CN112782696A - Sequence ISAR image scattering center multi-hypothesis tracking track correlation method - Google Patents
Sequence ISAR image scattering center multi-hypothesis tracking track correlation method Download PDFInfo
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Abstract
The invention discloses a sequence ISAR image scattering center multi-hypothesis tracking track correlation method, which comprises the steps of firstly, obtaining a high-resolution ISAR image sequence of an observed target and extracting two-dimensional projection coordinates of a scattering center in the image sequence on the basis of an ISAR target observation imaging model and a state transfer equation and an observation equation of a scattering center track position; then, using multi-hypothesis tracking method, the scattering center track association is performed from both forward and backward directions. Determining the only hypothesis of track association through N backtracking pruning hypothesis tree management, and generating an associated track; and finally, obtaining the final associated flight path through flight path fusion and Kalman filtering. The assumed tree framework adopted by the method has good response capability to false alarm, new generation and interruption situations in the association process, so that the robustness of the method is ensured.
Description
Technical Field
The invention relates to the technical field of radar image processing, in particular to a sequence ISAR image scattering center multi-hypothesis tracking track correlation method.
Background
The motion restoration structure algorithm is initially applied to three-dimensional imaging of an optical target, and the algorithm reconstructs a three-dimensional profile of the observed target based on a sequence of optical images acquired by a camera under multiple viewing angles. The algorithm comprises 4 main steps: feature extraction, feature association, three-dimensional estimation and error optimization. The feature association is used for realizing correct matching of the same feature in different images, and is a key point and difficulty for realizing accurate three-dimensional reconstruction. The ISAR image mainly comprises strong scattering points which are similar to the corner point characteristics in the optical image, so that a plurality of researchers transfer the motion recovery structure algorithm to ISAR three-dimensional imaging, and the three-dimensional reconstruction of the observed target is realized based on an image sequence obtained by single-base ISAR multi-view echo data imaging. The ISAR target three-dimensional reconstruction result can visually and clearly reflect the three-dimensional structure of the target, and has important application value in the aspects of battlefield target investigation, target automatic identification and the like.
The difficulty associated with scattering centers of an ISAR image sequence is that Radar backscattering coefficient (RCS) of the scattering centers are significantly anisotropic, and the occlusion of different scattering centers causes the interruption of the scattering center track in the image sequence. The correlation algorithms proposed by the existing research can be roughly divided into two types: an image feature based correlation method and a motion model based correlation method. The correlation method based on the image features is characterized in that the strong scattering center in the ISAR image is used as a feature point to be extracted of the image, and matching correlation of the same scattering center in different images is realized by designing a reasonable feature descriptor and a feature matching criterion. The KLT (Kanade Lucas Tomasi, KLT) feature tracking algorithm is a typical image feature-based correlation algorithm, but the algorithm is based on the assumption that the brightness of a target is constant, and the condition is difficult to guarantee in an ISAR image sequence, so that the application of the algorithm in ISAR images is fundamentally limited. The Speeded Up Robust Features (SURF) algorithm is another commonly used correlation algorithm based on image Features, and correlation of scattering centers can be completed under the condition that a small rotation angle is satisfied between two adjacent frames of images. However, the scattering center characteristics of the ISAR images are similar, and when the rotation angle between the two images is slightly larger, the algorithm performance is poor.
In view of the fact that ISAR images have obvious anisotropy among images and single features in the images compared with optical images, and the correlation method based on the image features is difficult to obtain ideal correlation effects, some scholars propose a motion model based on a scattering center track to solve the correlation problem from the research angle of multi-target tracking. Unlike the correlation method based on image features, the method needs to complete the coordinate extraction of the scattering center in advance. In the research field of solving the association problem based on a scattering center motion model, on the basis of a model-built track form, a scholars proposes to adopt a Markov Chain Monte Carlo (MCMC) data association algorithm and design a prior and likelihood model of Bayesian inference according to the motion characteristics of a scattering center track to realize the scattering center track association. However, the performance of the algorithm depends heavily on the design of the prior and likelihood probability models, and once the models are mismatched, the correlation effect is poor. Another scholars proposes a correlation method based on plane homography, which assumes that a target surface consists of several rectangular planes according to the same characteristic of flight path state transfer equations of scattering centers between two frames of ISAR images on the same plane, manually marks the positions of four corner points of each rectangular plane in an image sequence and forms coordinate pairs through matching, and then solves the state transfer equations of the scattering centers of each plane between the image sequences by adopting a least square method, thereby realizing the flight path correlation of the scattering centers. However, in practice, the ISAR target surface is usually not only formed by rectangular planes, and therefore, the method has limited application in the actual sequential ISAR image track correlation.
According to the analysis, the ISAR image characteristics have the characteristics of single characteristic type and severe intensity change compared with the optical image characteristics, so that the correlation method with good performance in the optical image is limited to be applied to the ISAR image; due to the fact that ISAR observation environment and a target structure are complex, the existing correlation algorithm based on the scattering center track state transition motion model lacks robustness. In summary, for the present time, the problem of ISAR image sequence scattering center track correlation still lacks an accurate and robust solution.
