A Method for Detecting Ship Swaying Targets in Medium and High Earth Orbit SAR

Abstract

The present invention discloses a method for detecting swaying targets of medium- and high-orbit SAR (Radio Frequency Assisted Satellite) ships, belonging to the technical field of radar signal processing. The present invention first groups and accumulates swaying targets in azimuth, extracts one-dimensional range feature information for each fixed azimuth, then performs CFAR detection on the extracted one-dimensional signal, performs over-detection point aggregation on each group of over-detection points, intercepts target center point and range width value information, then re-aggregates the over-detection groups, and finally performs refocusing processing on the target row and column numbers provided after detection, thereby obtaining accurate two-dimensional imaging of the swaying target, thereby verifying the detection result. The present invention can perform target detection on swaying targets that are in the form of long strips after imaging and have lost the two-dimensional feature information of the ship.

Description

Technical Field

[0001] The present invention belongs to the technical field of radar signal processing, and in particular relates to a method for detecting ship swaying targets in a medium- and high-orbit SAR (SAR) system. Background Art

[0002] When surface ships are sailing on the sea, they will experience spatial translation and complex swaying driven by waves. The synthetic aperture time of traditional low-orbit SAR is usually around a few seconds. Surface ships can usually be approximately considered to have only spatial translation, ignoring swaying. Medium- and high-orbit SAR has a high orbit altitude and a long synthetic aperture time, up to tens of seconds. The swaying of surface ships cannot be ignored. The swaying of the target causes large Doppler differences at different points on the target, which will cause serious envelope offset and phase deviation in the echo, and then cause the imaging of the swaying ship target to be severely defocused, appearing as a long strip in the two-dimensional image, making the position of the ship target difficult to detect and locate. Therefore, how to detect the swaying ship target of medium- and high-orbit SAR in an efficient way is of great significance. Summary of the Invention

[0003] In view of this, the present invention provides a medium- and high-orbit SAR ship swaying target detection method, which can perform target detection on the swaying target that is in the shape of a long strip after imaging and has lost the two-dimensional characteristic information of the ship.

[0004] The technical solutions for implementing the present invention are as follows:

[0005] A method for detecting ship swaying targets using a medium- and high-orbit SAR system comprises the following steps:

[0006] S1: Divide the swaying target into azimuth regions and perform incoherent accumulation in azimuth to extract one-dimensional range feature information;

[0007] S2: Detection of the one-dimensional signal of the shaking target based on the unit average constant false alarm rate (CA-CFAR);

[0008] S3: Agglomerate the inspection points and intercept the target center point and distance width value information;

[0009] S4: The inspection group gathers and determines the row and column number of the target center;

[0010] S5: According to the row and column numbers of the target center, the ISAR imaging processing method is selected for refocusing to obtain an accurate two-dimensional image of the shaking target, thereby verifying the detection results.

[0011] Furthermore, the specific process of condensing the checkpoints is as follows: the coordinates of each checkpoint are subtracted from its subsequent coordinates. If the threshold condition is met, they are considered to belong to the same target and the same target flag is set. If the condensation threshold is not met, a new target flag is set. After all targets are traversed, the flag information of all coordinates after condensation is obtained, and then the isolated points are removed from the condensed coordinate information, the target center point and distance width value information are intercepted, and finally the information of all targets after condensation is obtained.

[0012] Furthermore, the strategy for intercepting the target center point is: in the group signals obtained after point aggregation, the maximum signal of each group is extracted as the center point of the shaking target distance. If there are several maximum values at the same time, the middle position value between the first maximum value and the last maximum value in each group is taken as the center point.

[0013] Furthermore, the strategy for intercepting the distance width value information is: save the maximum value of each group after the inspection points are condensed, and record the row numbers of the first and last values in each group that differ from the maximum value by a fixed pixel value, and the difference is used to obtain the distance width value of the shaking target.

