Front downward-looking imaging image processing method and device based on synthetic aperture radar

Abstract

The invention discloses a front downward-looking imaging image processing method and device based on a synthetic aperture radar. The method comprises the following steps: preprocessing an echo signal received by a synthetic aperture radar to obtain low-frequency echo data; performing imaging processing on the low-frequency echo data, applying Doppler sharpening imaging to a front downward-looking imaging scene, and fully utilizing echo information to obtain a target two-dimensional imaging image; and performing image processing on the target two-dimensional imaging image to obtain a target contour image. The time from data acquisition to result generation is greatly shortened, ground processing delay is avoided, the problem that data transmission is interrupted or lost due to the fact that a traditional mode depends on a ground station and a communication link is solved, and the method has more remarkable advantages especially in a complex geographical environment.

Description

Technical Field

[0001] The present invention relates to the field of radar imaging technology and image processing technology, and in particular to a method and device for processing forward downward imaging based on synthetic aperture radar. Background Art

[0002] In remote sensing technology, target detection in complex terrain areas has always been a hot topic and difficulty in research and application. Although traditional terrain detection methods, such as optical remote sensing, lidar and aerial photography surveys, can meet the detection needs to a certain extent, they are limited by light, cloud cover and surface reflection in complex terrain, making it difficult to obtain high-precision terrain data. Synthetic aperture radar, as an active microwave remote sensing technology, has the advantage of working all day and all weather, and can penetrate clouds, rain, fog and vegetation cover to obtain high-resolution surface information. However, in complex terrain environments, the decrease in coherence, multipath effects and shadow areas caused by the dramatic terrain fluctuations pose challenges to detection accuracy.

[0003] In recent years, the accuracy and reliability of complex terrain target detection have been significantly improved by optimizing the data processing process and adding subsequent image processing parts. These technologies, combined with terrain features and the imaging characteristics of synthetic aperture radar, have broad application prospects in national defense, disaster monitoring, resource management and engineering planning.

[0004] At present, the images generated by synthetic aperture radar are characterized by high resolution and large data volume. The data processing process usually includes the implementation of imaging algorithms and subsequent image processing, which places high demands on computing performance and real-time performance. Compared with traditional general-purpose processors or graphics processors, the integrated solution based on field programmable gate arrays has shown significant advantages in the field of synthetic aperture radar image processing. Summary of the invention

[0005] In order to overcome the shortcomings of traditional target detection schemes in complex terrain, the present invention proposes a method and device for forward downward imaging based on synthetic aperture radar, uses a field programmable gate array as the control processing core, and deploys all imaging algorithms and image processing algorithms on the field programmable gate array and integrates them into one module, which greatly simplifies the signal processing steps and reduces the operation complexity, so that the results of complex terrain target detection can be obtained more quickly.

[0006] In a first aspect, the present invention provides a method for processing a forward downward imaging image based on a synthetic aperture radar, the method comprising:

[0007] Preprocess the echo signal received by the synthetic aperture radar to obtain low-frequency echo data;

[0008] Perform imaging processing on low-frequency echo data, apply Doppler sharpening imaging in the forward-looking imaging scene, make full use of echo information, and obtain a two-dimensional imaging image of the target;

[0009] The target two-dimensional imaging image is processed to obtain a target contour image.

[0010] Preferably, the specific implementation process of preprocessing the echo signal received by the synthetic aperture radar is:

[0011] The synthetic aperture radar transmits a high-frequency linear frequency modulation pulse signal. When the synthetic aperture radar signal encounters the ground and the target object, an echo signal is generated. The echo signal received by the synthetic aperture radar is down-converted to obtain a low-frequency echo signal.

[0012] Use an analog-to-digital converter to sample the low-frequency echo signal, convert the analog signal into a digital signal, and divide the digital signal into multiple data frames in the order of sampling time to obtain two-dimensional low-frequency echo data. ,in represents the distance unit, Indicates the pulse time.

[0013] Preferably, the echo signal frequency reduction processing includes reducing the center frequency of the echo signal and controlling the bandwidth of the echo signal.

