A moving target imaging method for spaceborne multi-channel SAR

Abstract

The present invention relates to a satellite-borne multi-channel SAR moving target imaging method, which belongs to the field of synthetic aperture radar and comprises the following steps: step S1: based on internal calibration data, the inter-channel amplitude and phase error estimation is performed, and the spectrum reconstruction algorithm and the linear frequency modulation variable calibration algorithm are adopted to perform coarse imaging of sea surface clutter and moving targets; step S2: the moving target and its false target in the coarse imaging are detected by the constant false alarm detection method; step S3: the quantitative relationship between the inter-channel phase error, the false target amplitude and the moving target movement speed caused by the moving target movement is established; step S4: the inverse imaging is performed to obtain the fine imaging result of each moving target; step S5: the fine imaging results of all moving targets and the coarse imaging results of the sea surface clutter are superimposed to obtain the final imaging result without false targets. The present invention realizes the effective suppression of false targets and improves the imaging quality.

Description

Technical Field

[0001] The invention relates to the technical field of synthetic aperture radar, and in particular to a spaceborne multi-channel SAR moving target imaging method. Background Art

[0002] Synthetic Aperture Radar (SAR) is a new radar system with two-dimensional high-resolution imaging capability. It can observe the area of ​​interest all day and all weather, and has been widely used in geological exploration, vegetation survey, marine research, mapping, deformation detection and other fields. With the continuous development of spaceborne SAR technology, obtaining high-resolution and wide-band spaceborne SAR images has become an inevitable development trend.

[0003] The multi-channel SAR system adopts the offset phase center azimuth multi-beam technology. Through the azimuth antenna, a single sub-antenna transmits a wide azimuth beam at a lower pulse repetition frequency, and multiple sub-antennas receive signals at different azimuth positions, thereby obtaining echo signals at multiple spatial sampling positions, improving the spatial sampling of the signal, exchanging spatial sampling for time sampling, and achieving high-resolution wide-band imaging. However, for multi-channel moving target imaging, the target motion will cause phase errors between channels, resulting in a large number of false targets in the imaging results, which seriously affects the image quality.

[0004] Therefore, a spaceborne multi-channel SAR moving target imaging method is needed. Summary of the invention

[0005] The object of the present invention is to provide a spaceborne multi-channel SAR moving target imaging method to accurately estimate the inter-channel phase error introduced by target motion and achieve effective suppression of false targets.

[0006] In order to solve the above technical problems, the present invention provides a spaceborne multi-channel SAR moving target imaging method, comprising:

[0007] Step S1: Estimating the inter-channel amplitude and phase errors based on the internal calibration data, compensating the inter-channel amplitude and phase errors introduced by hardware differences, and using the spectrum reconstruction algorithm and the linear frequency modulation scaling algorithm to perform coarse imaging of sea surface clutter and moving targets;

[0008] Step S2: using a constant false alarm detection method to detect moving targets and false targets in the coarse image, and grouping the detected moving targets and false targets according to the positional relationship between the real targets and the false targets;

[0009] Step S3: establishing a quantitative relationship between the inter-channel phase error caused by the moving target motion, the false target amplitude and the moving target motion speed, and estimating the inter-channel phase error of each moving target respectively, and obtaining an estimated value of the inter-channel phase error of each moving target according to the measured false target amplitude;

[0010] Step S4: performing inverse imaging, based on the inverse process of the linear frequency modulation scaling algorithm and the spectrum reconstruction algorithm, inversely transforming the rough imaging image domain of each group of moving targets and their corresponding false targets into the echo domain, respectively compensating the inter-channel phase error of each moving target, and using the spectrum reconstruction algorithm and the linear frequency modulation scaling algorithm to obtain the fine imaging result of each moving target;

[0011] Step S5: superimpose the fine imaging results of all moving targets and the coarse imaging results of sea clutter to obtain the final imaging result without false targets.

[0012] Optionally, if there are still residual false targets in the refined imaging result obtained in step S4, then after step S4 and before step S5, step S41 is further included:

[0013] Step S41: iteratively perform steps S3 and S4 according to the residual false target amplitude until the false target is no longer visible in the refined imaging result.

[0014] Optionally, in step S2, each group includes a real target of a moving target and a corresponding false target.

