Data Set Information
|
| DATA_SET_NAME |
DAWN GRAND CALIBRATED (RDR) VESTA COUNTS V1.0
|
| DATA_SET_ID |
DAWN-A-GRAND-3-RDR-VESTA-COUNTS-V1.0
|
| NSSDC_DATA_SET_ID |
|
| DATA_SET_TERSE_DESCRIPTION |
The GRaND Reduced Data Records (RDR) contain a time series of calibrated
spectra, counting data, ephemeris, pointing, and geometry information
for use in geochemical studies of Vesta.
|
| DATA_SET_DESCRIPTION |
Acronyms and Abbreviations
==========================
BGO Bismuth Germanate
CDP Claudia Double Prime coordinate system
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). A description of GRaND
can be found in the instrument catalog (included in the archive) and in
the literature [PRETTYMANETAL2003, PRETTYMANETAL2011]. The archive
includes data from all phases of Dawn's encounter with Vesta. GRaND
data were used to study Vesta's geochemistry [PRETTYMANETAL2012,
LAWRENCEETAL2013, PEPLOWSKIETAL2013, PRETTYMANETAL2013,
YAMASHITAETAL2013]. 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.
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 ; Prettyman et al.,
2014, 45th Lunar and Planetary Science conference, Abstract 2565].
Energy-calibrated and corrected gamma ray spectra included in this
volume support studies of Vesta's geochemistry. 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 Vesta.
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 for
Vesta encounter (from 3-May-2011 to 9-Aug-2012):
GRD-L1B-110503-120809_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 Vesta 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 Vesta 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. VSL is
Vesta Science LAMO; 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 Vesta encounter were downloaded
from the NASA Planetary Data System (dawnsp_1000). The planetary
constants kernel for Vesta (dawn_vesta_v04.tpc) for the Claudia
coordinate system was modified with the latest pole position, per
private communication by R. Gaskell. The modified PCK file
(dawn_vesta_v04b.tpc) can be found in the EXTRAS directory.
Coordinate System
=================
Longitudes are given in the International Astronomical Union (IAU)
Claudia Double Prime (CDP) coordinate system. A method to convert from
CDP to the Claudia coordinate system, used by the Dawn mission, can be
found in the EPG label file and data processing document.
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 Vesta 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 |
3000-01-01T00:00:00.000Z
|
| START_TIME |
2011-05-03T04:36:19.000Z
|
| STOP_TIME |
2012-08-09T01:41:33.000Z
|
| MISSION_NAME |
DAWN MISSION TO VESTA AND CERES
|
| MISSION_START_DATE |
2007-09-27T12:00:00.000Z
|
| MISSION_STOP_DATE |
2017-06-30T12:00:00.000Z
|
| TARGET_NAME |
4 VESTA
|
| TARGET_TYPE |
ASTEROID
|
| INSTRUMENT_HOST_ID |
DAWN
|
| INSTRUMENT_NAME |
GAMMA-RAY AND NEUTRON DETECTOR
|
| INSTRUMENT_ID |
GRAND
|
| INSTRUMENT_TYPE |
GAMMA RAY SPECTROMETER
NEUTRON 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 EPG and BGO data have been used in studies
carried out by the Dawn Geochemistry team [PRETTYMANETAL2012,
LAWRENCEETAL2013, PEPLOWSKIETAL2013, PRETTYMANETAL2013,
YAMASHITAETAL2013]. 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 as 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.
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 Vesta can be obtained by subtracting a spectrum accumulated during
approach (VSA) from one accumulated in low altitude mapping orbit (VSL).
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) VESTA
APPROACH AND ORBITAL COUNTS V1.0, DAWN-A-GRAND-3-RDR-VESTA-COUNTS-V1.0.
NASA Planetary Data System, 2014.
|
| ABSTRACT_TEXT |
The GRaND Reduced Data Records (RDR) contain a time series of calibrated
spectra, counting data, and ephemeris, pointing, and geometry (EPG)
data. The data set is specific to Vesta encounter. The EPG data are
contained in a single file, with an entry for every science data record
acquired by GRaND at Vesta. Each record has a unique identifier,
spacecraft clock ticks, which is used as a serial number to identify
data from the same record in other files containing spectra and counting
data. The RDR data set includes a time series of corrected gamma ray
spectra acquired by GRaND's bismuth-germanate (BGO) scintillator.
|
| PRODUCER_FULL_NAME |
THOMAS H. PRETTYMAN
|
| SEARCH/ACCESS DATA |
SBN PSI WEBSITE
|
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