An active sonar response method and a transponder

Abstract

The present invention provides an active sonar response method and a transponder. First, the direct wave of the sonar transmission is searched, the spectrum of the direct pulse signal is measured, and based on this, the center frequency and bandwidth of the FIR band-pass filter are accurately determined. Then, the widths of the leading and trailing edges of the pulse envelope are accurately measured through differentiation, and an analog target echo signal that is not affected by multipath and reverberation spreading is generated by the internal signal of the transponder. The beneficial effects of the present invention are as follows: The present invention solves the problem that the conventional response output signal of the hull sonar with high-power transmission in shallow sea marine experiments cannot meet the accuracy calibration requirements of sonar performance detection; by utilizing the flexibility of the internal FPGA programmable signal generation of the transponder, the frequency, pulse width, and bandwidth can be set arbitrarily, greatly improving the test efficiency of marine experiments.

Description

Technical Field

[0001] The present invention belongs to the field of overall ship hull sonar and circuit design, and mainly relates to an active sonar response method and a transponder. Background Art

[0002] Due to the limitations of the test objectives and test environment, large hull sonars generally need to use transponders to simulate target echoes to achieve static performance tests such as sonar direction finding, distance measurement, speed measurement accuracy and measurement range during identification tests. Conventional transponders generally receive active sonar direct waves through hydrophones, and after amplification and filtering, the received signal is sent by the transmitting transducer to achieve a response. However, when receiving direct waves, there are also scattering (reverberation) from the seabed and the sea surface, multipath fading, and background noise. Usually, the response signal also contains such reverberation and noise, resulting in low signal-to-mix ratio and poor stability during sonar static detection, affecting the calibration of the sonar static performance. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings of the existing technology. While maintaining the normal (passive) response function, an active sonar response method and transponder are provided according to shallow sea channels and propagation conditions. The method is used to simulate the reflected echo of targets in the marine environment and to check the ranging, direction finding and speed measurement performance of the active sonar. By detecting the short-time spectrum characteristics and pulse width of the direct wave, a programmable response signal is generated using FPGA. The method has good environmental adaptability and is portable and flexible.

[0004] The objectives of the present invention are achieved through the following technical solutions: an active sonar response method, which first searches for direct waves emitted by sonar and measures the direct pulse signal spectrum, accurately determining the center frequency and bandwidth of the FIR bandpass filter based on this spectrum, then accurately measuring the leading and trailing edge widths of the pulse envelope through differentiation, and generating a simulated target echo signal from the transponder's internal signal that is not affected by multipath and reverberation expansion.

[0005] The beneficial effects of the present invention are as follows: the present invention solves the problem that the conventional response output signal of the high-power emission of the hull sonar in shallow sea tests cannot meet the sonar performance detection accuracy calibration problem; by utilizing the flexibility of the programmable signal generation of the FPGA inside the transponder, the frequency, pulse width and bandwidth can be set arbitrarily, which greatly improves the efficiency of marine test. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Attachment Figure 1 The transponder response principle block diagram of the present invention;

[0007] Attachment Figure 2 Schematic diagram of information flow and parameters of the transponder implementation process;

[0008] Attachment Figure 3 Pulse signal short-time FFT effect diagram;

[0009] Appendix Figure 4 Schematic diagram for detecting the leading and trailing edge moments of a quasi-square wave pulse signal using the differential (gradient) method;

[0010] Appendix Figure 5 Block diagram of generating a hybrid response signal based on FPGA;

[0011] Appendix Figure 6 Schematic diagram of the transponder implementing a human-machine interface;

[0012] Appendix Figure 7 Screenshot of the sonar display when a certain type of sonar in the South China Sea area uses the hybrid response method to conduct a sea trial for evaluating the speed measurement accuracy and speed measurement range;

[0013] Appendix Figure 8 Block diagram of the transponder;

[0014] Appendix Figure 9 Schematic diagram of the operation control process of the transponder. Specific implementation manners

[0015] The present invention will be described in detail below with reference to the accompanying drawings and embodiments:

[0016] The transponder of the present invention mainly consists of a receiving transducer, a transmitting transducer, a receiving acquisition and processing unit, a control unit, signal conditioning, a linear power amplifier, etc. Its response principle and functional module composition are as shown in the appendix Figure 1 as shown.

[0017] The transponder of the present invention realizes passive response and hybrid response through hardware design and software development. The passive response is realized at the signal simulation end. After the hybrid response passes through analog signal conditioning and A / D conversion, the control unit completes short-time spectrum analysis, FIR filter design, and envelope differentiation. After measuring the time-frequency characteristics of the transmitted signal, it controls the FPGA to generate various response pulse signals. The block diagram is as shown in the appendix Figure 8 as shown.

