Figure 1 is a schematic view of a radiometer viewing the Earth’s surface from a
remote platform. The surface area viewed by the radiometer antenna may be considered to
include bare soil, vegetation, or a heterogeneous mixture of soil and vegetation types. We
begin by discussing microwave emission from bare, smooth, homogeneous soils, with
uniform subsurface moisture and temperature, in order to illustrate the main concepts of
thermal radiation and emissivity, and to discuss the effects of soil moisture, soil texture,
and surface temperature on the emitted radiation. Subsequently we discuss the effects of
nonuniform soil profiles, surface roughness, and vegetation on the emitted radiation. The
effects of propagation through the atmosphere will not be dealt with since at the lower
microwave frequencies of interest in soil moisture sensing atmospheric effects are small
and may be safely neglected in most cases. Similarly, volume scattering in the soil is
negligible at these wavelengths and will not be considered hem