Orthogonal frequency division multiplexing signal synthetic aperture imaging method based on Omega-K algorithm

Abstract

The invention discloses a synthetic aperture imaging method for an orthogonal frequency division multiplexing signal based on an Omega-K algorithm, and belongs to the technical field of radar imaging. According to the method, in the process of performing synthetic aperture imaging by using an OFDM signal, the advantage of high range resolution of the OFDM signal is fully considered, range Fourier transform is performed on an echo signal, range matched filtering is performed in a range frequency domain and an azimuth time domain, then azimuth Fourier transform is performed, and data is converted into a two-dimensional frequency domain, so that the synthetic aperture imaging is realized. According to the synthetic aperture imaging method based on the orthogonal frequency division multiplexing signal, azimuth matched filtering is carried out in a two-dimensional frequency domain, then the phase of range-azimuth high-order coupling is corrected through Stolt interpolation in the two-dimensional frequency domain, and finally two-dimensional inverse Fourier transform is carried out to obtain an imaging result, so that the synthetic aperture imaging method based on the orthogonal frequency division multiplexing signal is efficient. A good focusing effect can be obtained at a large squint angle, and high-precision and high-resolution imaging can be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of radar imaging, and in particular relates to a synthetic aperture imaging method of orthogonal frequency division multiplexing signals based on an Omega-K algorithm. Background Art

[0002] Synthetic aperture radar (SAR) is a high-resolution and high-precision imaging technology that can be used all day and all weather. Through the movement of the radar antenna relative to the target, a virtual antenna that is longer than the actual antenna is formed, which greatly improves the resolution and achieves high-precision imaging. It is widely used in earth observation, military reconnaissance, meteorological monitoring and disaster warning. In recent years, as the radar frequency band and the communication frequency band are getting closer and closer, the demand for radar communication integration continues to grow while fully considering spectrum resources and hardware resources. Radar communication integration combines radar detection functions and communication functions, and simultaneously realizes target detection and information transmission by sharing hardware resources and spectrum resources.

[0003] OFDM (Orthogonal Frequency Division Multiplexing) divides the spectrum into multiple orthogonal subcarriers to transmit data, reducing inter-symbol interference, and has strong anti-multipath interference capabilities. At the same time, the cyclic prefix is ​​used to further reduce the multipath effect. Using OFDM signals to transmit communication signals can not only achieve rapid information transmission, but also greatly improve the utilization efficiency of the spectrum, which makes OFDM signals one of the best candidate signals for radar communication integration. The existing algorithm for synthetic aperture imaging using OFDM signals is generally the range Doppler algorithm (RDA). The range Doppler algorithm is generally used for imaging with low oblique angles or positive side views. RDA uses interpolation in the range Doppler domain to decouple the range and azimuth directions. Geometric distortion may occur when the oblique angle is large. The RDA imaging accuracy may not be accurate enough in some high-precision situations. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a synthetic aperture imaging method of orthogonal frequency division multiplexing signals based on the Omega-K algorithm. In the process of using OFDM signals for synthetic aperture imaging, the high range resolution advantage of OFDM signals is fully considered, and the echo signal is transformed into a two-dimensional frequency domain by performing a range Fourier transform, performing a range matched filter in the range frequency domain and azimuth time domain, and then performing an azimuth Fourier transform, transforming the data into a two-dimensional frequency domain, performing an azimuth matched filter in the two-dimensional frequency domain, and then correcting the phase of the high-order coupling in the range and azimuth in the two-dimensional frequency domain by Stolt interpolation, and finally performing a two-dimensional Fourier inverse transform to obtain the imaging result. This is an efficient synthetic aperture imaging method based on OFDM signals, which can obtain a good focusing effect even at a large oblique angle, and can achieve high-precision and high-resolution imaging.