Disclosure of Invention
Aiming at the problems in the prior art, the invention aims to provide a sequence ISAR image scattering center multi-hypothesis tracking track association method, and the adopted hypothesis tree framework has good response capability to false alarms, new generations and interruption situations in the association process, so that the robustness of the method is ensured; the forward and reverse correlation fusion operation solves the problem of inaccurate correlation caused by lack of prior information during initial correlation, eliminates the influence of the correlation direction on the correlation result, and further improves the accuracy of the method.
The technical principle of the invention is as follows: on the basis of an ISAR (inverse synthetic aperture radar) target observation imaging model and a state transfer equation and an observation equation of a scattering center track position, firstly, a distance-Doppler imaging algorithm is adopted to obtain a high-resolution ISAR image sequence of an observation target, and two-dimensional projection coordinates of a scattering center in the image sequence are extracted through an ESPRIT (rotation invariant subspace) algorithm; then, using multi-hypothesis tracking method, the scattering center track association is performed from both forward and backward directions. Continuously updating the flight path state transition parameters of the scattering center by using the information of the existing hypothesis trees in the association process to generate two-direction hypothesis trees; then, determining a unique hypothesis of track association through N backtracking pruning hypothesis tree management, and generating an associated track; and finally, realizing information fusion of the flight paths obtained by association in the positive direction and the negative direction through flight path fusion, and performing Kalman filtering on the fused flight paths to obtain the final associated flight paths.
In order to achieve the purpose, the invention is realized by adopting the following technical scheme.
A sequence ISAR image scattering center multi-hypothesis tracking track correlation method comprises the following steps:
and 5, fusing the correlation tracks in the positive direction and the negative direction to realize the complementation of the correlation information in the positive direction and the negative direction to obtain a fused track, and performing Kalman filtering on the fused track to obtain a final track correlation result.
Compared with the prior art, the invention has the beneficial effects that:
the method of the invention provides an accurate robust track association scheme based on the characteristics and key problems of the ISAR image sequence scattering center track association: aiming at the characteristics of single characteristic type and severe intensity change of a scattering center imaging result, only depending on coordinate motion information projected by a scattering center to carry out correlation, and eliminating the influence of unreliable redundant information on the correlation result; aiming at the problem that the flight path state transition model parameters are unknown, parameter estimation is carried out by combining a flight path theoretical model, and the limitation that the flight path state cannot be predicted and associated due to the fact that the state transition model is unknown is broken through; aiming at the complex situations of track interruption, track regeneration and false alarm existing in the track association, multi-hypothesis association and delayed decision judgment are carried out on the basis of a hypothesis tree frame, and association accuracy is guaranteed; and the track fusion filtering is carried out aiming at the problem of inaccurate parameter estimation of the initial model, so that the accuracy of initial association is improved. The comprehensive analysis of the characteristics and the symptoms of the scattering center track correlation problem enables the method to have accurate and steady performance in the process of processing the scattering center track correlation problem.
Drawings
The invention is described in further detail below with reference to the figures and specific embodiments.
FIG. 1 is a schematic flow chart of a sequential ISAR image scattering center multi-hypothesis tracking track association method provided by the present invention;
FIG. 2 is a schematic diagram of an ISAR target observation and imaging model according to an embodiment of the present invention;
FIG. 3 is a diagram of an approximate circular motion estimation model according to an embodiment of the present invention;
FIG. 4 is a hypothetical management process schematic of an embodiment of the present invention;
FIG. 5 is a diagram of an aircraft model composed of 16 ISAR target points according to an embodiment of the present invention;
FIG. 6 is a high-resolution two-dimensional ISAR image of a portion of an aircraft target point model obtained by the method of the present invention; wherein 6(a) is the 1 st sub-aperture imaging result; 6(b) is the 10 th sub-aperture imaging result; 6(c) is the 20 th sub-aperture imaging result;
FIG. 7 is a diagram of a track coordinate extraction result of an image sequence corresponding to different sub-apertures;
FIG. 8 is a result diagram of a track correlation performed on the extracted results of FIG. 7;
FIG. 9 is a result diagram of a target three-dimensional reconstruction of the track correlation result of FIG. 8; wherein, 9(a) is a comparison graph of a space target point model reconstruction result and a real structure thereof in a three-dimensional space; 9(b) is a comparison graph of the spatial target point model reconstruction result and the projection of the real structure on the XOY plane; FIG. 9(c) is a comparison graph of the reconstruction result of the spatial object point model and the projection of the real structure thereof on the XOZ plane; and 9(d) is a comparison graph of the space target point model reconstruction result and the projection of the real structure on the YOZ plane.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to examples, but it will be understood by those skilled in the art that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention.
