Radar anti-clutter method based on combined design of initial phase agility and filter

Abstract

The invention discloses a radar anti-clutter method based on initial phase agility and filter joint design, and belongs to the technical field of radar signal processing, and the method comprises the following steps: S1, initial phase agility waveform; s2, dividing distance sections; s3, performing range gating and pulse compression processing; and S4, filtering processing. According to the invention, through combined design of the transmitting end and the receiving end of the pulse radar, the low-speed target echo and the folded clutter which are originally aliasing in the Doppler domain and the distance domain are enabled to be separable by using the range gating characteristic of the agile waveform, so that the clutter suppression performance of the pulse radar in a low-speed target detection scene is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of radar signal processing, and in particular to a radar anti-clutter method based on the joint design of initial phase agility and filter. Background Art

[0002] In the process of radar target detection, clutter refers to the objective radar echo reflected by the background around the target, which includes various ground objects, clouds, rain, waves, vehicles, buildings, bird flocks, etc. Some of these backgrounds are completely motionless, and some are slowly moving. When the clutter power is too strong, the clutter will disrupt the normal operation of the radar, drown out the real target, and make target detection difficult. In order to reduce the clutter component of the echo received by the radar receiver, the main measures currently taken are: (1) increase the tilt angle of the radar antenna and install an anti-clutter net to reduce the clutter component entering the antenna; (2) improve the radar resolution. Clutter is the result of the synthesis of a large number of scattering unit echoes. The smaller the resolution unit, that is, the higher the resolution of the radar system, the weaker the clutter power entering the radar receiver; (3) when working at low repetition rate, use STC to suppress close-range clutter; (4) apply MTI and MTD technology in the Doppler frequency domain, suppress clutter power through Doppler frequency domain filters, and improve the signal-to-clutter ratio; (5) use constant false alarm detection technology and clutter map auxiliary processing to reduce the impact of clutter on detection; (6) airborne array radar uses space-time adaptive processing to suppress ground clutter.

[0003] Pulse radar can use the Doppler effect to design a Doppler frequency domain filter to detect moving targets under strong clutter interference. The pulse repetition frequency of pulse radar is divided into three levels: high, medium, and low. The selection of pulse repetition frequency depends on the pulse radar working mode. There is no range ambiguity at low repetition frequency, but there is Doppler ambiguity. The clutter suppression methods that can be used include: STC, MTI, and MTD. There is both range ambiguity and Doppler ambiguity at medium repetition frequency, and the main clutter suppression method is MTD. There is range ambiguity at high repetition frequency, but no Doppler ambiguity. The main clutter suppression method is MTD. When pulse radar works at a high repetition frequency, it has the following advantages: (1) it can reduce background fluctuations; (2) it can increase the radar blind speed when using MTI technology for clutter suppression; (3) the data rate is high, the number of accumulated pulses increases, and the signal-to-noise ratio can be improved. However, traditional pulse radar has range ambiguity when the pulse repetition frequency is high. At this time, the echoes of different range segments (the range segments are divided by the maximum unambiguous distance) are folded with each other. When there are strong short-range clutter, the weak echoes of these strong short-range clutter and long-range small targets will be folded into the same distance segment for vector superposition in the distance domain. This phenomenon is also called clutter folding. At this time, the long-range small target needs to compete with the strong short-range clutter, which seriously affects the radar's detection performance for long-range small targets.

[0004] In order to reduce the impact of strong clutter on weak target detection, the most commonly used signal processing method in pulse radar engineering is to design MTI or MTD filters, which use the Doppler effect of moving targets to filter out relatively static clutter in the Doppler frequency domain. However, when the target moves slowly and its Doppler frequency is close to the clutter spectrum, the filter notch will suppress the target energy while suppressing the clutter. Therefore, improving the improvement factor of the clutter suppression filter in the slow target detection scenario has become an urgent problem to be solved by pulse radar. To this end, the present invention proposes a radar anti-clutter method based on the joint design of initial phase agility and filter. Summary of the invention

[0005] The technical problem to be solved by the present invention is how to solve the problem of insufficient clutter suppression capability of traditional pulse radar in slow target detection scenario. A radar anti-clutter method based on the joint design of initial phase agility and filter is provided. By jointly designing the pulse radar transmitter and receiver, the range gating characteristics of the agile waveform are utilized to make the slow target echo and folded clutter originally mixed together in the Doppler domain and the range domain become separable, thereby improving the clutter suppression performance of the pulse radar in the slow target detection scenario.

