CN113156381A - Multipath clutter suppression method for airborne external radiation source radar - Google Patents

Multipath clutter suppression method for airborne external radiation source radar Download PDF

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CN113156381A
CN113156381A CN202110357411.6A CN202110357411A CN113156381A CN 113156381 A CN113156381 A CN 113156381A CN 202110357411 A CN202110357411 A CN 202110357411A CN 113156381 A CN113156381 A CN 113156381A
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signal
multipath
clutter
multipath clutter
snapshot
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邓亚琦
李加升
肖卫初
李稳国
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Hunan City University
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Hunan City University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/36—Means for anti-jamming, e.g. ECCM, i.e. electronic counter-counter measures
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
    • G01S7/411—Identification of targets based on measurements of radar reflectivity
    • G01S7/412—Identification of targets based on measurements of radar reflectivity based on a comparison between measured values and known or stored values
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
    • G01S7/418—Theoretical aspects

Abstract

The invention discloses a multipath clutter suppression method for an airborne external radiation source radar, which comprises the following steps: acquiring a plurality of echo signals and impure reference signals of an airborne external radiation source radar; respectively carrying out uniform segmentation on the reference signal and the echo signals received by the observation antennas to obtain an equivalent reference pulse signal and an equivalent echo pulse signal; performing distance matching by using the equivalent reference signal and the equivalent echo signal to obtain a space-time snapshot signal of the distance unit to be detected; constructing a multipath clutter observation model; converting the problem of multi-path clutter suppression into the problem of optimization of weight vectors in a multi-path clutter observation model; solving an optimization problem by adopting a minimum mean square error algorithm based on an exponential forgetting window to obtain a snapshot signal of the distance unit to be detected after multipath clutter suppression; and carrying out target detection by utilizing the snapshot signal of the distance unit to be detected after multipath clutter suppression. The method of the invention can obtain stable multipath clutter suppression performance with low complexity.

Description

Multipath clutter suppression method for airborne external radiation source radar
Technical Field
The invention belongs to the technical field of communication, and particularly relates to a multipath clutter suppression method for an airborne external radiation source radar.
Background
The external radiation source radar does not emit signals, and a third-party radiation source is used as a transmitting station, so that the radar has the advantages of small size, good concealment, strong viability and the like. The airborne external radiation source radar is a radar system applying the external radiation source radar technology to an airborne platform. Due to the fact that the receiving platform is lifted, the airborne external radiation source radar not only has various advantages of the traditional external radiation source radar, but also has the advantage of being high in perspective and far-paying attention. Therefore, research based on the airborne external radiation source radar system is widely concerned by researchers at home and abroad.
Due to the movement of the receiving platform, ground object echo signals received by the airborne external radiation source radar have a space-time coupling characteristic, and the traditional one-dimensional (time domain or space domain) adaptive cancellation method cannot achieve effective suppression of clutter. The Space-Time Adaptive Processing (STAP) technology provides an effective way for the airborne radar to realize clutter suppression. In the STAP algorithm, a direct wave signal and an echo signal are matched and filtered to obtain space-time snapshot data, a clutter covariance matrix of a distance unit to be detected is estimated by using training sample data, and clutter suppression is finally achieved. Based on the assumption that the reference signal is pure (the reference channel receives a pure direct wave signal), the airborne external radiation source radar can suppress clutter and detect a target by using a STAP method. However, in an actual airborne external radiation source radar system, the existence of a large main lobe width and a station address error is difficult to ensure that a reference antenna receives a pure direct wave signal. At this time, the clutter snapshot signal obtained after the matched filtering includes two parts: the first part is the matching result of the multipath in the reference channel and the ground object echo (multipath clutter); the second part is the matching result of the direct wave and the ground object echo (direct wave clutter). The clutter suppression and target detection performance of the STAP technology can be seriously influenced by the existence of multipath clutter, so that the research on a clutter suppression method under an impure reference signal is the key for realizing target detection of an airborne external radiation source radar.
