A method of inverse synthetic aperture imaging based on frequency diversity array

Abstract

The present invention discloses an inverse synthetic aperture imaging method based on a frequency diversity array, which intercepts the effective time period within the scanning cycle of FDA for a certain target, repeatedly transmits the effective time period, and uses the formed repeated signal as the transmitting beam of ISAR. The method utilizes the time period pointing to other useless directions, and continuously transmits a focused beam in the direction that needs to be detected, thereby achieving a stable tracking effect. In addition, the beam generated by this method also solves the distance angle coupling problem existing in FDA. At the same time, the continuous FDA beam can obtain the distance resolution capability, and obtain the azimuth image resolution capability through the relative motion between the target and the radar, which enables the radar imaging system to have the ability of two-dimensional imaging, and finally uses the imaging algorithm to perform two-dimensional imaging on the target.

Description

Technical Field

[0001] The present invention belongs to the technical field of radar signal processing, and specifically uses a frequency diversity array (FDA) to complete an inverse synthetic aperture radar (ISAR) imaging method of a target. Background Art

[0002] Phased arrays are widely used in traditional radar imaging. However, since the beam pattern of a phased array is only related to angle and not to distance, it is impossible to distinguish and identify targets at different distances in the same azimuth directly from the beam pattern. FDA, which can generate distance-angle dependent beam patterns, has great advantages in radar imaging. FDA does not need to adjust the phase to control the beam pointing. It will automatically scan the entire scene periodically, but the uncontrollable time-varying nature of its automatic scanning has also become a major problem. Especially when tracking targets in ISAR imaging, the rapidly changing beam pattern shortens the target reflection time and weakens the accumulated reflection energy, resulting in unstable tracking or loss of the target.

[0003] In view of the above problems, the present invention intercepts the effective time period of FDA scanning cycle for a certain target, repeatedly transmits the effective time period, and uses the formed repeated signal as the transmitting beam of ISAR. This method utilizes the time period pointing to other useless directions, and continuously transmits a focused beam in the direction that needs to be detected, thereby achieving a stable tracking effect. In addition, the beam generated by this method also solves the distance angle coupling problem of FDA. At the same time, the continuous FDA beam can obtain the distance resolution capability, and obtain the azimuth image resolution capability through the relative motion between the target and the radar, which enables the radar imaging system to have the ability of two-dimensional imaging. Summary of the invention

[0004] The technical problem to be solved by the present invention is how to use FDA to complete ISAR imaging of moving targets and ensure that the accumulation of imaging energy is not lost due to FDA periodic scanning. When the linear frequency-biased FDA transmission beam automatically scans the entire plane, for sparse space targets, it not only causes energy waste, but also has a great impact on the accumulation of reflected energy for imaging of the target. To this end, the present invention provides an inverse synthetic aperture imaging method based on a frequency diversity array, which uses FDA to form a continuous irradiation beam for a moving target, achieves sufficient accumulation of reflected energy, and thus accurately perceives the two-dimensional distribution of target scattering, and finally uses an imaging algorithm to perform two-dimensional imaging of the target.

[0005] The present invention provides an inverse synthetic aperture imaging method based on a frequency diversity array, comprising the following steps:

[0006] Step 1: Initialize the frequency diversity array parameters. The total number of frequency diversity array elements is N, the distance between adjacent elements is d, the carrier frequency is f0, the fixed frequency offset is Δf, and the single frequency signal transmitted by the nth element is expressed as:

[0007] S n (t)=exp(j2π(f0+nΔf)t),n=0,1,2,…,N-1

[0008] Step 2: Use the signal transmitted by the frequency diversity array in step 1 to scan the space and obtain the angle θ where the target is at time i i and distance R i , where R i is the distance from the far-field target to the first array element, and the distance from the nth array element to the far-field target is R n :

[0009] R n =R i -ndsinθ i

[0010] Step 3: extract the main lobe period that is useful to the target at time i. The main lobe period that needs to be extracted is:

[0011]

[0012] Where k is an integer and the propagation speed of electromagnetic waves is c;

[0013] Step 4: Calculate the phase to obtain the directional beam at the target angle. For different times, calculate the angle θ that is only with the target. i The initial phases involved are:

[0014]

[0015] Step 5: Set the main lobe period T e As a pulse width, the signal of this period is repeatedly transmitted;

[0016] Step 6: Receive echo data. All signals of different frequencies transmitted by all array elements are received. Then the echo signal received by the nth array element is:

[0017]

[0018] Step 7: Perform phase compensation on the echo signal. i The echo signal after compensation is:

[0019]

[0020] Step 8: The phase difference caused by the array element spacing to the echo signal is expressed as:

[0021]

[0022] The echo signal obtained by compensating the phase difference between array elements is:

[0023]

[0024] Step 9, perform inverse discrete Fourier transform on the echo signal n in step 8 to obtain a one-dimensional range image, and further obtain an azimuth image through the relative motion between the target and the radar, and finally obtain a two-dimensional ISAR image of the target.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] It avoids the time-varying problem of traditional FDA, can produce continuous beam irradiation in a certain direction, and the accumulated reflected energy is much greater than that of traditional FDA, which can achieve stable tracking effect for moving targets. It also retains the distance-angle dependence of FDA beam pattern, so it can obtain higher distance resolution, and at the same time, by utilizing the characteristics of repeated pulses to generate strip-shaped transmission beams similar to phased arrays, it solves the problem of distance and angle coupling and achieves the effect of accurate reconstruction of different target positions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A diagram showing the relationship between the radar system and the target geometry of the embodiment;

[0028] Figure 2 It is a multi-objective simulation scene diagram of the embodiment;

[0029] Figure 3 This is a diagram of multi-target simulation imaging results of an embodiment. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to the embodiments and drawings, but the present invention is not limited thereto.