[0014] Preferably, the imaging process of the low-frequency echo data is performed, and Doppler sharpening imaging is applied in the forward-looking imaging scene to fully utilize the echo information. The specific implementation process is:

[0015] In the forward-looking scenario, low-frequency echo data Perform a two-dimensional Fourier transform to obtain a range Doppler image , represents the straight-line distance between the synthetic aperture radar and the target, represents the Doppler frequency;

[0016] Based on the range Doppler image , obtain the imaging image of the target in the two-dimensional spatial domain.

[0017] Preferably, the specific implementation process of imaging the target in the two-dimensional space domain is:

[0018] According to the flight speed of the onboard system , get the radial flight speed of the synthetic aperture radar relative to the target ;

[0019] The Doppler frequency is deduced from the synthetic aperture radar flight trajectory diagram ;

[0020] Range Doppler image Perform geometric correction by using the geometric correction relationship and Get the imaging image of the target in the two-dimensional space domain.

[0021] Preferably, the radial flight speed of the synthetic aperture radar relative to the target is The calculation process is:

[0022]

[0023]

[0024] in represents the horizontal distance between the synthetic aperture radar and the target, Indicates the straight-line distance between the synthetic aperture radar and the target.

[0025] Preferably, the Doppler frequency The calculation process is:

[0026] ,in is the operating frequency of the synthetic aperture radar antenna.

[0027] Preferably, the geometric correction relationship satisfies ,in Indicates the height distance between the synthetic aperture radar and the target.

[0028] Preferably, the specific implementation process of performing image processing on the target two-dimensional imaging image is:

[0029] Perform bilateral filtering on the target two-dimensional imaging image to reduce the noise introduced during the imaging process;

[0030] Binarization processing is performed on the filtered image data;

[0031] Perform morphological operations on the image after binarization processing; extract the contour of the target area of ​​the image after morphological operation to obtain the target contour image.

[0032] In a second aspect, the present invention provides an image processing device, comprising:

[0033] A radar module is used to transmit a high-frequency linear frequency modulation pulse signal to the measured ground using a synthetic aperture radar of an airborne system, and to receive echo signals reflected from the ground and target objects;

[0034] The numerical control module uses a field programmable gate array to receive instructions from the host computer and control the start and end of the acquisition process; it also performs echo signal preprocessing, imaging processing, and image processing on the echo signals collected from the radar module, and stores the processed target contour image;

[0035] A display module, used for displaying the target contour image processed by the numerical control module;

[0036] Remote control radio module, used for communication between the host computer and the CNC module;

[0037] Power module, used to provide power support for radar module and CNC module to ensure stable operation of ground system and airborne system;

[0038] The host computer is used to send start and stop commands to the remote control radio module.

[0039] The present invention solves the shortcomings of traditional synthetic aperture radar (SAR) in target detection in complex terrain through the above technical means and achieves significant beneficial effects:

[0040] The present invention uses a Doppler sharpening imaging algorithm to reduce the interference of strong reflectors on the detection results, transforms the traditional one-dimensional distance measurement into two-dimensional imaging, and accurately maps the target position through geometric correction to overcome the imaging distortion caused by terrain undulations.

[0041] The present invention uses Doppler sharpening imaging algorithm to switch the imaging perspective to the side of the target, improving the ability to detect ground targets. It uses field programmable gate array as the numerical control core, making full use of its high-performance parallel computing capabilities and high-speed memory interface, which can make it suitable for real-time and high-performance application scenarios. The data processing and imaging process is completed on the airborne end, avoiding ground station processing delays, reducing data transmission dependence, and is suitable for complex geographical environments.

[0042] The integrated design adopted by the present invention integrates the imaging and subsequent image processing of the synthetic aperture radar into the field programmable gate array; other methods mostly use the traditional method of first collecting radar echo data and then performing imaging and image processing on the ground, which makes the entire experimental process very complicated and takes a lot of time. The synthetic aperture radar integrated imaging and image processing mode implemented by the field programmable gate array can significantly improve the real-time, reliability and efficiency of the system, especially in tasks with large data volumes and high real-time performance. Compared with the traditional mode, it not only greatly shortens the time from data collection to result generation, avoiding ground processing delays, but also solves the problem of traditional mode relying on ground stations and communication links, and the problem of data transmission interruption or loss. Especially in complex geographical environments, its advantages are more significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Other features, objectives and advantages of the present invention will become more apparent from a reading of the detailed description of non-limiting embodiments made with reference to the following accompanying drawings.

