| dc.description.abstract | Due to the Columbia Space Shuttle Accident of February 2003, the Columbia
Accident Investigation Board determined the need for an on-orbit inspection system
for the Thermal Protection System that accurately determines damage depth to 0.25". NASA contracted the Spacecraft Technology Center in College Station, Texas, for a
proof-of-concept photogrammetric system. This system involves a high quality digital
camera placed on the International Space Station, capable of taking high fidelity
images of the orbiter as it rotates through the Rendezvous Pitch Maneuver. Due to
the pitch rotation, the images are tilted at different angles. The tilt causes the damage
to exhibit parallax between multiple images. The tilted images are therefore registered
to the near-vertical images using visually striking features on the undamaged surface
of the Thermal Protection System that appear in multiple images taken at different
tilt angles. The images become relatively oriented after registration, and features in
one image are ensured to lie on the epipolar line in the other images. Features that
do not lie on the undamaged surface, however, are shifted in the tilted images. These
pixels are matched to the near-vertical image using a sliding-window area-matching
approach. The windows are matched using a least-squares error method. The change
in location for a pixel in a tilted image from its expected location on the undamaged
surface is called the pixel disparity. This disparity is linearly scaled using the tilt
angle and the pixel sampling to determine the depth of the damage at that pixel location. The algorithm is tested on a set of damaged tiles at the Johnson Space
Center in Houston and the photogrammetric damage depth is then compared to a set
of truth data provided by NASA. The photogrammetric method shows promise, with
the 0.25" error limit being exceeded in only a few pixel locations. Once the camera
properties are fully known from calibration, this systematic error should be reduced. | en |