Referring to fig. 1, the method for correlating a sequential ISAR image scattering center multi-hypothesis tracking track provided by the present invention includes the following steps:
the method specifically comprises the following steps:
(1.1) continuously observing a target for a long time by an ISAR system to obtain echo data of the target in a wide observation angle range;
(1.2) carrying out sub-aperture division on the obtained target echo data to obtain K-segment sub-aperture data;
(1.3) constructing a reference signal, and performing pulse compression on each echo data in each sub-aperture to obtain a high-resolution one-dimensional range profile sequence;
the reference signal is generated as: taking the distance from the inverse synthetic aperture radar to the scene center as a reference distance, and constructing a linear frequency modulation signal with the same parameter as the transmitted signal as a reference signal;
(1.4) for each sub-aperture pulse pressure data, namely a high-resolution one-dimensional range profile sequence, firstly, envelope alignment is carried out on each sub-aperture pulse pressure data by adopting an envelope alignment method based on adjacent correlation, and envelope offset caused by target translation is compensated; then, compensating echo initial phase errors by adopting a self-focusing method based on a minimum entropy criterion; and finally, carrying out azimuth Fourier transform on the compensated data to obtain a two-dimensional high-resolution image sequence.
as shown in fig. 1, a translation compensated target coordinate system O-XYZ is established, where O is the target center and LOS is the translation compensationThe latter equivalent radar line of sight, l, is the projection of the radar line of sight on the XOY plane. Azimuth angle theta and pitch angle of radar equivalent visual line direction
And (4) defining. The target motion is equivalent to a rotation with an angular velocity ω about the fixed axis OZ.
And dividing the whole observation period into K sub-apertures, and correspondingly, obtaining the image frame number of the ISAR image sequence as K. Let us note that the time interval between adjacent sub-apertures is Δ t, and the coherent processing time of each sub-aperture is tCPI. Consider the imaging situation in the kth sub-aperture, let tkRepresenting the instant of the sub-aperture centre for any nth scattering centre s on the targetn＝(xn，yn，zn)TThe theoretical projection coordinate on the k frame image is
The two satisfy the following projection relation:
scattering center snThe flight path is as follows:
the scattering center trajectory is in the form of an ellipse, where (x)n)2＝(xn)2+(yn)2Compensating for the equivalent radius of rotation of the back scattering center for translational motion. During the actual observation, the parameter rn，znAnd radar line of sight equivalent pitch angle
Estimation acquisition is usually required.
Establishing a state transition equation of the projection position of the scattering center between two adjacent frames of images:
wherein,
are respectively a parameter zn、
An estimate of ω. Target three-dimensional equivalent rotation angular velocity omega and imaging effective rotation angular velocity omegaeApproximately satisfy the relation
Obtaining an estimate of effective rotational angular velocity by image orientation scaling
Thereafter, the estimated values are further combined
Calculating an estimated value of equivalent rotational angular velocity
And
the estimation and updating of (b) is performed during the hypothesis tree growing process.
The one-step state transition equation of the scattering center track between two frames of images is as follows:
wherein,
for noise in the state transition process, white Gaussian noise is usually assumed, and the correlation matrix is Qp＝E[χk(χk)T]＝diag{[δ1，δ2]}，δ1And delta2Representing the noise covariance of the distance and azimuth position during the transfer, respectively.
The position state observation of the scattering center is obtained as follows:
wherein,
denotes the scattering center snAt the observation position in the image of the k-th frame,
a representation of an observation matrix is shown,
to observe noise; the observed noise is assumed to be white Gaussian noise
To know
Representing the noise covariance of the range and azimuth positions during the observation, respectively.
Adopting an ESPRIT algorithm to carry out scattering center extraction operation on each frame of image in the calibrated ISAR image sequence to obtain the observation position of each scattering center track at the corresponding moment of each frame of image
the method specifically comprises the following steps:
(3.1) assume tree initialization:
for a chronological sequence of ISAR images, it is defined that the correlation is backward from the first frame as a forward correlation and forward from the last frame as a backward correlation. Selecting any one correlation direction, setting the optimal assumed nodes reserved in each layer of the assumed tree to be M, assuming the tree depth to be N, setting the maximum Life of the flight path to be k, wherein k is an integer larger than 0, and considering that the flight path is terminated if the Life of a certain flight path is equal to 0. When the hypothesis tree is initialized, all observations are initialized to new tracks, and the initial innovation covariance of each new track is P0. At this time, the tree is assumed to have only one root node, and the nodes are the only assumptions of the scattering center track association.
(3.2) state transition model estimation and state prediction:
suppose a new hypothesis is being generated based on a hypothesis node corresponding to the k-1 frame image, let
Representing the set of tracks for which an association has been assumed in the node,
representing the nth track, L, in the setk-1Representing the total number of tracks in the set. And defining the flight path length as the total number of observation points and prediction points of the flight path. From Tk-1Zhongren flight path
According to
The length of the path completes the state transition parameter estimation and state prediction of the path.
The length is divided into three cases:
(a)
the length is 1:
due to the lack of more track information, the state prediction is performed as follows:
wherein,
to represent
The position state prediction result in the k-th frame image,
representing the theoretical observed coordinates of the flight path in the k-th frame image,
to represent
And observing the actual position in the k-1 frame image.
