Data Set Overview
  =================
    All PROFILE 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.  PROFILE
    data are provided  in a table labeled Pyyyy.tab located in its
    respective orbit folder. PROFILE data are provided at one second
    resolution.
 
 
  Parameters
  ==========
    Time From Periapsis:            Units = seconds
    Latitude:                       Units = degrees
    Longitude:                      Units = degrees
    Local Solar Time:               Units = N/A
    Solar Zenith Angle:             Units = degrees
    Altitude:                       Units = km
    7 Second Density:               Units = kg/km**3
    Sigma 7 Second Density:         Units = kg/km**3
    40 Second Density               Units = kg/km**3
    Sigma 40 Second Density:        Units = kg/km**3
 
 
  Data
  ====
    For each orbit, PROFILE data consist of an n-by-1 array in the file
    folder Pyyyy, where n is the number of seconds of data received
    during the aerobraking pass.  Column 1 contains the time from
    periapsis.  Columns  2 through 10 contain Latitude, Longitude, Local
    Solar Time, Solar Zenith Angle, Altitude, 6.7 Second Density, Sigma
    6.7 Second Density, 40 second density, and Sigma 40 Second Density.
 
 
  Processing
  ==========
    Accelerometer data were processed in the PROFILE stage through
    several steps and the entire process with examples is given in
    [TOLSONETAL1999B].  First, traditional Euler terms were removed
    using low pass rates and low pass shifted rates.  The shifted rates
    are utilized to eliminate erroneous acceleration due to thruster
    induced, nearly instantaneous changes in rates.  Next, high
    frequency terms due to SAM vibration were removed using a linear
    regression on both high pass angular rates and angular acceleration.
    Third, accelerometer data at the time of thruster firings are
    replaced with a local exponential fit or with a constant equal to
    the bias.  A seven point running mean is then performed to remove
    any remnants of the SAM vibration.
 
    The 7 second averaged counts are converted into density values.
    Because of considerable wave structure in data which do not
    represent the 'mean' thermosphere, a second data set is also
    archived.  A 40 point running mean of the 7 second running mean
    density is the final data product.
 
    Density averaging was conducted in two forms. The first form,
    seven-second running averages, are included in the data files for
    only those instances following the final negative density inbound,
    and prior to the first negative density value outbound.  (These
    negative count values arise due to the offsets and biases of the
    system at very small decelerations).  These seven second running
    mean values account for all instantaneous measurements during this
    seven second window.  Thus, seconds 1-7, seconds 2-8, seconds 3-9,
    etc., are included in consecutive average values.
 
    The second averaging is a running forty 7-second average mean. Here,
    the individual 7-second average density values are accumulated for
    40 consecutive points (seconds 1-7, 2-8, 3-9,....33-39, 34-40).
    These 40-point averages DO include all 7-second points (including
    those which covered negative count values; recall, the individual
    7-second average values included in the data tables are set to zero
    for all instances where a negative count value existed in the
    seven-second accumulation prior to averaging). Thus, in the data
    table, the 40-point mean values commence in the first record, while
    the 7-second average values are zero at the beginning of and the end
    of the table.
 
    These averaging procedures, and their motivation, are described more
    completely in [TOLSONETAL1999B].
 
 
  Coordinate System
  =================
    Latitude and longitude are in the IAU Mars Centered Mars Equatorial
    System.  Altitude is above the IAU reference ellipsoid, with
    a=3393.4 km and f=0.0052083 and including corrections for (4,4)
    gravitational potential.

Confidence Level Overview
  =========================
    Standard deviations for the 7 second and 40 second running means are
    sample standard deviations and include both natural variability and
    data noise.
 
 
  Review
  ======
    Comparisons were made with operational data sets.  Deviations were
    within expectations founded on archiving analysis that is based on
    (4,4) gravitational potential and numerically integrated precision
    trajectories that were not included in operational software.
 
 
  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.  Not all orbits have yet
    been included in current archiving data sets.  Omitted orbits
    include orbits where automatic data processing procedures did not
    produce realistic answers.  If required, many of these orbits can be
    recovered by manual manipulation of data and data analysis
    procedures.