The Mars Global Surveyor MOLA Archive
1. Introduction
This document provides an overview of the Mars Global Surveyor
MOLA Archive, including a complete listing of the contents of this
archive volume, below.
This volume and the preceding volume, MGSL_0001, contain science
data products from observations acquired by the Mars Global Surveyor
(MGS) Mars Orbiter Laser Altimeter (MOLA) instrument during the Orbit
Insertion Phase of the mission. They include data products from the
Aerobraking Hiatus and the Science Phasing Orbit 1 and 2 (SPO-1 and
SPO-2) Subphases. The products from the Aerobraking Hiatus were
published previously on the Mars Global Surveyor Science Sampler
CD-ROM. See the file MISSION.CAT in the CATALOG directory for a
description of the various phases of the mission.
MOLA data products include Aggregated Experiment Data Records, or
AEDRs (raw data), Precision Experiment Data Records, or PEDRs (data
from AEDRs with precision orbit corrections applied), and Experiment
Gridded Data Records, or EGDRs (gridded products derived from
PEDRs). This volume contains AEDRs and PEDRs, but no EGDRs. Data
collection on each revolution has been limited to at most 25 minutes
around periapse. Because coverage is sparse and limited to the
northern hemisphere, a gridded product will not be released until the
first month of mapping data are released. A polar topographic gridded
dataset will then be made available.
MOLA AEDRs and PEDRs are binary data files. To make these data
more easily accessible, an ASCII (i.e. plain text) table has been
generated for each PEDR file. The ASCII tables contain the data fields
from the PEDRs most likely to be of interest; some of the engineering
data fields have been omitted from the ASCII tables. For users who
prefer to generate their own ASCII tables, two software methods are
provided: a Fortran program that generates ASCII tables containing
user-specified fields, and a script for use with the commercial
software IDL that generates both ASCII tables and topography plots.
Also on this volume is a set of images of topography profiles
derived from the PEDRs, to be used as a means of browsing the PEDR
data set. To view these profile images in a web browser, open the file
BROWSE.HTM in the BROWSE directory.
2. Notes and Caveats for Volumes MGSL_0001 and MGSL_0002
Our current understanding of the Martian environment, the
capabilities of MGS, and its suite of instruments is changing rapidly.
MOLA has met or exceeded its design expectations. It has demonstrated
a measurement precision of 30 centimeters over flat terrain. While
designed for nadir-looking operation in a circular, 365- to
445-km-high orbit, MOLA has ranged successfully to Mars at distances
from 170 to 786 km, and to surface slopes up to 60 degrees. MOLA has
ranged to the surfaces of clouds lying at elevations of a few hundred
meters above the surface, to over 15 km high, and returned
measurements of atmospheric opacity greater than 2 during dust
storms. MOLA returned 628 ranges to the moon Phobos in an orbital
fast-flyby. The planetary range detection rate in clear atmosphere has
exceeded 99% over smooth and rough terrain.
The MOLA ranges and precision orbit data are preliminary, and
will be revised as our knowledge of the spacecraft and the Martian
gravity field improves. Important details of the instrument design and
the progress of the mission are found in the files INST.CAT and
MISSION.CAT. The orbital, atmospheric and thermal environment of the
Orbit Insertion phase has introduced uncertainties in the data
quality. The eccentric orbits and frequent off-nadir pointing during
ranging cause a greater sensitivity to errors in spacecraft timing and
attitude knowledge than expected in mapping orbit.
Orbital location is derived from radio observations and a host of
dynamic variables, most important of which is the gravitational
attraction of Mars. Improvements in the gravity field are best obtained
from tracking at low elevations, now being obtained from MGS. The
gravity model used to calculate the orbits is an interim solution,
internally designated mgm0827e, derived from Goddard Mars Model 1. This
model is given in the software directory as GMM1.2 for the purpose of
defining an equipotential topographic reference surface. GMM1.2 is
necessarily constrained and lacks detailed resolution of the polar
regions, so that unmodeled orbital perturbations accumulate. At the
same time, the areoid reference surface may vary by tens of meters
depending on the choice of gravity model. The altimetric error budget is
currently dominated by orbital uncertainty, and does not yet meet our
goal of 30 m accuracy.
The spacecraft radial distance from Mars may change up to 1.6
meters in a millisecond due to orbital eccentricity, and up to 8 meters
between the time the pulse is fired and it is received. Altimetric
processing therefore depends strongly on timing accuracy and knowledge
of the direction in which the laser is fired. MOLA data are time-tagged
once per packet with a spacecraft time code, calibrated to ground time.
