Radio Frequency Interference Suppression Method for Low-Frequency Ultra-Wideband SAR Based on Two-Dimensional Frequency Domain Notching

Abstract

The invention provides a low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching. The method performs two-dimensional Fourier transform on an echo signal received by a low-frequency ultra-wideband SAR to obtain an echo signal in the two-dimensional frequency domain; calculates the average value and standard deviation of the two-dimensional frequency domain echo signal, and takes the sum of the average value and three times the standard deviation as a suppression threshold. In this process, the size of the threshold can also be adjusted according to the energy of the radio frequency interference signal to achieve a better suppression effect; then, a notch process is performed by using a "1" method, and a value greater than the threshold is set as the average value to achieve the purpose of suppressing the radio frequency interference signal; finally, the two-dimensional frequency domain echo signal after the radio frequency interference suppression is transformed into a two-dimensional time domain to complete the suppression of the radio frequency interference signal.

Description

Technical Field

[0001] The present invention relates to the field of radar technology, and more specifically to a low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching. Background Art

[0002] Synthetic aperture radar (SAR) has the characteristics of all-day, all-weather operation and large-scale imaging, and has very important applications in national economic construction and military reconnaissance. Due to its low operating frequency, low-frequency ultra-wideband SAR has a long wavelength, so the radar system has strong penetration ability, can penetrate vegetation and shallow surface and detect and image targets hidden in the jungle or buried in the shallow surface. However, many areas in my country are densely covered with jungles and have complex geographical environments. It is difficult to effectively detect and scout hidden targets in the jungle using high-frequency radars, so low-frequency ultra-wideband SAR has great application value.

[0003] However, there are a large number of radio frequency signals such as broadcasting, wireless television, and mobile communications in the low frequency band, which will cause serious radio frequency interference to low-frequency ultra-wideband SAR. Compared with the target echo signal received by the radar, the radio frequency interference signal has a very high power. After these radio frequency interference signals and the target echo signal are mixed together, obvious lines or noise will be generated in the imaging results. Therefore, the existence of radio frequency interference will seriously affect the detection performance and imaging quality of low-frequency ultra-wideband SAR.

[0004] At present, the main methods for RFI suppression are parametric methods and non-parametric methods. The parametric method models the RFI signal, and then estimates the parameters such as frequency, phase and amplitude of the RFI signal model by using methods such as minimum mean square error or maximum likelihood estimation, and reconstructs the RFI signal. Finally, the echo signal received by the radar is subtracted from the reconstructed RFI signal to achieve the purpose of RFI suppression. The condition for using the parametric method to suppress RFI signals is to assume that the RFI signal is stable. However, with the development of information technology, RFI signals will show strong non-stationarity. In the case of non-stationary RFI signals, it is difficult to accurately establish the model of RFI signals using the parametric method, and the parameters of RFI signals are also difficult to accurately estimate. At the same time, the parametric method will generate a huge amount of calculation when estimating the parameters of the RFI signal, which seriously affects the efficiency of the RFI suppression algorithm. The non-parametric method mainly uses the different characteristics between the RFI signal and the target echo signal received by the low-frequency ultra-wideband SAR system for filtering. Compared with the parametric method, the non-parametric method has a simpler structure and a smaller amount of calculation. However, the non-parametric method has a poor suppression effect on RF interference signals and is difficult to meet the requirements of high-precision imaging of low-frequency ultra-wideband SAR. In addition, the existing non-parametric methods almost all suppress RF interference signals in a single pulse echo one by one. When the imaging range is large and the number of pulse echoes is large, the algorithm has a large amount of calculation.

[0005] Most of the existing RFI suppression methods suppress narrowband RFI signals, but the suppression effect on broadband RFI signals is not ideal. However, in real environments, narrowband RFI signals and broadband RFI signals often exist at the same time, so it is difficult for existing RFI suppression methods to effectively suppress narrowband and broadband RFI signals. Therefore, how to effectively and quickly suppress narrowband and broadband RFI signals in the echo of low-frequency ultra-wideband SAR system is still a technical problem to be solved. Summary of the invention

[0006] The present invention provides a low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching, which can effectively and quickly suppress narrowband and broadband radio frequency interference signals in the echo of the low-frequency ultra-wideband SAR system.

