Microwave radiometry

Microwave radiometry is the passive measurement of naturally emitted thermal microwave radiation from Earth's surface and atmosphere, quantified as brightness temperature (TB, in Kelvin). Because surface emission at microwave frequencies is governed by physical temperature and emissivity rather than reflected sunlight, the technique operates in all weather conditions, day and night.

Sensors measure emitted power at frequencies of approximately 1-90 GHz (wavelengths approximately 3 mm to 30 cm). Brightness temperature is related to physical temperature and surface emissivity via the Rayleigh-Jeans approximation: TB equals emissivity times physical temperature, plus a reflected sky contribution. Emissivity varies with soil dielectric constant (sensitive to moisture content), surface roughness, vegetation structure, snow, and ice type. Multi-frequency TB observations are inverted using radiative transfer models (the tau-omega model for vegetation canopy, MEMLS for snowpack) to retrieve geophysical state variables. L-band (1.4 GHz) is optimal for soil moisture, probing approximately 5 cm depth; shorter wavelengths probe snow water equivalent, sea ice, and precipitation.

Representative missions include SMOS (ESA, L-band 1.4 GHz interferometric radiometer, launched 2009, soil moisture and ocean salinity, 40 km resolution), SMAP (NASA, L-band 1.4 GHz, launched January 2015, soil moisture and freeze/thaw state, 36 km native resolution), AMSR2 (JAXA, 6.9-89 GHz multi-frequency, 2012 onward), SSM/I and SSMIS (DMSP series, 19.3-91.7 GHz, operational since 1987), and GMI (NASA/JAXA on GPM Core Observatory, 10-183 GHz, 2014 onward). The upcoming Copernicus CIMR mission extends multi-band coverage with L+C+X+K+Ka channels.

Key applications include global soil moisture (SMAP unbiased RMSE approximately 0.04 m3/m3, correlation 0.7-0.81), ocean salinity, sea ice concentration and extent, snow water equivalent, precipitation rate, sea surface temperature, atmospheric water vapour and cloud liquid water, vegetation optical depth as an above-ground biomass proxy, and freeze-thaw state detection.

The technology's principal strengths are all-weather penetration through clouds and light rain, day/night operation, and sub-surface sensitivity at long wavelengths. Limitations include coarse spatial resolution at long wavelengths (approximately 10-100 km), radio frequency interference particularly at L-band from ground transmitters, and signal attenuation from dense vegetation that can overwhelm the soil moisture signal.