| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Mars Global Surveyor spacecraft included 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 Spherical Harmonics Topography Model (SHADR) is a
numerical transform of the MOLA Experiment Gridded Data Record
(EGDR), a global topography data set. See the MOLA EGDR Software
Interface Specification [MOLAEGDRSIS1999] and the PDS Catalog
entry for the EGDR data set (MGS-M-MOLA-5-IEGDR-L3-V1.0) for a
description of the EGDRs.
The MOLA SHADR follows the general format of Spherical Harmonics
ASCII Data Records defined in the SHADR SIS [SIMPSON1999]. The
SHADR was defined initially for gravity models derived from
Magellan and Mars Observer radio tracking data, but the format has
been generalized to be suitable for spherical harmonic expansions
of gravity, topography, magnetic, and other fields.
This Mars topography model was produced by Greg Neumann under the
direction of David E. Smith, MOLA Principal Investigator.
Further improvements to the model are expected as additional MGS
data are incorporated.
Data
====
The MOLA SHADR is in the form of two ASCII tables, a header table
and a coefficients table, contained in a single file with a
detached PDS label. The header table consists of a single record
of eight data columns that contain descriptive information
about the coefficients table. The columns include the assumed
gravitational constant, the uncertainty in the constant, the
degree and order of the model field, the reference longitude and
latitude for the spherical harmonic expansion, and a normalization
indicator. A column for the assumed reference radius of the planet
is also included for consistency with the general SHADR format,
but it is not used or needed for the model.
The EGDR that was the source of the SHADR data set included all
MOLA nadir observations from the Orbit Insertion phase, plus
Mapping Phase nadir observations from the beginning of mapping
through the end of August 1999. Additionally, off-nadir
observations of the north pole were included from 87 N latitude
and northward, taken during the spring of 1998, and of both poles
taken in July of 1999, from 87 N and S to the poles. Orbits 355
and 358 of the Orbit Insertion Phase and parts of orbits 10709
through 10716, inclusive, of the Mapping Phase were excluded
because solutions for these orbits were deemed to be poor. (Note:
subtract 10000 from a MOLA mapping phase orbit number to determine
the equivalent MGS Project orbit number.) Also excluded were
shots more than 1 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 approximately 123 million
observations are represented in the EGDR.
Parameters
==========
N/A
Processing
==========
See the PDS Catalog entries for the MOLA EGDR and PEDR data sets
for a description of the processing applied to the data from which
this SHADR is derived.
Ancillary Data
==============
N/A
Coordinate System
=================
The coordinate system is IAU 1991 (Davies et al., Celestial
Mechanics and Dynamical Astronomy, 53, 377-397, 1992
[DAVIESETAL1992B]), areocentric, with longitudes measured positive
east.
Software
=======
N/A
Media/Format
============
The MGS MOLA SHADR dataset is available on CD-ROM and
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).
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