(b)
Length greater than 1 and less than 4:
to wind the center o of the image (0, 0)TThe circular motion of (2) is approximate to track motion, and estimation of state transition parameters is carried out. As shown in FIG. 3, assume that
The observation in the image of the (k-1) th frame is an observation point 2, the observation in the image of the (k-2) th frame is an observation point 1, the motion from the observation point 1 to the observation point 2 is approximate to the circular motion with the center o of the image as the center and the radius of the distance r from the observation value 2 to the o as the radius, and the corner
With counterclockwise being positive. The motion of the flight path from the (k-1) th frame to the (k) th frame is still approximate to the circular motion and the corner with o as the center of a circle and r as the radius according to the stable and continuous motion characteristic of the flight path
Calculating the position prediction of the flight path in k frames as follows:
if it is
If there is no corresponding observation in the k frame image, then order
(c)
A length of 4 or more
And directly adopting least square fitting ellipse parameters to calculate a transfer equation of the track position state. The scattering center projection trajectory satisfies the elliptical motion equation,
the corresponding equation of motion is:
the elliptical trajectory is converted into a more general form:
the following relationship is further obtained:
e2＝e4＝0
in the formula
To know
Are the parameters that need to be estimated in the state transition equations.
Let e be [ e1，e3，e5，e6]TFormation matrix W [ u ], [ v ], [ 1 ]]Wherein u and v respectively represent an azimuth coordinate vector and a distance coordinate vector composed of all scattering center coordinates on the associated track, < > represents a Hadamard product, and 1 represents a column vector of all elements 1. Assuming the flight path length is xi, then
Defining an error function
F(e)＝(We)TWe＝eTWTWe
When the e parameter is the best fit parameter of the ellipse, the value of F (e) is minimized. Thus, the solution of the ellipse parameters is translated into the solution of the following constraint problem:
arg min F(e)＝eTWTWe
s.t.eTFe＝1
wherein,
solving the equation constraint optimization problem by adopting a Lagrange multiplier method, introducing a Lagrange multiplier lambda and performing derivation to obtain the equation constraint optimization problem
2Fe-2λWTWe＝0
eTFe＝1
Let S be WTW, further obtaining
Fe＝λSe
eTFe＝1
Solving by adopting a generalized eigenvalue solving method to obtain an eigenvalue and eigenvector pair (lambda)i，ei) It is clear that for any non-zero real number μ, (μ λ)i，μei) For a set of solutions that still satisfy the constraints, there are
Further obtaining an ellipse parameter set
Then the elliptical optimal parameter estimate eoptIs composed of
By means of eoptEach element of (A) and
to know
Is calculated from the relationship of
And
the value of (c). At this time, A in the state transition equationk|k-1、bnAll estimates are obtained, and the state prediction of the target at the next moment is calculated according to the estimation:
traverse T as abovekL in (1)kAnd (4) tracking the flight path to complete the state prediction of all the flight paths. The covariance is predicted in one step:
(3.3) hypothesis generation:
(a) and calculating the probability of the old flight path continuation, the new flight path and the false alarm of any current observation.
Recording probability P of track point detected in imagedThe nth observation in the kth frame image
The probability associated with the mth track is:
wherein, define
Respectively representing the innovation and innovation covariance corresponding to the association:
Furthermore, let PnewRepresenting the probability that a track point is a new track, PfThe probability that the track point is a false alarm is represented.
(b) Hypothesis assignments are made to all current observed sources, and probabilities of hypothesis assignments being established are calculated.
Suppose an observation set D of k frame imageskHas a total of NsAn observation, i group association hypothesis at previous time
The number of tracks in is NTIn a
On the basis of a priori assumptions on NsAssigning each observation, and constructing an assignment matrix omega:
Ω1(m，n)＝-ln(Pmn)
Ω2(m，n)＝-ln(Pnew)δ(m，n)
Ω3(m，n)＝-ln(Pf)δ(m，n)
Ω＝[Ω1，Ω2，Ω3]
and adopting a Murty algorithm to make an assignment hypothesis with the maximum M groups of possibilities for all current observations, wherein the j group of possibilitiesSuppose that
Middle to NsOne observation is assigned: old track continuation NtNew born target NnewFalse alarm NfAnd (4) respectively. Based on
Computing
Probability of being true:
the probability that the remaining M-1 groups are assigned is calculated in the same manner.
(c) Taking into account the probability that the previous assumption holds
Probability of being true:
wherein c is a normalization coefficient, D1：kSet of all observations up to the k-th frame, hypothesis set
Included
And the related assumptions made at all previous times IIk-1. The probability that the remaining M-1 groups are assigned to be established is calculated in the same way, and a new layer of hypothetical tree nodes is formed on the hypothetical tree.
(3.4) status update:
and updating the positions, the innovation covariance, the Life and other states of the targets at the previous moment to the current moment. If a certain observation is assumedIf the state is false alarm, the observation is removed from the state set; if a certain observation is assumed to be a new flight path, the position of the observation is taken as the initial position of the scattering center flight path, Life is set to be k, and innovation covariance is set to be P0And incorporating the observation into a track set at the current moment; if a certain observation hypothesis is associated with a certain track at the previous moment, updating the track position to be the current observed position, and updating the innovation covariance:
if the flight path at the previous moment is not associated with any observation hypothesis at the current moment, the flight path state classification is carried out according to the Life value of the flight path: if Life is greater than 0, considering that the flight path is temporarily interrupted, predicting and updating the position of the target at the current moment to be in the current position state, keeping the innovation covariance unchanged, and reducing Life by 1; and if Life is 0, the track is considered to be terminated, and the state of the track is not updated.
