Data Set Overview
  =================
    The Mars Global Surveyor spacecraft includes a laser altimeter
    instrument. The primary objective of the Mars Orbiter Laser
    Altimeter (MOLA) is to determine globally the topography of Mars at
    a level suitable for addressing problems in geology and geophysics.
 
    The MOLA Experiment Gridded Data Record (EGDR) is a topographic map
    of Mars based on altimetry data acquired by the MOLA instrument and
    accumulated over the course of the mission. Two types of EGDR
    products are produced: the Initial Experiment Gridded Data Record
    (IEGDR), consisting of data accumulated through at least the first
    30 days of the mapping mission, and the Mission Experiment Gridded
    Data Record (MEGDR), consisting of data accumulated over the whole
    primary mission. Different resolutions of the IEGDR and MEGDR may
    be released, and multiple versions of each product may be released.
    See the MOLA EGDR Software Interface Specification [MOLAEGDRSIS1999]
    for details.
 
    The MOLA Precision Experiment Data Records (PEDRs) are the source
    for the EGDRs. See the MOLA PEDR Software Interface Specification
    [MOLAPEDRSIS1998] and the PDS Catalog entry for the PEDR data set
    (MGS-M-MOLA-3-PEDR-L1A-V1.0) for a description of the PEDRs.
 
 
  Data
  ====
    The MEGDR product is a global map of planetary radius, areoid,
    topography, and number of observations, derived from MOLA PEDR
    products and aggregated into latitude-longitude bins. The binned
    data include all MOLA nadir observations from the Mapping Phase
    through the Primary and Extended missions, from the end of
    aerobraking in February 1999 through June 2001. Additionally,
    off-nadir observations of the north pole are included from 87 N
    latitude and northward, taken during the spring of 1998, and of
    both poles taken during Mapping from 87 N and S to the poles.
    Data are adjusted using a first-order crossover solution for
    radial, along-track, and across-track position. Parts of orbits
    are excluded where solutions for these orbits are deemed to be
    poor. Also excluded are shots more than 1.2 degree off-nadir
    (except as noted above), channel 4 returns, and any returns not
    classified as ground returns, e.g. clouds or noise, according to
    the SHOT_CLASSIFICATION_CODE. A total of nearly 600,000,000
    observations are represented.
 
    Each image is a binary array of 8- or 16-bit integers in
    most-significant-byte-first storage order. The image file name is
    in the form MEGkxxdyyyrv.IMG, where
 
      k   = A for areoid, C for counts, R for radius, T for topography
      xx  = latitude of pixel in upper left corner of the image
      d   = N for north latitude, S for south
      yyy = longitude of the pixel in the upper left corner of the image
      r   = map resolution in pixels per degree, e.g.
                 C =   4 pix/deg
                 E =  16 pix/deg
                 F =  32 pix/deg
                 G =  64 pix/deg
                 H = 128 pix/deg
      v    = version letter.
 
 
  Parameters
  ==========
    N/A
 
 
  Processing
  ==========
    The PEDRs incorporate the best multi-arc orbital solutions derived
    from the Goddard Mars potential model GMM1.6, and the available
    tracking. The latest spacecraft SCLK timing corrections have been
    applied.  The ranges account for instrument delays and the leading
    edge timing biases, estimated by the receiver model of
    [ABSHIREETAL2000]. This model assumes a Gaussian shape for the
    transmitted and surface-scattered pulse waveforms, using the
    detector threshold settings and the observed pulse width and energy
    measurements between the threshold crossings to infer the true pulse
    centroid, width, and amplitude.  The eccentric orbit brought MOLA
    much closer to the surface of Mars than the design called for, thus
    the pulse width and energy measurements were saturated for much of
    each pass.  Caution must be exercised when interpreting these
    measurements.  Laser energies are calculated according to the
    transmitter model of [AFZALETAL1997].  A post-launch calibration to
    the MOLA oscillator frequency has been applied, based on the
    difference between the spacecraft high-resolution timer and the MOLA
    clock, resulting in an estimated frequency of f=99,996,232 +/- 5 Hz.
    This frequency is given in the PEDR and may change due to clock
    drift.  The interval between shots, as well as the shot
    time-of-flight, is controlled by this frequency. The shot interval
    in seconds, delta_t = 10,000,000 / f.
 