[0031] Example

[0032] A method for inverse synthetic aperture imaging based on a frequency diversity array comprises the following steps:

[0033] Step 1: Initialize the frequency diversity array parameters, such as Figure 1 As shown, the total number of frequency diversity array elements is N, the distance between adjacent elements is d, the carrier frequency is f0, the fixed frequency offset is Δf, and the single-frequency signal transmitted by the nth element is expressed as:

[0034] S n(t)=exp(j2π(f0+nΔf)t),n=0,1,2,…,N-1

[0035] Step 2: Use the signal transmitted by the frequency diversity array in step 1 to scan the space and obtain the angle θ where the target is at time i i and distance R i , where R i is the distance from the far-field target to the first array element, and the distance from the nth array element to the far-field target is R n :

[0036] R n =R i -ndsinθ i

[0037] Step 3: extract the main lobe period that is useful to the target at time i. The main lobe period that needs to be extracted is:

[0038]

[0039] Where k is an integer and the propagation speed of electromagnetic waves is c;

[0040] Step 4: Calculate the phase to obtain the directional beam at the target angle. For different times, calculate the angle θ that is only with the target. i The initial phases involved are:

[0041]

[0042] Step 5: Set the main lobe period T e As a pulse width, the signal of this period is repeatedly transmitted;

[0043] Step 6: Receive echo data. All signals of different frequencies transmitted by all array elements are received. Then the echo signal received by the nth array element is:

[0044]

[0045] Step 7: Perform phase compensation on the echo signal. i The echo signal after compensation is:

[0046]

[0047] Step 8: The phase difference caused by the array element spacing to the echo signal is expressed as:

[0048]

[0049] The echo signal obtained by compensating the phase difference between array elements is:

[0050]

[0051] Step 9, perform inverse discrete Fourier transform on the echo signal n in step 8 to obtain a one-dimensional range image, and further obtain an azimuth image through the relative motion between the target and the radar, and finally obtain a two-dimensional ISAR image of the target.

[0052] The technical solution of the present invention is further illustrated by experimental simulation:

[0053] (1) Experimental scene setting

[0054] Frequency diversity inverse synthetic aperture radar imaging system, using Figure 1 The target and radar structure diagram shown in the figure, where the total number of frequency diversity array elements N = 60, the spacing between adjacent elements d = λ / 2 = 0.15m, the radar signal reference carrier frequency f0 = 10GHz, and the fixed frequency offset Δf = 3.33MHz.

[0055] Imaging area range: x-axis direction is -20m~20m, y-axis direction is -20m~20m, the position (0,0) in the grid is the target center (i.e. the origin of the coordinate axis). Now set 5 point targets at (0,0), (0,10), (-10,0), (0,-10), (10,0), assuming that the target scattering coefficient is 1, the target is 5 kilometers away from the radar, and the target rotation speed in turntable mode is w=0.02rad / s. The target setting position is as follows Figure 2 shown.

[0056] (2) Simulation results

[0057] The final imaging result is Figure 3 As shown, by duplicating and moving the effective time period to obtain the transmitting beam, a strip directional beam similar to a phased array is formed in azimuth, which not only retains the resolution of the frequency diversity array in terms of distance, but also enhances the energy accumulation in azimuth, thereby accurately sensing the two-dimensional distribution of target scattering, enhancing the tracking capability for dynamic targets, achieving the imaging effect of accurately reconstructing the position of the target, and avoiding the problem of distance and angle coupling of the frequency diversity array.

Claims

1. A method for inverse synthetic aperture imaging based on a frequency diversity array, characterized in that: The following steps are involved: Step 1: Initialize the frequency diversity array parameters. The total number of frequency diversity array elements is N, the distance between adjacent elements is d, the carrier frequency is f0, the fixed frequency offset is Δf, and the single frequency signal transmitted by the nth element is expressed as: S n (t)=exp(j2π(f0+nΔf)t),n=0,1,2,…,N-1 Step 2: Use the signal transmitted by the frequency diversity array in step 1 to scan the space and obtain the angle θ where the target is at time i i and distance R i , where R i is the distance from the far-field target to the first array element, and the distance from the nth array element to the far-field target is R n : R n =R i -nd sinθ i Step 3: extract the main lobe period that is useful to the target at time i. The main lobe period that needs to be extracted is: Where k is an integer and the propagation speed of electromagnetic waves is c; Step 4: Calculate the phase to obtain the directional beam at the target angle. For different times, calculate the angle θ that is only with the target. i The initial phases involved are: Step 5: Set the main lobe period T e As a pulse width, the signal of this period is repeatedly transmitted; Step 6: Receive echo data. Receive all signals of different frequencies transmitted by all array elements. Then the echo signal received by the nth array element is: Step 7: Perform phase compensation on the echo signal. i The echo signal after compensation is: Step 8: The phase difference caused by the array element spacing to the echo signal is expressed as: The echo signal obtained by compensating the phase difference between array elements is: Step 9, perform inverse discrete Fourier transform on the echo signal n in step 8 to obtain a one-dimensional range image, and further obtain an azimuth image through the relative motion between the target and the radar, and finally obtain a two-dimensional ISAR image of the target.

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