Robust weak target detection with high gain using continuous wave MIMO sonar and doppler filtering

Abstract

In order to overcome the problems of short weak target detection distance, long detection time and few detection periods of the existing traditional active sonar, the application provides a weak target robust high-gain detection method using continuous wave MIMO sonar and Doppler filtering.The method first designs a frequency diversity continuous wave pulse signal with low cross-correlation according to the number of transmitting transducers and the number of detection periods of the MIMO sonar, then continuously transmits the designed frequency diversity continuous wave pulse signal by using the MIMO sonar and the multi-period transmission mode, then performs Doppler filtering processing on the echo, obtains the Doppler filtering output of all frequency diversity continuous wave pulse signals, finally performs coherent accumulation and incoherent accumulation on the Doppler filtering output, and obtains the robust high-gain accumulated output.In this process, the Doppler filtering of the continuous wave pulse signal can be fully utilized to suppress reverberation, and the multi-period accumulation can be utilized to suppress noise, so that the weak target detection performance of the traditional active sonar can be obtained.

Description

Technical Field

[0001] This invention relates to the field of array signal processing, and more specifically to a robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering. Background Technology

[0002] As the active stealth capabilities of underwater acoustic targets continue to improve, target intensity decreases significantly, and the effective range of active sonar also decreases significantly. This is because when the target is at a greater distance, the target echo becomes very weak, and active sonar faces both low signal-to-mixing ratio and low signal-to-noise ratio operating conditions. For these reasons, how to use active sonar to detect targets of interest at greater distances has become a current research hotspot and challenge.

[0003] Currently, in the field of active sonar detection technology, the main active sonar system adopted is the single-input multiple-output (SIMO) system, which uses a single element to transmit and multiple elements to receive. This type of traditional active sonar primarily employs a repetitive, periodic detection method. To avoid interference between echoes from adjacent periods, traditional active sonar requires a sufficiently long detection period interval to ensure that the scattered signals from distant targets have all returned before transmitting the next detection signal. While a sufficiently long detection period can improve target detection capability, excessively long intervals allow non-cooperative targets to counteract the signal, resulting in a limited number of accumulated periods within a finite timeframe. This leads to low cumulative gain over multiple periods, making it difficult to effectively detect weak targets in a short time, ultimately resulting in detection failure.

[0004] Some researchers utilize continuous active sonar (CAS) to transmit signals with high duty cycles to achieve higher data update rates, thereby improving the detection capability of active sonar. However, existing CAS sonar technologies have a limited number of detection cycles obtained from a single transmitted signal and require a long detection time, making it difficult to meet the needs of weak target detection. To obtain multiple detection cycles in a short time and improve the potential performance of weak target detection, multiple-input multiple-output (MIMO) sonar can also be used. However, the number of signals transmitted by existing MIMO sonar technologies in a single cycle is limited by the number of transmitting transducers, making it difficult to meet the needs of weak target detection.

[0005] In summary, existing target detection methods using active sonar are limited by shortcomings such as short detection range, long detection time, and few detection cycles. Therefore, we urgently need to propose a method with weak target detection performance that is superior to traditional active sonar. Summary of the Invention

[0006] To overcome the problems of short detection range, long detection time, and few detection cycles in existing traditional active sonar, this invention proposes a robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering. This method can make full use of Doppler filtering of continuous wave pulse signals to suppress reverberation and use multi-cycle accumulation to suppress noise, thereby achieving weak target detection performance superior to that of traditional active sonar.

[0007] The technical solution of the invention is as follows:

[0008] A robust, high-gain detection method for weak targets using continuous-wave MIMO sonar and Doppler filtering is characterized by the following steps:

[0009] Step 1: Design a frequency diversity continuous wave pulse signal with low cross-correlation based on the number of transmitting transducers and the number of detection cycles of the MIMO sonar;

[0010] Step 2: Continuously transmit the designed frequency diversity continuous wave pulse signal using MIMO sonar and multi-cycle transmission mode;

[0011] Step 3: Perform Doppler filtering on the received wave to obtain the Doppler filtered output of the diversity continuous wave pulse signal of all frequencies;

[0012] Step 4: Perform coherent accumulation and incoherent accumulation on the Doppler filter output to obtain a robust, high-gain accumulated output.

