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Space-borne passive sensing of the Earth’s surface and atmosphere has an essential and increasing importance in Earth Observation. The impressive progress recently made or shortly expected in weather analysis, warning and forecasts (in particular for dangerous weather phenomena as rain and floods, storms, cyclones, droughts) as well as in the study and prediction of climate change, is mainly attributable to the spaceborne observations. On this basis, economic studies show that meteorological services have a high positive impact on a wide range of economic activities, notwithstanding safety of life and property aspects. Space-borne passive sensing feeds crucial observational data to numerical weather predction models run on the most advanced super-computers that are operated by a few global forecasting centers. All meteorological and environmental satellite organizations operate these crucial remote-sensing missions as part of the GOS of the World Weather Watch and others... Spaceborne passive sensing for meterological applications is performed in frequency bands allocated to the Earth Exploration-Satellite Service. This is named "EESS passive" in the ITU-R Radio Regulations. The appropriate bands are uniquely determined by the physical properties (e.g. molecular resonance) of constituents of the atmosphere, and are therefore one of the unique natural resources (similarly to Radio Astronomy bands). Passive measurements at several frequencies in the microwave spectrum must be made simultaneously in order to extract the individual contribution of the geophysical parameter of interest. Bands below 100 GHz are of particular importance to provide an "all-weather" capability since many clouds are almost transparent at these frequencies. Along this line, the two first figures below about zenithal opacity describes respectively the atmosphere optical thickness due to water vapor and dry components in the frequency range 1 to 275 GHz and 275 GHz to 1000 GHz on which have been based the definition of most of the current allocations to EESS (passive) that are listed, as currently specified in ITU-R Rec. SA.515-3 summarized below. Interference criteria and performance criteria of passive sensors are indicated in ITU-R Rec(s) SA.1028-2 and 1029-2, respectively. A common summary of these two Rec(s) is also available below.
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