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. 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. 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 resolution and the field of view (FOV). The second more innovative method called aperture synthesis uses interferometric techniques and offers higher resolution and larger fields 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.