An improved method for suppressing direct waves in time domain of high-frequency external radiation source radar

Abstract

The present invention discloses an improved time-domain direct wave suppression method for high-frequency external radiation source radar. The present invention relates to a direct wave suppression method based on an orthogonal subspace projection theory algorithm, namely an extended phase cancellation algorithm. The purpose of the present invention is to obtain ionospheric information through channel estimation to achieve accurate reconstruction of direct wave signals and improve the suppression effect of direct waves. The process is: 1. Reconstruction of standard direct wave signals and extraction of pilot position information; 2. Extraction of gain pilot information from echoes; 3. Channel estimation using cubic spline interpolation to obtain ionospheric information, and then accurate reconstruction of direct waves using ionospheric information; 4. Adopting an extended phase cancellation algorithm, using the reconstructed direct wave signal as a reference signal, and performing direct wave suppression on the echo signal in the monitoring channel. The present invention is used in the field of high-frequency external radiation source radar signal processing.

Description

Technical Field

[0001] The invention belongs to the field of signal processing, and in particular relates to an improved method for suppressing direct waves in the time domain of a high-frequency external radiation source radar. Background Art

[0002] High-frequency exo-radiation radar is a radar system that detects targets by passively receiving electromagnetic signals emitted by a third-party exo-radiation source. It works in the shortwave band (HF, 3-30MHz) with a wavelength of 10-100m. High-frequency exo-radiation radar combines the advantages of high-frequency over-the-horizon radar and exo-radiation radar. It has the advantages of long detection distance, large observation range, all-weather, anti-stealth, strong concealment, anti-interference, and low cost. It has important research significance in the military field. The increasingly popular digital AM broadcasting signal has provided conditions for the research of exo-radiation radar due to its low operating frequency and good low-altitude coverage performance, and has become a research hotspot. Exo-radiation radar includes a reference channel and a monitoring channel. The signal of the reference channel refers to the beam of the antenna array pointing to the transmitting station, that is, the direct wave direction, and the signal of the monitoring channel is to point the beam to the target direction. Since the radiation source signal comes from a third party, on the one hand, the content and parameters of the transmitted signal are uncontrollable, and on the other hand, there is strong direct wave interference in the monitoring channel, which makes target detection difficult.

[0003] DRM signal is a high-frequency digital broadcast used worldwide, using orthogonal frequency division multiplexing (OFDM) modulation technology. OFDM technology modulates several orthogonal subcarriers separately so that each subcarrier in the spectrum meets orthogonality. OFDM technology can solve some problems caused by multipath, such as symbol crosstalk, frequency selective attenuation, Doppler frequency drift, etc. The external radiation source radar using DRM signal can purify the signal through demodulation-reconstruction to obtain the reconstructed reference signal. For the direct wave interference in the monitoring channel, direct wave suppression processing is required. In this regard, some scholars proposed an algorithm based on the orthogonal subspace projection theory, namely the Extended Cancellation Algorithm (ECA). The idea is to project the monitoring channel signal containing the target echo onto a subspace orthogonal to the direct wave subspace, thereby effectively suppressing the direct wave. On this basis, later generations proposed ECA-B, which reduces the subspace dimension through segmented calculation, reduces the storage space consumption, and speeds up the update of the filter coefficients, but the ECA-B algorithm has a significant truncation effect, resulting in Doppler ambiguity. Based on this, some scholars proposed ECA-S, which uses the sliding window segmentation method to ensure low storage space consumption and achieve parallelization. Other scholars proposed the ECA-C algorithm based on ECA, which uses the time domain carrier processing method to effectively reduce the amount of calculation and is also suitable for the situation where the clutter expansion is serious. Many scholars have applied the ECA series algorithm to direct wave suppression processing. Zhao Zhixin. Research on several key technologies of new system and signal processing of high-frequency external radiation source radar [D]. Doctoral dissertation, 2013: 77-93. The ECA series algorithm was used to suppress the direct wave of external radiation source radar. Zhang Jian, Wan Xianrong, Liu Yuqi. Parallel implementation of sliding window expansion cancellation algorithm for external radiation source radar [J]. Radar Science and Technology, 2017, 15(02): 115-119+125. The ECA-S algorithm is used to suppress the direct wave of external radiation source radar.