[0044] Figure 1 The module diagram of the forward downward imaging image processing device based on synthetic aperture radar according to an embodiment of the present invention.

[0045] Figure 2 The present invention is a flowchart of a method for processing a forward-looking downward imaging image based on a synthetic aperture radar according to an embodiment of the present invention.

[0046] Figure 3 This is the result of range Doppler imaging of synthetic aperture radar echo data.

[0047] Figure 4 The result of imaging geometry correction for synthetic aperture radar image data.

[0048] Figure 5 The result of adjustable dilation and erosion of synthetic aperture radar image data.

[0049] Figure 6 This is the result of multi-directional edge detection on synthetic aperture radar image data. DETAILED DESCRIPTION

[0050] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form.

[0051] See attached Figure 1 This embodiment provides a forward downward imaging image processing device based on synthetic aperture radar, including:

[0052] The radar module is located in the airborne system and is responsible for transmitting high-frequency linear frequency modulation pulse signals to the measured ground using the synthetic aperture radar of the airborne system during the flight of the UAV, and receiving the echo signals reflected by the ground and the target object;

[0053] The numerical control module uses a field programmable gate array and is located in the airborne system. It is used to receive instructions from the host computer and control the start and end of the acquisition process; it also performs echo signal preprocessing, imaging processing, and image processing on the echo signals collected from the radar module, and stores the processed target contour image;

[0054] A display module, used for displaying the target contour image processed by the numerical control module;

[0055] Remote control radio module, used for communication between the host computer and the CNC module;

[0056] Power module, used to provide power support for radar module and CNC module to ensure stable operation of ground system and airborne system;

[0057] The host computer is used to send start and stop commands to the remote control radio module.

[0058] See attached Figure 2 This embodiment also provides a method for processing a front downward imaging image based on a synthetic aperture radar, the method comprising:

[0059] Step S1, placing a corner mirror in the detection area, with the corner mirror and trees as detection targets.

[0060] Step S2: The image processing device is powered on and powered on to complete the initialization of all systems. The drone is lifted off to a height of about 35 m from the ground, reaches the designated area, and is ready to fly to the target area.

[0061] Step S3: When the UAV is flying toward the target area, the host computer of the ground system sends a start command, which is transmitted to the airborne system through communication between the remote control radio modules. The numerical control module receives the command and starts working, preprocessing the echo signal received by the synthetic aperture radar to obtain low-frequency echo data.

[0062] Step S4: perform imaging processing and geometric correction on the low-frequency echo data to obtain a two-dimensional imaging image of the target in a two-dimensional spatial domain:

[0063] 4.1 Echo data Doppler sharpening imaging is performed. The result of range Doppler imaging of synthetic aperture radar image data is shown in the attached figure. Figure 3 .

[0064] In the forward-looking downward scene, the Doppler sharpening imaging algorithm shows that the low-frequency echo data sampled back Arrange into a two-dimensional matrix, and then perform a two-dimensional Fourier transform on the two-dimensional matrix of echo data to obtain the range Doppler image .

[0065] 4.2 Doppler image data For the geometric correction of synthetic aperture radar image data, please refer to the attached figure for the result of geometric correction of imaging. Figure 4 .

[0066] Step S5: performing image processing on the target two-dimensional imaging image to obtain a target contour image.

[0067] Step S6: The upper computer of the ground system sends an end instruction. After communication between the remote control radio modules, the start instruction is transmitted to the airborne system. The numerical control module receives the end instruction and the entire system stops working.

[0068] Furthermore, the specific implementation process of preprocessing the echo signal received by the synthetic aperture radar in step S3 is:

[0069] The synthetic aperture radar transmits a high-frequency linear frequency modulated pulse signal. When the synthetic aperture radar signal encounters the ground and the target object, an echo signal is generated. The echo signal received by the synthetic aperture radar is down-converted to obtain a low-frequency echo signal.

[0070] In one implementation manner, the echo signal frequency reduction processing includes reducing the center frequency of the echo signal and limiting the bandwidth of the signal by filtering, intermediate frequency down-conversion and other methods.

[0071] Use an analog-to-digital converter to sample the low-frequency echo signal, convert the analog signal into a digital signal, and divide the digital signal into multiple data frames in the order of sampling time to obtain two-dimensional low-frequency echo data. ,in represents the distance unit, Indicates the pulse time.

