CN107085212A - A kind of spin target time-varying three-D imaging method based on linearly modulated stepped frequency - Google Patents
A kind of spin target time-varying three-D imaging method based on linearly modulated stepped frequency Download PDFInfo
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Abstract
The present invention provides a kind of spatial spin target time-varying three-D imaging method based on linearly modulated stepped frequency, including：The first step, passes through " Dechirp " and translation compensation deals by data echo, enters the slow time image of row distance, and obtained coarse resolution Range Profile is handled, and synthesis obtains smart resolution distance picture；Second step, by Hough transform, extracts the echo curve parameter of each scattering point in smart resolution distance picture, and then obtain the echo track of each scattering point；3rd step, interference treatment is carried out to triantennary echo along course bearing, obtains two-dimensional coordinate, then the distance represented to track carries out range walk compensation to position, obtains third dimension coordinate, and finally comprehensive obtained three-dimensional coordinate, that is, obtain the three-dimensional image of scattering point；Linearly modulated stepped frequency has narrower instant bandwidth, but can be applied to Narrow-band Radar to improve the burden of image quality and reduction hardware system again with larger synthetic bandwidth simultaneously.
Description
Technical Field
The invention relates to a signal and information processing technology, in particular to a spinning target time-varying three-dimensional imaging method based on linear frequency modulation stepping signals.
Background
In recent years, with the rapid increase of the number of space targets such as space fragments, satellites, ballistic missiles and the like, the space environment is increasingly complex, and the development of space target tracking, measurement, classification, identification and the like has very important significance for guaranteeing the space safety of China, promoting the development of national space technology and the average of space and utilization.
Radar imaging technology plays a significant role in object classification and recognition as an important means of spatial surveillance. Since the space target mostly has various micromotion forms of spin, precession, rolling and the like, each scattering point on the target can generate over-range unit walking in the distance direction and the azimuth direction in the coherent accumulation time, and the ISAR image focusing is difficult to realize by the traditional distance-Doppler (RD) algorithm. Compared with two-dimensional imaging, the three-dimensional imaging technology can provide more abundant target characteristics, and has remarkable advantages in the aspects of obtaining the appearance, the volume, the micro-motion parameters and the like of a target. The existing three-dimensional imaging technology for spatial targets mainly includes a three-dimensional imaging technology based on a single-base radar (see royal ' fast precession target high-resolution three-dimensional radar imaging ' published in the science and remote sensing academy of earth's republic of china, 2008, stage forty-six) and a three-dimensional imaging technology based on a double (see zhouhai's double-base micro-doppler research on spinning tail warheads, published in the electronics and information academy, 2012, stage thirty-four)/multi-base radar (see lianshuai's spatially asymmetric spinning target radar three-dimensional imaging method based on micro-motion characteristic association, published in the electronics and information academy, 2014, stage thirty-six) for extracting precession angle and real length characteristics of a precession target from a distance image sequence. However, when the above method is used to obtain a real three-dimensional imaging result and three-dimensional micro-motion characteristics of a target, joint processing needs to be performed on echoes of multiple radars at different viewing angles, and in practical application, problems of radar synchronization, scattering center anisotropy on the target, shielding effect and the like also exist, and the system implementation is complex. In order to solve the problems in the above three-dimensional imaging method for spatial targets, an interferometric three-dimensional imaging method in a narrow-band radar is proposed (refer to "three-dimensional imaging of spatial rotating targets of narrow-band radar processed by multi-antenna interference", published in "university of air force engineering" (nature science edition), 2016, seventeenth stage), and by performing interferometric calibration on echoes of different scattering points in time-frequency domain, but because the distance resolution of the narrow-band radar is low, the real coordinates of each scattering point in the distance direction cannot be obtained.
At present, in the extraction of the micro-motion features of the spatial target, in order to obtain higher resolution, a broadband radar has become a development trend, meanwhile, since the track height of the spatial target is generally over hundreds to thousands of km, the maximum detection distance of the radar must meet the condition to monitor the spatial target, and since the narrow-band radar has a narrower bandwidth and a smaller noise coefficient, the system sensitivity is higher, and the maximum detection distance is farther.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provides a spinning target time-varying three-dimensional imaging method based on linear frequency modulation stepping signals.
