Data Set Information
DATA_SET_NAME ODY MARS GAMMA RAY SPECTROMETER 5 AHD V1.0
DATA_SET_ID ODY-M-GRS-5-AHD-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : The Mars Odyssey Gamma-Ray Spectrometer (GRS) is a suite of three instruments working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The suite of three instruments, the gamma-ray sensor head (GS), the Los Alamos National Laboratory (LANL) neutron spectrometer (NS) and the Russian Academy of Science Institute for Space Research (IKI) high-energy neutron detector (HEND), are a complementary set of instruments in that the neutron instruments have better counting statistics and sample to a greater depth than the GS, but the GS determines the abundance of many more elements. A full description of the Mars Odyssey Gamma-Ray Spectrometer instrument can be found in [BOYNTONETAL2004]. The ODY MARS GAMMA RAY SPECTROMETER 5 AHD (AHD) data set is a collection of data tables that contain an average counting rate and the associated engineering data for each of the five neutron signals obtained by HEND's four detectors that have been summed by signal over 5-degree by 5-degree latitude longitude cells on the surface of Mars over a time period of 15-degrees of solar longitude (Ls). The five neutron signals of interest are the: Small Detector, SD, energy range 0.4 eV to 1 keV; Medium Detector, MD, energy range 10 eV to 100 keV; Large Detector, LD, energy range 10 eV to 1 MeV; Inner Scintillator, IN_SC_LOW, neutron pulse height 1 MeV to 2.5 MeV; Inner Scintillator, IN_SC_HIGH, neutron pulse height > 2.5 MeV; Each of the GRS HEND detectors collects a new spectrum (pixel or frame) approximately every 20 seconds, 360 times per orbit. Approximately 4200 spectra are expected to be received every day. The data (both science and engineering) are downloaded from the spacecraft by the Jet Propulsion Laboratory (JPL) into the Telemetry Data System (TDS). The TDS sends data to a process that translates data packets and examines instrument health via messages. Data are output to a spooler that passes it to both the University of Arizona (UA) database ingest process and to IKI. The UA ingest process inputs raw data into the UA database. IKI maintains a separate database that is used to process the derived neutron data set that is the basis for the averaged neutron data set. HEND data are processed by IKI through a number of processes to yield background subtracted counts for the five neutron signals obtained by HEND's four detectors. The background subtracted counts are used in the computation of average normalized count rates and subsequently orbital epithermal and fast neutron fluxes for 5x5 degree latitude longitude grids over a time interval of 15 degrees of Ls. The AHD is intended to be the second intermediate data product available for the HEND portion of the GRS. These data should be useful to those scientists who are experienced in neutron spectroscopy. Parameters : The AHD data set is composed of a single data type (AHD). The objective of compiling the AHD is to create a table of HEND data table with average normalized counting rates and neutron fluxes for the five HEND neutron signals in an easily mappable form. Each AHD product data file will contain a 2592-row table. The table will contain 1 data record (row) for each of the 5x5 degree latitude longitude cells. The data files will be grouped by time, so that for each 15 degrees of Ls there will be one data directory. Processing : A full description of the HEND data processing can be found in the Mars Odyssey Gamma-ray Spectrometer HEND Data Processing document located in the document folder that accompanies this release. The following paragraphs are a summary of how the data are processed from data receipt through averaging, items in all capitols are references to data column names. GRS HEND data are downloaded from the Mars Odyssey spacecraft by JPL into the Telemetry Data System (TDS). The TDS sends the data to a process called GRS_tl that translates GRS packets from any source to any destination and examines instrument health via messages. The data are checked for packet types, header information, messages, errors, engineering and channelized data values and are output to a spooler and to a data display IDL program called GRS Displays. Data sent to the spooler are passed to the UA database ingestion process, which inputs raw data into the UA database. Data sent to the GRS Displays program are recorded at IKI and held in a data repository. All additional steps of HEND data processing are done at IKI. All data packets received by the GRS Displays program are processed for timing and spatial information. Once time data are extracted from the data packet, UTC time is calculated from the spacecraft event time (sc_ev_time, time at the middle of the collection interval in 256th seconds) by a SPICE function. The UTC time is then inserted into the database. The spatial portion of the data packet is calculated using other SPICE routines. The returned values are all the spatial elements of an observation, including latitude and longitude at the mid-point of the observation. Once raw HEND data, with associated timing, spatial and engineering data is entered into the database, a series of processing steps are started to correct, normalize and average data. The HEND instrument collects five neutron signals from four independent detectors whose response functions are sensitive to different energy regions. Raw neutron counts are reported in the HEND portion of the EDR data set as a 16-channel spectra for each of the four detectors (SD_SPECTRUM, MD_SPECTRUM, LD_SPECTRUM,NEUTRON_SPECTRUM). In the DHD data set, raw neutron counts are reported as the sums of channels 1-16 in the SD (Small Detector), MD (Medium Detector), and LD (Large Detector). The inner scintillator records two neutron signals, LOW (sum of channels 1 -4 in the inner scintillator) and HIGH (sum of channels 5-15 in the inner scintillator). The raw neutron counts for the LOW and HIGH signals are reported in the DHD as the sum of channels 1-4 and 5-15 respectively, of the NEUTRON_SPECTRA field of the HEND portion of the EDR data set. A background term that is a combination of the galactic cosmic-ray flux induced spacecraft background and a spacecraft orbital geometry term, which accounts for shadowing by the planet of the galactic cosmic-ray flux, is