====================================================================
    Data Set Overview
    =================

      The Electron Reflectometer Data Record (ERDR) is a time ordered
      series of electron measurements from the Mars Global Surveyor (MGS)
      Mission. Each record consists of a time tag with 19 scalar data
      points representing measurements of the electron flux in 19
      different energy channels, ranging from 10 eV to 20 keV, with an
      energy resolution of 25%.  Each data point is a measure of the
      electron flux (cm-2 sec-1 ster-1 eV-1) averaged over a 360 x 14
      degree disk-shaped field of view (FOV).  (Parts of this FOV are
      masked because of spacecraft obstructions, as described below.)
      During the Science Phasing Orbits (SPO), the spacecraft was in
      Array Normal Spin (ANS) configuration, for which the ER field of
      view sweeps out the entire sky (4-pi ster) every 50 minutes, which
      is much longer than the integration time per record (2 to 48 sec,
      depending on energy and telemetry rate) and much longer than most
      timescales of interest in Mars' plasma environment.

      The ERDR is intended to be used in conjunction with MGS
      Magnetometer (MAG) data records, which provide the magnetic field
      vector and spacecraft ephemeris data as a function of time.
      Electrons travel along the magnetic field lines in tight helices
      (few km radius) at high speed (roughly one Mars diameter per
      second).  Thus the electron data contain information about the
      plasma environment as well as the large-scale configuration of the
      magnetic field, which is sampled locally by the MAG.


    ===================================================================
    Parameters
    ==========

      The Mars Global Surveyor ER data set consists of a time ordered
      series of electron flux measurements in 19 energy channels, ranging
      from 10 eV to 20 keV.  The ER data are organized into ''packets,''
      each of which contains 12, 24, or 48 seconds of data, respectively,
      for high, medium, and low spacecraft telemetry rates.  Each packet
      is further subdivided into samples.  There are from 1 to 6 samples
      per packet, depending on the energy channel, as given in the table
      below.  The ER data set is generated at the rate of 6 samples per
      packet, regardless of energy.  When there are fewer than 6 samples
      per packet at a particular energy, data values are repeated in
      order to maintain a uniform table.  The time listed for each record
      is the center of the sampling interval.  When records are repeated,
      taking an average of the times for all repeated records provides
      the center time for that sample.

      The energy channels and sampling intervals are as follows:


          Channel Number        Energy Range     Samples per Packet
      ---------------------------------------------------------------
                 0               13 - 20 keV            1
                 1              8.0 - 12 keV            1
                 2              4.9 - 7.5 keV           1
                 3              2.9 - 4.6 keV           3
                 4              1.8 - 2.8 keV           3
                 5              1.1 - 1.7 keV           3
                 6              680 - 1046 eV           6
                 7              415 - 639 eV            6
                 8              253 - 390 eV            6
                 9              153 - 237 eV            6
                10               92 - 144 eV            6
                11               72 - 87 eV             2
                12               56 - 67 eV             2
                13               43 - 52 eV             1
                14               33 - 40 eV             2
                15               25 - 30 eV             1
                16               18 - 23 eV             2
                17               14 - 17 eV             1
                18               10 - 13 eV             2
      ---------------------------------------------------------------


      The ER has a 360 x 14 degree disk-shaped field of view.  The 360
      degrees are divided into 16 angular sectors, each with a separate
       counter that is read out in telemetry.  The sizes and look
       directions of these sectors are programmable to within an accuracy
      of 1.4 degrees.  The ER can operate in one of two modes: PAM-fixed
      or PAM-variable.  In PAM-variable mode, the sizes and look
      directions of the 16 angular sectors are dynamically chosen by the
      DPU (using onboard MAG data) in order to map the FOV into fixed
      pitch angle bins. PAM-variable mode is used only in the mapping
      orbit.  Throughout the SPO period, the ER was in PAM-fixed mode,
      meaning that the FOV was divided into 16 equally sized 22.5 x 14
      degree sectors that remained fixed in spacecraft coordinates.