[0005] The technical problem proposed by the present invention is solved in this way:

[0006] A synthetic aperture imaging method of orthogonal frequency division multiplexing signals based on the Omega-K algorithm comprises the following steps:

[0007] Step 1: The echo data s of SAR imaging based on OFDM signal r After removing the center carrier frequency from (τ, η), Fourier transform is performed in the distance direction; Fourier transform is performed on the distance reference signal s0(τ); the conjugate multiplication of the two Fourier transformed signals can obtain the distance compression frequency domain signal S ic (f τ , η);

[0008] Step 2: Signal S ic (f τ ,η) perform azimuth Fourier transform to obtain a two-dimensional frequency domain signal S ic (f τ , f η );

[0009] Step 3: Two-dimensional frequency domain signal S ic (f τ , f η ) and the two-dimensional frequency domain filter H(f τ , f η ) to obtain the two-dimensional frequency domain signal S of the uncorrected high-order coupling phase in the range and azimuth. b (f τ , f η );

[0010] Step 4: Use Stolt interpolation to correct the high-order coupling phase between the range and azimuth directions, and obtain the two-dimensional frequency domain signal S of the corrected high-order coupling phase in the range and azimuth directions. l (f τ ′,f η );

[0011] Step 5: For signal S l (f τ ′,f η ) performs a two-dimensional inverse Fourier transform to obtain the time domain data s out (τ′, η) is taken as the imaging result.

[0012] Furthermore, the specific process of step 1 is:

[0013] The range radar reference signal s0(τ) is expressed as:

[0014]

[0015] Among them, τ is the fast time variable, w ris the range window function, R0 is the shortest distance between the radar and the target, c is the speed of light; N is the number of OFDM signal subcarriers; 0≤k≤N-1, S k is the information transmitted by the kth subcarrier; T is the radar pulse width, j is the sign of the imaginary part,

[0016] Echo data of SAR imaging based on OFDM signal r (τ, η) is expressed as:

[0017]

[0018] in, v is the speed of the radar platform, η is the slow time variable; w a is the azimuth envelope window function, η c is the beam center deviation time; g m is the target reflection coefficient, f c is the carrier frequency,

[0019] After removing the center carrier frequency, s r (τ, η) is expressed as:

[0020]

[0021] Respectively r (τ, η) and s0(τ) are transformed to the distance direction and conjugate multiplied to obtain the frequency domain signal S after distance compression ic (f τ , n):

[0022]

[0023] Among them, f τ is a fast time frequency domain variable, sinc is the sinc function, 0≤n≤N-1, S n is the information transmitted by the nth subcarrier, W r is the frequency domain expression of the distance window function.

[0024] Furthermore, in step 2, the two-dimensional frequency domain signal S is obtained by the stationary phase principle. ic (f τ , f η ), specifically:

[0025]

[0026] In the above formula, S ic (f τ , f η ) has a phase of According to the stationary phase principle Then S ic (f τ , f η ) is expressed as:

[0027]

[0028]

[0029] Among them, f η is the slow time frequency domain variable, W a is the frequency domain expression of the azimuth envelope window function; is the Doppler frequency center, and t0 is the zero Doppler moment.

[0030] Furthermore, in step 3, the two-dimensional frequency domain filter H(f τ , f η )for:

[0031]

[0032] Among them, R ref is the reference distance;

[0033] The two-dimensional frequency domain signal S after azimuth pulse compression b (f τ , f η )for:

[0034]

[0035] Furthermore, the specific process of step 4 is as follows:

[0036] The following corrections are achieved through the Stolt interpolation mapping method:

[0037] For S b (f τ , f η ) Carry out the above operation to obtain S l (f τ ′,f η ) is:

[0038]

[0039] Among them, f τ ′ is the fast time frequency domain variable after Stolt interpolation.

[0040] Furthermore, the specific process of step 5 is as follows:

[0041] In S l (f τ ′,f η) Lieutenant General S is a constant. k is coded as 1 and -1, then the time domain data s out The expression of (τ′, η) is:

[0042]

[0043] Among them, τ′ is the fast time variable after Stolt interpolation.