The invention is realized by the following modes:
the method comprises the following steps: performing translational compensation on three-antenna echo signals, performing line-releasing frequency modulation processing, performing one-dimensional range imaging to obtain a coarse resolution range image of each antenna echo, and extracting range unit data of the target scattering point echo to synthesize a fine resolution range image;
step two: separating the echo track of each scattering point by adopting Hough transformation according to the obtained fine resolution range profile;
step three: and performing interference processing on the echo of each scattering point according to the echo track to obtain a two-dimensional coordinate position, and performing distance walking compensation on the echo track to obtain a third-dimensional coordinate position, thereby realizing three-dimensional imaging on the scattering point target.
The third step specifically comprises the following steps:
step1) respectively carrying out interference processing on echoes of the antenna A and the antenna B and echoes of the antenna A and the antenna C on a distance-slow time plane according to the echo track of each scattering point to obtain a spatial two-dimensional coordinate of the scattering point;
step2) performing distance walking compensation on the distance curve represented by the extracted scattering point track to obtain a third dimensional coordinate;
step3) obtaining the space three-dimensional position of the scattering point by combining the obtained three-dimensional coordinates, wherein the three-dimensional coordinates of each slow time moment form a time-varying three-dimensional image of the scattering point.
The invention has the beneficial effects that: in the extraction of the micro-motion features of the spatial target, in order to obtain higher resolution, broadband radar has become a development trend, and since the track height of the spatial target is generally over hundreds to thousands of km, the maximum detection distance of the radar must reach the condition to be able to monitor the spatial target. The narrow-band radar has a narrow bandwidth and a small noise coefficient, so that the system sensitivity is high, and the maximum detection distance is long. The chirp stepping signal has a narrow instantaneous bandwidth, but also has a large synthesis bandwidth, and can be applied to narrow-band radars to improve the imaging quality and reduce the burden of a hardware system.
Drawings
FIG. 1 is a geometric diagram of a three-antenna interferometric imaging system;
FIG. 2 is a flow chart of the present imaging method;
FIG. 3(a) is a rough-resolved range profile of the echo processing results, and FIG. 3(b) is a high-resolved range profile of the echo processing results;
FIG. 4 shows the result of the separation of the spin trajectory of scattering points;
fig. 5(a) is a three-dimensional time-varying x-axis coordinate reconstruction result of the scattering point, fig. 5(b) is a three-dimensional time-varying z-axis coordinate reconstruction result of the scattering point, and fig. 5(c) is a three-dimensional time-varying y-axis coordinate reconstruction result of the scattering point;
FIG. 6 shows a time-varying three-dimensional imaging result of a scattering point target;
fig. 7(a) shows a range image when the SNR is 5dB and a coarse-resolution range image of an interference processing result, fig. 7(b) shows a range image when the SNR is 5dB and a high-resolution range image of an interference processing result, fig. 7(c) shows a range image when the SNR is 5dB and an x-axis time-varying coordinate reconstruction result of an interference processing result, and fig. 7(d) shows a range image when the SNR is 5dB and a z-axis time-varying coordinate reconstruction result of an interference processing result.
Detailed Description
The invention will be further described with reference to the drawings and examples of the invention.
