A method for estimating the target azimuth of a broadband split beam sonar

Abstract

The present invention discloses a method for estimating the target azimuth of a broadband split-beam sonar. The transducer used in this method includes four quadrants that are centrosymmetric. In this method, the transmitted signal is down-converted to obtain the analytic signal z(t). For the 4 received signals s a (t), s b (t), s c (t), s d (t) received by the four transducer quadrants are respectively down-converted to obtain 4 analytic signals: z a (t), z b (t), z c (t), z d (t); each received analytic signal is respectively subjected to matched filtering with the transmitted analytic signal to obtain m a (t), m b (t), m c (t), m d (t); the received signals of each quadrant are added to obtain four sub-array signals of up, down, left, and right; the delay and the target azimuth are calculated according to the signals of each sub-array. This method has a small amount of computation, is easy to implement in engineering, and can accurately estimate the target azimuth when the target interval is greater than or equal to ΔR = C / 2B (C is the sound speed, B is the bandwidth of the detection signal).

Description

Technical Field

[0001] The present invention relates to the field of underwater acoustic equipment, and in particular to a method for estimating the target azimuth of a broadband split beam sonar. Background Art

[0002] Accurately measuring the target strength is a necessary condition for target size estimation, which requires determining the position of the target in the beam space to correct the measurement deviation caused by the beam directivity. When there is only one transducer, only the target distance can be estimated, and the target azimuth cannot be determined. The split beam technology divides the transducer into four quadrants, and determines the azimuth of the target through the signal differences received by each quadrant. The narrowband split beam sonar uses a pulse continuous (CW) detection signal and estimates the angle by calculating the phase difference of the received signal, while the broadband split beam sonar uses a linear frequency modulation (LFM) signal, whose phase is time-varying, and generally determines the target azimuth through time delay estimation (TDE). The commonly used time delay estimation methods in engineering include the cross-correlation method, adaptive filtering, and cross-power spectrum method. These methods use a sliding window to intercept data and then calculate. The time length of the window is generally the pulse width of the transmitted signal. When there are target echoes at different angles in the data window, these methods will produce large errors. Shortening the data window length can reduce the overlap probability, but it will cause a decrease in the estimation accuracy. Therefore, accurately estimating the target azimuth requires the target interval to be greater than ΔR = CT / 2 (C is the sound speed, T is the pulse width of the transmitted signal). Summary of the Invention

[0003] In view of the above-mentioned defects of the prior art, the present invention provides a method for estimating the target azimuth of a broadband split beam sonar, which can overcome the problems existing in the above-mentioned prior art.

[0004] To achieve the above object, the present invention provides a method for estimating the target azimuth of a broadband split beam sonar. The transducer array used in this method includes four quadrants a, b, c, and d that are symmetric about the center; the transmitted signal s(t) = A(t)cos(2πf c t + πKt 2 ), where f c is the carrier frequency, K is the frequency modulation slope, K = B / T; B is the bandwidth, and T is the pulse width of the transmitted signal; The process of signal processing includes:

[0005] (S1) Mix down the transmitted signal to obtain the analytic signal z(t) of the transmitted signal;

[0006] (S2) Mix down the 4 received signals s a (t), s b (t), s c (t), s d (t) received by the four quadrants of the transducer array respectively to obtain 4 analytic signals: z a(t), z b (t), z c (t), z d (t);

[0007] (S3) Match-filter each received parsed signal with the parsed signal z(t) of the transmitted signal to obtain m a (t), m b (t), m c (t), m d (t);

[0008] (S4) Obtain the four sub-array signals y in the up, down, left, and right directions with reference to the following formula u (t), y b (t), y l (t), y r (t):

[0009] y u (t) = m a (t) + m b (t)

[0010] y b (t) = m c (t) + m d (t)

[0011] y l (t) = m a (t) + m c (t)

[0012] y r (t) = m b (t) + m d (t)

[0013] (S5) Calculate the time delay, and the expression is:

[0014] Time delay in the Y direction

[0015] Time delay in the X direction

[0016] In this step, phase estimation is performed using the result after match filtering, expanding the phase time delay estimation from narrowband to broadband and avoiding the influence of phase time variation of broadband signals;

[0017] (S6) Calculate the target azimuth, and the expression is:

[0018] Y-direction angle

[0019] Where: C is the speed of sound, and D is the acoustic center distance between the ab sub-array and the cd sub-array; the ab sub-array is the upper sub-array, including quadrant a and quadrant b; the cd sub-array is the lower sub-array, including quadrant c and quadrant d;

