This document provides an overview of the Mars Global Surveyor MOLA PEDR and EGDR Archives, including a complete listing of the contents of this archive volume, below.
This volume contains science data products from observations acquired by the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) instrument during the Mapping Phase 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 PEDRs and EGDRs.
In addition, this volume contains a spherical harmonic model (SHADR) of Mars topography, a numerical transform of the MOLA 0.5-degree EGDR.
The PEDR, EGDR and SHADR data sets are described in the files PEDRDS.CAT, IEGDRDS.CAT and SHADRDS.CAT respectively, in the CATALOG directory. Also in this directory are files describing the MOLA instrument (INST.CAT), the MGS spacecraft (INSTHOST.CAT) and mission (MISSION.CAT), personnel associated with this archive volume (PERSON.CAT), and a bibliography of references mentioned in these files (REF.CAT). The files in this directory will become entries in the Planetary Data System online catalog.
Additional documentation is found in the DOCUMENT directory, including Software Interface Specification (SIS) documents for the data sets and for the MOLA archive volumes. A description of calibration procedures applied to MOLA data is also given.
MOLA data products are identified by orbit number in the file name and in the product ID found in the label. MOLA products from the Orbit Insertion Phase of the mission contain one orbit per file. Mapping Phase products are aggregated by day, so that one file contains a variable number of orbits. For Mapping Phase products, the orbit number in the file name and the product ID refers to the first orbit covered by that product. (Note: subtract 10000 from a MOLA mapping phase orbit number to determine the equivalent MGS Project orbit number.)
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 derived from Goddard Mars Model 1. This model is given in the software directory as GMM1.6 for the purpose of defining an equipotential topographic reference surface. GMM1.6 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 current areoid is defined by GMM1.6 evaluated to degree and order 50.
During orbit insertion, the spacecraft radial distance from Mars was subject to a change of up to 1.6 meters in a millisecond due to orbital eccentricity, and up to 8 meters between the time the pulse was fired and the time it was received. Altimetric processing therefore depends strongly on timing accuracy and knowledge of the direction in which the laser was 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 provide 1/256 second resolution timing. The PEDRs contain interpolated laser transmit time to a precision of a tenth of a millisecond.
Altimetric crossovers are 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 the 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. In addition it is assumed that the time tag of the attitude kernel provided by the MGS Project is 1.15 seconds later than the time of the spacecraft attitude sensor readings, due to a software filter delay.
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. 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 approximately 3% 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.
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.
PEDRs are binary files with attached PDS labels. The format of these files is described in the PEDR Software Interface Specification (SIS) document 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.
The EGDR products on this volume are version 2 of this dataset. They are images, whereas version 1 products were ASCII tables. The image files have detached PDS labels. More information is found in the PDS labels and in the Software Interface Specification document in the DOCUMENT directory. The images can be displayed using NASAview image display software, available from the PDS at no charge. NASAview can be downloaded from the PDS Web site at http://pds.jpl.nasa.gov.
The SHADR product is an ASCII tabular file with a detached PDS label that describes the columns in the table. More information is found in the PDS label and in the Software Interface Specification document in the DOCUMENT directory.
All text 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 (HTML), which can be viewed with a Web browser, and in the proprietary Adobe Portable Document Format (PDF), which can be viewed with the free software Acrobat Reader from Adobe Systems Incorporated.
Files on this volume are organized into a series 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 Plain text version of AAREADME.HTM.
|- AAREADME.HTM The file you are reading.
|- AAREADME.LBL PDS label for both AAREADME files.
|- ERRATA.TXT Comments and errata on this volume.
|- VOLDESC.CAT Description of the contents of this CD-ROM
| volume in object format.
|
|- [CATALOG] PDS catalog objects.
| |- CATINFO.TXT Description of files in the CATALOG directory.
| |- DSMAP.CAT Description of map projection referenced in
IEGDR image labels.
| |- IEGDRDS.CAT Description of IEGDR data set.
| |- 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.
| |- SHADRDS.CAT Description of SHADR data set.
| |- TARGET.CAT Description of the observation target (Mars).
|
|- [DATA]
| |- DATAINFO.TXT Description of files in the DATA directory.
| |
| |- [EGDR] EGDR data directory.
| | |- IEG0125A.IMG 1/8 degree resolution EGDR areoid image.
| | |- IEG0125A.LBL PDS label for IEG0125A.IMG.
| | |- IEG0125C.IMG 1/8 degree resolution EGDR counts image.
| | |- IEG0125C.LBL PDS label for IEG0125C.IMG.
| | |- IEG0125R.IMG 1/8 degree resolution EGDR radius image.
| | |- IEG0125R.LBL PDS label for IEG0125R.IMG.
| | |- IEG0125T.IMG 1/8 degree resolution EGDR topography image.
| | |- IEG0125T.LBL PDS label for IEG0125T.IMG.
| | |- IEG025A.IMG 1/4 degree resolution EGDR areoid image.
| | |- IEG025A.LBL PDS label for IEG025A.IMG.
| | |- IEG025C.IMG 1/4 degree resolution EGDR counts image.