[0007] In order to achieve the above technical effects, the technical solution of the present invention is as follows:

[0008] A method for suppressing radio frequency interference of a low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching comprises the following steps:

[0009] S1: transform the echo signal into two-dimensional frequency domain;

[0010] S2: Set the threshold and perform notch processing;

[0011] S3: transform the two-dimensional frequency domain echo signal after RF interference suppression into two-dimensional time domain.

[0012] Furthermore, in the step S1, the received low-frequency ultra-wideband SAR echo data is transformed into a two-dimensional frequency domain, and radio frequency interference suppression is performed in the two-dimensional frequency domain using a two-dimensional notch.

[0013] Furthermore, in step S1, the frequency band of the broadband RF interference signal in the two-dimensional frequency domain is also narrower than the echo signal received by the low-frequency ultra-wideband SAR system, so transforming the echo signal into the two-dimensional frequency domain can further concentrate the energy of the RF interference signal.

[0014] Furthermore, in step S2, the echo signal in the two-dimensional frequency domain is notched to suppress the radio frequency interference signal:

[0015] First, the mean value and standard deviation of the two-dimensional frequency domain echo signal are calculated; then, the sum of the mean value and three times the standard deviation is used as the threshold value; finally, the part of the two-dimensional frequency domain echo signal greater than the threshold value is set to the mean value, thereby completing the suppression of the RF interference signal in the two-dimensional frequency domain.

[0016] Furthermore, in step S2, the sum of the average value and three times the standard deviation is used as the threshold value.

[0017] Furthermore, in step S2, the portion of the two-dimensional frequency domain echo signal that is greater than the threshold value is set as the average value by using the "1" method.

[0018] Furthermore, in step S3, the two-dimensional frequency domain echo signal after radio frequency interference suppression is transformed into a two-dimensional time domain to obtain a final echo signal after radio frequency interference suppression, that is, echo data for low-frequency ultra-wideband SAR imaging.

[0019] Furthermore, the specific process of step S1 is:

[0020] Assume that the echo signal received by the low-frequency ultra-wideband SAR system is x(t,τ), where t is the slow time and τ is the fast time. Perform a two-dimensional Fourier transform on x(t,τ) to obtain a two-dimensional frequency domain signal X(f,f τ ).

[0021] Furthermore, the specific process of step S2 is:

[0022] Calculate the two-dimensional frequency domain signal X(f,f τ ) and standard deviation δ, then the notch threshold is set to:

[0023]

[0024] The two-dimensional frequency domain signal X(f,f τ ) are set to the average value if the value is greater than the threshold value. The radio frequency interference signal can be suppressed in the two-dimensional frequency domain.

[0025] Furthermore, in step S2, when setting the threshold, the multiplication factor of the standard deviation is adjusted according to the energy of the radio frequency interference signal, so that the radio frequency interference signal can be better suppressed.

[0026] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0027] The present invention first performs a two-dimensional Fourier transform on the echo signal received by the low-frequency ultra-wideband SAR to obtain an echo signal in the two-dimensional frequency domain; then, the average value and standard deviation of the two-dimensional frequency domain echo signal are calculated, and the sum of the average value and three times the standard deviation value is used as the suppression threshold. In this process, the size of the threshold can also be adjusted according to the energy of the radio frequency interference signal to achieve a better suppression effect; then, the "1" method is used to perform notch processing, and the value greater than the threshold is set as the average value to achieve the purpose of suppressing the radio frequency interference signal; finally, the two-dimensional frequency domain echo signal after radio frequency interference suppression is transformed into a two-dimensional time domain to complete the suppression of the radio frequency interference signal. The two-dimensional frequency domain notch method proposed by the present invention is suitable for suppressing narrowband and broadband radio frequency interference signals when processing low-frequency ultra-wideband SAR echo data, and can effectively suppress radio frequency interference signals while greatly improving the suppression efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of the method of the present invention;

[0029] Figure 2 (a) is the target imaging result when there is no RF interference signal;

[0030] Figure 2 (b) is the point target imaging result after adding narrowband RF interference signal;

[0031] Figure 2 (c) is the point target imaging result after adding broadband RF interference signal;

[0032] Figure 3 (a) is the result of point target imaging after suppressing narrowband RF interference signal using the traditional notch filter method;

[0033] Figure 3 (b) is the result of point target imaging after suppressing broadband RF interference signal using the traditional notch filter method;

[0034] Figure 4 (a) is a diagram showing the imaging result of a point target after the narrowband radio frequency interference signal is suppressed by the method of the present invention;

[0035] Figure 4 (b) is a diagram showing the point target imaging result after the broadband RF interference signal is suppressed by the method of the present invention. DETAILED DESCRIPTION

[0036] The drawings are for illustrative purposes only and should not be construed as limiting the present patent;

[0037] In order to better illustrate the present embodiment, some parts in the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product;

[0038] It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0039] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0040] Example 1

[0041] like Figure 1 As shown, a low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching includes the following steps:

[0042] S1: transform the echo signal into two-dimensional frequency domain;

[0043] S2: Set the threshold and perform notch processing;

[0044] S3: transform the two-dimensional frequency domain echo signal after RF interference suppression into two-dimensional time domain.

