Data Set Information
DATA_SET_NAME ODY MARS GAMMA RAY SPECTROMETER 4 DND V1.0
DATA_SET_ID ODY-M-GRS-4-DND-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 sensor head (GS), the neutron spectrometer (NS) and the 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 [BOYNTONETAL2003]. The ODY MARS GAMMA RAY SPECTROMETER 4 DND data set is a time series collection of derived neutron data (DND) measurements from the NS sub-system of the Mars Odyssey Gamma-Ray Spectrometer. The DND are an intermediate data product. These data are the neutron data that must be derived from the raw Level-0 data before a set of calculations can be run that will result in thermal, epithermal, and fast neutron fluxes. The GRS collects a new spectrum (pixel) approximately every 20 seconds, 360 times per orbit. Approximately 4200 neutron measurements are expected to be received every day. The data 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 the University of Arizona (UA) database ingest process. The ingest process inputs raw data into the UA database. Neutron data are processed through a number of programs to yield the derived neutron data. The derived neutron data are retrieved from the UA database to build the DND data. The DND product is intended to be the first intermediate data product available for the NS portion of the GRS data. These data should be useful to those scientists who are experienced in neutron spectroscopy. Parameters : The DND data set is composed of a single data type (DND). The objective of compiling the DND is to create a series of neutron data that have been processed to yield the intermediate results necessary for final processing. Each DND product data file will contain a time series of neutron data collected over a 24-hour time period. The data files are labeled as Earth days. Each DND data file is a table containing 63 columns, with one row of data for each approximately 19.7 second collection interval. Processing : The product of our data reduction algorithms is a time-series data set, corrected for variations in the instrument response and the cosmic ray background, that can be used to produce thermal, epithermal, and fast neutron global maps of Mars binned on coarse or fine time intervals (as short as two weeks for high latitude studies). An overview of these algorithms is provided here. References to the DND or EDR column names are in capitals, and a lower case x in the name refers to either prism 1,2,3,or 4. See [PRETTYMANETAL2003] for details of the neutron data reduction process. See [FELDMANETAL2002] and [BOYNTONETAL2003] for details of the neutron spectrometer design and operation. The NS detector consists of a cubical block of boron-loaded plastic scintillator. It is segmented into four prism-shaped quadrants that are optically isolated from each other. Thermal- and epithermal-neutrons are detected via the 10B(n,alpha)7Li* reaction, which produces a distinct peak in the pulse height spectrum at 93 keV electron equivalent light output due to the recoil of the reaction products. Fast neutrons are detected by construction of a pulse height spectrum from the early and late time histograms recorded in the NS EDR data. The first step in processing NS DND data is a calculation using the NS EDR EVENTS array. The results of this processing are used to monitor the time dependence of the gain of each prism of the NS by calculating the centroid (PRISMx_CENTROID) of the 93 keV electron equivalent peak in the second-pulse spectrum constructed from event mode data. (Additional information regarding the event-mode parameters can be found in [FELDMANETAL2002].) For each collection interval, the event mode data is accumulated over coarse time intervals, 721 x ~19.75s. The event mode data is accumulated using a sliding scale, i.e. by summing the event data from 360 collection intervals before and 360 collection intervals after the collection interval of interest with the collection interval of interest. The event values are added to the accumulation if the time to second pulse values are between 3 to 53, the event values are subtracted from the accumulation if the time to second pulse values are between 199 and 259. If the time to second pulse is not within either range than nothing is added or subtracted to the total. The resulting spectrum reveals the 93 keV electron equivalent peak as the second peak in the spectrum. The peak location is proportional to the gain of the prism, which is found to change with time. The centroid (PRISMx_CENTROID) of the peak is calculated using a gaussian fitting algorithm. This centroid is recorded, and used later in the processing. For each prism, the net count rate (peak area) for the pulse height spectrum at 93 keV electron equivalent light output is determined from the Category 1 pulse height spectrum (NS EDR CAT1_x_HIST), a spectrum of single-interaction events recorded every ~19.75s. During a single measurement interval, the spacecraft traverses approximately one degree of arc length (roughly 60 km). The algorithm used to generate time series Category 1 peak area data (CAT1_PRISMx_PEAK_AREA) is implemented as a sliding window 3 x ~19.75s long, with one record before and one record after being summed with the collection interval of interest. The time interval is selected such that counting precision is sufficient to reliably determine the background and the distance traveled by the spectrometer is small compared to the footprint (approximately 10-degrees of arc length). The background under the peak for each prism is determined by fitting a line to points above and below the peak (CAT1_PRISMx_BACKGROUND). The thermal neutron count rate (CAT1_PRISM4_PEAK_AREA, CAT1_PRISM2_PEAK_AREA) is determined by subtracting 0.85 times the count rate measured by the backward looking prism (P4) from the forward looking prism (P2). For the purpose of making a thermal neutron map, use the latitude/ longitude values projected 4.5-degrees ahead of the spacecraft (LAT_4_5, LON_4_5). The rationale for selecting the factor 0.85 is described in [PRETTYMANETAL2003]. The epithermal neutron count rate is taken to be the count rate for P1 (CAT1_PRISM1_PEAK_AREA). The 1-degree projected latitude/ longitude values should be used to make epithermal neutron maps (LAT_01, LON_01). For fast neutrons, histogram data (EARLY_HIST, LATE_HIST in NS EDR data) for which first pulses with magnitude above a pre-set threshold are binned into histograms for early and late time windows are used for the fast