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.