Data Set Information
DATA_SET_NAME MOLA PRECISION RADIOMETRY DATA RECORD
DATA_SET_ID MGS-M-MOLA-3-PRDR-L1A-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION
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. Secondary objectives include characterizing the 1.064 micrometer wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes. The ability of MOLA to determine the surface albedo actively is limited by the precision and dynamic range of the energy measurement of returned pulse echoes, and by the changing characteristics of the atmosphere. There is a passive measurement capability as well, that depends on the background light received by the detector from solar illumination reflected off the surface. This capability has been enhanced in resolution by replacing the altimetry in the MOLA telemetry packet with high-rate background counts. The detector images an area on the surface approximately 300 m by 300 m. The detector was calibrated prior to launch while in thermal vacuum testing with a series of noise vs. threshold tests, as well as for noise response vs.temperature. Packet data include detector threshold voltages and average noise counts once per second, as well as temperature readings from an interface plate on which the detector is mounted. The noise counts on channels 1 and 2 are now recorded 8 times per second, during which time MOLA travels approximately 375 m, for an along-track resolution of approximately 1/2 km. The measurements have been made continuously throughout the mission at 3-km resolution along-track, with a lower average background count rate and therefore lower overall precision. A Precision Radiometry Data Record (PRDR) contains MOLA science mode telemetry data that has been converted to engineering and physical units. Each PRDR contains a 1.25 second span of data, called a half-frame, that is retrieved from the MOLA science mode telemetry packet. Therefore, 14 PRDRs are generated from each MOLA packet during a 17.5 second interval. Additional packet information, e.g., time, thresholds, temperature etc, are stored in the PRDR. These values are used to process the telemetry data into the PRDR. The raw telemetry data continue to be stored in the AEDR data set. Contained in a PRDR are range value, target position, and the planetary radius at the half-frame mid-point, interpolated from precision orbit data, spacecraft attitude data, and a MOLA shape model of Mars at 1/16 degree per pixel resolution. Also included are solar illumination data and season. These data are for convenience in interpretation and do not constitute independent planetometric data. There are 10 radiometric values or 'shots' in each half-frame. The location of individual shots may be obtained by interpolation for a pair of half-frames via the generic formula: shot_location : x * mid_pt_location1 + (1.-x) * mid_pt_location2, where x : (shot-5.5)/10. for half-frame 1, and x : (shot-15.5)/10. for half-frame 2. 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 2000 coordinate system [SEIDELMANNETAL2002]. A complete listing of all parameters contained in a PRDR can be found in the accompanying detached label. Data : The Precision Radiometry Data Record (PRDR) was not defined prior to the end of the altimetry mission. The files are in ASCII text format with detached PDS labels. The PRDRS consist of 38 columns of instrument science and engineering data, sub-spacecraft location, and ancillary illumination data. For further information see SUNETAL2006 in the DOCUMENT directory. Parameters : The MOLA instrument measures the spectral radiant flux from the Martian surface at 1064 nm wavelength, in a 2-nm-wide waveband. Such measurements have been normalized to a constant mean solar distance for convenience. They may be interpreted as a spectral albedo using the solar constant of roughly 290 milliwatts per meter squared per nanometer at 1064 nm wavelength, given appropriate corrections for the illumination angle of the sun and for surface scattering. Processing : The PRDRs are processed using the threshold calibrations in the receiver model of [ABSHIREETAL2000]. The thresholds and noise counts are inverted for radiant power at the detector using an analytic model [SUNETAL1992, SUNETAL2001] corrected for measurement biases. The model assumes a Gaussian noise characteristic convolved with the response of a 5-pole Bessel lowpass filter, using detector response and noise characteristics. A further correction for changes in the detector sensitivity with respect to temperature is applied, allowing for phase shifts between the temperature of the detector and the measurement point. Caution must be exercised when interpreting these measurements pending inter-instrument comparisons. Time tags are given in ET seconds of MOLA fire time. Timing of the shots is synchronized to the spacecraft 8-Hz real-time interrupt (RTI) signal, as well as the spacecraft one-second time marks. The latest spacecraft SCLK timing corrections have been applied. In the software version RTIB, the RTI signal counter was stored for each shot in the storage once allocated for laser transmit energy. Following instrument reset on 19 January 2002, after current anomalies suggested that the laser might occasionally be firing, the laser energy record was restored and the RTI counter was stored once per frame in an unused location. This packet format change was implemented in the processing software version RTIB2. The ground location and planetary radius is calculated in inertial (J2000) coordinates by projecting the MOLA boresight vector to the surface of Mars, using project-supplied spacecraft attitude kernels and the boresight calibration of the instrument with respect to the spacecraft. An iterative solution is obtained for the planetary range using a 1/16 degree per pixel MOLA shape model. The ground point position vector is transformed into planetocentric body-fixed coordinates, using the IAU 2000 planetary rotation model [SEIDELMANNETAL2002]. Ancillary Data : N/A Coordinate System : The areocentric coordinate system is used to describe data products on this volume. The areocentric coordinate system, 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. Software : N/A Media/Format : The MGS MOLA PRDR dataset will be available on DVD media 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).
DATA_SET_RELEASE_DATE 2002-07-10T00:00:00.000Z
START_TIME 2001-10-10T12:40:07.585Z
STOP_TIME N/A (ongoing)
MISSION_NAME MARS GLOBAL SURVEYOR
MISSION_START_DATE 1994-10-12T12:00:00.000Z
MISSION_STOP_DATE 2007-09-30T12:00:00.000Z
TARGET_NAME MARS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID MGS
INSTRUMENT_NAME MARS ORBITER LASER ALTIMETER
INSTRUMENT_ID MOLA
INSTRUMENT_TYPE LASER ALTIMETER
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE
Overview : The resolution of the data is about 300 m across-track and 500 m along-track. 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. The radiometry calibrations are accurate to approximately one milliwatt per meter squared per steradian per nanometer, subject to further calibration. Review : The volume containing the MOLA PRDR radiometry dataset was reviewed by MOLA science team members and the PDS. Data Coverage/Quality : On June 30, 2001, the Mars Orbiter Laser Altimeter (MOLA) ceased altimetry operations. Operation in radiometry mode commenced on October 10, 2001, and have continued with some interruptions due to spacecraft anomalies and investigation of a current anomaly on the MOLA instrument on January 19, 2002. A total of 134 days of data were collected up to MGS entry into contingency mode on February 27, 2002. Operations resumed and are ongoing as of July 2002. Limitations : The use of MOLA as a radiometer was not part of the initial instrument requirements. Such calibrations were part of routine checkout. The long-term stability of radiometric measurements has not been quantified.
CITATION_DESCRIPTION Smith, D., G. Neumann, E. A. Guinness, and S. Slavney, Mars Global Surveyor Laser Altimeter Precision Radiometry Data Record, NASA Planetary Data System, MGS-M-MOLA-3-PRDR-L1A-V1.0, 2004.
ABSTRACT_TEXT The Mars Orbiter Laser Altimeternot only provides surface topography from the laser pulsetime-of-flight, but also two radiometric measurements, the activemeasurement of transmitted and reflected laser pulse energy, and thepassive measurement of reflected solar illumination. The passiveradiometry measurement is accomplished in a novel fashion bymonitoring the noise density at the output of the photodetector andsolving for the amount of background light. The passive radiometrymeasurements provide images of Mars at 1064-nm wavelength over a 2 nmbandwidth with sub-km spatial resolution and with 2% or betterprecision under full illumination.
PRODUCER_FULL_NAME P. FORD
DAVID E. SMITH
MARIA T. ZUBER
GREGORY A. NEUMANN
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