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Soil Moisture Active Passive (SMAP) is an United States environmental research satellite launched on 31 January 2015. It was one of the first Earth observation satellites developed by NASA in response to the National Research Council’s Decadal Survey.
SMAP provides measurements of the land surface soil moisture and freeze-thaw state with near-global revisit coverage in 2–3 days. SMAP surface measurements are coupled with hydrologic models to infer soil moisture conditions in the root zone. These measurements enable science applications users to:
SMAP observations will be acquired for a period of at least three years after launch. A comprehensive validation, science, and applications program will be implemented, and all data are publicly available through the NASA archive centers.
The SMAP observatory includes a dedicated spacecraft and instrument suite in a near-polar, Sun-synchronous orbit. The SMAP measurement system consists of a radiometer (passive) instrument and a synthetic aperture radar (active) instrument operating with multiple polarizations in the L-band range. The combined active and passive measurement approach takes advantage of the spatial resolution of the radar and the sensing accuracy of the radiometer.[1]
The active and passive sensors provide coincident measurements of the surface emission and backscatter. The instruments sense conditions in the top 5 cm of soil through moderate vegetation cover to yield globally mapped estimates of soil moisture and its freeze-thaw state.
The satellite carries two scientific instruments: a radar and a radiometer, that share a single feed and deployable 6m reflector antenna system that rotates around the nadir axis making conical scans of the surface. The wide swath provides near-global revisit every 2–3 days.
Characteristic | Radar | Radiometer |
---|---|---|
Frequency | 1.2 GHz | 1.41 GHz |
Polarizations | VV, HH, HV | V, H, U |
Resolution | 1–3 km[1] | 40 km |
Antenna diameter | 6 m | |
Rotation rate | 14.6 rpm | |
Incidence angle | 40° | |
Swath width | 1000 km | |
Orbit | Polar, Sun-synchronous | |
Local time asc. node | 6 am | |
Altitude | 670 km |
SMAP is a directed mission of the National Aeronautics and Space Administration NASA. The SMAP project is managed for NASA by the Jet Propulsion Laboratory JPL, with participation by the Goddard Space Flight Center GSFC. SMAP builds on the heritage and risk reduction activities of NASA's cancelled ESSP Hydros Mission.[2]
SMAP observations are used to characterize hydrologic and ecosystem processes including land-atmosphere exchanges of water, energy, and carbon. Among the users of SMAP data are hydrologists, weather forecasters, climate scientists and agricultural and water resource managers.[3] Additional users include fire hazard and flood disaster managers, disease control and prevention managers, emergency planners and policy makers.[3] SMAP soil moisture and freeze-thaw information will directly benefit several societal applications areas, including:
Initialization of numerical weather prediction models and seasonal climate models with accurate soil moisture information will extend forecast lead times and enhance prediction skill.
SMAP soil moisture information will improve the monitoring and forecasting of drought conditions, enabling new capabilities for mitigating drought impacts.
Hydrologic forecast systems calibrated and initialized with high-resolution soil moisture fields will lead to improved flood forecasts and will provide essential information on the potential for landslides.
Soil moisture observations from SMAP will lead to improvements in crop yield forecasts and will enhance the capabilities of crop water stress decision support systems for agricultural productivity.[3]
Improved seasonal soil moisture forecasts will directly benefit famine early warning systems. Benefits will also be realized through improved predictions of heat stress and virus spread rates, and improved disaster preparation and response.
In August 2015, scientists completed their initial calibration of the two instruments on board, however, SMAP's radar stopped transmitting July 7 due to an anomaly that was investigated by a team at JPL.[4] The team identified the anomaly to the power supply for the radar's high-power amplifier.[5][6] On 2 September 2015, NASA announced that the amplifier failure meant that the radar could no longer return data. The science mission continues with data being returned only by the radiometer instrument.[7]