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

    The Mars Global Surveyor (MGS) Electron Reflectometer Angular Data
    (ERAD) consist of time-ordered series of 100-600-eV electron flux
    measurements for 16 look directions spanning a 360 x 14 degree field
    of view.  These data are intended to supplement existing MGS ER
    data, which cover the entire instrumental energy range (10 eV -
    20 keV), but are omni-directional.

    Separate data files are provided for each of four energy channels,
    116, 191, 314, and 515 eV (dE/E = 25%).  These energies were chosen
    because:

        1) Electron trajectories are not significantly bent by the
           spacecraft floating potential at energies > 100 eV.

        2) Count rates at energies < 600 eV are typically > 100 times
           greater than instrument background.

        3) Sunlight contamination can be safely neglected in this
           energy range, except when sunlight directly enters the
           instrument aperture, which rarely occurs in the mapping
           configuration.

    Each record consists of a time tag (UTC, spacecraft event time) with
    16 scalars representing measurements of electron energy flux in 16
    look directions, 16 scalars indicating the uncertainties in those
    measurements, 1 scalar indicating instrument background, 2 scalars
    for the magnetic field azimuth and elevation angles in sensor
    coordinates, and 1 scalar indicating the size of the angular
    uncertainty cone around the magnetic field direction.  Electron
    fluxes, uncertainties, and background are in units of electron
    differential energy flux, eV/(cm^2 sec ster eV).  Angles are in
    degrees.

    The Electron Reflectometer field of view is a 360 x 14 degree fan
    that is divided into sixteen 22.5 x 14 degree sectors, numbered
    0 through 15, each with its own counter.  The ER is mounted on the
    nadir deck of the spacecraft.  Its orientation is shown in the
    MGS ER mounting description currently located in the DOCUMENT
    directory of the PDS volume containing these data.  ER azimuth is
    defined in the ER X-Y plane with zero at the boundary between
    sectors 0 and 15.  Note that ER coordinates are left handed,
    so that azimuth increases with sector number.  ER elevation is
    measured out of the ER X-Y plane.  The field of view extends to
    +/- 7 degrees elevation.

    During the Mapping Phase, as the spacecraft orbits the planet, the
    ER field of view sweeps out the entire sky (4-pi ster) every 58.5
    minutes, which is much longer than the integration time per record
    (2-8 sec, depending on telemetry rate) and much longer than most
    timescales of interest in Mars' plasma environment.  Thus, the
    field of view for each data record spans ~12% of the sky.  However,
    since electrons are constrained to travel along magnetic field
    lines, it is more important to consider an electron's motion with
    respect to the magnetic field.

    In a uniform field, electrons gyrate around magnetic field lines on
    helical paths of constant radius (typically a few km) and pitch
    angle, which is the angle between an electron's velocity and the
    magnetic field.  With knowledge of the ambient magnetic field
    direction measured by the MGS Magnetometer (MAG), the ER field of
    view can be mapped into pitch angle.  In the ER X-Y plane, the
    relationship between azimuth (az) and pitch angle is:


        cos(Pitch Angle) = cos(az - Baz) * cos(Bel)


    where Baz and Bel are the azimuth and elevation of the magnetic field
    in ER sensor coordinates.  When Bel = 0, the ER measures the entire
    pitch angle distribution (0-180 degrees) twice, once for each half of
    the field of view.  When Bel = 90 degrees, the ER measures only pitch
    angles of 90 degrees.  All values of Bel are possible, but the ER
    field of view is oriented in such a way that when the magnetic field
    has a large radial component (with respect to the planet), Bel is
    small, and most of the pitch angle distribution is observed.  The
    magnetic field tends to have a large radial component on the night
    hemisphere and in the vicinity of crustal magnetic fields.

    ER Angular Data is intended to be used in conjunction with ER omni-
    directional data, MAG data, and spacecraft ephemeris data.  Time tags
    are provided to synchronize the ERAD with all of these data sets.
    The electron energy distribution (omni-directional data) contains
    information about the plasma environment (i.e., whether the
    spacecraft is in the magnetosheath, magnetotail, or ionosphere).  The
    MAG data provide the strength and direction of the local magnetic
    field (from crustal sources or induced by the Mars-solar wind
    interaction).  The ERAD provide information about the large-scale
    configuration of the magnetic field -- for example, whether the
    spacecraft is on a closed crustal magnetic field loop, on an open
    crustal field line connected to the solar wind, or on a solar wind
    field line.  Electron reflectometry can be used on open field lines
    to determine the magnetic field strength at altitudes near the
    exobase (~180 km) and to probe the atmospheric density between the
    spacecraft and the exobase.


