Data Set Overview
    =================
      The data set consists of advanced data products, also known as
      DDR/DAPs.

      For all the DDR products there is a detached PDS label file that
      describes the contents of one data file. Each label file will have
      the same base name as the data file it is describing, with the
      extension .LBL to denote a label file. The label file defines the
      start time and end of the observation, product creation time, and
      the structure of the binary (or ASCII) tables. Each data product
      is identified within the PDS label by a STANDARD_DATA_PRODUCT_ID
      value, shown in parentheses in the table below.

      The FIPS portion of the data archive currently consists of eight
      DDR data products (one retired). The following table shows the
      different FIPS data products and their files.
      Following is a description of each
      of the current FIPS DDRs. A more complete description of each may
      be found in the EPPS DDR SIS document, located in the DOCUMENT
      directory of the document volume.

      Current FIPS Data Products:

        FIPS Observed Density (FIPS_NOBS)

          Contains observed density (n_obs) for all major ion species as
          well as UTC and MESSENGER position.

        FIPS Energy Spectra (FIPS_ESPEC)

          Contains differential energy flux at each E/q value for selected ion
          species during selected time periods.

        FIPS Pitch Angle Distribution (FIPS_PCHANG)

          Contains histograms of pitch angle in 5 degree bins for selected ion
          species during selected time periods.

        FIPS Energy-Resolved Pitch Angle Distribution (FIPS_ERPCHANG)

          Contains histograms of pitch angle in 5 degree bins for selected ion
          species during selected time periods with the additional separation
          of energy/charge (i.e., E/q).

        FIPS Pitch Arrival Direction (FIPS_ARRDIR)

          Contains ion flux as a function of arrival direction in units of
          (cm^2 s keV/keV sr)^-1. NOTE: This product is contained only in
          version 1.0 of this data set and is retired as of the Mercury Orbit
          Year 2 Phase of the mission.

        FIPS Kinetic Properties (FIPS_NTP)

          Contains number density (cm^-3), temperature (K) and pressure (nPa)
          for selected major ion species over selected time periods.

        FIPS Viewing Normalization (FIPS_ROTMSO)

          Contains rotation matrix from FIPS cartesian to MSO for each energy
          scan.

        FIPS Angular Flux Map (FIPS_FLUXMAP)

          Contains a mapping of the integrated ion flux as a function of flow
          direction for selected ions and regions around Mercury.

    Instrument Overview
    ===================
      The FIPS sensor consists of an electrostatic analyzer (ESA), located
      at the entrance to the sensor, a post-acceleration chamber between
      the output of the ESA and the carbon foil, and a time-of-flight
      telescope. The ESA at the entrance to the FIPS acts as a wide-angle
      lens for ions. It only allows ions with a specific energy/charge band
      to enter through its output plane.

      See the EPPSINST.CAT file for more information and
      [ANDREWSETAL2007] for full details.


    Calibration Overview
    ====================

      This data set is calibrated and noise-corrected.  Individual ion events
      (pulse height analyzed, PHA) have been converted to observed
      densityCount rates have been converted from raw counts to counts/sec
      and, where possible, to fluxes.  See the DDR SIS document for details.


    Parameters
    ==========
      The energy per charge (E/q) tables are programmable as described in
      the CDR SIS document and EDR to CDR calibration document.  The
      various adjustable parameters used in the noise removal process are
      described in the DDR SIS.


    Data
    ====
      The FIPS DDR data set products are described as follows (the STANDARD_
      DATA_PRODUCT_ID value is given in parentheses). The notes provided
      in brackets give the current status of the data product in version 2
      of the data set.  Each file has an index column which contains values
      unique for the day.  This index can be used to exactly match data in
      one file to data in the other two.

      FIPS Observed Density (FIPS_NOBS) is the main DDR data product. It
      contains observed density (n_obs) for all major ion species: H+,
      He2+, He+, O+ group and Na+ group.  This data has been noise corrected.
      MESSENGER position information in MSO and altitude, latitude and
      local time is also included., as well as UTC converted from MET by
      SPICE.

      FIPS Energy Spectra (FIPS_ESPEC) contains differential energy flux at
      each of the 64 E/q values (0.046 - 13.3 keV/e) for selected major
      ion species during selected time periods.

      FIPS Pitch Angle Distribution (FIPS_PITCHANG) contains a histogram of
      pitch angle in 5 degree bins for selected major ion species during
      selected time periods.  Pitch angle is the angle between the particle
      incident and the local magnetic field vector (from MAG).

      FIPS Arrival Direction (FIPS_ARRDIR) contains ion flux as a function of
      arrival direction in units of (cm^2 s keV/keV sr)^-1.  This product is
      provided for selected major ion species over selected time periods.
      When necessary, this product is provided at a lower time resolution than
      the fundamental instrument scan, to allow accumulation of enough raw
      data to facilitate scientific interpretation.

