PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "S. SLAVNEY, 1998-09-10; S. SLAVNEY, 1999-02-24" RECORD_TYPE = STREAM OBJECT = DATA_SET DATA_SET_ID = "MGS-M-MOLA-3-PEDR-L1A-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "MOLA PRECISION EXPERIMENT DATA RECORD" DATA_SET_COLLECTION_MEMBER_FLG = "N" START_TIME = 1997-212T19:10:00.000 STOP_TIME = UNK DATA_OBJECT_TYPE = FILE DATA_SET_RELEASE_DATE = 1998-10-01 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. A Precision Experiment Data Record (PEDR) contains MOLA science mode telemetry data that has been converted to engineering and physical units. Each PEDR contains a 2 second span of data, called a frame, that is retrieved from the 14 second MOLA science mode telemetry packet. Therefore, seven PEDRs are generated from each MOLA science mode telemetry packet. In addition to the frame data, the packet's engineering and housekeeping data are retained and subcommutated among the seven PEDRs that comprise a packet. Additional packet information, e.g., packet header, are stored in the PEDR as well as data correction values which were used to process the telemetry data into the PEDR data. Storing the data correction values ensures that the telemetry data can be re-created from the PEDR data. Contained in a PEDR are the range value computed at the frame mid- point, the planetary radius at the frame mid-point, and the planetary radius for each shot. There are 20 possible shots in a 2 second frame, numbered from 1-20. Additionally, ground and spacecraft location, i.e., latitude, longitude, and radial distance, obtained from the precision orbit data for the frame mid-point, is stored in the PEDR. The change in latitude and longitude per frame is also stored, so that the location of individual shots may be obtained by interpolation via the generic formula: shot_location = mid_pt_location + (shot_no - 10.5)/20 * delta_location. The range and precision orbit data are given with respect to the Mars Global Surveyor center of mass. The planetary radius values are computed with respect to the center of mass of Mars. Ground locations are given in the IAU 1991 coordinate system prevailing at the time of Mars Observer. Additional information describing the instrument and its configuration are included in the PEDR. A complete listing of all parameters contained in a PEDR can be found in Table 1 of the PEDR Software Interface Specification (SIS) document [MOLAPEDRSIS1998]. A description of the parameters contained in a PEDR is found in Table 2. The engineering/housekeeping data are listed in Table 3; this table also describes the location of the engineering/ housekeeping data among the seven PEDRs that constitute a MOLA telemetry packet. Additionally, the PEDR format and contents are described in the PEDR Data Dictionary in Appendix B of the SIS. The topography of Mars relative to the gravitational potential was determined by the MOLA instrument during 18 assessment orbits over the northern hemisphere, i.e., near periapsis. Overall more than 221,000 valid laser ranges were collected. While the instrument was designed to collect data in a mapping orbit of approximately 400 km altitude, MOLA performed well out to its 785 km maximum range. This science volume contains the complete archival delivery of MOLA data for the Assessment Subphase of the MGS Mission, SPO-1 and SPO-2. 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 ==== One of the primary standard products are the Precision Experiment Data Record (PEDR) files. The files are in binary format with an attached PDS label. The SIS document describing this standard product is included on this volume. The PEDRs contain instrument science data, sub-spacecraft location, estimates of the planetary radii, and radii of an areoid equipotential surface. Areoid radii were calculated using the interim gravity model mgm0827e with a mean equatorial radius of 3396 km. The MOLA topography is the shot planetary radius minus the areoid radius at a given location. 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 will produce ranging measurements. Surface topography estimates can be derived from these data, given appropriate corrections for the position and attitude of the spacecraft. Processing ========== The PEDRs incorporate the best multi-arc orbital solutions derived from the Goddard Mars potential model mgm0827e, and the available tracking. The latest (January 20, 1998 or later) 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 TBD. 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 Software ======= Software for accessing the PEDR data products is provided on the archive volumes and on 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. 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 ====== The volume containing the MOLA PEDR topography 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. 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 [ABSHIRE&SUN1998], 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_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