Radar covert communication system and method

Abstract

The invention relates to the technical field of satellite communication, in particular to a radar covert communication system and method, and the system comprises a signal interception device which is used for obtaining level control information and a synthetic aperture radar signal; performing time-varying amplitude modulation on the synthetic aperture radar signal according to the level control information to obtain echo information; the signal receiving device is used for acquiring the echo information and performing two-dimensional imaging processing according to the echo information to obtain imaging result information; performing false target information extraction on the imaging result information to obtain selected image information; performing inverse imaging processing on the selected image information to obtain restored echo information; and threshold processing is carried out on the recovered echo information to obtain code element information, so that a target with a relatively high communication concealment requirement is protected, and the security and concealment of communication are improved.

Description

Technical Field

[0001] This application relates to the field of satellite communication technologies, and in particular, to a radar stealth communication system and method. Background Art

[0002] Satellite communication is a technology for information transmission via artificial satellites. The communication process is as follows: First, the information to be transmitted is converted into a signal form suitable for transmission through a modulator. For example, after encoding the information, it is propagated through a carrier signal. The modulated signal is amplified by a power amplifier so that the coverage range of the signal is large enough. The amplified signal is transmitted to the target satellite through an antenna. Then, the receiving antenna on the satellite receives the weak signal, amplifies it through a low-noise amplifier to improve the signal-to-noise ratio, and then restores it to the original information through a demodulator.

[0003] In current technologies, the spectral resources used for satellite communication are limited, and the spectra in each region are strictly managed, resulting in restrictions on the allocation and use of spectral resources. Moreover, satellite communication links are vulnerable to interception and monitoring. Existing satellite communication methods cannot handle usage scenarios with high requirements for communication stealth. The above problems need to be solved. Summary of the Invention

[0004] In order to protect targets with high requirements for communication stealth and improve the security and stealth of communication, this application provides a radar stealth communication system and method, adopting the following technical solutions:

[0005] In a first aspect, this application provides a radar stealth communication system, including:

[0006] A signal interception device, configured to obtain level control information and synthetic aperture radar signals; perform time-varying amplitude modulation on the synthetic aperture radar signals according to the level control information to obtain echo information;

[0007] A signal receiving device, configured to obtain echo information, perform two-dimensional imaging processing on the echo information to obtain imaging result information; extract false target information from the imaging result information to obtain selected image information; perform inverse imaging processing on the selected image information to obtain restored echo information; perform threshold processing on the restored echo information to obtain symbol information.

[0008] Preferably, the level control information is:

[0009]

[0010] where rect is a rectangular pulse signal, α is the duty cycle, T s is the modulation period, δ is the impulse response function, and n is a positive integer.

[0011] Preferably, the synthetic aperture radar signal is:

[0012]

[0013] Among them, the range - direction time is τ, the azimuth - direction time is t, and the pulse duration is T p , and the carrier frequency is f 0 , and the linear frequency modulation rate is K r .

[0014] Preferably, the echo information is as follows:

[0015]

[0016] Preferably, the imaging result information is as follows:

[0017]

[0018] Among them, τ 0 = 2R 0 / c is the range - direction position of the target, R 0 is the time delay, B dop is the Doppler bandwidth, K a is the azimuth - direction frequency modulation rate, T L is the synthetic aperture time, and n is the order of the false target.

[0019] Preferably, the specific steps of performing time - varying amplitude modulation on the synthetic aperture radar signal according to the level control information are as follows:

[0020] Perform inter - pulse modulation on the synthetic aperture radar signal according to the level control information.

[0021] In a second aspect, the present application provides a radar covert communication method, including:

[0022] Obtain level control information and a synthetic aperture radar signal;

[0023] Perform time - varying amplitude modulation on the synthetic aperture radar signal according to the level control information to obtain echo information.

[0024] In a third aspect, the present application provides a radar covert communication method, including:

[0025] Obtain echo information, perform two - dimensional imaging processing according to the echo information to obtain imaging result information;

[0026] Extract false target information from the imaging result information to obtain selected image information;

[0027] Perform inverse imaging processing on the selected image information to obtain restored echo information;

[0028] Perform threshold processing on the restored echo information to obtain symbol information.