neutron spectrum calculation. The early time interval is selected so that it is sensitive to neutrons slowing down in the spectrometer. The late time interval is selected so that it is well beyond the die-away time of neutrons in the spectrometer and thus contains accidental events. Subtraction of the late from early time histograms yields a pulse height spectrum that is sensitive to fast neutrons. Only the sum of the first two energy bins (which correspond to energies between 0.7 and 1.6 MeV) are used for the fast_sum_counts (CAT2_PRISMx_FAST_SUM). Counting data can, for each prism, be normalized to the average count rate observed between latitudes plus or minus 40-degrees (the 'belly-band') every 12 orbits, which is about one Martian day. An average, belly-band count rate is determined for each measurement location using a sliding window that extends plus or minus 6 orbits relative to the location (PRISMx_FAST_NORM, PRISMx_PEAK_NORM). We recommend that each count rate be divided by its corresponding belly-band average to produced a normalized count rate. For example, the normalized thermal neutron count rate would be: (CAT1_PRISM2_PEAK_AREA/PRISM2_PEAK_NORM) - (0.85 * CAT1_PRISM4_PEAK_AREA/PRISM4_PEAK_NORM) The 'belly-band' normalization is designed to minimize variations in count rate due to instrumental drift, variations in the galactic cosmic ray flux, and diurnal and seasonal changes in globally averaged atmospheric mass. This approach to normalization assumes that the mid-latitude composition does not change significantly with time, which enables precise determination of seasonal variations in count rates at polar latitudes. The normalization period of 12-orbits was selected so that nearly full coverage of the mid-latitudes was achieved by the spectrometer. To get a time series of absolute counting rates, multiply the normalized count rate by that observed at a reference time or the average count rate observed for a selected time interval (see ancillary data for these values). For example, the absolute thermal neutron count rate would be: ((CAT1_PRISM2_PEAK_AREA/PRISM2_PEAK_NORM) * 9.681) - (0.85 * ((CAT1_PRISM4_PEAK_AREA/PRISM4_PEAK_NORM) * 3.148)) Data : The DND data set is composed of a series of date stamped files that contain 24-hours worth of data. Derived Neutron Data -------------------- Derived Neutron Data are composed of the derived neutron data and the associated timing, spatial, and engineering information for each ~19.7 second collection interval. The derived neutron data consists of 39 columns of data that are output from the derived neutron data processing. The timing and spatial data provided with the DND includes 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. The associated engineering data is obtained from a linear interpolation of the engineering data packet received just before and just after the neutron data collection interval of interest. Ancillary Data : The 'absolute' normalization constants are as follows: N1: 7.325 (CAT1, Prism 1) N2: 9.681 (CAT1, Prism 2) N3: 2.684 (CAT1, Prism 3) N4: 3.148 (CAT1, Prism 4) F1: 1.445 (Fast Histogram, Prism 1) F3: 1.183 (Fast Histogram, Prism 3) The units are counts per second. Coordinate System : The coordinate system used for all GRS data is a Mars aerocentric system following the IAU convention [SEIDELMANNETAL2002], with east longitudes from 0 to 360. Software : A library of source code to parse the DND data product files is included in the software directory. This library allows a programmer to build applications that display or manipulate DND 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_DR_CODE_DOC.ZIP. The contents of this file are described in the label GRS_DR_CODE_DOC.LBL and the source and the binary classes that make up the library are in the file DR_CODE.JAR. Media/Format : The DND will be delivered using CDROM media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR2001].
DATA_SET_RELEASE_DATE 2003-07-01T00:00:00.000Z
START_TIME 2002-02-19T12:00:00.000Z
STOP_TIME 2003-01-01T12:00:00.000Z
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 DND is intended to be the first intermediate data set released for the neutron spectrometer sub-system of the GRS. Data presented here are a highly processed representation of the neutron data as received from the spacecraft. It is possible that changes will be made in the neutron processing algorithms if errors are found. If errors are found, the data will have to be regenerated from the raw unprocessed neutron data set. Review : The DND was reviewed internally by the GRS team prior to release to the PDS. PDS will also perform an external peer review of the DND. Data Coverage and Quality : In order to consider data for inclusion in the time series, we required that the instrument be aligned along the bore-site vector. This eliminated instances such as when the spacecraft was in event mode in which P1 was not pointed in the nadir direction. We also required that the data in the 3 x 19.75s buffer be contiguous, separated by no more than 1-degree of arc length. This avoided the introduction of errors in the counting data when orbital data were missing. Finally, we did not process data for time intervals in which the overload monitor of the neutron spectrometer exceeded 7000 counts per second, which corresponded to solar energetic particle events. Limitations : The major limitation of this data set is that it is derived from the raw minimally processed data. The data is received from spacecraft telemetry, and ingested into a database. If gaps exist in the telemetry, data is lost. Timing and spatial components of the data set rely on the accuracy of the NAIF SPICE kernels. The validity of the derived neutron data is based on the 'correctness' of each step in the processing. It is possible that changes will be made in the processing 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 DND data set.
CITATION_DESCRIPTION Boynton, W.V., Mars 2001 Odyssey Gamma Ray Spectrometer Derived Neutron Data V1.0, ODY-M-GRS-4-DND-V1.0, NASA Planetary Data System, 2003.
ABSTRACT_TEXT The Odyssey GRS Derived Neutron Data (DND) data set includes prism counting rates, normalizations, corrections, and thermal, epithermal, and fast neutron counting rates derived from neutron data collected by the Neutron Spectrometer of the Odyssey GRS instrument suite.
PRODUCER_FULL_NAME WILLIAM BOYNTON
SEARCH/ACCESS DATA
  • Geosciences Web Services
  • Geosciences Online Archives
  • GRS Data Node Services
    		•