[0014] Furthermore, the specific process of the inspection group cohesion is as follows: first, set the number of distance-related association channels, divide the targets in the inspection group with close association positions into a group, traverse each group of pixels to be fused, and compare the amplitude difference between the pixel to be fused and its next pixel with the cohesion threshold, so as to determine whether it is cohesive; if it is less than the cohesion threshold, it means cohesion, otherwise it is not cohesive; after all targets in the group are traversed, the target information after cohesion in the group is obtained; then determine the row and column numbers of the target center point in each group, first determine the row, count the number of pixels in each row whose pixel amplitude exceeds the set threshold, take the row with the largest number as the row where the target center point is located, and then take the column where the pixel with the largest amplitude in this row is located as the column where the target center point is located; if the number of values that meet the conditions in each row is the same, directly take the row and column where the maximum amplitude is located, so as to more accurately determine the row and column numbers of the target center point.

[0015] Beneficial effects:

[0016] 1. The present invention uses the over-detection point aggregation technology to eliminate isolated points and redundant targets detected by CFAR, further improving the target detection effect.

[0017] 2. The present invention utilizes the inspection group aggregation technology to simplify the associated targets, thereby obtaining the target position more accurately.

[0018] 3. The present invention uses the over-detection point aggregation technology, over-detection group aggregation technology, and refocusing verification technology to eliminate isolated points and redundant targets detected by CFAR, streamline related targets, and further accurately obtain target positions, thereby solving the problem of severe defocusing in the imaging of swaying targets of medium and high orbit SAR ships and the difficulty in detecting and locating the target position, greatly improving the detection effect of swaying targets. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the overall flow chart of the shaking target detection of the present invention.

[0020] Figure 2 It is a one-dimensional image of the shaking target after incoherent superposition in the azimuth direction.

[0021] Figure 3 is a two-dimensional image of the shaking target.

[0022] Figure 4 This is the flow chart of the CFAR detection technology for shaking targets.

[0023] Figure 5 To pass the inspection, condense the overall flow chart.

[0024] Figure 6 Re-compile the overall flow chart for the inspection team.

[0025] Figure 7 This is an example of the ship swaying target detection results.

[0026] Figure 8 This is an example image of a ship target after refocusing. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0028] The present invention provides a method for detecting swaying targets of medium and high orbit SAR ships. First, the swaying targets are grouped and accumulated in azimuth direction, and the one-dimensional range feature information is extracted for each fixed azimuth. Then, the extracted one-dimensional signal is subjected to CFAR detection, and each group of over-detection points is subjected to over-detection point aggregation, and the target center point and range width value information are intercepted. Next, the over-detection group is re-aggregated, and finally the target row and column numbers provided after detection are refocused to obtain accurate two-dimensional imaging of the swaying target, thereby verifying the detection result. The overall process is as follows: Figure 1 shown.

[0029] S1: One-dimensional feature information extraction: Due to the synthetic aperture time of medium and high orbit SAR, the shaking of ship targets will cause serious image defocusing, affecting the detection performance of ship targets. Therefore, the imaging processing of the two-dimensional NCS algorithm is used to reduce the degree of shaking and defocusing of ship targets to a certain extent and improve the detection performance of targets.

[0030] After imaging, the swaying target of the ship appears as a long strip, and the two-dimensional characteristic information of the ship has been lost. In order to detect the swaying target, the present invention designs a strategy for extracting the one-dimensional characteristic information of the swaying target. First, the azimuth accumulation method of the swaying target is adopted, that is, the image is first divided into small blocks with fixed azimuth areas in the azimuth direction, and then incoherently superimposed along the azimuth direction to obtain the one-dimensional range information of the swaying target. Figure 2 It is a one-dimensional image after incoherent superposition in azimuth. The targets from top to bottom are shaking target, stationary target, and translation target. Figure 3 It is a two-dimensional image of the shaking target, and the three targets correspond one to one.

[0031] S2: One-dimensional signal CFAR detection: The one-dimensional signal of the shaking target is detected using the detection technology based on the unit average constant false alarm rate (CA-CFAR). The technical flow chart is as follows: Figure 4 shown.