And (4) repeating the steps (3.1) to (3.4) and continuously generating new hypothesis nodes to form a hypothesis tree.
the process is shown in fig. 4: assuming that the Murty algorithm selects M to be 2 optimal hypotheses each time, assuming that the tree depth N to be 3, when the depth of the assumed tree is greater than N, selecting a node (a node 11 in the figure) with the minimum assumed cost in all the current leaf nodes, tracing back to the N-1 layer upwards, taking the traced node as a new root hypothesis, and pruning the side branches (subtracting all the nodes connected with the node 3). The hypothetical case of track association in the newly generated root node is determined to be the scattering center track case.
And 5, fusing the correlation tracks in the positive direction and the negative direction to realize the complementation of the correlation information in the positive direction and the negative direction to obtain a fused track, and performing Kalman filtering on the fused track to obtain a final track correlation result.
The method is implemented by the following steps:
(a) track matching
Assume that the forward correlated track results in
The number is the ith' track, and I is the total number of tracks; similarly, the inversely correlated track results in
Definition of
Representing flight paths
To know
And counting the number of the coincident tracks of the two tracks. Definition ai′j′Representing flight paths
To know
A matching relationship ofi′j′1 represents
Matching
ai′j′Is 0 represents
And
and not matched. The following optimization problem is solved by the hungian algorithm,
obtaining a flight path matching relation set A ═ { a ═ ai′j′＝1}；
(b) Taking any element a in the set A i′j′1, first rejecting
To know
The first four frame results of (1); then go through
To know
In the mutual exclusion frame number, the track point (including the predicted value) in the mutual exclusion frame number is used as the fusion track ti′j′Track points of; then go through
And
in the common frame number, the track points (including the predicted values) with the same coordinates in the corresponding frame number are taken as the fusion track ti′j′Taking the coordinate mean value of track points (including predicted values) with different coordinates in corresponding frame times as a fusion track ti′j′Predicting track points; wherein mutually exclusive frames are next to
And
of the frames appearing only once, the total number of frames being
To know
The number of frames present in each;
(c) repeating the step (b) until all the elements A are traversed to obtain a track fusion result Tfuse＝{ti′j′}；
(d) For TfusePerforming Kalman filtering along the forward direction and then along the reverse direction to obtain a filtering result T for each track in the systemfilter。
The method of the invention provides an accurate robust track association scheme based on the characteristics and key problems of the ISAR image sequence scattering center track association: aiming at the characteristics of single characteristic type and severe intensity change of a scattering center imaging result, only depending on motion information of a scattering center projection track to carry out correlation, and eliminating the influence of unreliable redundant information on the correlation result; aiming at the problem that the flight path state transition model parameters are unknown, parameter estimation is carried out by combining a flight path theoretical model, and the limitation that the flight path state cannot be predicted and associated due to the fact that the state transition model is unknown is broken through; aiming at the complex situations of track interruption, track regeneration and false alarm existing in the track association, multi-hypothesis association and delayed decision judgment are carried out on the basis of a hypothesis tree frame, and association accuracy is guaranteed; and the track fusion filtering is carried out aiming at the problem of inaccurate parameter estimation of the initial model, so that the accuracy of initial association is improved. The comprehensive analysis of the characteristics and the symptoms of the scattering center track correlation problem enables the method to have accurate and steady performance in the process of processing the scattering center track correlation problem.
Simulation experiment
The effectiveness of the present invention is further illustrated by point target simulation imaging experiments.
(1) Simulation conditions
The ISAR target point model is an airplane model composed of 16 points as shown in figure 5, the equivalent radar sight azimuth angle theta is 45 degrees after translational motion compensation, and the pitch angle is
(2) Simulation experiment content and result analysis
Based on the ISAR echo data of the space target with long time and large angle, the method of the invention can obtain a high-resolution two-dimensional ISAR image sequence. FIG. 6 shows a high-resolution two-dimensional ISAR image of a target point model of an airplane obtained by the method of the present invention. Wherein, fig. 6(a) is the 1 st sub-aperture imaging result; FIG. 6(b) shows the result of the 10 th sub-aperture imaging; fig. 6(c) shows the 20 th sub-aperture imaging result. And (3) extracting the track coordinates in the sequence image, wherein the extraction result is shown in FIG. 7, and the light and dark points in the graph represent the sub-aperture sequence of the observation.
Then, the extracted scattering center track observation is correlated, and the obtained track correlation result is shown in fig. 8. And further performing target three-dimensional reconstruction based on the obtained track association result, wherein the obtained reconstruction result is shown in fig. 9. FIG. 9(a) is a comparison graph of the reconstruction result of the model of the target point in space and the real structure in three-dimensional space; FIG. 9(b) is a comparison graph of the spatial object point model reconstruction result and the projection of the real structure on the XOY plane; FIG. 9(c) is a comparison graph of the reconstruction result of the spatial object point model and the projection of the real structure thereof on the XOZ plane; FIG. 9(d) is a comparison of the spatial object point model reconstruction and its real structure projected on the YOZ plane.
