The earth's atmosphere, clouds, and surface continually emit thermal radiation throughout the electromagnetic spectrum (refer to figure above). A relatively small portion of that radiation energy is emitted at microwave wavelengths. Microwaves are considerably longer than those within the visible and infrared portions of the spectrum.

"Passive" microwave sensors or radiometers are designed to measure microwave energy at frequencies between 19 and 200 GHz (1.6 to 0.15 cm). The term "passive" is used to refer to sensors that only measure energy emitted by another source. "Active" sensors, such as weather radars, measure the return signal from a pulse of energy emitted by the sensor itself. From here on, microwave energy is referred to at various "frequencies" as opposed to "wavelengths." This terminology is commonly used by the remote sensing community.

Microwave instruments have to date only been deployed on polar-orbiting spacecraft because large antennas are needed to observe earth-emitted energy at the very long wavelengths associated with microwaves. Both military (DMSP) and civilian (NOAA) POES currently make use of microwave sensing technology. In spite of the differences noted, the similarities between the two capabilities allow us to discuss the important basics of microwave remote sensing as they apply to both the DMSP's SSM/I and NOAA's AMSU microwave instruments.