MICROWAVE RADIOMETRY AND APPLICATIONS

The radiometry in general is a method of detecting the radiation of matter. All material bodies and substances radiate energy in the form of electromagnetic waves according to Planck s Law. The frequency spectrum of such thermal radiation is determined, beyond the properties of a blackbody, by the emissivity of surfaces and by the temperature of a particular body. Also, its reflectivity and dispersion take part. Investigating the intensity of radiation and its spectral distribution, one may determine the temperature and characterize the radiating body as well as the ambient medium, all independently of distance. With the above possibilities, the radiometry represents a base of scientific method called remote sensing. Utilizing various models, temperature of distant bodies and images of observed scenes can be determined from the spatial distribution of radiation. In this method, two parameters are of paramount importance: - the temperature resolution, which flows out from the detected energy, and - the spatial resolution (or, angular resolution), which depends upon antenna size with respect to wavelength. An instrument usable to conduct radiometric observations thus consists of two basic elements: a detector or radiometer, which determines the temperature resolution, and an antenna which determines the angular or spatial resolution. For example, a photographic camera consists of an objective lens (antenna) and of a sensitive element (a film or a CCD). In remote sensing, different lenses and reflectors and different sensors are employed, both adjusted to a particular spectrum region in which certain important features of observed bodies and scenes are present: frequently, UV and IR bands are used. The microwave radiometry utilizes various types of antennas and detectors and provides some advantages in observing various scenes: the temperature resolution is recently being given in milikelvins, while the range extends from zero to millions of Kelvins. Microwaves also offer a chance to penetrate surfaces of non-metallic objects down to some wavelengths, by which it is advantageous in certain applications over e.g. IR waves. An extreme example of capabilities of the microwave radiometry is found in radio astronomy, where it determines temperatures and spectral features of bodies so remote that their distance from us is measured in millions of light years. Other apparatus serve in remote observation of Earth s resources: soils, water regions and atmosphere. Similar systems also have found applications in medical studies of human body, e.g. in cancer and inflammation diagnostics. The paper presents a background of the radiometric method, comments to equipment design and outlines some of the applications. The last Fig.52 shows a simple demonstration radiometer which can be built at very low cost: it uses a C- or Ku- band LNB (low-noise block downconverter) used in satellite TVRO receivers, a simple 10 dB UHF IF amplifier, a detector and a DC millivoltmeter. Using a small parabolic dish (say, 30 - 50 cm diameter), or a horn having an aperture at least 20 x 20 cm, radiation from luminescent lights and even human body can be clearly demonstrated. A larger antenna allows to observe even solar noise.