Data Set Overview
   =================
     This data set consists of the GRS advanced products (DAPs). There
     is one DAP product derived from calibrated data that maps either
     gamma-ray count rates or count-rate-derived elemental abundances
     over the northern hemisphere.
 
 
   Instrument Overview
   ===================
     The GRS detector is a high-resolution coaxial germanium crystal
     50 mm in diameter and 50 mm in length, chosen for its radiation
     damage resistance and annealing capabilities. The detector is
     rigidly clamped in a hermetically sealed Al capsule pressurized
     with clean, dry nitrogen. The capsule is cooled to an operating
     temperature in the 80-95 K range by a mechanical cryocooler. A
     plastic scintillator anti-coincidence shield surrounds the
     germanium detector in its sides and back, for rejection of
     cosmic-ray background. Galactic cosmic rays continuously bombard
     the surface of Mercury, and through interactions with the
     surface, gamma-rays of discrete energies that are characteristic
     of specific elements are created. A fraction of these gamma-rays,
     as well as those from the decay of radiogenic elements escape
     from the surface, where they can be detected by the orbiting GRS.
     Gamma-ray fluxes are measurable at altitudes up to 1000 km and
     for gamma-rays up to about 10 MeV that emanate from depths of up
     to tens of centimeter beneath the surface. Detected fluxes are
     generally low and require numerous orbital passes over a specific
     region to obtain a statistically well-defined energy spectrum.
     The measurements of elements such as Fe, Si, Mg, Ca, Ti, K and Th
     by GRS will provide insight into distinguishing between different
     formation models for Mercury as well as other planetary evolution
     issues.
 
     See the INSTRUMENT.CAT file for more information and
     [GOLDSTENETAL2007] for full details.
 
 
   Calibration Overview
   ====================
     The DAPs have been corrected for deadtime, efficiency, and
     attitude-dependent count rate and abundance dependencies. They
     represent spatially resolved measurements and therefore include
     spatial processing.
 
 
   Parameters
   ==========
     The principal parameters when observing with the GRS are as
     follows:
 
     * Accumulation Time: The accumulation time, in seconds, of the Ge
       detector.
     * Sub-spacecraft latitude/longitude.
     * Observing geometry.
 
 
   Data
   ====
     The GRS-derived maps are either gamma-ray count rates or count-
     rate derived elemental abundances. These were created from
     calibrated gamma-ray spectra, represent spatially resolved
     measurements, and have been highly processed to remove
     variability originating from detector altitude and viewing
     geometry. The measurements were also corrected for deadtime.
 
     Each map pixel corresponds to 0.5 deg. x 0.5 deg. surface
     elements, and range from -180 deg. to 180 deg. longitude and -90
     deg. to 90 deg. latitude. The data are mapped in larger surface
     elements whose size is variable but are typically larger than 15
     deg. x 15 deg. The value for each surface element was derived
     from multiple data records (GRS_CAL_AC) summed using the process
     described for deriving GRS_RDR_SUM records.
 
     The maps were created using multiple GRS_CAL_AC spectra that were
     summed according to the sub-spacecraft latitude and longitude and
     were limited to data acquired when the angle between the detector
     boresight and the spacecraft-to-planet-center vectors was less
     than 15 deg.
 
     There are a total of 8 GRS_DAP maps: a potassium (K) abundance
     map, silicon (Si), oxygen (O), and K gamma-ray count rate maps,
     along with 4 maps of the statistical significance of these
     measurements. The maps have spatial coverage that is limited to
     the northern hemisphere as a result of the highly elliptical
     orbit of MESSENGER about Mercury (with periapse location at
     mid-to-high northern latitudes) and the strong
     altitude-dependence of the GRS measurements. The spatial
     resolution of the measurements as shown in the maps is variable
     to ensure comparable statistical significance for each
     measurement pixel [PEPLOWSKIETAL2012B].
 
     The data reduction process used to create these maps is presented
     in the document MESSGRS_PROCESSING.PDF, located in the DOCUMENT
     directory. Also see the GRS CDR-RDR-DAP SIS in the DOCUMENT
     directory for further details.

Confidence Level Overview
   =========================
     The GRS DAP data is the advanced data set released for the GRS.
     There may be further recalibrated deliveries of these data if
     needed and future data may require calibration changes.
 
 
   Review
   ======
     The GRS CDR/RDR/DAP SIS and data were reviewed internally by the
     GRS team prior to release to the PDS. PDS also performed an
     external review of the GRS CDR/RDR/DAP. The RDR and DAP data are
     based on the uncalibrated EDR data and the calibrated data, which
     have been previously reviewed, and GRS science spectra have been
     analyzed based on the EDR data. However, GRS science analysis
     based on these data has not yet taken place; when this occurs, it
     may be found that a new release of these data, possibly
     recalibrated, is needed.
 
