Research On The Key Technology Of Acoustic Doppler Current Velocity Measurement

Acoustic Doppler Current Profiler (ADCP) is sonar equipment. It can measure both the bottom velocity and the current velocity. ADCP can be used for ship positioning and navigation; it can also be widely applied to marine and inland river measurement. As an instrument of velocity measurement, ADCP has been applied more and more to the river and marine engineering, and the research on technology of ADCP arouse people's attention for increasingly. The major work of this paper is to research several key technologies of acoustic Doppler current velocity measurement, including the signal analysis and modeling; current velocity measurement modes and performance differences in estimates; broadband beamforming technology; and calibration technology of transducer installation error. Specific contents and the results obtained are as follows:(1) Transmit signal is studied based on the target resolution theory. Analyze three kinds of signals such as single CW pulse, coherent pulse train and coherent coding pulse train in the form of the application in the ADCP. Doppler shift measurements are scattering inside the beam space, based on this fact the beam scattering model is proposed, and the bottom echo and the stratospheric echo are simulated. Compared with the experimental data, results show the correctness of the beam scattering model, and this is ready for performance evaluation of ADCP.(2) Three current velocity measurement modes are summarized, namely incoherent, coherent and broadband. Generally, incoherent mode is suit for low spatial resolution and large range of measurement applications; coherent mode is suit for shallow water and high-resolution applications; and broadband mode focus on the advantages of the incoherent and the coherent mode, it can be thought as a combination of above two modes. Based on the beam scattering model and the complex autocorrelation algorithm, three modes are simulated. Statistics results show that: the standard error of the incoherent mode is in order of cm/s; standard error of the coherent mode can be attained a few mm/s; and standard error of the broadband mode can be up to a few cm/s of magnitude. Compared to incoherent mode, broadband mode increase signal time-bandwidth product and improve the current velocity measurement accuracy.(3) Beamforming technology is researched. Generally, the phase shift beamforming uses a narrowband signal periodicity, and the phase shift is equivalent to the time delay at this time. When broadband signal is used, the phase shift makes the beam width broadened and the signal distortion produced. Delay beamforming solution is proposed for broadband signals. Simulation results show that the delay beamforming solution technology can distortionless send and receive both narrowband and broadband signals, which makes it possible to use beamforming technology to broadband signal transceiver.(4) Transducer installation error calibration is researched. Using ADCP bottom velocity data, attitude data and DGPS data, Least squares method is proposed to the acoustic transducer calibration of the three dimensional installation errors, and complete formula for calculating the parameters is established. It can be seen that three installation angle work as rotation matrix, there is no need to solved specific angles, but made the rotation matrix as a direct solution for dead reckoning. The results show that the least squares method greatly reduces systematic error due to the transducer installation, and significantly improves current measurement accuracy.