PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM
OBJECT                = TEXT
  PUBLICATION_DATE    = 2017-02-27
  NOTE                = "Geometry information and description of the contents
                         of the GEOMETRY directory."
END_OBJECT            = TEXT
END


Contents of the GEOMETRY directory
----------------------------------

The Bloomsburg University Goniometer (BUG) was used to make bidirectional
reflectance distribution function (BRDF) measurements of Apollo samples at
several wavelengths. The instrument is described in "Shepard, M.K., The
Bloomsburg University Goniometer (B.U.G.) Laboratory: An Integrated
Laboratory for Measuring Bidirectional Reflectance Functions, Lunar and
Planetary Science Conference XXXII, Abstract 1015, 2001". This abstract can
be accessed at: http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1015.pdf.

The GEOMETRY directory contains 2 images, BUG_LAB_GONIOMETER.JPG and
GEOMETRY_SKETCH.JPG. BUG_LAB_GONIOMETER.JPG is a labeled image of the
instrument used to acquire the archive data. GEOMETRY_SKETCH.JPG is a sketch
used to explain the geometry of the instrument (see below).

GEOMETRY Information
--------------------

The geometry of the observations requires some explanation (refer to
GEOMETRY_SKETCH.JPG in this directory). The system records incidence and
emission angles as positive or negative. The emission arm is fixed in one
plane while the incidence arm rotates in its own independent plane that can
rotate in azimuth, also positive or negative. Because of this, the same
geometry can be measured using different combinations of incidence (i),
emission (e), and azimuth (az). For example, i = +30 deg, e = -40 deg, az = 0
deg and i = -30 deg, e = -40 deg, az = 180 deg have the source and detector
in the same exact positions. The data set can have two congruent
measurements, such as one at i = +60 deg, e = +30 deg, az = -20 deg, phase
angle = 88.5 deg and another at i = -60 deg, e = +30 deg, az = 160 deg, phase
angle = 88.5 deg. In this case, the relative geometry is the same, but the
arms have switched sides of the table. Phase angle is calculated based on the
relative positions of the two arms. Twenty-seven identical or congruent
geometry measurements were made throughout a sample run to check the fidelity
of our data. In the 3D plots of the data, it was assumed that scattering was
mirrored across the principal plane (azimuth = 0 deg to 180 deg line).

Samples are illuminated at 5 different incidence angles (i = 0, 15, 30, 45,
and 60 deg) and emission angles from (absolute) 0 to 80 deg in 10 deg or
finer increments. Azimuth varied from (absolute) 0 to 170 deg in varying
steps, depending upon the incidence angle. When i is small (<30 deg or so),
larger azimuth steps are taken between measurements than when i > 30 deg.
This is because even large changes in azimuth do not significantly change the
phase angle when i is small. In essence, measurements are made at
approximately equal solid angles around the scattering hemisphere. The
resulting range in phase angle is 3 deg to 150 deg.

Sample preparation is discussed in the BUGAPOLLO_DS.CAT file.