The purpose of this paper is to develop a high resolution imaging acoustic signal simulator. The receiving array of the imaging sonar is a 48 element equidistant linear array with an operating frequency of 800KHz, an operating distance of 0.5-25m and an angular resolution of 0.35 °. Imaging sonar performs beamforming on the received array signal to achieve the acquisition of acoustic image.
The incident sound wave is incident parallel to the normal direction of the array at an angle of θ, and the elements are numbered 1, 2,… From left to right i、t+1、… N. The distance between primitives is d. If the No.1 element is selected as the time reference point and the received signal is acos2 π ft, then there is a sound path difference between the two adjacent elements Δ = DSIn θ. Where C is the velocity of sound. Because the imaging sonar is a narrow-band active sonar, the phase difference between the received signals of I-element and no.1-element is φ I = 2 π (i-1) d / λ sin θ, where λ is the wavelength. Therefore, in order to orient the linear array in the direction of θ 0, we only need to delay the signal of the ith element by τ I (θ 0) = 2 π (i-1) d / λ sin θ 0.
The above is the basic principle of linear array beamforming, but it is only an approximation in the far field. For the near-field condition, such an approximation will produce a large error. In this paper, focusing sonar must be used in the whole range of high frequency imaging. The basic principle is the same as above, but the delay (or phase shift) of each primitive signal is not linear, which is not detailed in this paper.
Generally, the number of analog signals used in imaging sonar is the same as that of the basic elements, and the output of each channel simulates the signal of one basic element in sonar array. Due to the short working distance of imaging sonar and the low noise level of high frequency in underwater acoustic environment, the received signal-to-noise ratio is usually high. For this reason, there is no additional noise in the output of the signal simulator. Imaging sonar works in a strong reverberation environment. Because the simulation of reverberation is difficult and the impact on imaging is not serious, the reverberation simulation is not considered in the design, and only focuses on the simulation of near-field target echo.
According to the azimuth and distance of the point target to be simulated input by the user, the signal simulator calculates the phase difference of the corresponding target echo to each element of the receiving array, and then generates the corresponding multi-channel sinusoidal signal according to the phase difference. These signals are added to the input of the imaging sonar to replace the real array output, so that the imaging sonar can be easily debugged and measured in the land laboratory.
Traditional sonar signal simulator usually uses a fixed oscillator to generate sinusoidal signal with the same working frequency as sonar system. The LO signal is passed through a group of multi tap analog delay lines, and then the signal is extracted from different taps of the delay line as the output of the simulator. There are some defects in the structure of this signal simulator.
Firstly, the minimum variable delay length is limited due to the tapped delay line structure composed of analog devices. Especially considering the hardware scale and cost of the system, the number of taps of the general delay line is small, which leads to a large error between the delay time and the theoretical value, thus reducing the accuracy of the simulator.
Secondly, in order to simulate the echo signals of targets in different directions, the outputs of different tapped delay lines must be switched or combined, and then output to sonar equipment as a signal of a primitive. Therefore, the scale of the whole simulator is huge, and it can only simulate a number of discrete azimuth and range targets, and can not simulate the echo of point target at any azimuth and range, otherwise the complexity will not increase, it will be difficult to achieve.
In addition, it is difficult to guarantee the channel consistency and debug the tapped delay line composed of analog devices. And the frequency range of delay line is narrow, if the frequency parameters change, it will not be used normally, so the application range is narrow, and the cost performance is very low.
In order to overcome these defects of traditional sonar signal simulator, a new signal simulator is designed and implemented by DDS technology. This DDS based simulator structure can accurately simulate the echo signal of point target at any azimuth and range, which is suitable for different frequency parameters and has certain expansion ability, so it has high cost performance.