Data Set Information
DATA_SET_TERSE_DESCRIPTION MGS Laser Altimeter Passive Radiometry Measurements
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

  A complete listing of all parameters contained in a PRDR can be
  found in the accompanying detached label.

  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.

  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

  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

  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

  Ancillary Data

  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.


  The MGS MOLA PRDR dataset will be available on DVD media and
  electronically via the PDS Geosciences Node web site at and the MOLA Science Team web site at  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 3000-01-01T12:00:00.000Z
MISSION_START_DATE 1994-10-12T12:00:00.000Z
MISSION_STOP_DATE 2007-09-30T12:00:00.000Z
NODE_NAME Geosciences
  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.

  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.

  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
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 Altimeter not only provides surface topography from the laser pulse time-of-flight, but also two radiometric measurements, the active measurement of transmitted and reflected laser pulse energy, and the passive measurement of reflected solar illumination. The passive radiometry measurement is accomplished in a novel fashion by monitoring the noise density at the output of the photodetector and solving for the amount of background light. The passive radiometry measurements provide images of Mars at 1064-nm wavelength over a 2 nm bandwidth with sub-km spatial resolution and with 2% or better precision under full illumination.
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