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 following volume, MGSL_0002, 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.