[0044] The beneficial effects of the present invention are:

[0045] The method of the present invention can accurately compress the reference distance, and remove the high-order coupling phase in the range and azimuth directions through Stolt interpolation for the non-reference distance, so as to achieve high-precision and high-resolution imaging, and has a good imaging effect even in the case of large oblique viewing angles, and the implementation process does not involve complicated calculation processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a schematic diagram of the process of the method of the present invention;

[0047] Figure 2 It is a single point target imaging diagram in the method described in the embodiment;

[0048] Figure 3 A line slice diagram of the distance between a point target and a point target in the method described in the embodiment;

[0049] Figure 4 A slicing diagram of the midpoint target azimuth in the method described in the embodiment;

[0050] Figure 5 This is a single point target imaging diagram of the prior art RDA algorithm;

[0051] Figure 6 A multi-point target distribution map configured for the method described in the embodiment;

[0052] Figure 7 A diagram showing the imaging results of multiple target points in the method described in the embodiment;

[0053] Figure 8 The figure is an imaging result of the method described in the embodiment when the oblique viewing angle is 10 degrees. DETAILED DESCRIPTION

[0054] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0055] This embodiment provides a synthetic aperture imaging method based on an orthogonal frequency division multiplexing signal of the Omega-K algorithm, and its flow chart is as follows: Figure 1As shown, the original echo of SAR imaging based on OFDM signal is input, and after imaging processing, a two-dimensional image of SAR imaging based on OFDM signal is obtained, which specifically includes the following steps:

[0056] Step 1: The echo data s of SAR imaging based on OFDM signal r After removing the center carrier frequency from (τ, η), the distance Fourier transform is performed, and then the distance reference signal s0(τ) is Fourier transformed. The conjugate multiplication can obtain the distance compression signal S ic (f τ , η);

[0057] Radar reference signal s0(τ):

[0058]

[0059] Where τ is the fast time variable, j is the imaginary symbol, c is the speed of light; N is the number of subcarriers of the OFDM signal; 0≤k≤N-1, S k is the information transmitted by the kth subcarrier; T is the radar pulse width, R0 is the shortest distance between the radar and the target, w r (τ) is the distance window function,

[0060] Echo data of SAR imaging based on OFDM signal r (τ, η) is:

[0061]

[0062] Among them, g m is the target reflection coefficient, f c is the carrier frequency, η is the slow time variable; v is the moving speed of the radar platform; w a (η) is the azimuth envelope window function, η c is the beam center deviation time.

[0063] After removing the center carrier frequency, s r (τ, η) becomes:

[0064]

[0065] Respectively r (τ, η) and s0(τ) are transformed to the distance direction and conjugate multiplied to obtain the frequency domain signal S after distance compression ic (f τ , n):

[0066]

[0067] Among them, f τ is a fast time frequency domain variable, sinc is the sinc function, 0≤n≤N-1, S n is the information transmitted by the nth subcarrier, W r (f τ ) is w r (τ) is the frequency domain expression of

[0068] Step 2: S ic (f τ ,η) signal is transformed into the two-dimensional frequency domain to obtain S ic (f τ , f η );

[0069] In step 2, the two-dimensional frequency domain signal S is obtained by the stationary phase principle. ic (f τ , f η ), specifically:

[0070]

[0071] S ic (f τ , f η ) has a phase of According to the stationary phase principle Then S ic (f τ , f η ) is expressed as:

[0072]

[0073] Among them, f η is the slow-time frequency domain variable, is the antenna pattern w a (η-η c ) in the frequency domain, the two are roughly the same in form. is the Doppler frequency center, and t0 is the zero Doppler moment.

[0074] Step 3, performing azimuth pulse compression in the two-dimensional frequency domain;

[0075] The specific implementation process of step 3 is:

[0076] The two-dimensional frequency domain signal S ic (f τ , f η ) and the two-dimensional frequency domain filter H(f τ , f η ) is multiplied to obtain the uncorrected two-dimensional frequency domain signal S of the high-order coupling phase in the range and azimuth.b (f τ , f η );

[0077] Among them, the two-dimensional frequency domain filter H(f τ , f η )for:

[0078]

[0079] Among them, R ref is the reference distance.

