PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
   PUBLICATION_DATE    = 2007-06-08
   NOTE                = "Description of contents of DATA directory"
END_OBJECT            = TEXT
END

  Files 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.
  
Please see lrm_ds.cat for information about known problems with the data.