Soil Moisture Active Passive (SMAP) is an observatory orbiting Earth and mapping its soil moisture content and freeze/thaw state. Moisture in the topsoil (top 5cm) is measured via radar and radiometer on the observatory, while freeze/thaw state is measured via radar. Radio waves are beamed down to an area on Earth and the radar measures the echoes that return almost immediately. These echoes are then used to measure the soil moisture content. Radio waves emitted from the ground are detected by the radiometer; the strength of these waves is determined by the temperature of the ground, giving a measure of the freeze/thaw state. The radar actively emits and detects radio waves, while the radiometer passively measures temperatures. Unfortunately, after two and a half months operation in 2015, the radar malfunctioned and could not be repaired. Despite this, the radiometer is still able to produce highly accurate global soil moisture data, although at a coarse resolution.
The main objective of SMAP is to generate global maps of soil moisture to gain a deeper understanding of the water and carbon cycles, by estimating the surface flux of water and energy, and measuring carbon flux in sub-polar climates. The data has many practical applications it can be used in climate and weather studies to enhance predictive capability, resulting in more accurate forecasts; to help predict and monitor natural disasters such as droughts, wildfires, floods and landslides; to improve agricultural productivity; in improving public health via enhancing early warning systems related to famine and the spread of disease vectors such as mosquitoes; and to evaluate ground suitability for civil and military purposes.