To overcome the effects of impure reference signals, a multipath clutter cancellation method is typically used before the STAP. Existing Blind equalization methods (BEM, Blind)Equalization Method) utilizes the prior information of direct wave Doppler to construct an optimization problem of phase constraint, and the multi-path clutter suppression is realized by solving the problem. However, the multipath clutter suppression performance of the method depends on the accuracy of the priori knowledge, when the priori knowledge has errors, the multipath clutter suppression capability is reduced, and the target detection capability of the STAP method is reduced. Already has L1Norm constrained recursive least squares (L)1-RLS，L1-based Recursive Least square) algorithm to construct a multipath clutter sparse observation model, to determine a reconstruction problem of sparse constraint, and to solve a weight vector by using an RLS method to realize multipath clutter suppression.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides an airborne external radiation source radar multipath clutter suppression method, which can obtain stable multipath clutter suppression performance with low complexity. The technical problem to be solved by the invention is realized by the following technical scheme:
the invention provides a multipath clutter suppression method for an airborne external radiation source radar, which comprises the following steps:
s1: acquiring a receiving signal of an airborne external radiation source radar, wherein the receiving signal comprises a plurality of echo signals received by a multi-array element observation antenna and impure reference signals received by a reference antenna;
s2: respectively carrying out uniform segmentation on the reference signal and the echo signals received by the observation antennas to obtain an equivalent reference pulse signal and an equivalent echo pulse signal;
s3: performing distance matching by using the equivalent reference signal and the equivalent echo signal to obtain a space-time snapshot signal of a distance unit to be detected;
s4: constructing a multipath clutter observation model;
s5: converting the suppression problem of the multipath clutter into the optimization problem of the weight vector in the multipath clutter observation model based on the multipath clutter observation model and the sparse characteristic of the multipath clutter snapshot in the distance-Doppler two-dimensional plane;
s6: solving the optimization problem by adopting a minimum mean square error algorithm based on an exponential forgetting window to obtain a snapshot signal of the distance unit to be detected after multipath clutter suppression;
s7: and carrying out target detection by utilizing the snapshot signal of the distance unit to be detected after multipath clutter suppression.
In an embodiment of the present invention, the S2 includes:
s21: in a coherent accumulation time, uniformly dividing signals received by all antenna array elements into M sections, equivalently receiving M pulse signals by a reference antenna and an observation antenna respectively, wherein the repetition period of the equivalent pulse signals is TrThe number of fast time samples is L;
s22: an independent reference channel is formed in the reference antenna, and the reference channel contains a direct wave signal and NTA multipath signal.
In an embodiment of the present invention, the S3 includes:
s31: performing matched filtering by using the equivalent reference pulse signal and the equivalent echo pulse signal corresponding to each observation antenna to obtain a direct wave clutter snapshot signal x of the distance unit l to be detecteddc,lAnd NTFast shooting x of multipath clutterpc,l；
Step 32: obtaining an interference snapshot signal of a distance unit l to be detected:
Figure BDA0003004268590000031
wherein x isnRepresenting noise.
In an embodiment of the present invention, the S4 includes:
s41: using the received interference snapshot signal { x ] to the multipath clutter signal located in the range unit ll+1,xl+2,…,xl+DWhere D is the distance cancellation order,
s42: dividing the Doppler plane into MtA grid cell, obtainObtaining the divided Doppler frequency omegajAnd corresponding modified time-oriented vector
Figure BDA0003004268590000041
S43: and constructing a multipath clutter observation model of the multipath clutter signal positioned in the distance unit l according to the corrected time guide vector and the D received interference snapshot signals:
Figure BDA0003004268590000042
wherein x isl+iFor disturbing snap-shot signals, x, located at distance units l + il+i,jFor the jth modified snap signal, α, located at a distance unit l + ii,jRepresents the complex amplitude of the jth modified snapshot signal located at the distance cell l + i;
s44: let the correction time-oriented matrix be
Figure BDA0003004268590000043
Expressing the multipath clutter observation model as:
Figure BDA0003004268590000044
wherein the content of the first and second substances,
Figure BDA0003004268590000045
with a representation dimension of MtThe weight vector of the D x 1 vector,
Figure BDA0003004268590000046
represents MN × MtD-dimensional distance-Doppler dictionary matrix, where SD,l＝[xl+1,xl+2,…,xl+D]。
In an embodiment of the present invention, the S5 includes:
converting the estimation problem of the multipath clutter into an optimization problem of sparse constraint by utilizing a minimum output power criterion according to the multipath clutter observation model and the weight vector sparsity analysis to obtain a cost function:
Figure BDA0003004268590000051
wherein, E [. C]Expressing the mean value, | · | | luminance1And | · | non-conducting phosphor2Respectively represent L1Norm and L2Norm, Re {. cndot } represents the real part, η is the regularization parameter, ρ ═ σ211×MNΦ，
Figure BDA0003004268590000052
σ2Representing the noise power.
