Imaging technologies and applications of microwave radiometry

Abstract

Microwave radiometry deals with the measurement of the natural thermally caused electromagnetic radiation of matter at a physical temperature above 0K. For imaging purposes no transmitter is needed as in the case of a radar, but the receiver requires a very low noise figure because the signals to be measured are in the same order of magnitude as its own noise. In the case of Earth observation significant contrasts can be observed between reflective and absorbing materials due to the impact of reflected sky radiation of cosmic origin. The space outside the terrestial atmosphere acts like a blackbody radiator having a physical temperature of about 3K. This cold appearance can be observed more or less strongly depending on the reflective properties of an object and the frequency dependent absorption and self-emission of the atmosphere. The main MMW windows for sufficient atmospheric penetration and low sky brightness temperatures are at frequencies around 35, 94, 140 and 220GHz. For Earth observation, an approximate brightness temperature range from 3K to more than 300K can be observed. Due to the sky, which acts as a huge and extended illumination source, and because of the incoherent character of thermal radiation, radiometric signatures don’t suffer from fading, speckle, shadowing, and dominant scattering centers as in the case of radar imaging. In the microwave region the spatial two-dimensional brightness temperature distribution can be used as a daytime and almost weather independent indicator for many different physical phenomena. Hence, interesting application areas incorporate geo science, climatology, agriculture, pollution and disaster control, detection, reconnaissance, surveillance, and status registration in general. Many of those applications require high spatial and radiometric resolution, high precision, large fields of view, quasi real-time imaging, and high frame rates. The rapid developments in receiver and computer electronics of the past ten to twenty years allow the advanced design and construction of more complex and more powerful measurement systems and widen the application areas of each technology. Today, in principle three radiometric imaging methods are considered. The first more classical one is based on a linescanner approach and is relatively easy to implement, but it has strong limitations concerning the spatial resolution and the field of view (FOV). The second more innovative method called aperture synthesis uses interferometric techniques and offers higher resolution, real-time imaging and a larger field of view at the cost of much more expense. It is a subject of actual research. The third principle uses a focal plane array and a focusing aperture as in many optical systems. Thus real-time imaging is possible, but for present technologies high resolution systems with larger FOVs are associated with a high expense. Coherent signal processing for a very high and almost distance independent spatial resolution as in the case of synthetic aperture radar is not applicable to radiometric imaging. The presentation illustrates the phenomenology of microwave radiometry and the at present mostly considered imaging principles. Typical examples from current practice and experimental basics for future applications are shown.