CN106646472A - Method for restraining SAR radio-frequency interference based on space-frequency cascading filtering - Google Patents

Abstract

The invention provides a radio frequency interference suppression method based on space-frequency cascade filtering. The technical solution includes the following steps: Step 1: DOA estimation of RFI. The DOA of RFI in the received signal is estimated by using the MUSIC algorithm, and the spatial steering vectors of all RFI are determined according to the DOA. The second step: frequency domain filtering. The received signal is transformed into the frequency domain by FFT, and the spectral support area of RFI is separated from the received signal by using the 3δ discriminant method. The third step: spatial filtering. Calculation of radio frequency interference by spatial filtering to filter out interference. The invention can effectively reduce the loss of useful signals and has better interference suppression effect.

Description

SAR Radio Frequency Interference Suppression Method Based on Space-Frequency Cascade Filtering

technical field

The invention belongs to the intersecting technical field of aerospace and microwave remote sensing, in particular to a method for suppressing RFI (Radio Frequency Interference, radio frequency interference) in SAR (Synthetic Aperture Radar, synthetic aperture radar) data.

Background technique

SAR (Synthetic Aperture Radar) operating in the low-band is susceptible to interference from electromagnetic signals emitted by television networks, communication networks and other civilian equipment in the same frequency band, which is the so-called RFI (Radio Frequency Interference, radio frequency interference). The presence of RFI seriously affects image quality as well as interpretation and interpretation. For the current spatial filtering method, see the reference "Zhu Feng, Zhang Youyi. Interference Research on Adaptive Spatial Filtering [J]. Modern Electronic Technology, 2013,36(11):67～74". For full-band filtering, useful signals in the same direction as RFI are completely suppressed, and while suppressing RFI, useful signals are also suppressed.

Aiming at this situation, according to the high-power and narrow-band characteristics of radio frequency interference, a radio frequency interference suppression method based on space-frequency cascade filtering is proposed. Before the spatial filtering, the received signal is filtered in the frequency domain to separate the frequency band where the RFI is located, and then the spatial filtering is performed on the frequency band where the RFI is located. Compared with the existing spatial filtering methods, the proposed method limits the loss of useful signals to the narrow frequency band where RFI is located, effectively reducing the loss of useful signals.

Contents of the invention

The object of the present invention is to propose a radio frequency interference suppression method based on space-frequency cascaded filtering, which can effectively reduce the loss of useful signals and has better interference suppression effect.

The idea of the technical solution of the present invention is: first use the MUSIC (Multiple Signal Classification, multiple signal classification) algorithm to estimate the DOA (Direction of Arrival, angle of arrival) of the RFI, and then construct the spatial orientation vector of the RFI according to the DOA. Next, the received signal is transformed into the frequency domain through FFT, and the frequency domain filtering is performed on the received signal according to the high power and narrow frequency band characteristics of radio frequency interference to separate the frequency band where the RFI is located. Finally, the spatial domain filter is constructed by using the spatial steering vector of RFI, and the spatial domain filter is used to realize filtering for the frequency band signal where RFI is located.

The technical solution of the present invention is: a kind of SAR radio frequency interference suppression method based on space-frequency cascade filtering, assuming that x (t, η _L ) represents the received signal of SAR at slow time η _L , wherein t represents fast time, it is characterized in that, Include the following steps:

Step 1: DOA estimation of RFI:

Use the MUSIC algorithm to estimate the DOA of the RFI in the received signal x(t,η _L ), determine the spatial steering vectors of all RFIs according to the DOA, and record the spatial steering vector of the i-th RFI as d(θ _i ), i=1,2, ..., N, where N represents the number of RFIs.