[0006] The present invention solves the above technical problems through the following technical solutions, and the present invention comprises the following steps:

[0007] S1: Initial phase agile waveform

[0008] The radar transmits an initial phase agile waveform between pulses, and the initial phase of each transmitted pulse waveform is randomly selected in the range of [0-2π];

[0009] S2: Divide distance segments

[0010] After determining the radar transmission waveform parameters, according to the radar transmission waveform pulse repetition period T r Divide the observation distance into segments;

[0011] S3: Range gating and pulse compression processing

[0012] Set the C hannel Distance segment, from C hannel The baseband echo is compressed after the first echo pulse.

[0013] S4: Filtering

[0014] According to the radar transmission waveform parameters and the gating distance segment C hannel The filter weight coefficients are designed, and the echo after pulse compression is filtered based on the coefficients.

[0015] Furthermore, in step S1, it is assumed that the linear frequency modulation signal is transmitted in the pulse, and the i-th transmission pulse s iThe waveform expression is:

[0016] s i =exp(j(πμt 2 +φ(i)))i=1,...,M

[0017] Where exp(·) is an exponential function, j represents an imaginary unit, t is a sampling time series, μ is the frequency modulation slope of the linear frequency modulation signal, φ(i) is the initial phase of the i-th transmitted pulse, and M is the number of pulses contained in a coherent processing cycle.

[0018] Furthermore, in step S2, the range of each distance segment is as follows:

[0019] Distance segment 1: (0R max ]

[0020] Distance segment 2: (R max 2R max ]

[0021] Distance segment 3: (2R max 3R max ] ...

[0023] Among them, R max is the maximum unambiguous observation distance:

[0024]

[0025] Here, c represents the speed of light.

[0026] Furthermore, in step S3, pulse compression is performed by frequency domain matched filtering, and the nth baseband echo pulse x n The output after pulse compression for:

[0027]

[0028] Among them, fft(·) represents Fourier transform, ifft(·) represents inverse Fourier transform, conj(·) represents conjugation, represents the Hadamard product, s 1 The expression is as follows:

[0029] s 1 =exp(jπμt 2 ).

[0030] Furthermore, in step S4, the specific processing process is as follows:

[0031] S41: Filter Coefficient Design

[0032] Performing inter-pulse phase compensation on the pulse compressed data, constructing a clutter covariance matrix in different distance segments, and calculating filter weight coefficients based on the constructed clutter covariance matrix, wherein the filter is an MTD filter;

[0033] S42: Filtering

[0034] The echo data after pulse compression and pulse-to-pulse phase compensation is filtered based on the obtained filter weight coefficient to obtain the filtering result of the kth filter.

[0035] Furthermore, in step S41, the specific processing process is as follows:

[0036] S411: Perform inter-pulse phase compensation on the pulse compressed data. The compensation expression is:

[0037]

[0038] Where φ(i) is the initial phase of the i-th transmitted pulse, i=1,...,MC hannel +1;

[0039] S412: Assume that the standard deviation of the velocity to be used to suppress clutter is Δv, then generate a random velocity sequence v with a standard deviation of Δv, corresponding to a random Doppler frequency sequence f d for:

[0040] f d =2v / λ

[0041] Where λ is the wavelength;

[0042] S413: Set the pulse number to M, select the C hannel distance segment, then it is in distance segment C o The clutter slow time vector is:

[0043] t c =[[(C hannel -C o +1):(MC o +1)]-1]T r ;

[0044] S414: After phase compensation between meridians, from distance segment C o The clutter is expressed as:

[0045]

[0046] Among them, φ o From distance segment C o The residual initial phase of the clutter after inter-pulse phase compensation;

[0047] S415: Obtain the clutter covariance matrix based on the constructed clutter:

[0048]

[0049] After diagonal loading:

[0050]

[0051] Where δ is the diagonal loading coefficient and I is the unit matrix.

[0052] Furthermore, in step S41, the filter weight coefficient w k as follows:

[0053]

[0054] Among them, f k is the passband center frequency of the kth filter, a(f k ) is the pilot vector:

[0055] a(f k )=exp(-j2πf k t r )

[0056] Among them, t r =[(1:(MC hannel +1))-1]T r .

