Instrument Overview
  ===================
    The Electron Reflectometer system consists of a ''symmetric
    quadrisphere'' imaging electrostatic analyzer, followed by a
    microchannel plate (MCP) detector system with a resistive imaging
    anode.  The analyzer has a narrow energy pass band (about 25%), which
    can be set by the inner hemisphere potential.  This potential is
    generated by a programmable high voltage supply, which is swept
    through its range to measure electrons from 10eV to 20KeV.  Electrons
    are imaged onto the MCP, which multiplies individual electrons by a
    factor of about a million.  This cloud of electrons then hits a
    resistive anode.  The relative signal level on each end of the anode
    is measured by the Pulse Position Analyzer (PPA) to determine the
    location on the anode that the electron cloud landed.  This location
    translates into the direction in the field-of-view plane the incident
    electron was coming from.  The output of the PPA is an 8 bit digital
    value proportional to the incident direction, from 0 to 360 deg.
 
    This information is run through a Pitch Angle Mapper (PAM) which
    sorts the events into 16 pitch angle bins, which are then counted in
    a bank of counters.  The PAM is programmed by the main electronics
    package to convert the PPA output to pitch angle bins based on the
    measured magnetic field vector provided by the magnetometer sensors.
    The counters are read out to the main electronics package over a
    serial interface 32 times a second, synchronized to the telemetry
    clock (RTI).  The analyzer control voltages and PAM are programmed by
    the main electronics package via another serial interface.
 
 
  Platform Mounting Descriptions
  ==============================
    The ER is mounted on the instrument deck.  The ER's symmetry axis
    (Z, which is orthogonal to its FOV) is orthogonal to the spacecraft
    Z axis.  The projection of the symmetry axis onto the spacecraft XY
    plane is 10 degrees from the -Y axis and 80 degrees from the +X
    axis.
 
 
  Principal Investigator
  ======================
    The Principal Investigator for the MAG/ER experiment is Mario
    Acuna.  The Lead Investigator for the Electron Reflectometer is
    Robert P. Lin.
 
    For more information on the Electron Reflectometer see
    [ACUNAETAL1992].
 
 
  Scientific Objectives
  =====================
    See the MISSION_OBJECTIVES_SUMMARY item in the file MISSION.CAT.
    (This file is in the CATALOG directory on this disk.)
 
 
  Operational Considerations
  ==========================
    Parts of the spacecraft are within the instrument's FOV.  During pre-
    mapping, the stowed high gain antenna (HGA) blocked ~70 degrees.
    Once the HGA was deployed, only small amounts of blockage remained,
    which 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 vary over the course of the mission.
    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 once per spacecraft spin
    (~100 minutes during pre-mapping, ~120 minutes during mapping).  The
    contamination disappears as abruptly as it appears.
 
    For a duration of ~4 minutes every half-spin of the spacecraft,
    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.
 
 
  Calibration
  ===========
    The ER may be put into one of 5 automated calibration modes by
    command. These modes (1, 2, 3, 4, and 6) are designed to measure
    specific instrument performance characteristics.  Each mode
    consists of a sequence of steps, each of which lasts one packet
    collection interval. A calibration mode sequence can be programmed
    to repeat from 1 to 255 times.  After the selected number of
    iterations has been performed, the system is returned to its
    original state, with a few minor exceptions. These exceptions
    include:
 
      1. The test pulser is turned off at the end of Cal mode 1,
         independent of its original state.
      2. The ER_PAM_FIX table and ER_PAM_OFFSET value are modified in
         all Cal modes.
 
    In all modes except mode 6, the telemetry format is identical to
    the normal format, but the instrument is operated differently, as
    described below, except that Case Current telemetry data is usually
    garbled.  In mode 6, a block of each packet is used for the ADC
    measurements, and the rest of the packet is scrapped.
 
    In all Calibration modes, the PAM_FIXED mode is used (see below),
    and the PAM_FIX table and PAM_OFFSET value are modified.  The
    nominal values for the table are described below, but all can be
    reprogrammed.
 
 
    (1) ER Calibration Mode 1 (Test Pulser)
 
 
    This mode is used to stimulate the instrument when the analyzer is
    not functional (high voltages off).  It can also be used to
    calibrate the PPA anode.  If the instrument high voltage is turned
    on, real particle counts will be mixed with the test pulser counts.
    The PAM is loaded with a fixed map which maps the region near the
    pulser input with highest resolution (1 PPA bin per counter), with
    the rest of the anode going into counter number 15 (the last pitch
    angle bin).  Next the test pulser is turned on.  Its amplitude is
    increased linearly from zero to full scale synchronous with the
    normal one second analyzer voltage sweep, so that what is normally
    energy step in the telemetry is now test pulser amplitude.  This is
    repeated for one packet duration, and then the cycle is repeated
    for the next packet with the next test pulser.  The whole sequence
    lasts four ER packet intervals, as shown in the table below:
 
 
        Packet Counter            Test Pulser    PAM_OFFSET
            0                     A (30  deg)        13
            1                     B (180 deg)       120
            2                     C (330 deg)       227
            3                     B (180 deg)       120
 
 
    PAM_OFFSET is set to center the nominal test pulser location on the
    high resolution part of the PAM.  The values of PAM_OFFSET and the
    sequence of test pulsers are programmable.
 