    Time tags are given in ET seconds of MOLA fire time.  Timing of the
    shots is interpolated to ~100 microseconds. This step is essential
    in the highly elliptical orbit insertion geometry because the
    spacecraft may change its radial distance by as much as 1600 meters
    per second.
 
    The spacecraft time, from which the shot time is derived, is subject
    to further timing corrections. The range observations have been
    registered with orbital position by assuming that the actual time of
    observation is 117 milliseconds later than the time tag of the MOLA
    range as derived from the spacecraft clock.
 
    The ground location and planetary radius is calculated in inertial
    (J2000) coordinates as the difference between the spacecraft
    position vector and the MOLA one-way range vector. The direction of
    the MOLA vector is obtained from project-supplied spacecraft
    attitude kernels and the boresight calibration of the instrument
    with respect to the spacecraft. The one-way range of the laser shot
    to the planet is obtained from the two-way range by correcting for
    the change in spacecraft position during laser shot
    time-of-flight.
 
    Due to the inverse-square-law energy return in the link equation
    [ZUBERETAL1992], the instrument detector was saturated during a part
    of the periapsis approach.  Received pulse energy and pulse width
    are resolved during the portion of the pass when the detector is not
    saturated.  The absolute accuracy of these quantities is about 5%.
 
    There is a table entry for each non-zero shot range detection for
    all in-range packets in the data stream.  Occasional corrupted range
    values occur due to transmission errors, and some packets are lost
    entirely. A packet sequence number is generated by MOLA. The
    sequence number was initialized to 0 just before the planet came
    within range during the SPO-1 and 2 data passes via a restart
    command, while during the Hiatus subphase the restart occurred
    earlier.
 
    Some MOLA ranges are either clouds or false detections due to the
    intrinsic noise characteristics of the receiver. The MOLA ranges
    that are true ground hits are flagged with a positive number in the
    tables.
 
 
  Ancillary Data
  ==============
    N/A
 
 
  Coordinate System
  =================
    The MEGDR products use the areocentric coordinate system
    [DAVIESETAL1994B], more generally described as planetocentric. The
    areocentric system is body-centered, using the center-of-mass as
    the origin.  Areocentric latitude is defined by the angle between
    the equatorial plane and a vector extending from the origin of the
    coordinate system to the relevant point on the surface.  Latitude
    is measured from -90 degrees at the south pole to +90 degrees at
    the north pole.  Longitude extends from 0 to 360 degrees, with
    values increasing eastward (i.e., it is a right-handed coordinate
    system) from the prime meridian [DAVIESETAL1994B].  This
    coordinate system is preferred for use in geophysical studies in
    which, for example, estimates of elevation or gravitational
    potential are generated mathematically.
 
    The MEGDR coordinate system is based on the IAU2000 cartographic
    standard, which differs from the IAU1991 standard used for previous
    MOLA products [SEIDELMANNETAL2002, DUXBURYETAL2001].
 
 
  Software
  =======
    N/A
 
 
  Media/Format
  ============
    The MGS MOLA MEGDR dataset is available electronically
    via the PDS Geosciences Node web site at
    http://wwwpds.wustl.edu and the MOLA Science Team web site at
    http://ltpwww.gsfc.nasa.gov/tharsis/mola.html.  Formats are
    based on standards established by the Planetary Data System (PDS).

Confidence Level Overview
  =========================
    The resolution of the altimetry data is about 40 cm vertically,
    and about 300 m along-track, limited by the 10 Hz firing rate
    of the laser.  The absolute, long-wavelength radial orbit error
    is estimated to be about 30 m. The uncertainty in absolute ground
    spot location is limited by the attitude knowledge of the
    spacecraft, and is estimated to be about 400 m at a nominal
    range of 400 km.
 
 
  Data Coverage/Quality
  =====================
    The MEGDR product is based on altimetry measurements acquired by
    the MOLA instrument from Mars orbit insertion on September 15,
    1997, through the aerobraking phase, two sets of Science Phasing
    Orbits, the regular Mapping Mission, and the Extended Mission, up
    to June 30, 2001 when the MOLA laser stopped operating.