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 interpret-
    ation.  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 space-
    craft.  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.