In an embodiment of the present invention, the S6 includes:
s61: and obtaining the deviation of the cost function by calculating:
Figure BDA0003004268590000053
wherein sign (·) represents a sign function (·)ΗThe representation is taken of the conjugate transpose,
Figure BDA0003004268590000054
Figure BDA0003004268590000055
s62: order to
Figure BDA0003004268590000056
Obtaining an expression of the optimal weight vector:
Figure BDA0003004268590000057
s63: the weight vector is iteratively estimated by using the minimum mean square error algorithm based on the exponential forgetting window to obtain the estimated value of the weight vector
Figure BDA00030042685900000511
S64: based on the estimated value of the weight vector
Figure BDA00030042685900000512
Obtaining snap-shot signal y of distance unit to be detected after multipath clutter suppressionl：
Figure BDA0003004268590000058
In an embodiment of the present invention, the S7 includes:
s71: 2L adjacent to distance unit L to be detected after multipath clutter suppression1Estimating a direct wave clutter covariance matrix by using the snapshot signals:
Figure BDA0003004268590000059
s72: solving an STAP weight vector based on the estimated direct wave clutter covariance matrix:
Figure BDA00030042685900000510
wherein, v (ω)t,θt) For a target space-time steering vector, ωtAnd thetatRespectively the doppler frequency and the spatial frequency of the target;
s73: obtaining an output signal at a distance unit l to be detected:
Figure BDA0003004268590000061
compared with the prior art, the invention has the beneficial effects that:
the method for restraining the multipath clutter of the airborne external radiation source radar constructs the optimization problem of sparse constraint on the basis of the multipath clutter sparse observation model, and utilizes the method based on index residualAnd estimating the multipath clutter by using an EFWLMS algorithm of a forgetting window, finally realizing the suppression of the multipath clutter and eliminating the influence of the multipath clutter on the detection performance of the STAP target. Compared with the existing BEM method, the method does not need prior information, and can obtain stable multipath clutter suppression performance under the condition that the error exists in the prior knowledge; with the existing L1Compared with the RLS method, the method solves the optimization problem by using the EFWLMS algorithm, has the calculation complexity far lower than that of the RLS method, and can reduce the calculation amount of multipath clutter suppression. Therefore, the proposed method can achieve robust multipath clutter suppression performance with low complexity.
The present invention will be described in further detail with reference to the accompanying drawings and examples.
Drawings
Fig. 1 is a flowchart of a multipath clutter suppression method for an airborne external radiation source radar according to an embodiment of the present invention;
fig. 2 is a schematic view of a bistatic configuration of an airborne external radiation source radar according to an embodiment of the present invention;
FIG. 3a shows STAP, BEM, L with accurate a priori knowledge1-signal to interference plus noise ratio loss performance comparison results for RLS and the method proposed in the embodiments of the present invention;
FIG. 3b shows STAP, BEM, L with accurate a priori knowledge1-comparing results of target detection of RLS and the method as proposed in the examples of the present invention;
FIG. 4a is a BEM, L under inaccurate a priori knowledge1-the signal to interference and noise ratio performance comparison results of the RLS and the method proposed in the embodiments of the present invention;
FIG. 4b is BEM, L for the case where the prior knowledge is inaccurate1-comparing results of target detection of RLS and the method as proposed in the examples of the present invention;
FIG. 5a is BEM, L1-the additive complexity of RLS and the method proposed by the embodiments of the present invention contrasts the results;
FIG. 5b is BEM, L1-the result of the multiplicative complexity comparison of RLS and the method proposed by the embodiment of the invention.
Detailed Description
In order to further explain the technical means and effects of the present invention adopted to achieve the predetermined object, a method for suppressing multipath clutter of an airborne external radiation source radar according to the present invention is described in detail below with reference to the accompanying drawings and the detailed description.
The foregoing and other technical matters, features and effects of the present invention will be apparent from the following detailed description of the embodiments, which is to be read in connection with the accompanying drawings. The technical means and effects of the present invention adopted to achieve the predetermined purpose can be more deeply and specifically understood through the description of the specific embodiments, however, the attached drawings are provided for reference and description only and are not used for limiting the technical scheme of the present invention.
It is noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that an article or device that comprises a list of elements does not include only those elements but may include other elements not expressly listed. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of additional like elements in the article or device comprising the element.
Example one
Referring to fig. 1, fig. 1 is a flowchart of a method for suppressing multipath clutter for an airborne external radiation source radar according to an embodiment of the present invention. The method comprises the following steps:
s1: and acquiring a receiving signal of the airborne external radiation source radar, wherein the receiving signal comprises a plurality of echo signals received by the multi-array element observation antenna and impure reference signals received by the reference antenna.
Prior to S1, the method further includes a scene setting process. Referring to fig. 2, fig. 2 is a schematic view of a bistatic configuration of an airborne external radiation source radar according to an embodiment of the present invention. The scene setting includes: transmitting stationT is arranged in the far field of an airborne external radiation source radar receiving station R, and the airborne plane is parallel to the ground at a speed vRThe uniform linear flight is carried out, and the flying height of the carrier is HRThe reference antenna and the N observation antennas are erected on the aerial carrier platform, the reference antenna points to the transmitting station to obtain a reference signal, and the phase center distance of the N array element observation antennas is d.
S2: and uniformly segmenting the reference signal and the echo signals respectively to obtain an equivalent reference pulse signal and an equivalent echo pulse signal.
Specifically, in a coherent accumulation time, the continuous signals received by all antenna elements (including the reference antenna and the N observation antennas) are respectively and uniformly divided into M segments, equivalently, the reference antenna and the observation antennas respectively receive M pulse signals, wherein the repetition period of the equivalent pulse signals is TrThe number of fast time samples is L. Each antenna forms an independent processing channel, i.e. the reference antenna forms a reference channel and the observation antenna forms an observation channel. The reference channel contains a direct wave signal and NTA multipath signal, wherein the amplitude, delay unit and Doppler frequency of the direct wave are respectively gammad、ldAnd ωdNo. (p ═ 1,2, …, N)T) The amplitude, delay unit and Doppler frequency of each multipath signal are respectively gammap、lpAnd ωp。
S3: and performing distance matching by using the equivalent reference pulse signal and the equivalent echo pulse signal to obtain a space-time snapshot signal of the distance unit to be detected.