An instrument clock synchronized to the Payload Data System provides
1/256 second resolution timing. The PEDRs contain interpolated laser
transmit time to a precision of a tenth of a millisecond.
Altimetric crossovers are being used to assess the accuracy of
the data. It has been determined that the observations have a
systematic timing bias, further, that the attitude knowledge of the
spacecraft is offset. The range observations have been registered
with orbital position by assuming that the time tag of the MOLA range,
as derived from the spacecraft clock, is 113 milliseconds earlier than
the actual transmit time. In addition it is assumed that the time tag
of the attitude kernel provided by the MGS Project is one second later
than the time of the spacecraft attitude sensor readings, due to a
software filter delay. The precise causes and amounts of offset are
under investigation.
Range measurements are affected by the counting frequency
standard, electronic delays, and spreading of the returned pulse due
to ground slope and detector characteristics. The MOLA timing interval
unit has a an accuracy of ~2.5 nanoseconds, its precision being
extended from the 10 ns clock rate by two interpolator bits. However,
"range walk" due to variable threshold settings, pulse amplitude and
shape, can be many times greater than measurement precision,
especially over rough terrain. The MOLA instrument records the pulse
width and amplitude during the time that the signal exceeds a
software-controlled threshold. Shot ranges are corrected in processing
via a mathematical receiver model assuming linear instrument
behavior. Flat and highly reflective terrain, short ranges, and
abnormal atmospheric conditions can drive the electronics into
saturation, increasing detected pulse width and invalidating the
instrument model. The range corrections for saturated returns are
limited to their equivalents for terrain with a slope of one in
sixteen. Meter-level changes in topography must be interpreted in the
context of the range correction values in the PEDR files.
The returned-optical-pulse-width and energy measurements must
also be interpreted with caution, in view of the above-mentioned
effects. Moreover, the detectors were not calibrated for the
unusually cold conditions experienced during Orbit Insertion. Energy
values are slightly higher than measured by test equipment under
optimal conditions. The unsaturated return energy and reflectivity
measurements were only designed for 5% accuracy in any case.
Lastly, the presence of highly reflective clouds, and a level of
noise returns consistent with instrument tradeoffs, has necessitated
an empirical classification of shots as to their origin. The first
shot of every 140 is likely to be triggered by an internal test
source, but may be a valid ground return, while 0.5% of the shots
result from detector noise exceeding the triggering threshold. The
probable ground returns have been flagged based on a combination of
measurements and a stochastic model of topographic variability. An
unambiguous classification is often impossible, given clouds that
often follow the surface, and the dramatic variability of Martian
terrain. The classification should be used only as a guide.
3. CD-ROM Format
The CD-ROM has been formatted so that a variety of computer
systems (e.g., PC, Macintosh, and Sun) may access the
data. Specifically, it is formatted according to the ISO-9660 level 1
Interchange Standard. For further information, refer to the ISO-9660
Standard Document: RF# ISO 9660-1988, April 15, 1988.
This CD-ROM does not contain any Extended Attribute Records
(XAR). Thus, VAX/VMS users may have some problems accessing files on
this volume.
4. File Formats
AEDRs and PEDRs are binary files with attached PDS labels. The
formats of these files are described in their respective Software
Interface Specification (SIS) documents in the DOCUMENT directory.
Additional information about PDS labels is found in the Planetary Data
System Standards Reference, version 3.2, July 24, 1995, JPL D-7669.
Topography profiles derived from the PEDRs are in the form of GIF
images in the BROWSE directory. GIF images can be viewed with most
commercial image processing software packages and with Web browsers.
All text and hypertext (HTML) files on this volume are stream
format files, with a carriage return (ASCII 13) and a line feed
character (ASCII 10) at the end of the record. This allows the files
to be read by the MacOS, DOS, and UNIX operating systems. The SIS
documents in the DOCUMENT directory are given as hypertext, which can
be viewed with a Web browser, and as PDF files, which can be viewed
with Adobe Acrobat Reader.
5. Volume Contents
Files on this volume are organized into a set of subdirectories
below the top-level directory. The following table shows the structure
and content of these directories. In the table, directory names are
enclosed in square brackets ([]).
