This paper describes an instrument designed to distinguish frozen from thawed land surfaces from an Earth satellite by bouncing signals back to Earth from deployable mesh antennas.

The Soil Moisture Active-Passive (SMAP) L-band microwave radiometer is a conical scanning instrument designed to measure soil moisture with 4% volumetric accuracy at 40-km spatial resolution. SMAP is NASA's first Earth Systematic Mission developed in response to its first Earth science decadal survey. Here, the design is reviewed and the results of its first year on orbit are presented. Unique features of the radiometer include a large 6-m rotating reflector, fully polarimetric radiometer receiver with internal calibration, and radio-frequency interference detection and filtering hardware. The radiometer electronics are thermally controlled to achieve good radiometric stability. Analyses of on-orbit results indicate that the electrical and thermal characteristics of the electronics and internal calibration sources are very stable and promote excellent gain stability. Radiometer NEDT < 1 K for 17-ms samples. The gain spectrum exhibits low noise at frequencies >1 MHz and 1/f noise rising at longer time scales fully captured by the Piepmeier et al.

A geosynchronous synthetic aperture radar (SAR) with an orbit inclination of 50-65° can provide daily coverage of all of North and South America. Longitudinally, the width of the mapped area would be on the order of ±50° at the Equator, somewhat more at the most northern/southern latitudes. Within the area mapped, very good temporal coverage can be obtained -up to several mappings during the 12 hours per day where the satellite is in the "right" hemisphere. This would be a key capability in relation to disaster management, tectonic mapping and modeling, vegetation and soil moisture mapping, and for operational and semi-operational requirements. A constellation of geosynchronous satellites could provide global coverage.

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