 
   Data Coverage and Quality
   =========================
     Data reported are the derived data received from the spacecraft
     during the first year of Mercury Orbit operations.
     This mission phase is defined as:
 
                           Start time         End time
     Phase Name            Date (DOY)         Date (DOY)
     -------------------   -----------------  -----------------
     Mercury Orbit         04 Mar 2011 (063)  17 Mar 2012 (077)
 
     The specific operational periods during the Mercury Orbit phase
     were:
 
                           Start time         End time
     Phase Name            Date (DOY)         Date (DOY)         Sensor
     --------------------  -----------------  -----------------  ------
     Mercury Orbit         23 Mar 2011 (082)  03 Jun 2011 (154)   Ge
     Mercury Orbit         11 Jun 2011 (162)  14 Jun 2011 (165)   Ge
     Mercury Orbit         16 Jun 2011 (167)  05 Jul 2011 (186)   Ge
     Mercury Orbit         20 Jul 2011 (201)  25 Jul 2011 (206)   Ge
     Mercury Orbit         28 Jul 2011 (209)  06 Sep 2011 (249)   Ge
     Mercury Orbit         08 Sep 2011 (251)  17 Sep 2011 (260)   Ge
 
     In this list, 'Ge' means Ge and shield.
 
     During orbital operation, the GRS remained on continuously,
     except for periods when the high voltage was down, for various
     reasons, including an X-class SPE (Solar Particle Event) that
     safed the detector, OCMs (Orbital Correction Maneuvers), and
     cryocooler temperature setpoint changes. Periods having
     conditions that lead to broad energy resolution, such as
     radiation damage revealed by a Ge temperature increase or large
     very low-energy noise, have also been removed. Not shown in the
     table of operational periods are periods of SPEs that did not
     safe the detector: SPE2 08-02-2011, SPE3 08-04/05-2011,
     SPE4 08-09-2011, and SPE5 09-08/09-2011. These time periods
     should probably be eliminated from analysis, due to count-rate
     saturation and strong activation of gamma-ray peaks.
 
     Beginning on 11 October, 2011, the HPGe detector began
     registering anomalous counts in the low-energy (< 300 keV)
     portion of the spectrum. While these events are lower in energy
     than the gamma-ray photopeaks of interest, the resulting count
     rate during these periods is sufficiently high to degrade the
     energy resolution for the detection of all gamma rays, regardless
     of energy. The magnitude of the anomalous counts and
     corresponding degradation of the energy resolution varies, and
     all data acquired after 11 October, 2011 should be carefully
     evaluated for its energy resolution prior to analysis. Particular
     care should be taken prior to summing individual gamma-ray
     spectra with differing energy resolution to produce summed
     spectra for analysis. Post 11 October 2011 data was not used for
     the generation of the DAPs.
 
     The GRS operated in anomalous count rate mode for the remainder
     of its life.
 
     On 15 June, 2012 the GRS cryocooler failed after approximately
     9,500 hours of operation. This compares favorably to the 8,000
     hour mean lifetime for these coolers. The failure occurred during
     a routine detector cool down following the 8 June 2012 annealing
     activity. Several unsuccessful attempts to revive the cooler have
     been made. Without the ability to maintain the HPGe detector at
     cryogenic temperature, gamma-ray measurements are not possible.
     Post-June 15 HPGe data will not be generated or delivered to the
     PDS for archiving. The instrument was repurposed with a software
     upload to enhance neutron and charged particle measurements from
     the shield. These new products may be delivered to the PDS in
     future deliveries.
 
     DAPs were created using the highest-quality GRS measurements. Data
     acquired during solar particle events (SPE) or unusual instrument
     operations were omitted. These include:
 
     3-16 June, 2011; (SPE, including instrument safing and subsequent
     recovery)
     6-16 July, 2011; (HV off, followed by GRS Annealing)
     26 July, 2011; (HV off for Orbital Correction Maneuver)
     2 August, 2011; (SPE)
     4-5 August, 2011; (SPE)
     9 August, 2011; (SPE)
     7-8 September, 2011; (SPE and instrument safing)
     21-23 September, 2011; (SPE)
 
     Data acquired after 28 September 2011, just prior to the onset of
     the anomalous count low energy count rates was likewise omitted
     due to its compromised energy resolution.
 
   Limitations
   ===========
     Though calibrated, this data set is based on minimally processed
     data received from the spacecraft telemetry and ingested into the
     MESSENGER Science Operations Center (SOC). No data gaps or
     corrupted data have been identified for any of the GRS
     operational periods. Although there may be some data not
     identified as missing or corrupted, such data should be minimal
     and a very small fraction of the available data.