[0057] Furthermore, in step S42, the filtering result of the kth filter is:

[0058] z k =w k Y out1

[0059] Among them, Y out1 It is the echo data after pulse compression and inter-pulse phase compensation.

[0060] Compared with the prior art, the present invention has the following advantages:

[0061] 1. Compared with traditional pulse radar technology, the present invention distinguishes the clutter and target originally aliased in the range domain and Doppler domain through the joint design of initial phase agility and filter, so that the receiving end filter can suppress the energy of short-range strong clutter while ensuring the gain of long-range slow targets.

[0062] 2. Compared with traditional pulse radar technology, the present invention adopts a pulse-to-pulse agile transmission waveform strategy, combined with the receiving end signal processing design, so that the radar system has a range gating characteristic, which can improve the range ambiguity problem in the high repetition rate working mode.

[0063] 3. The present invention requires the radar transmitter to have agile initial phase change between pulses, and the radar system receiving end processing steps are consistent with the existing pulse radar processing steps, and only the parameters of the receiving end processing module need to be changed. Therefore, the anti-clutter method involved in the present invention is easy to implement in engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 1 is a flow chart of a radar anti-clutter method based on initial phase agility and filter joint design in an embodiment of the present invention;

[0065] Figure 2 is an initial phase diagram of an initial phase agile waveform transmitting pulse in an embodiment of the present invention;

[0066] Figure 3 is T in the embodiment of the present invention r = 200us, receiving end distance division diagram;

[0067] Figure 4 1 is a diagram of the initial phase agile waveform echo pulse compression result in an embodiment of the present invention;

[0068] Figure 5 1 is a filter response diagram of the initial phase agile joint filter design in different distance segments in an embodiment of the present invention, wherein (a) is distance segment 1, (b) is distance segment 2, and (c) is distance segment 3;

[0069] Figure 6 is the traditional pulse radar filter response diagram;

[0070] Figure 7 1 is a diagram of the filtering processing results in an embodiment of the present invention, wherein (a) is the initial phase agile joint filter design, and (b) is the traditional pulse radar. DETAILED DESCRIPTION

[0071] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiment.

[0072] like Figure 1 As shown, this embodiment provides a technical solution: a radar anti-clutter method based on initial phase agility and filter joint design, comprising the following steps:

[0073] (1) Radar transmits initial phase agile waveform

[0074] Traditional pulse radar transmits linear frequency modulation signal within pulse, and the waveform parameters between pulses remain unchanged. Different from traditional pulse radar, in the method of the present invention, the radar transmits an initial phase agile waveform between pulses, and the initial phase of each pulse waveform can be randomly selected in the range of [0-2π]. The i-th transmitted pulse s iThe waveform expression is:

[0075] s i =exp(j(πμt 2 +φ(i)))i=1,...,M

[0076] Wherein, exp(·) is an exponential function, j represents an imaginary unit, t is a sampling time series, μ is the frequency modulation slope of the linear frequency modulation signal, φ(i) is the initial phase of the i-th transmission pulse, and M is the number of pulses contained in a coherent processing cycle. In this embodiment, M=32, the transmission pulse width is 20us, the transmission waveform bandwidth is 8MHz, and the initial phases of 32 transmission pulses are as follows: Figure 2 shown.

[0077] (2) Divide distance segments

[0078] After determining the radar transmission waveform parameters, according to the radar transmission waveform pulse repetition period T r Treat observation distance R ob Divide the distance into segments. The maximum unambiguous observation distance is:

[0079]

[0080] The range of each distance segment is as follows

[0081] Distance segment 1: (0R max ]

[0082] Distance segment 2: (R max 2R max ]

[0083] Distance segment 3: (2R max 3R max ] ...

[0085] In the subsequent signal processing, different distance segments correspond to different echoes and filter coefficients to be processed. Therefore, before performing pulse compression and filtering at the receiving end, it is determined which distance segment to select for processing. When it is unknown which distance segment the target is located in, each distance segment is selected in turn for signal processing. In this embodiment, T r = 200us, corresponding to the maximum unambiguous distance R max = 30km, then the distance segments are divided as follows Figure 3 shown.