By observing the graph 9, the three-dimensional scattering center position of the target point model can be accurately reconstructed based on the scattering center track correlation result obtained by the method provided by the invention, and the accuracy of the track correlation result is reflected. The method can accurately process the complex conditions of false alarm, new generation, interruption and the like in the association, and has obvious robustness compared with the prior method.
Although the present invention has been described in detail in this specification with reference to specific embodiments and illustrative embodiments, it will be apparent to those skilled in the art that modifications and improvements can be made thereto based on the present invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (10)
1. A sequence ISAR image scattering center multi-hypothesis tracking track correlation method is characterized by comprising the following steps:
step 1, obtaining target echoes of ISAR (inverse synthetic aperture radar) long-time continuous observation, and performing sub-aperture division on target echo data to obtain K-segment sub-aperture data; sequentially performing distance compression, envelope alignment and self-focusing operation on each section of sub-aperture data to obtain a target high-resolution two-dimensional ISAR image sequence;
step 2, establishing a target coordinate system and a radar observation imaging model after translation compensation; establishing a state transition equation of the scattering center track between two frames of images and a position state observation equation of the scattering center; extracting the scattering center projection coordinates in the target high-resolution two-dimensional ISAR image sequence by utilizing an ESPRIT algorithm to obtain the observation position of each scattering center track at the corresponding moment of each frame of image;
step 3, performing scattering center track association frame by frame from the positive direction and the negative direction by using a multi-hypothesis tracking algorithm, namely continuously updating scattering center track state transition parameters by using the generated hypothesis trees of the positive direction and the negative direction to generate hypothesis trees of the positive direction and the negative direction;
step 4, performing hypothesis tree management by using an N backtracking pruning method, and determining scattering center tracks generated by association in the positive and negative directions, namely association tracks in the positive and negative directions;
and 5, fusing the correlation tracks in the positive direction and the negative direction to realize the complementation of the correlation information in the positive direction and the negative direction to obtain a fused track, and performing Kalman filtering on the fused track to obtain a final track correlation result.
2. The method for correlating multiple hypothesis tracking tracks of scattering centers of sequential ISAR images according to claim 1, wherein the distance compression, envelope alignment and self-focusing operations are sequentially performed on each segment of sub-aperture data, specifically:
(1.1) constructing a reference signal: taking the distance from the inverse synthetic aperture radar to the scene center as a reference distance, and constructing a linear frequency modulation signal with the same parameter as the transmitted signal as a reference signal;
(1.2) performing pulse compression on each echo data in each sub-aperture based on a reference signal to obtain each sub-aperture pulse pressure data, namely a high-resolution one-dimensional range profile sequence;
(1.3) for each sub-aperture pulse pressure data, firstly, carrying out envelope alignment on each sub-aperture pulse pressure data by adopting an envelope alignment method based on adjacent correlation, and compensating envelope offset caused by target translation; then, compensating echo initial phase errors by adopting a self-focusing method based on a minimum entropy criterion; and finally, carrying out azimuth Fourier transform on the compensated data to obtain a high-resolution two-dimensional image sequence.
3. The sequence ISAR image scattering center multi-hypothesis tracking track association method according to claim 1, wherein a translation-compensated target coordinate system and a radar observation imaging model are established; accordingly, a state transition equation of the scattering center track between two frames of images and a position state observation equation of the scattering center are established, and the method specifically comprises the following steps:
(2.1) establishing a translation compensated target coordinate system O-XYZ, wherein O is a target center; the azimuth angle of the equivalent sight line direction of the radar in the radar observation imaging model is theta, and the pitch angle is
The target motion is equivalent to rotation with an angular velocity omega around a fixed axis OZ;
(2.2) correspondingly obtaining K frames of target high-resolution two-dimensional ISAR images by K sections of sub-aperture target data, wherein the K frames of images form an image sequence; let us note that the time interval between adjacent sub-apertures is Δ t, and the coherent processing time of each sub-aperture is tCPI(ii) a Let tkRepresenting the instant of the sub-aperture centre for any nth scattering centre s on the targetn＝(xn，yn，zn)TThe theoretical projection coordinate on the k frame ISAR image is
The two satisfy the following projection relation:
scattering center snThe flight path is as follows:
wherein, the superscript T is the transposition of the matrix; (r)n)2＝(xn)2+(yn)2Compensating the equivalent rotation radius of the back scattering center for translation;
during the actual observation, the parameter rn，znEquivalent pitch angle with radar sight line
Obtained by estimation;
(2.3) establishing a state transition equation of the projection position of the scattering center between two adjacent frames of images:
wherein,
are respectively a parameter zn、
An estimate of ω; a. thek|k-1、bnAre respectively asA scattering center track state transition parameter; by estimating parameters
Obtaining a scattering center track state transition parameter;
(2.4)
the estimation process of (2) is: target three-dimensional equivalent rotation angular velocity omega and imaging effective rotation angular velocity omegaeSatisfy the relation
Obtaining an estimate of effective rotational angular velocity by image orientation scaling
Thereafter, the estimated values are further combined
Calculating an estimated value of equivalent rotational angular velocity
And
the estimation and updating of (2) is carried out during the growth of the hypothetical tree;
(2.5) the one-step state transition equation of the scattering center track between two frames of images is as follows:
wherein,
noise that is the process of state transition;
the position state observation of the scattering center is obtained as follows:
wherein,
denotes the scattering center snAt the observation position in the image of the k-th frame,
a representation of an observation matrix is shown,
is a scattering center snObservation noise in the k-th frame image.