[0029] Fourthly, the present application provides a radar stealth communication device, including a memory and a processor. The memory stores a computer program, and the processor is configured to run the computer program to execute the radar stealth communication method as described above.

[0030] Fifthly, the present application provides a computer-readable storage medium, in which a computer program is stored. The computer program is configured to execute the radar stealth communication method as described above when running.

[0031] In summary, compared with the prior art, the beneficial effects brought by the technical solution provided by the present application at least include:

[0032] In the present application, a signal interception device is used to obtain level control information and synthetic aperture radar signals. The synthetic aperture radar signals are subjected to time-varying amplitude modulation through the level control information, and information is hidden in the synthetic aperture radar signals to obtain echo information, thereby improving its confidentiality. The information is transmitted to the signal receiving device in the form of echo information. After receiving the echo information, the signal receiving device performs two-dimensional imaging processing on it to obtain imaging result information, extracts false target information from the imaging result information to obtain selected image information, and then performs inverse imaging processing on the selected image information to obtain restored echo information, and finally restores the code elements, which plays a role in protecting targets with high requirements for communication stealth and improves the security and stealth of communication. Description of the Drawings

[0033] Figure 1 is a schematic module diagram of a radar stealth communication system according to an embodiment of the present application.

[0034] Figure 2 is a first flowchart of a radar stealth communication method according to an embodiment of the present application.

[0035] Figure 3 is a second flowchart of a radar stealth communication method according to an embodiment of the present application.

[0036] Description of the Reference Numerals:

[0037] 1. Signal interception device; 2. Signal receiving device. Detailed Embodiments

[0038] The following Figures 1 - 3 further describes the present application in detail. The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to be limiting.

[0039] Referring to Figure 1 , a radar stealth communication system involved in the present application specifically includes:

[0040] A signal interception device, which is used to obtain level control information and synthetic aperture radar signals; perform time-varying amplitude modulation on the synthetic aperture radar signals according to the level control information to obtain echo information;

[0041] A signal receiving device, which is used to obtain echo information, perform two-dimensional imaging processing on the echo information to obtain imaging result information; extract false target information from the imaging result information to obtain selected image information; perform inverse imaging processing on the selected image information to obtain restored echo information; perform threshold processing on the restored echo information to obtain symbol information.

[0042] Specifically, in order to protect targets with high requirements for communication concealment such as submarines, the present application proposes a synthetic aperture radar (SAR) stealth communication method based on an active frequency selective surface (AFSS). Spaceborne SAR has the characteristic of large beam coverage. Therefore, various ground and sea clutter components in its echo signal account for a high proportion, and it is difficult to detect some parasitic communication signals in it, making it suitable as a carrier for stealth communication. In order to carry information into the SAR signal, electromagnetic regulation technology needs to be used. The birth of electromagnetic metamaterials provides a new idea for electromagnetic regulation technology. It is a kind of material and has characteristics that natural materials do not have, and can be flexibly regulated to achieve amplitude modulation, phase modulation, frequency modulation, etc. of electromagnetic waves. The active frequency selective surface is an electrically controlled time-varying electromagnetic material in electromagnetic metamaterials, which can achieve switch-type electromagnetic regulation through an external bias circuit, so as to achieve the time-varying selection characteristics of electromagnetic waves. The present application uses AFSS to implement electromagnetic regulation technology, carries the information to be transmitted into the SAR signal through amplitude modulation, and restores it.

[0043] The present application obtains level control information and synthetic aperture radar signals through a signal interception device, performs time-varying amplitude modulation on the synthetic aperture radar signals according to the level control information, hides the information in the synthetic aperture radar signals to obtain echo information, thereby improving its confidentiality. The information is transmitted to the signal receiving device in the form of echo information. After receiving the echo information, the signal receiving device performs two-dimensional imaging processing on it to obtain imaging result information, extracts false target information from the imaging result information to obtain selected image information, then performs inverse imaging processing on the selected image information to obtain restored echo information, and finally restores the symbol, which plays a role in protecting targets with high requirements for communication concealment and improves the security and concealment of communication.

