## 1 Introduction  
The Pearl River Estuary (PRE), located in the central-southern region of Guangdong Province in China, is an estuary where delta channels and residual bay coexist (Figure 1). It features a large discharge, minimal tidal range, and relatively low sediment concentration. The estuarine area is characterized by the development of river channels and a dense distribution of water networks. Due to the abundance of natural resources and navigational conditions around the PRE, detecting circulatory conditions is of significant importance for regional economic development and disaster mitigation efforts. As a new means of observing the ocean, high-frequency (HF) radar can obtain information about wind, waves, and currents on the ocean surface through the inversion of electromagnetic waves based on the Bragg scattering principle [1; 2; 3; 4]. The accuracy of HF radar measurements of ocean currents has gradually been recognized [5; 6; 7; 8; 9; 10]. The detection ability of HF radar is mainly evaluated by comparing the velocity obtained from mooring and cruising observations with radar data. In areas with a good electromagnetic environment, such as open ocean surface, the observation results of the HF radar are quite close to the mooring observations. An acoustic Doppler current profiler (ADCP) is a marine monitoring equipment that applies sound waves to measure current velocity and direction. As one of the most reliable instruments for observing ocean currents, ADCP has been developed and verified for decades. Early ADCP technology in the 1970s used a single-point measurement method that could only measure the water flow velocity at a specific point. This technique had limitations and could not comprehensively capture the dynamics of water flow. With the development of computer technology and multi-point measurement, relatively mature ADCP technology began to be applied in the field of ocean science research. [PERSON] et al. [11] applied ADCPs to long-term monitoring of suspended sediment concentrations (SSCs) in rivers. [PERSON] [12] measured tidal phase and amplitude using ADCP in Cook Strait, New Zealand. [PERSON] and [PERSON] [13] used ADCP to measure the velocity field of an ebb tidal jet. Not only stationary ADCP but also shipborne devices provide important data in many studies. [PERSON] et al. [14] observed the flow structure through the Minch channel with a ship-mounted 150 kHz ADCP. [PERSON] et al. [15] described the current patterns in the Inner Sound of Pentland Firth through underway ADCP measurement. [PERSON] et al. [16] reported a cruise survey that used an RDI 600 kHz ADCP on the Pearl River Estuary (PRE) and compared observations with model results. Compared with traditional ADCP, shipborne ADCP has the advantages of low cost and wide data coverage, although the observation duration is usually shorter. In numerous studies, ADCPs are utilized as a trustworthy device for evaluating the performance of HF radar detection. [PERSON] et al. [6] evaluated the current-mapping ability of the CODAR SeaSonde and WERA HF Radars on the West Florida Shelf. [PERSON] et al. [9; 17] compared the barotropic tidal currents detected by HF radars and ADCP with those simulated by a model. [PERSON] et al. [18] evaluated radial current measurements from HF radars and moored current meters. [PERSON] et al. [19] conducted an accurate assessment of current velocities observed by the OSMAR-S HF radar system. Despite variations in the study area and the methods used to evaluate radar data, all of the aforementioned studies have demonstrated a strong correlation between radar and ADCP measurements. Machine learning methods are widely applied in the field of ocean science, where they are used to find the nonlinear processes in data in order to achieve data fusion and calibration and improve data accuracy.