[0018] The control unit uses the Getac B300 rugged portable computer, which is built with an i5-2520M vPro processor at 2.5 GHz and 8 GB of DDR3 memory, and has good ruggedness, dust resistance, and vibration resistance. The functions are realized through the network interface as follows:

[0019] (a) Complete the selection of the transponder working mode;

[0020] (b) In the passive response working mode, the receiving transducer receives the direct wave sound signal, the control unit controls to complete the bandwidth selection and amplitude control, and then controls the output power control of the linear power amplifier, and the transmitting transducer completes the transmission.

[0021] (c) In the hybrid response mode, detect the time-frequency characteristics of the transmitted signal, perform FIR filtering, send the frequency and pulse width information to the FPGA, and control the FPGA to implement complex signal response;

[0022] (d) Complete the input and output signal display of the transponder receiving channel and transmitting channel; display the working status;

[0023] (e) Have the function of test and calibration.

[0024] The operation control software flow is as shown in the appendix Figure 9 as follows.

[0025] The hybrid response method proposed by the present invention searches for the direct wave transmitted by the sonar, measures the spectrum of the direct wave through short-time spectrum analysis, selects the signal conditioning and programmable band-pass filter parameters of the subsequent stage based on this to improve the signal-to-noise ratio; then differentiates the envelope of the filtered direct wave to measure the pulse front / back edge and pulse width; controls the internal signal source of the transponder to generate an analog target echo signal that is not affected by multipath and reverberation expansion; the signal generation is flexible, and the frequency, pulse width, and bandwidth can be set arbitrarily, which can meet the requirements of the high-power working performance test and accuracy calibration of the hull sonar in shallow sea sea trials, and greatly improve the efficiency of sea trials.

[0026] The transponder of the present invention is divided into passive response and hybrid response. The process information flow of the implementation is shown in Figure 2 , and mainly includes the signal reception conditioning, short-time FFT of the pulse signal, pulse signal envelope detection information flow and key parameter settings.

[0027] (1) Passive response

[0028] That is, according to the command of the control unit, receive the direct sound wave and reverberation through the receiving transducer, amplify, band-pass filter, amplitude adjust, and set the threshold of the received signal, and then excite the linear power amplifier to drive the transmitting transducer to send out the response signal, so as to realize the real-time simulation of the active target echo signal.

[0029] (2) Hybrid response

[0030] In shallow water, complex terrain or long-range test responses, where the signal-to-noise (mix) ratio is relatively low, receive the unknown form of the transmitted direct wave through the receiving transducer, first perform short-time spectrum analysis and pulse front and width detection on the direct wave, select a reasonable FIR filter bandwidth to improve the signal-to-noise ratio, then digitally differentiate the pulse envelope to measure the pulse front and back edge widths, obtain the pulse width, and use the rising edge (positive gradient) to trigger the internal signal source to generate a pulse response signal with the same pulse width as the direct wave signal, and adjustable frequency, amplitude, and threshold, so as to realize the simulation of target strength and Doppler.

[0031] a) Signal extraction

[0032] In order to measure the spectrum of the direct wave, considering that the actual response signal pulse width is not less than 50 ms, the pulse signal length of the short-time FFT is set to τ = 31.25 ms, with a 50% overlap, and the accumulation time T is not less than 10 s. The calculation of the short-time FFT is as follows:

[0033] Using f s as the sampling frequency, sample x(t) to obtain the sample sequence:

[0034] x(0), x(1), …, x(N - 1),

[0035] where N is the sample length, and its DFT is:

[0036]

[0037] When f s = 32768 Hz and N = τ × f s = 1024, the frequency resolution is: This resolution is sufficient for the filter setting accuracy. The gain obtained through FFT

[0038] By increasing the accumulation times and taking advantage of the uncorrelation of noise, the gain of FFT can be further improved. For this purpose, divide x(k) into M segments, which also covers the spectrum analysis of the active signal pulse width length T. T = M × τ = 10 s, so M = 320.

[0039] Let the m-th segment signal be:

[0040] x((m - 1)N + k) for k = 0, 1, …, N - 1

[0041] Each segment has a length of N. Perform DFT on the m-th segment signal to obtain the spectrum

[0042]

[0043] Accumulate |X m (l)| 2 to obtain:

[0044]

[0045] The above formula can further improve the gain. As shown in the appendix Figure 3 , the short-time FFT before (left) / after (right) accumulating 320 times, LFM signal frequency (5 - 5.5) kHz, pulse width 200 ms, sampling rate 32.768 kHz.