[0013] Furthermore, step 1 specifically includes:

[0014] First, construct a monostatic MIMO sonar: select M transmitting transducers to form a transmitting array, M is an integer; select N hydrophones to form a multi-element receiving array, the hydrophones are arranged in a straight line, N is an integer; set the axis of the multi-element receiving array as the x-direction, and the direction perpendicular to the x-direction as the y-direction, then the transmitting array and the receiving array together form a MIMO sonar.

[0015] Redesign the transmitted waveform: Design the number of transmitting transducers to be M and the number of detection cycles to be K. Then the number of frequency diversity continuous wave pulse signals is the product of the number of transmitting transducers and the number of detection cycles, which is MK. The frequency diversity continuous wave pulse signals have the same pulse width T and the same frequency interval Δf. The frequency interval Δf needs to be large enough to ensure that the peak value of the cross-correlation function is not greater than 0.001 times the peak value of the autocorrelation function. At the same time, a sufficiently large frequency interval Δf needs to be designed according to the maximum Doppler frequency shift generated by the target's motion velocity to ensure that the Doppler frequency shift of the target echo will not cause echo confusion between adjacent frequency diversity continuous wave pulse signals.

[0016] Furthermore, in step 2:

[0017] M transmitting array elements simultaneously transmit M frequency diversity continuous wave pulse signals. The expression for the frequency diversity continuous wave pulse signal transmitted by the m-th transmitting transducer in the k-th detection period is:

[0018]

[0019] In the formula,

[0020] s m,k (t) represents the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle, where m = 1, 2, ..., M and k = 1, 2, ..., K;

[0021] f m,k This represents the center frequency of the pulse signal from the m-th transmitting transducer during the k-th detection period;

[0022] t represents time;

[0023] T represents the pulse width of the transmitted signal;

[0024] ΔT represents the time interval between adjacent detection periods, and the value of ΔT is much smaller than the time interval used by traditional active sonar, that is, the range of ΔT is ΔT∈(0, 6T];

[0025] A represents the signal amplitude, and Where F represents the sampling frequency;

[0026] j represents the imaginary number symbol.

[0027] Furthermore, step 3 specifically includes:

[0028] Step 3.1: The MIMO sonar simultaneously transmits M mutually orthogonal continuous pulse signals in each detection cycle. After illuminating the target, it forms a target echo. The multi-element receiver array receives the target echo and performs time-domain beamforming on the received echo to obtain the time-domain beam output.

[0029] Step 3.2: Define the possible range of radial velocity values ​​for the target, and discretize the radial velocity of the target within the range of velocity values ​​to obtain L discrete radial velocity values;

[0030] Step 3.3: Combining the parameters of the MK frequency diversity continuous wave pulse signals transmitted, a matched filter is designed at each discrete velocity, so that each frequency diversity continuous wave pulse signal can obtain a matched filter group consisting of L matched filters.

[0031] Step 3.4: Design MK matched filter banks for the MK frequency diversity continuous wave pulse signals to be transmitted, with L matched filters in each bank. Use the MK matched filter banks to process the echo signals to obtain the results of MK detection cycles.

[0032] Step 3.5: Combining the outputs of the L matched filters yields the processing result of the matched filter group designed for the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle and the received wave.

[0033] Furthermore,

[0034] The result of processing the received wave by the matched filter bank designed for the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle in step 3.5 is as follows:

[0035] Y m,k (t,v)=[y m,k,1 (t,v) y m,k,2 (t,v) … y m,k,L (t,v)]

[0036] In the formula,

[0037] Y m,k (t,v) represents the processing result of the m,k-th matched filter group on the docking return wave;

[0038] y m,k,l Let l be the matched filter output obtained by processing the received signal from the l-th matched filter of the matched filter bank designed for the pulse signal transmitted by the m-th transmitting transducer in the k-th detection period. Its expression is:

[0039]

[0040] In the formula,

[0041] This represents the received echoes corresponding to MK transmitted pulse signals;

[0042] * indicates a convolution operation;

[0043] h m,k,l (t)=s m,k,l c (Tt) represents the impulse response function of the l-th matched filter designed with the parameters of the pulse signal emitted by the m-th transmitting transducer in the k-th detection period, where the superscript c represents taking the conjugate and the subscripts m,k,l represent the sequence numbers corresponding to the pulse signal emitted by the m-th transmitting transducer in the k-th detection period and the l-th discrete velocity value.

