## 2 Site Study and Database
### Data  
As described, there have been numerous remote sensing satellite missions for measuring and monitoring soil moisture around the world. This paper focuses on data sets derived from two recent missions: SMAP and CYGNSS. The SMAP satellite was launched on 31 January 2015 with the objective of producing global soil moisture mapping and landscape freeze/thaw state via the L-band radar and radiometer employed [36]. The radiometer observes microwave emissions from the Earth's surface through a 6 m antenna, measuring both vertical and horizon polarization. The observation results are provided by four levels of data product with various special resolutions being derived by the scientific objective for each level and available to access through the Alaska Satellite Facility (ASF) and the National Snow and Ice Data Center (NSIDC) website [37]. This research uses Level 3 version 6 soil moisture data with 36 km \(\times\) 36 km spatial resolution being drawn on same distance of geophysical latitude and longitude, called the Equal-Area Scalable Grid (EASEGrid) [38]. Additionally, a quality flag indicating the condition of the data accompanying SMAP metadata is used to acquire qualified and validated dielectric constant. The detailed process can be found in Section 4. CYGNSS performing GNSS-R technology consists of eight nanosatellites for observing wind speed over the Earth's tropical oceans to understand and predict the characteristics of hurricanes. The satellites were launched on 15 December 2016, orbiting an equatorial region with an inclination of 35\({}^{\circ}\), 514 km perigee, and 536 km apogee. Each satellite's orbit limits the instrument surface coverage to a latitude range of \(\pm\)38\({}^{\circ}\). The observations from each satellite have been made available by the Physical Oceanography Distributed Active Archive Center (PODAAC) ([[https://podaac.jpl.nasa.gov](https://podaac.jpl.nasa.gov)]([https://podaac.jpl.nasa.gov](https://podaac.jpl.nasa.gov)) (accessed on 15 May 2021)) [39] since March 2017. Each CYGNSS satellite contains a Delay Doppler Mapping Instrument (DDMI), which measures both the direct GPS signals and the GPS signals that are reflected from Earth's surface. The DDMI can process up to four reflections simultaneously. The antenna on each satellite is capable of sensing the Right Hand Circular Polarization (RHCP) signals in the zenith direction and the Left Hand Circular Polarization (LHCP) signals in the nadir direction for the GPS L1 frequency. The observed signals are compressed and downloaded to the ground station, and then a post-processing procedure is applied to generate scientific observables satisfying the mission objective. There are three accessible data levels published for the community. Level 1 (L1), version 2.1 data are used for the quantitative dielectric constant analysis in this study. Each  
Figure 1: Global image of International Geosphere–Biosphere Programme (IGBP) Land Classification, including water, categorized by using Moderate Resolution Imaging Spectroradiometer (MODIS) data. observation file contains a number of observables and metadata, including Delay Doppler Maps (DDM--scattered power from specular point), as well as geometry information arranged by GPS satellite as transmitter, receiver, and specular power stored in daily NetCDF files. Detailed specifications and processing schemes can be found in [40]. The DDM is the fundamental product that is related to scattering physics affected by scattering geometry and conditions; this information is essential in progressing the analysis of GNSS-R data. Figure 2 provides a DDM example using CYGNSS data from 1 January 2019. CYGNSS data have been used for a variety of geophysical research, because the provided metadata include reflection geometry. The GNSS-R bistatic configurability differs from other active/passive remote sensing instruments, allowing for high surface spatial resolution. The first Fresnel zone or first glistening zone is regarded as the reflection zone for obtaining spatial resolution for a GNSS-R instrument.