W -band radiometry for the non-invasive investigation of materials.

Presented here is the development of passive remote sensing techniques for the detection of air pockets within a low-loss multi-layer dielectric, applied to the Thermal Protection System (TPS) of the Space Shuttle Orbiter. Also presented is an investigation into the extinction rate of dense media based on a simplified approach. In this investigation we ask the following questions: what are the properties of a material (i.e., emissivity, extinction, complex permittivity)? Our approach involves the development of transfer functions, models, and algorithms, that convert output radiometer voltages to the quantities of interest. Verification based on comparison of models to measured data concludes the work. The contributions of this dissertation include the following: (1) The design, construction, and calibration of a 94-GHz Dicke radiometer, for use in laboratory and field measurements. (2) The presentation of an alternate calibration method for millimeter wave radiometers used in a laboratory setting. This technique permits more accurate calibration, and alleviates the historically common and difficult need for two calibration sources of known brightness temperatures. Instead, the requirements are that the hot load calibration target have a physical temperature that is reasonably close to the physical temperature of the target under test, and that the cold load calibration target be stable over a time frame of 30 seconds. (3) We developed a novel technique for the detection of impurities (air pockets) within a layered dielectric, such as the TPS. (4) An algorithm was developed for transforming millimeter-wave radiometric emission measurements from planar low-loss dielectrics, into the complex dielectric permittivities of the materials. The process was demonstrated for acrylic and may generally be applied to all low-loss materials. And finally, (5) a simple approach to determining the increase in emission in a dense medium due to multiple scattering was hypothesized and tested. The theory compensated for multiple scattering by increasing the extinction in the propagating wave, proportional to the scattering coefficient derived from dense media radiative transfer. This technique was shown to be accurate for thin dense media samples with volume fractions below 35%.