Data Set Overview
  =================
    All ALTITUDE data are packaged by periapsis number for each
    aerobraking orbit.  ALTITUDE data are provided in a table labeled
    Pyyyy.tab where 'yyyy' is the four digit periapsis number.
 
 
  Parameters
  ==========
    Altitude:               Units = km
    Latitude:               Units = degrees
    Longitude:              Units = degrees
    Local Solar Time:       Units = N/A
    Longitude of the Sun:   Units = degrees
    Solar Zenith Angle:     Units = degrees
    Density:                Units = kg/km**3
    Sigma Density:          Units = kg/km**3
    Scale Height:           Units = km
    Sigma Scale Height:     Units = km
 
 
  Data
  ====
    For each orbit, ALTITUDE data consist of an 11 by 10 array in the
    file folder Pyyyy.  Each row gives atmospheric characteristics at a
    constant altitude.  When applicable, inbound and outbound legs are
    in separate rows.  The altitude levels going from row 1 to row 11
    are:  periapsis, 1.26 nbar inbound, 1.26 nbar outbound, 130 km
    inbound, 130 km outbound, 140 km inbound, 140 km outbound, 150 km
    inbound, 150 km outbound, 160 km inbound, 160 km outbound.  The
    columns from one to ten are separated as follows:  altitude,
    latitude, longitude, local solar time, longitude of the sun, solar
    zenith angle, density, sigma density, scale height, and sigma scale
    height.
 
 
  Processing
  ==========
    Density and density scale heights are provided at reference
    altitudes of periapsis, 1.26 nbar, 130, 140, 150, and 160 km.  The
    latter 4 are obtained by a least squares fit to the 40 point running
    mean data sets that span 5 km on either side of a reference altitude
    for both the inbound and outbound portions of the orbit.  The values
    at periapsis are obtained by a similar fit to all data within 10km
    of the periapsis altitude. See [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 on density and scale height are based on the
    formal covariance matrix for the linear regression of log density
    and altitude.  Data noise is assumed to be the standard deviation of
    the fit.
 
 
  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.