[0044] τ m,k = (k-1)(ΔT+T)+2R / c, k=1,2,…K represents the time delay of the signal emitted by the m-th transmitting transducer in the k-th detection cycle, where R represents the distance between the target and the sonar;

[0045] v represents the radial velocity of the target; c represents the speed of sound; r(t) represents reverberation; n(t) represents noise.

[0046] This represents the Doppler frequency shift signal designed using the pulse signal emitted by the m-th transmitting transducer in the k-th detection period and the l-th discrete velocity value, where v l This represents the l-th discrete radial velocity.

[0047] Furthermore, step 4 specifically includes:

[0048] The complex domain outputs of all MK matched filter banks are summed to obtain the coherent cumulative output of the continuous wave MIMO sonar, which is expressed as:

[0049]

[0050] In the formula,

[0051] Y C (t,v) represents the coherent cumulative output of the continuous wave MIMO sonar;

[0052] The incoherent cumulative output of the continuous wave MIMO sonar is obtained by summing the absolute values ​​of the outputs of all MK matched filter banks. Its expression is:

[0053]

[0054] In the formula,

[0055] Y InC (t,v) represents the incoherent cumulative output of a continuous wave MIMO sonar.

[0056] The beneficial effects of this invention are as follows:

[0057] This invention addresses the problems of traditional active sonar, which mainly employs repetitive periodic detection and requires a sufficiently long detection period, resulting in insufficient accumulation of cycles within a limited time and difficulty in detecting weak targets. It proposes a method that utilizes MIMO sonar and a multi-cycle transmission mode to continuously transmit a frequency diversity continuous wave pulse signal. This is combined with Doppler filtering, incoherent accumulation, and coherent accumulation methods to fully utilize Doppler filtering to suppress reverberation and multi-cycle accumulation to suppress noise, thereby obtaining a robust, high-gain cumulative output.

[0058] The basic principle of this invention has been theoretically derived, and the implementation scheme has been verified by computer numerical simulation. The results of weak target detection obtained by traditional methods and by the method proposed in this invention are presented respectively. The results show that the method proposed in this invention can be applied to various MIMO sonar arrays, and the signal used can be designed according to the detection requirements, such as detecting weak targets, detecting long-range targets, and detecting slow-moving small underwater targets. It can simultaneously suppress reverberation and noise, and obtain weak target detection performance superior to traditional active sonar. In practical applications, different numbers of transmitting array elements and detection periods can be designed according to requirements to achieve robust and high-gain detection of weak targets by active sonar. Attached Figure Description

[0059] The above and / or other features and advantages of the present invention will become more readily understood from the following description with reference to the accompanying drawings, which are not drawn to scale and some features are enlarged or reduced to show details of specific parts.

[0060] Figure 1 The flowcharts are of the main steps of the present invention, wherein (a) is a flowchart of the main steps of coherent accumulation processing, and (b) is a flowchart of the main steps of incoherent accumulation processing.

[0061] Figure 2 This is a schematic diagram of the transmission mode of the transmitting transducer of the present invention;

[0062] Figure 3 The diagram shows the array structure of a MIMO sonar, where (a) is the array structure of a MIMO sonar in Implementation Example Simulation 1, (b) is the array structure of a MIMO sonar in Implementation Example Simulation 2, (c) is the array structure of a MIMO sonar in Implementation Example Simulation 3, and (d), (e), and (f) are examples of array structures that can be used in this invention.

[0063] Figure 4 To implement the MIMO sonar transmission signal spectrum used in Example Simulation 1, (a) is the transmission of a total of 10 FDCW signals, (b) is the transmission signal of transmitter transducer 1, and (c) is the transmission signal of transmitter transducer 2;

[0064] Figure 5 To implement the simulation of the autocorrelation function and the highest cross-correlation function of the transmitted signal used by the MIMO sonar in Example 1;

[0065] Figure 6 The detection results of Example Simulation 1 are shown in (a) for single-cycle detection, (b) for coherent accumulation processing, and (c) for incoherent accumulation processing.

[0066] Figure 7To implement the MIMO sonar transmission signal spectrum used in Example Simulation 2, (a) is the transmission of a total of 12 FDCW signals, (b) is the transmission signal of transmitter transducer 1, (c) is the transmission signal of transmitter transducer 2, and (d) is the transmission signal of transmitter transducer 3.