And performing distance matching by using the equivalent reference pulse signal and the equivalent echo pulse signal to obtain three-dimensional data blocks of slow time sampling, space sampling and fast time sampling, and stacking all data of the same distance unit door into a column vector according to a space dimension to obtain an empty-time snapshot signal.
In this embodiment, step S3 specifically includes:
s31: performing matched filtering by using the equivalent reference pulse signal and the equivalent echo pulse signal corresponding to each observation antenna array element to obtain clutter snapshot which is divided into two partsDividing into: firstly, the matching result of the direct wave signal and the echo signal reflected by a scattering point is the direct wave clutter snapshot; and secondly, a matching result of the multipath signal and an echo signal reflected by the scattering point, namely, the multipath clutter snapshot. And the influence of distance ambiguity is not considered, and the clutter snapshot of a single distance unit is formed by jointly superposing clutter reflection signals of a single distance ring. Therefore, at delay unit l + ldThe echo signal reflected by the scattering point and the direct wave signal are subjected to matched filtering to obtain the direct wave clutter snapshot x of the distance unit l to be detecteddc,l(ii) a At delay unit l + lpThe scattering point of (1) and (2) reflects the echo signal and the p (p-1, …, N)T) The multipath clutter snapshot x of the distance unit l to be detected can be obtained by performing matched filtering on the multipath signalspc,l。xdc,lAnd xpc,lAre respectively:
Figure BDA0003004268590000091
Figure BDA0003004268590000092
wherein N iscRepresents the number of clutter scattering points in a single range ring,
Figure BDA0003004268590000093
indicating that it is in delay unit l + ldThe complex amplitude of the ith clutter scattering point of (1),
Figure BDA0003004268590000094
indicates a Kronecker product,. mu.l indicates a Hadamard product,. theta.iAnd ωd,iRespectively representing the spatial frequency of the ith clutter scattering point and the Doppler frequency of the relative direct wave, vs(θi) And vt(ωd,i) Respectively representing a spatial frequency of thetaiHas a spatial steering vector and a Doppler frequency of omegad,iTime-oriented vector of, v (ω)d,i,θi) Representing spatial frequency as θiDoppler frequencyIs omegad,iThe space-time steering vector.
Figure BDA0003004268590000095
Representing the complex amplitude of the p-th multipath clutter snapshot located at the l-th range bin, p 1, …, NTDenotes the conjugate of the complex number,
Figure BDA0003004268590000101
the complex amplitude conjugate of the p-th multipath signal,
Figure BDA0003004268590000102
is the conjugate of the amplitude of the direct wave,
Figure BDA0003004268590000103
to be located in delay unit l + lpComplex amplitude of the ith clutter scattering point of (c), ωp,iIs the Doppler frequency, v (ω), of the ith clutter scattering point relative to the p-th multipath signalp,i,θi) Representing spatial frequency as θiDoppler frequency of omegap,iThe space-time steering vector of (a),
Figure BDA0003004268590000104
indicates a location in the distance cell l + (l)p-ld) Direct wave clutter snapshot signal of rpRepresenting modified time-oriented vectors
Figure BDA0003004268590000105
1A×BIs an A × B dimensional matrix with elements all 1.
S32: obtaining interference snapshot signal of distance unit to be detected including three portions of direct wave clutter, multipath clutter and noise
Figure BDA0003004268590000106
Wherein x isnRepresenting noise.
S4: and constructing a multipath clutter observation model.
Dividing a Doppler plane into a plurality of grid units, constructing a correction time guide matrix according to the Doppler frequency corresponding to each grid point, and simultaneously selecting proper snapshot data to construct a distance-Doppler matrix, thereby constructing an observation model of the multipath clutter;
firstly, analyzing the characteristics of a multipath snapshot signal to obtain the correlation between the multipath clutter snapshot and two factors: firstly, the received snapshot data after the unit to be detected and secondly, the correction time guide vector related to the Doppler frequency.