[0046] By determining the center frequency and bandwidth of the transmitted signal as above, the design parameters of the FIR filter can be determined, including the center frequency, bandwidth, and attenuation characteristics.

[0047] b) Envelope and Pulse Width Detection

[0048] Through the above-filtered signal, the signal-to-noise ratio is improved. After envelope detection and differentiation, the leading and trailing edges of the transmitted pulse signal can be accurately extracted to determine the pulse width of the response signal.

[0049] The calculation formula for digital differentiation is relatively complex. For convenience, the gradient function in Matlab is called here:

[0050] f y = gradient(y(j)),

[0051] MAX(ABS(f y ))

[0052] The occurrence times of the rising edge and the falling edge are obtained through the maximum value of the gradient, and the signal source is triggered accordingly to generate a transmitted signal with the same pulse width as the transmitted signal. The processing results are shown in the appendix Figure 4 , appendix Figure 4 In the appendix, the differentiation of the rising edge is "positive", and the differentiation value of the falling edge is "negative". The left figure is the envelope of the received transmitted signal, and the right figure shows the rising edge and falling edge times obtained by taking the modulus after differentiation. The interval between the leading / trailing edges is the pulse width, which has higher accuracy compared to the pulse width obtained by amplitude comparison.

[0053] C) Response Signal Generation

[0054] Based on the signal forms and pulse widths of three consecutive transmissions measured in a) and b), the average values are used as the parameters for generating the response signal. The rising edge (maximum positive gradient) measured in real time is used to trigger and synchronize the built-in NCO of the Altera chip EP3C120F780I7 to generate the response signal. The NCO is a numerically controlled oscillator that generates a signal by setting the frequency word. At a fixed operating clock F mclk , to obtain a digital signal with a frequency of f clk , the frequency word with a bit width of N is defined by the following formula:

[0055] f phi = 2 N f clk F mclk

[0056] The functional block diagram for generating the hybrid response signal is shown in the appendix Figure 5 , with flexible signal generation and high precision.

[0057] In the circuit, 1 # the NCO generates a carrier signal, 2 #The NCO generates a modulation envelope. The single-frequency signal and the modulation signal frequency word are obtained by looking up a table. The frequency word of the frequency-modulated signal is calculated and updated in real time according to the pulse width and frequency parameters by formulas (1) and (2). Through the above design, the pulse width control accuracy of the response signal reaches 1 ms, and the frequency control accuracy reaches 1 Hz.

[0058] The bi-phase frequency-modulated transmission signal with a pulse width of T is:

[0059]

[0060] t0 = (T / B)f m , k = (f m -B / 2)×(t0 + T / 2), f m is the center frequency, T is the pulse width of the signal, B is the bandwidth of the signal, and the instantaneous frequency of the hyperbolic frequency modulation is:

[0061]

[0062] Generation of the single-frequency pulse signal:

[0063] s(t) = exp(-j2πf o t) t = 0 to T (2)

[0064] Through the above design, the following performance indicators are achieved as follows:

[0065] (1) The frequency range of the received and transmitted signals: 1 kHz to 10 kHz;

[0066] (2) Under the rated resistive load condition, at the main operating frequency point f0, the maximum transmitted electric power is not less than 50 W, and the total harmonic distortion (THD) is not greater than 5%.

[0067] (3) In the passive response working mode, the response output power adjustment range is -10 dB to +20 dB, the output electric power is 50 W, and the adjustment range is -40 dB to 0 dB. Two types of analog filter bandwidths can be selected through the control unit, namely narrowband (3 - 6) kHz and broadband (1 - 10) kHz.

[0068] (4) In the hybrid response working mode, the response output power, signal form, frequency, bandwidth are controllable and adjustable. The frequency setting accuracy is 1 Hz, and the pulse width setting step is 1 ms; the filter center frequency and bandwidth are automatically set.

[0069] The transponder implements a human-machine interface such as Figure 6 , and in the hybrid response working mode and the parameter selection interface, the input and output signal amplitudes are controllable, the filter can be set, and the signal bandwidth and pulse width can be set.

[0070] The response echo display screen for participating in the sea trial of a certain type of equipment is as attached Figure 7 .

[0071] It is understood that, for those skilled in the art, equivalent substitution or modification of the technical solution and inventive concept of the present invention should fall within the protection scope of the appended claims of the present invention.