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


    Each Mars Global Surveyor ER angle data file contains a time ordered
    series of electron differential energy flux measurements at a given
    energy (ranging from 100-600 eV).  ER data are organized into
    ''packets'', each of which contains 12, 24, or 48 seconds of data, for
    high, medium, and low spacecraft telemetry rates, respectively.  Each
    packet is further subdivided into samples.  There are 6 samples per
    packet for each of the relevant energy channels, as displayed in the
    table below, resulting in sample periods of 2, 4, or 8 seconds,
    depending on telemetry rate.  The time listed for each record is the
    center of the sampling interval.

    The relevant energy channels and sampling intervals are as follows:


        Channel Number        Energy Range     Samples per Packet
    ---------------------------------------------------------------
               7              415 - 639   eV           6
               8              253 - 390   eV           6
               9              153 - 237   eV           6
              10               92 - 144   eV           6
    ---------------------------------------------------------------


    In-flight calibrations are available for each of the pre-mapping
    mission phases (AB1, SPO1, SPO2, and AB2), and are performed about
    once per month in the Mapping and Extended phases.  This calibration
    is used to determine the relative instrumental sensitivity around
    the FOV to an accuracy of 5-10%.  The sensitivity varies slowly
    with time primarily because of aging of the microchannel plate (MCP),
    which is used to amplify the signal from a single electron into an
    electrical pulse that can be detected on the anode.  This information
    is taken into account when converting count rates to calibrated
    electron differential energy fluxes.

    The local magnetic field measured by the Magnetometer is rotated
    into ER sensor coordinates (see the MGS ER mounting description
    mentioned above) to determine the pitch angle range spanned by each
    angular sector of the instrument.  The magnetic field direction
    changes continuously, so the pitch angle map must be determined
    separately for every data record.


  ====================================================================
  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 electron energy flux eV/(cm^2 s ster eV).
    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 (4 elements).

    The data array is converted from raw counts to differential energy
    flux as follows.  Raw count rate (R) is 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 0.5 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 instrumental background, which is
    caused primarily by high energy particles that penetrate the
    ~2-mm-thick instrument casing and impact the MCP.  Electrons > few
    MeV and protons > 20 MeV have sufficient energy to do this. (Lower
    energy particles are also detected if any of the secondaries they
    produce reach the detector.)  Since these particles bypass the
    electostatic analyzer section, they produce a count rate that is
    independent of the instrument's energy sweep, typically dominating
    the signal in the highest 2-3 energy channels.  During quiet times,
    the background count rate over the entire anode is 7-10 counts/sec,
    which translates to 350-500 eV/(cm^2 s ster eV) in the ERAD.  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 or interplanetary shock crossings (when significant fluxes
    of >10 keV electrons are present) or during energetic solar events,
    when bursts of solar energetic particles can increase the flux of
    penetrating particles by several orders of magnitude.  Assuming that
    the highest energy channel contains 100% background, the background
    level for the lower energy channels used in this data set can be
    estimated as B(E) = B(20 keV), where B(E) is the background level (in
    units of eV cm-2 s-1 ster-1 eV-1) at energy E.  The background is
    typically negligible at energies below about 1 keV.  Finally, we divide
    by the geometric factor (0.02 cm2 ster) to obtain the differential
    energy flux eV/(cm^2 s ster eV).

    In order for the user to calculate pitch angles for each sector and
    observation, the orientation of the magnetic field with respect to
    the ER instrument must be supplied.  To accomplish this, the magnetic
    field vectors recorded by MGS MAG, expressed in a payload coordinate
    system, are first resampled to the time resolution of the electron
    observations.  Then, the orientation of the local field vector in
    azimuth and elevation are recorded, along with an uncertainty.  The
    ER sectors are numbered 0-15 in a clockwise fashion as viewed from
    above the ER instrument.  Azimuth is defined from 0-360 degrees, also
    in a clockwise fashion as viewed from above ER, with 0 degrees
    located at the boundary between instrument sectors 0 and 15.
    Elevation is defined between -90 and 90 degrees, with 90 degrees
    indicating the local magnetic field is orthogonal to the instrument
    aperture and pointed ''up'' away from the instrument (in the +z
    direction for the left-handed coordinate system defined by the azimuth
    angles), 90 degrees pointing ''down'' toward the instrument, and 0
    degrees indicating that the local magnetic field is in the plane of
    the instrument aperture. The uncertainty in the local magnetic field
    vector is supplied as a single angle defining a cone around the
    nominal vector.  Therefore, the uncertainty for a given observation
    could be in the azimuth direction or the elevation direction or (most
    likely) both.