4. The method for associating the scattering center multi-hypothesis tracking tracks of the sequence ISAR images according to claim 3, wherein the method for associating the scattering center tracks frame by frame from the forward direction and the backward direction by using the multi-hypothesis tracking algorithm comprises the following steps:
(3.1) assume tree initialization:
for the ISAR image sequence arranged in time sequence, defining that the backward correlation is the forward correlation from the first frame and the forward correlation is the backward correlation from the last frame; for any correlation direction, setting the number of the reserved optimal assumed nodes of each layer in the assumed tree to be M, the assumed tree depth to be N, the maximum Life of the flight path to be k, and the k to be an integer greater than 0;
when the hypothesis tree is initialized, all observations are initialized to new tracks, and the initial innovation covariance of each new track is P0；
(3.2) target state transition parameter estimation and state prediction:
let a new hypothesis be generated based on a hypothesis node corresponding to the k-1 th frame of image; order to
Representing the set of tracks for which an association has been assumed in the node,
representing the nth track, L, in the setk-1Representing the total number of tracks in the set; defining the flight path length as the total number of observation points and prediction points of the flight path;
from Tk-1Zhongren flight path
According to
The length of the flight path is used for completing the state transition parameter estimation and state prediction of the flight path;
(3.3) hypothesis generation:
(3.3a) calculating the probability that any current observation is old track continuation, new track and false alarm;
(3.3b) making hypothesis assignments to all current observations based on the probabilities calculated in step (3.3a) and calculating the probability that each hypothesis assignment holds;
(3.3c) calculating the current hypothesis assignment based on the probability that the previous hypothesis assignment holds
A probability of being established;
(3.4) status update:
if a certain observation hypothesis is a false alarm, removing the observation from the state set; if a certain observation is assumed to be a new flight path, the position of the observation is taken as the initial position of the scattering center flight path, Life is set to be k, and innovation covariance is set to be P0And is combined withBringing the observation into a track set at the current moment; if a certain observation hypothesis is associated with a certain track at the previous moment, the track position is updated to be the current observed position, and the innovation covariance is updated according to the following formula:
if the flight path at the previous moment is not associated with any observation hypothesis at the current moment, carrying out flight path state classification according to the Life value of the flight path: if Life is greater than 0, considering that the flight path is temporarily interrupted, and updating the position prediction of the target at the current moment to be in the current position state, wherein the innovation covariance is unchanged, and Life is reduced by 1; if Life is equal to 0, the track is considered to be terminated, and the state of the track is not updated;
and (4) repeating the steps (3.1) to (3.4) and continuously generating new hypothesis nodes to form a hypothesis tree.
5. The sequential ISAR image scattering center multi-hypothesis tracking track association method according to claim 4, wherein the slave T is Tk-1Zhongren flight path
According to
The length of the path is used for completing the state transition parameter estimation and state prediction of the path, and specifically comprises the following steps:
(3.2a) when
Length 1:
due to the lack of more track information, the state prediction is performed as follows:
wherein,
to represent
The position state prediction result in the k-th frame image,
representing the theoretical observed coordinates of the flight path in the k-th frame image,
to represent
Observing the actual position in the k-1 frame image;
(3.2b) when
When the length is greater than 1 and less than 4, the estimation of the state transition parameters is carried out by approximating the track motion by the circular motion around the image center o, specifically: is provided with
The motion of the observation point in the (k-2) th frame to the observation point in the (k-1) th frame image is approximately circular motion with the center o of the image as the center and the radius of the distance r from the observation point in the (k-1) th frame image to the point o, and the rotation angle thetak-1|k-2Taking the anticlockwise direction as positive; the motion of the flight path from the (k-1) th frame to the (k) th frame is approximated to be circular motion with o as the center of a circle and r as the radius, and the motion corners of adjacent frames are the same, namely thetak|k-1＝θk-1|k-2And calculating the position prediction of the flight path in the k frames as follows:
if it is
If there is no corresponding observation in the k frame image, then order
(3.2c) when
When the length is greater than or equal to 4, fitting the ellipse parameters by adopting a least square method, and calculating a transfer equation of the track position state;
traverse T as abovekL in (1)kAnd (4) tracking the flight path to complete the state prediction of all the flight paths.
