What Is the Smap Mission?

The Soil Moisture Active Passive (SMAP) mission is a NASA Earth-observing satellite program dedicated to measuring and mapping soil moisture and freeze/thaw state across the globe. Launched on January 31, 2015, from Vandenberg Air Force Base aboard a Delta II rocket, SMAP addresses a fundamental gap in Earth system science: reliable, high-resolution observations of the water held in the top layer of soil. Those observations link the water cycle, the carbon cycle, and the energy budget of the land surface in ways that ground-based networks cannot replicate at global scale.

The mission grew out of a National Research Council Decadal Survey recommendation that soil moisture ranked among the highest-priority Earth observations. NASA's Jet Propulsion Laboratory leads mission operations with support from the Canadian Space Agency.

Science Objectives and Instruments

SMAP carries two L-band microwave instruments: an active radar and a passive radiometer. The L-band frequency (1.26 GHz for the radar, 1.41 GHz for the radiometer) penetrates moderate vegetation cover and reaches roughly the top five centimeters of soil, the depth most relevant to surface hydrology and land-atmosphere exchange. The radiometer measures naturally emitted microwave radiation from the surface, providing high sensitivity to soil moisture with coarser spatial resolution. The radar transmits pulses and records the reflected energy, yielding finer spatial detail. A rotating mesh reflector antenna spanning six meters produces a conical scan that sweeps a 1,000-kilometer swath.

The radar transmitter failed in July 2015, several months after launch. Engineers repurposed the radar receiver to capture reflected signals from Global Navigation Satellite System (GNSS) constellations, making SMAP the first full-polarimetric GNSS reflectometer in space. The radiometer continues to operate nominally and delivers the primary soil moisture data products. Details on the current instrument configuration and data products are maintained by NASA's SMAP mission team at JPL.

Orbital Operations and Data Coverage

SMAP flies in a near-polar, sun-synchronous orbit at an altitude of approximately 685 kilometers, with a 98.5-degree inclination. The orbit geometry causes the satellite to cross each point on Earth's surface at roughly the same local solar time on successive passes, reducing confounding effects from diurnal temperature variation. The 1,000-kilometer swath width means the satellite achieves global coverage in two to three days, with an exact 8-day repeat cycle.

Data are acquired continuously and downlinked through NASA's ground stations. The mission produces several standard data products at spatial resolutions ranging from 3 kilometers (radar-enhanced) to 36 kilometers (radiometer-only), with daily and time-averaged composites. All Level-2 through Level-4 products are freely available through the National Snow and Ice Data Center, which serves as the primary archive and distribution point.

Scientific Applications

SMAP data have found operational and research use across hydrology, agriculture, weather forecasting, and climate science. The U.S. Department of Agriculture uses SMAP soil moisture observations operationally to monitor global crop conditions and assess drought severity. Numerical weather prediction centers at NOAA and the European Centre for Medium-Range Weather Forecasts have assimilated SMAP measurements to improve initial soil moisture states in atmospheric models, with measurable gains in forecast accuracy over land. Flood prediction models use SMAP data to identify saturated soils that raise runoff risk before storm events. More broadly, the mission contributes to NASA's program of Earth science missions that track long-term changes in the global water cycle.

SMAP has applications across a range of fields, including:

• Agricultural drought monitoring and crop yield forecasting
• Operational numerical weather prediction
• Flood risk assessment and early warning
• Permafrost and freeze/thaw monitoring in high latitudes
• Carbon flux modeling for climate research