      Some of these PAM-fixed sectors are masked because of obstructions
      in the FOV, most notably the stowed high gain antenna (HGA), which
      blocks 3 sectors. Three additional sectors are partially obstructed
      by the -Y solar array gimbal/yoke assembly and corners of the
      spacecraft bus. These obstructions are minimal, so data from these
      sectors is still considered to be of good quality in PAM-fixed
      mode.  Finally, one sector was damaged in the interval between
      SPO-1 and SPO-2, and its efficiency dropped by about a factor of 4.
       For this SPO data set, the 3 sectors obstructed by the HGA and the
      one damaged sector are masked, and we sum data from all other
      sectors to form a scalar ''omnidirectional'' value.  The effective
      FOV for each record consists of two 135 x 14 degree fans.


    ====================================================================
    Processing
    ==========

      Processing is carried out at the Space Sciences Laboratory (SSL) of
      the University of California, Berkeley, (UCB) to convert the raw
      data to measurements of the omnidirectional electron flux (cm-2 s-1
      ster-1 eV-1).  Because of the instrument's high dynamic range (six
      decades), the onboard digital processing unit (DPU) compresses the
      raw counts in a logarithmic scale.  The first step is to decompress
      the raw counts and construct a three-dimensional data array, where
      the first dimension is time (6 elements per telemetry packet), the
      second dimension is direction around the FOV (16 elements), and the
      third dimension is energy (19 elements).

      The next step is to sum over unobstructed angular sectors to
      produce a two-dimensional time/energy array.  Raw count rate (R) is
      then obtained by dividing the raw counts by the integration time
      (0.0625 sec per energy step).  The data are next corrected for
      deadtime. During the time it takes the instrument to process a
      single electron (known as the ''deadtime'', which is about 0.4
      microsec for the ER), it ignores any other electrons.  The raw
      count rate is multiplied by the factor 1/(1 - RT), where T is the
      deadtime, to obtain corrected count rate.  Data values are masked
      (set to -9.999e-9) when the deadtime correction factor exceeds
      1.25.  These data are NOT CORRECTED for a background count rate due
      to cosmic rays and noise in the electronics (about 10 counts/sec).
      Most of the time, the signal in the highest energy channel (13-20
      keV) is dominated by background.  Exceptions to this sometimes
      occur during bowshock crossings or during energetic solar events.
      Assuming that the highest energy channel contains 100% background,
      the background level for the lower energy channels can be estimated
      as follows:


        Channels  0-10 (92 eV - 20 keV): B(E) = B(20 keV) * (20 keV/E)
        Channels 11-18 (10 eV - 87 eV):  B(E) = B(20 keV) * (20 keV/E) *
        43.5


      where B(E) is the background level (in units of cm-2 s-1 ster-1
      eV-1) at energy E.  The background is typically negligible at
      energies below about 1 keV.  Background correction is essential at
      higher energies.

      Data are collected through two separate apertures that cover the
      same field of view but differ in their transmission by a factor of
      43.5. At low energies (10 eV to 100 eV), the smaller aperture is
      used to attenuate high fluxes, and at high energies (100 eV to 20
      keV), the larger aperture is used to maximize the sensitivity to
      low fluxes. The corrected count rates in energy channels 11-18
      (100-10 eV) are multiplied by the factor 43.5 to compensate for the
      smaller aperture size.  Finally, we divide by the geometric factor
      (0.02 cm2 ster) and the center energy (eV) to obtain the
      differential flux (cm-2 s-1 ster-1 eV-1).

      The data are organized into a table with a uniform time step for
      all energy channels.  Since the sampling interval is different for
      different energy channels, data values are repeated within each
      packet, as necessary, to enforce a uniform time step.  The data are
      sent via FTP to the Principal Investigator (Mario Acuna) at Goddard
      Space Flight Center (GSFC), where they are incorporated with the
      magnetometer data.


    ====================================================================
    Data
    ====

      The ERDR data set consists of a single time-ordered table.  Each
      record contains a time stamp and 19 data values, representing the
      omnidirectional electron flux in 19 different energy channels
      ranging from 10 eV to 20 keV.


    ====================================================================
    Ancillary Data
    ==============

      No additional ancillary data is required beyond that described for
      the MAG.


    ====================================================================
    Coordinate System
    =================


      The data are presented in omnidirectional format.  The time tags
      contained in the ER data set should be used to obtain the
      corresponding spacecraft ephemeris information from the MAG data
      set.


    ====================================================================
    Software
    ========

      Data reduction software for the ER is written in IDL.


    ====================================================================
    Media/Format
    ============

      The ER data are provided in the form of 20-column ascii tables.
      Storage media are described in the MAG documentation.

