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MOLA PRECISION EXPERIMENT DATA RECORD (PDS)

NSSDCA ID: PSPG-00856

Availability: Archived at NSSDC, accessible from elsewhere

Time span: 1990-09-15 to 1994-10-12

Description

This description was generated automatically using input from the Planetary Data System.

Data Set Overview

The Mars Global Surveyor (MGS) 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. The Aggregated Experiment Data Record (AEDR) is the source for the science data, while ancillary information is provided by the Radio Science investigation. Precision orbit, geometric, and calibration data have been incorporated, as well as an equipotential datum (areoid), so that topographic profiles may be obtained directly from the PEDR. Each PEDR contains a 2 second span of data, called a frame, that is retrieved from the 14 second MOLA science mode telemetry packets. Therefore, seven PEDRs are generated from each MOLA AEDR record. 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. Some of the engineering data, namely the background noise counters, have recently been used as a narrow-band radiometric data type and will be archived separately. 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. A complete listing of all parameters contained in a PEDR can be found in Table 1 of the PEDR Software Interface Specification (SIS) document [MOLAPEDRSIS2003]. 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. Contained in a PEDR are the range value computed at the frame midpoint, the planetary radius at the frame mid-point, and the planetary radius for each shot. Range counts are converted to units of length following [ABSHIREETAL2000]. 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, are 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. In view of the parallax introduced by offnadir observations, there is an added first-order correction for deviations in radius from the frame midpoint radius to the location of each individual shot in a frame. 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. The locations of the MOLA shots are in areocentric coordinates, with the longitude positive Eastward. Each footprint represents the centroid of a pulse returned within a spot approximately 168 m in diameter, although the majority of the energy detected from an individual shot may come from a smaller area 75 m in diameter [NEUMANNETAL2003]. The cartographic reference frame and rotational model describing ground locations used in this release is the IAU2000 model [SEIDELMANNETAL2002] based on Viking, Pathfinder, and MGS data. This frame differs from the IAU1991 coordinate system used in earlier MOLA releases [DAVIESETAL1992B]. The most significant change is the longitude of the prime meridian in the J2000 epoch and reference frame, which has decreased by 0.238 degrees from IAU1991. The effect of this change is to place the center of a landmark, the crater Airy-0, as close as possible to 0 longitude, and reduce the discrepancy between Viking-era maps and the MOLA locations introduced by a mistaken location for the Viking-1 Lander [ZEITLER&OBERST1999]. On Earth, it is customary to define topography in terms of a height above or below 'sea level', given that the ocean is very nearly an equipotential. surface. Beneath continents, an ellipsoidal datum is used that closely matches the mean static oceanic surface, or geoid. In contrast to terrestrial practice, the topography of Mars is referenced to an equipotential surface, or areoid, described below. The areoid departs by nearly 2 km from an ellipsoid of revolution and cannot be described as a simple function of latitude. An earlier such datum [WU1991] was defined by a spherical harmonic expansion to degree and order four, thought to represent the height at which the mean atmospheric pressure would equal 6.1 mbar (the triple point of water). The martian atmosphere exchanges nearly 30% of its mass with the poles annually, so that pressure is highly variable with season. MOLA uses an areoid defined by a gravitational potential model derived from satellite tracking, and an equatorial mean planetary radius. MOLA topography is the difference between planetary radius and areoid at a given longitude and latitude. The average 6.1 mbar pressure surface lies about 1.6 km below the MOLA areoid [SMITH&ZUBER1998], but is expected to vary by 1.5 - 2.5 km with season. The areoid is defined as a surface of constant gravitational plus rotational potential. The inertial rotation rate of Mars is assumed to be 0.70882187E-4 rad/s. This potential is the mean value at the equator at a radius of 3396.000 km, namely 12652804.7 m^2/s^2, calculated from Goddard Mars Gravity Model mgm1025 [LEMOINEETAL2001] evaluated to degree and order 50. Coefficients of mgm1025 are archived by the Radio Science investigation. At a given longitude and latitude, zero elevation has a potential equal to the mean equatorial potential at a radius of 3396 km. The choice of 3396 km is convenient but somewhat arbitrary, as the radius of Mars at the equator varies from 3390.219 to 3411.522 km, with mean 3396.195 km, median 3396.068, and mode 3394.324 km. In practice the radius of the areoid is calculated iteratively using spherical harmonics so that the potential due to gravity and rotation at that radius is equal to the equatorial value. Earlier releases used the Goddard Mars potential model mgm0964c20 areoid, based on preliminary MGS tracking and the IAU1991 coordinate system. The mgm1025 areoid differs by 3 m root-mean-square (RMS) from previous releases, with changes as large as 27 m over the Tharsis volcanoes and elsewhere. Users