[0044] As one of the implementation manners, the level control information is:

[0045]

[0046] where rect is a rectangular pulse signal, α is the duty cycle, T s is the modulation period, δ is the impulse response function, and n is a positive integer.

[0047] Specifically, in the embodiment of the present application, the protected target equipped with the AFSS serves as the transmitting end of communication, and binary symbols are sent as information. The symbols correspond to level control signals of different frequencies, forming a method similar to frequency shift keying (FSK) to transmit digital information. The high level of the level control signal corresponds to the on state of the AFSS bias circuit, and the low level corresponds to the off state of the AFSS bias circuit. The two states respectively correspond to the two working modes of wave absorption and reflection of the AFSS. The active frequency selective surface will perform time-varying amplitude modulation on the SAR signal, that is, the synthetic aperture radar signal.

[0048] In the embodiment of the present application, the reflection coefficients of the AFSS when powered off and powered on are defined as 1 and x respectively, where 0 ≤ x < 1; rect() is a rectangular pulse signal with a duty cycle of α and a modulation period of T s , represents the convolution operation, δ() is the impulse response function, and n is a positive integer.

[0049] As one of the implementation manners, the synthetic aperture radar signal is:

[0050]

[0051] where the range time is τ, the azimuth time is t, the pulse duration is T p , the carrier frequency is f 0 , and the linear frequency modulation rate is K r .

[0052] The specific steps for performing time-varying amplitude modulation on the synthetic aperture radar signal according to the level control information are as follows:

[0053] Perform inter-pulse modulation on the synthetic aperture radar signal according to the level control information.

[0054] Specifically, the modulation of the SAR signal can be divided into intra-pulse modulation and inter-pulse modulation. Since short pulses are used in some scenarios such as spaceborne SAR, and the pulse width is only on the order of microseconds, the requirements for the modulation equipment are relatively high for intra-pulse modulation. Therefore, the inter-pulse modulation method is adopted for the SAR signal.

[0055] In the embodiment of the present application, the range time, that is, the fast time, is τ, the azimuth time, that is, the slow time, is t, the pulse duration is T p , the carrier frequency is f 0 , and the linear frequency modulation rate is K r .

[0056] As one of the implementation manners, the echo information is as follows:

[0057]

[0058] Specifically, in the embodiments of the present application, by hiding information in the SAR signal, the behavior of communication is hidden in the signal, so that its confidentiality is improved.

[0059] In the communication process implemented by the signal interception device according to the embodiments of the present application, the information source does not emit signals, and it is difficult to be detected by other eavesdropping means. It is suitable for applications with extremely high concealment requirements such as submarine communication. Moreover, the signal is parasitic in the SAR satellite signal, and no additional transceiver device is required, which greatly reduces the communication cost.

[0060] As one of the implementation manners, the imaging result information is as follows:

[0061]

[0062] Where τ 0 = 2R 0 / c is the range position of the target, R 0 is the time delay, B dop is the Doppler bandwidth, K a is the azimuth modulation frequency, T L is the synthetic aperture time, and n is the order of the false target.

[0063] Specifically, the imaging result information in the embodiments of the present application is imaged by the backend system, where n = ±1, ±2,..., ±N represents the order of the false target.

[0064] Due to modulation, a series of false targets appear in the imaging result. The false targets carry the information to be transmitted through AFSS regulation, and it is necessary to collect them and then recover the information. Moreover, the SAR image information contains noise and ground clutter, and the noise and clutter are an interference to the communication signal. The receiving end of the communication needs to select the area carrying the information in the image for retention and set other positions to zero, so that the modulated information is not interfered.

[0065] The embodiments of the present application are operations of the receiving end. Only after the receiving end acquires the signal and images, can there be the feature of the appearance of false targets. The finally obtained information symbol has been carried in the SAR signal after AFSS modulation and is hidden through the sar signal. Finally, the signal symbol needs to be recovered in the embodiments of the present application.

[0066] As one of the implementation manners, before signal extraction, it is first necessary to preprocess the SAR image to reduce the interference of noise and clutter on the signal. The SAR image contains various noises, such as Gaussian noise, scattering noise, etc. Mean filtering, median filtering, Wiener filtering and other methods are used to reduce the influence of noise on the signal.