[0086] (3) Range gating and pulse compression processing

[0087] After the radar transmits the RF signal, the echo received by the radar receiver can be converted into a baseband echo. The baseband echo is then pulse compressed. hannelThe nth baseband echo pulse x is compressed by frequency domain matched filtering. n The output after pulse compression processing for:

[0088]

[0089] Among them, fft(·) represents Fourier transform, ifft(·) represents inverse Fourier transform, conj(·) represents conjugation, represents the Hadamard product, s 1 The expression is as follows:

[0090] s 1 =exp(jπμt 2 )

[0091] From the above pulse compression process, we can see that compared with the traditional linear frequency modulation pulse radar, the pulse initial phase agility radar has a high pulse width at the receiving end. hannel The first echo pulse begins to be compressed because the previous C hannel -1 echo pulse does not contain the target echo. In this embodiment, the target distance is set to 72km, the target speed is 4m / s, the clutter distribution is 10km~15km, the amplitude follows the Rayleigh distribution, the speed range is 0m / s~4m / s, the clutter power spectrum is Gaussian, the signal-to-clutter ratio is -30dB, and C is set hannel =3, the echo after pulse compression is as follows Figure 4 As shown in Figure 2, it can be seen that since both clutter and target echo can obtain pulse compression gain, the target echo after pulse compression processing is submerged in the folded clutter.

[0092] (4) Filtering

[0093] (4a) Filter coefficient design

[0094] According to the expression of the transmitted pulse waveform, there is an agile phase between pulses after the echo is compressed, and the phase compensation between pulses is required before pulse accumulation. The compensation expression is:

[0095]

[0096] Where φ(i) is the initial phase of the i-th transmitted pulse, i=1,...,MC hannel +1.

[0097] After phase compensation, it is located in the gate distance segment C hannel The pulse-to-pulse agility initial phases of the target echo and clutter from the range segment C are complemented, but the pulse-to-pulse phases of the target echo and clutter from other range segments are not complemented. oAfter the above phase compensation, the residual initial phase between pulses of the echo is:

[0098] φ o (i) = φ(C hannel -C o +i)-φ(i)i=1,...,MC hannel +1

[0099] It can be seen that the gating distance segment C hannel After the clutter from different distance segments is compensated, the initial phase of the residual pulse-to-pulse agility is different. Therefore, the clutter covariance matrix needs to be constructed by distance segment. The clutter covariance matrix construction method of the present invention is as follows:

[0100] Assuming the standard deviation of the velocity for suppressing clutter is Δv, a random velocity sequence v with a standard deviation of Δv is generated, corresponding to a random Doppler frequency sequence f d for:

[0101] f d =2v / λ

[0102] Assume the number of pulses is M, and the gate distance segment is C hannel , then it is located in distance segment C o The clutter slow time vector is:

[0103] t c =[[(C hannel -C o +1):(MC o +1)]-1]T r

[0104] In summary, after phase compensation, the o The clutter is expressed as:

[0105]

[0106] Based on the clutter constructed above, the clutter covariance matrix can be obtained:

[0107]

[0108] After diagonal loading:

[0109]

[0110] Where δ is the diagonal loading coefficient and I is the unit matrix. The weight vector expression of the kth filter in the MTD filter bank is:

[0111]

[0112] Among them, f k is the passband center frequency of the kth filter, a(fk ) is the pilot vector:

[0113] a(f k )=exp(-j2πf k t r )

[0114] in,

[0115] t r =[(1:(MC hannel +1))-1]T r

[0116] In engineering applications, the phase compensation coefficient can be incorporated into the filter weight coefficient, that is, the pulse compressed data can be directly processed through the following weight coefficient:

[0117]

[0118] In the filter weight coefficient design of this embodiment, the standard deviation of the clutter velocity is set to 2m / s, the number of MTD filters is 64, and the clutter of distance segment 1 is constructed to generate the clutter covariance matrix, and then the pilot vector when selecting the distance segment 3 is combined to form the filter weight coefficient. The response of the filter weight coefficient in different distance segments is as follows: Figure 5 As shown, the conventional pulse radar filter response is Figure 6 It can be seen that the initial phase agile joint filter design can form a notch near 0 Doppler in the short-range range where the clutter is located, and form a passband in the low-speed area of ​​the range where the target is located. However, the traditional pulse radar filter cannot form a notch in the range segment, so it cannot guarantee the target gain while suppressing the clutter.