accounted for in the derived HEND data (see the DHD data set description) yielding background subtracted counts for each HEND neutron signal. Background subtracted count values for each of the five neutron signals (SD, MD, LD, LOW, HIGH) are temporally and spatially binned to compare seasonal changes in the neutron flux. The temporal binning is 15-degrees of LS and the spatial binning is a grid of 5-degrees latitude by 5-degrees longitude. Membership in a temporal bin is determined by calculating the fraction of the collection interval time spent in each temporal cell. Membership in a spatial bin is determined by calculating the fraction of the collection interval length in a given spatial cell along the orbital trajectory projection on the Martian surface. The temporally and spatially binned counts for each of the five signals (SD, MD, LD, LOW, HIGH) are calculated by: 1) multiplying the background subtracted counts by the collection interval faction and summing the result for each collection interval within the 15-degree LS interval 2) multiplying the duration (in seconds) of each collection interval by the collection interval faction and summing the result for each collection interval within the 15-degree LS interval 3) dividing the summed counts by the summed time to yield a counting rate in units of counts per second. The summed time is the EXPOSURE. The averaged counting rates are then normalized by the average counting rate from the Solis Planum region of Mars defined as 245E to 295E and 15S to 55S. The normalization values for each signal are the SD_NORMALIZATION, MD_NORMALIZATION, LD_NORMALIZATION, IN_SC_HIGH_NORMALIZATION, and IN_SC_LOW_NORMALIZATION fields given in the AHD data set. The normalization calculation yields the SD_RATE, MD_RATE, LD_RATE, IN_SC_HIGH_RATE, and IN_SC_LOW_RATE fields, which can be used to create signal specific neutron maps. Because the rates are ratios of counting rates, they are unitless. Error fields, SD_ERR, MD_ERR, LD_ERR, IN_SC_HIGH_ERR, IN_SC_LOW_ERR are also present for each of the signals. The errors are simply based on the counting statistics for the raw HEND counts. The conversion of normalized counting rates to neutron fluxes requires knowledge of the efficiency of each of the neutron detectors. Efficiency functions, derived from model calculations and direct calibrations, describe the directional and energy dependent response of each of the four HEND detectors, SD, MD, LD, and the inner scintillator, to bombardment by neutrons. A series of Monte-Carlo simulations were run to calculate the efficiency of each HEND detector. The simulations accounted for the composition and geometry of the HEND instrument. In parallel, a series of experimental calibration measurements were carried out to understand the quality of the Monte-Carlo simulations. Two types of experiments were performed. The first experiment measured the HEND response to monoenergetic neutron sources with known spectra and the second measured the HEND response to energetic neutron fluxes. The combined results of the numerical simulations and the experiments yield detector specific efficiency functions. As a further analysis of the data, calculations have been made with MCNPX for various models of Mars. The neutron flux at the spacecraft predicted by these calculations was fitted with a power law, AE^-alpha. The power law spectrum is then multiplied by the five efficiency functions to obtain computed counting rates in the HEND detectors. These computed counting rates are compared to the real counting rates in via a Pearson minimizing procedure. The values, A and alpha, for the model that best satisfy the minimization criterion, are inserted into AHD as NEUTRON_NORMALIZATION and ALPHA fields. The NEUTRON_NORMALIZATION and ALPHA values are used to estimate the neutron flux (n/(cm^2-s)) at the Odyssey orbit in different ranges of epithermal and fast neutrons (EPITHERMAL_FLUX1 100 eV - 10 KeV, EPITHERMAL_FLUX2 10 KeV - 1 MeV, FAST_FLUX1 1 MeV - 2.5MeV, and FAST_FLUX_2 2.5 MeV - 10 MeV) for each 5-degree by 5-degree map grid by taking the integral of the neutron spectrum as described by A and alpha over the energy interval. See Equation 9 in the HEND Processing document located in the Documents directory that accompanies this release for further details. Data : The AHD data set is composed of a series of data tables labeled with Mars year, starting Ls and ending Ls. Each file contains one data table with 2592 rows of data, corresponding to the 2592 5-degree latitude longitude bins on the planet. Averaged HEND Data ------------- Averaged HEND Data products are composed of averaged normalized counting rates, orbital neutron fluxes and the associated timing, spatial and engineering information. The AHDs consists of the cumulative background subtracted and normalized counts of neutrons at the four HEND detectors averaged over the 5x5 degree latitude longitude, 15-degree Ls. Please refer to the HEND processing document located in the documents directory that accompanies this release for a full description of how HEND spectra are processed to yield averaged normalized counting rates and orbital neutron fluxes. The timing and spatial data provided with each AHD record include the time and spatial range over which the average was generated. Ancillary Data : Ancillary data needed to understand the HEND data processing can be found in the Mars Odyssey Gamma-ray Spectrometer HEND Data Processing document located in the document folder that accompanies this release. Coordinate System : The coordinate system used for all GRS data is a Mars areocentric system following the IAU convention [SEIDELMANNETAL2002], with east longitudes from 0 to 360. Software : A library of source code to parse the AHD data product files is included in the software directory. This library allows a programmer to build applications that display or manipulate AHD data. This source is written in the Java language, and requires version 1.3 of the Java Runtime Environment (JRE) or Java Software Development Kit (SDK). Documentation for the code is located in the software directory in the file GRS_CODE_DOC.ZIP. The contents of this file are described in the label GRS_CODE_DOC.LBL. Media/Format : The AHD will be delivered electronically. Formats will be based on standards established by the Planetary Data System (PDS) [PDSSR2001].