[0067] The false target is the target regulated by AFSS and frequency modulation continuous wave frequency modulation synthesis, and usually carries specific modulation information. In order to extract the false target, all potential targets in the detected image are detected, and which ones are false targets containing modulation information are identified.

[0068] Specifically, by setting a threshold or using a target detection algorithm, such as target detection based on the convolutional neural network CNN, to identify the target area in the image, and the area may be a potential false target.

[0069] Or use time-frequency analysis methods such as wavelet transform and short-time Fourier transform to analyze the signal characteristics, find the position where the false target appears, and through the time-frequency characteristics of these signals, the spatial position of the false target can be located.

[0070] One of the embodiments is that in the next step, inverse imaging is performed to restore the image signal to the signal form before imaging processing. Then, binarization is required to convert the restored complex signal into a rectangular signal, that is, a threshold th is set according to the amplitude characteristics of the signal, points greater than the threshold are set to 1, and points less than the threshold are set to 0 to obtain the final rectangular signal p r :

[0071]

[0072] In the formula, L is the total length of the signal, and s is the signal to be processed.

[0073] Finally, the rectangular signal is demodulated. In this covert communication method, the method of matched filtering is adopted, that is, all carrier signals are multiplied by the restored signal, the output of each filter is calculated, and the code element is restored according to the comparison of the energy magnitude.

[0074] After the embodiment of this application identifies the false target area containing modulation information, the information carried therein needs to be extracted next. The signal of the false target will be modulated by a certain modulation, such as frequency modulation or phase modulation. Therefore, demodulation is required to restore the original information. For example, the coherent demodulation method is used to perform demodulation operations on the signals at the false target positions to restore the modulated information. Or by performing Fourier analysis or other feature extraction techniques on the false target signals, such as principal component analysis PCA, feature selection algorithms, etc., the information features carried are extracted.

[0075] In order to reduce the interference of noise and clutter, it is necessary to suppress other irrelevant areas in the image and only retain the false target area carrying information.

[0076] After the false targets are extracted and the noise and interference are removed in the embodiments of the present application, the last step is to recover the modulation information. The information carried by the false targets is usually transmitted through a certain modulation method. The method for recovering the modulation information depends on the modulation method used. If frequency modulation is used, the original information is recovered through frequency demodulation.

[0077] The main application field of the present application is the communication field. The AFSS is used to directly regulate the SAR signal. After the signal contacts the AFSS, it is directly reflected. This solution does not require storing the radar signal, reducing the processing cost. And amplitude modulation is adopted, and the amplitude of the modulation signal is based on the reflection coefficient of the AFSS.

[0078] As one of the implementation manners, the embodiment of the present application is the simulation result of MATLAB. The simulation parameters are as follows: the signal carrier frequency is 1.26 GHz, the signal bandwidth is 60 MHz, the pulse width is 80 μs, the pulse repetition frequency is 2000 Hz, the effective speed of the radar is 7349 m / s, the slant range of the scene center is 640 km, and the point target is located at the scene center. The Chirp Scaling algorithm, that is, CSA, is used to process the SAR signal. The information symbol sequence sent is set as "01", corresponding to the signals with modulation frequencies of 50 Hz and 80 Hz respectively. The duty cycle is 0.5, and the amplitude coefficient is 0.1.

[0079] The simulation result shows that the modulation method of the AFSS switches once within the synthetic aperture time. In addition to the real target, there are two groups of false targets symmetrically distributed on both sides of the real target, with intervals of 518 meters and 829 meters respectively, which is consistent with the theoretical analysis value. In addition, range migration correction can only accurately correct the zero-order target during the imaging process. The higher the order, the worse the correction effect, resulting in a defocusing effect. In order to retain most of the information, the false target object selected by inverse imaging has a result very similar to the echo information. Then, an image slice centered on the range direction is extracted, and thresholding is performed with a threshold set to 0.55 according to the mean value of the amplitude coefficient, and then the signal is demodulated. Finally, the matched filtering result is used to determine the symbol, and the final result "01" is obtained. That is, a simple 2-bit communication is completed.

