USA: NASA is set to launch Soil Moisture Active Passive (SMAP) satellite as part of the ESSP (Earth System Science Pathfinder) program. The overall objective of SMAP is to monitor global soil moisture mapping with unprecedented resolution, sensitivity, area coverage, and revisit times of 3days or less.
The radiometer provides more accurate soil moisture but a coarse resolution of about 40 kilometers [25 miles] across," said JPL's Eni Njoku, a research scientist with SMAP. "With the radar, you can create very high resolution, but it's less accurate. To get both an accurate and a high-resolution measurement, we process the two signals together.
The SMAP data will help characterise the relationship between soil moisture, its freeze/thaw state, and the associated environmental constraints to ecosystem processes including land-atmosphere carbon, water and energy exchange, and vegetation productivity.
The instrument’s three main parts are a radar, a radiometer and the largest rotating mesh antenna ever deployed in space.
SMAP's radar, uses the antenna to transmit microwaves toward Earth and receive the signals that bounce back, called backscatter. The microwaves penetrate a few inches or more into the soil before they rebound. Changes in the electrical properties of the returning microwaves indicate changes in soil moisture, and also tell whether or not the soil is frozen.
SMAP’s radiometer detects differences in Earth's natural emissions of microwaves that are caused by water in soil. The mission also tries to address the problems radio frequency interference. The microwave wavelengths that SMAP uses are officially reserved for scientific use, but signals at nearby wavelengths that are used for air traffic control, cell phones and other purposes spill over into SMAP's wavelengths unpredictably. NASA Goddard engineers devised a new way to delete only the small segments of actual interference, leaving much more of the observations untouched.
Source: NASA Jet Propulsion Lab