Claims

1. An active sonar response method, characterized in that: First, the direct wave is emitted by the search sonar, and the spectrum of the direct pulse signal is measured. Based on this, the center frequency and bandwidth of the FIR band-pass filter are accurately determined. Then, the widths of the front and rear edges of the pulse envelope are accurately measured through differentiation. An analog target echo signal that is not affected by multipath and reverberation spread is generated from the internal signal of the transponder. This method includes passive response and hybrid response, and the specific steps are as follows: (1) Passive response According to the command of the control unit, the direct sound wave and reverberation are received through the receiving transducer. The received signal is amplified, band-pass filtered, amplitude adjusted, and threshold set, and then the linear power amplifier is excited to drive the transmitting transducer to emit a response signal, realizing real-time simulation of the active target echo signal. (2) Hybrid response The direct wave of the unknown form of the transmitted signal is received through the receiving transducer. First, short-time spectrum analysis, pulse front edge and width detection are performed on this direct wave, and a reasonable FIR filter bandwidth is selected to improve the signal-to-noise ratio. Then, the widths of the front and rear edges of the pulse are measured by digitally differentiating the pulse envelope to obtain the pulse width. The rising edge is used to trigger the internal signal source to generate a pulse response signal with the same pulse width as the direct wave signal, adjustable frequency, amplitude, and threshold, realizing target strength and Doppler simulation. The specific steps of the hybrid response are as follows: a) Signal extraction In order to measure the spectrum of the transmitted direct wave and considering that the actual response signal pulse width is not less than 50 ms, it is assumed that the pulse signal length of the short-time FFT is taken as τ = 31.25 ms, with a 50% overlap, and the cumulative time T is not less than 10 s. The calculation of the short-time FFT is as follows: With f s as the sampling frequency, sample x(t) to obtain a sample sequence: x(0), x(1), …, x(N - 1), where N is the sample length, and its DFT is: When f s = 32768 Hz, N = τ × f s = 1024, the frequency resolution is: The gain obtained by FFT By increasing the number of accumulations and using the non-correlation of noise, the gain of the FFT is further improved. The x(k) is divided into M segments, which simultaneously cover the spectrum analysis of the active signal pulse width length T. T = M × τ = 10 s, then M = 320; Let the m-th segment signal be: x((m - 1)N + k) k = 0, 1, …, N - 1 Each segment has a length of N. The DFT is performed on the m-th segment signal to obtain the spectrum Put | X m (l)| 2 Accumulate them to get: Through the above, the center frequency and bandwidth of the transmitted signal are determined, the design parameters of the FIR filter are determined, and the center frequency, bandwidth, and attenuation characteristics are determined. b) Envelope and pulse width detection Through the above-filtered signal, the signal-to-noise ratio is improved. After envelope detection and differentiation, the front and rear edges of the transmitted pulse signal are accurately extracted to determine the pulse width of the response signal. The gradient function in Matlab is called, and the moments of the rising edge and falling edge are obtained through the maximum value of the gradient to trigger the signal source to generate a transmitted signal with the same pulse width as the transmitted signal. C) Response signal generation The signal forms and pulse widths of three consecutive transmissions are measured through a) and b), and after averaging, they are used as the response signal generation parameters; the rising edge measured in real time, that is, the maximum positive gradient, triggers and synchronizes the built-in NCO of the Altera chip EP3C120F780I7 to generate the response signal; the NCO is a numerically controlled oscillator that generates signals by setting the frequency word. At a fixed operating clock F mclk below, to obtain a digital signal with a frequency of f clk , the frequency word with a bit width of N is defined by the following formula: f phi = 2 N f clk F mclk In circuit 1 # The NCO generates a carrier signal, 2 # The NCO generates a modulation envelope; the single-frequency signal and modulation signal frequency words are obtained by looking up a table, and the frequency word of the frequency-modulated signal is calculated and updated in real time according to the pulse width and frequency parameters by formulas (1) and (2); The two-tone frequency modulation transmitted signal with a pulse width of T is: t0 = (T / B)f m , k = (f m - B / 2)×(t0 + T / 2), f m is the center frequency, T is the pulse width of the signal, B is the bandwidth of the signal, and the instantaneous frequency of hyperbolic frequency modulation is: Generation of the single-frequency pulse signal: s(t) = exp(-j2πf o t) where t = 0 to T (2).

2. A transponder using the active sonar response method as described in claim 1, characterized in that: The transponder consists of a receiving transducer, a transmitting transducer, a receiving acquisition and processing unit, a control unit, signal conditioning, and a linear power amplifier. The transponder realizes passive response and hybrid response through hardware design and software development. The passive response is implemented at the signal analog end. After the mixed response undergoes analog signal conditioning and A / D conversion, the control unit completes short-time spectrum analysis, FIR filter design, and envelope differentiation. After measuring the time-frequency characteristics of the transmitted signal, it controls the FPGA to generate various response pulse signals.

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