FILE CONTENTS
Top-level directory
|
|- AAREADME.TXT The file you are reading.
|- AAREADME.HTM Hypertext version of AAREADME.TXT.
|- AAREADME.LBL PDS label for both AAREADME files.
|- ERRATA.TXT Comments and errata on this volume.
|- VOLDESC.CAT Description of the contents of this
| volume in object format.
|
|- [AEDR]
| |
| |- [AAnnnXX] AEDR data directories. "nnnXX" is a group
| | of orbits (e.g. 002XX = orbits 200-299).
| |- AAnnnnnF.B AEDR data files, where nnnnn = orbit number.
|
|- [ASCII]
| |
| |- [APnnnXX] ASCII table directories. "nnnXX" is a group
| | of orbits (e.g. 002XX = orbits 200-299).
| |- APnnnnnK.LBL PDS labels for ASCII tables.
| |- APnnnnnK.TAB ASCII tables, where nnnnn = orbit number.
|
|- [BROWSE] Directory of browse files.
| |
| |- [APnnnXX] Browse directories. "nnnXX" is a group
| | | of orbits (e.g. 002XX = orbits 200-299).
| | |- APnnnnnK.GIF GIF images of topography profiles derived
| | from PEDRs, where nnnnn = orbit number.
| |- BROWINFO.TXT Description of files in the BROWSE directory.
| |- BROWSE.HTM Hypertext interface to browse images.
| |- BROWSE.LBL PDS label for BROWSE.HTM.
|
|- [CATALOG] PDS catalog objects.
| |- AEDRDS.CAT Description of AEDR data set.
| |- ASCIIDS.CAT Description of PEDR ASCII data set.
| |- CATINFO.TXT Description of files in the CATALOG
| | directory.
| |- INST.CAT Description of the MOLA instrument.
| |- INSTHOST.CAT Description of the MGS spacecraft.
| |- MISSION.CAT Description of the MGS mission.
| |- PEDRDS.CAT Description of PEDR data set.
| |- PERSON.CAT Description of personnel associated with
| | production of this volume.
| |- REF.CAT Reference objects mentioned in *.CAT files.
|
|- [DOCUMENT] Documentation files.
| |- AEDRSIS.CSS HTML style sheet used by AEDRSIS.HTM.
| |- AEDRSIS.HTM Hypertext version of MOLA AEDR Software
| | Interface Specification (SIS).
| |- AEDRSIS.LBL PDS label for AEDRSIS.HTM and AEDRSIS.PDF.
| |- AEDRSIS.PDF PDF version of MOLA AEDR SIS.
| |- ARCHSIS.HTM MOLA Archive Volume SIS as hypertext.
| |- ARCHSIS.LBL PDS label for ARCHSIS.HTM and ARCHSIS.PDF.
| |- ARCHSIS.PDF PDF version of MOLA Archive Volume SIS.
| |- DOCINFO.TXT Description of files in DOCUMENT directory.
| |- FIGURE1.GIF Figure 1 used in ARCHSIS.HTM.
| |- MOLALOGO.GIF MOLA logo image used in *.HTM files.
| |- PEDRSIS.CSS HTML style sheet used by PEDRSIS.HTM.
| |- PEDRSIS.HTM Hypertext version of MOLA PEDR Software
| | Interface Specification (SIS).
| |- PEDRSIS.LBL PDS label for PEDRSIS.HTM and PEDRSIS.PDF.
| |- PEDRSIS.PDF PDF version of MOLA PEDR SIS.
|
|- [INDEX] Index files.
| |- INDXINFO.TXT Description of files in the INDEX directory.
| |- AEDCMIDX.LBL PDS label for AEDCMIDX.TAB.
| |- AEDCMIDX.TAB Cumulative index table of AEDR products on
| | this and preceding MOLA archive volumes.
| |- AEDINDEX.LBL PDS label for AEDINDEX.TAB.
| |- AEDINDEX.TAB Index table of AEDR products on this volume.
| |- ASCCMIDX.LBL PDS label for ASCCMIDX.TAB.
| |- ASCCMIDX.TAB Cumulative index table of PEDR ASCII products
| | on this and preceding MOLA archive volumes.
| |- ASCINDEX.LBL PDS label for ASCINDEX.TAB.
| |- ASCINDEX.TAB Index table of PEDR ASCII products on this volume.
| |- PEDCMIDX.LBL PDS label for PEDCMIDX.TAB.
| |- PEDCMIDX.TAB Cumulative index table of PEDR products on
| | this and preceding MOLA archive volumes.
| |- PEDINDEX.LBL PDS label for PEDINDEX.TAB.