[0067] Figure 8 To implement the autocorrelation function and the highest cross-correlation function of the transmitted signal used by the MIMO sonar in Example Simulation 2;

[0068] Figure 9 The detection results of Example Simulation 2 are shown in (a) for single-cycle detection, (b) for detection results of coherent accumulation processing, and (c) for detection results of incoherent accumulation processing.

[0069] Figure 10 To implement the MIMO sonar transmission signal spectrum used in Example 3, (a) shows a total of 10 transmitted FDCW signals, and (b) shows the time-frequency diagram of the transmitted signals, with 5 transmitting transducers simultaneously transmitting FDCW pulse signals;

[0070] Figure 11 To implement the simulation of the MIMO sonar in Example 3, the autocorrelation function and the highest cross-correlation function of the transmitted signal are calculated.

[0071] Figure 12 The detection results of Example Simulation 3 are shown in (a) for single-cycle detection, (b) for coherent accumulation processing, and (c) for incoherent accumulation processing. Detailed Implementation

[0072] The present invention will now be described in detail with reference to the accompanying drawings. It should be noted that the following detailed description of the present invention is for illustrative purposes only and is not intended to limit the present invention.

[0073] This invention proposes a robust, high-gain detection method for weak targets using continuous-wave MIMO sonar and Doppler filtering, such as... Figure 1 The diagram shows the main steps of this invention, including two types: coherent accumulation processing and incoherent accumulation processing. (a) is the main steps flowchart for coherent accumulation processing, and (b) is the main steps flowchart for incoherent accumulation processing. Figure 2 The diagram shown illustrates the transmission mode of the transmitting transducer of the present invention. Each step of the present invention will be described in detail below:

[0074] Step 1: Design a frequency diverse continuous wave (FDCW) pulse signal with low cross-correlation based on the number of transmitting transducers and the number of detection cycles of the MIMO sonar.

[0075] First, construct a monostatic MIMO sonar: Select M transmitting transducers to form a transmitting array, where M is an integer; select N hydrophones to form a multi-element receiving array, where the hydrophones are arranged in a straight line, and N is an integer; set the axis of the multi-element receiving array as the x-direction, and the direction perpendicular to the x-direction as the y-direction, then the transmitting array and the receiving array together form a MIMO sonar.

[0076] Redesign the transmission waveform: Design the number of transmitting transducers to be M, and the number of detection cycles to be K. Then the number of FDCW pulse signals is the product of the number of transmitting transducers and the number of detection cycles, which is MK. The FDCW pulse signals have the same pulse width T and the same frequency interval Δf. The frequency interval Δf needs to be large enough to ensure that the peak value of the cross-correlation function is not greater than 0.001 times the peak value of the autocorrelation function. At the same time, a sufficiently large frequency interval Δf needs to be designed according to the maximum Doppler frequency shift generated by the target's motion velocity to ensure that the Doppler frequency shift of the target echo will not cause echo confusion between adjacent FDCW pulse signals.

[0077] Step 2: Continuously transmit the designed FDCW pulse signal using MIMO sonar and multi-cycle transmission mode.

[0078] The transmitting array simultaneously transmits M mutually orthogonal continuous wave pulse signals. Taking the FDCW pulse signal as an example, M transmitting array elements simultaneously transmit M FDCW pulse signals, and so on. All MK FDCW pulse signals are transmitted over K detection cycles. Therefore, the expression for the pulse signal transmitted by the m-th transmitting transducer in the k-th detection cycle is:

[0079]

[0080] In the formula,

[0081] s m,k (t) represents the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle, where m = 1, 2, ..., M and k = 1, 2, ..., K;

[0082] f m,k This represents the center frequency of the pulse signal from the m-th transmitting transducer during the k-th detection period;

[0083] t represents time;

[0084] T represents the pulse width of the transmitted signal;

[0085] ΔT represents the time interval between adjacent detection periods, and the value of ΔT is much smaller than the time interval used by traditional active sonar, that is, the range of ΔT is ΔT∈(0, 6T];

[0086] A represents the signal amplitude, and Where F represents the sampling frequency;

[0087] j represents the imaginary number symbol.

[0088] Step 3: Perform Doppler filtering on the received waves to obtain the Doppler filtered output of all FDCW pulse signals.

