# Research on Shore-Based River Flow Velocity Inversion Model Using GNSS-R Raw Data
## 3 Shore-Based Velocity Inversion Experiment
### Experimental Configuration  
The hardware used in the experiment was a miniature GNSS-R detector designed and manufactured by Shanghai Aerospace Electronics Institute, which is made of a left-hand circular polarization antenna (LHCP), right-hand circular polarization antenna (RHCP), and hardware delay/Doppler-mapping receiver (DDMR). The device can receive GPS L1 and BDS B1I signals simultaneously. A contact velocity-detecting instrument was used to record the average flow velocity from 9:30 a.m. to 11:30 a.m. (LT) every 5 min, which will be viewed as real velocity to assess inversion precision. To meet the experimental requirements, the reflecting antenna of the receiver can receive the reflected signals in the area from negative 90 degrees to positive 90 degrees, \begin{table}
\begin{tabular}{c c} \hline \hline
**Attributes** & **Value** \\ \hline Antenna latitude & 31\({}^{\circ}\)57\({}^{\prime}\)43\({}^{\prime\prime}\) N \\ Antenna longitude & 118\({}^{\circ}\)38\({}^{\prime}\)27\({}^{\prime\prime}\) E \\ Antenna azimuth angle & 304\({}^{\circ}\) (northwest) \\ Tilt angle of reflecting antenna & 45.8\({}^{\circ}\) \\ Height of antenna & 5.45 m \\ Vertical height difference of direct/reflective antenna & 0.38 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental parameters. Figure 7: Experimental set-up. and the reflecting antenna was set with a direction angle of 304 degrees northwest, so the azimuth of the region of the specular reflection point was chosen from 214 degrees to 34 degrees. Combining the zenith map with the screening scheme above, the experimental satellites selected in this paper are GPS PRN 4, PRN 9/BDS GEO 2, and IGSO 10. Figure 8 shows the zenith map of the selected experimental satellite at 10:00 a.m. (LT) on 22 April. Figure 9 shows the movement track of the selected satellites' specular points on 22 April. The red mark is the location of the erected antenna and the blue mark is the current meter location. The green circles are the starting points of GPS satellites' specular points, and the orange circles are the starting points of BDS satellites' specular points. Triangles represent the terminal of the corresponding specular reflection points. The white arrow represents the movement track of the satellite's specular point. The white and red dashed lines delineate areas with the current meter at the center and a radius of 25 m and 50 m, respectively. Figure 8: Satellite zenith at 10:00 a.m. (LT), 22 April 2021. Figure 9: Movement trajectory of specular points. Circle points represent the beginning of specular points and triangle points represent the end of specular point. In this paper, the specular points in the red dotted line region are selected for inversion, and the “close points” in the white dotted line region are selected. In the figure, “\(\backslash\)” represents the 1 st period of GPS PRN4 inversion, and “\(\copyright\)”represents the second stage of GPS PRN4 inversion. As can be seen from Figure 9, since only one current meter is used in this experiment, there will be some difference between estimated river flow velocity on specular points and in situ measurements. We just chose the specular points within 50 m of the current meter (Figure 9, in the red dashed area) to reduce the difference impact in precision evaluation. Specular points within 25 m from the current meter are considered as the \"close points\" (Figure 9, in the white dotted area). Among specular points in Figure 9, PRN 4 specular points are divided into two periods due to environmental impact. In the 1 st period, the specular points are almost \"close points\"; in the 2 nd period, the specular points are obscured by platform obstacles. Figures 10 and 11 record the elevation and elevation change rate of the selected satellites during the experimental period (GPS PRN 4, GPS PRN 9; BDS GEO 2, BDS IGSO 10). Surface water fluctuation is an important factor affecting the river's surface roughness, and the wind speed affects the river's surface roughness as well as the flow velocity of the river.