A photogrammetric on-orbit inspection for orbiter thermal protection system

摘要

Due to the Columbia Space Shuttle Accident of February 2003, the ColumbiaAccident Investigation Board determined the need for an on-orbit inspection systemfor the Thermal Protection System that accurately determines damage depth to 0.25". NASA contracted the Spacecraft Technology Center in College Station, Texas, for aproof-of-concept photogrammetric system. This system involves a high quality digitalcamera placed on the International Space Station, capable of taking high fidelityimages of the orbiter as it rotates through the Rendezvous Pitch Maneuver. Due tothe pitch rotation, the images are tilted at different angles. The tilt causes the damageto exhibit parallax between multiple images. The tilted images are therefore registeredto the near-vertical images using visually striking features on the undamaged surfaceof the Thermal Protection System that appear in multiple images taken at differenttilt angles. The images become relatively oriented after registration, and features inone image are ensured to lie on the epipolar line in the other images. Features thatdo not lie on the undamaged surface, however, are shifted in the tilted images. Thesepixels are matched to the near-vertical image using a sliding-window area-matchingapproach. The windows are matched using a least-squares error method. The changein location for a pixel in a tilted image from its expected location on the undamagedsurface is called the pixel disparity. This disparity is linearly scaled using the tiltangle 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 SpaceCenter in Houston and the photogrammetric damage depth is then compared to a setof truth data provided by NASA. The photogrammetric method shows promise, withthe 0.25" error limit being exceeded in only a few pixel locations. Once the cameraproperties are fully known from calibration, this systematic error should be reduced.