# L-band microwave radiometry
*sensing . methodologies*

Measures naturally emitted microwave radiation in the protected L-band near 1.4 GHz (about 21 cm wavelength), where the atmosphere is nearly transparent and the signal carries information from the top few centimetres of soil. The primary spaceborne source of global surface soil moisture and sea surface salinity.

## Specifications
- **family**: Passive microwave
- **requirements envelope**: {"kind":"spectral","spectral_range_nm":{"min":210000000,"max":215000000},"bands_min":1,"calibration_tier_min":"radiometric","snr_min":10,"daylight_required":false,"cloud_tolerant":true}
- **entity type**: methodology
- **last verified date**: 2026-06-05
- **verified by**: sw
- **claim status**: unclaimed
- **subtype**: sensing
- **attributes**: {"family":"Passive microwave","kind":"sensing","summary":"Measures naturally emitted microwave radiation in the protected L-band near 1.4 GHz (about 21 cm wavelength), where the atmosphere is nearly transparent and the signal carries information from the top few centimetres of soil. The primary spaceborne source of global surface soil moisture and sea surface salinity.","requirements_envelope":"{\"kind\":\"spectral\",\"spectral_range_nm\":{\"min\":210000000,\"max\":215000000},\"bands_min\":1,\"calibration_tier_min\":\"radiometric\",\"snr_min\":10,\"daylight_required\":false,\"cloud_tolerant\":true}"}
- **technology**: microwave-radiometry

## Editorial
L-band radiometry measures natural thermal emission in the protected radio-astronomy band from 1.400 to 1.427 GHz, a wavelength near 21 cm. At this frequency the atmosphere is almost transparent and the emission originates several centimetres below the surface, deeper than the higher imaging frequencies reach. That depth of penetration is what makes the band uniquely sensitive to near-surface soil moisture and, over the ocean, to sea surface salinity.[^smap-handbook]

The retrieval inverts measured brightness temperature against a radiative-transfer emission model, correcting for vegetation water content and surface temperature. The long wavelength fixes a coarse native resolution of roughly 35 to 40 km, so the method underpins global and regional water-cycle monitoring rather than field-scale mapping. It is independent of cloud cover and solar illumination, giving consistent day-and-night, all-weather revisit.

Two spaceborne architectures reach the same measurement by different routes. A real-aperture design uses a large rotating mesh reflector to sweep a wide swath (NASA SMAP).[^smap-handbook] An aperture-synthesis design reconstructs the brightness-temperature field interferometrically from a sparse Y-shaped array of small antennas, removing the need for a single large dish (ESA SMOS, carrying the MIRAS payload).[^smos]

## Sources
- [smap-handbook] | NASA SMAP Mission (Soil Moisture Active Passive) | https://smap.jpl.nasa.gov/ | tier=agency-doc | accessed=2026-06-05
- [smos] | ESA SMOS Mission (Soil Moisture and Ocean Salinity) | https://www.esa.int/Applications/Observing_the_Earth/SMOS | tier=agency-doc | accessed=2026-06-05

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Source: https://eo-atlas.org/methodologies/l-band-microwave-radiometry
Maintainer: SpectraWorks B.V. (CC-BY 4.0)