[0004] The reference signal used for cancellation by the ECA series of algorithms is usually a pure direct wave obtained by demodulating and reconstructing the echo signal in the monitoring channel. However, since the echo signal passes through the ionosphere, the direct wave contained in the echo is modulated in amplitude and phase by the ionosphere, and the pure direct wave obtained by demodulation and reconstruction does not contain ionospheric modulation information. Therefore, when the pure direct wave obtained by demodulation and reconstruction is directly used for direct wave suppression, the suppression effect is not good. In view of the above shortcomings, the present invention proposes an improved ECA algorithm, which uses cubic spline interpolation technology to add ionospheric modulation information to the demodulated and reconstructed direct wave, thereby greatly improving the direct wave suppression effect. Summary of the invention

[0005] The present invention aims to solve the technical problems existing in the background technology, and aims to provide an improved method for suppressing direct waves in the time domain of high-frequency passive bistatic radar, and uses an extended cancellation algorithm (ECA) to suppress the direct waves of high-frequency passive bistatic radar in the field of high-frequency passive bistatic radar (HF Passive Bistatic Radar, HFPBR) signal processing of digital radio mondiale (DRM) using orthogonal frequency division multiplexing (OFDM) modulation. The invention has reference significance for suppressing direct waves of passive bistatic radar using other signal types for detection.

[0006] In order to solve the technical problem, the technical solution of the present invention is:

[0007] An improved method for suppressing direct waves in the time domain of a high-frequency external radiation source radar, the method comprising:

[0008] S1: Demodulate and reconstruct the DRM signal to obtain the standard direct wave time domain signal, then perform Fourier transform to obtain the spectrum of each symbol of the standard direct wave signal, extract the gain pilot position information and gain pilot information from the spectrum, and obtain the gain pilot information and gain pilot position information of the standard direct wave signal;

[0009] S2: According to the spectrum of each symbol of the standard direct wave signal, simulate and generate the ionosphere and the echo containing the strong direct wave signal, and extract the gain pilot information of the echo from the echo containing the strong direct wave signal;

[0010] S3: performing cubic spline interpolation on the gain pilot information of the strong direct wave signal in the echo to perform channel estimation to obtain ionosphere information; then using the ionosphere information to distort the spectrum of each symbol of the standard direct wave signal to obtain an ionosphere modulated direct wave signal;

[0011] S4: Using the extended cancellation algorithm, the obtained direct wave signal is used as the reference signal to suppress the direct wave of the simulated echo and obtain the RD diagram after suppression.

[0012] Further, the step S1 is specifically as follows:

[0013] S11: Demodulate and reconstruct the DRM measured echo signal to obtain the standard direct wave time domain signal S sta (n);

[0014] S12: For S sta (n) Perform Fourier transform on each symbol to obtain the spectrum S of each symbol of the standard direct wave signalsta (s,k);

[0015] S13: Extract the spectrum S of each symbol of the standard direct wave signal according to the arrangement of the gain pilot in the protocol sta The gain pilot information G of the standard direct wave signal in (s,k) sta (s,p) and gain pilot position information.

[0016] Further, the step S2 is specifically as follows:

[0017] S21: The spectrum S of each symbol of the standard direct wave signal obtained in step S1 sta (s,k), simulation generates ionospheric frequency domain information I sta (s,k);

[0018] S22: S sta (s,k) and I sta The corresponding position (s, k) is multiplied and then the inverse Fourier transform is used to obtain the ionospheric modulated direct wave S iosta (n), using S iosta (n) Generate a simulated target S with a certain delay and Doppler frequency tar (n) Simulated echo S rec (n), the simulated echo contains a strong direct wave signal;

[0019] S23: For simulated echo S rec (n) Perform Fourier transform on each symbol to obtain the spectrum S of each symbol rec (s, k), extract the gain pilot information G of the strong direct wave signal in the echo according to the gain pilot position information in step S1 rec (s,p).

[0020] Further, the step S3 is specifically as follows:

[0021] S31: Gain pilot information G for the strong direct wave signal in the echo rec (s,p) and the gain pilot information G of the standard direct wave signal sta The ionospheric information I at the location of the gain pilot can be obtained by dividing the corresponding position of (s, p) iono (s,p), for I iono (s,p) is interpolated with cubic spline in the time axis direction, and then the interpolation result is interpolated with cubic spline in the frequency axis direction to obtain the ionospheric information I in the frequency domain iono (s,k);

[0022] S32: The standard direct wave spectrum S in step S1 is sta (s, k) is multiplied by the ionospheric information I according to the corresponding position iono(s, k) to obtain the ionospheric modulated direct wave frequency domain signal S iono (s,k), then S iono (s, k) is transformed by inverse Fourier transform to obtain the ionospheric modulated direct wave time domain signal S iono (n).