Specifically, the relationship between the p-th multipath clutter snapshot located in the distance unit l to be detected and the interference snapshot signal can be approximately expressed as:
Figure BDA0003004268590000107
wherein the content of the first and second substances,
Figure BDA0003004268590000108
indicates a location in the distance cell l + (l)p-ld) The interference of (2) with the snapshot signal,
Figure BDA0003004268590000109
indicates a location in the distance cell l + (l)p-ld) To (1) a
Figure BDA00030042685900001010
The fast-shooting of the multipath clutter is carried out,
Figure BDA00030042685900001011
indicates the position at l + (l)p-ld) The complex amplitude of the p-th multipath spur for each range bin,
Figure BDA0003004268590000111
indicates a location in the distance cell l + (l)p+lp-2ld) The interference of (2) with the snapshot signal,
Figure BDA0003004268590000112
represents a modified time-oriented vector, in which
Figure BDA0003004268590000113
Figure BDA0003004268590000114
Is shown as
Figure BDA0003004268590000115
An
Figure BDA0003004268590000116
Doppler frequency of the multipath signal. At the same time, the time-oriented vector is corrected
Figure BDA0003004268590000117
Can be rewritten as:
Figure BDA0003004268590000118
from the above analysis, it can be seen that the p-th multipath clutter snapshot located at the distance unit l to be detected is related to two factors, namely, the received interference snapshot data located behind the distance unit l, that is, the received interference snapshot data
Figure BDA0003004268590000119
And
Figure BDA00030042685900001110
second, a modified steering vector related to the Doppler frequency, i.e. rpAnd
Figure BDA00030042685900001111
thus, an observation model of multipath clutter may be constructed by selecting appropriate snapshot data and doppler frequencies.
To obtain an observation model of multipath clutter, first, assume that a multipath clutter signal located at a range bin l can be captured from a received interfering snapshot signal { xl+1,xl+2,…,xl+DD is defined as the distance cancellation order, i.e., D received snapshot signals are selected to estimate multipath clutter. Then, the Doppler frequency plane is divided into MtMore than M grids, and the divided Doppler frequency is set as omegaj，j＝1,2,…,MtModified time-oriented vector of corresponding MN x 1 dimension
Figure BDA00030042685900001112
The multipath clutter observation model of the multipath clutter signal located at range unit l may be expressed as
Figure BDA00030042685900001113
Wherein x isl+iFor a received interfering snapshot signal located at a distance unit l + i, x is definedl+i,jFor the jth modified snap signal, α, located at a distance unit l + ii,jRepresenting the complex amplitude of the jth modified snapshot signal located at distance cell l + i.
Further, let the correction time steering matrix be
Figure BDA00030042685900001114
The multipath clutter observation model may be expressed as
Figure BDA0003004268590000121
Wherein the content of the first and second substances,
Figure BDA0003004268590000122
with a representation dimension of MtThe weight vector of the D x 1 vector,
Figure BDA0003004268590000123
represents MN × MtD-dimensional distance-Doppler dictionary matrix, where SD,l＝[xl+1,xl+2,…,xl+D]。
S5: and analyzing the sparse characteristic of the multipath clutter snapshot on a distance-Doppler two-dimensional plane based on the multipath clutter observation model, converting the suppression problem of the multipath clutter into a solving problem of a weight vector in the multipath clutter observation model, and constructing an optimization problem of sparse constraint by using the minimum output power as a criterion.
In particular, since the number of scattering points in the reference beam is quite limited, and in the limited multipath signals, almost all the multipath signals are different from the doppler frequency of the direct wave signal by only a few millihertz, the influence of such multipath signals on the division of the doppler plane is negligible. Only when a scattering point in the reference beam has a certain height or speed, the extremely small part of scattering points has a doppler frequency different from that of the direct wave, corresponding to the distribution of the doppler dimension in the dictionary matrix. Therefore, the number of non-zero values in the weight vector is far smaller than the dimension of the dictionary matrix, which indicates that the multipath clutter snapshots to be recovered are sparsely distributed in the range-Doppler plane.
According to a multipath clutter observation model and weight vector sparsity analysis, the estimation problem of the multipath clutter can be converted into the optimization problem of the following sparse constraint by using the minimum output power criterion:
Figure BDA0003004268590000124
wherein, E [. C]Expressing the mean value, | · | | luminance1And | · | non-conducting phosphor2Respectively represent L1Norm and L2The norm, η, is the regularization parameter that determines the number of near-zero elements in the weight vector. Because it is difficult to obtain pure multipath clutter snapshot in practice
Figure BDA0003004268590000125
It is difficult to solve the weight vector α by solving the above-described optimization problem. By analysis of
Figure BDA0003004268590000126
And
Figure BDA0003004268590000127
the following conclusions can be found in relation to each other:
Figure BDA0003004268590000131
wherein Re {. cndot } represents a real part. Because the amplitudes of the electromagnetic waves are statistically independent and uncorrelated between different components, i.e. between clutter and thermal noise, it suffices
Figure BDA0003004268590000132
Figure BDA0003004268590000133
And is
Figure BDA0003004268590000134
Wherein
Figure BDA0003004268590000135
Figure BDA0003004268590000136
Λn＝diag(xn) Sign (·) is a sign function. In addition, by analysis
Figure BDA0003004268590000137
Wherein sigma2Representing the noise power, equation (7) can be simplified to
Figure BDA0003004268590000138
Where ρ ═ σ211×MNΦ，
Figure BDA0003004268590000139
Is a constant independent of the weight vector. Therefore, the optimization problem represented by equation (6) can be converted into the following form
Figure BDA00030042685900001310
At this time, the suppression of multipath clutter is equivalent to solving the weight vector α in the above equation.
