SMAP Radiometer

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 ^[1]. 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 ^[2].

SMAP measures all four Stokes parameters, making it a fully polarimetric L-band radiometer ^[2]. The instrument frequency range spans 1.4015 to 1.4255 GHz, within the protected passive band ^[2]. Individual footprints are 36 by 47 km (effective 39 by 47 km after along-scan averaging) ^[2]. 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) ^[2].

The radiometer was designed and built by NASA Goddard Space Flight Center ^[2]. Science requirements specify a radiometric uncertainty of 1.3 K absolute accuracy ^[2] to support soil moisture and freeze-thaw retrieval. Primary geophysical products are surface soil moisture to 0.04 cubic metres per cubic metre accuracy ^[2] and a binary freeze-thaw state flag ^[2], both delivered as global gridded products at approximately 40-km spatial resolution ^[3].