A spaceborne high-resolution frequency-stepped SAR image domain sub-band synthesis method

Abstract

The application belongs to the technical field of synthetic aperture radar, and discloses a kind of spaceborne high-resolution frequency step SAR imaging processing method.The implementation steps are as follows: step one, each sub-band is respectively imaged in time domain to obtain a range coarse resolution image;Step two, the range coarse resolution image is subjected to range Fourier transform to obtain a range spectrum;Step three, the range spectrum of each sub-band is subjected to zero padding, shifting, phase compensation and superposition to complete spectrum splicing;Step four, the spliced range spectrum is subjected to inverse range Fourier transform to obtain a full-resolution image.The application solves the problems of differences in Doppler history between spaceborne high-resolution frequency step SAR sub-bands and severe parameter space variation, and realizes high-quality and efficient imaging processing of spaceborne high-resolution frequency step SAR.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of synthetic aperture radar (SAR), and particularly relates to a spaceborne high-resolution frequency-stepped SAR image domain sub-band synthesis method. BACKGROUND

[0002] Spaceborne synthetic aperture radar (SAR) is a two-dimensional high-resolution microwave remote sensing imaging system. Compared with optical sensors, spaceborne SAR is not limited by sunlight and weather conditions, and can perform ground surface observation under complex weather conditions, thus playing an important role in battlefield reconnaissance, disaster monitoring, topographic mapping, resource exploration and the like.

[0003] Resolution is a core index of a spaceborne SAR system, and the higher the resolution is, the stronger the ground feature representation capability is. Super high resolution can make a SAR image obtain an effect close to that of an optical image, but at the same time, it will also increase the complexity of spaceborne SAR system design and signal processing. The range resolution of spaceborne SAR depends on the bandwidth of radar signals. In order to reduce the hardware pressure, it is necessary to adopt a frequency-stepped system, divide the wideband signal into sub-band signals of different carrier frequencies, and perform sub-band splicing through signal processing means.

[0004] The existing sub-band splicing method usually splices the sub-band signals in the signal domain before imaging. This method has a simple process, but is only suitable for lower resolution imaging, and is not suitable for spaceborne super high resolution frequency-stepped SAR imaging. In order to avoid energy leakage between sub-bands, the frequency-stepped SAR system usually transmits the sub-band signals in time division, resulting in an indiscernible difference in Doppler history of each sub-band. Compensation for the difference before imaging will greatly increase the algorithm complexity. In summary, how to realize efficient and high-precision sub-band synthesis of spaceborne high-resolution frequency-stepped SAR is a problem to be solved at present. SUMMARY

[0005] Therefore, the present application provides a spaceborne high-resolution frequency-stepped SAR image domain sub-band synthesis method to solve the problem of the difference in Doppler history between sub-bands of spaceborne super high resolution frequency-stepped SAR.

[0006] The present application is realized by the following technical solutions:

[0007] Step one, each sub-band is respectively imaged in time domain to obtain a range coarse resolution image.

[0008] Step two, the range coarse resolution image is subjected to range direction Fourier transform to obtain a range spectrum.

[0009] Step three, the range spectrum of each sub-band is subjected to zero padding, shifting, phase compensation and superposition to complete spectrum splicing.

[0010] Step four, inverse Fourier transform of the distance spectrum to get the full resolution image.

[0011] The beneficial effects of the present application are:

[0012] (1) The time domain algorithm is used for single sub-band imaging, which is simple and fully utilizes the accuracy of the time domain algorithm. In the single sub-band imaging process, fast time domain algorithm, hardware parallel acceleration and other means can be used to improve the time efficiency of the algorithm. Finally, high-quality and efficient single sub-band imaging can be realized.

[0013] (2) Image domain sub-band synthesis is used instead of traditional signal domain sub-band synthesis. The image domain sub-band synthesis method can accurately compensate the Doppler history of each sub-band during the imaging stage, thereby effectively solving the problem of Doppler history difference between sub-bands. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The flow chart of the present application is shown in the figure.