[0045] In step S1, the received low-frequency ultra-wideband SAR echo data is transformed into a two-dimensional frequency domain, and radio frequency interference suppression is performed in the two-dimensional frequency domain using a two-dimensional notch.

[0046] In step S1, the frequency band of the broadband RF interference signal in the two-dimensional frequency domain is also narrow compared with the echo signal received by the low-frequency ultra-wideband SAR system, so transforming the echo signal into the two-dimensional frequency domain can further concentrate the energy of the RF interference signal.

[0047] In step S2, the echo signal in the two-dimensional frequency domain is notched to suppress the radio frequency interference signal:

[0048] First, the mean value and standard deviation of the two-dimensional frequency domain echo signal are calculated; then, the sum of the mean value and three times the standard deviation is used as the threshold value; finally, the part of the two-dimensional frequency domain echo signal greater than the threshold value is set to the mean value, thereby completing the suppression of the RF interference signal in the two-dimensional frequency domain.

[0049] In step S2, the sum of the mean value and three times the standard deviation is used as the threshold value.

[0050] In step S2, the portion of the two-dimensional frequency domain echo signal that is greater than the threshold value is set as the average value by using the "1" method.

[0051] In step S3, the two-dimensional frequency domain echo signal after radio frequency interference suppression is transformed into a two-dimensional time domain to obtain a final echo signal after radio frequency interference suppression, that is, echo data for low-frequency ultra-wideband SAR imaging.

[0052] Example 2

[0053] like Figure 1 As shown, a low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching includes the following steps:

[0054] S1: transform the echo signal into two-dimensional frequency domain;

[0055] S2: Set the threshold and perform notch processing;

[0056] S3: transform the two-dimensional frequency domain echo signal after RF interference suppression into two-dimensional time domain.

[0057] The specific process of step S1 is:

[0058] Assume that the echo signal received by the low-frequency ultra-wideband SAR system is x(t,τ), where t is the slow time and τ is the fast time. Perform a two-dimensional Fourier transform on x(t,τ) to obtain a two-dimensional frequency domain signal X(f,f τ ).

[0059] The specific process of step S2 is:

[0060] Calculate the two-dimensional frequency domain signal X(f,f τ ) and standard deviation δ, then the notch threshold is set to:

[0061]

[0062] The two-dimensional frequency domain signal X(f,f τ ) are set to the average value if the value is greater than the threshold value. The radio frequency interference signal can be suppressed in the two-dimensional frequency domain.

[0063] In step S2, when setting the threshold, the multiplication factor of the standard deviation is adjusted according to the energy of the radio frequency interference signal, so as to achieve better suppression of the radio frequency interference signal.

[0064] In step S3, the two-dimensional frequency domain echo signal after radio frequency interference suppression is transformed into a two-dimensional time domain to obtain a final echo signal after radio frequency interference suppression, that is, echo data for low-frequency ultra-wideband SAR imaging.

[0065] Example 3

[0066] like Figure 1 As shown, a low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching includes the following steps:

[0067] S1: transform the echo signal into two-dimensional frequency domain;

[0068] S2: Set the threshold and perform notch processing;

[0069] S3: transform the two-dimensional frequency domain echo signal after RF interference suppression into two-dimensional time domain.

[0070] In step S1, the received low-frequency ultra-wideband SAR echo data is transformed into a two-dimensional frequency domain, and radio frequency interference suppression is performed in the two-dimensional frequency domain using a two-dimensional notch.

[0071] In step S1, the frequency band of the broadband RF interference signal in the two-dimensional frequency domain is also narrow compared with the echo signal received by the low-frequency ultra-wideband SAR system, so transforming the echo signal into the two-dimensional frequency domain can further concentrate the energy of the RF interference signal.