[0015] Optionally, in step S2, the grouping is performed in the following manner:

[0016] Step S21: grouping all detected targets in the same range gate into a large group;

[0017] Step S22: determining the unmarked target with the strongest energy in the large group as the real target of a certain moving target;

[0018] Step S23: Determine the false target. Determine Nc-1 false targets at both ends of the real target along the azimuth. The azimuth time interval between adjacent real targets and false targets is, where is the number of channels, is the single-channel signal pulse repetition frequency, and is the Doppler modulation frequency; obtain the false target corresponding to the real target;

[0019] Step S24: grouping the certain moving target and the corresponding false target obtained; and marking the certain moving target and the corresponding false target;

[0020] Step S25: iterate the above steps S22 to S24 until all moving targets in the large group are traversed;

[0021] Step S26: Select the next range gate, iterate the above steps S21 to S25, and complete the grouping in step S2.

[0022] Optionally, all detected targets include different moving targets and false targets thereof.

[0023] Optionally, after step S2 and before step S3, the following steps are further included:

[0024] The coarse imaging results of each moving target and the corresponding false target are extracted.

[0025] The satellite-borne multi-channel SAR moving target imaging method provided by the present invention quantitatively constructs the relationship between the phase error introduced by target motion and the amplitude of false targets, accurately estimates the inter-channel phase error introduced by target motion, effectively suppresses false targets, and improves the imaging quality; the present invention solves the problem of ambiguity in the imaging result caused by the inter-channel phase error introduced by target motion, and makes up for the shortcomings of the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the flow of the spaceborne multi-channel SAR moving target imaging method provided by the present invention;

[0027] Figure 2 It is a schematic diagram of the relationship between the false target amplitude and the target radial velocity;

[0028] Figure 3 It is a schematic diagram of the coarse imaging results of the simulation of the spaceborne multi-channel SAR moving target imaging method provided by the present invention;

[0029] Figure 4 It is a schematic diagram of the final imaging result of the spaceborne multi-channel SAR moving target imaging provided by the present invention. DETAILED DESCRIPTION

[0030] The following is a further detailed description of a satellite-borne multi-channel SAR moving target imaging method proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description and claims. It should be noted that the accompanying drawings are all in a very simplified form and are not in precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0031] The present invention provides a spaceborne multi-channel SAR moving target imaging method, the process of which is as follows: Figure 1 As shown, the following steps are included:

[0032] Step S1: Estimating the inter-channel amplitude and phase errors based on the internal calibration data, compensating the inter-channel amplitude and phase errors introduced by hardware differences, and using the spectrum reconstruction algorithm and the chirp scaling (CS) algorithm to perform coarse imaging of sea surface clutter and moving targets;

[0033] Step S2: using a constant false-alarm rate (CFAR) detection method to detect moving targets and their false targets in the coarse image, and grouping the detected moving targets and their false targets according to the positional relationship between the real targets and the false targets of the moving targets, wherein each group includes a real target of the moving target and a corresponding false target;

[0034] Specifically, the real target of the moving target and the false target are in the same range gate. First, all detected targets in the same range gate are divided into a large group. Here, all detected targets may include different moving targets and their false targets. Since the amplitude of the real target of the moving target is the strongest, the unmarked target with the strongest energy in the large group is the real target of a certain moving target. At the same time, the inter-channel phase error introduced by the motion of the moving target will cause the real target of the moving target to have N at both ends along the azimuth. c -1 false target, the bearing time interval between adjacent real targets and false targets is Among them, N c is the number of channels, PRF is the pulse repetition frequency of a single channel signal, f dr The method comprises the following steps: performing Doppler modulation frequency, thereby obtaining a false target corresponding to the real target; grouping the moving target and the corresponding false target into a group; marking the moving target and the corresponding false target; looping the above process until all the moving targets in the large group are traversed; then, selecting another range gate different from the above range gate, and continuously looping the above process, grouping all the moving targets and their corresponding false targets in turn, and then extracting the coarse imaging results of each moving target and its corresponding false target according to the grouping.