[0072] Furthermore, the specific implementation process of step S4.1 is:

[0073] In the forward-looking downward scene, the Doppler sharpening imaging algorithm shows that the low-frequency echo data sampled back Arrange into a two-dimensional matrix, and then perform a two-dimensional Fourier transform on the two-dimensional matrix of echo data to obtain the range Doppler image ; represents the straight-line distance between the synthetic aperture radar and the target, Indicates the Doppler frequency.

[0074] Furthermore, the specific implementation process of step S4.2 is:

[0075] According to the flight speed of the onboard system , get the radial flight speed of the synthetic aperture radar relative to the target :

[0076]

[0077]

[0078] in represents the horizontal distance between the synthetic aperture radar and the target, Indicates the straight-line distance between the synthetic aperture radar and the target.

[0079] The Doppler frequency is deduced from the synthetic aperture radar flight trajectory diagram : ,in is the operating frequency of the synthetic aperture radar antenna.

[0080] Range Doppler image Perform geometric correction by using the geometric correction relationship and Get the imaging image of the target in the two-dimensional space domain.

[0081] The geometric correction relationship satisfies ,in Indicates the height distance between the synthetic aperture radar and the target.

[0082] Furthermore, the specific implementation process of performing image processing on the target two-dimensional imaging image in step S5 is:

[0083] S5.1. Perform bilateral filtering on the corrected target two-dimensional image to reduce the speckle noise introduced during the imaging process. The filtering algorithm is based on the traditional Gaussian operator, on the other hand, based on the similarity of the image pixel values, the closer the pixel value is to the center pixel, the greater the weight; then the image input Matrix, Gaussian operator Matrix and pixel value similarity The convolution operation is performed on the matrix, and the final convolution result is used to replace the value of the original pixel center point; this method combines the weights of spatial distance and pixel intensity similarity, which can better retain the edge information and detail features of the image while suppressing noise, providing high-quality input images for subsequent operations.

[0084] S5.2, binarize the filtered image data. The pixels in the image input in the previous step are divided into two categories, corresponding to the target area and the background area, respectively. A fixed threshold is used for segmentation, and the pixel points greater than the threshold are assigned a value of 1, otherwise 0; this ensures that the target area can be clearly separated from the complex background, and significantly simplifies the subsequent processing of the image. The image after binarization is convenient for further analysis of morphology and edge features.

[0085] S5.3 applies morphological operations to the binarized result, including dilation and erosion. The dilation operation is an OR operation on the pixels in the target area. The formula is:

[0086]

[0087] The corrosion operation is to perform an AND operation on the pixels in the target area. The formula is:

[0088]

[0089] in for The image data in the pixel matrix; P is the final calculated image data.

[0090] The dilation operation expands the boundary of the target area so that smaller gaps are filled and broken parts are reconnected; the erosion operation reduces the boundary of the area, removes noise and redundant details in the image, and strengthens the coherence of the target outline; the adjustable dilation and erosion operation controls the degree of dilation and erosion by counting the number of pixel values ​​1 in the target area, which can further optimize the shape and integrity of the target area and ensure the accuracy of subsequent processing; the result of the adjustable dilation and erosion of the synthetic aperture radar image data is shown in the attached figure Figure 5 .

[0091] S5.4 extracts the contour of the target area from the image after morphological operation to obtain the target contour image. That is, multi-directional edge detection is performed on the image to extract the edge information of the target area. The operator can be convolved to get the result, using The operator can effectively detect strong gradient changes at the target boundary and suppress noise interference. The traditional operator can only detect horizontal and vertical angles. The multi-directional edge detection includes operators in eight directions, which can further enhance the contour features of the target area, which is of great significance for subsequent image recognition. The result of target contour extraction from synthetic aperture radar image data is shown in the attached figure. Figure 6 .

[0092] Figure 3 , Figure 4 , Figure 5 and Figure 6 The results of the verification of the original data are shown. Figure 3 It is the result of Doppler sharpening imaging of low-frequency echo data. Figure 4 is the result of geometric correction of Doppler sharpening imaging. Figure 5 is the result after the adjustable dilation erosion operation, Figure 6 It is the result after multi-directional edge detection; it can be seen from the figure that the overall process of the front downward imaging image processing method proposed in the present invention is feasible and has good results.

