Millimeter-wave detection of localized anomalies in the space shuttle external fuel tank insulating foam

The Space Shuttle Columbia´s catastrophic accident emphasizes the growing need for developing and applying effective, robust, and life-cycle-oriented nondestructive testing (NDT) methods for inspecting the shuttle external fuel tank spray on foam insulation (SOFI). Millimeter-wave NDT techniques were one of the methods chosen for evaluating their potential for inspecting these structures. Several panels with embedded anomalies (mainly voids) were produced and tested for this purpose. Near-field and far-field millimeter-wave NDT methods were used for producing images of the anomalies in these panels. This paper presents the results of an investigation for the purpose of detecting localized anomalies in several SOFI panels. To this end, continuous-wave reflectometers at single frequencies of 33.5, 70, or 100 GHz representing a relatively wide range of millimeter-wave spectrum [Ka-band (26.5-40 GHz) to W-band (75-110 GHz)] and utilizing different types of radiators were employed. The resulting raw images revealed a significant amount of information about the interior of these panels. However, using simple image processing techniques, the results were improved in particular as it relates to detecting the smaller anomalies. This paper presents the results of this investigation and a discussion of these results.