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 4 DHD (DHD) data set is a time series collection of counts and backgrounds for the five neutron signals obtained by HEND's four detectors.  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 DHD data. HEND data are processed by IKI through a number of processes to yield counts and backgrounds for the five neutron signals obtained by HEND's four detectors. The derived HEND data are retrieved from IKI and inserted into the UA database. The DHDs are prepared for PDS from the UA database.  The DHD is intended to be the first 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 DHD data set is composed of a single data type (DHD). The objective of compiling the DHD is to create a series of neutron data processed to yield local spacecraft background subtracted counts for each of the five distinct neutron signals.  Each DHD product data file will contain a time series of derived neutron counts collected over an Earth Day. The 24-hour data files will be grouped by Earth years, i.e., 365 data files per year.   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 background subtraction.  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 is 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.  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 spectrum 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.  The background term reported for each collection interval is a combination of the galactic cosmic-ray flux induced spacecraft background and a spacecraft orbital geometry term to account for the shadowing by the planet of the galactic cosmic-ray flux. The galactic cosmic-ray flux induced background was measured in each detector during the cruise phase of the mission when the spacecraft was far from both the Earth and Mars. The observed background count for the detector of interest is multiplied by (1-omega(t)/4pi) where omega(t) is the solid angle subtended by Mars for the current position in the spacecraft orbit. The resulting term is the background counts in the particular collection interval for the detector of interest.  The background subtracted counts reported for each signal in the DHD data set are simply the background counts subtracted from the summed raw counts.   Data : The DHD data set is composed of a series of date stamped files that contain 24-hours worth (00:00:00 UTC to 23:59:59 UTC) of data.   Derived HEND Data ------------- Derived HEND data are composed of raw counts, a background component, background subtracted counts and the associated timing and spatial information. The five raw counts fields included in the DHDs consist of the cumulative counts of neutrons at the detector of interest over one collection interval. The collection interval is approximately 19.7 seconds, but may vary over the course of mapping. The five background count fields are derived from spatial data and count data collected during the cruise phase of the mission. The five background subtracted count fields are the background subtracted counts at the detector of interest over one collection interval. Timing and spatial data provided with the counts include spacecraft clock values and spacecraft geometry data. The sc_ev_time, utc time and spatial fields are all recorded at the center of the collection interval.   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 DHD data product files is included in the software directory. This library allows a programmer to build applications that display or manipulate DHD 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 DHD will be delivered using DVD media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR2001].

Confidence Level Overview : The data presented in the DHD are intended to be the first intermediate data set released for the HEND portion of the GRS. Data presented here are a highly processed representation of the GRS HEND data as received from the spacecraft. It is possible that changes will be made in the data processing procedures if errors are found. If errors are found, the data will have to be regenerated from the raw unprocessed data set.   Review : The DHD was reviewed internally by the GRS team prior to release to the PDS. PDS will perform an external peer review of the DHD.   Data Coverage and Quality : Data reported are the processed raw 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-ray background signal from the spacecraft. The HEND instrument was always in the same spacecraft position. 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 being greater than 1, are suspect.  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 DHD data set.