# Research on Shore-Based River Flow Velocity Inversion Model Using GNSS-R Raw Data
## 1 Introduction  
River flow velocity detection is of great significance in hydrology work. It is also an essential fundamental link in hydrologic detection, hydrologic prediction, hydrodynamic research, flood control engineering design, and ecological environment assessment [1]. Traditional river flow velocity detection methods, such as artificial detection, are work-intensive, time consuming, and inefficient. Additionally, the more extreme the hydrological situation is, the more difficult it is to measure the river flow velocity, such as during floods and typhoons [2, 3].  
Global navigation satellite system reflectometry technology (GNSS-R) possesses the characteristics of being global, all-weather-accessing, and low-cost. At present, GNSS-R technology has been widely used in sea surface altimetry [4, 5, 6], sea surface wind fields [7, 8, 9],sea ice sensing [10; 11; 12], inland water detection [13; 14], sea surface oil slicks [15; 16], sea surface effective wave height [17; 18], and other application fields. Better yet, it still has great potential in the applications of remote sensing inversion.  
As in traditional radar remote sensing, GNSS-R carrier phase observation has been widely used to measure sea surface height (SSH). Since the first lake altimetry through carrier phase observation under shore-based conditions [19], attributed to its better measurement accuracy, carrier phase observation has also been introduced for sea-level [20] and sea ice experiments [21]. Meanwhile, GNSS-R carrier phase observation altimetry has been expanded in different airborne experiments [22; 23; 24], and more recently, with space-borne data collected by the U.K.'s TechDemoSat-1 (TDS-1) [25; 26] and NASA's Cyclone GNSS (CYGNSS) missions [27; 28]. Based on the carrier phase observations, a different method for deriving Doppler observations is proposed and applied to altimetry [29]. In 2015, [PERSON] et al. published a new potential application of GNSS-R through carrier phase observation [30], inversion of river flow velocity, and verified its feasibility by using GPS L1 satellite reflected signals, with MAE reaching 0.0271 m/s. However, the inversion lasted for only minutes, without further systematic analysis of the accuracy and principle of river flow velocity inversion. Above all, few experiments are using GNSS-R technology to detect river flow velocity, and in particular, there is no research on river flow velocity inversion currently using BeiDou Navigation Satellite System reflectometry technology (BDS-R).  
BDS consists of geosynchronous orbit (GEO) satellites, inclined geosynchronous orbit (IGSO) satellites, and medium orbit (MEO) satellites. The GEO satellites have low angular velocity and almost no change in elevation. Their maximum speed is between 80 m/s and 90 m/s. The trajectory of an IGSO satellite is in the shape of \"8\", and the elevation potentially changes at a considerable rate. The maximum speed will reach about 2800 m/s. The maximum velocity of an MEO satellite is about 3780 m/s. For BDS receivers under shore-based experimental conditions, the full Doppler frequency shift of BDS GEO/IGSO/MEO satellites are 70.8 Hz, 2207.4 Hz, and 4496.5 Hz, respectively [31].  
Here, the combination of B1I GEO satellite and IGSO satellite of BDS is used to carry out river flow velocity inversion for the first time. GPS/BDS dual-mode GNSS-R detection equipment was used to obtain the raw IF data of GPS L1/BDS BII direct and reflected signals in the actual experiment. Meanwhile, self-developed soft receiver process data was used to obtain Doppler frequency extracted by interferometric carrier phase and realize river flow inversion. Finally, the velocity inversion results are compared with the actual velocity, and the influence of satellite elevation angle change rate and satellite signal intensity on the accuracy of river flow velocity inversion is further explored.