DATA_SET_RELEASE_DATE 2004-07-01T00:00:00.000Z
START_TIME 2002-02-19T12:00:00.000Z
STOP_TIME N/A (ongoing)
MISSION_NAME 2001 MARS ODYSSEY
MISSION_START_DATE 2001-01-04T12:00:00.000Z
MISSION_STOP_DATE N/A (ongoing)
TARGET_NAME MARS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID ODY
INSTRUMENT_NAME GAMMA RAY/NEUTRON SPECTROMETER/HIGH ENERGY NEUTRON DETECTOR
INSTRUMENT_ID GRS
INSTRUMENT_TYPE SPECTROMETER
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED_ACCUMULATING
CONFIDENCE_LEVEL_NOTE Confidence Level Overview : The data presented in the AHD are intended to be the second intermediate data set released for the HEND portion of the GRS. Data presented here are a highly processed and re-binned representation of the GRS HEND data. It is possible that changes will be made in the processing procedures that the AHD products are based on if errors are found. If errors are found, the data will have to be regenerated from the corrected data set. Review : The AHD was reviewed internally by the GRS team prior to release to the PDS. PDS will also perform an external review of the AHD. Data Coverage and Quality : Except for background estimates, data reported are the processed and re binned HEND data received from the spacecraft during the mapping mission stage. Data coverage and quality are addressed in the following sections. Primary Mapping --------------- The mapping portion of the mission began February 19, 2002, and is expected to last until August, 2004. Primary mapping is broken into two phases: stowed mapping and deployed mapping. Stowed Mapping -------------- The stowed mapping configuration began on February 19, 2002 and lasted until June 1, 2002. The stowed mapping configuration is with the Odyssey spacecraft in a mapping orbit, with the GRS in data collection configuration without the 6-meter boom deployed. The reason for data collection in the stowed configuration is to measure the gamma background signal from the spacecraft. HEND data collection was not affected. On June 1, 2002 the GRS was shutdown to prepare for boom deployment, which occurred on June 4, 2002. No data were collected during this time period. There are short periods of missing data due to missing telemetry that cannot be recovered. Deployed Mapping ---------------- Deployed Mapping began on June 5, 2002 and is expected to continue until at least August, 2004. A number of solar particle events, and solar flare events have been recorded. HEND data collected during these times, as indicated by the SUN_ACTIVITY field in the DHD data product being greater than 1, are suspect. These data have not been included in the AHD data product. There are short data gaps due to missing telemetry that cannot be recovered. Limitations : The major limitation of this data set is that it is derived from the raw minimally processed HEND data. The data are received from spacecraft telemetry, and ingested into a database. If gaps exist in the telemetry, data are lost. Timing and spatial components of the data set rely on the accuracy of the NAIF SPICE kernels. The validity of the derived HEND counts is based on the 'correctness' of each step in the processing. It is possible that changes will be made in the processing procedures if errors are found. If errors are found, the data will have to be regenerated from the raw unprocessed data set. Data Compression : No compression is used on the AHD data set.
CITATION_DESCRIPTION Boynton, W.V., Mars 2001 Odyssey Gamma Ray Spectrometer Averaged High Energy Neutron Detector Data V1.0, ODY-M-GRS-5-AHD-V1.0, NASA Planetary Data System, 2004.
ABSTRACT_TEXT The Odyssey Gamma Ray Spectrometer (GRS) Averaged HEND Data (AHD) products are temporally and spatially binned neutron counting rates and fluxes that can be used to create global maps. These products are generated from the Derived HEND Data (DHD) product, which in turn are derived from HEND EDRs.
PRODUCER_FULL_NAME WILLIAM BOYNTON
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