Radiometry, Noise Temperature, and Interferometry

This chapter first discusses radiometry, which is the passive detection of natural radiation from various media, targets, and objects. Included in this discussion are brightness, antenna temperature, radiative transfer, and emissivity. Next, the chapter discusses the effects of the receiving system on the system noise temperature and the minimum detectable temperature, and deals with the use of interferometry for mapping the brightness distribution. The fundamental quantity in radiometry is the brightness (B). The chapter considers the propagation of the brightness in a medium emitting electromagnetic radiation. All objects emit electromagnetic energy. A perfect absorber that absorbs electromagnetic energy at all wavelengths is called a blackbody. The brightness of the electromagnetic radiation from a blackbody depends only on its temperature and frequency. The chapter further describes how the noise temperature is emitted, propagated, absorbed, and scattered, and then reaches the antenna. Finally, it discusses what happens in the receiver.