6. The sequence ISAR image scattering center multi-hypothesis tracking track correlation method according to claim 5, wherein the method for fitting the ellipse parameters by using the least square method and calculating the transfer equation of the track position state comprises the following specific steps: the scattering center projection trajectory satisfies the elliptical motion equation,
the corresponding equation of motion is:
converting the above equation to a general form:
the following relationship is further obtained:
e2＝e4＝0
in the formula,
to know
Parameters to be estimated in the state transition equation;
let e be [ e1，e3，e5，e6]TFormation matrix W [ u ], [ v ], [ 1 ]]Wherein u and v respectively represent an azimuth coordinate vector and a distance coordinate vector formed by all scattering center coordinates on the associated track, < > represents a Hadamard product, and 1 represents a column vector of all elements 1; if the flight path length is xi, then
Defining an error function:
F(e)＝(We)TWe＝eTWTWe
when the e parameter is the best fitting parameter of the ellipse, the value of F (e) is minimized, thus converting the solution of the ellipse parameter into the solution of the following constrained optimization problem:
arg min F(e)＝eTWTWe
s.t.eTFe＝1
wherein,
the constraint optimization problem is solved by adopting a Lagrange multiplier method, and a Lagrange multiplier lambda is introduced and subjected to derivation to obtain the constraint optimization problem
2Fe-2λWTWe＝0
eTFe＝1
Let S be WTW, further obtaining
Solving by adopting a generalized eigenvalue solving method to obtain an eigenvalue and eigenvector pair (lambda)i，ei) It is clear that for any non-zero real number μ, (μ λ)i，μei) For a set of solutions that still satisfy the constraints, there are
Further obtaining an ellipse parameter set table
Then the elliptical optimal parameter estimate eoptComprises the following steps:
by means of eoptEach element of (A) and
and
by obtaining the relationship of
And
a value of (d); further obtaining a parameter A in the state transition equationk|k-1、bnThereby obtaining the state prediction of the target at the next moment
7. The method according to claim 4, wherein the calculating the probability that any current observation is an old track continuation, a new track and a false alarm specifically comprises: recording the probability that the track point is detected in the image as PdThe nth observation in the kth frame image
The probability associated with the mth track is:
wherein,
is Gaussian distributed and has
Wherein,
respectively representing the innovation and innovation covariance corresponding to the association:
Suppose an observation set D of k frame imageskHas a total of NsAn observation, i group association hypothesis at previous time
The number of tracks in is NTIn a
On the basis of a priori assumptions on NsAssigning each observation, and constructing an assignment matrix omega:
Ω1(m，n)＝-ln(Pmn)
Ω2(m，n)＝-ln(Pnew)δ(m，n)
Ω3(m，n)＝-ln(Pf)δ(m，n)
Ω＝[Ω1，Ω2，Ω3]
and adopting a Murty algorithm to assign M groups of hypotheses with the maximum possibility to all current observations, wherein the j group of possibility hypotheses
Middle to NsOne observation is assigned: old track continuation NtNew born target NnewFalse alarm NfA plurality of; based on
Computing
Probability of being true:
calculating the probability of the assignment of the rest M-1 groups in the same way;
device set
To comprise
And the related assumptions made at all previous timesk-1Then the probability that its assumption is true is:
wherein c is a normalization coefficient, D1：kRepresents a set of all observations up to the k-th frame;
the probability that the remaining M-1 groups are assigned to be established is calculated in the same way, and a new layer of hypothetical tree nodes is formed on the hypothetical tree.
8. The sequence ISAR image scattering center multi-hypothesis tracking track association method according to claim 1, wherein the hypothesis tree management is performed by using an N backtracking pruning method, specifically: setting a Murty algorithm to select M optimal hypotheses each time, assuming that the depth of a tree is N, when the depth of the assumed tree is greater than N, selecting a node with the highest probability of establishment assigned by the hypotheses in all current leaf nodes, tracing back an N-1 layer upwards, taking the traced node as a new root hypothesis, and cutting off all side branches irrelevant to the new root hypothesis; the path correlation hypothesis in the newly generated root node is determined as the scattering center path.
9. The sequence ISAR image scattering center multi-hypothesis tracking track correlation method according to claim 1, wherein the correlation tracks in the forward direction and the backward direction are fused to realize correlation information complementation in the forward direction and the backward direction, and the method is implemented by the following steps:
(5.1) track matching
Setting the forward correlation track result as
The number is the ith' track, and I is the total number of tracks; the inversely correlated track results are
Definition of
Representing flight paths
And
counting the number of the overlapped tracks of the two tracks; definition ai′j′Representing flight paths
And
a matching relationship ofi′j′1 represents
Matching
ai′j′Is 0 represents
And
mismatch is not achieved; the track matching problem is converted into the following optimization problem:
solving the following optimization problem through a Hungarian algorithm to obtain a track matching relation set A ═ ai′j′＝1}；
(5.2) taking any element a in the set Ai′j′1, first rejecting
And
the first four frame results of (1); then go through
And
in the mutual exclusion frame time, the track point in the mutual exclusion frame time is taken as the fusion track ti′j′Track points of; then go through
To know
The track points with the same coordinates in the corresponding frame times are taken as the fusion track ti′j′Taking the coordinate mean value of the track points with different coordinates in the corresponding frame number as the fusion track ti′j′Predicting track points;
wherein the mutually exclusive frame is next to
And
of the frames appearing only once, the total number of frames being
To know
The number of frames present in each;
(5.3) repeating the step (5.2) until all elements of the set A are traversed to obtain a fused track Tfuse。
10. The method of claim 9, wherein the sequential ISAR images scatter center multi-hypothesis tracking track correlation method is characterized in thatThe Kalman filtering is performed on the fusion track by using a mode of TfusePerforming Kalman filtering along the positive direction and then performing Kalman filtering along the negative direction to obtain a filtering result T for each flight path in the systemfilterAnd obtaining the final track correlation result.
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