[0035] Step S3: establishing a quantitative relationship between the inter-channel phase error caused by the moving target, the false target amplitude and the target movement speed, and estimating the inter-channel phase error of each moving target respectively, and obtaining an estimated value of the inter-channel phase error of each moving target according to the measured false target amplitude;

[0036] Taking the first channel as the reference channel, the phase error of the ith channel caused by the moving target is:

[0037]

[0038] Where λ is the wavelength of the electromagnetic wave, v ris the radial velocity of the moving target, d is the interval between the phase centers of adjacent receiving channels, v s is the satellite speed;

[0039] This phase error causes the real target of the moving target to have N c -1 false target, where the amplitude of the two false targets closest to the real target is |A left | and |A right The relationship between | and the moving target speed is:

[0040]

[0041] In the formula, |A real | is the true target amplitude of the moving target, C and z ki is a constant:

[0042]

[0043]

[0044] Among them, i and k are integers, j is an imaginary unit, and p 1 ~p n-1 is an integer, m is an integer, thus, the amplitude of the two false targets closest to the real target of the moving target |A left | and |A right The relationship curve between | and the radial velocity of the moving target is as follows Figure 2 As shown in the figure, when the radial velocity of the moving target is greater than 0, the amplitude of the false target on the left is smaller than that of the false target on the right; when the radial velocity of the moving target is less than 0, the amplitude of the false target on the left is larger than that of the false target on the right; after determining the radial velocity direction, the curve is fitted:

[0045]

[0046] Among them, a n For |A left |The polynomial fitting coefficients, b n For |A right | is the polynomial fitting coefficient, n is an integer, based on the measured amplitudes of the two false targets closest to the real target |A left | and |A right |, construct the cost function:

[0047]

[0048] Estimate the radial velocity of a moving target using the gradient descent method Then we get the phase error of the i-th channel:

[0049]

[0050] Step S4: performing inverse imaging, based on the inverse process of the linear frequency modulation scaling algorithm and the spectrum reconstruction algorithm, inversely transforming the rough imaging image domain of each group of moving targets and their corresponding false targets into the echo domain, respectively compensating the inter-channel phase error of each moving target, and using the spectrum reconstruction algorithm and the linear frequency modulation scaling algorithm to obtain the fine imaging result of each moving target;

[0051] If there are still residual false targets in the above-mentioned refined imaging result, iteratively perform steps S3 and S4 according to the residual false target amplitude until the false targets are no longer visible in the refined imaging result;

[0052] Due to the influence of target defocus, clutter and noise, there is an error between the measured false target amplitude and the theoretical value, and the estimated moving target radial velocity and phase error There are also errors, which may cause residual false targets in the imaging results; steps S3 and S4 are iterated in a loop to further estimate the residual moving target radial velocity and phase error according to the residual false target amplitude until the false targets in the refined imaging results are no longer visible.

[0053] Step S5: superimpose the fine imaging results of all moving targets and the coarse imaging results of sea clutter to obtain the final imaging result without false targets.

[0054] The effect of the present invention is further illustrated by a satellite-borne multi-channel SAR simulation test. The simulated radar parameters are shown in Table 1, and the target position and motion parameters are shown in Table 2.

[0055]

[0056] Table 1 Simulation parameters of spaceborne multi-channel SAR simulation test

[0057]

[0058]

[0059] Table 2 Target positions and motion parameters

[0060] First, the inter-channel amplitude and phase errors are estimated based on the internal calibration data, and the sea surface clutter and moving targets are roughly imaged. The rough imaging results are as follows: Figure 3 As shown, Figure 3In the figure, the target in the box is the real target of the moving target, and the other targets are false targets. The moving targets and the corresponding false targets are grouped, and the coarse imaging results of each moving target and the corresponding false target are extracted. Then, according to the amplitude of each real target and false target, the radial velocity of each moving target is iteratively estimated. The radial velocity estimation results of each moving target after iteration are shown in Table 3, and the estimation accuracy is better than 0.2m / s. Finally, after inverse imaging, the inter-channel phase error introduced by the radial velocity of each moving target is compensated, and the fine imaging is completed through spectrum reconstruction and CS algorithm. The fine imaging results of all moving targets and the coarse imaging results of sea clutter are superimposed to obtain the final SAR imaging result. The imaging result is shown in Figure 4 It can be seen that the imaging result obtained by the method proposed in the present invention has no azimuth ambiguity, no false targets, and good imaging quality.

[0061]

[0062] Table 3 Radial velocity estimation results of each target after iteration

[0063] It can be seen that the spaceborne multi-channel SAR moving target imaging method proposed in the present invention quantitatively constructs the relationship between the phase error introduced by target motion and the amplitude of false targets, accurately estimates the inter-channel phase error introduced by target motion, effectively suppresses false targets, improves imaging quality, and makes up for the shortcomings of the prior art.

[0064] The above description is only a description of the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention. Any changes or modifications made by a person skilled in the art in the field of the present invention based on the above disclosure shall fall within the scope of protection of the claims.