As shown in fig. 1 and 2, the present invention is implemented by the following steps: the three vertically disposed antennas A, B and C in the three-antenna three-dimensional imaging system constitute a ground-based imaging radar system. A radar coordinate system xyz is formed with the transmitting-receiving integrated antenna a as the origin of coordinates, and the receiving antennas B and C are located at (L,0,0) and (0,0, L), respectively. The position of the reference point O in the radar coordinate system is (X)c,Yc,Zc) And establishing a target local coordinate system by taking the O as an origin, wherein each coordinate axis is parallel to each coordinate axis of the radar coordinate system. The target local coordinate system has the same translation speed as the target. An object in space spins around a rotation axis at a rotational angular velocity ω, which can be resolved into angular velocity vectors (ω) along the x-axis, y-axis, and z-axisx,ωy,ωz). Performing one-dimensional range imaging on spatial spinning target echoes obtained by the three antennas after translational compensation to obtain a coarse resolution range image, synthesizing a high resolution range image by extracting a range unit signal where the target echo is located, performing track extraction on each echo curve of a range-slow time plane, performing interference processing on the echoes obtained by the three antennas, and obtaining x-axis coordinates and z-axis coordinates respectively; obtaining a y-axis coordinate by performing distance walking compensation on the obtained track; the existence of noise causes the coordinate values of the x axis and the z axis obtained by interference to fluctuate randomly, and the coordinate of the y axis is not a smooth curve in imaging time due to the limitation of distance resolution, and is smoothed. The concrete description is as follows:
the method comprises the following steps: performing one-dimensional range imaging
Assuming that a radar transmits a linear frequency modulation stepping signal, each pulse consists of M sub-pulses, the stepping frequency is delta f, the pulse width of the sub-pulses is tau, the interval of the sub-pulses is T, the echo of the mth sub-pulse received by an antenna A is
Wherein e (t) ═ rect (t/τ) · exp (j π μ t)2)，tkThe intra-pulse time, i.e. "fast time", mu is the modulation frequency, thetamIs the initial phase of the mth sub-pulse, RiAIs the distance, σ, from the i-th scattering point to the antenna AiThe scattering coefficient of the ith scattering point is shown, c is the wave speed, and n is the number of the scattering points. The reference signal may be denoted as E0(tk,m)＝e(tk-mT-2ROA/c)·exp(j2π(fc+m·Δf)·(tk-mT-2ROA/c)+jθm) (2)
The sub-pulses are subjected to a dechirp process, the result being
Wherein t ═ tk-mT-2ROA/c，ΔRiA＝RiA-ROA. Fourier transform is performed on t ' and the ' residual video phase term ' and the ' oblique ' term of the echo envelope are removed, and equation (3) becomes
Equation (4) is the "coarsely resolved range profile", and Δ R is assumed to have been accurately compensated for target translationiARepresenting the relative spin motion distance. For smaller size objects in space, the spin motion distance is always smaller than the coarse range resolution unitLength of (i.e. | Δ R)iAI < c/(2B), B ═ μ τ is the bandwidth of the sub-pulse. Therefore, the 'coarse resolution range images' of all scattering points on the target are gathered in the same range resolution unit, and the sequence of the 'coarse resolution range images' obtained by all pulse echoes in the imaging time is shown as a straight line on the range-slow time plane. As can be seen from the phase term in (4), although information for interference processing can be extracted from the phase of the coarsely resolved range image, a phase additional term is added between the sub-pulses due to the presence of m · Δ f, but the value of the additional phase term is relatively small. In order to obtain the distance images of different scattering points, a high-resolution distance image of each scattering point needs to be synthesized by starting from a 'coarse-resolution distance image'. For the spin motion of the ith scattering point, the displacement expression of the ith scattering point in the line-of-sight direction of the antenna A can be written as
ΔR0iA(t)＝RbA+ρiAcos(Ωt+φiA) (5)
wherein ,RbAIs the distance from the center of rotation to a reference point, Ω is the angular velocity of rotation, ρi and φiRespectively the rotation radius and the initial phase of the ith scattering point. From (5) the rotational speed of the scattering point in the line of sight of the antenna A is
viA(t)＝-ρiAΩsin(Ωt+φiA) (6)
The rotation speed v of the ith scattering point within one pulse durationiAApproximately constant, since the rotating scattering point also generates displacement in the pulse train, i.e. the distance in the pulse train moves, the displacement of the ith scattering point in the line of sight direction of the antenna A can be rewritten as
ΔRiA(t,m)＝ΔR0iA(t)+viA(t)mT (7)
Extracting the row signal of the "coarse resolution range image" in (4), i.e. making f-2 μ Δ RiAC, substituting (6) and (7) and performing Fourier transform on m
This is the high resolution range image sequence of the scattering point, whose peak is seen to be located
(9) Range image scaling can be achieved by dividing both sides equally by-2 af/c, but additional terms are generated due to intra-pulse range walk, i.e., the second and third terms in (9), the second term causing range image walk, and the third term widening the range image. However, it can be seen from (8) that although the high-resolution range image produces walk and spread, the phase term is not affected. Therefore, the phase term can be directly extracted during subsequent interference processing without considering the influence of distance walking in the pulse train.
