## Materials and Methods
### Remote Sensing Datasets
#### 2.2.2 Global Soil Moisture Products  
To better evaluate the performance of the soil moisture retrieved by the Sentinel-1A dataset, several typical microwave soil moisture products were selected for comparison and evaluation at a coarse resolution, including SMOS, SMAP, ASCAT, and AMSR-2. To ensure that the different products were compatible, the above products were resampled to 0.25\({}^{\circ}\) and were registered to the AMSR2 grid.  
Launched on 2 November 2009, SMOS's L-band Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) radiometer picks up microwave emissions from the Earth's surface to map levels of land soil moisture and ocean salinity. SMOS scans the global surface twice every three days at 6:00 a.m. (ascending) and 6:00 p.m. (descending) equatorial local crossing time [37]. The SMOS Level 3 daily soil moisture product with a resolution of 25 km on descending orbit was used in this study ([[http://www.catds.fr/Products/Products-access](http://www.catds.fr/Products/Products-access)]([http://www.catds.fr/Products/Products-access](http://www.catds.fr/Products/Products-access))).  
SMAP was launched on 31 January 2015 with radar and radiometer sensors both in the L-band at full polarization carried on the spacecraft. The instruments share a rotating 6-m mesh reflector antenna on a platform in a 685-km sun-synchronous near-polar orbit, viewing the Earth's surface at a constant 40-degree incidence angle with a 1000-km swath width [38]. The mission intends to provide a capability for the global mapping of soil moisture and landscape freeze/thaw state with a maximum revisit time of three days. However, the radar sensor failed in July 2015. Therefore, only the SMAP Level 3 passive soil moisture product with a resolution of 36 km at 6:00 A.M. local time was used in this study ([[https://nsidc.org/data/smap/smap-data.html](https://nsidc.org/data/smap/smap-data.html)]([https://nsidc.org/data/smap/smap-data.html](https://nsidc.org/data/smap/smap-data.html))).  
AMSR2, which continues the observations of AMSR-E, was launched on 18 May 2012 on the Global Change Observation Mission (GCOM)-W1 satellite. It operates at multiple dual-polarized frequencies ranging from 6.9 to 89 GHz in an ascending (13:30 local time) and descending (01:30 local time) mode. When compared with AMSR-E, the new mission has a larger main reflector (2.0 m), additional channels at the C-band (7.3 GHz), an improved calibration system, and an increased reliability [39; 40]. The AMSR-2 Level 3 soil moisture product in descending mode was used in this study ([[http://gcom-w1.jaxa.jp/index.html](http://gcom-w1.jaxa.jp/index.html)]([http://gcom-w1.jaxa.jp/index.html](http://gcom-w1.jaxa.jp/index.html))).  
The Advanced SCATreometer (ASCAT) is a radar instrument onboard the Meteorological Operational Platform (Metop) series of satellites: Metop-A was launched in October 2006 and Metop-B in September 2012; the third and last satellite (Metop-C) is expected to be launched in 2018 supersedingMetop-A [41]. The radar operates in the C-band (5.255 GHz) at VV polarization and scans the Earth's surface in a descending (9:30 local time) and ascending (21:30 local time) mode. The ASCAT soil moisture is estimated using the method developed by Technische Universitat Wien (TU Wien) [42; 43; 44], which provides a soil moisture fraction between completely dry conditions (0%) and full saturation (100%) for the topsoil. The ASCAT Level 2 soil moisture product with a resolution of 25 km on ascending orbit was used in this study ([[https://www.eumetsat.int/website/home/Data/Products/Land/index.html](https://www.eumetsat.int/website/home/Data/Products/Land/index.html)]([https://www.eumetsat.int/website/home/Data/Products/Land/index.html](https://www.eumetsat.int/website/home/Data/Products/Land/index.html))). To allow for a comparison with the other products, the ASCAT measurements were converted to volumetric soil moisture using the soil texture information.