| | |- IEG025C.LBL PDS label for IEG025C.IMG.
| | |- IEG025R.IMG 1/4 degree resolution EGDR radius image.
| | |- IEG025R.LBL PDS label for IEG025R.IMG.
| | |- IEG025T.IMG 1/4 degree resolution EGDR topography image.
| | |- IEG025T.LBL PDS label for IEG025T.IMG.
| |
| |- [PEDR] PEDR data directory.
| | |- APnnnnnK.B PEDR data files, where nnnnn = orbit number.
| | The MOLA orbit number is equal to the MGS
| | Project orbit number plus 10000, for
| | Mapping Phase products.
| |
| |- [SHADR] SHADR data directory.
| |- GTM070AA.LBL PDS label for GTM070AA.LBL.
| |- GTM070AA.SHA Spherical harmonics topography model.
|
|- [DOCUMENT] Documentation files.
| |- ARCHSIS.HTM MOLA Archive Volume SIS.
| |- ARCHSIS.LBL PDS label for ARCHSIS.HTM and ARCHSIS.PDF.
| |- ARCHSIS.PDF MOLA Archive Volume SIS in Adobe Portable
| | Document Format.
| |- CALFIGS.LBL PDS label for CALFIGS.PDF.
| |- CALFIGS.PDF Figures to accompany CALIB.PDF.
| |- CALIB.LBL PDS label for CALIB.PDF.
| |- CALIB.PDF MOLA calibration document.
| |- DOCINFO.TXT Description of files in DOCUMENT directory.
| |- EGDRSIS.HTM MOLA EGDR SIS.
| |- EGDRSIS.LBL PDS label for EGDRSIS.HTM and EGDRSIS.PDF.
| |- EGDRSIS.PDF MOLA EGDR SIS in Adobe Portable Document
| | Format.
| |- FIGURE1.GIF Figure 1 used in ARCHSIS.HTM.
| |- PEDRSIS.HTM MOLA PEDR SIS.
| |- PEDRSIS.LBL PDS label for PEDRSIS.HTM and PEDRSIS.PDF.
| |- PEDRSIS.PDF MOLA PEDR SIS in Adobe Portable Document
| Format.
| |- SHADRSIS.TXT MOLA SHADR SIS.
|
|- [INDEX] Index files.
| |- EGCMIDX.LBL PDS label for EGCMIDX.TAB.
| |- EGCMIDX.TAB Cumulative index table of EGDR products on
| | this and preceding MOLA archive volumes.
| |- EGINDEX.LBL PDS label for EGINDEX.TAB.
| |- EGINDEX.TAB Index table of EGDR 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.
| |- SHACMIDX.LBL PDS label for SHACMIDX.TAB.
| |- SHACMIDX.TAB Cumulative index table of SHADR products on
| | this and preceding MOLA archive volumes.
| |- SHAINDEX.LBL PDS label for SHAINDEX.TAB.
| |- SHAINDEX.TAB Index table of SHADR products on this volume.
| |- INDXINFO.TXT Description of files in the INDEX directory.
|
|- [LABEL] Format files referenced by data product labels.
| |- LABINFO.TXT Description of files in the LABEL directory.
| |- 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.
|
|- [SOFTWARE] Software for accessing PEDR files.
|- SOFTINFO.TXT Description of files in SOFTWARE directory.
|
|- [MPROF] Directory containing MPROF software to
| | generate PEDR topography profiles.
| |
| |- MPRFINFO.TXT Instructions for using MPROF.PRO.
| |- MPROF.PRO PC/Windows 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.
| | |- 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.
|
|- [SPLITORB] Directory containing SPLITORB software
| to separate a PEDR into single orbits.
|
|- SPLTINFO.TXT Instructions for using SPLITORB.PRO.
|- SPLITORB.PRO Script for use with UNIX or PC versions
of IDL.
For questions concerning this data set collection:
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.eduMOLA 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
MOLA data were provided by David Smith, MOLA Principal Investigator, with assistance from Greg 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.
The MOLA PEDR data set published on this volume may be cited in published literature using the following reference.
Smith, D., G. Neumann, P. Ford, R. E. Arvidson, E. A. Guinness, and S. Slavney, "Mars Global Surveyor Laser Altimeter Precision Experiment Data Record", NASA Planetary Data System, MGS-M-MOLA-3-PEDR-L1A-V1.0, 1999.
The MOLA IEGDR data set published on this volume may be cited in published literature using the following reference.
Smith, D., G. Neumann, P. Ford, R. E. Arvidson, E. A. Guinness, and S. Slavney, "Mars Global Surveyor Laser Altimeter Initial Experiment Gridded Data Record", NASA Planetary Data System, MGS-M-MOLA-5-IEGDR-L3-V1.0, 1999.
The MOLA SHADR data set published on this volume may be cited in published literature using the following reference.
Smith, D., G. Neumann, and R. Simpson, "Mars Global Surveyor Laser Altimeter Initial Experiment Gridded Data Record", NASA Planetary Data System, MGS-M-MOLA-5-IEGDR-L3-V1.0, 2000.