of MOLA data must be aware of two important differences between the planetocentric coordinate system and Viking-era coordinates. These differences are significant when comparing MOLA groundtracks to MDIMs, USGS DTMs, or maps. MOLA uses the areocentric coordinate system in IAU2000. MOLA areocentric coordinates may be converted to areographic coordinates by means of relations given below that depend on the flattening of the ellipsoid assumed. The IAU2000 recommended values for mean equatorial and polar radius are 3396.2 and 3376.2 km. Earlier IAU models gave values of 3397 and 3375 km. (Note that Viking data assumed equatorial radius = 3393.40 km, and polar radius = 3375.73 km). Owing to the 3-km offset between the Mars center of figure and its center of mass, the polar radii differ from north to south by 6 km. These considerations cause headaches when areographic latitudes, sometimes poorly specified, are compared with planetocentric latitudes. MOLA longitudes are areocentric, with positive degrees East. Areographic longitudes are given as positive degrees West. However, the Viking-era longitudes were less precise than MOLA's and there are additional offsets relative to the present IAU2000 frame. The magnitude of offset varies. More than one factor may contribute to this discrepancy; a 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]. Data ==== 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, spacecraft and sub-spacecraft location, estimates of the planetary radii, and radii of an areoid equipotential surface. 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 operated in a single autonomous mode, in which it produced 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 Goddard Mars potential models and the available tracking, supplemented by limited altimetric crossovers. Spacecraft clock conversions are applied to obtain obtain Ephemeris Time in seconds from the J2000 epoch, whence MGS state vectors are found for each shot and the corresponding pointing matrices from project-supplied C-kernels. Instrument timing biases and boresight offsets are given by the MOLA instrument kernel version 2.6. MOLA ranges account for instrument delays and the leading edge timing biases, estimated by the receiver model of [ABSHIREETAL2000]. This model uses the detector threshold settings and the pulse width and energy measurements between the threshold crossings to infer the true pulse centroid, width, and amplitude. During the aerobraking mission phases, the highly eccentric orbit brought MOLA much closer to the surface of Mars than the design called for. Due to the inverse-square-law energy return in the link equation [ZUBERETAL1992], the instrument detector was saturated during the periapsis approach. Received pulse energy and pulse width are resolved during the portion of these passes when the detector is not saturated. The absolute accuracy of these quantities is about 5%, and caution must be exercised when interpreting these measurements. [NEUMANNETAL2003] give a more recent analysis and calibration of the pulse width and energy measurements. Laser energies are calculated according to the transmitter model of [AFZALETAL1997]. During operation the laser energy declined gradually to about half its preflight output, as discussed in [SMITHETAL2001A]. Even so, returning pulses over bright terrain remained saturated. A bistatic measurement of albedo is possible from the link equation during portions of the observations that are not saturated, but is affected by the two-way atmospheric transmittance. Further details are given in [IVANOV&MUHLEMAN1998]. The MOLA range data are clocked by the 99.996 MHz frequency F of the MOLA timing oscillator, stable to 1 part in 10**8 per day, which in turn is calibrated with respect to the spacecraft clock and thereby to ground station standards. A post-launch calibration of the MOLA oscillator resulted in an estimated frequency F=99,996,232 +/- 5 Hz. This frequency changed due to clock drift and was updated daily in the PEDR to maintain longterm absolute calibration. The firing interval Delta_T was controlled by this clock so that Delta_T = 10,000,000 / F. Eventually the oscillator circuit gain degraded from aging and radiation so much that F began to decrease rapidly, and firing ceased. Despite these changes, range accuracy was limited only by the 37 cm precision of the timing measurement and by the estimate of the pulse centroid location with respect to the leading-edge time (range walk). Range walk is terrain- and link-dependent. A pulse width measurement is incorporated into the calibration of [ABSHIREETAL2000], but where the energy and pulse width measurements are saturated, a threshold-dependent leading-edge timing bias derived from system characteristics is used instead of pulse width. The leading-edge timing correction may underestimate centroid range over very bright or rough terrain by a few tenths of a meter. Time tags are given in ET seconds of MOLA fire time. Timing of the shots is interpolated to ~100 microseconds. Precision was essential in the highly elliptical orbit insertion geometry because the spacecraft radial distance changed by as much as 1600 meters per second. Further timing corrections are discussed in the MOLA instrument kernel version 2.6, but no adjustment to time tags has been applied in the PEDR. In other words, the time of MOLA observations used in a dynamical sense is 117 ms later than the stated laser fire time. 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 IAU2000 planetary model. 