SMAP

SMAP was designed to map near-surface soil moisture globally with complementary microwave instruments. The spacecraft carries a passive L-band radiometer and an active L-band radar; together, they were intended to combine the radiometer’s sensitivity with the radar’s finer spatial detail. SMAP launched from Vandenberg Air Force Base on 31 January 2015 into a near-polar, sun-synchronous orbit at roughly 685 kilometers. Its measurements target the top few centimeters of soil, where water availability changes rapidly and strongly affects evaporation, vegetation, and runoff. The mission’s orbital design enables near-global mapping every two to three days, while avoiding much of the interference that affects observations at lower microwave frequencies.¹

SMAP converts natural microwave emission from land into estimates of volumetric soil-water content. The radiometer provides broad-area observations at about 36-kilometer resolution, while the radar was intended to sharpen mapping to approximately 3 kilometers; an important planned product would have blended both measurements at roughly 9 kilometers. The radar stopped transmitting in July 2015 after an electronics failure, but the radiometer continued to produce operational soil-moisture observations.² Later processing has also supported enhanced-resolution radiometer products, generally near 9 kilometers, by combining measurements with spatial information from land-surface models and ancillary data. Public archives include calibrated brightness temperatures, soil moisture, freeze-thaw information, quality flags, and derived composites.¹

SMAP soil-moisture data improve estimates of drought, flood potential, crop stress, and land-atmosphere exchange. Soil water controls how rainfall is divided among infiltration, runoff, evaporation, and plant uptake, so assimilating SMAP observations into numerical models can improve weather, hydrologic, and agricultural forecasts.³ The data are also used to monitor the onset and recovery of drought, identify unusually wet ground before flooding, and constrain estimates of carbon and water cycling. A specialized SMAP product measures the freeze-thaw state of the landscape, which is relevant to boreal ecosystems and the seasonal timing of biological activity. Researchers commonly combine the satellite record with ground networks, vegetation observations, precipitation datasets, and land-surface models because microwave retrievals are affected by vegetation, surface roughness, frozen ground, and radio-frequency interference.

SMAP’s most distinctive legacy is that its science program continued after the loss of the radar, rather than ending with the instrument failure. The surviving radiometer has a relatively coarse footprint, but its stable global record remains valuable for climate studies and model evaluation.² Soil-moisture retrieval is also an indirect measurement: the instrument senses microwave brightness temperature, and algorithms infer water content while accounting for soil texture, vegetation, temperature, and surface conditions. Retrievals become more difficult under dense forests, deep snow, frozen soil, wetlands, and urban surfaces. SMAP data have additionally contributed to estimates of root-zone moisture and hydrologic states when assimilated into models, although those deeper-layer estimates are model-informed rather than direct observations. The mission therefore illustrates both the power and limits of satellite remote sensing.³

L-band radiometer

A passive microwave instrument operating near 1.4 gigahertz that measures natural thermal emission from Earth’s surface.

Soil moisture

The amount of water contained in soil, commonly expressed as volumetric water content.

Brightness temperature

A temperature-equivalent measure of microwave radiation emitted by a surface.

Freeze-thaw state

Whether land, especially near the surface, is frozen or unfrozen; SMAP derives this condition from microwave observations.