Study On The Real-Time Monitoring Techniques Of Water Depth In Port Channel Based On Bistatic Sonar

Along the 18000-kilometer mainland coastline of China, the muddy coast accounts for about 22%. Most of the ports are built on the muddy coast, some of which have the characteristics such as long voyage, narrow width, limited water depth and heavy siltati-on. Owing to the lack of real-time monitoring means, the dynamic law of silt flow and depth change of these channels is not grasped radically, so it often happens that economic losses for less load and ship grounding and channel blockage for over load. Therefore, according to the characteristics of shallow sea acoustic channel and acoustic propagation, a study on the real-time monitoring techniques of water depth in port channel based on bistatic sonar is carried out in this paper. The main contributions of this dissertation are summarized as follows:1. The multipath propagation characteristics of underwater acoustic channel in port channel are discussed and a signal transmission mode of bistatic sonar is established. The noise and reverberation interference is deeply analyzed, and the research is focused on the impacts of shallow sea and its boundaries on acoustic propagation. Numerical analysis of characteristics such as acoustic attenuation, reflection losses and scattering loss are performed. The results show that the short-term sound velocity change is so small that the trajectory of acoustic ray is basically a straight-line. In the case of small grazing angles of far field, the forward reflection loss of seafloor is very small regardle-ss of the types of seabed sediment, and at this point, the time domain extension of multipath is relatively short and regular. The interferences of sea surface reverberation can be inhibited by adopting a high-frequency, narrow-beam sound source.2. On the basis of the classical APL-UW model, a model namely "SS A-Volume" is provided for improving the performance of APL-UW model. By use of the small slope approximation (SSA) to solve the seafloor roughness scattering, the SSA-Volume model can be applied to higher frequencies and low grazing angles avoiding the acoustic frequency limitation for APL-UW model (10-100kHz). Numerical simulation and performance analysis are conducted on several typical types of seafloor, and the predictions are compared with the APL-UW model. Take a typical muddy port channel as an example, simulation experiments are performed to analyze the relationships betw- een scattering strength and model input parameters at low grazing angles. The results show that there is a better agreement between the predictions of SSA-Volume model and APL-UW model in the frequcny range of 10-100kHz, with relative value of their difference less than 3%. The bottom scattering strength has a strong dependence on the angle configuration, and the scattering strength is highest around specular scattering angle for the in-plane scattering. In the case of high frequency and small grazing angle, the scattering strength of specular reflection is larger (less than 5dB), and can be less affected by the frequency and sound velocity. Hence, there is a certain advantage of adopting the grazing incidence to realize the depth monitoring in port channel.3. Aiming at the current requirements and existing problems for realizing the real-time monitoring of water depth in port channel, a novel method based on bistatic sonar is put forward. By use of the multipath propagation characteristics of underwater acoustic channel, the method obtains the depth values by calculating the relative time delay of acoustic signals between the direct and the shortest bottom reflected path. Furthermore, the method can solve the field depth monitoring in real time without influencing the normal sailing of ships.4. A hybrid algorithm combing the MODEX algorithm and the NLS algorithm with penalty function is proposed to solve the high resolution time delay estimation of underwater acoustic sinusoidal signals. Numerical and experimental results demonstrate the dramatic performance improvement and precision enhancement achieved by the hybrid algorithm relative to classical matched filter method and WRELAX on dealing with the superposition of very closely spaced signals at low SNR values. The accuracy of proposed algorithm ensures a high level of microseconds and the corresponding depth accuracy can completely meet the requirement of engineering.5. A practical system based on bistatic sonar is developed for realizing the real-time monitoring of water depth. It is mainly composed of the following three parts: transmitting unit, receiving unit and monitoring center. The system uses a pair of bistat-ic high-frequency narrow-beamwidth transducers to transmit and receive the acoustic signals, and adopts the GPS time synchronization technology to synchronize the transm-ittting and receiving units. A circuit design combining the single-chip microcontroller with FPGA is employed to achieve the delay acquisition and processing of underwater acoustic signals. A performance experiment of system principle prototype is conducted in the indoor pool, and the results indicate that the system is stable and reliable, with sounding accuracy less than 0.04m and peak-peak stability 0.084m, respectively.Finally, sea trials are conducted to validate the performance of system principle prototype in certain harbor basin of Dalian sea. The experiment results show that when the horizontal distance l between bistatic transducer pairs is fixed, the smaller the depression angle a is, the greater the measurement error becomes. Similarly, when the height h that from transducer pairs to seafloor is fixed, the larger the horizontal distance l is, the greater the measurement error becomes. The measurement error increases as the silt thickness increases gradually. When the silt thickness ranges from 0-0.83m, the measurement error is less than 0.16m, which can meet the engineering requirements. Furthermore, simple installation and better real-time performance enable the system to possess high application value and broad application prospect for the real-time monitoring of water depth in narrow channel.