By the same processing as the echo of the antenna A, a high-resolution range image sequence obtained by the antenna B can be obtained as
wherein ,σ′iIs the scattering coefficient, Δ R, of the i-th scattering point relative to the antenna B0iAB(t)＝ΔR0iA(t)+ΔR0iB(t)，viAB(t)＝viA(t)+viB(t)，ΔRiB(t)＝RiB(t)-ROA(t)，RiB(t) is the distance from the i-th scattering point to the antenna B, viBAnd (t) is the movement speed of the ith scattering point in the line-of-sight direction of the antenna B.
Step two: trajectory separation
In order to optimize the time-varying three-dimensional imaging result of the scattering points of the target, the spin trajectories of the scattering points in the high-resolution range-finding sequence need to be separated before the interference processing, that is, the peak position of the high-resolution range-finding image of each scattering point at each slow time instant is found out. From (9), although the high-resolution range image sequence of the spin scattering point moves, it still appears as a sine curve, and if the parameters of each sine curve can be extracted, the spin trajectory of the scattering point can be reflected by the obtained sine curve. The Hough transform is firstly applied to image edge detection, is used for detecting straight lines and various curves meeting specific analytical expressions, and is then used in the SAR/ISAR imaging field for target identification and target micro-motion feature extraction. And extracting sinusoidal parameters in a high-resolution range profile sequence by adopting Hough transformation so as to obtain the spin trajectory of each scattering point. The Hough transform equation can be constructed as
wherein ,for the expression of the curve to be detected, RbIs the baseline of the sinusoid, C is the amplitude, Ω is the oscillation frequency, and θ is the phase. Thus, willThe problem of planar curve detection is converted into parameter (R)bC, Ω, θ) space.
Discretizing each obtained sine curve at a slow time sampling interval, and corresponding each discrete value to a distance resolution unit of a high-resolution range profile so as to obtain the spin trajectory of each scattering point in the high-resolution range profile sequence.
Step three: interferometric three-dimensional imaging
And on the basis of obtaining the spin trajectory of each scattering point in the high-resolution range image sequence, performing interference processing on the high-resolution range image sequence of each scattering point in each interference plane respectively. Suppose the ith scattering pointThe high-resolution range profile sequences obtained by antenna A, antenna B and antenna C are respectively EAi(t,fm)，EBi(t,fm) and ECi(t,fm). Subjecting it to an interference treatment, which can be expressed as
Where "angle" represents the phase angle of the complex number, and "+" represents the conjugate operation.
According to the geometrical relationship in FIG. 1
wherein ,ROA(t) is the reference distance, i.e. the distance of point O from antenna A. Since the phase term takes 2 pi as the period, in order to avoid phase ambiguity, it should be ensuredAccording to (14), a
From (15), the time-varying position of the point P in the azimuth direction can be obtained. Similarly, by performing interference processing on the echoes of the antenna A and the antenna C, the time-varying position of the point P in the pitching direction is obtained
The position of the y axis can be obtained by a high-resolution range image sequence, but the high-resolution range image sequence cannot directly reflect range direction information of a scattering point of a target due to the influence of range walk and spread, and correction processing is needed. Due to the far-field condition between the three-antenna structure and the target distance, the distance difference between the scattering point and the three antennas is generally smaller than the length of one fine distance resolution unit, so that the radial distance information of the target scattering point reflected by the three high-resolution range image sequences is basically the same, the time-varying position of the y axis can be reconstructed according to any one high-resolution range image sequence, and the high-resolution range image sequence obtained by the antenna A is adopted. According to (9), since the range broadening term is much smaller than the first two terms, it can be ignored in determining the peak position of the high-resolution range profile, and the substitution of (5) and (6) and the range scaling can be obtained
wherein ,(17) namely, the peak value of the high-resolution range image sequence generating range walk, compared with the theoretical value of (5) generating no range walk, the amplitude and the phase of the peak value are changed. Thus, the actual range-wise position of the scattering point of the object should be expressed as
Wherein, Δ t ═ 2 π - φa)/Ω′，R′bAAnd Ω' are the baseline and frequency of the target scattering point spin trajectory sinusoid extracted by the Hough transform, respectively.
