# Soil Moisture Retrieval from Multi-GNSS Reflectometry on FY-3E GNOS-II by Land Cover Classification
## 2 Materials  
### GNOS-II Data and Observable  
The retrieval model for soil moisture was developed based on GNOS-II Level 1 data spanning approximately one year, from July 2021 to June 2022. Data from July 2022 to October 2022 were than used for validation. The mean revisit time (temporal resolution) at 36 km is around 12 days when the AM/PM overpasses are combined for the retrieved GNOS-R soil moisture. The GNSS signals received by the instrument are GPS L1 C/A, BDSBII and GAL E1B. Table 1 shows the differences between the three types of signals. Note that GAL signals are received in a subcarrier mode, resulting an equivalent chipping rate of 2.046 MHz [26]. The coherent integration time is 1 ms, and the incoherent integration time is 1 s.  
In GNOS-II Level 1 data, the reflected GNSS signal is processed into Delay-Doppler Map (DDM) in raw counts. Different from previous GNSS-R missions, GNOS-II uses 122 \(\times\) 20 non-uniform Delay Doppler maps, where 1/8 chip delay interval is used for delay around the specular point between [\(-\)3, 2.875] chips, and the 1/4 chip interval is used for the rest delay range. This enables high precision measurement around the specular point while achieving the delay range of [\(-\)12.25, 12.125]. Some examples of DDMs over land from BDS/GPS/GAL channels are shown in Figure 1. The samples are collected from a plain area, so the energy is concentrated at the peak. Different from the DDMs of GPS and BDS systems, the GAL DDM waveform has two side-lobes due to the BOC modulation. In addition to the observed DDM waveforms, metadata could also be extracted from the Level 1 data, including the DDM observation time in UTC, the position of receiver and transmitter satellites, the latitude and longitude of specular points, the PRN code of GNSS satellites, the gain of the reflected antenna, the incidence angle of observations, etc.  
Generally, it is assumed that the coherent reflections predominate in the GNSS-R signal over land, so the spatial resolution will be restricted to the first Fresnel zone [12]. At an altitude of around 830 km, the cross-track spatial resolution of the GNOS-II measurements of land surface is about 1 km. However, due to the 1 s incoherent integration time and the fact that the specular point travels around 5.8 km on the earth surface in one second, the resolution in the along-track direction is around 6 km.  
A flag identifying the surface type in the Level 1 data was used to select DDMs acquired over land, along with various quality flags. Since GNOS-R prioritizes sea surface wind monitoring, the digital elevation map was not employed in the receiver during the in-orbit testing phase. Considering that observations of highlands are now unavailable, instead of using the delay offset flag in the Level 1 data to eliminate these observations, the delay offset and the difference between the estimated peak delay and observed DDM peak delay were calculated using Equations (1) and (2).  
\[delay\_offset\ =\ -2\cdot h\cdot\cos(\theta_{inc})/chip\_length \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **GPS L1 C/A** & **BDS BII** & **GAL E1B** \\ \hline Frequency (MHz) & 1575.42 & 1561.098 & 1575.42 \\ Modulation & BPSK & BPSK & BOC(1,1) \\ Chipping rate (Mcps) & 1.023 & 2.046 & 2.046 \\ Code length (ms) & 1 & 1 & 4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristic of different GNSS signals received by GNOS-II.  
Figure 1: DDMs over the land surface from (**a**) BDS C29, (**b**) GPS PRN20 and (**c**) GAL E11.  
where \(h\) represents the height from the Digital Elevation Model (DEM) at a resolution of 10 km, \(\theta_{inc}\) represents the incidence angle; the _chip_length_ is 150 m for BDS and GAL, and 300 m for GPS.  
The difference between the calculated _delay_offset_ and _ddm_peak_delay_ is  
\[delay\_diff=delay\_offset-ddm\_peak\_delay \tag{2}\]  
Only DDMs with an absolute value of _delay_offset_ less than 12 chips and an absolute value of _delay_diff_ less than 3 chips are exploited to ensure that the observations are valid.