[0023] Further, the step S4 is specifically as follows:

[0024] S41: The ionospheric modulated direct wave signal S obtained in step S3 is iono (n) as a reference signal for the extended cancellation algorithm, the simulated echo S obtained in step S2 rec (n) Perform direct wave suppression processing to obtain the RD spectrum after suppression.

[0025] Furthermore, the extended cancellation algorithm is derived from the least squares algorithm LS; the ECA direct wave suppression method is as follows, according to the cost function of LS:

[0026]

[0027] in,

[0028]

[0029] is the direct wave subspace, where s ref (n)=[s ref (n),s ref (n-1),…,s ref (nM-1)] T is the reference signal set, s surv (n) is the monitoring signal set, M is the number of direct wave suppression units, and the filter coefficients are obtained by solving:

[0030] W*=(X H X) -1 X H s surv

[0031] Then the filter output is obtained:

[0032] e=s surv -X(X H X) -1 X H s surv =(I N -X(X H X) -1 X H )s surv

[0033] That is, construct the direct wave subspace X according to the reference signal, and then transform the detection signal ssurv (n) Project it onto a subspace orthogonal to X to suppress the direct wave signal.

[0034] Compared with the prior art, the advantages of the present invention are:

[0035] The improved high-frequency external radiation source radar time domain direct wave suppression described in the present invention provides a more effective suppression method. Based on the original extended phase cancellation algorithm, channel estimation is performed through cubic spline interpolation, so as to obtain ionospheric information to achieve accurate reconstruction of direct wave signals, better restore direct wave information, provide a more accurate reference signal for the ECA algorithm, and improve the direct wave suppression effect in the traditional algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 , overall flow chart;

[0037] Figure 2 , the position of the gain pilot in each transmission frame (white part);

[0038] Figure 3a , simulate ionospheric amplitude information;

[0039] Figure 3b , simulate ionospheric phase information;

[0040] Figure 3c , simulated echo RD spectrum;

[0041] Figure 4a , interpolate ionospheric amplitude information;

[0042] Figure 4b , interpolate ionospheric phase information;

[0043] Figure 5a , S sta (n) RD spectrum after direct wave suppression as reference signal;

[0044] Figure 5b , S iono (n) RD spectrum after direct wave suppression as reference signal;

[0045] Figure 5c , Doppler profile at the target before and after direct wave suppression;

[0046] Figure 5d , a partial enlarged view of the Doppler profile at the target before and after direct wave suppression. DETAILED DESCRIPTION

[0047] The specific implementation mode of the present invention is described below in conjunction with embodiments:

[0048] It should be noted that the structures, proportions, sizes, etc. shown in this specification are only used to match the contents disclosed in the specification so that people familiar with this technology can understand and read them, and are not used to limit the conditions under which the present invention can be implemented. Any structural modification, change in proportional relationship or adjustment of size should still fall within the scope of the technical content disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0049] At the same time, the terms such as "upper", "lower", "left", "right", "middle" and "one" cited in this specification are only for the convenience of description and are not used to limit the scope of implementation of the present invention. Changes or adjustments to their relative relationships should be regarded as the scope of implementation of the present invention without substantially changing the technical content.

[0050] Embodiment 1:

[0051] The specific process of an improved time-domain direct wave suppression method for high-frequency external radiation source radar is as follows:

[0052] Step 1: Reconstruction of the standard direct wave signal and extraction of pilot position information; demodulate and reconstruct the DRM signal to obtain the reconstructed standard direct wave time domain signal, denoted as S sta (n), for S sta (n) Each symbol is Fourier transformed to obtain the spectrum of each symbol, denoted as S sta (s, k), s represents the sth OFDM symbol, and k represents the subcarrier number in a symbol. According to the protocol, the gain pilot position information and gain pilot information in the frequency domain of each OFDM symbol of the standard direct wave are extracted. The gain pilot information of the standard direct wave signal is recorded as G sta (s, p), s represents the sth OFDM symbol, and p represents the pilot number in an OFDM symbol.

[0053] Step 2: Extract the gain pilot information from the echo; Based on the gain pilot position information in each OFDM symbol obtained in step 1, the echo time domain signal is recorded as S rec (n), for S rec (n) Each symbol is Fourier transformed to obtain the spectrum of each symbol, denoted as S rec (s, k), extract the gain pilot information from the corresponding position of each OFDM symbol frequency domain of the echo, denoted as G rec (s,p).

