The SMAP mission will provide global mapping and monitoring of landscape freeze/thaw cycle and surface soil moisture content. The data will be used to model and estimate the earth carbon/ carbon dioxide fluxes and underlying environmental controls. Scientists hope that these observations will allow them to link together the terrestrial water, energy and carbon cycle processes and identify why the climate models have failed to predict the last 15 years of near stable global temperatures. Scientists hope to use this and other global data to identify the functioning of the “missing carbon sink” that has slowed the predicted rise in global temperatures. NASA hopes that SMAP will bring us some of the data we need to better understand our planet and predict its future. Though, the most immediate benefit will be drought prediction.
SMAP will accomplish its mission by measuring how much water is contained in the top layer of soil and identifying when the ground is frozen. This is accomplished using an instrument that combines an L-band radar and an L-band radiometer that both share a 6 meter in diameter aperture reflector. The reflector will rotate to scan a 621 mile wide swath of earth on each orbital pass of the observatory. The radiometer provides “passive” measurements of the microwave emissions of the upper soil. The radar makes the “active” back scatter measurements of the surface. Thus, the name of the mission- Soil Moisture Active Passive.
The ground processing systems run by the NASA Jet Propulsion Laboratory and the Goddard Space Flight Center will combine the data sets. Combining the active and passive datasets increases the resolution and accuracy of the data. Together the instruments will be able to measure the moisture in the top two inches of soil to a spacial resolution of under two miles.
The SMAP mission will also collect data on the frozen/thawed state of the soil. This information is important to understand the length of the plant growing season and will increase our understanding the contribution of the boreal forest to the global carbon balance and contribute to a better understanding of droughts and climate.
A high-resolution, space-based measurement of soil moisture is a new capability. Scientists will be able to better predict natural hazards of extreme weather, climate change, floods and droughts, and help reduce uncertainties and unknowns that now exist in our understanding of Earth's water, energy and carbon cycles. Several of the data gathering techniques grew out of previous NASA missions of this century including the Aquarius project and the cancelled Hydros project.
The microwave portion of the electromagnetic spectrum (wavelengths from a few centimeters to a meter) is used to estimate the surface soil moisture. Passive microwave sensors measure the natural thermal emission emanating from the soil surface. The intensity of this radiation depends on the dielectric properties and temperature of the target medium, which for the SMAP mission is the surface soil layer and is a function of the amount of moisture present. The low microwave frequencies used offer additional advantages; the atmosphere is almost transparent in that wavelength so weather does not impact the sensing, transmission of the data from the soil is also possible through sparse to moderate vegetation water content, and the microwaves measurements are not impacted by daylight to allow for 24 hour observations.
The SMAP mission will map the entire globe every two to three days for at least three years and provide the most accurate and highest-resolution maps of soil moisture ever obtained. The spacecraft's final circular polar orbit will be 426 miles (685 kilometers), at an inclination of 98.1 degrees. The spacecraft will orbit Earth once every 98.5 minutes and repeat the same ground track every eight days.
Weather forecasting, accurate modeling and forecast of climate variability and change, planning and predicting agricultural productivity, effective water resources management, drought prediction, flood area mapping, and ecosystem health monitoring all require information on the status of soil moisture. Soil moisture affects plant growth and agricultural productivity, especially during times of drought or water shortages. This can improve our ability to monitor and forecast agricultural productivity and allow for a famine early warning in the most food insecure regions of the earth.