Research On Acoustic Correlation Technology For Velocity Measurement

The principle of"waveform invariance"is used for the velocity measurement among acoustic correlation log (ACL), which is suitable to be equipped on submarines and autonomous underwater vehicles (AUV). The ACL has striking advantages compared with the acoustic Doppler log (ADL). It can work with a single beam pointing directly down at the sea bed and the return signal is much larger than from the ADL because of the normal incidence. Furthermore the beam can be fairly broad and, for the same size of transmit transducer, this enables the use of lower acoustic frequencies for which the attenuation is less. The ACL has the potential therefore for operating on the sea bed echoes at much greater depths than is possible with the ADL. The design and utilization of ACL become more and more important both for the defense of the national ocean sofereignty and for the scientific research and exploitation of islands and ocean rescource.In this dissertation, the overall scheme of the ACL simulation system is planned based on the principle of"waveform invariance". The whole system is divided into several modules, including seabed reverberation simulation module, waveform design module, bottom tracking module, transducer arrays design module, velocity measurement and optimization algorithm module. The final goal of the module design and research in ACL simulation system is developing a suit of software used in the whole design processing and the migration to hardware platform.According to the single transmitted pulse system in this dissertation, the truncating Gold sequences which contains the same number of 0's and 1's are used as code subsections to form the encoded pulse. The pulse coding technique used involves pseudo-random codes which are now generated by a new optimal code finding program. The nature of the coded pulse is such that the autocorrelation function of the pulse possesses a temporal autocorrelation peak at a predetermined time lag so as to achieve the same function as a repetitive pulse system. Thus, the trade-off between impulse width and pulse interval time has been obtained. A realization of the seafloor reverberation simulation based on the Kirchhoff approximation scattering model and the network model was presented. On the basis of the field investigation and research results of others, the case of a plane array is analyzed from one dimensional to three dimensional velocity and sufficient conditions to keep the normalized space-time correlation function of reverberation close to 1 are obtained. The conclusion provides guidance for the application to the seafloor reverberation simulation and the velocity measurement.The design method of transducer arrays assembly plays a key role in the velocity measurement for the ACL. The purpose is to make better use of the array aperture and fully utilization of sensors because spatial discrimination depends on differences in hydrophone locations rather than hydrophone locations. A design method of one dimension and two dimension arrays for the ACL is studied in this dissertation. Based on the method of restricted minimum-redundancy linear array (RMRLA), the redundancy factor is redefined, an ideal positional vector map is built, the conception of positional vector coverage rate is proposed and a fast two-dimensional arrays design method is obtained.By taking the second derivative of the filtered signal with respect to time in literature [75], the sonar system will compensate for signal losses due to spreading. As a result, the detection probability will increase although the seafloor reverberation signal is extremely slow-decaying. With the combination of the matched filtering method mentioned above and the pulse compress method, the dissertation solves the conflicting problems among ocean depth, range resolution and amplitude attenuation. Based on the likelihood function proposed as the cost function which is constructed by a model covariance matrix and a sample one, one of the main contributions of this dissertation is a hybrid algorithm being put forward and the convergence being proved. The algorithm is combined of the simplex method and the pattern search method for bound constrained function minimization which comes from the two subclasses of the direct search methods (DSM). Based on the study on the classical simulated annealing (CSA), which is a method of finding a global minimin, another main contribution is the adaptive simulated annealing (ASA) algorithm being proposed to improve the precision of measure velocity.