PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "S. SLAVNEY, 1999-11-29; S. SLAVNEY, 2000-03-20" RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "MGS-M-MOLA-5-IEGDR-L3-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MOLA INITIAL EXPERIMENT GRIDDED DATA RECORD" DATA_SET_COLLECTION_MEMBER_FLG = "N" START_TIME = 1997-09-15T19:10:00.000 STOP_TIME = UNK DATA_OBJECT_TYPE = FILE DATA_SET_RELEASE_DATE = 1999-10-01 PRODUCER_FULL_NAME = {"DAVID E. SMITH", "MARIA T. ZUBER", "GREGORY A. NEUMANN"} DETAILED_CATALOG_FLAG = "N" DATA_SET_DESC = " 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 Experiment Gridded Data Record (EGDR) is a topographic map of Mars based on altimetry data acquired by the MOLA instrument and accumulated over the course of the mission so far. Two types of EGDR products are to be produced: the Initial Experiment Gridded Data Record (IEGDR), consisting of data accumulated through at least the first 30 days of the mapping mission, and the Mission Experiment Gridded Data Record (MEGDR), consisting of data accumulated over the whole primary mission (one Mars year). Different resolutions of the IEGDR and MEGDR may be released, and multiple versions of each product may be released. The MOLA Precision Experiment Data Records (PEDRs) are the source for the EGDRs. See the MOLA PEDR Software Interface Specification [MOLAPEDRSIS1998] and the PDS Catalog entry for the PEDR data set (MGS-M-MOLA-3-PEDR-L1A-V1.0) for a description of the PEDRs. Users of MOLA data must be aware of two important differences between the MOLA coordinate system and the Viking-era coordinates. These differences are significant when comparing MOLA groundtracks to MDIMs, USGS DTMs, or maps. MOLA uses the areocentric coordinate frame (see below). MOLA areocentric latitudes should be converted to areographic latitudes using the equation provided below. (Note that Viking data was processed assuming different radii: equatorial radius = 3393.40 km and polar radius = 3375.73 km for a flattening of 1/192.) There appears to be a residual discrepancy in latitude of less than 0.1 degree (6 km) magnitude, and variable sign, between MGS and Viking coordinates. MOLA longitudes are also areocentric, with positive degrees East. However, there is an additional eastward offset of the Viking-era coordinate system relative to the present MGS inertial frame. The magnitude of this offset ranges from about 0.1 to 0.3 degrees (<20 km). More than one factor may contribute to this discrepancy; the primary reason is a change in the IAU coordinate system. Other possible effects are a drift of the prime meridian due to uncertainties in the martian rotation period or errors in the Viking spacecraft orbital position that propagated through the image processing [SMITHETAL1998]. Subtracting the Viking longitude West from 360.0 converts to longitude East. Subtracting 0.2 degrees from the Viking East longitude is a first-order correction for comparison to the MOLA data. Data ==== The MOLA IEGDR is in the form of an ASCII table with one row for each latitude-longitude bin, from 90 to -90 degrees latitude and from 0 to 360 degrees longitude. The binned data include all MOLA nadir observations from the Orbit Insertion phase, plus Mapping Phase nadir observations, plus off-nadir observations of the north pole above 86 degrees latitude acquired during spring 1998 and of both poles taken in July 1999. Orbits 355 and 358 of the Orbit Insertion Phase and orbits 10709 through 10716, inclusive, of the Mapping Phase are excluded because solutions for these orbits are deemed to be poor. (Note: subtract 10000 from MOLA mapping phase orbit number to determine the equivalent MGS Project orbit number.) Also excluded are 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. Most observations have been crossover-corrected. The polar observations have not been fully corrected and may be revised somewhat, as the pointing of the instrument is not known as accurately as the range measurement. The IEGDR may be released at various grid resolutions. Examples are 1 degree latitude by 1 degree longitude bins, 0.5 by 0.5 degree bins, and 0.25 by 0.25 degree bins. The IEGDR file name is in the form IEGnnn_v.TAB, where nnn represents the bin size and v the version. For example, the first release of the 0.5 by 0.5 degree IEGDR is named IEG050_A.TAB. Parameters ========== N/A Processing ========== The PEDRs incorporate the best multi-arc orbital solutions derived from the Goddard Mars potential model GMM1.6, and the available tracking. The latest spacecraft SCLK timing corrections have been applied. The ranges account for instrument delays and the leading edge timing biases, estimated by the receiver model of [ABSHIRE&SUN1998]. This model assumes a Gaussian shape for the transmitted and surface-scattered pulse waveforms, using the detector threshold settings and the observed pulse width and energy measurements between the threshold crossings to infer the true pulse centroid, width, and amplitude. The eccentric orbit brought MOLA much closer to the surface of Mars than the design called for, thus the pulse width and energy measurements were saturated for much of each pass. Caution must be exercised when interpreting these measurements. Laser energies are calculated according to the transmitter model of [AFZALETAL1997]. A post-launch calibration to the MOLA oscillator frequency has been applied, based on the difference between the spacecraft high-resolution timer and the MOLA clock, resulting in an estimated frequency of f=99,996,232 +/- 5 Hz. This frequency is given in the PEDR and may change due to clock drift. The interval between shots, as well as the shot time-of-flight, is controlled by this frequency. The shot interval in seconds, delta_t = 10,000,000 / f. Time tags are given in ET seconds of MOLA fire time. Timing of the shots is interpolated to ~100 microseconds. This step is essential in the highly elliptical orbit insertion geometry because the spacecraft may change its radial distance by as much as 1600 meters per second. The spacecraft time, from which the shot time is derived, is subject to further timing corrections. The range observations have been registered with 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. The