Foam shedding from the shuttle external tank remains a critical problem regarding Shuttle orbiter safety. Flaws present in the foam can result in initiation sites for foam loss, and NASA is continuing to look at improving existing NDE methods for foam inspection as well as developing new methods. Terahertz NDE, greatly enhanced over the last several years with respect to its use for external tank foam inspection, has been a focus for continued improvement through signal and image processing improvements. In this study, results from various signal processing approaches to improve resolution of defects in terahertz images of external tank foam were described. Centroid of the power spectral density for the main (substrate) reflection off of a metallic substrate under the foam shows promise versus conventional parameters of the signal used to form images. The centroid is reasonably simple and rapid computationally. Where it is suspected that extremely difficult-to-detect flaws exist, the use of an interactive gating method that allows near-real-time formation of images at any portion of the wavetrain might be warranted. Line profile and region-of-interest examination are suggested as ways to compare flaw resolvability/sensitivity when evaluating various signal processing methods. When applied to ice frost ramp components, the centroid image showed some indications but did not obviously reveal voids where they were known to exist. Other signal processing methods continue to be investigated. The NASA-developed software described in this article is applicable to any waveform-based image data and will be upgraded in the future. For example, it is expected that the automated image quality protocol described in ref 5, as well as protocols that allow image comparison to a gold standard, will be implemented within the NASA software at a future date. It is also planned to test and deploy additional joint timefrequency methods such as wavelets that can be useful in denoising and characterizing complicated signals.
Signal processing approaches for terahertz data obtained from inspection of the shuttle external tank thermal protection system foam
Signalverarbeitungsverfahren für Terahertzdaten aus der Prüfung des thermischen Schaumschutzsystems vom externen Shuttletank
2007
10 Seiten, 8 Bilder, 2 Tabellen, 5 Quellen
Aufsatz (Konferenz)
Datenträger
Englisch
Spray-on Foam Insulation Development for the Space Shuttle External Tank Thermal Protection System
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