S6: and solving the optimization problem by adopting a minimum mean square error algorithm based on an exponential forgetting window, and after multipath clutter suppression, detecting the snapshot signal of the distance unit to be detected.
Step S6 specifically includes:
s61: firstly, the cost function of the formula (9) is subjected to partial derivation to obtain:
Figure BDA00030042685900001311
wherein sign (·) represents a sign function (·)ΗIndicating taking the conjugate transpose. Then, the derivative is equal to zero to obtain the optimal weight vector of
Figure BDA00030042685900001312
Wherein (·)-1Which is expressed by the inverse of the matrix being evaluated,
Figure BDA0003004268590000141
because the optimal weight vector is not a closed analytic solution, the weight vector is iteratively estimated by using an EFWLMS (extended learning Window Least Mean Square error based on exponential Forgetting Window) algorithm with low complexity in the embodiment of the invention. Assuming a sliding window length of Q and a forgetting factor of β, then in the kth iteration, it is possible to utilize
Figure BDA0003004268590000142
And
Figure BDA0003004268590000143
respectively replacing the mean value
Figure BDA0003004268590000144
And
Figure BDA0003004268590000145
thus, the iterative formula for the weight vector can be expressed as:
Figure BDA0003004268590000146
wherein the content of the first and second substances,
Figure BDA0003004268590000147
representing the weight vector estimate for the kth iteration,
Figure BDA0003004268590000148
representing the weight vector estimate for the (k-1) th iteration, mu being the iteration step,
Figure BDA0003004268590000149
and updating the weight vector according to the iteration formula until the cost function output value of two adjacent iterations is not obviously changed or the change is smaller than a set threshold value, and stopping the iteration. The estimated value of the weight vector after the iteration is finished is
Figure BDA00030042685900001410
S62: based on the estimated value
Figure BDA00030042685900001411
Realizing the suppression of the multipath clutter, and then after the suppression of the multipath clutter, detecting the snapshot signal y of the distance unitlCan be expressed as
Figure BDA00030042685900001412
At the moment, the multipath clutter is filtered, and the influence of impure reference signals on the STAP performance can be effectively eliminated.
S7: and carrying out target detection by utilizing the snapshot signal of the distance unit to be detected after multipath clutter suppression.
And utilizing snapshot signal data near a distance unit to be detected to realize maximum likelihood estimation of a clutter covariance matrix, and inhibiting direct wave clutter according to a traditional STAP method to realize target detection.
Specifically, the vicinity of a distance unit l to be detected after multipath clutter suppression is utilized first2L of1Estimating the covariance matrix of the direct-wave clutter from the training sample data, i.e.
Figure BDA0003004268590000151
Then, an STAP weight vector is obtained based on the estimated direct wave clutter covariance matrix
Figure BDA0003004268590000152
Wherein, v (ω)t,θt) For a target space-time steering vector, ωtAnd thetatRespectively the doppler frequency and the spatial frequency of the object,
Figure BDA0003004268590000153
is the inverse of the covariance matrix of the direct wave clutter. Then, after the proposed multi-path clutter suppression method and STAP direct wave clutter suppression, the output signal at the distance unit to be detected l can be represented as
Figure BDA0003004268590000154
At this time, the multipath clutter and the direct wave clutter are both suppressed, and the target detection can be realized by the output signal.
The effect of the method for suppressing multipath clutter of the airborne external radiation source radar in the embodiment of the invention is further described through experiments.
(1) The experimental conditions are as follows:
in the experiment, the frequency of a signal source is 600MHz, the bandwidth is 8MHz, and the sampling frequency is 10 MHz. As shown in fig. 2, the aircraft moving speed is 100m/s, a reference antenna and an observation antenna are placed on the aircraft, the reference antenna is directed to a transmitting station to receive a reference signal, 10 antenna array elements receive echo signals, and the array element spacing is a half wavelength. In the coherent accumulation time, the reference signal and the echo signal are segmented and are equivalent to 10 pulse signals, and the pulse repetition period is 2.5 ms. The reference channel contains three multipath signals, threeThe relative delay units of the multipath signals are respectively 5, 12 and 15, the corresponding normalized Doppler frequencies are respectively 0.3, 0.4 and-0.4, and the ratio of the corresponding multipath signals to the direct wave energy is respectively-20 dB, -27dB and-23 dB. The normalized Doppler frequency of the direct wave is 0.5, the ratio of the direct wave ratio to the noise energy is 70dB, and the parameter MtD-20-11. The target to be detected is located at the 451 st distance unit, and the normalized relative (relative direct wave) Doppler frequency is-0.3. The multipath clutter suppression algorithms are BEM and L respectively1RLS and the method proposed in the examples of the present invention.