      FIPS Kinetic Properties (FIPS_NTP) contains number density (cm^-3),
      temperature (K) and pressure (nPa) for selected major ion species over
      selected time periods.  Also included are error measures for each
      quantity.  This product contains the full number density, rather than
      observed (partial) number density included in other products.  When
      necessary, this product is provided at a lower time resolution than the
      fundamental instrument scan, to allow accumulation of enough raw data to
      facilitate scientific interpretation.

      FIPS Viewing Normalization (FIPS_ROTMSO) data product contains a
      rotation matrix from FIPS cartesian to MSO coordinates, for each FIPS
      energy scan. This matrix can be used to rotate ion incident angles in
      CDR PHA data into MSO coordinates needed for producing normalized
      directional maps (e.g. FLUXMAP or PITCHANG) for arbitrary time
      resolutions, in multiples of 10s.

Confidence Level Overview
    =========================
      The EPPS DDR data are the processed data set released for the
      EPPS.  Data presented here are an accurate representation of the EPPS
      data as received from the spacecraft and have been processed from
      instrument unit to physical unit.


    Review
    ======
      The EPPS DDR was reviewed internally by the EPPS team prior to
      release to the PDS. PDS also performed an external review of the
      EPPS DDR.


    Data Coverage and Quality
    =========================
      Data reported are the processed data received from the
      spacecraft during the following mission phases: Launch,
      Earth Cruise, Earth Flyby, Venus 1 Cruise, Venus 1 Flyby,
      Venus 2 Cruise, Venus 2 Flyby, Mercury 1 Cruise, Mercury 1 Flyby,
      Mercury 2 Cruise, Mercury 2 Flyby, Mercury 3 Cruise,
      Mercury 3 Flyby, Mercury 4 Cruise, Mercury Orbit, Mercury Orbit
      Year 2, Mercury Orbit Year 3, Mercury Orbit Year 4, and Mercury
      Orbit Year 5.  These mission phases are defined as:

                            Start time         End time
      Phase Name            Date (DOY)         Date (DOY)
      -----------------     ----------------  ----------------
      Launch                3 Aug 2004 (216)  12 Sep 2004 (256)
      Earth Cruise         13 Sep 2004 (257)  18 Jul 2005 (199)
      Earth Flyby          19 Jul 2005 (200)  16 Aug 2005 (228)
      Venus 1 Cruise       17 Aug 2005 (229)   9 Oct 2006 (282)
      Venus 1 Flyby        10 Oct 2006 (283)   7 Nov 2006 (311)
      Venus 2 Cruise        8 Nov 2006 (312)  22 May 2007 (142)
      Venus 2 Flyby        23 May 2007 (143)  20 Jun 2007 (171)
      Mercury 1 Cruise     21 Jun 2007 (172)  30 Dec 2007 (364)
      Mercury 1 Flyby      31 Dec 2007 (365)  28 Jan 2008 (028)
      Mercury 2 Cruise     29 Jan 2008 (029)  21 Sep 2008 (265)
      Mercury 2 Flyby      22 Sep 2008 (266)  20 Oct 2008 (294)
      Mercury 3 Cruise     21 Oct 2008 (295)  15 Sep 2009 (258)
      Mercury 3 Flyby      16 Sep 2009 (259)  14 Oct 2009 (287)
      Mercury 4 Cruise     15 Oct 2009 (288)  03 Mar 2011 (062)
      Mercury Orbit        04 Mar 2011 (063)  17 Mar 2012 (077)
      Mercury Orbit Year 2 18 Mar 2012 (078)  17 Mar 2013 (076)
      Mercury Orbit Year 3 18 Mar 2013 (077)  17 Mar 2014 (076)
      Mercury Orbit Year 4 18 Mar 2014 (077)  17 Mar 2015 (076)
      Mercury Orbit Year 5 18 Mar 2015 (077)  30 Apr 2015 (120)


      A dark count signature was discovered in the FIPS PHA data. These
      counts appear in a time-of-flight -- E/q histogram of PHA data as
      straight tracks in E/q, centered at about 8 different time-of-flights.
      These dark counts significantly complicate the analysis of FIPS PHA
      data, though not significantly for low mass ions (H+, He+ and He2+).
      All data since launch is affected. This situation has been well
      characterized by the FIPS team and models/algorithms for removing
      these counts have been developed [Gershman et al., 2013,
      http://hdl.handle.net/2027.42/100358].  These algorithms have been
      used in the processing of this data.

      In orbit (Mercury_Orbit phase), FIPS is regularly taken out of
      nominal configuration for various spacecraft operations, such as
      commanded momentum dumps and eclipses, typically prevent possible
      high voltage discharge within FIPS or for power considerations.
      On occasion, the EPPS flight software autonomously takes FIPS out
      of nominal configuration in response to an anomaly.  There
      intervals are marked as bad in the data quality field with a
      value of 1.  (Good data contains a 0 in this field.)

