*2.3. Soil Moisture Active Passive Data*

The reference products utilized in this study primarily originate from the SMAP satellite, launched by NASA in 2015. The primary mission of this satellite is to monitor global surface SM and freeze-thaw states, aiming to gain a deeper understanding and knowledge of the Earth's surface water cycle, climate change, ecosystem dynamics, and the impact of human activities. The SMAP satellite employs an L-band radiometer for its observations, a device capable of penetrating clouds and most vegetation to directly measure microwave radiation from the ground, thereby inferring SM and freeze-thaw states. The SMAP satellite revisits each location every 2–3 days, offering a very short global coverage cycle. Notably, the SMAP satellite carries out two types of observations: ascending (6:00 a.m.) and descending (6:00 p.m.) [35]. This design allows for comparisons and analyses at different times for the same location, providing more comprehensive information.

The data used in this study is obtained from the National Snow and Ice Data Center (NSIDC, https://nsidc.org/, accessed on 15 April 2023). We selected the SMAP Level-3 (L3) Radiometer Global Daily 36 km EASE-Grid Soil Moisture (Version 8, SPL3SMP) as the reference product. This product offers daily estimates of global land surface conditions. The data derived from SMAP's L-band are resampled to a global, cylindrical, 36-km Equal-Area Scalable Earth Grid. The data period is from 1 January to 31 August 2019, providing ample samples for our study.

The original format of the SMAP product is HDF5. In this study, we use the HEG tool (HDFEOS To GeoTIFF Conversion Tool) to convert it into an easily processed Geotiff data format.