(2) And (4) analyzing results:
please refer to fig. 3a, fig. 3a shows STAP, BEM, L under the condition that the prior knowledge is accurate1Comparing the signal to interference plus noise ratio (SINR Loss) performance of the RLS and the method of the embodiment of the invention, wherein the STAP means that direct wave clutter and multi-path clutter are simultaneously suppressed by using the STAP method, and a single multi-path clutter suppression method is not included. As can be seen from fig. 3a, the STAP method not only has deeper nulls in the direct wave clutter regions (normalized doppler frequencies of-0.5 and 0.5), but also forms notches in the multipath clutter regions (normalized doppler frequencies of-0.3, -0.4, and 0.4). At this time, targets falling in this region will also be suppressed, indicating that impure reference signals severely affect the target detection capability of the STAP method. BEM, L of the prior art1The RLS and the method provided by the embodiment of the invention generate nulls only in the direct wave clutter region, still have higher output in other frequency ranges, and improve the target detection performance, thereby showing that the method provided by the embodiment of the invention can effectively overcome the influence of impure reference signals.
Please refer to fig. 3b, fig. 3b shows STAP, BEM, L under the condition that the prior knowledge is accurate1And comparing the target detection result of the RLS and the method of the embodiment of the invention, wherein the simulated target is positioned in the area where the multipath clutter is positioned. As can be seen from FIG. 3b, the STAP method cannot effectively detect the target, whereas the existing BEM, L1The RLS multipath clutter suppression method and the method of the embodiment of the invention can obtain better target detection performance, and prove that the method provided by the embodiment of the invention can effectively suppress the multipath clutter.
Referring to FIG. 4a, FIG. 4a shows BEM and L under the condition of inaccurate priori knowledge1The SINR Loss performance comparison results of RLS and the method proposed in the embodiment of the present invention, where the prior value of the normalized doppler frequency of the direct wave is 0.45 (the true value is 0.5). As can be seen from FIG. 4a, since L1Neither RLS nor the method proposed by the embodiments of the present invention need to use a priori information, so that L is accurate under conditions where a priori knowledge is inaccurate1The SINR Loss performance of the RLS and the method provided by the embodiment of the invention is superior to that of the BEM method. Please refer to fig. 4b, fig. 4b shows BEM and L under the condition of inaccurate a priori knowledge1Target detection comparison results of RLS and the method proposed in the examples of the present invention. The results in FIG. 4b show that L1The RLS and the method provided by the embodiment of the invention can still maintain better target detection performance, and the performance of the RLS and the method is better than that of the BEM method. The method provided by the embodiment of the invention can provide more stable multipath clutter suppression performance than the BEM method.
Referring to FIG. 5a and FIG. 5b, FIG. 5a shows BEM and L1Results of additive complexity comparison of RLS and the method proposed by the embodiment of the present invention, and BEM, L in FIG. 5(b)1-the result of the multiplicative complexity comparison of RLS and the method proposed by the embodiment of the invention. It can be seen that the addition complexity and the multiplication complexity of the method provided by the embodiment of the invention are both higher than those of the BEM algorithm, but far less than L1The RLS method. The following conclusions can be drawn from the simulation results of fig. 3a, 3b, 4a and 4 b: compared with the prior L1The RLS method can reduce the calculation amount of multipath clutter suppression without causing obvious suppression performance loss; compared with the existing BEM method, the method provided by the embodiment of the invention can provide stable multipath clutter suppression performance and target detection performance. Therefore, the method provided by the invention can realize compromise between the calculation complexity and the target detection performance in the multipath clutter suppression.
In summary, the airborne external radiation source radar multi-path clutter suppression method provided by the embodiment of the invention constructs the optimization problem of sparse constraint on the basis of the multi-path clutter sparse observation model, estimates the multi-path clutter by using the minimum mean square Error (EFWLMS) algorithm based on the exponential forgetting window, and finally realizes the suppression and elimination of the multi-path clutterAnd the influence of the multipath clutter on the STAP target detection performance is eliminated. Compared with the existing BEM method, the method does not need prior information, and can obtain stable multipath clutter suppression performance under the condition that the error exists in the prior knowledge; with the existing L1Compared with the RLS method, the method solves the optimization problem by using the EFWLMS algorithm, has the calculation complexity far lower than that of the RLS method, and can reduce the calculation amount of multipath clutter suppression. Therefore, the proposed method can achieve robust multipath clutter suppression performance with low complexity.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.

Claims (7)

1. A multipath clutter suppression method for an airborne external radiation source radar is characterized by comprising the following steps:
s1: acquiring a receiving signal of an airborne external radiation source radar, wherein the receiving signal comprises a plurality of echo signals received by a multi-array element observation antenna and impure reference signals received by a reference antenna;
s2: respectively carrying out uniform segmentation on the reference signal and the echo signals received by the observation antennas to obtain an equivalent reference pulse signal and an equivalent echo pulse signal;
s3: performing distance matching by using the equivalent reference signal and the equivalent echo signal to obtain a space-time snapshot signal of a distance unit to be detected;
s4: constructing a multipath clutter observation model;
s5: converting the suppression problem of the multipath clutter into the optimization problem of the weight vector in the multipath clutter observation model based on the multipath clutter observation model and the sparse characteristic of the multipath clutter snapshot in the distance-Doppler two-dimensional plane;
s6: solving the optimization problem by adopting a minimum mean square error algorithm based on an exponential forgetting window to obtain a snapshot signal of the distance unit to be detected after multipath clutter suppression;
s7: and carrying out target detection by utilizing the snapshot signal of the distance unit to be detected after multipath clutter suppression.
