# Research on Shore-Based River Flow Velocity Inversion Model Using GNSS-R Raw Data
## 5 Conclusions  
In this paper, the GNSS-R river current measurement was carried out on the south bank of Dashengguan Yangtze River in Nanjing, China, for nearly two h on 22 April 2021. The self-developed GNSS-R soft receiver was used to process satellites' direct and reflected signals. The Doppler frequency is extracted by the interferometric carrier phase of the direct signal, and the reflected signal is proposed for inverse river flow velocity. The accuracy of the inversion results is verified by using the actual obtained data. This shore-based experiment realized the detection of river flow velocity and achieved high accuracy. More importantly, the possibility of BDS-R inversion of river flow velocity was verified for the first time. This article qualitatively analyzes the inversion results from three perspectives: (1) relative position of specular points to current meter, (2) elevation angle, (3) elevation angle change rate, and (4) signal strength. The following conclusions can be drawn from the inversion results of this experiment:  
In the case that the specular points are far away from the current meter or the specular points are blocked by obstacles, the current meter may not accurately represent the flow velocity, which leads to a large error between the flow velocity results of experimental inversion and the flow velocity results recorded by the current meter, which affects the evaluation of results and the effectiveness of inversion. The MAE and RMSE of GPS PRN 4 (1 st period) inversion results are 0.028 m/s and 0.036 m/s, respectively. The MAE and RMSE of BDS GEO 2 and IGSO 10 inversion results are 0.049 m/s and 0.063 m/s, and 0.061 m/s and 0.073, respectively. Both achieve high inversion accuracy, which proves the effectiveness of the inversion model of river flow velocity. Figure 17: SNR of four satellites’ reflected signals (GPS PRN 4 (**a**), PRN 9 (**b**), BDS GEO 2 (**c**), and IGSO 10 (**d**)). Blue represents the instantaneous SNR, and red represents the average SNR in 2 min. The analysis of elevation results and the SNR of different satellites show that signal strength and elevation of satellites have obvious correlations with the inversion accuracy of river flow velocity in this shared-based experiment. Because the elevation angle of specular points of the BDS GEO 2 and IGSO 10 satellites is little changed during the period of the experiment, the Doppler frequency value generated by satellite motion is very small, while the elevation change rate of the GPS MEO satellite is larger, and the Doppler frequency generated by satellite motion is larger than the former. The inversion results show that the inversion results of BDS GEO 2 and IGSO 10 are superior to those of GPS PRN 4 and GPS PRN 9. The elevation angle change rate of GEO 2 is almost close to 0 degrees/minute. Its achieved inversion precision was the best. The decrease of elevation change rate directly reduces the Doppler frequency shift difference \(\Delta f_{T}\) between direct and reflected signals and is more conducive to inversion of river flow velocity. Apart from the factors discussed in this article, whether the differences between GPS and BDS systems in other aspects would impact the inversion accuracy of river flow velocity should be explored in future studies. Longer-term experimental data (more than 24 h), more river current meters, and more environmental data (such as wind speed, etc.) should be used in the future to verify the factors affecting river flow rate under different conditions. Conceptualization, Y.Z. ; methodology, Y.Z. and Z.Y. ; software, Z.Y. and S.Y. ; validation, W.M. and S.Y. ; formal analysis, Y.Z. and Z.Y. ; resources, W.M., S.G. and J.Q. ; data curation, Z.Y; writing--original draft preparation, Y.Z. and Z.Y; writing--review and editing, Y.Z., Z.Y., S.Y. and W.M. ; supervision, Y.H. and Z.H. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Natural Science Foundation of China (Grant No.