# SMAP Radiometer
*sensor . products*

L-band passive microwave radiometer on NASA's SMAP spacecraft. Conically scanning real-aperture instrument sharing a 6-m deployable mesh reflector with the now-failed radar. Measures fully polarimetric brightness temperature at 1.413 GHz across a 1000-km swath at ~40 km resolution to retrieve global surface soil moisture and freeze-thaw state.

## Specifications
- **manufacturer**: nasa
- **product status**: operational
- **deployment context**: orbital
- **sensor**: {"instrument_family":"radiometer","measurement_principle":"passive","frequency_bands_ghz":[1.413],"polarisation_modes":["H","V","T3","T4"],"scan_type":"conical","scan_rate_rpm":14.6,"incidence_angle_deg":40,"antenna_diameter_m":6,"swath_km":1000,"resolution_per_band_km":[{"frequency_ghz":1.413,"footprint_km":"36x47"}],"radiometric_sensitivity_k":1,"notes":"NEDT < 1 K for 17-ms samples; orbit-average 0.90 K (H-pol) and 0.96 K (V-pol). T3 and T4 NEDT is sqrt(2) larger. Shares 6-m offset mesh reflector with the SMAP SAR radar (which failed July 2015); radiometer continues nominal operations. Operating range 1.4015-1.4255 GHz. IFOV 36x47 km; effective FOV 39x47 km after along-scan averaging. Built by NASA GSFC."}
- **entity type**: sensor
- **last verified date**: 2026-06-05
- **verified by**: agency-doc
- **claim status**: agency-sourced
- **provider**: nasa
- **attributes**: {"kind":"sensor","manufacturer":"nasa","description":"L-band passive microwave radiometer on NASA's SMAP spacecraft. Conically scanning real-aperture instrument sharing a 6-m deployable mesh reflector with the now-failed radar. Measures fully polarimetric brightness temperature at 1.413 GHz across a 1000-km swath at ~40 km resolution to retrieve global surface soil moisture and freeze-thaw state.","product_status":"operational","deployment_context":"orbital","sensor":"{\"instrument_family\":\"radiometer\",\"measurement_principle\":\"passive\",\"frequency_bands_ghz\":[1.413],\"polarisation_modes\":[\"H\",\"V\",\"T3\",\"T4\"],\"scan_type\":\"conical\",\"scan_rate_rpm\":14.6,\"incidence_angle_deg\":40,\"antenna_diameter_m\":6,\"swath_km\":1000,\"resolution_per_band_km\":[{\"frequency_ghz\":1.413,\"footprint_km\":\"36x47\"}],\"radiometric_sensitivity_k\":1,\"notes\":\"NEDT < 1 K for 17-ms samples; orbit-average 0.90 K (H-pol) and 0.96 K (V-pol). T3 and T4 NEDT is sqrt(2) larger. Shares 6-m offset mesh reflector with the SMAP SAR radar (which failed July 2015); radiometer continues nominal operations. Operating range 1.4015-1.4255 GHz. IFOV 36x47 km; effective FOV 39x47 km after along-scan averaging. Built by NASA GSFC.\"}"}
- **technology**: microwave-radiometry

## Editorial
The SMAP Radiometer is the L-band passive microwave radiometer carried by NASA's Soil Moisture Active/Passive (SMAP) spacecraft. It operates at 1.413 GHz and uses a conically scanning real-aperture design to measure brightness temperature across the Earth's surface.

The instrument shares a 6-metre deployable mesh reflector with the SMAP synthetic aperture radar, which failed in July 2015. The radiometer has continued nominal operations since [^smap-jpl]. The reflector is offset and spin-stabilised at 14.6 rpm, producing a continuous conical scan at a fixed Earth incidence angle of 40 degrees. The 1000-km swath achieves near-global coverage every two to three days [^smap-radiometer-pmc].

SMAP measures all four Stokes parameters, making it a fully polarimetric L-band radiometer [^smap-radiometer-pmc]. The instrument frequency range spans 1.4015 to 1.4255 GHz, within the protected passive band [^smap-radiometer-pmc]. Individual footprints are 36 by 47 km (effective 39 by 47 km after along-scan averaging) [^smap-radiometer-pmc]. Radiometric noise equivalent delta-T is below 1 K for 17-millisecond samples, with orbit-average NEDT of 0.90 K (H-polarisation) and 0.96 K (V-polarisation) [^smap-radiometer-pmc].

The radiometer was designed and built by NASA Goddard Space Flight Center [^smap-radiometer-pmc]. Science requirements specify a radiometric uncertainty of 1.3 K absolute accuracy [^smap-radiometer-pmc] to support soil moisture and freeze-thaw retrieval. Primary geophysical products are surface soil moisture to 0.04 cubic metres per cubic metre accuracy [^smap-radiometer-pmc] and a binary freeze-thaw state flag [^smap-radiometer-pmc], both delivered as global gridded products at approximately 40-km spatial resolution [^smap-jpl-specs].

## Compositional position
- smap (missions) --[mission_payloads]--> SMAP Radiometer
- smap-rss-l3-sea-surface-salinity-8day (data_products) --[related]--> SMAP Radiometer

## Sources
- [smap-radiometer-pmc] | SMAP L-Band Microwave Radiometer: Instrument Design and First Year on Orbit | https://pmc.ncbi.nlm.nih.gov/articles/PMC7357195/ | tier=peer-reviewed | accessed=2026-06-05 | author=Piepmeier et al. | published=2017-01-01
- [eoportal-smap] | SMAP (Soil Moisture Active/Passive) - eoPortal | https://www.eoportal.org/satellite-missions/smap | tier=community | accessed=2026-06-05
- [smap-jpl] | SMAP Instrument - NASA JPL SMAP Mission | https://smap.jpl.nasa.gov/observatory/instrument/ | tier=agency-doc | accessed=2026-06-05
- [smap-jpl-specs] | SMAP Observatory Specifications - NASA JPL | https://smap.jpl.nasa.gov/observatory/specifications/ | tier=agency-doc | accessed=2026-06-05

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Source: https://eo-atlas.org/products/sensor/smap-radiometer
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