Data Set Information
DATA_SET_NAME CLEM1 LUNAR TOPOGRAPHY V1.0
DATA_SET_ID CLEM1-L-LIDAR-5-TOPO-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION 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. The profiles were then gridded and used to produce digital maps at 1 and 0.25 degree resolution. A 70th-degree and order spherical harmonic expansion of the data yielded the topographic model designated Goddard Lunar Topography Model 2B (GLTM-2B). This topographic model represents the first reliable global characterization of surface heights for the Moon. Data : There are 3 data types for the topography products found in this volume: tabular, array, and image data. The data file containing the orbital altimetry profiles is in tabular format, with each row containing altimetry data and information for a given point in an orbit. The file containing the spherical harmonic coefficients of the Moon's topography (GLTM-2B) is also in tabular format, with each row in the table containing the degree index m, the order index n, and the coefficients Cmn and Smn. The gridded digital maps of topography (at 1 and 0.25 degree resolution) are ASCII 2-D data arrays. There is also a byte- scaled image of each of the gridded products. 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 <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. In order to produce a digital topographic model of the Moon, it was necessary to develop a filter [SMITHETAL1995] which, when applied to the data, returned at most a single valid range value for each bounce point. Since the detailed topography is largely unknown, the filter was based on a priori knowledge of the surface properties. After filtering, a few dozen ranges, corresponding to known impact features, were manually included, and several suspect ranges excluded. While the altimetry along-track is fairly dense, the east-west coverage is limited by the 2.8 degree spacing of the groundtracks, repeating every 132 orbits. The filtered data were averaged and assembled into a 0.25x0.25 degree grid in the latitude range from -79 to +82 degrees by minimum-curvature interpolation. The elevations were referenced to a spheroid with flattening of 1/3234.93, which corresponds to the flattening term, C20 obtained for the lunar gravity field [LEMOINEETAL1995A]. This is the observed dynamical flattening of the planet. The LIDAR did not return ranging information at the poles, nor was data coverage complete at lower latitudes. Therefore before performing a spherical harmonic expansion of the lunar radii it was necessary to 'fill in' the polar regions (~3% of the planet's surface area) by minimum-curvature interpolation across the poles. A spherical harmonic model of topography, complete to degree and order 70, was then produced by numerically integrating the Fourier transform of the gridded data at each latitude times the normalized associated Legendre polynomial, with respect to the cossine of the latitude. This model is designated Goddard Lunar Topography Model 2B (GLTM-2B) [SMITHETAL1995]. Ancillary Data : N/A Coordinate System : The coordinate system for the topography data, and the coefficients in the GLTM-2B topography field, is selenocentric, center of mass, longitude positive east. The location of the pole and the prime meridian are defined as per the reference from [WILLIAMSETAL1993; DICKEYETAL1994], who used the latest Lunar Laser Ranging (LLR) data to define the lunar orientation. The locations of the pole and the prime meridian are defined using a series of chebychev polynomials, which are available upon request from NASA Goddard Space Flight Center (contact Frank G. Lemoine at 301-286-2460 or flemoine@olympus.gsfc.nasa.gov) or the PDS Geoscience Node (contact Jim Alexopoulos at 314-935-5365 or jim@wuzzy.wustl.edu). Software : N/A Media/Format : The Clementine topography dataset will be available electronically via the World-Wide Web and anonymous FTP transfer. File types include ASCII and binary formats. Formats will be based on standards established by the Planetary Data System (PDS).
DATA_SET_RELEASE_DATE 1996-01-01T00:00:00.000Z
START_TIME 1994-02-19T12:00:00.000Z
STOP_TIME 1994-05-03T12:00:00.000Z
MISSION_NAME DEEP SPACE PROGRAM SCIENCE EXPERIMENT
MISSION_START_DATE 1991-11-19T12:00:00.000Z
MISSION_STOP_DATE 1994-05-07T12:00:00.000Z
TARGET_NAME MOON
TARGET_TYPE SATELLITE
INSTRUMENT_HOST_ID CLEM1
INSTRUMENT_NAME LASER RANGEFINDER
INSTRUMENT_ID LIDAR
INSTRUMENT_TYPE ALTIMETER
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE 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 [LEMOINEETAL1995A], which is comparable to the single-shot ranging precision of the lidar (40 m). Review : The volume containing the Clementine gravity and topography datasets was reviewed by Clementine mission scientists and by PDS. 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]. Limitations : See Data Coverage/Quality above.
CITATION_DESCRIPTION Zuber, M. T., CLEM1 LUNAR TOPOGRAPHY V1.0, CLEM1-L-LIDAR-5-TOPO-V1.0, NASA Planetary Data System, 1996
ABSTRACT_TEXT 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. This data set contains topography data derived from the LIDAR measurements.
PRODUCER_FULL_NAME MARIA T. ZUBER
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