 
    (2) ER Calibration Mode 2 (MCP Bias)
 
 
    This mode is used to determine the optimum setting of the MCP bias
    voltage.  The instrument continues to operate in the normal mode,
    except that the MCP bias voltage is modified once per packet, over
    a cycle of 8 ER packets.  Also, the PAM_FIXED mode is used, with
    the PAM table set to 16 equal 22.5 deg fixed image plane bins.
    This allows a look at MCP efficiency as a function of location on
    the MCP.  If there is one particular point on the MCP that should
    be monitored, the PAM_FIX table generated for Cal mode 2 can be
    modified in ER_CAL_PAM table - see discussion above.
 
    Assuming conditions are stable over the test cycle time, the data
    from the different MCP voltage settings can be compared on the
    ground, and an optimum voltage can be selected and commanded up.
    The pattern of MCP voltages used is programmable using the
    ER_CAL2_MCPOFFSET command; the default values are +3, +6, 3, 0, -3,
    -6, -3, 0.  The values from this table are added to the current MCP
    DAC setting.  The last entry in this table should be 0; the MCP
    will be left at the voltage level of the final entry.  Note that
    the default table has steps of about 48 volts.
 
 
    (3) ER Calibration Mode 3 (Background)
 
 
    Calibration mode 3 is used to measure the instrument background
    counting rate from noise and cosmic rays.  The instrument sweep is
    stopped at about 10eV with the deflector attenuator set on.  The
    PAM_FIX table and MCP bias are set and sequenced identically to Cal
    mode 2, so that the background can be measured as a function of MCP
    bias voltage. Unfortunately, unlike the case of Mars Observer, one
    cannot completely shut off incoming electrons, so this mode is of
    marginal usefulness.
 
 
    (4) ER Calibration Mode 4 (Deflector Attenuator)
 
 
    This mode is used to inter-calibrate the instrument sensitivity
    with and without the deflector attenuator on.  This is done by
    disabling normal sweeps, and fixing the analyzer voltage at the
    place where the grid attenuator is normally turned on.  For the
    first packet, the attenuator is off, and for the second packet it
    is turned on (the cycle lasts 2 ER packet intervals).  The PAM_FIX
    table is loaded for 16 equally spaced 22.5 deg bins.
 
 
    (5) ER Calibration Mode 6 (Voltage Calibration)
 
 
    This mode is used to measure the various analyzer voltages which
    normally vary during a packet via the analog housekeeping ADC.  The
    voltages are swept over their range slowly, and the ER analog
    housekeeping ADC read-out time slot is dedicated to the measurement
    (giving 4 samples per second).  The voltage sweeps are synchronized
    to the ADC read-out times. The data from the ADC is loaded directly
    into the ER telemetry packet right after the header, taking 48 16
    bit words (48 12-bit ADC samples per packet, with the 4 MSB of each
    word set to zero).  Normal telemetry data is lost, and the rest of
    the packet will be garbled.  The mode consists of 2 different
    cycles, each looking at different voltages, and each taking 1
    packet interval to complete.
 
 
    (5a) ER Calibration Mode 6, Cycle 1 (Analyzer and Deflector
         Attenuator Voltages)
 
 
    This cycle measures 32 samples of the analyzer voltage, using every
    4th value of the normal analyzer sweep table.  Note that the gain
    of the housekeeping channel switches when the gain of the
    programming DAC for the analyzer is changed, and by the same
    amount, so to correctly interpret the measurements, the setting of
    the gain bit must be known for each measurement.  The MSB of each
    sample contains the gain bit of the DAC for this purpose.  The
    remaining 16 values are measurements of the attenuator grid made
    while the analyzer and grid voltages are programmed in their normal
    pattern over the second half of the sweep table, again using every
    4th entry of the table.
 
 
    (5b) ER Calibration Mode 6, Cycle 2 (Case Voltage)
 
 
    The second packet of ER Cal mode 6 contain measurements of Vcase
    (the case voltage), Icase (the case current), and Icase*80 (the
    high gain Icase measurement), while the Case Voltage DAC is ramped
    over its full range in 16 steps (starting at 0, 16 DAC steps each
    sample). The data is ordered as 16 Vcase samples, followed by 16
    Icase samples, and finally 16 Icase*80 samples.
 
