Data Set Overview
=================
  The Clementine spacecraft included a Light Imaging Detection
  and Ranging (LIDAR) instrument.  The LIDAR instrument measured
  the slant range from the spacecraft to the lunar surface.
  The instrument acquired high resolution profiles of lunar
  topography.  

  This data set contains aggregated topographic
  ranges, radii, and related data along each observational pass during which
  LIDAR laser range data were acquired. The data were created using 
  preliminary spacecraft timing, orientation, and orbital solutions. The 
  laser ranges have been converted from counts to meters using a calibration 
  factor of 39.972 m per count. Timing information may have substantial 
  errors owing to spacecraft computer resets and clock ambiguity. The 
  lidar electronics triggered on photon pulses continuously, and recorded up 
  to four pulses within a programmable range window. The last trigger before 
  and the first trigger after the range window were also recorded. Usually, 
  but not always, the first trigger within the range window was the valid 
  range. For a few laser shots, multiple pulses were detected within the 
  expected time interval for lunar reflections. Note: the fields 
  START_ORBIT_NUMBER and STOP_ORBIT_NUMBER in the product labels 
  refer to the Clementine revolution numbers, which increment at the start
  of the observational pass.

Parameters
==========
  The LIDAR instrument measured the slant range from the
  spacecraft to the lunar surface at spacecraft altitudes of 640
  km or less.  Range value was determined by the number of clock
  cycles between the laser start pulse and the received signal. 
  The spacecraft orbited within ranging distance (orbital
  altitudes less than 640 km) for approximately one-half hour per 5-hour
  orbit during the 2-month lunar mapping mission, and typically
  ranged at a rate of 1 shot per 1.6 seconds.  The elevations,
  in meters, were referenced to a spheroid with flattening of
  1/3234.93.


Processing
==========
  Range was determined by the number of clock cycles between the
  laser start pulse and returned signal, using a calibration
  factor of 39.972 m per cycle. Determining the range involved
  interpolating the spacecraft orbital trajectory to the time of
  the laser measurement, applying time-tag corrections to the
  original lidar data as supplied by the Naval Research
  Laboratory, including a 2 msec delay to the laser fire time. 
  Corrections were then made for the one-way light time to the
  surface, and the measured range from the spacecraft to the
  surface was then transformed to a lunar radius in a
  center-of-mass reference frame.  Lunar radii are thus
  expressed in selenocentric surface coordinates. The spacecraft
  quaternions were then used to correct the ranges for off-nadir
  pointing of the spacecraft.

  During the course of the mission the LIDAR typically ranged at
  a rate of 1 shot per 1.6 seconds and triggered on about
  123,000 shots, corresponding to 19% of the transmitted laser
  pulses.  Typical along-track shot spacings were on the order
  of 20 km, but this varied considerably. Ranging at 8 Hz was
  performed on pass 323 and successfully downloaded.  Much of
  the time, the first trigger in the range window was a true
  echo, but often, particularly over rough terrain, there were
  multiple triggers that did not correlate with lunar features. 
  
  This data set was used to create the digital topographic model of 
  the Moon, PDS data set CLEM1-L-LIDAR-5-TOPO-V1.0. See the documentation
  for that data set for more information.

Overview
========
  Orbits were characterized by a formal uncertainty in radial
  position of about 10 meters and have an accuracy with respect
  to the lunar center of mass of approximately 100 m
  [LEMOINEETAL1995], which is comparable to the single-shot
  ranging precision of the lidar (40 m).

Data Coverage/Quality
=====================
  Laser ranging was performed on revolutions 8 to 163 in the
  southern hemisphere during the first month, and revolutions
  165 to 332 in the north during the second month of the mapping
  phase of the mission.  During the first month, with spacecraft
  periselene at latitude -30 degrees, topographic profiles were
  obtained in the approximate latitude range -79 to +20 degrees. 
  In the second month of mapping, with spacecraft periselene at
  latitude +30 degrees, profiles were obtained in the
  approximate latitude range -20 to +82 degrees.  This resulted
  in near-globally distributed profiles of elevation around the
  Moon [ZUBERETAL1994].