PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "S. SLAVNEY, 1998-09-10; S. SLAVNEY, 1999-02-24; S. SLAVNEY, 1999-10-13; S. SLAVNEY, 2007-10-15; A. WHITE, 2020-09-18" RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "MGS-M-MOLA-1-AEDR-L0-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MOLA AGGREGATED EXPERIMENT DATA RECORD" DATA_SET_TERSE_DESC = "The Aggregated Experiment Data Record (AEDR) archive contains raw altimetry profile data acquired by the Mars Orbiter Laser Altimeter (MOLA) during the Mars Global Surveyor (MGS) mission." DATA_SET_COLLECTION_MEMBER_FLG = "N" START_TIME = 1997-212T19:10:00.000 STOP_TIME = 2003-138T23:59:59.999 DATA_OBJECT_TYPE = FILE DATA_SET_RELEASE_DATE = 1998-10-01 ARCHIVE_STATUS = "ARCHIVED" PRODUCER_FULL_NAME = {"DAVID E. SMITH", "MARIA T. ZUBER", "GREGORY A. NEUMANN", "PEGGY JESTER"} DETAILED_CATALOG_FLAG = "N" ABSTRACT_DESC = "The MOLA AEDR data set contains the raw altimetry measurements acquired by the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft. The AEDR data are used as input to the MOLA Precision Experiment Data Record (PEDR) data set, in which the altimetry data have been corrected according to precise measurements of the orbit. The PEDR data set is preferred for science analysis." CITATION_DESC = "Smith, D., G. Neumann, P. Ford, E. A. Guinness, and S. Slavney, Mars Global Surveyor Laser Altimeter Aggregated Experiment Data Record, NASA Planetary Data System, MGS-M-MOLA-1-AEDR-L0-V1.0, 1999." DATA_SET_DESC = " 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 Aggregated Experiment Data Record (AEDR) is an aggregation of MOLA telemetry packets (raw data). AEDR products generated during the Orbit Insertion phase of the mission are aggregated by orbit. AEDR products generated during the Mapping Phase are aggregated by sol (Martian day). The raw AEDR products are truncated and used to create the PEDRs. The process to create the AEDR data product is performed as part of MOLA mission operations. The telemetry packets are aggregated on a Mars Global Surveyor mapping orbit basis. There are approximately 12 orbits per day with each orbit taking 117 minutes 39 seconds to complete. The mapping mission will last for at least one Martian year, which is 687 Earth days. The AEDR data products will be produced continuously for the life of the mission. Each product will contain approximately 7000 seconds of data. A complete listing of all parameters contained in an AEDR can be found in Tables 1 and 2 of the AEDR Software Interface Specification (SIS) document [MOLAAEDRSIS1998]. Additionally, the AEDR format and contents are described in the AEDR Data Dictionary in Appendices A and B of the SIS. Data ==== The AEDR files are in binary format with attached PDS labels. The SIS document describing this standard product is included on this volume. The AEDRs contain essentially all the data contained in the PEDRs. Everything in the AEDR is archived in the PEDR after scaling. The AEDRs usually contain engineering data during instrument warm-up. Parameters ========== The MOLA instrument measures the round-trip time of flight of infrared laser pulses transmitted from the MGS spacecraft to the Martian surface. The instrument normally operates in a single autonomous mode, in which it produces ranging measurements. Surface topography estimates can be derived from these data, given appropriate corrections for the position and attitude of the spacecraft. Processing ========== The AEDR raw telemetry data are truncated to create PEDRs. Ancillary Data ============== N/A Coordinate System ================= N/A Software ======= N/A Media/Format ============ The MGS MOLA PEDR dataset will be 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 will be based on standards established by the Planetary Data System (PDS). " CONFIDENCE_LEVEL_NOTE = " Overview ======== The resolution of the data is about 40 cm vertically, and about 330 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. Review ====== MOLA AEDR and PEDR archive volumes are reviewed by MGS mission scientists and by PDS. Data Coverage/Quality ===================== On May 26, 1998, the Mars Global Surveyor (MGS) spacecraft entered into Phase 2 of the Science Phasing Orbit (SPO-2). SPO is a near-polar (92.869 degrees) inclination orbit with a period of 11.6 hours and a periapsis altitude of about 170 km. During SPO-2 MOLA will collect observations of Mars' northern hemisphere, with emphasis on detailed mapping of the north polar ice cap. Late June and early July 1998 is expected to be the period of maximum ice loading for the northern cap for the current Martian year and thus represents an especially exciting and crucial time for MOLA observations. We anticipate that the observations collected during this period will contribute significantly towards understanding the present-day Martian volatile budget. We have just completed a two-week period where the MGS spacecraft was tilted on alternating orbits so that MOLA could fill in the 2 degree coverage gap at the north pole that occurred because the spacecraft orbital inclination is not exactly 90 degrees. MOLA collected 61 topographic profiles of Mars' northern hemisphere during the first phase of the MGS Science Phasing Orbit (SPO-1) that spanned the period from March 26, 1998 until April 28, 1998. All of the MOLA data collected during SPO-1 were presented in thirteen talks and posters during the week of May 26, 1998 at the Spring Meeting of the American Geophysical Union in Boston. MOLA's SPO-1 observations were collected during orbital passes in which targeted imaging of surface features was not being attempted. Collection of images of target sites (Viking 1 & 2 and Pathfinder landing sites and Cydonia) resulted in a loss of about 25% of the data that MOLA could have been collected during that period. SPO-1 ended in mid-May, just before solar conjunction. During conjunction the sun is in the line of sight of the spacecraft, which interferes with communication, so commanding of the spacecraft is minimized. The Science Phasing Orbit represents a hiatus from aerobraking that is needed so that the spacecraft will achieve the desired local time for the mapping orbit that will be entered next spring. SPO will last until September 11, 1998, after which time MGS will resume aerobraking to circularize its current elliptical orbit. During aerobraking passes, the MOLA instrument does not collect data because the instrument is not pointed at the surface during the period of time when the spacecraft is within ranging distance. Previous MOLA data was collected during the capture orbit phase of the MGS mission shortly after orbit insertion on September 15, 1997. A further 17 passes were collected between October 14 and November 6, 1997, during a hiatus in the aerobraking phase necessitated by a study of the integrity of a solar panel that was slightly damaged after launch. Limitations =========== 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 [ABSHIREETAL2000], 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." END_OBJECT = DATA_SET_INFORMATION OBJECT = DATA_SET_TARGET TARGET_NAME = MARS END_OBJECT = DATA_SET_TARGET OBJECT = DATA_SET_HOST INSTRUMENT_HOST_ID = MGS INSTRUMENT_ID = MOLA END_OBJECT = DATA_SET_HOST OBJECT = DATA_SET_MISSION MISSION_NAME = "MARS GLOBAL SURVEYOR" END_OBJECT = DATA_SET_MISSION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MOLAAEDRSIS1998" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END