L-band microwave radiometry

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.^[1]

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).^[1] 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).^[2]