Active sensors send out a beam of acoustic or electromagnetic energy and measure how the medium perturbs it. Passive sensors emit no energy of their own. By listening at microwave frequencies, it is possible to measure the roughness of the ocean surface or moisture content of the atmosphere.
Atmospheric radiometers measure the emissions of the atmosphere itself .
For example, a microwave antenna pointed into the air receives an abundance of thermal radio emissions from the atmosphere's various constituents. Each constituent possesses a unique emission spectrum that corresponds exactly to its absorption spectrum. At 31 GHz, on the other hand, the ratio is roughly inverted. Thus, a dual-frequency radiometer provides the information required to solve two equations for the two unknown quantities. This instrument, operating at 23.87 and 31.65 GHz, was designed at ETL to provide a detailed knowledge of the spatial distribution of cloud liquid for research in meteorological areas such as cloud physics, climatology, severe storms, and aircraft icing.