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 and checksum 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. During mapping the sequence number increments continuously to its maximum of 16,383, followed by packets numbered 0, 0, 1, 2.... Where possible, packets with invalid checksums are allowed, since usually only the latter frames of 7 in the packet are affected. 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 shot_classification_code of 1 in the PEDR. A statistical and visual analysis of cloud features and morphology can be performed, revealing polar CO2 ice, water ice, and dust clouds at altitudes up to 20 km above the surface. Revision Notes -------------The final release of the PEDRs incorporates several minor changes in the data format. One purpose of these changes is to expand the information provided by crossover analysis, so that some estimate of orbit and attitude quality can be made. The intent was also to minimize any backward incompatibility with existing software, keeping the record length and label structure the same. Existing fields with marginally useful information were modified. These changes apply to PRODUCT_VERSION_TYPE = 'R010-CALIBRATED REL.', with the PRODUCT_ID and FILENAME version letter 'L'. The following two bytes were previously bit fields in the 'SHOT_QUALITY_DESCRIPTOR_FLAG' reserved for the 'transmit power test'. Since the MOLA laser never failed this test, these were identically 0. Iterating the crossover adjustments did not always converge, so that the final adjustment values were larger than expected at some times. These values may be used to edit records when significantly greater than 0. All values are scaled to integers. Bytes Type Range Usage 34 unsigned 0-255 radial crossover adjustment magnitude, m 35 unsigned 0-255 in-plane crossover adjustment, unit = 30 m The range window test, range comparison test, and return energy test were not implemented. These and spare bits have been used for the crossover adjustment delta-latitude and delta-longitude. These values may be used to recover the original frame-midpoint location in body-fixed (areocentric) coordinates, by subtracting them from the frame-midpoint values in bytes 337-340 and 341-344. Bytes 41-44 signed * crossover delta-latitude, degrees * 10^6 45-48 signed * crossover delta-latitude, degrees * 10^6 The atmospheric opacity field in bytes 549-552 was never implemented. These bytes were replaced with the total along-track and across-track crossover adjustments, in units of 3 cm. Where these exceed approximately one shot spacing along-track (10000 units, 300 m) or one half shot spacing across-track, records may be edited. Values of 32767 or -32768 denote invalid adjustments. No attempt was made to adjust tracks where attitude knowledge was missing, or where the laser beam incidence/emission angle exceeded 0.025 radians or 1.4 degrees, as measured from a planetary radial vector at the ground point. These values were described in the PEDRSIS-2.8 revision. 549-550 signed * crossover along-track delta 551-552 signed * crossover across-track delta The received optical pulse width, corrected, scaled received_pulse_energy, and surface reflectivity-transmission product have been recalculated following methods described in Neumann et al., 2003, Mars Orbiter Laser Altimeter pulse width measurements and footprint-scale roughness, Geophysical Research Letters, in press. The measured pulse width at channel threshold setting has been recalibrated based on inflight data as described therein. In particular, the inversion for received optical pulse width has been improved, to correspond more closely to pulse spreading due to surface slope, non-nadir incidence, and footprint-scale roughness. The bytes assigned are unchanged, but values will differ. Each field consists of 20 16-bit values. 145-184 unsigned 0-65535 Received energy, attojoules 185-224 unsigned 0-65535 Reflectivity-transmission fraction * 10^5 245-284 unsigned 0-65535 Pulse width at trigger threshold, ns * 10 285-324 unsigned 0-65535 Sigma-optical, one S.D., ns * 10. 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 [DAVIESETAL1996], 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. This coordinate system is preferred for use in navigation and 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 [DAVIESETAL1996]. This system was standard for cartography of Mars and most pre-MGS 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 [SEIDELMANNETAL2002]: Equatorial radius = 3396.2 km Polar radius = 3376.2 km Flattening = 0.0058889 Inverse flattening = 169.81 Note that the flattening is computed as one minus the ratio of the polar radius to the equatorial radius. The gravitational flattening of the planet is somewhat less, roughly 1/191. 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 Areocentric longitudes may be converted to areographic longitudes by the relation long_areographic = 360 - long_areocentric. 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 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).

These data are available on-line from the Planetary Data System (PDS) at:

http://pds-geosciences.wustl.edu/mgs/mgs-m-mola-3-pedr-l1a-v1/mgsl_21xx/

Alternate Names

  • MGS-M-MOLA-3-PEDR-L1A-V1.0

Archives

  • NASA Space Science Data Coordinated Archive
  • Planetary Data System

Discipline

  • Planetary Science: Geology and Geophysics

Additional Information

Spacecraft

Experiments

Questions or comments about this data collection can be directed to: Dr. David R. Williams.

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. David E. SmithData ProviderNASA Goddard Space Flight Centerdavid.e.smith@nasa.gov
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