In the far field and radar side view, there is 2 × (y)i(t)+Yc)≈RiA(t)+RiB(t), combinations (12) and (13), (15) and (16) may be rewritten as
From the spatial three-dimensional coordinates { x ] of the respective scattering pointsi(t),yi(t),zi(t) }, i is 1,2, …, n, the time-varying three-dimensional image of the object can be reconstructed. The complete time-varying three-dimensional imaging flowchart is shown in fig. 2.
Example (c): time-varying three-dimensional imaging simulation experiment
Simulation experiment: to verify the validity of the algorithm proposed by the present invention, we performed the following computer simulation. The radar transmits a chirp step signal. The parameters required for the data simulation are set forth in table 1.
Table 1 simulation parameter settings
Simulation 1: to verify the validity of the algorithm, the following simulation experiment is now performed. Assuming three scattering points on the object, the rotation frequency is 2.5Hz, the rotation radius is (0.95,1.52,1.99) m, and the scattering coefficient is 1.
The first step is as follows: the echo signal is processed by demodulating, one-dimensional range imaging is performed, the coarse resolution range image is used to synthesize the fine resolution range image, and the obtained sequences of the coarse resolution range image and the high resolution range image are shown in fig. 3(a) and fig. 3(b), respectively. Extracting the track of the echo curves of different scattering points of the distance-slow time plane, wherein the curve parameters extracted by Hough transformation are shown in a table 2, and the obtained track separation result is shown in a figure 4;
the second step is that: and respectively carrying out interference processing on each scattering point in the high-resolution range image sequence obtained by the antenna A, the antenna B and the antenna A and the antenna C along the track direction by utilizing the obtained spin trajectory of each scattering point in the high-resolution range image sequence, wherein an interference plane formed by the antenna A and the antenna B can obtain an x-axis time-varying coordinate, and an interference plane formed by the antenna A and the antenna C can obtain a z-axis time-varying coordinate. Due to the influence of high-resolution range profile sequence intersection, the situation that the reconstruction coordinates are seriously deviated from the coordinate track at the moment of intersection, namely, a bad value occurs, a threshold value needs to be set according to the area range of the reconstruction result aggregation, and the bad value is removed. Here, the x-axis reconstruction result threshold value is set to [ -2.5,4.5], and the z-axis reconstruction result threshold value is set to [ -4,3], and the results are obtained as shown in fig. 5(a) and 5(b), respectively. As can be seen from fig. 5(a) and 5(b), due to the influence of the range side lobe and the existence of the intersection point reconstruction result which is not eliminated, the position of some time in the reconstructed coordinates appears to fluctuate around the theoretical value. According to sine curve parameters extracted by Hough transformation, a specific numerical value of a fine resolution range profile sequence peak value of each scattering point can be obtained, amplitude and phase changes caused by distance walking are corrected according to a formula (18), a time-varying coordinate of a y axis of each scattering point is obtained, and a reconstruction result is shown in a figure 5 (c);
the third step: in order to further optimize the coordinate reconstruction result, the x-axis reconstruction coordinate and the z-axis reconstruction coordinate are fitted to obtain a smooth time-varying coordinate track. The MSE of the x and z axis and y axis coordinate values of the scattering points after fitting are shown in table 3. It can be seen that the reconstructed coordinates accurately approximate the theoretical values. After fitting, time-varying three-dimensional imaging results of the scattering point target can be obtained, and fig. 6(a) and 6(b) are three-dimensional imaging results with t being 0.4333s and t being 0.9333s respectively. The three-dimensional motion trajectory of each scattering point during the imaging time is shown in fig. 6 (c).