[0089] Step 3.1: The MIMO sonar simultaneously emits M mutually orthogonal continuous pulse signals in each detection cycle. After illuminating the target, it forms a target echo. The multi-element receiver array receives the target echo and performs time-domain beamforming on the received echo to obtain the time-domain beam output.

[0090] Assuming the target is located within a certain beam, and the distance between the target and the center of the sonar system is R, then the echo on this beam can be represented as the superposition of multiple continuous wave pulses with different parameters (center frequency, time delay) after passing through the channel. That is, the expression for the received echo corresponding to all MK transmitted pulse signals transmitted in K detection cycles is:

[0091]

[0092] In the formula,

[0093] x(t) represents the received echo corresponding to MK transmitted pulse signals;

[0094] τ m,k = (k-1)(ΔT+T)+2R / c, k=1,2,…K represents the time delay of the signal emitted by the m-th transmitting transducer in the k-th detection period;

[0095] v represents the radial velocity of the target;

[0096] c represents the speed of sound;

[0097] r(t) represents reverberation;

[0098] n(t) represents noise.

[0099] Step 3.2: Assume the possible range of values ​​for the target's radial velocity v (unless otherwise specified, velocities in the following text refer to the radial velocity of the target relative to the sonar) is v∈[v rd1 ,v rd2 ], where v rd1 v rd2 Let v represent the radial velocity of the target. Within the aforementioned velocity range, the radial velocity of the target is discretized to obtain L discrete radial velocity values. Then, the l-th discrete radial velocity v l The expression is:

[0100]

[0101] Step 3.3: Based on the parameters of the MK transmitted FDCW pulse signals, design a set of matched filters for each pulse signal, that is, design a matched filter at each discrete velocity, so that each FDCW pulse signal can obtain a matched filter group consisting of L matched filters.

[0102] The expression for the impulse response function of the l-th matched filter, designed using the parameters of the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle, is as follows:

[0103] h m,k,l (t)=s m,k,l c (Tt) (4)

[0104] In the formula,

[0105] h m,k,l (t) represents the impulse response function of the l-th matched filter designed with the parameters of the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle;

[0106] The superscript c represents taking the conjugate, and the subscripts m, k, l represent the sequence numbers corresponding to the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle and the l-th discrete velocity value;

[0107] s m,k,l (t) represents the Doppler frequency shift signal designed using the pulse signal emitted by the m-th transmitting transducer in the k-th detection period and the l-th discrete velocity value, and its expression is:

[0108]

[0109] Step 3.4: Design MK matched filter banks for the MK transmitted FDCW pulse signals, where each matched filter bank contains L filters, as represented by equations (4) and (5). Use the MK matched filter banks to process the echo signals to obtain the results of MK detection cycles. The corresponding matched filter output is:

[0110] y m,k,l =x(t)*h m,k,l (t) (6)

[0111] In the formula,

[0112] y m,k,l This represents the matched filter output obtained by processing the received signal using the l-th matched filter of the matched filter bank designed for the pulse signal emitted by the m-th transmitting transducer in the k-th detection period.

[0113] * indicates a convolution operation.

[0114] Since the transmitted signal is designed based on the maximum Doppler frequency shift caused by the target's motion velocity, and this transmitted signal has a sufficiently large frequency interval Δf, it ensures that the Doppler frequency shift of the target echo will not cause echo confusion between adjacent FDCW pulse signals. Therefore, the matched filter output obtained by processing the received signal by the l-th matched filter of the matched filter bank designed for the pulse signal transmitted by the m-th transmitting transducer in the k-th detection period can be expressed as:

[0115]

[0116] Step 3.5: Combining the outputs of the L matched filters yields the processing result of the matched filter bank designed for the pulse signal transmitted by the m-th transmitting transducer in the k-th detection cycle, and the received wave.

[0117] Y m,k (t,v)=[y m,k,1 (t,v) y m,k,2 (t,v) … y m,k,L (t,v)] (8)

[0118] In the formula,

[0119] Y m,k (t,v) represents the processing result of the m-th, k-th matched filter group receiving the echo.

[0120] The MK-group matched filter output, i.e. the results of MK detection cycles, was obtained using the MK-group matched filter for subsequent processing.

[0121] Step 4: Perform coherent accumulation and incoherent accumulation on the Doppler filter output to obtain a robust, high-gain accumulated output.