The second step: frequency domain filtering:

Carry out FFT transformation to the frequency domain by receiving signal x (t, η _L ), obtain receiving frequency domain signal X (f, η _L );

Use the 3δ discriminant method to separate the RFI spectrum support area X(f′,η′ _L ) from the received signal,

Among them, 0≤η′ _L ≤Δη, _{0≤f′≤BS} , Δη is the slow time width of radio frequency interference in the received signal x(t,η _L ), B _S is the received signal x(t,η _L ) Bandwidth of medium radio frequency interference;

Step 3: Spatial filtering:

Calculate radio frequency interference r(f′,η′ _L ′) using the following formula:

Wherein D ^H (θ _i ) is the complex conjugate vector in the frequency domain with the spatial steering vector d(θ _i );

Then subtract the radio frequency interference r(f′,η′ _L ) from the frequency domain signal X(f,η _L ) to obtain the received signal suppressed by the radio frequency interference.

Adopt the present invention can obtain following technical effect:

The SAR radio frequency interference suppression method based on space-frequency cascade filtering proposed by the present invention utilizes the frequency domain characteristics of RFI, and first performs frequency domain filtering on the received signal to separate the frequency band where the RFI is located. Then, use the spatial steering vector of RFI to construct a spatial domain filter, and use the spatial domain filter to achieve filtering for the frequency band signal where RFI is located. The present invention can limit the lost useful signal to the narrow frequency band where the RFI is located, effectively reducing the loss of the useful signal and improving the image quality.

Description of drawings

Fig. 1 is the principle flowchart of the SAR radio frequency interference suppression method provided by the present invention;

Figure 2 is the imaging result of the point target used in the simulation experiment;

Figure 3 is the imaging result of the point target after adding radio frequency interference to Figure 2;

Fig. 4 is the imaging result after interference suppression of Fig. 3 by using the existing spatial filtering method;

Fig. 5 is the imaging result of Fig. 3 after interference suppression using the method provided by the invention;

Fig. 6 is the imaging result of the measured data used in the simulation experiment;

Figure 7 is the imaging result of the data after adding radio frequency interference to Figure 6;

Fig. 8 is the imaging result after interference suppression of Fig. 7 by using the existing spatial filtering method;

Fig. 9 is the imaging result of Fig. 7 after interference suppression by using the method provided by the present invention.

detailed description

Fig. 1 is a principle flow chart of the spaceborne SAR radio frequency interference suppression method provided by the present invention. In order to describe the principle of the present invention in detail, the following is set forth from the perspective of theoretical derivation:

Assuming that x(t,η _L ) represents the received signal of SAR at slow time η _L , the received signal x(t,η _L ) can be expressed as the following form:

Among them, s _k (t, η _L ) is the k-th useful signal received by the antenna array corresponding to fast time t at slow time η _L , φ _k is the DOA of the k-th useful signal, and d(φ _k ) is the The spatial steering vectors of k useful signals, K is the number of useful signals, this variable is required for calculation, and does not need to be estimated in the following filtering. r _i (t,η _L ) is the i-th RFI received by the antenna array corresponding to fast time t at slow time η _L , θ _i is the DOA of the i-th RFI, d(θ _i ) is the i-th RFI Space steering vector, N is the number of RFI.

Carry out FFT transformation to the frequency domain with x (t, η _L ), as follows:

where D ^H (θ _i ) is the complex conjugate vector in the frequency domain with the spatial steering vector d(θ _i ).

According to the spectrum characteristics of RFI, the spectrum support area of RFI is separated from the received signal by using 3δ discriminant method. The spectrum support area of RFI is in the following form:

Among them, 0≤η' _L ≤Δη, _0≤f'≤BS , Δη is the slow time width of radio frequency interference in the received signal, and B _S is the bandwidth of radio frequency interference in the received signal.

Using the frequency-domain complex conjugate vector of the RFI spatial steering vector as a spatial filter, the RFI is extracted from the spectrum support area where the RFI is located, as follows:

Further, the extracted RFI form is as follows:

Spatial steering vectors have the following dependencies:

Further, the obtained RFI is in the following form:

It can be seen from the above formula that when φ _k ≠ θ _i , there is no useful signal loss, and when φ _k = θ _i , the lost useful signal is S _k (f′,η′), but the lost useful signal is limited to 0 In the range of _≤f′≤BS , compared with the full-band loss of the existing spatial filtering method, the anti-jamming performance of the system is effectively improved.

