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Because water is sensitive to microwave frequencies, microwave instruments and sounders are useful for detecting water vapor, precipitation, and ground moisture. The Global Precipitation Mission (GPM) uses the core GPM satellite along with a constellation of microwave imagers and sounders to estimate global precipitation. The SMAP satellite mission uses active and passive microwave sensors to observe surface soil moisture every two to three days. The GRACE‐FO satellite measures gravitational anomalies, that can be used to infer changes in global sea levels and soil moisture. All three hydrology missions were developed and operated by NASA.