[0054] Step 3: Use cubic spline interpolation (Spline) to perform channel estimation to obtain ionospheric information, and then use the ionospheric information to accurately reconstruct the direct wave; use the gain pilot information G of the echo obtained in step 2 rec(s, p) and the gain pilot information G of the standard direct wave signal obtained in step 1 sta (s, p) and the cubic spline interpolation method are used to estimate the channel to obtain the ionospheric information. The reconstructed time domain signal of ionospheric modulation is obtained according to the ionospheric information, which is recorded as S iono (n).

[0055] Step 31: Use cubic spline interpolation to estimate the channel; the gain pilot information G of the echo rec (s,p) divided by the gain pilot information G of the standard direct wave signal sta (s, p) to obtain the ionospheric information at the location of the gain pilot, denoted as I iono (s,p), using I iono (s, p) is interpolated with cubic spline in the time axis direction, and then the interpolation result is used to perform cubic spline interpolation in the frequency axis direction to obtain ionospheric information in multiple symbol frequency domains, which is recorded as I iono (s,k).

[0056] Step 32: Use ionospheric information to accurately reconstruct the direct wave; sta (s, k), multiply the ionospheric information I according to the corresponding position iono (s, k) to obtain the reconstructed frequency domain signal of ionospheric modulation, denoted as S iono (s,k), then S iono (s, k) is inversely transformed to obtain the reconstructed time domain signal S modulated by the ionosphere iono (n).

[0057] Step 4: Use the extended cancellation algorithm to reconstruct the ionospheric modulated time domain signal S obtained in step 3 iono (n) is used as a reference signal to monitor the echo signal S in the channel. rec (n) Perform direct wave suppression.

[0058] The extended cancellation algorithm is derived from the least squares algorithm (LS). The ECA direct wave suppression method is as follows, based on the LS cost function:

[0059]

[0060] in,

[0061]

[0062] is the direct wave subspace, where s ref (n)=[s ref (n),s ref (n-1),…,s ref (nM-1)]T is the reference signal set, s surv (n) is the monitoring signal set, M is the number of direct wave suppression units, and the filter coefficients are obtained by solving:

[0063] W*=(X H X) -1 X H s surv

[0064] Then the filter output is obtained:

[0065] e=s surv -X(X H X) -1 X H s surv =(I N -X(X H X) -1 X H )s surv

[0066] That is, construct the direct wave subspace X according to the reference signal, and then transform the detection signal s surv (n) Project it onto a subspace orthogonal to X to suppress the direct wave signal.

[0067] Embodiment 2:

[0068] This embodiment 2 is applied to an improved method for suppressing high-frequency external radiation source radar time-domain direct waves in embodiment 1 and is specifically implemented according to the following steps.

[0069] The improved high-frequency external radiation source radar time domain direct wave suppression method of this embodiment is specifically implemented according to the following steps:

[0070] The standard direct wave signal is reconstructed by demodulating the measured echo data. The main system parameters of the measured data are shown in Table 1. The subsequent processing is all simulation experiments, and the main parameters of the simulation target are shown in Table 2:

[0071] Table 1 Radar system parameters

[0072]

[0073] Table 2 Simulation target parameter settings

[0074]

[0075] Step 1: Demodulate and reconstruct the DRM measured echo signal to obtain the standard direct wave time domain signal S sta (n), then S sta (n) Perform Fourier transform on each symbol to obtain the spectrum S of each symbol sta(s, k), since the gain pilot position information is determined in each transmission frame, it is distributed in a fixed arrangement, such as Figure 2 As shown in the figure, the white part is the position of the gain pilot in each transmission frame. According to the arrangement of the gain pilot in the protocol, the gain pilot information G in the frequency domain of each OFDM symbol of the standard direct wave can be extracted. sta (s,p).

[0076] Step 2: Simulate and generate the ionosphere and echo, draw the RD spectrum, and extract the gain pilot information from the echo;

[0077] Step 21: Simulate and generate the ionosphere; Based on the frequency domain S of the standard direct wave signal obtained in step 1 sta (s,k), simulation generates ionospheric frequency domain information I sta (s,k), Figure 3a To simulate the ionospheric amplitude information, Figure 3b To simulate the ionospheric phase information.

[0078] Step 22: Simulate and generate echoes and draw RD spectrum; sta (s,k) and I sta The direct wave S modulated by the ideal ionosphere is obtained by multiplying the corresponding positions of (s, k) and then inverse Fourier transforming. iosta (n), using S iosta (n) Generate a simulated target S with a certain delay and Doppler frequency tar (n) Simulated echo S rec (n), Figure 3c is the RD spectrum of the simulated echo with the simulated target.