[0072] The specific process of step S2 is:

[0073] Calculate the two-dimensional frequency domain signal X(f,f τ ) and standard deviation δ, then the notch threshold is set to:

[0074]

[0075] The two-dimensional frequency domain signal X(f,f τ ) are set to the average value if the value is greater than the threshold value. The radio frequency interference signal can be suppressed in the two-dimensional frequency domain.

[0076] In step S2, when setting the threshold, the multiplication factor of the standard deviation is adjusted according to the energy of the radio frequency interference signal, so as to achieve better suppression of the radio frequency interference signal.

[0077] In step S3, the two-dimensional frequency domain echo signal after radio frequency interference suppression is transformed into a two-dimensional time domain to obtain a final echo signal after radio frequency interference suppression, that is, echo data for low-frequency ultra-wideband SAR imaging.

[0078] In order to solve the problem that narrowband RF interference signals and broadband RF interference signals coexist in the echo signals received by the low-frequency ultra-wideband SAR system, the present invention transforms the echo signals into the two-dimensional frequency domain to suppress RF interference. This method can concentrate the energy of RF interference signals better than only transforming the echo signals into the range frequency domain or the azimuth frequency domain, and therefore has a better suppression effect than the traditional notch method. The method of the present invention can not only effectively suppress narrowband RF interference signals, but also suppress broadband RF interference signals. At the same time, the method of the present invention can perform notch processing on all echo signals at one time, greatly improving the suppression efficiency of RF interference signals. The method of the present invention is simple in principle, easy to implement, and requires very little computational effort to implement.

[0079] Through simulation experiments, the low-frequency ultra-wideband SAR radio frequency interference suppression method based on two-dimensional frequency domain notching of the present invention is verified, and the theoretical analysis and simulation experiment results prove the effectiveness of the present invention.

[0080] In the simulation experiment, the parameters of the low-frequency ultra-wideband SAR system and the radio frequency interference signal using the method of the present invention are shown in Table 1. Among them, the narrowband radio frequency interference signal and the wideband radio frequency interference signal have the same parameters except for the bandwidth.

[0081] Table 1 Parameters of low-frequency ultra-wideband SAR system and RF interference signal

[0082]

[0083]

[0084] Figure 2 It is the result of imaging the echo signal of the original point target (i.e., without adding the RF interference signal) and the echo signal after adding the narrowband RF interference signal and the broadband RF interference signal. The horizontal direction is the azimuth direction (unit: meter), and the vertical direction is the distance direction (unit: meter). Figure 2 (a) is the imaging result of the point target when no RF interference signal is added. Figure 2 (b) is the imaging result of the point target after adding the narrowband RF interference signal. Figure 2 (c) is the imaging result of the point target after adding the broadband RF interference signal. Figure 2 It can be seen that adding narrowband RF interference signals will have a serious impact on imaging, and adding broadband RF interference signals will have an even more serious impact on imaging, with point targets almost obliterated by the interference.

[0085] Figure 3 It is the imaging result after suppressing narrowband RF interference signals and broadband RF interference signals respectively by using the traditional notch method. Among them, the horizontal direction is the azimuth direction (unit: meter), and the vertical direction is the distance direction (unit: meter). Figure 3(a) is the imaging result after suppressing narrowband RF interference signal using traditional notch method. Figure 3 (b) is the imaging result after suppressing the broadband RF interference signal using the traditional notch method. Figure 3 It can be seen that the traditional notch method has a suppressive effect on both narrowband and broadband RF interference signals, and can filter out most RF interference signals. However, the RF interference suppression effect of the traditional notch method is not good, especially after suppressing the broadband RF interference signal, there are still a large number of residual RF interference signals, which leads to poor quality of imaging results. The traditional notch method suppresses RF interference on a single pulse echo one by one, which has a large amount of calculation, and the larger the imaging range and the more pulse echoes, the lower its suppression efficiency.

[0086] Figure 4 It is the imaging result after the narrowband radio frequency interference signal and the broadband radio frequency interference signal are suppressed respectively by the present invention. Among them, the horizontal direction is the azimuth direction (unit: meter), and the vertical direction is the distance direction (unit: meter). Figure 4 (a) is the imaging result after suppressing the narrowband radio frequency interference signal by the method of the present invention, Figure 4 (b) is the imaging result after suppressing the broadband radio frequency interference signal using the method of the present invention. Figure 4 It can be seen that the method of the present invention has a strong inhibitory effect on both narrowband RF interference signals and broadband RF interference signals, and can filter out most of the RF interference signals, with almost no residual interference signals. At the same time, compared with the traditional notch method, the method of the present invention can effectively suppress broadband RF interference signals and ultimately achieve better imaging results. The method of the present invention converts the two-dimensional time domain echo signal into the two-dimensional frequency domain, which can further concentrate the energy of the RF interference signal, thereby having a better suppression effect. In addition, the method of the present invention can suppress the RF interference signals in all echo signals at one time, with a small amount of calculation, thereby having a higher suppression efficiency.