[0093] The experimental results show that the method of the present invention verifies the feasibility of synthetic aperture radar in the forward downward scene, and deploys the imaging algorithm and image processing algorithm into the field programmable gate array, which can reduce the complexity of manual operation, significantly improve the real-time performance, and the experimental results are reliable. The above results verify the effectiveness and superiority of the present invention.

[0094] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for processing a forward downward imaging image based on synthetic aperture radar, characterized in that: The method comprises: Preprocess the echo signal received by the synthetic aperture radar to obtain low-frequency echo data; Perform imaging processing on low-frequency echo data, apply Doppler sharpening imaging in the forward-looking imaging scene, make full use of echo information, and obtain a two-dimensional imaging image of the target; The target two-dimensional imaging image is processed to obtain a target contour image.

2. The method according to claim 1, characterized in that: The specific implementation process of preprocessing the echo signal received by the synthetic aperture radar is: The synthetic aperture radar transmits a high-frequency linear frequency modulation pulse signal. When the synthetic aperture radar signal encounters the ground and the target object, an echo signal is generated. The echo signal received by the synthetic aperture radar is down-converted to obtain a low-frequency echo signal. The low-frequency echo signal is sampled using an analog-to-digital converter, the analog signal is converted into a digital signal, and the digital signal is divided into multiple data frames in sequence according to the sampling time to obtain two-dimensional low-frequency echo data.

3. The method according to claim 2, characterized in that: The echo signal frequency reduction processing includes reducing the center frequency of the echo signal and controlling the bandwidth of the echo signal.

4. The method according to claim 1, characterized in that: The specific implementation process of performing imaging processing on the low-frequency echo data and applying Doppler sharpening imaging in the forward-looking imaging scene to fully utilize the echo information is as follows: In the forward-looking downward scenario, the low-frequency echo data is subjected to a two-dimensional Fourier transform to obtain a range Doppler image; According to the range Doppler image, an imaging image of the target in the two-dimensional space domain is obtained.

5. The method according to claim 4, characterized in that: The specific implementation process of the imaging image of the target in the two-dimensional space domain is: According to the flight speed of the onboard system , get the radial flight speed of the synthetic aperture radar relative to the target ; The Doppler frequency is deduced from the synthetic aperture radar flight trajectory diagram ; The range Doppler image is geometrically corrected by using the geometric correction relationship and Doppler frequency Get the imaging image of the target in the two-dimensional space domain.

6. The method according to claim 5, characterized in that: The radial flight speed of the synthetic aperture radar relative to the target The calculation process is: ； ； in represents the horizontal distance between the synthetic aperture radar and the target, Indicates the straight-line distance between the synthetic aperture radar and the target.

7. The method according to claim 5, characterized in that: The Doppler frequency The calculation process is: ,in is the operating frequency of the synthetic aperture radar antenna.

8. The method according to claim 5, characterized in that: The geometric correction relationship satisfies ,in Indicates the height distance between the synthetic aperture radar and the target.

9. The method according to claim 1, characterized in that: The specific implementation process of performing image processing on the target two-dimensional imaging image is: Perform bilateral filtering on the target two-dimensional imaging image to reduce the noise introduced during the imaging process; Binarization processing is performed on the filtered image data; Perform morphological operations on the binarized image; The contour of the target area is extracted from the image after morphological operation to obtain the target contour image.

10. An image processing device for implementing the method according to any one of claims 1 to 9, characterized in that: include: A radar module is used to transmit a high-frequency linear frequency modulation pulse signal to the measured ground using a synthetic aperture radar of an airborne system, and to receive echo signals reflected from the ground and target objects; The numerical control module uses a field programmable gate array to receive instructions from the host computer and control the start and end of the acquisition process; it also performs echo signal preprocessing, imaging processing, and image processing on the echo signals collected from the radar module, and stores the processed target contour image; A display module, used for displaying the target contour image processed by the numerical control module; Remote control radio module, used for communication between the host computer and the CNC module; Power module, used to provide power support for radar module and CNC module to ensure stable operation of ground system and airborne system; The host computer is used to send start and stop commands to the remote control radio module.

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