非對稱分布式目標的聲成像方法研究畢業論文-資料下載頁

　　

【正文】 c. In order to solve these problems, we developed the sonar target echo signal simulator.2 Mathematic Models Model for cean Environment NoiseOcean environment noise is generally considered Gaussian and independent, and also has large fluctuation. Our goal is not to simulate actual ocean environment, but test performance of sonar systems. Therefore, suppose that the noise is statistically independent and x(1),x(2),...and x(k) and independent random numbers evendistributed between(1/2,1/2).Furthermore x(k+l)=x}1}+x(k)(mod 1 / 2)From (1) we know that x(k+1) is a random between (1/2,1/2). ing to next operation, x(k+1) will replace x(k), and x(k) will replace x(k1),and so on. By this way we get { x n} . According to the proper even distribute noisedAnd { xm = x (2 m ) + 1 / 2 } are both independent and evendistributed between (0,1) respectively. Transform we can achieve that:Therefore is mutually independent and subject to Gaussian distribution Model for Target echo Signal1) the model for target echo acoustic signal For the single frequency continuous wave transmit signal, it can be described as S ( t ) = A cos(2 πf 0t ) , Where A is the amplitude of s(t) and f0 is its frequency. After s(t) and target strength suffer from transmission loss of the given distance, the echo signal is given by (3):S ( t ) = A cos( 2 π f0 t + ? ) (3)where ? is phaseshifting and determined by the relative distance between target and sonar, and A39。 is the amplitude of target signal and determined by target strength, environment noise, transmission loss, etc. the model for target echo electronic signal. receivertransducer is a circular array posed of 24routereceiverarray. Assuming that the radius of circular array is }，the wavelength of signal is，the speed of wave under water is v，the beam angle range of every receiverarray is“，the target azimuth is B，and the received signals y (t ) , i ∈ (1 , 24 ) , then Model for Reverberation Signal The average amplitude of reverberation signal can be approximated as reduction by power function with the increasing time. Assuming that H is the depth of seawater, that R is the distance, thatS2 is shooting angle, that } is fleeting angle, that y is sloping angle in seabed, that I。 is intensity of sound in transmit,that z is breadth of transmit signal, that“is the backscattering coefficient,is the absorption coefficient, then reverberation strength in receiver inseabed is given as follows:After simplification,Transform (6), we can achieve the relation between reverberation strength and time:3 Hardware Design and ImplementationThe structure block diagram of designed sonar signal simulator is depicted in . The main parts are TMS320C6710 and EPM7160SLC84, which are used as DSP and FPGA respectively. The basic operational principle of the system is as follows: When using keyboard to set sonar signal simulator in active conditions, we first use keyboard and LCD to set the distance, the pulse width, the transmitting frequency, etc.If the output of sonar signal simulator is target echo acoustic signal, the outputs of sonar equipments will be set in pseudoload, and the transmitting pulsetriggered signal from sonar will be connected with the simulator. When sonar transmits an active detection signal, the system will output a pulsetriggered signal. Once the sonar signal simulator receives it, DSP and FPGA will generate a delay, which sets the distance, the pulse width, and the modulated frequency of target signal. This signal is synthesized with sea clutter and reverberation signal from the generator in signal synthesizer. In this way we generate the active target echo acoustic digital signal, which passes through a D/A converter, an active switch capacitor filter to convert the modulated signal into the standard sine wave, and then an amplifier and a transducer network to realize the match between the output circuit and the transducer load, and last output the signal to the transmitter transducer to generate an acoustic signal. After the realize a qualitative test of the receiver and the processor and the display of sonar system, and also a quantitative test of the ranging system. If the target echo signal from the sonar signal simulator is an electronic signal, the underwater extension of the sonar will be dismounted. Using its interface to connect the sonar signal simulator and the sonar system, we set the sonar system in practical transmission status. The transmitting signal of sonar is absorbed by equivalent loads, which absorb energy of transmitting signals, at the same time partial signals are radiated. Using detection and procession of the radiated signal, we can get its envelope message, which DSP and FPGA use to produce a 24route target echo signal for sonar receiver transducer at the delay of designed distance. The target echo signal with different delays is synthesized with sea clutter and reverberation signal in electronic signal synthesizer. In this way we produce the above 24route signal. The modulator modulates it at 3 different carrier frequencies orderly, which are identical to practical underwater extension, and modulate an 8route signal respectively. The synthesizer synthesizes the 24routesignal and converts it to 8route signal. The synthesized 8route signal passes through an active switch capacitor filter and outputs as the sonar target echo electronic signal output. In meanwhile, the gyro signal simulator output the analogue signal of 80 Hz in a definite rule.When the simulator is set as a passive sonar target echo simulator, we can use keyboard and LCD to set parameters of passive targets. Then the simulator generates the required acoustic and electronic signals. Compared with the active sonar target echo signal, the passive target echo acoustic signal is mainly radiated noise from underwater targets, which outp