[0119] (4b) Filtering

[0120] Based on the filter weight coefficient w obtained in the previous step k The data Y after pulse compression and pulse phase compensation out1 Filtering is performed, and the filtering result of the kth filter is:

[0121] z k =w k Y out1

[0122] The initial phase agile joint filter design and the filtering results of the traditional pulse radar are shown in Figure 7 As shown. Obviously, in the slow target detection scenario, the traditional filter suppresses the target while suppressing clutter, but the initial phase agile joint filter design can suppress short-range clutter while retaining long-range targets. In addition, it can be seen from the results that the range ambiguity problem under high pulse repetition frequency can be solved by range gating.

[0123] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A radar anti-clutter method based on initial phase agility and filter joint design, characterized in that: The following steps are involved: S1: Initial phase agile waveform The radar transmits an inter-pulse initial phase agile waveform, and the initial phase of each transmit pulse waveform is randomly selected in the range of [0-2π]; S2: Divide distance segments After determining the radar transmission waveform parameters, according to the radar transmission waveform pulse repetition period T r Divide the observation distance into segments; S3: Range Gating and Pulse Compression Processing Set the C hannel Distance segment, from C hannel The baseband echo is compressed after the first echo pulse. S4: Filtering According to the radar transmission waveform parameters and the gating distance segment C hannel The filter weight coefficients are designed, and the echo after pulse compression is filtered based on the coefficients.

2. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 1 is characterized in that: In step S1, it is assumed that the linear frequency modulation signal is transmitted in the pulse, and the i-th transmission pulse s i The waveform expression is: s i =exp(j(πμt 2 +φ(i)))i=1,...,M Where exp(·) is an exponential function, j represents an imaginary unit, t is a sampling time series, μ is the frequency modulation slope of the linear frequency modulation signal, φ(i) is the initial phase of the i-th transmitted pulse, and M is the number of pulses contained in a coherent processing cycle.

3. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 1 is characterized in that: In step S2, the range of each distance segment is as follows: Distance segment 1: (0R max ] Distance segment 2: (R max 2R max ] Distance segment 3: (2R max 3R max ] ... Among them, R max is the maximum unambiguous observation distance: Here, c represents the speed of light.

4. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 1 is characterized in that: In step S3, pulse compression is performed by frequency domain matched filtering, and the nth baseband echo pulse x n The output after pulse compression for: Among them, fft(·) represents Fourier transform, ifft(·) represents inverse Fourier transform, conj(·) represents conjugation, represents the Hadamard product, s 1 The expression is as follows: s 1 =exp(jπμt 2 )。 5. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 4, characterized in that: In step S4, the specific processing process is as follows: S41: Filter Coefficient Design Performing inter-pulse phase compensation on the pulse compressed data, constructing a clutter covariance matrix in different distance segments, and calculating filter weight coefficients based on the constructed clutter covariance matrix, wherein the filter is an MTD filter; S42: Filtering The echo data after pulse compression and pulse-to-pulse phase compensation is filtered based on the obtained filter weight coefficient to obtain the filtering result of the kth filter.

6. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 5, characterized in that: In step S41, the specific processing process is as follows: S411: Perform phase compensation between pulses on the data after pulse compression. The compensation expression is: Where φ(i) is the initial phase of the i-th transmitted pulse, i=1,...,MC hannel +1; S412: Assume that the standard deviation of the velocity to be used to suppress clutter is Δv, then generate a random velocity sequence v with a standard deviation of Δv, corresponding to a random Doppler frequency sequence f d for: f d =2vλ Where λ is the wavelength; S413: Set the pulse number to M, select the C hannel distance segment, then it is in distance segment C o The clutter slow time vector is: t c =[[(C hannel -C o +1):(M-C o +1)]-1]T r ; S414: After phase compensation between meridians, from distance segment C o The clutter is expressed as: Among them, φ o From distance segment C o The residual initial phase of the clutter after inter-pulse phase compensation; S415: Obtain the clutter covariance matrix based on the constructed clutter: After diagonal loading: Where δ is the diagonal loading coefficient and I is the unit matrix.

7. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 6, characterized in that: In step S41, the filter weight coefficient w k as follows: Among them, f k is the passband center frequency of the kth filter, a(f k ) is the pilot vector: a(f k )=exp(-j2πf k t r ) Among them, t r =[(1:(MC hannel +1))-1]T r .

8. The radar anti-clutter method based on initial phase agility and filter joint design according to claim 7, characterized in that: In step S42, the filtering result of the kth filter is: z k =w k Y out1 Among them, Y out1 It is the echo data after pulse compression and inter-pulse phase compensation.

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