[0032] The pulse compression processing of the one-dimensional distance feature information of the shaking target is performed to obtain the signal to be detected. Assume that the signal to be detected is Contains the sampling values of n distance units in total If there is no ship target at the i-th distance unit, the assumption is satisfied. , otherwise the assumption is satisfied , as shown below:

[0033]

[0034] Among them, the clutter amplitude of different distance units are independent of each other and their amplitudes follow the same Rayleigh distribution, s is the target signal, which represents the signal component generated by the target in the i-th range unit when there is a ship target.

[0035] After square law detection, the square value of the clutter amplitude obeys the same exponential distribution. Assuming that the parameter of the Rayleigh distribution is b, the parameter of the exponential distribution is ,but The square value of the signal amplitude at each distance unit The corresponding probability density function is:

[0036]

[0037] In the assumption When , the range unit only contains the clutter part. Assume When established, the range unit contains two parts: clutter and ship target. , SCR is the ratio of the target signal to the average power of the clutter. Reference units and protection units, assuming that the signal in the reference unit contains only the clutter part, the square values of the amplitudes obey the same exponential distribution and are independent of each other, it can be considered that The sum of the squared amplitudes of the reference units z follows the Gamma distribution , the corresponding probability density function is:

[0038]

[0039] CA-CFAR Assay Selection As the detection threshold, For factorial symbols, T is a number with the reference unit and false alarm probability If the range cell to be detected does not contain the ship target, the false alarm probability is:

[0040]

[0041] in, x is the signal amplitude value of the distance unit to be detected. It can be seen from the above formula that the false alarm probability It has nothing to do with the parameters and intensity of the clutter distribution, but only with the constant T and the number of reference units. The detection threshold can be adjusted as the background clutter changes, thus ensuring that the false alarm probability remains approximately unchanged.

[0042] S3: Checkpoint aggregation: After CFAR detection, there are multiple checkpoints, but multiple checkpoints may belong to the same target. The checkpoint aggregation flow chart is as follows: Figure 5 As shown, the coordinates of each checked point are subtracted from its subsequent coordinates. If the threshold condition is met, they are considered to belong to the same target and a same target flag is set. If the cohesion threshold is not met, a new target flag is set. After all targets are traversed, the flag information of all the coordinates after cohesion is obtained. Then, isolated points are removed from the cohesive coordinate information, and the target center point and distance width value information are intercepted. Finally, the information of all the targets after cohesion is obtained. The strategy for intercepting the target center point and distance width value information is as follows:

[0043] (1) Intercepting the center point: In the group signals obtained after point aggregation, the maximum value of each group is extracted as the center point of the shaking target distance. If there are several maximum values at the same time, the middle position value between the first maximum value and the last maximum value in each group is taken as the center point.

[0044] (2) Intercept the width value of the shaking target distance: save the maximum value of each group after the check points are condensed, and record the row numbers of the first and last values in each group that differ from the maximum value by a fixed pixel value, and obtain the width value of the shaking target distance by the difference.

[0045] S4: Cohesion of over-inspection groups: After the above steps, multiple sliding areas with fixed values in the orientation area can be obtained. In order to distinguish between shaking targets and horizontal and stationary targets, a threshold for the number of target orientation areas is set. When the target length is greater than the threshold, these orientation areas are determined to be an over-inspection group, and the information of the maximum amplitude point is taken as the target information of the over-inspection group. These over-inspection groups contain one or more targets. In order to eliminate the same targets, a group cohesion strategy is adopted. The over-inspection group re-cohesion flow chart is as follows: Figure 6 shown.

[0046] The over-inspection group cohesion first sets the number of distance-related channels, divides the targets in the over-inspection group with close associated positions into a group, traverses each group of pixels to be fused, and compares the amplitude difference between the pixel to be fused and its next pixel with the cohesion threshold to determine whether it is cohesive; if it is less than the cohesion threshold, it means cohesion, otherwise it is not cohesive; after all targets in the group are traversed, the target information after cohesion in the group is obtained; then determine the row and column numbers of the target center point in each group, first determine the row, count the number of pixels in each row whose pixel amplitude exceeds the set threshold, take the row with the largest number as the row where the target center point is located, and then take the column where the pixel with the largest amplitude in this row is located as the column where the target center point is located; if the number of values that meet the conditions in each row is the same, directly take the row and column where the maximum amplitude is located, so as to more accurately determine the row and column numbers of the target center point.

