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.