the following information: Index Description ------------ ----------0 Dead time counts 1 BGO overload events 2 CZT overload events 3 +Z phoswich overload events 4 -Y BLP overload events 5 +Y BLP overload events 6 -Z phoswich overload events 7 +Z phoswich CAT4 events 8 -Y BLP CAT4 events 9 +Y BLP CAT4 events 10 -Z phoswich CAT4 events 11 Early second interaction events 12 Multiple-crystal CZT events 13 Valid CZT events (CAT10) 14 Coincidence BGO and CZT events (CAT7) 15 Coincidence of three or more sensor elements 16 Total events processed by GRaND 17 Number of single CZT events (CAT10) in the gamma ray event buffer 18 Number of BGO-CZT coincidence events (CAT7) in the gamma ray event buffer 19 Number of events (CAT4) in the neutron event buffer 20 Total number of events allowed in the gamma ray event buffer 21 Number of single CZT events (CAT10) allowed in the gamma ray event buffer 22 Number of events allowed in the neutron event buffer Note that indices 0 through 19 are for 16-bit counters, which are reset at the end of every science accumulation interval specified by TELREADOUT. If the state-of-health accumulation interval is adjusted to subsample the science accumulation interval (for example, TELREADOUT = n * TELSOH, where n is a whole number), then the scalers will monotonically increase during each acquisition interval, unless overflow occurs. A rollover counter is not provided; however, for situations in which the counting rate is high or the accumulation intervals are large, the number of rollovers for individual scalers can be determined from the SOH scaler data if TELSOH is set to subsample the science accumulation interval. In situations where the counting rate is changing, abrupt changes in the scaler varues can also indicate that rollover has occurred. Rollover is treated in production of the Level1b RDR data. Indices 20 through 21 are maximum values for the number of events that can be recorded in the event buffers. The number of gamma ray and neutron events is commandable and can be adjusted. The total number of gamma ray and neutron events must be less than 6677. The LEVEL1A_GAMMA directory contains the following science data files: GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BGO.TAB This file contains a time-ordered list of pulse height spectra (1024 channels with units of uncorrected counts/channel) acquired by the BGO sensor. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-EMG.DAT This file contains gamma ray event data as a binary time series. The LEVEL1A_NEUTRON directory contains the following science data files: GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-PHOS_MZ.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-PHOS_PZ.TAB These files contain time ordered lists of the 256-channel CAT1 pulse height spectra for the +Z and -Z phoswiches. Note that the naming convention for the top, bottom, and side scintillators is determined by the instrument coordinate system. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BGO2_MZ.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BGO2_PZ.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BGO2_MY.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BGO2_PY.TAB These files contain time ordered lists of the 64-channel CAT2 BGO pulse height spectra for coincidences with the BGO and the four BLP sensors. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BLP2_MZ.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BLP2_PZ.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BLP2_MY.TAB GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-BKP2_PY.TAB These files contain time ordered lists of the 64-channel CAT2 BLP pulse height spectra for coincidences with the BGO and the four BLP sensors. GRD-L1A-Y1M1D1-Y2M2D2_YCMCDC-EMN.DAT This file contains the neutron event data as a binary time series.

Processing

The Level 1A data are automatically processed using a pipeline, which operates on files queried by the DSC over selected time intervals. Each DSC query separates the GRaND data into files containing state-of-health and science data records, in the order in which they were received on the ground and with corrupted packets removed. The state-of-health data are further divided into real time telemetry data and playback data. The science data are stored in a single raw data file. The pipeline merges the state-of-health data from the playback and realtime files to produce a time-ordered-list of records. Selected data are extracted to produce the Level 1A AUX files. Internal temperature readings are converted from data numbers (DN) to engineering units using a linear function determined during ground calibration: T (degrees C) = 0.4354 DN - 0.4354. The high voltage readings for the high voltage power supplies are reported in engineering units using the conversion V (Volts) = 1500 DN/255. The CZT differential bias voltage is converted using V (Volts) = 0.405 DN. The science data are decompressed, decoded, separated by functionality and written as time-ordered ASCII tables and binary time series. The raw histograms (CAT1, CAT2, and CAT9) are represented as 8 bit numbers which are decompressed and reported as 16 bit, unsigned integers. The gamma ray event buffer can store up to 3876 events for each science accumulation interval. Each event is packed into 3 bytes, which contain the ID of the CZT sensor, the CZT pulse amplitude, and the BGO pulse amplitude. The vales for each event are extracted and stored as a binary time series. When the gamma ray event buffer is not full, null events are reported as zeros, such that each row of the Level 1A time series contains 3876 events. The neutron event buffer can store up to 2800 events for each science data accumulation interval. Each event is packed into 3 bytes, which are contain the BLP sensor ID and pulse amplitude for the first interaction, the BLP sensor ID and pulse amplitude for the second interaction, and the time between pulses. The time between pulses has units of 100 nanoseconds/DN. The vales for each event are extracted and stored as a binary time series. The pulse amplitudes are uncalibrated for Level 1A. When the gamma ray event buffer is not full, null events are reported as zeros, such that each row of the Level 1A time series contains 2800 events. Ancillary

Data

The Level 1A data include ancilliary data in the form of SCET UTC strings reported in each row of the Level 1A data tables and time series. The UTC strings are determined from the spacecraft clock ticks recorded in each state-of-health packet and for the first packet in each science data record using NAIF SPICE (leap seconds kernel). This information is used in Level 1B processing to accurately determine the mid-point of each science accumulation interval, which is needed for mapping.

Coordinate System

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). ................. . 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

Proprietary software is not needed in order to use the EDR data; however, an Interactive Data Language (IDL) functions (GRD_READ_L1A_SOH.PRO and GRD_READ_L1A_SCIENCE.PRO) are provided to read selected EDR data into a structure for analysis and visualization. The IDL functions are compatible with IDL Version 7.0, distributed by ITT Visual Information Solutions, Boulder, CO (2008). The documentation is included at the beginning of the functions.

Media/Format

The EDR label and data files are delivered by electronic transmission to the PDS. The neutron and gamma ray binary event data were written in big endian IEEE binary format (MSB order).

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

http://sbn.psi.edu/archive/dawn/grand/DWNXGRD_1A/

Alternate Names

  • DAWN-X-GRAND-2-EDR-CRUISE-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. William C. FeldmanData ProviderLos Alamos National Laboratorywfeldman@lanl.gov
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