Instrument Overview
  ===================
    Each MER entry capsule contained two LITTON LN-200S Inertial
    Measurement Units (IMU).  One IMU was located within the rover,
    with the other IMU located on the backshell.  The IMU within the
    rover was not located at the rover's center of mass, and the IMU
    within the backshell was not located at the backshell's center of
    mass.  Neither of the two IMU's within the entry capsule was
    located near either the entry shell's center of mass nor the
    entry shell's spin axis.  The entry shell consisted of the heat
    shield and backshell inside of these two joined components.
 
    The four LN-200S units employed in the mission were selected from
    a larger population for improved stability.
 
    These LN-200S IMU's provide three-axis rotation rate and
    acceleration measurements.  The rotation rate is provided by a
    Fiber Optic Gyro (FOG) sensor, while the accelerations are
    provided by silicon accelerometers.  Each IMU contains three FOGs
    and three accelerometers, oriented orthogonally to provide
    measurements in three dimensions simultaneously.  The resultant
    measurements from each of the accelerometers and each of the FOGs
    is spatially transformed to the center of the IMU prior to being
    output by the instrument.
 
    Each IMU has a mass of 0.75 kg (1.65 pound weight), is ~9 cm by 9
    cm in size, and is operated at 12 watts.  These LN-200S IMU's
    have space flight heritage, having been successfully flown on the
    Clementine mission.
 
    The accelerometers within each IMU had a dynamic range of 80g
    (where g here represents Earth's standard surface gravitational
    acceleration of 9.80665 meters per second per second) and a
    resolution of 2.4 milli g's, with a noise level of 1.6 milli g's
    when sampled at 400 Hz (the nominal instrument sampling rate).
    The spacecraft could not handle 400 Hz sampling, so the IMU data
    were summed over 50 measurements, resulting in an 8 Hz sampling
    by the spacecraft.  This summing has the effect of reducing the
    the effective noise to 300 micro g's, with an effective
    resolution of 50 micro g's.  These instrument attributes are
    clearly discussed in [CRISPETAL2003].
 
    Atmospheric entry occurred at a nominal height of 128 km above
    the Mars surface at a nominal atmospheric relative velocity of
    5400 m/s.  The frictional drag of the atmosphere upon the entry
    vehicle results in a reduction in the speed of the entry vehicle.
    This deceleration is measured by the accelerometers within the
    IMU, while the orientation of the entry vehicle is provided by
    the gyroscope measurements.
 
    The FOGs also provided relative orientation information for both
    the backshell and the lander after parachute deployment, which
    occurred approximately 240 seconds after entry at an altitude
    approximately 8.5 kilometers above the surface.  The lander
    separated from the backshell and descended on a tether
    approximately 30 seconds after parachute deployment.  Subsequent
    simultaneous backshell and lander (rover) IMU measurements in
    this tethered condition allow for the 'swing' of the lander on
    the tether to be determined.  This enabled removal of this motion
    from the accelerometer measurements, permitting a determination
    of the net motion of the lander toward the surface.  This allows
    for atmospheric profile reconstruction while the lander is still
    attached to the parachute.  These measurements (as well as the
    descent imaging) also served as part of the guidance for the
    horizontal motion reduction system which was included to minimize
    lander horizontal motion upon initial impact with the surface.
    The bridle connecting the lander to the backshell was severed
    approximately three seconds prior to surface impact.  The
    backshell IMU measurements ceased upon the severing of this
    connection.
 
 
  Platform Mounting Description
  =============================
    Each entry assemblage included two IMUs.  One IMU was included
    within the rover while the other was included on the backshell.
    Neither of these was located at either the center of mass of its
    parent entry vehicle component nor the center of mass of the
    entire entry vehicle.
 
 
  Principal Investigator
  ======================
    No Science Team was selected for the IMU/EDL aspect of the MER
    mission, thus there is no Principal Investigator.  The IMU system
    was part of the engineering instrumentation and was selected,
    configured, installed, and operated by members of the engineering
    team.  They provided entry vehicle deceleration and position and
    orientation information that was used for tracking EDL and
    triggering EDL events on the entry vehicle (parachute deployment,
    lander and backshell relative attitudes, retro- rocket firing,
    etc.).
 
 
  Scientific Objectives
  =====================
    The IMUs were not science instruments and thus nominally there
    was no science objective for their inclusion on the spacecraft.
    The measured deceleration values provided by the IMU, as a
    function of time (which needs to be used to determine the height
    above the surface).  the surface, can be employed to deduce the
    vertical structure of density and pressure, and ultimately
    temperature of the atmospheric column traversed by the lander as
    it descended through the atmosphere.
 
 
  Operational Considerations
  ==========================
    The nominal data rate provided by the IMUs was 400 Hz, a rate
    faster than the on-board spacecraft processing could deal with.
    The 400 Hz rate was reduced to 8 Hz via averaging of 50
    consecutive measurements (measurements 1-50, 51-100, ...) and it
    was these 8 Hz data that were ingested into the onboard software
    and from which the accelerations and rotation rates were
    calculated.  It is these on-board calculated values of
    accelerations (or velocity change) and rotation rates (or
    quaternion elements) that are included in this archive.  EDL is a
    mission critical phase (if EDL fails, the mission is completely
    lost) and thus the IMU data collection/processing needed to adapt
    to the EDL and mission requirements.  Furthermore, since the IMUs
    were a key component (as an active sensor) of the EDL process,
    their operational characteristics were driven by EDL engineering
    considerations.  This included the amount and type of in-flight
    processing to the IMU data, the sampling frequency, and the data
    collection rate.  These choices have, in some ways, limited the
    scientific return of the instruments.
 
 
  Calibration
  ===========
    The IMUs were delivered with a 'factory calibration' based upon
    individual unit testing after assembly.  Based upon later
    testing, this is good, but not perfect.  Among other issues, it
    does not account for unit aging and instrument drift.  It is only
    intended to ensure the instrument remains within the formal
    specifications as they (the IMUs) performed their critical tasks
    during EDL.  The onboard calculated parameters (the TRANSFORMED
    dataset) are based upon the IMU output and nominal (not measured)
    entry-vehicle parameters (IMU position and orientation, etc.).
    There was no laboratory testing of the IMU performance and
    quality while in the entry configuration.  A more accurate
    calibration based upon prelaunch measurements, cruise
    observations, as well as EDL data themselves is in progress.  It
    is anticipated that this calibration effort will result in an
    improved IMU EDL dataset.
 
 
  Operational Modes
  =================
    From the time of IMU turn on throughout the total EDL process
    (atmospheric entry through the cessation of bouncing and
    rolling), IMU data were provided at 400 Hz and integrated to 8 Hz
    measurements used by the on-board software.  There were no gain
    changes or offset changes throughout this time period.  The
    interval between saved samples (and whether or not the HIGHRATE
    data were collected) varied over the course of EDL.  This was
    based partly upon absolute time and partly upon spacecraft
    events.  One anomaly was a periodic resetting to a value of zero
    of the accumulated velocity change (in each of the three axes) of
    the Rover IMU.