2. The method for multipath clutter suppression for an airborne external radiation source radar according to claim 1, wherein said S2 comprises:
s21: in a coherent accumulation time, uniformly dividing signals received by all antenna array elements into M sections, equivalently receiving M pulse signals by a reference antenna and an observation antenna respectively, wherein the repetition period of the equivalent pulse signals is TrThe number of fast time samples is L;
s22: an independent reference channel is formed in the reference antenna, and the reference channel contains a direct wave signal and NTA multipath signal.
3. The method for multipath clutter suppression for an airborne external radiation source radar according to claim 2, wherein said S3 comprises:
s31: performing matched filtering by using the equivalent reference pulse signal and the equivalent echo pulse signal corresponding to each observation antenna to obtain a direct wave clutter snapshot signal x of the distance unit l to be detecteddc,lAnd NTFast shooting x of multipath clutterpc,l；
Step 32: obtaining an interference snapshot signal of a distance unit l to be detected:
Figure FDA0003004268580000021
wherein x isnRepresenting noise.
4. The method for multipath clutter suppression for an airborne external radiation source radar according to claim 3, wherein said S4 comprises:
s41: using the received interference snapshot signal { x ] to the multipath clutter signal located in the range unit ll+1,xl+2,…,xl+DWhere D is the distance cancellation order,
s42: dividing the Doppler plane into MtA grid unit for obtaining the divided Doppler frequency omegajAnd corresponding modified time-oriented vector
Figure FDA0003004268580000022
S43: and constructing a multipath clutter observation model of the multipath clutter signal positioned in the distance unit l according to the corrected time guide vector and the D received interference snapshot signals:
Figure FDA0003004268580000023
wherein x isl+iFor disturbing snap-shot signals, x, located at distance units l + il+i,jFor the jth modified snap signal, α, located at a distance unit l + ii,jRepresents the complex amplitude of the jth modified snapshot signal located at the distance cell l + i;
s44: let the correction time-oriented matrix be
Figure FDA0003004268580000024
Expressing the multipath clutter observation model as:
Figure FDA0003004268580000031
wherein the content of the first and second substances,
Figure FDA0003004268580000032
with a representation dimension of MtThe weight vector of the D x 1 vector,
Figure FDA0003004268580000033
represents MN × MtD-dimensional distance-Doppler dictionary matrix, where SD,l＝[xl+1,xl+2,...,xl+D]。
5. The method for multipath clutter suppression for an airborne external radiation source radar according to claim 4, wherein said S5 comprises:
converting the estimation problem of the multipath clutter into an optimization problem of sparse constraint by utilizing a minimum output power criterion according to the multipath clutter observation model and the weight vector sparsity analysis to obtain a cost function:
Figure FDA0003004268580000034
wherein, E [. C]Expressing the mean value, | · | | luminance1And | · | non-conducting phosphor2Respectively represent L1Norm and L2Norm, Re {. is used for representing a real part, eta is a regularization parameter,
Figure FDA0003004268580000035
σ2representing the noise power.
6. The method for multipath clutter suppression for an airborne external radiation source radar according to claim 5, wherein said S6 comprises:
s61: and obtaining the deviation of the cost function by calculating:
▽Jα＝-rSx+RSα+ηsign(α)+ρΗ，
wherein sign (·) represents a sign function (·)ΗThe representation is taken of the conjugate transpose,
Figure FDA0003004268580000036
Figure FDA0003004268580000037
s62: let Jα0, get the expression of the optimal weight vector:
Figure FDA0003004268580000038
s63: the weight vector is iteratively estimated by using the minimum mean square error algorithm based on the exponential forgetting window to obtain the estimated value of the weight vector
Figure FDA0003004268580000041
S64: based on the estimated value of the weight vector
Figure FDA0003004268580000042
Obtaining snap-shot signal y of distance unit to be detected after multipath clutter suppressionl：
Figure FDA0003004268580000043
7. The method for multipath clutter suppression for an airborne external radiation source radar according to claim 6, wherein said S7 comprises:
s71: 2L adjacent to distance unit L to be detected after multipath clutter suppression1Estimating a direct wave clutter covariance matrix by using the snapshot signals:
Figure FDA0003004268580000044
s72: solving an STAP weight vector based on the estimated direct wave clutter covariance matrix:
Figure FDA0003004268580000045
wherein, v (ω)t,θt) For a target space-time steering vector, ωtAnd thetatRespectively the doppler frequency and the spatial frequency of the target;
s73: obtaining an output signal at a distance unit l to be detected:
Figure FDA0003004268580000046
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