are passive in that they do not transmit any signals or communications; they only receive naturally occurring signals. Passive instruments are thus much different from active devices, with which people are most familiar. Active devices include cellular telephones, wireless Internet networks, garage-door openers—anything that emits a signal, whether purposefully or not. Owing to the unique purposes for which passive instruments are used, these instruments also have unique designs and needs, which are discussed in detail in this report. These passive devices are used by Earth scientists for economically and scientifically important observations of Earth’s environment and by astronomers to observe the vast reaches of the cosmos beyond this planet.
The use of passive microwave sensors allows rainfall, clouds, ocean surface winds, temperature, and moisture distributions—the primary variables of meteorology—to be quantified over the globe, under clear and cloudy conditions and during both day and night. Data from passive microwave sensors are now a vital component in the complex calculations used for weather prediction.
In addition to enabling new understanding of atmospheric processes, passive microwave observations1 have brought about a new understanding of Earth’s surface processes. The distinct microwave signatures produced by water in its various phases (liquid, ice, vapor) permit all-weather measurements of environmental variables such as snowpack depth, soil moisture, sea ice extent, sea surface temperature, and sea surface salinity. These and other related passive microwave observables, including biomass and vegetation water content, are becoming increasingly important as drivers of industry and agriculture, particularly as global resources of freshwater, arable land, and fisheries are further stretched to satisfy an increasingly large and demanding global population. The role played by passive microwave observations from space as well as from surface-based and airborne platforms in the management of these resources and the understanding of their interactions with other natural systems cannot be overstated. Currently, 21 satellites carrying passive microwave sensors are orbiting Earth.
During roughly the same era in which Earth remote sensing was developed—but preceding it by about a decade—passive radio observations were used to study the makeup of the cosmos under what is now the discipline of radio astronomy. Radio astronomy is a young science, about 60 years old, but it has contributed enormously
The terms passive microwave observations, microwave brightness, microwave emission, and similar terms that are used in Chapter 3 of this report, on the EESS (remote sensing), are synonymous with the terms radio observations, radio brightness, and radio emission that are used throughout Chapter 4, on radio astronomy. The popular use of microwave within the passive remote sensing community may have arisen in an attempt to distinguish microwave observations from visible and infrared remote sensing observations in which the Rayleigh-Jeans approximation is not applicable, and radiance power is expressed as an equivalent brightness temperature. The EESS passive microwave measurements are referenced to absolute temperature.