From this altitude variations can be calculated. Although active microwave systems, i.e., radar, are the more commonly used sensors for this region of the spectrum, passive microwave sensors also have provided information about the Earth's surface, its oceans, and its atmosphere. Note, too, that the peaks of the curves shift systematically to the left as the objects increase in kinetic temperature. This shift is the consequence of Wien's Displacement Law that we examine in more detail on page 9-2 in the Thermal Remote Sensing Section. Technically, the radiation shown above is that of blackbodies at different temperatures.
Natural materials are graybodies whose temperatures depart somewhat from perfect blackbodies.
This radiation, which is emissive, is generally much weaker in intensity compared with shorter wavelength outputs but is still detectable by sensitive instruments and also is not much attenuated by the atmosphere. The temperatures measured by these instruments are brightness temperatures . . In this family of curves, land, water, air, and ice have different brightness temperatures and thus we can separate them in many cases, and sometimes uniquely identify them. The wavelength segment of the blackbody curves employed in passive microwave detectors is generally between 0.15 and 30 cm. On moving platforms, the fixed antenna operates along a single linear track so that it generates intensity profiles rather than images.