[0047] S5: Refocusing: The medium- and high-orbit SAR (SAR) orbit is high, and the synthetic aperture time is long, up to tens of seconds. The target sways and moves relative to the satellite, making it impossible for traditional SAR imaging methods to focus on the ship target. Therefore, the present invention adopts an ISAR imaging method, using the Doppler variation caused by the rotation of the ship relative to the radar line of sight to compensate for the translational component, and using the space-varying Doppler component of the ship target to acquire an ISAR image. Based on the row and column number of the target center corresponding to the ship target detection result, the corresponding echo of the ship target is intercepted and processed using the ISAR imaging method to refocus the ship target, obtaining an accurate two-dimensional image of the swaying ship target, thereby verifying the detection result.

[0048] S6: During the algorithm verification process, the detection performance of the algorithm was verified by simulating the echo of the ship surface target under the conditions of medium and high orbit and the measured data, and performing imaging detection on the echo and refocusing the detection results. Figure 7 As shown in the figure, the result of ship target after refocusing the detected row and column numbers is shown in the figure. Figure 8 shown.

[0049] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for detecting ship swaying targets using medium and high orbit SAR, characterized in that: The following steps are involved: S1: Divide the swaying target into azimuth regions and perform incoherent accumulation in azimuth to extract one-dimensional range feature information; S2: Detection of the one-dimensional signal of the shaking target based on the unit average constant false alarm rate; S3: Agglomerate the inspection points and intercept the target center point and distance width value information; The specific process of over-check point aggregation is as follows: the coordinates of each over-check point are subtracted from its subsequent coordinates. If the threshold condition is met, they are considered to belong to the same target and the same target flag is set. If the aggregation threshold is not met, a new target flag is set. After all targets are traversed, the marker information of all coordinates is obtained. Then, isolated points are removed from the condensed coordinate information, and the target center point and distance width value information are intercepted to finally obtain the information of all targets after condensation. S4: The inspection group gathers and determines the row and column number of the target center; The specific process of over-detection group condensation is as follows: first, set the number of distance-related correlation channels, group the objects in the over-detection group with close correlation positions, traverse each group of pixels to be fused, and compare the amplitude difference between the pixel to be fused and the next pixel with the condensation threshold to determine whether it is condensed; If it is less than the cohesion threshold, it means cohesion, otherwise it means no cohesion; after all targets in the group are traversed, the target information after cohesion in the group is obtained; then the row and column numbers of the target center point are determined in each group. First, the row is determined, and the number of pixels in each row whose pixel amplitude exceeds the set threshold is counted. The row with the largest number is taken as the row where the target center point is located, and then the column where the pixel with the largest amplitude in this row is taken as the column where the target center point is located; if the number of values that meet the conditions in each row is the same, the row and column where the largest amplitude is located is directly taken, so as to more accurately determine the row and column number where the target center point is located; S5: According to the row and column numbers of the target center, the ISAR imaging processing method is selected for refocusing to obtain an accurate two-dimensional image of the shaking target, thereby verifying the detection results.

2. The method for detecting a shaking target according to claim 1, wherein: The strategy for intercepting the target center point is: in the group signals obtained after point aggregation, the maximum signal of each group is extracted as the center point of the shaking target distance. If there are several maximum values at the same time, the middle position value between the first maximum value and the last maximum value in each group is taken as the center point.

3. The method for detecting a shaking target according to claim 1 or 2, wherein: The strategy for intercepting the distance width value information is: save the maximum value of each group after the inspection points are condensed, and record the row numbers of the first and last values in each group that differ from the maximum value by a fixed pixel value, and the difference is used to obtain the distance width value of the shaking target.

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