[0020] X-direction angle

[0021] Where C is the speed of sound, and D is the acoustic center distance between the ac sub-array and the bd sub-array; the ac sub-array is the left sub-array, including quadrant a and quadrant c; the bd sub-array is the right sub-array, including quadrant b and quadrant d;

[0022] A further improvement of the present invention lies in that in step S3, the received signals are respectively subjected to matched filtering, and its time-domain calculation formula is:

[0023]

[0024]

[0025]

[0026]

[0027] Where, z a (u), z b (u), z c (u), z d (u) are the input signals of the matched filter, which are the analytic signals of each received signal; m a (t), m b (t), m c (t), m d (t) are the output signals of the matched filtering; z * (t) is the conjugate signal of the analytic signal z(t) of the transmitted signal; the above integral formula represents a convolution calculation.

[0028] The method provided by the present invention has the following technical effects: The method has a small amount of computation. The matched filtering can be calculated through time-domain convolution or frequency FFT, which is easy to implement in engineering. There is no need to intercept data for calculation, and the target azimuth can be accurately estimated when the target interval is greater than or equal to ΔR = C / 2B (C is the speed of sound, and B is the signal bandwidth). BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a schematic diagram of the transducer array adopted by the method of the present invention;

[0030] Figure 2 is a schematic diagram of the output of the matched filtering of the LFM signal;

[0031] Figure 3 is a schematic diagram of split-beam angle estimation. Detailed implementation manners

[0032] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0033] It should be noted that the diagrams provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner. Therefore, only the components related to the present invention are shown in the diagrams, rather than being drawn according to the number, shape, and size of the components in actual implementation. The types, quantities, and proportions of the components in actual implementation can be arbitrarily changed, and the component layout type may also be more complex.

[0034] For the purpose of illustration, some exemplary embodiments of the present invention are described. It should be understood that the present invention can be implemented in other ways not specifically shown in the drawings.

[0035] An embodiment of the present invention provides a method for estimating the target azimuth of a broadband split beam sonar, as Figure 1 shown. The transducer used in this method includes four quadrants a, b, c, and d that are symmetric about the center; the sonar uses a transmitted signal s(t) = A(t)cos(2πf c t + πKt 2 ), where f c is the carrier frequency, K is the frequency modulation slope, K = B / T; B is the bandwidth, and T is the pulse width of the transmitted signal;

[0036] The downmixing method used in this embodiment processes the input signal s(t) as follows:

[0037] (S01) Mix the input signal s(t) separately, and its expression is:

[0038] x(t) = s(t)cos(2πf c t)

[0039] y(t) = s(t)sin(2πf c t)

[0040] (S02) Pass x(t) and y(t) through a low-pass filter with a cut-off frequency of 1.5B respectively to obtain the downmixing result, and the expression is:

[0041] z(t) = filter[x(t)] + 1jfilter[y(t)]

[0042] Where filter[·] represents a low-pass filter with a cut-off frequency of 1.5B, and j is the imaginary symbol.

[0043] In the matched filtering method adopted in this embodiment, in step S3, the analytic signal z(u) of the transmitted signal, the matched filter response h(u) = z * (-u), and the output of the matched filtering of the input signal s(u) As Figure 2 shown, for the LFM signal, the range resolution after matched filtering is ΔR = C / 2B, and the range resolution after matched filtering is independent of the transmitted pulse width.

[0044] In the specific implementation process:

[0045] The transmitted signal is A(t)cos(2πf c t + πKt 2 ), where f c is the carrier frequency, K is the frequency modulation slope, K = B / T; B is the bandwidth, and T is the transmitted signal pulse width; Assume that the time delay of the received signal is τ, and ignoring the signal amplitude change, the received signal is:

[0046] s(t) = A(t - τ)cos[2πf c (t - τ) + πK(t - τ) 2

[0047] Perform down-conversion on the received signal:

[0048] m 1 (t) = 2A(t - τ)cos[2πf c (t - τ) + πK(t - τ) 2 cos(2πf c t) = A(t - τ){cos[4πf c t - 2πf c τ + πK(t - τ) 2 + cos[πK(t - τ) 2 - 2πf c τ]}

[0049] m 2 (t) = -2A(t - τ)cos[2πf c (t - τ) + πK(t - τ) 2 sin(2πf c t) = A(t - τ){-sin[4πf c t - 2πf​c τ + πK(t - τ) 2 + sin[πK(t - τ) 2 -2πf c τ]}