    Finally, data for sectors 9 and 10 are supplied as for all other
    sectors in this data set.  However, in-flight intercalibration
    indicates that sectors 9 and 10 have significantly higher
    uncertainties than the other sectors.  Therefore we recommend that
    sectors 9 and 10 be disregarded in any science analysis using these
    data.

    After processing, data are written to ASCII tables, described below.


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


    The ERDR data set consists of four time-ordered tables containing
    electron fluxes in each of four different instrument energy ranges as
    a function of instrument sector.  Each table contains 43 columns, as
    follows:

     COLUMN       CONTAINS
     ------       ----------------------------------------------------
      1-6         time stamp (yr, day of year, hr, min, sec, msec)
      7-22        electron energy fluxes for 16 ER sectors
      23-38       one-sigma error estimates for 16 ER sectors
      39          estimate of instrument background (energy flux units)
      40-41       azimuth and elevation of local B-field wrt ER
      42          angular uncertainty in local B-field direction


    (Note:  The column numbering given here differs from the column
    numbering in the PDS labels for these data.  This is not a real
    discrepancy; it reflects the fact that the PDS labels treat the
    time stamp as a single multi-part column.)

    File names in this data set currently follow the format

    MyyDddd_PAD_eee.STS

    where yy = 2-digit year, ddd = 3-digit day of year, and eee =
    3-digit energy channel in eV as described under ''Data Set Overview''
    above.


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


    No additional ancillary data is required for use of these data.
    However, the user may wish to use MAG data and MGS spacecraft
    ephemeris information in conjunction with this dataset, also
    available from the PDS.


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


    The data are presented in a coordinate system tied to the ER
    instrument. Each of the 16 energy fluxes and error bars are
    associated with a single angular sector of the instrument.  Azimuth
    and elevation angles necessary to compute pitch angles for each
    sector are supplied with respect to the instrument, as described in
    the sections above.


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


    No software is provided with this PDS data set.

    Data reduction software for the ERAD is written in IDL.

    Users are advised that the following line of pseudocode can be used
    to compute the pitch angle range spanned by each of the ER's 16
    angular sectors, given the azimuth (Baz) and elevation (Bel) of the
    ambient magnetic field in sensor coordinates:


               cos(pitch angle) = cos(az - Baz) * cos(Bel)


    where az represents the azimuth range spanned by one of the sectors.
    For example, to compute the pitch angle range spanned by Sector 0,
    one would use a range of 0-22.5 degrees for az, together with values
    of Baz and Bel in sensor coordinates provided for each data record.
    Note that Baz and Bel vary continuously, so a separate pitch angle
    map must be computed for each data record.  Also note that the
    maximum and minimum pitch angles sampled by a given sector need not
    be at the edges of the sector.  One way to present these data on a
    plot of energy flux (Y axis) vs. pitch angle (X axis), is to plot
    horizontal ''error bars'' representing the pitch angles spanned by
    each sector and vertical error bars for the one-sigma flux
    uncertainties.

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


    Currently these data are stored on media at the PDS for online
    distribution.

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


    The 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 (once per
    orbit), each data record covers only a small region of the sky.
    Any application of these data that requires an unbiased average over
    all look directions (4-pi ster) is NOT RECOMMENDED.  Instead, use
    the formula above for calculating a separate pitch angle map for
    each data record.  Complete (0-180 degrees) pitch angle coverage is
    rarely achieved, but the pitch angle coverage is often sufficient to
    determine magnetic field topology and to detect loss cones caused by
    atmospheric absorption of electrons.