[0122] This invention employs coherent and incoherent accumulation processing respectively to fuse the results of MK detection cycles to obtain better weak target detection results than a single detection cycle, wherein:

[0123] The complex domain outputs of all MK matched filter banks are summed to obtain the coherent cumulative output of the continuous wave MIMO sonar, which is expressed as:

[0124]

[0125] In the formula,

[0126] Y C (t,v) represents the coherent cumulative output of the continuous wave MIMO sonar.

[0127] The incoherent cumulative output of the continuous wave MIMO sonar is obtained by summing the absolute values ​​of the outputs of all MK matched filter banks. Its expression is:

[0128]

[0129] In the formula,

[0130] Y InC (t,v) represents the incoherent cumulative output of a continuous wave MIMO sonar.

[0131] Implementation Examples

[0132] The present invention will now be described in detail with the aid of exemplary embodiments, namely, using computer numerical simulations to verify the effectiveness of the method proposed in the present invention.

[0133] Simulation 1: M=2, K=5

[0134] The MIMO sonar has 2 transmitting array elements (M=2), with the center of the transmitting array elements located at the origin. The receiving array (ULA) has 8 elements (N=8). A schematic diagram of the array structure is shown below. Figure 3 As shown in (a), the number of transmitting transducers is set to M=2, the number of detection cycles is K=5, the sampling frequency is 10kHz, the frequency interval between adjacent FDCW signals is Δf=50Hz, the pulse width of a single pulse is T=2 seconds, and the signal amplitude is A=0.0071.

[0135] Performing a Fourier transform on the transmitted signal yields a combination of 10 FDCW signals, the spectrum of which is shown below. Figure 4 As shown; calculate the autocorrelation function and cross-correlation function of the transmitted signal, and the results are as follows. Figure 5 As shown, the peak height of the cross-correlation function is about 50 dB lower than the peak height of the autocorrelation function.

[0136] Assuming a target exists in the detection scenario, the distance between the target and the sonar is R = 20 km, and the target's radial velocity is v = 3 m / s, 100 scattering points are set to simulate reverberant echoes, the signal-to-mixing ratio is approximately -20 dB, and complex Gaussian white noise with a power of 10 dB is added to each received frame. The processing result of a single frame received echo is as follows. Figure 6 As shown in (a), the results of coherent and incoherent accumulation of the 10 frames of received echoes are as follows: Figure 6 (b) Figure 6 As shown in (c).

[0137] The detection results show that traditional single-cycle detection has a high background, making it difficult to detect targets. However, coherent and incoherent accumulation processing can reduce the background and detect targets with radial velocity v = 3 m / s at 20 km.

[0138] Simulation 2: M=3, K=4

[0139] The MIMO sonar has 3 transmitting array elements (M=3), with the center of the transmitting array elements located at the origin. The receiving array (ULA) has 8 elements (N=8). A schematic diagram of the array structure is shown below. Figure 3 As shown in (b), the number of transmitting transducers is set to M=3, the number of detection cycles is K=4, and other parameters are the same as in simulation 1.

[0140] Performing a Fourier transform on the transmitted signal yields a combination of 12 FDCW signals, the spectrum of which is shown below. Figure 7 As shown; calculate the autocorrelation function and cross-correlation function of the transmitted signal, and the results are as follows. Figure 8 As shown, the peak height of the cross-correlation function is about 50 dB lower than the peak height of the autocorrelation function.

[0141] Assuming a target exists in the detection scenario, the distance between the target and the sonar is R = 20 km, and the target's radial velocity is v = 3 m / s, 100 scattering points are set to simulate reverberant echoes, the signal-to-mixing ratio is approximately -20 dB, and complex Gaussian white noise with a power of 13 dB is added to each received frame. The processing result of a single frame received echo is as follows. Figure 9 As shown in (a), the results of coherent and incoherent accumulation of the 12 frames of received echoes are as follows: Figure 9 (b) Figure 9 As shown in (c).

[0142] The detection results show that traditional single-cycle detection has a high background due to insufficient reverberation and noise suppression capabilities, making it difficult to detect targets. However, the coherent accumulation and incoherent accumulation processing proposed in this invention can detect moving targets at a distance of 20km and a radial velocity of v = 3m / s.