Claims

1. A spaceborne multi-channel SAR moving target imaging method, characterized in that: include: Step S1: Estimating the inter-channel amplitude and phase errors based on the internal calibration data, compensating the inter-channel amplitude and phase errors introduced by hardware differences, and using the spectrum reconstruction algorithm and the linear frequency modulation scaling algorithm to perform coarse imaging of sea surface clutter and moving targets; Step S2: using a constant false alarm detection method to detect moving targets and false targets in the coarse image, and grouping the detected moving targets and false targets according to the positional relationship between the real targets and the false targets; Step S3: establishing a quantitative relationship between the inter-channel phase error caused by the moving target motion, the false target amplitude and the moving target motion speed, and estimating the inter-channel phase error of each moving target respectively, and obtaining an estimated value of the inter-channel phase error of each moving target according to the measured false target amplitude; Taking the first channel as the reference channel, the phase error of the ith channel caused by the moving target is: Where λ is the wavelength of the electromagnetic wave, v r is the radial velocity of the moving target, d is the interval between the phase centers of adjacent receiving channels, v s is the satellite speed; This phase error causes the real target of the moving target to have N c -1 false target, where the amplitude of the two false targets closest to the real target is |A left | and |A right The relationship between | and the moving target speed is: In the formula, |A real | is the true target amplitude of the moving target, C and z ki is a constant: Among them, i and k are integers, j is an imaginary unit, p1~p n-1 is an integer, m is an integer. When the radial velocity of the moving target is greater than 0, the amplitude of the false target on the left is smaller than that of the false target on the right. When the radial velocity of the moving target is less than 0, the amplitude of the false target on the left is larger than that of the false target on the right. After determining the radial velocity direction, the curve is fitted: Among them, a n For |A left |The polynomial fitting coefficients, b n For |A right | is the polynomial fitting coefficient, n is an integer, based on the measured amplitudes of the two false targets closest to the real target |A left | and |A right |, construct the cost function: Estimate the radial velocity of a moving target using the gradient descent method Then we get the phase error of the i-th channel: Step S4: performing inverse imaging, based on the inverse process of the linear frequency modulation scaling algorithm and the spectrum reconstruction algorithm, inversely transforming the rough imaging image domain of each group of moving targets and their corresponding false targets into the echo domain, respectively compensating the inter-channel phase error of each moving target, and using the spectrum reconstruction algorithm and the linear frequency modulation scaling algorithm to obtain the fine imaging result of each moving target; Step S5: superimpose the fine imaging results of all moving targets and the coarse imaging results of sea clutter to obtain the final imaging result without false targets.

2. The spaceborne multi-channel SAR moving target imaging method according to claim 1, characterized in that: If there are still residual false targets in the refined imaging result obtained in step S4, then after step S4 and before step S5, step S41 is further included: Step S41: iteratively perform steps S3 and S4 according to the residual false target amplitude until the false target is no longer visible in the refined imaging result.

3. The spaceborne multi-channel SAR moving target imaging method according to claim 1 or 2, characterized in that: In step S2, each group includes a real target of a moving target and a corresponding false target.

4. The spaceborne multi-channel SAR moving target imaging method according to claim 3, characterized in that: In step S2, the grouping is performed in the following manner: Step S21: grouping all detected targets in the same range gate into a large group; Step S22: determining the unmarked target with the strongest energy in the large group as the real target of a certain moving target; Step S23: Determine the false target. Determine Nc-1 false targets at both ends of the real target along the azimuth. The azimuth time interval between adjacent real targets and false targets is Among them, N c is the number of channels, PRF is the pulse repetition frequency of a single channel signal, f dr To adjust the Doppler frequency; to obtain the false target corresponding to the real target; Step S24: grouping the certain moving target and the corresponding false target obtained; and marking the certain moving target and the corresponding false target; Step S25: iterate the above steps S22 to S24 until all moving targets in the large group are traversed; Step S26: Select the next range gate, iterate the above steps S21 to S25, and complete the grouping in step S2.

5. The spaceborne multi-channel SAR moving target imaging method according to claim 4, characterized in that: All detected targets include different moving targets and false targets.

6. The spaceborne multi-channel SAR moving target imaging method according to claim 5, characterized in that: After step S2 and before step S3, the method further includes the following steps: Extract the coarse imaging results of each moving target and the corresponding false target.

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