    The ER is mounted on the spacecraft body, where measurements are
    susceptible to the following effects:

    1. Spacecraft charging: The spacecraft typically charges to several
       volts positive in sunlight and several tens of volts negative in
       shadow.  Since electrons have to cross this potential before
       reaching the ER aperture, the energy scale of the ER is effectively
       shifted by the amount of the spacecraft potential.  In addition,
       it is possible for different parts of the spacecraft to charge
       to different potentials, because of differences in material
       properties.  Differential charging sets up small-scale electric
       fields that can bend the trajectories of low energy electrons.
       At the >100 eV energies used in this data set, these effects are
       relatively small and can be safely neglected.  For increased
       confidence, the user may compare pitch angle distributions at
       100 and 500 eV.

    2. Spacecraft photoelectrons: Parts of the spacecraft that are
       illuminated by the sun will emit photoelectrons.  Most of these
       spacecraft photoelectrons have energies below ~10 eV, but a
       small part of the distribution extends up to ~60 eV.  Since
       parts of the spacecraft are close to the ER field of view, some
       of these spacecraft photoelectrons can enter the ER aperture
       and contaminate the measurements.  The flux and angular
       distribution of spacecraft photoelectrons as seen by the ER
       depends on the illumination pattern of the spacecraft.  At the
       >100 eV energies of this data set, spacecraft photoelectron
       contamination can be safely neglected.

    3. Field-of-view blockage:  Parts of the field of view are partially
       obstructed by the spacecraft.  In the mapping configuration (with
       the high gain antenna deployed) these obstructions include the -Y
       solar array gimbal and corners of the spacecraft bus.  To first
       order, these fixed obstructions are accounted for in the
       calibration of the ER field of view.  (Sectors that contain an
       obstruction are assigned a lower effective sensitivity.)

    4. Solar energetic particle (SEP) events:  The instrument background
       is dominated by penetrating particles due to galactic cosmic rays
       (GCRs) and solar energetic particles (SEPs), which are produced
       during solar flares and associated coronal mass ejections (CMEs).
       The GCR background is negligible in the 100-500 eV energy range --
       the fluxes of ambient electrons dominate by several orders of
       magnitude.  However, during large solar events (''space weather''),
       SEPs can increase the background level enough to be significant
       even in the 100-500 eV range.  The user is cautioned to use the
       background data provided in this data set to identify SEP events.
       Large events may prevent the reliable use of these data.


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


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


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


    ER data are recorded continuously.  Data coverage depends almost
    entirely on the fraction of the spacecraft telemetry that can be
    received by the DSN.  The mapping orbit lies close to the ionopause
    altitude.  Because of spatial and temporal variations in the
    ionopause, the ER can sample several different plasma environments,
    including the ionosphere, the magnetosheath, the magnetotail, and
    closed magnetic field lines anchored to remanent crustal sources.
    Data quality is best when the spacecraft is within the planet's
    shadow.  In sunlight, data quality is a function of spacecraft
    rotation phase, since photoelectron contamination depends on the
    illumination pattern.  Additionally, pitch angle coverage is
    controlled by a combination of ER look direction and the local
    magnetic field direction. In some instances it is possible that the
    local magnetic field is orthogonal to the ER field of view, so that
    the pitch angle coverage is limited to a small range around 90 deg.
    However, a much more typical situation is partial pitch angle
    coverage, which is often sufficient to establish the topology of
    the magnetic field and to identify loss cones caused by atmospheric
    absorption of electrons.  The ER field of view is oriented in such
    a way that the pitch angle coverage is best whenever there is a
    large radial component (with respect to the planet) of the ambient
    magnetic field.  This situation often occurs on the night hemisphere
    (because of the draped magnetotail) and over strong crustal magnetic
    fields.


  ====================================================================
  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 mapping, the ER is 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 an orbit.

    Parts of the spacecraft are within the instrument's FOV.  The high
    gain antenna (HGA), which blocked ~70 degrees of the FOV during
    aerobraking is not in the FOV during mapping.  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 throughout
    the mapping phase. Photoelectron contamination has not been removed
    from the data; however, observations that were likely to have been
    contaminated have not been included in this dataset.  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 1/2 spacecraft spin (when the
    spacecraft is illuminated) sunlight can directly enter the ER
    aperture and scatter inside the instrument, creating secondary
    electrons.  (Note: the spacecraft spins once per orbit to keep the
    nadir deck pointed at the planet.)  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,
    outside the range of energies included with this dataset.  Sunlight
    pulses, therefore, 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 the higher energies used in this dataset.