Figures 2 to 5 are the results of point target simulation experiments. Figure 2 is the imaging result of the received signal of the original point target. It can be seen from the image that the point target has a good focusing effect. Figure 3 is the imaging result of the received signal after adding radio frequency interference in Figure 2. It can be seen that the point target has been annihilated by the interference signal. The original images in Figure 2 and Figure 3 are color images, and the contrast is not obvious after changing to grayscale images. Figure 4 is the imaging result of Figure 3 after using the existing spatial filtering method to suppress the interference. It can be seen from the image that the interference is well focused, but the detailed information is destroyed. Fig. 5 is the imaging result after interference suppression using the method provided by the present invention. It can be seen that the point target is in good focus and has less interference residue.

Figures 6 to 9 are the results of surface target simulation experiments. The simulation experiment selects a piece of uninterferenced airborne X-band three-channel SAR measured data.

Figure 6 is the imaging result of the raw data used in the simulation experiment. As can be seen from the figure, the image is clear and the contrast is high. Figure 7 is the imaging result of the data in Figure 6 after adding radio frequency interference. It can be seen from the figure that due to the existence of interference, the image target information is covered and the image contrast decreases. Figure 8 is the imaging result of Figure 7 after the existing spatial filtering method performs interference suppression. After interference suppression, the image quality is improved, but the contrast of the image decreases, and bright and dark areas appear, and the details are not obvious. Fig. 9 is the imaging result of Fig. 7 after interference suppression by using the method provided by the present invention. After the interference suppression, the image quality is significantly improved, the detail information recovery is more obvious, and the image contrast is significantly improved.

The above simulation experiment shows that the method of the present invention can effectively make up for the deficiency of the existing method in interference suppression, and effectively improve the quality of the SAR image after interference suppression.

Claims (1)

1. a kind of SAR Radio frequency interference suppressing methods based on null tone cascade filtering, SAR refers to synthetic aperture radar, it is assumed that x (t, η_L) represent SAR in slow moment η_LReception signal, wherein t represents the fast time, it is characterised in that comprise the steps：

The first step：The DOA of RFI estimates：

Estimate to receive signal x (t, η using MUSIC algorithms_L) in RFI DOA, according to DOA determine all RFI steric direction arrow Amount, the steric direction vector for remembering i-th RFI is d (θ_i), i=1,2 ..., N, N represents the number of RFI；Wherein, RFI is referred to and penetrated Frequency is disturbed, and DOA refers to direction of arrival；

Second step：Frequency domain filtering

Signal x (t, η will be received_L) Fast Fourier Transform (FFT) is carried out to frequency domain, obtain receiving frequency domain signal X (f, η_L)；

Frequency spectrum Support X (f ', η ' of RFI are isolated from reception signal using 3 δ diagnostic methods_L),

Wherein, 0≤η '_L≤ Δ η, 0≤f '≤B_S, Δ η is to receive signal x (t, η_L) in there is the slow time width of Radio frequency interference, B_STo receive signal x (t, η_L) in Radio frequency interference bandwidth.

3rd step：Airspace filter

Radio frequency interference r (f ', η ' are calculated using following formula_L)：

$r(f^{\′},{\mathrm{\η}}_L^{\′})=X(f^{\′},{\mathrm{\η}}_L^{\′})\underset{i=1}{\overset{N}{\Σ}}{D}^H\left({\mathrm{\θ}}_i\right)$

Wherein D^H(θ_i) it is that steric direction vector is d (θ_i) frequency domain complex conjugate vector；

Again from frequency domain signal X (f, η_L) in deduct Radio frequency interference r (f ', η '_L), obtain the reception signal of Jing Radio frequency interferences suppression.