[0143] Simulations 1 and 2 demonstrate that the proposed method achieves superior reverberation and noise suppression compared to traditional methods, thus effectively improving the weak target detection performance of active sonar. In practical applications, different numbers of transmitter array elements and detection periods can be designed according to requirements to achieve robust and high-gain detection of weak targets by active sonar.

[0144] Simulation 3: Slow small target detection with M=5 and K=2

[0145] The MIMO sonar has 5 transmitting array elements (M=5), with the center of the transmitting array elements located at the origin. The receiving array (ULA) has 8 elements (N=8). A schematic diagram of the array structure is shown below. Figure 3 As shown in (c), the number of transmitting transducers is set to M=5, the number of detection cycles is K=2, the sampling frequency is 100kHz, the frequency interval between adjacent FDCW signals is Δf=500Hz, the signal frequency band is 30.5kHz-35kHz, the pulse width of a single pulse is T=0.2 seconds, and the signal amplitude is A=0.0071.

[0146] Performing a Fourier transform on the transmitted signal yields a combination of 10 FDCW signals, the spectrum of which is shown below. Figure 10 As shown. Calculate the autocorrelation function and cross-correlation function of the transmitted signal, and the results are as follows. Figure 11 As shown, the peak height of the cross-correlation function is about 50 dB lower than the peak height of the autocorrelation function.

[0147] Assume there is a slow-moving small target in the detection scenario, the distance between the target and the sonar is R = 2 km, the radial velocity of the target is v = 1 m / s, 100 scattering points are set to simulate reverberant echoes, the signal-to-mixing ratio is approximately -20 dB, and complex Gaussian white noise with a power of 10 dB is added to each received frame. The result of processing the single-frame received echo is as follows. Figure 12 As shown in (a), the results of coherent and incoherent accumulation of the 10 frames of received echoes are as follows: Figure 12 (b) Figure 12 As shown in (c).

[0148] The detection results show that traditional single-cycle detection has a high background, making it difficult to detect targets. However, coherent and incoherent accumulation processing can reduce the background and detect targets with a radial velocity of v = 1 m / s at 2 km. Therefore, the method proposed in this invention can also be used for the detection of slow-moving small targets.

[0149] Based on the implementation examples, it can be concluded that the method proposed in this invention is applicable to various MIMO sonar arrays, and the signal used can be designed according to detection requirements, such as detecting weak targets, detecting long-range targets, and detecting slow-moving small underwater targets. This invention utilizes continuous wave MIMO sonar, Doppler filtering, and multi-period accumulation for weak target detection, effectively solving the problem that traditional active sonar struggles to effectively detect weak targets.

Claims

1. A robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering, characterized in that, Includes the following steps: Step 1: Design a frequency diversity continuous wave pulse signal with low cross-correlation based on the number of transmitting transducers and the number of detection cycles of the MIMO sonar; Step 2: Continuously transmit the designed frequency diversity continuous wave pulse signal using MIMO sonar and multi-cycle transmission mode; Step 3: Perform Doppler filtering on the received wave to obtain the Doppler filtered output of the diversity continuous wave pulse signal of all frequencies; Step 4: Perform coherent accumulation and incoherent accumulation on the Doppler filter output to obtain a robust, high-gain accumulated output.

2. The robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering as described in claim 1, characterized in that, Step 1 specifically includes: First, construct a monostatic MIMO sonar: select M transmitting transducers to form a transmitting array, M is an integer; select N hydrophones to form a multi-element receiving array, the hydrophones are arranged in a straight line, N is an integer; set the axis of the multi-element receiving array as the x-direction, and the direction perpendicular to the x-direction as the y-direction, then the transmitting array and the receiving array together form a MIMO sonar. Redesign the transmitted waveform: If the number of transmitting transducers is M and the number of detection periods is K, then the number of frequency diversity continuous wave pulse signals is the product of the number of transmitting transducers and the number of detection periods, which is MK. The frequency diversity continuous wave pulse signals have the same pulse width T and the same frequency interval. f, where the frequency interval f needs to be large enough to ensure that the peak value of the cross-correlation function is no greater than 0.001 times the peak value of the autocorrelation function; at the same time, a sufficiently large frequency interval needs to be designed based on the maximum Doppler frequency shift caused by the target's motion velocity. f, to ensure that the Doppler frequency shift of the target echo does not cause echo confusion of adjacent frequency diversity continuous wave pulse signals.