TABLE 2 Hough transform extracted parameters
TABLE 3 reconstruction results MSE
As can be seen from the simulation result, the spatial three-dimensional coordinates of the scattering points are respectively reconstructed, and the spatial time-varying imaging result of the target, namely the spatial three-dimensional motion trail, is obtained according to the obtained three-dimensional coordinates.
Simulation 2: in this section, gaussian white noise is added to the simulated echo data in order to verify the noise immunity of the algorithm. When the signal-to-noise ratio (SNR) is 5dB, as shown in fig. 7(b), the high-resolution range profile sequence is obtained by performing fourier transform twice, and thus has better noise immunity. Therefore, when the Hough transformation is used for extracting the parameters, the robustness is good, and the accuracy of the reconstruction result of the y-axis coordinate is not affected. However, noise has a large influence on the interference phase, and the x-axis and z-axis reconstruction coordinates fluctuate significantly as shown in fig. 7(c) and 7 (d). The MSE of the three-dimensional reconstruction coordinates obtained after the x-axis reconstruction coordinates and the z-axis reconstruction coordinates are smoothed by fitting is shown in the table 4, and it can be seen that under the condition of 5dB noise, the algorithm can still accurately obtain the three-dimensional coordinate information of the target scattering point. But when the noise is larger than 5dB, the interference reconstruction coordinate result will be severely distorted. For a spatial environment, the noise is generally at a low level, so the algorithm is applicable to three-dimensional imaging of a spatial object.
TABLE 4 reconstruction coordinate value and true value MSE when SNR is 5dB
The space spinning target time-varying three-dimensional imaging method based on the linear frequency modulation stepping signal can realize the precise resolution three-dimensional imaging of the space target under the condition of a narrow-band radar, and can obtain a longer detection distance and a larger bandwidth under the condition of reducing the hardware load, thereby accurately imaging the space micro-motion target in real time.
Claims (2)
1. A spinning target time-varying three-dimensional imaging method based on a linear frequency modulation stepping signal comprises the following steps:
the method comprises the following steps: performing translational compensation on three-antenna echo signals, performing line-releasing frequency modulation processing, performing one-dimensional range imaging to obtain a coarse resolution range image of each antenna echo, and extracting range unit data of the target scattering point echo to synthesize a fine resolution range image;
step two: separating the echo track of each scattering point by adopting Hough transformation according to the obtained fine resolution range profile;
step three: and performing interference processing on the echo of each scattering point according to the echo track to obtain a two-dimensional coordinate position, and performing distance walking compensation on the echo track to obtain a third-dimensional coordinate position, thereby realizing three-dimensional imaging on the scattering point target.
2. The method of claim 1, wherein the method comprises: the third step specifically comprises the following steps:
step1) respectively carrying out interference processing on echoes of the antenna A and the antenna B and echoes of the antenna A and the antenna C on a distance-slow time plane according to the echo track of each scattering point to obtain a spatial two-dimensional coordinate of the scattering point;
step2) carrying out distance walking compensation on the distance curve represented by the extracted scattering point trajectory to obtain a third dimensional coordinate;
step3) obtaining the space three-dimensional position of the scattering point by combining the obtained three-dimensional coordinates, wherein the three-dimensional coordinates of each slow time moment form a time-varying three-dimensional image of the scattering point.
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| CN108303694A (en) * | 2018-02-09 | 2018-07-20 | 周口师范学院 | A kind of narrowband passive radar three-D imaging method and system |
| CN108983190A (en) * | 2018-07-19 | 2018-12-11 | 中国科学院国家空间科学中心 | The method for obtaining rotating object difference scattering center fine motion track based on interferometric phase |
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