[0080] The two-dimensional frequency domain signal S after azimuth pulse compression b (f τ , f η )for:

[0081]

[0082] Step 4: Use Stolt interpolation to correct the high-order coupling phase between the range and azimuth directions, and obtain the two-dimensional frequency domain signal S of the corrected high-order coupling phase in the range and azimuth directions. l (f τ ′,f η );

[0083] The following corrections are achieved through the Stolt interpolation mapping method:

[0084] For S b (f τ , f η ) Carry out the above operation to obtain S l (f τ ′,f η ) is:

[0085]

[0086] Among them, f τ ′ is the fast time frequency domain variable after Stolt interpolation.

[0087] Step 5: S l (f τ ′,f η ) performs a two-dimensional inverse Fourier transform to obtain the time domain data s out (τ′, η) is taken as the imaging result.

[0088] In S l (f τ ′,f η )middle can be approximated as a constant, S k is coded as 1 and -1, then s outThe expression of (τ′, η) is:

[0089]

[0090] Among them, τ′ is the fast time variable after Stolt interpolation.

[0091] In the method described in this embodiment, the specific settings of the simulation parameters are as follows: a linear OFDM signal-based SAR imaging mode is adopted, the distance between the radar and the center of the scene is 12,000 meters, the central carrier frequency of the transmitted signal is 16 GHZ, and the movement speed of the radar platform is 20 m / s; the number of subcarriers is 400, the bandwidth is 400 MHz, and the synthetic aperture time is 20 s.

[0092] Figure 2 The imaging of a single point target is given, and it can be seen that the echo data of SAR imaging based on OFDM signals is compressed into a point after imaging processing;

[0093] Figure 3 The range slice of the point target is given, and it can be seen that its side lobe is relatively low, the first side lobe is less than -20db, and the range resolution is high; Figure 4 Azimuth slices of point targets are given, with low side lobes and good imaging effects.

[0094] right Figure 2 The PSLR and ISLR of the midpoint target range slice and azimuth slice are shown in Table 1:

[0095] Table 1: Range slices and azimuth slices of midpoint target PSLR and ISLR in the method described in the embodiment

[0096]

[0097] From the data in Table 1, it can be seen that the imaging method has good resolution.

[0098] Figure 5 The result diagram of point target imaging under the same conditions using the traditional RDA is given. The point target can be focused into a point, but the effect is not as good as the method described in the embodiment.

[0099] right Figure 5 The PSLR and ISLR of the midpoint target range slice and azimuth slice are shown in Table 2:

[0100] Table 2 PSLR and ISLR using RDA point target range slices and azimuth slices

[0101]

[0102] From the comparison between Table 1 and Table 2, it can be seen that the PSLR of the point target slice in the method described in the embodiment is better than the PSLR of the point target slice obtained by using the commonly used RDA. It can be seen that the method described in the embodiment has high-resolution characteristics.

[0103] Figure 6 A distribution diagram of multiple point targets configured by the method described in this embodiment is given, showing the distribution of 9 point targets in the center of the scene; Figure 7 The method described in the embodiment is for Figure 6 The imaging result after the multi-point target is oversampled. Figure 7 It can be seen that the imaging results are Figure 6 The configured multi-point target distribution map is highly consistent, with good imaging effect and high imaging accuracy.

[0104] Figure 8 The imaging results of the method described in the embodiment are given when the oblique angle is 10 degrees. In standard positive side-view imaging, the range side lobe and the azimuth side lobe are arranged orthogonally to the range direction and the azimuth direction respectively. Figure 8 For a midpoint target, due to the existence of the oblique angle, the orthogonality changes, resulting in the range side lobes and the azimuth side lobes no longer maintaining their original arrangement directions, but still having a good focusing effect. Therefore, it can be seen that the method described in the embodiment also has a relatively good focusing effect at a larger oblique angle.