3. The robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering as described in claim 1, characterized in that, In step 2: M transmitting transducers simultaneously transmit M frequency diversity continuous wave pulse signals. The expression for the frequency diversity continuous wave pulse signal transmitted by the m-th transmitting transducer in the k-th detection period is: In the formula, This represents the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle, where m = 1, 2, ... M, ; This represents the center frequency of the pulse signal from the m-th transmitting transducer during the k-th detection period; t represents time; T represents the pulse width of the transmitted signal; T represents the time interval between adjacent detection periods, and The value of T is much smaller than the time interval used in traditional active sonar, that is The range of values ​​for T is: T (0, 6T]; A represents the signal amplitude, and , where F represents the sampling frequency; j represents the imaginary number symbol.

4. The robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering as described in claim 1, characterized in that, Step 3 specifically includes: Step 3.1: The MIMO sonar simultaneously transmits M mutually orthogonal continuous pulse signals in each detection cycle. After illuminating the target, it forms a target echo. The multi-element receiver array receives the target echo and performs time-domain beamforming on the received echo to obtain the time-domain beam output. Step 3.2: Define the possible range of radial velocity values ​​for the target, and discretize the radial velocity of the target within the range of velocity values ​​to obtain L discrete radial velocity values; Step 3.3: Combining the parameters of the MK frequency diversity continuous wave pulse signals emitted in K detection cycles, a matched filter is designed at each discrete velocity, so that each frequency diversity continuous wave pulse signal can obtain a matched filter group consisting of L matched filters. Step 3.4: Design MK matched filter banks for the MK frequency diversity continuous wave pulse signals to be transmitted, with L matched filters in each bank. Use the MK matched filter banks to process the echo signals to obtain the results of MK detection cycles. Step 3.5: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full context.] The outputs of the matched filters can be combined to obtain the processing result of the received wave from the matched filter group designed for the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle.

5. The robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering as described in claim 4, characterized in that: The result of processing the received wave by the matched filter bank designed for the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle in step 3.5 is as follows: In the formula, Indicates the first The processing results of the returned wave from the matched filter group; This represents the matched filter bank designed for the pulse signal emitted by the m-th transmitting transducer in the k-th detection period. The expression for the matched filter output obtained by processing the received signal using a matched filter is: In the formula, This represents the received echo corresponding to MK transmitted pulse signals, where This represents the center frequency of the pulse signal from the m-th transmitting transducer during the k-th detection period; This represents the convolution operation; The parameters of the pulse signal emitted by the m-th transmitting transducer in the k-th detection period are designed as follows: The impulse response function of a matched filter, where the superscript c represents taking the conjugate, and the subscripts m, k, ... This represents the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle and the pulse signal emitted by the m-th transmitting transducer in the k-th detection cycle. The serial number corresponding to each discrete velocity value, where T represents the pulse width of the transmitted signal; Let R represent the time delay of the signal emitted by the m-th transmitting transducer in the k-th detection period, where R represents the distance between the target and the sonar. T represents the time interval between adjacent detection periods, and The value of T is much smaller than the time interval used in traditional active sonar, that is The range of values ​​for T is: T (0, 6T]; Indicates the radial velocity of the target; r(t) represents the speed of sound; r(t) represents reverberation; n(t) represents noise. This represents the pulse signal emitted by the m-th transmitting transducer during the k-th detection cycle and the pulse signal emitted by the m-th transmitting transducer during the k-th detection cycle. A Doppler frequency shift signal designed with discrete velocity values, wherein Indicates the first A discrete radial velocity.

6. The robust high-gain detection method for weak targets using continuous wave MIMO sonar and Doppler filtering as described in claim 1, characterized in that, Step 4 specifically includes: The coherent cumulative output of the continuous wave MIMO sonar is obtained by summing the complex domain outputs of all MK matched filter banks designed by combining M transmitting transducers with MK frequency diversity continuous wave pulse signals emitted within K detection periods. The expression for this coherent cumulative output is as follows: In the formula, This represents the coherent cumulative output of a continuous-wave MIMO sonar; Indicates the first The processing results of the returned wave from the matched filter group; The incoherent cumulative output of the continuous wave MIMO sonar is obtained by summing the absolute values ​​of the outputs of all MK matched filter banks. Its expression is: In the formula, This represents the incoherent cumulative output of a continuous-wave MIMO sonar.

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