[0087] Table 2 shows the operation time of suppressing RF interference signals by the traditional notch method and the method of the present invention, as well as the signal-to-interference ratio of the imaging result after suppressing the RF interference signal (i.e., the ratio of the target echo signal energy to the residual interference signal energy in the imaging result). As can be seen from Table 2, the time taken by the traditional notch method to suppress RF interference signals is 1.035041s, while the time taken by the method of the present invention to suppress RF interference signals is only 0.305909s, indicating that the method of the present invention has a high suppression efficiency. As can be seen from the signal-to-interference ratio of the imaging results, the RF interference suppression performance of the method of the present invention is far superior to that of the traditional notch method, whether it is for suppressing narrowband RF interference signals or broadband RF interference signals. Therefore, the method of the present invention is a fast and high-performance RF interference suppression method.

[0088] Table 2 Comparison of the indicators of the traditional notch method and the present invention for suppressing RF interference signals

[0089]

[0090] The same or similar reference numerals correspond to the same or similar components;

[0091] The positional relationships described in the drawings are only for illustrative purposes and should not be construed as limiting the present patent;

[0092] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching, characterized in that: The following steps are involved: S1: transforming the echo signal into a two-dimensional frequency domain; wherein, in the step S1, the received low-frequency ultra-wideband SAR echo data is transformed into a two-dimensional frequency domain, and radio frequency interference suppression is performed in the two-dimensional frequency domain using a two-dimensional notch; the frequency band range of the broadband radio frequency interference signal in the two-dimensional frequency domain is also narrow compared with the echo signal received by the low-frequency ultra-wideband SAR system, so transforming the echo signal into the two-dimensional frequency domain can further concentrate the energy of the radio frequency interference signal; S2: Setting a threshold and performing notch processing; wherein, in step S2, notch processing is performed on the echo signal in the two-dimensional frequency domain to achieve the purpose of suppressing the radio frequency interference signal; when setting the threshold, the multiplier of the standard deviation is adjusted according to the energy of the radio frequency interference signal, so as to achieve better suppression of the radio frequency interference signal; S3: transform the two-dimensional frequency domain echo signal after RF interference suppression into two-dimensional time domain.

2. The method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching according to claim 1, characterized in that: In step S2: First, the mean value and standard deviation of the two-dimensional frequency domain echo signal are calculated; then, the sum of the mean value and three times the standard deviation is used as the threshold value; finally, the part of the two-dimensional frequency domain echo signal greater than the threshold value is set to the mean value, thereby completing the suppression of the RF interference signal in the two-dimensional frequency domain.

3. The method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching according to claim 2, characterized in that: In step S2, the sum of the mean value and three times the standard deviation is used as the threshold value.

4. The method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching according to claim 3 is characterized in that: In step S2, the part of the two-dimensional frequency domain echo signal that is greater than the threshold value is set as the average value by using the "1" method.

5. The method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching according to claim 4, characterized in that: In the step S3, the two-dimensional frequency domain echo signal after the radio frequency interference is suppressed is transformed into the two-dimensional time domain to obtain the final echo signal after the radio frequency interference is suppressed, that is, the echo data for low-frequency ultra-wideband SAR imaging.

6. The method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching according to claim 5, characterized in that: The specific process of step S1 is: Assume that the echo signal received by the low-frequency ultra-wideband SAR system is x(t,τ), where t is the slow time and τ is the fast time. Perform a two-dimensional Fourier transform on x(t,τ) to obtain a two-dimensional frequency domain signal X(f,f τ ).

7. The method for suppressing radio frequency interference of low-frequency ultra-wideband SAR based on two-dimensional frequency domain notching according to claim 6, characterized in that: The specific process of step S2 is: Calculate the two-dimensional frequency domain signal X(f,f τ ) and standard deviation δ, then the notch threshold is set to: The two-dimensional frequency domain signal X(f,f τ ) are set to the average value if the value is greater than the threshold value. The radio frequency interference signal can be suppressed in the two-dimensional frequency domain.

Images