Instrument Overview
  ===================
    During aerobraking, the accelerometers measure the change in velocity of
    the spacecraft due to aerodynamic forces. The dominant force is along the
    spacecraft y-direction. The spacecraft y-axis is approximately into the
    wind. Data are provided at 1 second intervals and recorded in units of
    m/s^2. Data from all three accelerometers will be provided in Version 2.
    Higher altitude data may also be provided after certain corrupting
    influences are corrected.
 
 
  Scientific Objectives
  =====================
    Accelerometer data were used to characterize the nature of the atmosphere,
    to determine the effect of the atmosphere on each orbit, and to predict
    the effect of the atmosphere on future orbits.
 
 
  Calibration
  ===========
    The instrument was calibrated on each orbit to determine drift in
    instrument bias.  Bias is determined by monitoring the accelerometer
    instrument during periods of inactivity before and after entering the
    atmosphere. The bias acceleration is then estimated over the entire pass
    by trending the data from the pre- and post-atmospheric entry periods.
    The pre- and post-atmospheric periods were defined by instrument turn-on
    and turn-off times and a lower limit on the altitude of data used for
    calibration, typically 250 km.
 
 
  Operational Considerations
  ==========================
    The instrument readings are affected by changes in temperature.  The
    instrument is mounted in the inertial measurement unit (IMU) and the
    temperature of the IMU and the accelerometers are monitored by temperature
    sensors.
 
 
  Operational Modes
  =================
    The data from the accelerometer are passed to the telemetry deck during an
    aerobraking pass from the time the s/c reaches aerobraking orientation
    until the s/c returns to nominal orbit attitude.
 
 
  Measured Parameters
  ===================
    An accelerometer is an instrument that measures the acceleration of the
    case of the sensor due to external forces.  All accelerometers have a
    'proof mass' and it is the tendency of the proof mass to move relative to
    the case that is a measure of the acceleration of the case.  Early
    accelerometers produced output that was directly related to acceleration;
    but modern sensors integrate the internally measured signal to reduce
    noise and the output is proportional to the change in velocity over the
    integration time.  In high precision accelerometers, like those on MRO,
    the proof mass is an electronically floating mass.  The electromagnetic
    field is varied to keep the proof mass stationary relative to the case.
    The current required to accomplish this is proportional to the
    acceleration. The accelerometers on MRO are sensitive to acceleration of
    the center of mass (c.m.) of the s/c, pseudo-accelerations (i.e.
    centrifugal) due to rigid motion of the s/c about the c.m., and
    differences in gravitational force at the proof mass and the c.m. of the
    s/c (gravity gradient).