 
  Operational Modes
  =================
 
    (1) Energy sweep and attenuator
 
 
    Data is accumulated over one or more energy sweeps.  Each energy
    sweep takes 1 second, and is divided into 128 equal steps
    (synchronized to the RTI).  The analyzer high voltage is swept from
    high to low energy during the sweep in an approximately exponential
    decay.  Data is collected for 16 pitch angle bins 30 times each
    sweep, dividing phase space into 16 pitch angles by 30 energy bands
    over the energy range of the instrument. Note that data is not
    collected during the first 4 energy steps when the analyzer high
    voltage is re-charged, or during the 4 steps that the attenuator
    grid voltage is charged up.
 
    The sweep voltage is controlled via the ER sweep registers to a 12
    bit DAC with a gain switch.  The gain switch changes the sweep
    voltage by a factor of 16 to increase the accuracy of the voltage
    setting at the low end.  The software generates a log sweep pattern
    on turn-on (which may be over-written by ground command), and
    automatically sets the gain bit appropriately.
 
    At a selectable point in the sweep, the deflection attenuator turns
    on (default is at step 124, which means it never turns on).  This
    decreases the sensitivity of the instrument by a factor of 43.5.
    This is needed to avoid saturating the instrument at the low energy
    end (where there are typically a lot of electrons), while allowing
    maximum sensitivity at the high energy (where there are few
    electrons).  When energized, the deflectors bend electrons from the
    normal aperture out of the analyzer field of view, while
    simultaneously bending electrons from the lower, attenuated
    aperture into the analyzer field of view.  The deflection supply
    runs at 8 times the analyzer high voltage,  but the supply tops out
    at about 600 volts, and works only in the low gain range of the
    analyzer high voltage.  The sweep voltage pauses while the grid
    attenuator voltage comes on for one accumulation sector time, and
    the data collected from that interval is discarded.
 
 
    (2) PAM-variable Mode: (onboard pitch angle sorting)
 
 
    The events are converted into 16 pitch angle bins, corresponding to
    the 16 counters, using the PAM table.  This table is generated by
    the software based on the direction of the magnetic field vector.
    The PAM table is updated every 2, 4, or 8 seconds at 1296, 648, and
    324bps respectively.  A one second average of the magnetic field
    samples is computed for this purpose.  Offsets are then subtracted
    from a programmable table.  Next, the vector is rotated into ER
    sensor coordinates using a variable rotation array set by the solar
    array motion model (see below), followed by a second fixed rotation
    matrix.  The vector is then normalized and the azimuthal angle
    (PHI) and cosine of the elevation angle (COSTH) are computed.  PHI
    is the angle around the FOV plane, coded in an 8 bit number such
    that 256=360 deg, zero degrees being at the anode break point,
    increasing clockwise as viewed from the top of the analyzer (the
    anode break point is 135 deg clockwise from the RPA aperture).
    COSTH is the cosine of the elevation angle out of the FOV plane,
    zero degrees being is the image plane.  COSTH is always positive,
    since the sign is unimportant for PAM table computation.  These
    quantities are the basis of the PAM table generation, and are
    transmitted in the ER packet so that the PAM table can be
    reconstructed on the ground for computing the bin weighting.
 
    The PAM table is generated by computing the image plane location of
    the pitch angle bin boundaries, and then filling in the rest of the
    table, using the relationship:
 
 
      Bin = PHI +- Cos-1(COSPAi/COSTH)
 
 
      where:
       Bin is the image plane bin corresponding to pitch angle boundary # i
       COSPAi is the cosine of the i-th pitch angle boundary
       +- is the 'plus-or-minus' sign, normally represented by a + sign
            above a - sign, but written here as +- for typographical
            reasons.
 
 
    The COSPAi are programmable.
 
 
    (3) PAM-fixed Mode.
 
 
    This computation can be bypassed by going to a 'Fixed' PAM table,
    which is independent of the magnetic field direction.  This can be
    done by command or automatically during calibration cycles.  The
    'fixed' map is characterized by a set of 16 image plane boundaries
    and a rotation of that pattern similar in function to PHI.
 
    Before the PAM table is loaded into the ER, the table is masked to
    remove a programmable set of 'bad' regions.  These are parts of the
    image plane which are noisy or which have objects in the FOV
    distorting the trajectories.  This table defaults to 'none'.
 
 
  Measured Parameters
  ===================
    The ER measures particle counts from 1 to 507904.  The instrument
    integrates for 0.0625 seconds at each energy channel every 2
    seconds. The noise level is about 10 counts per second at all
    energies.