Data Set Information
DATA_SET_NAME ARECIBO/NRAO MOON RTLS/GBT 4/5 70CM V1.0
DATA_SET_ID ARCB/NRAO-L-RTLS/GBT-4/5-70CM-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION The data products in the DATA directory are organized into folders based on NASA data processing level. Data in the 'level1' folder correspond to NASA Level 1B, or PDS Level 4 Resampled Data. According to the PDS Standards Reference, these are 'data that have been resampled in time or space domains in such a way that the original edited data cannot be reconstructed.' Data in the 'level2' folder correspond to NASA Level 2, or PDS Level 5 Derived Data. According to the PDS Standards Reference, these are 'derived results, as maps, reports, graphics, etc.' The 'level1' and 'level2' folders contain 80 and 100 image files, respectively. Each image has an accompanying detached PDS label. The 'level1' data set consists of 4 image files for each of 20 lunar 'quads.' The images are named as follows: xxxx_dep_level1.img (depolarized map), xxxx_pol_level1.img (polarized map), xxxx_beam_level1.img (map of beam angle in radians), and xxxx_inc_level1.img (map of incidence angle in radians), where xxxx is an abbreviation of the FEATURE_NAME (named for a prominent feature within each image 'quad') listed in the PDS label. The 'level2' data set consists of 5 image files for each of the 20 lunar 'quads.' The naming convention is the same as that of 'level1' for the 'dep', 'pol', 'beam', and 'inc' maps, however the 'level2' data set also includes a circular polarization ratio image, xxxx_rat_level2.img. North is at the top of the frame in all of these images. Level 1: For the polarized ('pol') and depolarized ('dep') maps, the floating point values represent the relative backscatter power from the lunar surface at a radar wavelength of 70 cm (430 MHz frequency). 'Polarized' refers to energy scattered from the lunar surface in the opposite circular polarization sense to that transmitted (the behavior expected of a flat, mirror-like reflecting surface). 'Depolarized' refers to reflected energy with the same sense of circular polarization (the behavior expected from a surface with abundant wavelength-scale objects that randomize the reflected polarization). The data were collected by transmitting a circular polarized signal from the Arecibo Observatory, and receiving the lunar echoes at the Robert C. Byrd Green Bank Telescope in West Virginia. The transmitted signal was a series of 3 microsecond pulses separated by 15 ms to allow for the full possible range of echo time delays over the Moon's surface. No pulse compression techniques (for example, Barker coding or a chirped signal) were used. A patch focusing method was used to correct for time-varying Doppler changes at lunar surface points distant from the radar pointing target. Multiple independent integration periods (looks) were added together to reduce radar speckle. Image power values were normalized to the effective scattering area that contributes to each pixel (based on a reference spherical shape) and to the thermal background noise measured at the Green Bank Telescope. Results are thus proportional to the backscatter coefficient, but have not been calibrated for the transmitted power, variation in antenna gain across the scene, or the absolute power represented by the Green Bank Telescope thermal noise. Ratios between the two circular polarization channels are well calibrated. The stated location of the sub-radar point and the apparent Doppler angle and center-to-limb bandwidth (Hz) at 430 MHz are approximate values at the center of the multi-look observing period. The incidence angle value for each pixel represents the angle (in radians) between a vector from the Moon's center of mass (COM) to the surface location and the vector from the COM that passes through the sub-radar point. Values near the radar-visible limb of the Moon thus approach 90 degrees. This angle is often used to correct a radar backscatter image for variations in brightness due to the 'scattering law' of the surface. The beam angle value for each pixel represents the angle (in radians) between a vector from the observer to the surface location and the vector from the observer to the radar pointing target given for each 'quad.' This angle may be used to correct for the decrease in incident power and receiving antenna sensitivity with offset from the pointing target. Level 2: The Level 2 images and ancillary data (incidence and beam angle) have been warped to match the Clementine basemap, adjusted for the average net Arecibo-GBT beam pattern, and calibrated to our best estimate of absolute backscatter coefficient ('sigma zero'). These maps are also normalized to the average lunar power scattering behavior (set to unity, or zero dB, at zero incidence angle), given for the polarized and depolarized channels by: Pol : 10^([-1.4372 P + 0.02545 P^2 - 0.000168 P^3]/10) Dep : cos(P), where P is the incidence angle in degrees. In addition to the 'pol' and 'dep' images, the archive also includes 'rat', 'beam', and 'inc' images. The 'rat' image files, found only in the 'level2' data set, are circular polarization ratio images formed by averaging 'valid' pixels in a 5x5 grid and dividing the averaged 'dep' data by the averaged 'pol' data. No other normalization is performed, so the polarization ratio images have a strong decrease toward the center of the Moon (lower incidence angle). Values of unity and greater are typical of rough surfaces, particularly at high incidence angles where the diffuse scattering component dominates the echo. The other image files ('beam' and 'inc') display the beam angle and incidence angle, respectively, in radians for each pixel. These co-registered maps, in floating point format, will allow a user to reverse the applied calibration steps if desired.