| |- PEDINDEX.TAB Index table of PEDR products on this volume.
|
|- [LABEL] Format files referenced by data product
| | labels.
| |- LABINFO.TXT Description of files in the LABEL directory.
| |- MOLAMNT.FMT Describes the format of an AEDR Maintenance
| | Mode table.
| |- MOLASCCT.FMT Describes the format of the COUNTS
| | container in an AEDR frame.
| |- MOLASCFR.FMT Describes the format of an AEDR frame.
| |- MOLASCI.FMT Describes the format of an AEDR Science
| | Mode table.
| |- PEDRENGn.FMT Describe the format of the second section
| | of a PEDR, where n=1 through 7. Seven
| | different record formats are possible,
| | depending on which of seven frames is
| | recorded.
| |- PEDRSEC1.FMT Describes the format of the first section
| | of a PEDR.
| |- PEDRSEC3.FMT Describes the format of the third section
| of a PEDR.
|
|- [PEDR]
| |
| |- [APnnnXX] PEDR data directories. "nnnXX" is a group
| | of orbits (e.g. 002XX = orbits 200-299).
| |- APnnnnnK.B PEDR data files, where nnnnn = orbit number.
|
|- [SOFTWARE] Software for accessing PEDR files.
|
|- SOFTINFO.TXT Description of files in SOFTWARE directory.
|
|- [MPROF] Directory containing MPROF software to
| | generate PEDR topography profiles.
| |
| |- [PC] Directory containing PC/Windows version.
| | |
| | |- MPRFINFO.TXT Instructions for using MPROF.PRO.
| | |- MPROF.PRO PC/Windows version of IDL script.
| |
| |- [SUN] Directory containing Sun/UNIX version.
| |
| |- MPRFINFO.TXT Instructions for using MPROF.PRO.
| |- MPROF.PRO Sun/UNIX version of IDL script.
|
|- [PEDR2TAB] Directory containing PEDR2TAB software
| to generate PEDR ASCII tables.
|
|- [MAC] Directory containing Macintosh version.
| |
| |- MOLA.LBL Example PDS label for PEDR2TAB output.
| |- P2TAB68K.HQX BINHEX file containing executable
| | program compiled for Macintosh 68K.
| |- P2TABFPU.HQX BINHEX file containing executable
| | program compiled for Macintosh 68K FPU.
| |- P2TABPPC.HQX BINHEX file containing executable
| | program compiled for PowerPC.
| |- P2TINFO.TXT Instructions for using PEDR2TAB.
| |- PEDR2TAB.F Fortran 77 source code for Macintosh.
| |- PEDR2TAB.PRM Preferences file to be edited by user.
|
|- [PC] Directory containing PC/Windows version.
| |
| |- MOLA.LBL Example PDS label for PEDR2TAB output.
| |- P2TINFO.TXT Instructions for using PEDR2TAB.
| |- PEDR2TAB.EXE Executable file compiled for MS-DOS.
| |- PEDR2TAB.F Fortran 77 source code for PC/Windows.
| |- PEDR2TAB.PRM Preferences file to be edited by user.
|
|- [SUN] Directory containing Sun/UNIX version.
|
|- MOLA.LBL Example PDS label for PEDR2TAB output.
|- P2TINFO.TXT Instructions for using PEDR2TAB.
|- PEDR2TAB.EXE Executable file compiled for Solaris 2.4.
|- PEDR2TAB.F Fortran 77 source code for Sun/UNIX.
|- PEDR2TAB.PRM Preferences file to be edited by user.
6. Whom to Contact for Information
PDS Geosciences Node
Washington University
Dept. of Earth and Planetary Sciences
1 Brookings Drive
St. Louis, MO 63130
314-935-5493
WWW Site: http://wwwpds.wustl.edu
Electronic mail address: geosci@wunder.wustl.edu
MOLA Science Team
Code 920
NASA / Goddard Space Flight Center
Greenbelt, MD 20771
WWW Site: http://ltpwww.gsfc.nasa.gov/tharsis/mola.html
Electronic mail address: Gregory.Neumann@GSFC.NASA.GOV
7. Cognizant Persons
MOLA data were provided by David Smith, MOLA Principal
Investigator, with assistance from Gregory Neumann, both of
NASA/Goddard Space Flight Center, and Peter Ford, MIT.
This volume was designed and produced by Susan Slavney, Planetary
Data System Geosciences Node, Washington University, St. Louis,
Missouri.