## 1 Introduction  
Spatial patterns and the timing of landscape freeze/thaw (FT) state transitions have a major influence on eco-hydroological processes where frozen temperatures are a significant part of the annual cycle [1; 2; 3]. The landscape FT signal determined from microwave remote sensing is related to changes in the surface energy budget, which also influences permafrost extent and stability, terrestrial carbon and water budgets, and land-atmosphere trace gas exchanges [3; 4; 5]. The FT state parameter is a key component of the National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) and European Space Agency (ESA) Soil Moisture Ocean Salinity (SMOS) missions [6; 7; 8]. Both missions provide L-band (~1.4 GHz) microwave radiometer brightness temperature retrievals with potentially enhanced sensitivity to soil FT conditions compared to higher frequency passive microwave observations from other current and historical satellite sensors [7; 9], but with different sensor configurations and sampling footprints. The NASA SMAP mission, launched in January 2015, included both fine resolution (~1 to 3 km) L-band radar (1.26 GHz) and moderate resolution (~40 km) radiometer (1.41 GHz) measurements. The SMAP satellite has a 685 km altitude and sun-synchronous orbit with local 6 AM/PM equator crossings. The SMAP radar and radiometer share a single feedhorn and parabolic mesh reflector that provides a conical-scan antenna beam. A constant 40\({}^{\circ}\) measurement incidence angle across the swath simplifies data processing and enables accurate repeat-pass estimation of FT dynamics. Unfortunately, the SMAP radar failed in July 2015. The SMAP mission subsequently shifted to the production of operational algorithms and land products emphasizing the use of L-band brightness temperature (Tb) retrievals from the SMAP radiometer, which continues normal operations. The SMAP radiometer has an approximate 40 km sensor footprint and provides global coverage and temporal repeat observations of 3 days or less between 35\({}^{\circ}\)N and 35\({}^{\circ}\)S, and 2 days or less poleward of 55\({}^{\circ}\)N/S. The initial SMAP Level 3 FT algorithm and operational product (L3_FT_A) was derived from SMAP radar backscatter retrievals and encompassed a northern (\(\geq\)45\({}^{\circ}\)N) polar-grid domain [7]. Following the SMAP radar instrument failure, the operational FT algorithms were modified to utilize the radiometer Tb retrievals as primary inputs for the FT classification. The SMAP radiometer-based FT products continue to be refined to better address the mission requirements and science objectives [8]. The latest version (R16) of the SMAP operational L3_FT products are derived solely from the SMAP Tb retrievals using a consistent FT algorithm framework, but with different product formats designed for diverse applications. The L3_FT product formats include both global and northern polar-grid domains, and spatial gridding at 36 km and 9 km resolutions. The 36 km grid is closer to the native footprint of the SMAP Tb retrievals, whereas the enhanced resolution (9 km) grid product is derived from optimally interpolated SMAP Tb retrievals [10]. The satellite microwave FT signal detects the frequency-dependent shift in surface dielectric properties associated with changes in the relative abundance of liquid water that occur as the landscape transitions between predominantly frozen and unfrozen conditions [1]. The SMAP retrieval represents the bulk landscape FT signal and does not distinguish individual contributions from soil, snow, vegetation, and surface water components within the sensor footprint. The SMAP FT retrieval is expected to have a larger contribution of soil emissions at lower (L-band) microwave frequencies [7] compared to higher frequency (~37 GHz) Tb measurements from other available sensors, which are more sensitive to surface temperature related FT variations [11].