Data Set Overview
  =================
    All level zero accelerometer data are packaged by periapsis
    number for each aerobraking orbit.  Each orbit is identified by a
    folder with name Pyyyy where 'yyyy' is the four digit periapsis
    number.  Level 0 z-axis accelerometer data are provided every 0.1
    seconds during an interval of time that generally assures that
    the initial and final data points are taken at least 200 km above
    the surface of Mars.  Additional data, required to reduce
    accelerometer counts to acceleration on the spacecraft, are
    provided at lower sampling rates.
 
 
  Parameters
  ==========
  Accelerometer counts: Units = counts  (1 count = 0.332 mm/s change in
                                         velocity)
                        Sampling Interval = 0.1 seconds
 
  Quaternions:          Units = dimensionless
                        Sampling Interval = 1 second
 
  Filtered body rates:  Units = rad/s
                        Sampling Interval = 1 second
 
  Thruster on-times:    Units = sec (cumulative time thruster has fired)
                        Sampling Interval = 8 seconds
 
  Orbital elements:     Units = various
                        Sampling Interval = once per orbit
 
 
  Data
  ====
    For each orbit, level 0 data consist of four arrays in four files
    in folder Pyyyy.  The array in 'counts.tab' is n-by-11 in size,
    where n is the number of seconds of data received during the
    aerobraking pass.  Column 1 contains the time in UTC and follows
    the PDS format YYYY-DDDTHH:MM:SS.SSSZ where YYYY = four digit
    year, DDD = day of year, HH = hour, MM = minute, and SS.SSS =
    seconds.  T is a separator for date and time and the Z is the UTC
    Z.  Columns 2 through 11 contain the 0.1 second accelerometer
    counts for the second beginning at the time stamp.
 
    The array in 'ratequat.tab' is n-by-8 in size.  Column 1 is the
    time corresponding to the filtered rates and quaternions in the
    same UTC format as described for the counts above.  Columns 2-4
    contain the angular rates about the x, y and z axes respectively.
    Columns 5-8 contain the quaternions.
 
    The third file, 'thruster.tab', is a k-by-13 array, where k is
    within 1 of n/8.  The first column is time as described above.
    The next 12 columns are the cumulative reading of how long each
    thruster has been fired during the mission.  Column two
    corresponds to thruster number one, column 3 with thruster 2, and
    so on.  Column 13 corresponds with thruster 12.  Thrusters 1-8
    produce moments about the 'x-y' axes by forces along the s/c
    z-axis and thereby corrupt the accelerometer measurements.
    Thrusters 9-12 provide roll about the z-axis and no detectible
    corruption of z-acceleration has been found.
 
    The fourth file, 'orbelem.tab', is a 1-by-6 array of osculating
    elements at periapsis in the order semi-major axis (km),
    eccentricity, inclination (radians), longitude of the ascending
    node (radians), argument of periapsis (radians), and universal
    time of periapsis (seconds past J2000).
 
 
  Coordinate System
  =================
    Spacecraft body coordinate system has the origin at the center of
    mass.  The z-axis is normal to the main engine nozzle etiz plane
    and positive in the direction of the science instrument deck.
    Positive x-axis is in the direction of the high gain antenna.
    See DOCUMENT/JSR01_04.TIF or [CANCROETAL1998] for graphic.
 
    Acceleration and rates are given in the MGS body system with the
    z-axis along the centerline of the bus, y being along the solar
    array inner gimbal rotation axis, and positive x on the same side
    of the bus as the high gain antenna.  The quaternions define the
    orientation of the body axes with respect to the IAU Mars
    Centered Mars Equatorial at Time of Jan 1, 2000 12:00.  Orbital
    elements are given relative to the IAU system.  The reference
    geoid used is a (4,4) representation of the Mars gravitational
    field.  There is a negligible difference between current Mars
    gravitational models up to fourth order and degree.  The
    objective is to determine the areo-potential altitude to within
    100m.
 
 
  Timing
  ======
    The times in the data files are UTC.  For orbit calculations,
    ephemeris time was converted to UTC and the spacecraft data were
    already in UTC.  During the early part of operations it was found
    that the time tags on the three fundamental data types -
    accelerometer counts, attitude rates, and thruster firing times -
    were not synchronized.  Information on the telemetry maps for all
    these data were not available.  Five of the 10 accelerometer
    channels came from each of the two onboard computers and special
    care was taken to assure that these data were properly ordered in
    time.  Accelerometer data time tags were taken as the fundamental
    reference and all orbital velocities and positions are
    interpolated to this time.  Between Phase 1 and Phase 2 of
    aerobraking, it was found that the accelerometer time tags were
    delayed 1.5 seconds on the s/c and subsequently all accelerometer
    data were shifted 1.5 seconds relative to UTC.  As mentioned
    above, attitude rates have a frequency dependent delay in
    addition to any basic uncertainty in time tagging.  Finally,
    there is uncertainty in the proper time tags for thruster firing.
    To synchronize the rates and thruster data with the accelerometer
    data, numerous firings of the x- and y- thrusters throughout the
    mission were studied.  Such thruster firings should produce
    simultaneous effects in rates and acceleration.  By these means
    it was found that for the purposes herein, the rate time tags
    required shifting by -2 seconds to be in sync with the already
    shifted accelerometer data.  By studying thruster firings that
    occurred on either side of the 8 sec thruster data interval, it
    was concluded that the thruster time tag needed to be shifted by
    -0.5 seconds.

Confidence Level Overview
  =========================
    After the effects of rate filtering are included (see Limitations
    below), the confidence level for the rate and quaternion data is
    sufficient for reducing the accelerometer data to density at the
    noise level of the accelerometer.  Accelerometer data would most
    likely be corrupted by changes in temperature of the instrument.
    The temperature of the accelerometer environment is actively
    controlled, and short term (order 10 seconds) variations are
    expected to be less than 1/6 count.  Variations in accelerometer
    bias due to temperature changes over a pass have been less than
    0.1 counts.  Except for the time of periapsis, errors in
    remaining orbital elements are of sufficient accuracy for data
    interpretation.  During phase 1 of aerobraking, Sept.  1997
    through March 1998, every orbit was reconstructed using DSN
    tracking data on each side of periapsis.  This provided errors in
    the time of periapsis of less than 1 second.  During phase 2,
    this was not the case and larger errors may result.
 
 
  Review
  ======
    All of the data types included in the level 0 product are
    utilized by the MGS operations to monitor the health of the
    spacecraft.  These data are reviewed in near real time to assure
    MGS performance.
 
 
  Data Coverage and Quality
  =========================
    Data coverage during an aerobraking pass has varied throughout
    the mission.  Early in the mission, data were received for 500
    seconds on either side of periapsis.  Later, the data started
    between 200 and 250 seconds before periapsis.  This change was
    made to reduce propulsion usage.  Though this change somewhat
    reduced the accuracy of determining the accelerometer bias, it
    had the advantage of reducing the corruption of the data set with
    thruster firings while still inside the detectable atmosphere.
    The data for the early orbits of MGS have an eight second sample
    time.  This does not agree with software devised to archive this
    data, therefore there is no plan, at this time, to archive the
    early orbit data.
 
 
  Limitations
  ===========
    The rate and quaternion data are calculated onboard the
    spacecraft.  Raw rate information from the rate gyros are
    filtered to remove a potential interaction with a 2 Hz structural
    vibration mode.  This produces an approximate 2 second delay in
    the output rates.  Quarternions are obtained by onboard
    integration of rates, providing a delay of about 1 second.