[0080] Refer to Figure 2 , an embodiment of the present application provides a radar stealth communication method, which is applied to a signal interception device, including:

[0081] Step S1: Obtain the level control information and the synthetic aperture radar signal;

[0082] Step S2: Perform time-varying amplitude modulation on the synthetic aperture radar signal according to the level control information to obtain the echo information.

[0083] Refer to Figure 3, this application embodiment provides a radar stealth communication method, which is applied to a signal receiving device and includes:

[0084] Step S3: Obtain echo information, perform two-dimensional imaging processing according to the echo information, and obtain imaging result information;

[0085] Step S4: Extract false target information from the imaging result information to obtain selected image information;

[0086] Step S5: Perform inverse imaging processing on the selected image information to obtain restored echo information;

[0087] Step S6: Perform threshold processing on the restored echo information to obtain symbol information.

[0088] This application embodiment provides a radar stealth communication device, including a memory and a processor. The memory stores a computer program, and the processor is configured to run the computer program to execute the radar stealth communication method as described above.

[0089] This application embodiment provides a computer-readable storage medium, in which a computer program is stored. The computer program is configured to execute the radar stealth communication method as described above when running.

[0090] Those skilled in the art can clearly understand that for the convenience and conciseness of description, the specific working processes of the devices and products described above can refer to the corresponding processes in the foregoing method embodiments, and will not be elaborated herein.

[0091] In several embodiments provided by this application, it should be understood that the disclosed methods, systems, devices, and program products can be implemented in other ways.

[0092] In addition, in each embodiment of this application, each functional unit can be integrated into a processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above integrated units can be implemented in the form of hardware or in the form of software functional units.

[0093] As mentioned above, the above embodiments are only used to illustrate the technical solutions of this application, rather than to limit them; although this application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of each embodiment of this application.

Claims

1. A radar concealed communication system, characterized in that: include: Signal interception device, used to obtain level control information and synthetic aperture radar signals; Performing time-varying amplitude modulation on the synthetic aperture radar signal according to the level control information to obtain echo information; The signal receiving device is used to obtain echo information, perform two-dimensional imaging processing according to the echo information to obtain imaging result information; extract false target information from the imaging result information to obtain selected image information; perform inverse imaging processing on the selected image information to obtain restored echo information; Threshold processing is performed on the restored echo information to obtain code element information.

2. The radar concealed communication system according to claim 1, characterized in that: The level control information is: Among them, rect is a rectangular pulse signal, α is the duty cycle, T s is the modulation period, δ is the impulse response function, and n is a positive integer.

3. The radar concealed communication system according to claim 2, characterized in that: The synthetic aperture radar signal is: Among them, the range time is τ, the azimuth time is t, and the pulse duration is T p , the carrier frequency is f0, and the linear modulation frequency is K r .

4. The radar concealed communication system according to claim 3, characterized in that: The echo information is:

5. The radar concealed communication system according to claim 4, characterized in that: The imaging result information is: Among them, τ0=2R0 / c is the distance position of the target, R0 is the delay, B dop is the Doppler bandwidth, K a is the azimuth modulation frequency, T L is the synthetic aperture time, and n is the order of false targets.

6. The radar concealed communication system according to claim 1, characterized in that: The specific steps of performing time-varying amplitude modulation on the synthetic aperture radar signal according to the level control information are: The synthetic aperture radar signal is inter-pulse modulated according to the level control information.

7. A radar covert communication method, applied to a signal interception device, characterized in that: include: Acquire level control information and synthetic aperture radar signals; The synthetic aperture radar signal is subjected to time-varying amplitude modulation according to the level control information to obtain the echo information.

8. A radar covert communication method, applied to a signal receiving device, characterized in that: include: Acquire echo information, perform two-dimensional imaging processing according to the echo information, and obtain imaging result information; Extract false target information from the imaging result information to obtain selected image information; Performing inverse imaging processing on the selected image information to obtain restored echo information; Threshold processing is performed on the restored echo information to obtain code element information.

9. A radar concealed communication device, characterized in that: The invention comprises a memory and a processor, wherein the memory stores a computer program, and the processor is configured to run the computer program to execute the radar covert communication method according to any one of claims 7 or 8.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, wherein the computer program is configured to execute the radar covert communication method according to any one of claims 7 or 8 when executed.

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