[0050] After passing through a low - pass filter, the high - frequency part is removed to obtain the analytic signal:

[0051] z(t) = cos[πK(t - τ) 2 -2πf c τ] + j sin[πK(t - τ) 2 -2πf c τ] = exp[jπK(t - τ) 2 -j2πf c τ]

[0052] Use the filter h(u) = A(t)exp(-jπKt 2 ), h(u) is the conjugate signal of the transmitted signal, and its output is

[0053]

[0054] When t ≥ τ,

[0055] y(t) = exp(-j2πf c τ)∫ t τ+T exp[jπK(2u - τ - t)(t - τ)]du

[0056] Let v = 2u - τ - t;

[0057]

[0058] Similarly, when t < τ,

[0059]

[0060]

[0061] Before matched filtering, the received signal is exp[jπK(t - τ) 2 -j2πf c τ]. The phase difference between the two signals is a function of time and changes with time t. By observing the expression of the above - mentioned matched filtering y(t), it can be seen that after matched filtering, the phase difference between the two signals is fixed, and the accurate time delay is easily obtained.

[0062] Suppose there are 2 receiving channels, and the outputs after matched filtering are y 1 and y 2 , let

[0063] r(t) = y 1 y 2 * = A 1 (t)A 2 (t)exp[jπB(τ 2 -τ 1 ) + j2πf c (τ 2 -τ 1 )]

[0064] where A 1 (t) and A 2 (t) are the amplitude functions after matched filtering. The expression for the time delay difference of the received signal is:

[0065]

[0066] where imag[r(t)] is the imaginary part of r(t), and real[r(t)] is the real part of r(t).

[0067] Angle estimation principle:

[0068] Since there is a time difference when the echo reaches transducers in different quadrants, the position of the target in the beam is determined accordingly. As Figure 3 shown, there are two sub-arrays A and B with a spacing D. Obviously, the relative time delay between xA(t) and xB(t) corresponds to the signal incident angle θ,

[0069] In this embodiment, through matched filtering, the reflected signals of targets that are relatively close or even partially overlapping can be distinguished, solving the problem of large errors in signal delay calculation when the traditional method processes the reflected signals of close-range or partially overlapping targets. In specific applications, the method of this application can resolve targets with a distance greater than or equal to C / 2B. For example, when the bandwidth B = 60k, as long as the target distance interval is greater than or equal to 12.5mm, the angle can be resolved.

[0070] The above embodiments are only illustrative of the principles and effects of the present invention, and are not used to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A method for estimating the target azimuth of a broadband split beam sonar, characterized in that, The transducer array used in this method includes four quadrants a, b, c, and d that are symmetric about the center; the transmitted signal s(t) = A(t)cos(2πf c t + πKt 2 ), where f c is the carrier frequency, K is the frequency modulation slope, and its expression is K = B / T; B is the bandwidth, and T is the pulse width of the transmitted signal; The process of signal processing includes: (S1) Perform downmixing on the transmitted signal to obtain the analytical signal z(t) of the transmitted signal; (S2) Perform downmixing on the four 4-channel received signals s a (t), s b (t), s c (t), s d (t) received by the four quadrants of the transducer array to obtain four analytical signals: z a (t), z b (t), z c (t), z d (t); (S3) Match-filter each received parsed signal with the parsed signal of the transmitted signal to obtain m a (t), m b (t), m c (t), m d (t); (S4) Obtain the four sub-array signals y in the up, down, left, and right directions according to the following formula u (t), y b (t), y l (t), y r (t): y u (t) = m a (t) + m b (t) y b (t) = m c (t) + m d (t) y l (t) = m a (t) + m c (t) y r y(t) = m b y(t) + m d y(t) (S5) Calculate the time delay, and the expression is: Time delay in the Y direction Time delay in the X direction (S6) Calculate the target azimuth, and the expression is: Y-direction angle where: C is the speed of sound, and D is the acoustic center distance between the ab subarray and the cd subarray; X-direction angle where: C is the speed of sound, and D is the acoustic center distance between the ac subarray and the bd subarray.

2. The method for estimating the target azimuth of a broadband split beam sonar according to claim 1, characterized in that, In step S3, perform a matched filter on the analytical signals of the received signals respectively, and its time-domain calculation formula is: Among them, z a (u), z b (u), z c (u), z d (u) is the input signal of the matched filter, m a (t), m b (t), m c (t), m d (t) is the output signal of the matched filtering; z * (t) is the conjugate signal of the analytic signal z(t) of the transmitted signal.

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