[0015] Figure 2 The imaging geometry of the present application is shown in the figure.

[0016] Figure 3 The point array distribution diagram of the simulation experiment of the present application is shown in the figure.

[0017] Figure 4 The two-dimensional contour plot of the imaging result of the simulation experiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0018] The present application will be described in detail below with reference to the accompanying drawings and examples.

[0019] The present application is a kind of spaceborne high-resolution frequency stepping SAR image domain sub-band synthesis method, and its flow chart is shown in the figure. Figure 1 The specific steps include:

[0020] Step one, each sub-band is respectively imaged in time domain to obtain a distance coarse resolution image.

[0021] Let the number of system sub-bands be N, then the echo signal of the nth sub-band after demodulation is:

[0022]

[0023] Where, τ is the distance time axis, τ0(t) = 2(R(t)-R0) / c, R(t) is the target instantaneous slant range, R0 is the starting sampling slant range, T is the pulse width, t is the time, and c is the speed of light. min min p ​​​d For the transmit time delay, f c (n) = f0+ (n-1) Af is the center frequency of the nth sub-band, Af is the sub-band interval, and c is the speed of light.

[0024] Subsequently, an imaging grid is established with the beam illumination center as the origin. The algorithm uses a rectangular coordinate grid, the geometry of which is shown in Figure 2 The grid X-axis represents the satellite flight direction, and the Y-axis represents the slant range direction. The distance grid interval is slightly smaller than the single sub-band distance resolution, and the azimuth grid interval is slightly smaller than the azimuth resolution.

[0025] Pulse compression, upsampling, and time domain alignment are performed on the echo signal shown in (1) to obtain the pulse compression signal shown in (2). It should be noted that the upsampling multiple affects the azimuth processing accuracy, so the interpolation multiple needs to reach more than 8 times to ensure image quality.

[0026]

[0027] Subsequently, the pulse compression signal is subjected to Doppler phase compensation, projected onto the image grid, and subjected to phase-preserving processing to obtain the distance coarse resolution image corresponding to each sub-band. In actual situations, the sampling rate and the starting sampling distance of the distance coarse resolution image are usually different from the echo, and its expression is shown in (3).

[0028]

[0029] wherein τ' is the image domain distance time axis, τ0' = 2 (R0-R min ') / c, R0 is the shortest slant range of the target, and R min ' is the image domain starting sampling distance. Since only single sub-band echoes are used, the image distance resolution is still low, and a distance high resolution image needs to be generated through sub-band splicing.

[0030] Step two, the distance coarse resolution image is subjected to distance Fourier transform to obtain the distance spectrum.

[0031] The distance coarse resolution image shown in (3) is subjected to distance Fourier transform to obtain the distance spectrum shown in (4).

[0032]

[0033] wherein B is the sub-band bandwidth, and f is the distance frequency axis.

[0034] Step three, the sub-band distance spectrum is subjected to zero padding, shifting, phase compensation, and superposition to complete the spectrum splicing.

[0035] To restore the relative position relationship between the sub-band spectrums, it is necessary to first zero-fill and shift each sub-band spectrum, and the zero-fill multiple is the same as the number of sub-bands. The shifted spectrum is shown in (5).

[0036]

[0037] Substitute Δf(n), f c (n) into and simplify to obtain

[0038]

[0039] The first phase term is linear phase, which determines the position of the target in the image; the second phase term is constant phase, which has no effect on the image; and the third phase term is the inter-sub-band jump phase, which will cause the appearance of grating lobes in the stitched image, and therefore, the phase needs to be compensated, and the compensation factor is shown in (7).

[0040]

[0041] After compensating the phase, the sub-band spectrums are superimposed, and the superimposed spectrum is shown in (8).

[0042]

[0043] Step four, inverse Fourier transform the stitched range spectrum in the range direction to obtain the full-resolution image as shown in (9).

[0044]

[0045] Example

[0046] The positive effects of the present application will be demonstrated below in combination with a specific embodiment.