[0105] In summary, judging from the processing results, the method of the present invention has practical value.

Claims

1. A synthetic aperture imaging method for orthogonal frequency division multiplexing signals based on the Omega-K algorithm, characterized in that: The following steps are involved: Step 1: The echo data s of SAR imaging based on OFDM signal r After removing the center carrier frequency from (τ, η), Fourier transform is performed in the distance direction; Fourier transform is performed on the distance reference signal s0(τ); the conjugate multiplication of the two Fourier transformed signals can obtain the distance compression frequency domain signal S ic (f τ , η); Step 2: Signal S ic (f τ ,η) perform azimuth Fourier transform to obtain a two-dimensional frequency domain signal S ic (f τ , f η ); Step 3: Two-dimensional frequency domain signal S ic (f τ , f η ) and the two-dimensional frequency domain filter H(f τ , f η ) to obtain the uncorrected two-dimensional frequency domain signal S of the high-order coupling phase in the range and azimuth. b (f τ , f η ); Step 4: Use Stolt interpolation to correct the high-order coupling phase between the range and azimuth directions, and obtain the two-dimensional frequency domain signal S of the corrected high-order coupling phase in the range and azimuth directions l (f τ ′,f η ); Step 5: For signal S l (f τ ′,f η ) performs a two-dimensional inverse Fourier transform to obtain the time domain data s out (τ′, η) is taken as the imaging result.

2. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on Omega-K algorithm according to claim 1, characterized in that: The specific process of step 1 is: The range radar reference signal s0(τ) is expressed as: Among them, τ is the fast time variable, w r is the range window function, R0 is the shortest distance between the radar and the target, c is the speed of light; N is the number of OFDM signal subcarriers; 0≤k≤N-1, S k is the information transmitted by the kth subcarrier; T is the radar pulse width, j is the sign of the imaginary part, Echo data of SAR imaging based on OFDM signal r (τ, η) is expressed as: in, v is the speed of the radar platform, η is the slow time variable; w a is the azimuth envelope window function, η c is the beam center deviation time; g m is the target reflection coefficient, f c is the carrier frequency, After removing the center carrier frequency, s r (τ, η) is expressed as: Respectively r (τ, η) and s0(τ) are transformed to the distance direction and conjugate multiplied to obtain the frequency domain signal S after distance compression ic (f τ , n): Among them, f τ is a fast time frequency domain variable, sinc is the sinc function, 0≤n≤N-1, S n is the information transmitted by the nth subcarrier, W r is the frequency domain expression of the distance window function.

3. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on Omega-K algorithm according to claim 2, characterized in that: In step 2, the two-dimensional frequency domain signal S is obtained by the stationary phase principle. ic (f τ , f η ), specifically: In the above formula, S ic (f τ , f η ) has a phase of According to the stationary phase principle Then S ic (f τ , f η ) is expressed as: Among them, f η is the slow time frequency domain variable, Wa 为 Frequency domain expression of the azimuth envelope window function; is the Doppler frequency center, and t0 is the zero Doppler moment.

4. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on Omega-K algorithm according to claim 3, characterized in that: In step 3, the two-dimensional frequency domain filter H(f τ , f η )for: Among them, R ref is the reference distance; The two-dimensional frequency domain signal S after azimuth pulse compression b (f τ , f η )for:

5. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on Omega-K algorithm according to claim 4, characterized in that: The specific process of step 4 is: The following corrections are achieved through the Stolt interpolation mapping method: For S b (f τ , f η ) Carry out the above operation to obtain S l (f τ ′,f η ) is: Among them, f τ ′ is the fast time frequency domain variable after Stolt interpolation.

6. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on Omega-K algorithm according to claim 5, characterized in that: The specific process of step 5 is: In S l (f τ ′,f η ) Lieutenant General S is a constant. k is coded as 1 and -1, then the time domain data s out The expression of (τ′, η) is: Among them, τ′ is the fast time variable after Stolt interpolation.

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