      FIPS regularly experiences penetrating radiation which serves as an
      additional noise source in the start and stop single-coincidence rates.
      These counts are identified in start and stop rate spectra as relatively
      constant streaks across the energy spectrum, because they bypass the
      electrostatic analyzer energy selction when they pass through the FIPS
      solid structures to trigger the detectors.  Typically, these signals do
      not occur in double-coincidence and therefore do not affect the science
      data. (That is, they do not generate PHA events or add to the valid
      event or proton rate.) When double-coincidences do occur, they are often
      so short in duration (< 1 min) that they can be ignored in all but the
      most fine time scale analysis. Occasionally, events occur that are of
      sufficient intensity and duration as to make the double coincidence data
      unusable, such as long duration solar energetic particle (SEP) events.
      These events are not suitable for normal science analysis and are
      excluded from this data set.

      In some scans, the pitch angle distributions show very high values in
      one of the edge bins, 0 or 180 degrees.  These values are artifacts
      of the processing and should not be used.

      As of the 10th PDS delivery, FIPS CDR data has a new quality flag
      system, which  automatically flags SEP events and other issues which
      would affect data quality.  This quality flag is a bit field, with each
      bit set signifying the condition  detailed below:

     Flag                               Bit Position        Decimal Value
     ------------------------------     ------------        -------------
     Bad data (reason undefined)         0                      1
     PHA hot pixel*                      1                      2
     Scan hard limit reached*            2                      4
     SEP event                           3                      8
     Start rate anomaly*                 4                     16
     Instrument off                      7                    128
     Bad data (manually marked bad)     15                  32768

     Multiple conditions set multiple bits, effectively adding the numbers in
     the value column.  (This is the same as performing a logical OR on the
     binary values.)  For example, if PHA hot pixel and SEP event are both
     set, the flag will have the decimal value 10 (binary: 1010).  If the
     start rate anomaly and scan hard limit reached flags are set in
     addition, the decimal value would be 30 (binary: 11110).

     For most science uses, only data with a quality flag value of 0 should be
     used.  However, data with the flags marked with '*' above is often often
     usable for science. Users should consult with the instrument team for
     details.

     From May-Sept 2013, MCP voltages higher than the nominal -2520V were
     tested on FIPS, with a goal of recovering counting efficiency lost
     through use thus far in the mission.  Since the counting efficiency is a
     little different for each of these voltages, users are advised to consult
     the actual MCP voltage stored in housekeeping when using data from this
     period.  This effect is expected to be small.  Corrections for these
     variations will be applied in the next release.

      In this version of the dataset, many of these products are provided
      in regions determined by hand-picked or modeled bow shock and
      magnetopause boundaries.  Boundary picking was done by the MAG team at
      a level of accuracy sufficient for the relatively low time resolution
      FIPS data (10s - 60s).  Where these were not available, the model given
      by Slavin et al. [Science, 2010] is used.

      The recovered density, temperature and pressure (NTP) product is
      particularly sensitive to the region of Mercury's space environment in
      which the data was taken. The reasons for this, based on validity of
      required assumptions, are described in detail in the DDR SIS.  This
      data is provided within the magnetosphere, where the assumptions are
      likely to hold for averages of 1 minute ore more.  It is is also
      provided in the magnetosheath within a 45 degree cone angle of the X_MSO
      axis.  The validity of the assumptions is much more variable here,
      though they will often hold. USERS ARE STRONGLY CAUTIONED to read the
      NTP section of the DDR SIS follow the recommendations for using these
      quantities.

      The region and type of boundary used is encoded in the NTP data quality
      flag as a bitfield according to the table below.  Since it is a bit
      field, the bits are cumulative.  For example, when the spacecraft is in
      the magnetosheath flanks, which is outside of the magnetosphere and
      outside the 45 deg cone angle of X_MSO, both bits 0 and 1 would be set
      (value of 1), leading to a value of 3. Individual bits can be extracted
      by performing a bit-wise AND on the quality flag value with a value with
      a single bit set, e.g. 1, 2, 4, and 8, for bits 0-3, resp.  Further
      information on handling bit fields can be easily found in programming
      resources on the web.


      Bit       Value   Meaning when set
      ---       -----   ----------------
      0           0     Inside Magnetosphere
                  1     Outside Magnetosphere
      1           0     Magnetosheath, inside 45 cone angle of X_MSO
                  1     In magnetosheath, outside 45 cone angle of X_MSO
      2           0     Hand-picked bow shock used
                  1     Bow shock model used
      3           0     Hand-picked magnetopause used
                  1     Magnetopause model used

      NTP data is not produced when the corresponding CDR scans are marked
      with the SEP flag (see above), when the method does not converge or when
      the minimum count value (20) is not reached for a given accumulation.
      These time steps are simply omitted in the data file.


    Limitations
    ===========
      This data set is fully processed, science quality data.  It has been
      reviewed at high level by the FIPS instrument team.  It has not been,
      however, manually validated on a point by point.  Users are
      encouraged to contact the FIPS instrument team when questions about
      the data arise.  Some analysis of this data requires detailed
      knowledge of FIPS pointing which can be extracted from SPICE.