[0047] Point array target simulation experiments are performed for the typical scene of spaceborne high-resolution frequency-stepped SAR, the point array is set as shown in (10), and the simulation parameters are shown in Table 1. Figure 3

[0048] Table 1 Simulation parameters of spaceborne high-resolution wide-width video SAR

[0049]

[0050] After obtaining the simulation echo, the proposed algorithm is used for imaging and the imaging results are evaluated. The evaluation results of the point array targets in the amplitude and width are shown in Table 2, and it can be seen that the resolution, peak side lobe ratio and integrated side lobe ratio of all targets meet the imaging requirements. Figure 4 The two-dimensional contour maps of the imaging results of each point target are shown. It can be seen that the range and azimuth imaging of each point target is very ideal. ​

[0051] Table 2 Dot target simulation evaluation results

[0052]

[0053] To sum up, the above is only the preferred embodiment of the present application, not for limiting the scope of protection of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A space-borne high-resolution frequency-stepped SAR imaging processing method, characterized in that, The method comprises: Step one, each sub-band respectively performs time domain imaging to obtain a distance coarse resolution image; Step two, performing distance Fourier transform on the distance coarse resolution image to obtain a distance spectrum; Step three, performing zero padding, shifting, phase compensation and superposition on the distance spectrum of each sub-band to complete spectrum splicing; To restore the relative position relationship between the sub-band spectrums, it is necessary to first zero-pad and shift each sub-band spectrum, and the zero-padding multiple is the same as the number of sub-bands; let the shift amount satisfy and the shifted spectrum is: ； Substituting , and simplifying gives ； Wherein, the first phase term is a linear phase, which determines the position of the target in the image; the second phase term is a constant phase, which has no effect on the image; the third phase term is a sub-band jump phase, which will cause grating lobes in the spliced image, therefore, the phase needs to be compensated, and the compensation factor is: ； After compensating the phase, the frequency spectra of each sub-band are superposed, and the superposed frequency spectrum is: ； Step four, performing inverse distance Fourier transform on the spliced distance spectrum to obtain a full resolution image.

2. The space-borne high-resolution frequency-stepped SAR imaging processing method according to claim 1, characterized in that In step one, the number of system sub-bands is N, and the echo signal after demodulation of the nth sub-band is: ； where τ is the distance to the time axis, , Rmin is the initial sampling slant range, Tp is the pulse width, and td is the transmission time delay, is the center frequency of the nth sub-band, is the inter-sub-band frequency interval, and c is the speed of light. An imaging grid is established with the beam irradiation center as the origin; the algorithm adopts a rectangular coordinate grid, and the geometric configuration is: ； Wherein, the X-axis of the grid represents the satellite flight direction, and the Y-axis represents the slant range direction; the distance grid interval is slightly smaller than the single sub-band distance resolution, and the azimuth grid interval is slightly smaller than the azimuth resolution; The pulse compression signal is subjected to Doppler phase compensation, projected onto the image grid, and subjected to phase-preserving processing to obtain the distance coarse resolution image corresponding to each sub-band; in actual situations, the sampling rate and the initial sampling distance of the distance coarse resolution image are usually different from the echo, and at this time, the expression is: ； wherein, is the image domain distance to time axis, , is the target minimum slant range, is the image domain start sampling distance; since only single sub-band echo is used, the image distance resolution is still low at this time, and it is necessary to generate a high-resolution distance image by sub-band splicing.

3. The space-borne high-resolution frequency-stepped SAR imaging processing method according to claim 1, characterized in that In step two, the distance Fourier transform is performed on the distance coarse resolution image to obtain a distance spectrum: ； wherein is a sub-band bandwidth, is a distance to the frequency axis.

4. The space-borne high-resolution frequency-stepped SAR imaging processing method according to claim 1, characterized in that In step four, the inverse distance Fourier transform is performed on the spliced distance spectrum to obtain a full resolution image: 。

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