====================================================================
    Confidence Level Overview
    =========================

      The ER is mounted on the spacecraft body, where measurements are
      susceptible to spacecraft charging and FOV blockage.  This ER
      instrument design is typically used on a rapidly spinning
      spacecraft (few seconds period), on which the disk-shaped FOV would
      sweep out the entire sky in a time that is short compared with most
      timescales of interest.  However, since MGS spins slowly (100
      minute period) during SPO, each data record covers only a small
      region of the sky. Despite this limitation, the scalar flux
      provided in the ERDR is suitable for identification of plasma
      boundaries (bow shock, magnetic pile-up boundary, ionopause) and
      following the evolution of the electron energy distribution, which
      is useful for evaluation of the plasma environment and
      interpretation of the magnetometer data.  Any application of these
      data that requires an unbiased average over all look directions
      (4-pi ster) is NOT RECOMMENDED.


    ====================================================================
    Review
    ======

      The ERDR will be reviewed internally by the MGS MAG/ER team prior
      to release to the planetary community. The ERDR will also be
      reviewed by PDS.


    ====================================================================
    Data Coverage and Quality
    =========================

      ER data are recorded throughout most of the 12-hr elliptical SPO
      orbits of MGS.  This orbit carries the spacecraft from the
      unperturbed solar wind to well inside the ionosphere.  Data quality
      does not depend on the position of MGS along the orbit, although
      the telemetry rate was highest near periapsis. Data quality is a
      function of spacecraft rotation phase, since photoelectron
      contamination depends on the illumination pattern.


    ====================================================================
    Limitations
    ===========

      The ER is mounted on the spacecraft instrument deck and has a
      disk-shaped FOV that is orthogonal to the spacecraft XY plane and
      nearly orthogonal to the spacecraft Y axis.  (There is a 10-degree
      rotation about the Z axis to minimize spacecraft obstructions in
      the FOV.)  This 360-degree FOV is divided into 16 angular sectors,
      each 22.5 degrees wide.  Throughout SPO, the ER was in
      ''fixed-sector'' mode, meaning that these 16 angular sectors
      remained constant in the spacecraft reference frame, sweeping out
      the entire sky every 1/2 of a spacecraft spin.

      Parts of the spacecraft are within the instrument's FOV -- most
      notably the stowed high gain antenna (HGA), which blocks ~70
      degrees. Smaller amounts of blockage are caused by attitude control
      thrusters and the -Y solar array gimbal and yoke assembly.  One
      effect this has on the measurements is to block ambient electrons
      from the directions of the obstacles. This is most clearly seen at
      high energies (> 100 eV), which are only slightly deflected by the
      spacecraft floating potential.  In addition, when these obstacles
      are illuminated by the sun, they emit photoelectrons up to ~50 eV,
      which can enter the ER aperture and elevate the counting rate at
      low energies.  The detailed signature of this effect depends on the
      illumination pattern as the spacecraft rotates, which is a function
      of the angles between Earth, Mars, and the Sun.  These angles
      varied during the course of SPO. Photoelectron contamination has
      not been removed from the data; however, the presence of
      contamination is readily identified in the low energy channels (<
      50 eV) by a sharp (nearly discontinuous) increase in counting rate
      which appears at regular 100-minute intervals.  The contamination
      disappears as abruptly as it appears.

      For a duration of 4 minutes every 50 minutes, sunlight can directly
      enter the ER aperture and scatter inside the instrument, creating
      secondary electrons. A tiny fraction of these photons and secondary
      electrons can scatter down to the anode and create a ''pulse'' of
      spurious counts.  This sunlight pulse appears at all energies, but
      is most noticeable from 10 to 80 eV and above 1 keV.  Sunlight
      pulses have not been removed from the data.

      The instrument's energy scale is referenced to spacecraft ground.
      In sunlight, spacecraft ground floats a few volts positive relative
      to the plasma in which the spacecraft is immersed.  Electrons are
      accelerated by the spacecraft potential before they can enter the
      ER aperture, thus all energies are shifted upward by a few eV.  In
      addition to shifting the electron energy, the trajectories of low
      energy electrons can be significantly bent by electric fields
      around the spacecraft.  Thus, the energy scale and imaging
      characteristics are relatively poor at the lowest energies (10-30
      eV), becoming much more accurate at higher energies.