ground location and planetary radius is calculated in inertial (J2000) coordinates as the difference between the spacecraft position vector and the MOLA one-way range vector. The direction of the MOLA vector is obtained from project-supplied spacecraft attitude kernels and the boresight calibration of the instrument with respect to the spacecraft. The one-way range of the laser shot to the planet is obtained from the two-way range by correcting for the change in spacecraft position during laser shot time-of-flight. The ground point position vector is transformed into planetary body-fixed coordinates at a time midway between the MOLA laser fire time and the shot receive time, using the IAU 1991 planetary model. Due to the inverse-square-law energy return in the link equation [ZUBERETAL1992], the instrument detector was saturated during a part of the periapsis approach. Received pulse energy and pulse width are resolved during the portion of the pass when the detector is not saturated. The absolute accuracy of these quantities is about 5%. There is a table entry for each non-zero shot range detection for all in-range packets in the data stream. Occasional corrupted range values occur due to transmission errors, and some packets are lost entirely. A packet sequence number is generated by MOLA. The sequence number was initialized to 0 just before the planet came within range during the SPO-1 and 2 data passes via a restart command, while during the Hiatus subphase the restart occurred earlier. Some MOLA ranges are either clouds or false detections due to the intrinsic noise characteristics of the receiver. The MOLA ranges that are true ground hits are flagged with a positive number in the tables. Ancillary Data ============== N/A Coordinate System ================= The diverse processing and display requirements for various observational quantities necessitates flexibility in the choice of coordinate system. Two systems are used to describe data products on this volume: 1. The areocentric coordinate system [DAVIESETAL1994B], more generally described as planetocentric, is body-centered, using the center-of-mass as the origin. Areocentric latitude is defined by the angle between the equatorial plane and a vector extending from the origin of the coordinate system to the relevant point on the surface. Latitude is measured from -90 degrees at the south pole to +90 degrees at the north pole. Longitude extends from 0 to 360 degrees, with values increasing eastward (i.e., it is a right-handed coordinate system) from the prime meridian [DAVIESETAL1994B]. This coordinate system is preferred for use in geophysical studies in which, for example, estimates of elevation or gravitational potential are generated mathematically. 2. The areographic system (more generally, the planetographic system) uses the same center-of-mass origin and coordinate axes as the areocentric coordinate system. Areographic latitudes are defined by a vector normal to a reference ellipsoid surface. Longitudes are measured from the prime meridian and increase toward the west since Mars is a prograde rotator [DAVIESETAL1994B]. This system is standard for cartography of Mars and most existing maps portray locations of surface features in areographic coordinates. For MGS, the following data have been adopted as standard for defining the reference spheroid for computing the areographic latitudes [DAVIESETAL1994B]: Equatorial radius = 3397 km Polar radius = 3375 km Flattening = 0.0064763 Note that the flattening is computed as one minus the ratio of the polar radius to the equatorial radius. The relationship between areographic and areocentric latitudes is approximated as: tan(lc) = (1-f) * (1-f) * tan(lg) where: f = flattening lg = areographic latitude lc = areocentric latitude While the official MGS Project coordinate system is the IAU 1994 convention specified in DAVIESETAL1994B, the MOLA data are located in the IAU 1991 system, which differs only in the prime meridian W0 at J2000. To convert MOLA east longitude from IAU 1991 to IAU 1994, one must subtract 0.033 degrees; i.e., areocentric_longitude_East_94 = areocentric_longitude_East_91 - 0.033. Software ======= N/A Media/Format ============ The MGS MOLA EGDR 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). " 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 ====== The volume containing the MOLA EGDR dataset was 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. The Science Phasing Orbit was a hiatus from aerobraking that was needed so that the spacecraft could achieve the desired local time for the mapping orbit. 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. SPO-1 ended April 28, 1998, 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 second phase of the Science Phasing Orbit (SPO-2) began May 29, 1998, and continued through September 23, 1998, at which time aerobraking was resumed to bring the spacecraft into a near-circular mapping orbit. During the aerobraking passes the MOLA instrument did not collect data because the instrument was not pointed at the surface during the period of time when the spacecraft is within ranging distance. MOLA's Science Phasing Orbit 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. Aerobraking ended in early February, 1999, and was followed by a transition period extending up to the beginning of the Mapping Phase on March 9, 1999. MOLA collected data in mapping mode beginning February 28, 1999. From this date through May 31, 1999, MOLA completed 750 orbital passes in which data were collected. 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 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." 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_REFERENCE_INFORMATION REFERENCE_KEY_ID = "ABSHIRE&SUN1998" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "AFZALETAL1997" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "DAVIESETAL1994B" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "MOLAPEDRSIS1998" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "SMITHETAL1998" END_OBJECT = DATA_SET_REFERENCE_INFORMATION OBJECT = DATA_SET_REFERENCE_INFORMATION REFERENCE_KEY_ID = "ZUBERETAL1992" END_OBJECT = DATA_SET_REFERENCE_INFORMATION END_OBJECT = DATA_SET END