channels used in retrieving many EDRs can make this a complicated quantity to determine exactly, the values in Figures 2.11 and 2.12 generally reflect the sensitivity from the primary channels influencing errors in a particular EDR. This ratio permits the accuracy requirements of a particular EDR to be related to the accuracy requirements of the associated radiometric system. Alternately, radio frequency interference levels (K) can be related to resulting errors in EDRs. For example, the sensitivity of sea surface temperature (SST) to the vertically polarized 5 GHz brightness temperature is roughly 0.5 K(Tb)/K(SST). Since current scientific requirements for climate studies include retrievals of SST accurate to within 0.5 K or better, radio frequency interference that causes a 0.25 K change in 5 GHz brightness temperatures would pose a major problem for the retrieval of accurate sea surface temperatures. Similar quantitative statements can be made regarding other EDRs.
It is important to recognize that the EDR products shown in Figures 2.11 and 2.12 are simply unavailable on a global scale from any other type of sensor, particularly for all-weather conditions. These products include critical atmospheric parameters for NWP such as atmospheric temperature, humidity profiles, and precipitation rate. Considering global cloud conditions, surface infrared measurements are possible over an average of 5 percent of Earth’s surface and over 30 percent of Earth for the upper troposphere. At somewhat higher altitudes, atmospheric temperature and moisture profiles from microwave measurements (e.g., AMSU) are possible over 70 percent of Earth’s surface and 95 percent for the upper troposphere.39
Table 2.1 provides a summary of the common geophysical products and the microwave frequencies used for their measurement in current, future, and proposed missions, with an indication of the potential impact of those measurements from radio frequency interference (RFI) based on the current radio frequency environment.
Because of the wide range of EESS applications of microwave radiometry, numerous space-based missions are currently in operation or are planned for the near future. Table 2.2 provides a detailed list of such missions, including their planned spectrum usage and intended applications of Environmental Data Records. It is evident that microwave radiometry is widely used by the space agencies of the United States and other nations for sensing both atmospheric and surface properties and that passive microwave radiometry will continue to be widely