DATA_SET_RELEASE_DATE 2007-07-18T00:00:00.000Z
START_TIME 2006-01-01T12:00:00.000Z
STOP_TIME N/A (ongoing)
MISSION_NAME
MISSION_START_DATE
MISSION_STOP_DATE
TARGET_NAME MOON
TARGET_TYPE SATELLITE
INSTRUMENT_HOST_ID ARCB
NRAO
INSTRUMENT_NAME ROBERT C. BYRD GREEN BANK TELESCOPE
RADIO TELESCOPE
INSTRUMENT_ID GBT
RTLS
INSTRUMENT_TYPE RADAR
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Known Problems : (a) The individual radar looks are combined with a second-order rubbersheet transform that compensates for minor variations in the apparent origin of the radar coordinate system. The edges of a Level 1 map may thus have pixels that do not reflect a sum of data values from every look. These low-quality border areas are less than 10 pixels in extent. The edges of each Level 2 quad are 'trimmed' of these pixels, so the maps can be more readily mosaicked. (b) Separate quad maps may have spatial offsets of order 4 km with respect to the Clementine image base, but it is unknown to what degree these offsets are due to errors in the lunar radar ephemeris or the Clementine lunar reference grid. (c) There is considerable uncertainty (up to a factor of 2) in the absolute transmitted power at Arecibo. This translates into up to 3 dB of offset between mosaicked Level 2 image products. Good-quality mosaics may be obtained by a simple multiplicative shift between images, but the absolute backscatter coefficient must be regarded as uncertain by this 3 dB factor. The polarization ratios are much better calibrated, and are accurate to at least the 10% level. (d) Delay-Doppler radar images of a planet have a north-south ambiguity about the line on the plane of the sky (the 'Doppler equator') perpendicular to the apparent spin axis. This means that some observations, particularly those collected for areas near the center of the Moon, may have blurred bright patches associated with echoes from rugged features in the undesired hemisphere. In general, these are only visible in quads near the center of the Moon (for example, with the bright crater Tycho appearing as a blurred patch near the crater Copernicus). Careful mosaicking of overlapping coverage can avoid these unwanted contributions. (e) The polarized (opposite-sense circular) data for 'quads' 'Canon' and 'Faraday' suffered from saturation of the analog/digital converter at low incidence angles (closer to the center of the Moon). This led to an apparent reduction in echo power with decreasing incidence angle. The error was corrected in an approximate sense by comparison of the overlap between these two 'quads' and other data collected without saturation effects. The polarization ratio values for incidence angles less than 45 degrees for these two 'quads' should be treated as only approximate. This data set underwent external peer review from April to July 2007.
CITATION_DESCRIPTION Campbell, B.A., and J. Ward, Dual- Polarization Calibrated Radar Map of the Moon, ARCB/NRAO-L-RTLS/GBT-4/5-70CM-V1.0, NASA Planetary Data System, 2007.
ABSTRACT_TEXT Radar backscatter power from the lunar surface was collected at a wavelength of 70 cm, using the 305 m Radio Telescope at Arecibo to transmit and the NRAO's 105 m Robert C. Byrd Green Bank Telescope to receive.
PRODUCER_FULL_NAME BRUCE A. CAMPBELL
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