Introduction

In chapters 5 and 6 we considered passive remote sensing systems in which the diffraction resolution limit λ/D, while important, was not usually a critical parameter of the operation. In this chapter, we consider our last major class of passive remote sensing system, the passive microwave radiometer. This is a device that measures thermally generated radiation in the microwave (usually 5–100 GHz) region. As we discussed in section 2.6, the long ‘tail’ to the Planck distribution at relatively low frequencies means that measurable amounts of radiation are emitted even in this range of frequencies.

Because microwave wavelengths are so much greater than those of visible or even of thermal infrared radiation, the resolution limit plays a much more important role, and we shall need to give careful attention to the factors that determine it. The treatment that follows in this chapter is similar to that of Robinson (1994), and is expanded upon by Ulaby et al. (1981, 1982, 1986). Much of the technology and nomenclature of passive microwave radiometry was originally developed in the field of radio astronomy, and further details can also be found in works on that subject.

Antenna theory

Angular response and spatial resolution

As we have remarked before, electromagnetic radiation is detected through its influence on electrons, which are excited to higher energy states by the incident photons. The energy of a microwave photon is typically only a few microelectron-volts, which is too small to excite an electron across an atomic or molecular band-gap.