Determination of Backscattering Sources in Various Targets

The objectives of this research are to identify the primary contributors to 10 GHz radar backscatter from various natural and man-made surfaces and objects, and to use this information in developing new and better models for the scatter. When the true sources are known for the scattering that leads to variation in intensity on radar images, the images (and sets of them) may be interpreted more meaningfully in terms of the variation of parameters of interest for science or application. For example, better interpretation of vegetation images may be possible for yield forecasting and stress detection

Microwave and Millimeter Wave Testing for the Inspection of the Space Shuttle Spray on Foam Insulations (SOFI) and the Acreage Heat Tiles

The utility of microwave and millimeter wave nondestructive testing and evaluation (NDT&E) methods, for testing the Space Shuttle's external he1 tank spray on foam insulation (SOFI) and the acreage heat tiles has been investigated during the past two years. Millimeter wave NDE techniques are capable of producing internal images of SOFI. This paper presents the results of testing several diverse panels with embedded voids and debonds at millimeter wave frequencies. Additionally, the results of testing a set of heat tiles are also presented. Finally, the attributes of these methods as well as the advantageous features associated with these systems are also provided

Sources of and Remedies for Removing Unwanted Reflections in Millimeter Wave Images of Complex SOFI-Covered Space Shuttle Structures

In the recent years, continuous-wave near-field and lens-focused millimeter wave imaging systems have been effectively used to demonstrate their utility for producing high-resolution images of metallic structures covered with spay on foam insulation (SOFI) such as the Space Shuttle external fuel tank. However, for some specific structures a certain interference -pattern may be superimposed on the produced images. There are methods by which the influence of this unwanted interference can be reduced, such as the incorporation of an incidence .angle and the proper use of signal polarization. This paper presents the basics of this problem and describes the use of the methods for reducing this unwanted influence through specific examples

Comparison of Focused and Near-Field Imaging of Spray on Foam Insulation (SOFI) at Millimeter Wave Frequencies

Millimeter wave imaging techniques can provide high spatial-resolution images of various composites. Lens antennas may be incorporated into the imaging system to provide a small incident beam footprint. Another approach may involve the use of horn antennas, which if operating in their near-fields, images with reasonably high spatial-resolutions may also be obtained. This paper gives a comparison between such near-field and focused far-field imaging of the Space Shuttle Spray on Foam Insulation (SOFI) used in its external fuel tank at millimeter wave frequencies. Small horn antennas and lens antennas with relatively long depth of focus were used in this investigation

3D Microwave Camera for Concrete Delamination and Steel Corrosion Detection

Corrosion of embedded steel reinforcement in concrete leads to concrete cracking and delamination, followed by increased salt and moisture permeation and further damage. Invisibility of the embedded rebar in combination with physical inaccessibility in elevated bridges presents a challenge in the assessment of RC bridge elements. Wideband (3D) microwave synthetic aperture radar (SAR) imaging techniques that can be integrated into a UAV offer a practical solution to overcome this challenge. This project aims to develop and optimize a 3D microwave camera for bridge inspection on a UAV platform, quantify its performance for steel corrosion evaluation and concrete delamination detection in reinforced concrete (RC) bridge elements, and build a microwave camera prototype that can be installed on a UAV for field applications

Inspection Of Spray On Foam Insulation (SOFI) Using and Microwave and Millimeter Wave Synthetic Aperture Focusing and Holography

The Space Shuttle Columbia's catastrophic failure is thought to have been caused by a dislodged piece of external tank spray on foam insulation (SOFI) striking the left wing of the orbiter causing significant damage to some of the reinforced carbon/carbon leading edge wing panels [1]. Microwave and millimeter wave nondestructive evaluation methods have shown great potential for inspecting SOFI for the purpose of detecting anomalies such as small air voids that may cause separation of the SOFI from the external tank during a launch [2,3]. These methods are capable of producing relatively high-resolution images of the interior of SOFI. Although effective, there are some advantages in using synthetic focusing methods as opposed to real focusing methods such as reduced probe size, the ability to determine depth from multiple views, and the ability to slice images due to sufficient range resolution. To this end, synthetic aperture focusing techniques (SAFT) were first pursued for this purpose and later wide-band microwave holography was implemented [4-7]. This paper presents the results of this investigation using frequency domain synthetic aperture focusing technique (FD-SAFT) and wide-band microwave holography methods illustrating their potential capabilities for inspecting the space shuttle's SOFI at millimeter wave frequencies

On the Mutual Coupling Between Circular Resonant Slots

For near- and far-field microwave imaging purposes, array of circular resonant slots can be utilized to sample the electric field at a given reference plane. In general, the sensitivity of such array probes is impaired by the mutual coupling present between the radiating elements. The mutual coupling problem poses a design tradeoff between the resolution of the array and its sensitivity. In this paper, we investigate the mutual coupling between circular resonant slots in conducting ground plane both numerically and experimentally. Based on the analysis of the dominant coupling mechanism, i.e., the surface currents, various remedies to reduce the slots' mutual coupling are proposed and verified

3D Microwave Camera for Concrete Delamination and Steel Corrosion Detection

Corrosion of embedded steel reinforcement in concrete leads to concrete cracking and delamination, followed by increased salt and moisture permeation and further damage. Invisibility of the embedded rebar in combination with physical inaccessibility in elevated bridges presents a challenge in the assessment of RC bridge elements. Wideband (3D) microwave synthetic aperture radar (SAR) imaging techniques that can be integrated into a UAV offer a practical solution to overcome this challenge. This project aims to develop and optimize a 3D microwave camera for bridge inspection on a UAV platform, quantify its performance for steel corrosion evaluation and concrete delamination detection in reinforced concrete (RC) bridge elements, and build a microwave camera prototype that can be installed on a UAV for field applications

Millimeter Wave Detection of Localized Anomalies in the Space Shuttle External Fuel Tank Insulating Foam and Acreage Heat Tiles

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) and its protective acreage heat tiles. 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 millimeter wave images of the anomalies in SOFI panel and heat tiles. This paper presents the results of an investigation for the purpose of detecting localized anomalies in two SOFI panels and a set of heat tiles. To this end, reflectometers at a relatively wide range of frequencies (Ka-band (26.5 - 40 GHz) to W-band (75 - 110 GHz)) and utilizing different types of radiators were employed. The results clearly illustrate the utility of these methods for this purpose

Inspection of the Space Shuttle External Tank SOFI Using Near-Field and Focused Millimeter Wave Nondestructive Testing Techniques

The Space Shuttle Columbia's catastrophic failure has been attributed to a piece of external tank SOFI (Spray On Foam Insulation) striking the left wing of the orbiter causing significant damage to some of the reinforced carbon/carbon leading edge wing panels. Subsequently, several nondestructive testing (NDT) techniques have been considered for inspecting the external tank. One such method involves using millimeter waves which have been shown to easily penetrate through the foam and provide high resolution images of its interior structures. This paper presents the results of inspecting three different SOFI covered panels by reflectometers at millimeter wave frequencies, specifically at 100 GHz. Each panel was fitted with various embedded anomalies/inserts representing voids and unbonds of diferent shapes, sizes and locations within each panel. In conjunction with these reJqectome&rs, radiators including a focused lens antenna and a small horn antenna were used. The focused lens antenna provided for a footprint diameter of approximately 1.25 cm (0.5") at 25.4 cm (10") away from the lens surface. The horn antenna was primarily operated in its near-field for obtaining relatively high resolution images. These images were produced using 2 0 scanning mechanisms. Discussions of the difference between the capabilities of these two types of antennas (radiators) for the purpose of inspecting the SOFI as it relates to the produced images are also presented