[0079] Step 23: Extract the gain pilot information from the echo; rec (n) Perform Fourier transform on each symbol to obtain the spectrum S of each symbol rec (s, k), extract the echo gain pilot information G according to the gain pilot position in step 1 rec (s,p).

[0080] Step 3: Use cubic spline interpolation to estimate the channel to obtain ionospheric information, and then use the ionospheric information to accurately reconstruct the direct wave;

[0081] Step 31: Use cubic spline interpolation to perform channel estimation; convert the echo gain pilot information G rec (s,p) and the gain pilot information G of the standard direct wave signal sta The ionospheric information I at the location of the gain pilot can be obtained by dividing the corresponding position of (s, p) iono (s,p), first iono(s,p) is interpolated with cubic spline in the time axis direction, and then the interpolation result is interpolated with cubic spline in the frequency axis direction to obtain the ionospheric information I in the frequency domain iono (s,k), such as Figure 4a is the ionospheric amplitude information obtained by interpolation, Figure 4b is the interpolated ionospheric phase information.

[0082] Step 32: Use ionospheric information to accurately reconstruct the direct wave; convert the standard direct wave spectrum S sta (s, k) is multiplied by the ionospheric information I according to the corresponding position iono (s, k) to obtain the ionospheric modulated direct wave frequency domain signal S iono (s,k), then S iono (s, k) is transformed by inverse Fourier transform to obtain the ionospheric modulated direct wave time domain signal S iono (n).

[0083] Step 4: Transform the standard direct wave signal S in step 1 sta (n) is used as the reference signal of the extended cancellation algorithm to simulate the echo S obtained in step 2. rec (n) Perform direct wave suppression processing, such as Figure 5a is the suppressed RD spectrum; the ionospheric modulated direct wave signal S obtained in step 3 is iono (n) is used as the reference signal of the extended cancellation algorithm to simulate the echo S obtained in step 2. rec (n) Perform direct wave suppression processing, such as Figure 5b is the RD spectrum after inhibition; compare the inhibition effects of the two as follows Figure 5c as well as Figure 5c Partial magnification Figure 5d .

[0084] Table 3 Direct wave suppression results

[0085]

[0086] Table 4 Target signal-to-noise ratio improvement

[0087]

[0088] The simulation results show that:

[0089] contrast Figure 3a , 3b As can be seen from Figures 4a and 4b, using cubic spline interpolation to restore the ionospheric information at all subcarriers at the ionospheric information at the gain pilot position has achieved good results. The amplitude and phase information of the ionosphere are well restored, laying the foundation for better direct wave suppression processing in the future.

[0090] contrast Figure 3c , 5a , 5b, at the 277th Doppler unit and 401th distance unit, Figure 3c The target cannot be seen before the direct wave is suppressed. The traditional direct wave suppression process is used. Figure 5a The target can be seen, but it is not obvious. After the direct wave is suppressed by the algorithm proposed by the present invention, Figure 5b The target can be clearly seen.

[0091] observe Figure 3c , 5a , 5b, 5c, 5d, the direct wave suppression results of the two methods are shown in Table 3, and the target signal-to-noise ratio improvement is shown in Table 4. The traditional direct wave suppression algorithm has a suppression degree of 9.5211dB, and the target signal-to-noise ratio is improved by 11.5846dB; and the direct wave suppression degree after the gain pilot information interpolation improvement reaches 21.6033dB, and the target signal-to-noise ratio is improved by 26.6474dB; the direct wave suppression degree of the improved algorithm is 12.0822dB more than the traditional direct wave algorithm, and the target signal-to-noise ratio is improved by 15.0628dB. The method used in the present invention has a better effect on the suppression of direct waves, greatly improves the direct wave suppression degree and the target signal-to-noise ratio, and at the same time, the simulated target is clearly revealed at the 277th Doppler unit and the 401st distance unit, and the noise floor is reduced, which can prove the effectiveness of the method.

[0092] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

[0093] Many other changes and modifications may be made without departing from the concept and scope of the present invention.It should be understood that the present invention is not limited to the specific embodiments, and the scope of the present invention is defined by the appended claims.

Claims

1. An improved method for suppressing direct waves in the time domain of high-frequency external radiation source radar, characterized in that: The method comprises: S1: Demodulate and reconstruct the DRM signal to obtain the standard direct wave time domain signal, then perform Fourier transform to obtain the spectrum of each symbol of the standard direct wave signal, extract the gain pilot position information and gain pilot information from the spectrum, and obtain the gain pilot information and gain pilot position information of the standard direct wave signal; S2: According to the spectrum of each symbol of the standard direct wave signal, simulate and generate the ionosphere and the echo containing the strong direct wave signal, and extract the gain pilot information of the echo from the echo containing the strong direct wave signal; S3: performing cubic spline interpolation on the gain pilot information of the strong direct wave signal in the echo to perform channel estimation to obtain ionosphere information; then using the ionosphere information to distort the spectrum of each symbol of the standard direct wave signal to obtain an ionosphere modulated direct wave signal; S4: Using the extended cancellation algorithm, the obtained direct wave signal is used as the reference signal to suppress the direct wave of the simulated echo and obtain the RD diagram after suppression.

2. The improved high-frequency external radiation source radar time domain direct wave suppression method according to claim 1 is characterized in that: The step S1 is specifically as follows: S11: Demodulate and reconstruct the DRM measured echo signal to obtain the standard direct wave time domain signal S sta (n); S12: For S sta (n) Perform Fourier transform on each symbol to obtain the spectrum S of each symbol of the standard direct wave signal sta (s,k); S13: Extract the spectrum S of each symbol of the standard direct wave signal according to the arrangement of the gain pilot in the protocol sta The gain pilot information G of the standard direct wave signal in (s,k) sta (s,p) and gain pilot position information.

3. The improved high-frequency external radiation source radar time domain direct wave suppression method according to claim 2 is characterized in that: The step S2 is specifically as follows: S21: The spectrum S of each symbol of the standard direct wave signal obtained in step S1 sta (s,k), simulation generates ionospheric frequency domain information I sta (s,k); S22: S sta (s,k) and I sta The corresponding position (s, k) is multiplied and then inverse Fourier transformed to obtain the ionospheric modulated direct wave S iosta (n), using S iosta (n) Generate a simulated target S with a certain delay and Doppler frequency tar (n) The simulated echo S rec (n), the simulated echo contains a strong direct wave signal; S23: For simulated echo S rec (n) Perform Fourier transform on each symbol to obtain the spectrum S of each symbol rec (s, k), extract the gain pilot information G of the strong direct wave signal in the echo according to the gain pilot position information in step S1 rec (s,p).

4. The improved high-frequency external radiation source radar time domain direct wave suppression method according to claim 3 is characterized in that: The step S3 is specifically as follows: S31: Gain pilot information G for the strong direct wave signal in the echo rec (s,p) and the gain pilot information G of the standard direct wave signal sta The ionospheric information I at the location of the gain pilot can be obtained by dividing the corresponding position of (s, p) iono (s,p), for I iono (s,p) is interpolated with cubic spline in the time axis direction, and then the interpolation result is interpolated with cubic spline in the frequency axis direction to obtain the ionospheric information I in the frequency domain iono (s,k); S32: The standard direct wave spectrum S in step S1 is sta (s, k) is multiplied by the ionospheric information I according to the corresponding position iono (s, k) to obtain the ionospheric modulated direct wave frequency domain signal S iono (s,k), then S iono (s, k) is transformed by inverse Fourier transform to obtain the ionospheric modulated direct wave time domain signal S iono (n).

5. The improved high-frequency external radiation source radar time domain direct wave suppression method according to claim 4 is characterized in that: The step S4 is specifically as follows: S41: The ionospheric modulated direct wave signal S obtained in step S3 is iono (n) as a reference signal for the extended cancellation algorithm, the simulated echo S obtained in step S2 rec (n) Perform direct wave suppression processing to obtain the RD spectrum after suppression.

6. The improved high-frequency external radiation source radar time domain direct wave suppression method according to claim 5 is characterized in that: The extended cancellation algorithm is derived from the least squares algorithm LS; the ECA direct wave suppression method is as follows, according to the cost function of LS: in, is the direct wave subspace, where s ref (n)=[s ref (n),s ref (n-1),…,s ref (nM-1)] T is the reference signal set, s surv (n) is the monitoring signal set, M is the number of direct wave suppression units, and the filter coefficients are obtained by solving: W*=(X H X) -1 X H s surv Then the filter output is obtained: e=s surv -X(X H X) -1 X H s surv =(I N -X(X H X) -1 X H )s surv That is, construct the direct wave subspace X according to the reference signal, and then transform the detection signal s surv (n) Project it onto a subspace orthogonal to X to suppress the direct wave signal.

Images