Azimuth Decoupling Method for Ionospheric Scintillation Phase Error in Low-band Spaceborne SAR

Abstract

The present invention discloses an azimuth decoupling method for ionospheric scintillation phase error of low-band satellite-borne SAR, comprising: S1, constructing a phase screen projection function; S2, multiplying the phase screen projection function with the azimuth frequency domain of the satellite-borne SAR image to obtain azimuth re-frequency modulation data; S3, performing inverse Fourier transform on the azimuth re-frequency modulation data to obtain azimuth re-frequency modulation time domain data; S4, constructing a phase screen projection de-skewing function; S5, multiplying the azimuth re-frequency modulation time domain data with the phase screen projection function to obtain azimuth de-skewing phase screen distance compression domain data. The present invention can obtain phase screen distance compression domain data corresponding to the scintillation phase error one by one, solves the coupling problem of the scintillation phase error and the azimuth data, avoids tedious azimuth splicing, and improves processing efficiency.

Description

Technical Field

[0001] The present invention relates to the cross-technical field of space-based microwave remote sensing and earth space weather, and more specifically, to an azimuth decoupling method for ionospheric scintillation phase error of low-band spaceborne SAR. Background Art

[0002] Low-band spaceborne SAR operating in the L-band and P-band has great application potential and value in global biomass inversion, carbon-water cycle monitoring, and concealed target reconnaissance, and has become one of the important development directions of spaceborne SAR technology. However, due to its low operating frequency, low-band spaceborne SAR is extremely susceptible to the influence of the ionosphere, among which the ionospheric scintillation effect can cause serious azimuth defocusing of low-band spaceborne SAR. Studies have shown that under strong scintillation conditions, the azimuth resolution of P-band SAR can deteriorate by hundreds of meters. So far, there is no P-band SAR system in orbit. The reason is that the ionospheric scintillation effect is one of the key bottlenecks hindering its development.

[0003] In view of the space-varying characteristics of ionospheric SPE that are staggered and overlapped along the azimuth direction, the applicant previously proposed to use azimuth splicing to deal with the azimuth space-varying problem of SPE. Although good results have been achieved, its implementation process is complicated and the processing efficiency is low. Summary of the invention

[0004] The purpose of the present invention is to provide a method for azimuthally decoupling the ionospheric scintillation phase error of a low-band spaceborne SAR, so as to overcome the defects existing in the prior art.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The method for azimuth decoupling of ionospheric scintillation phase error of low-band spaceborne SAR includes the following steps:

[0007] S1, construct phase screen projection function;

[0008] S2, multiplying the phase screen projection function with the azimuth frequency domain of the spaceborne SAR image to obtain azimuth re-frequency modulation data;

[0009] S3, performing inverse Fourier transform on the azimuth re-frequency modulation data to obtain azimuth re-frequency modulation time domain data;

[0010] S4, constructing a phase screen projection de-skewing function;

[0011] S5. Multiply the azimuth re-modulated time domain data with the phase screen projection function to obtain the phase screen distance compression domain data after azimuth de-skewing.

[0012] Furthermore, the formula of the phase screen projection function in step S1 is:

[0013]

[0014] Among them, K a is the Doppler modulation frequency, α i =H p / H s is the mapping coefficient, and α i <1,H p is the ionosphere PS height, H s is the satellite altitude, i represents the coefficient corresponding to the ionosphere altitude, and j is a complex number.

[0015] Furthermore, the calculation method of the azimuth frequency domain of the spaceborne SAR image is:

[0016] Input the single-view complex image of the spaceborne P-band SAR affected by the ionospheric scintillation effect. Assuming it works in strip mode, the azimuth frequency domain of the spaceborne SAR is:

[0017]

[0018] Among them, f a is the Doppler frequency, PRF is the pulse repetition frequency, σ n is the backscatter coefficient of the nth target in the scene, is a standard rectangular window function, η n is the azimuth center moment corresponding to the nth target, Φ0 is the constant phase, ΔΦ i,n SPE experienced for the nth target.

[0019] Furthermore, the frequency domain expression of the azimuth re-modulation data in step S2 is:

[0020]

[0021] Furthermore, the expression of the azimuth re-frequency modulation time domain data is:

[0022]

[0023] Among them, η is the azimuth slow time, T a is the synthetic aperture time, K′ a =K a / α i is the Doppler modulation frequency after mapping, α i T a is the time width of the azimuth linear frequency modulation signal, and the Doppler bandwidth B a =α i T a K′ a Same as original bandwidth.

[0024] Furthermore, the expression of the phase screen projection de-skewing function is:

[0025] H deramp (η) = exp(-jπK′ a η 2 ).

[0026] Furthermore, the expression of the phase screen distance compression domain data after azimuth de-slanting is:

[0027]

[0028] Compared with the prior art, the advantages of the present invention are: the present invention can obtain phase screen distance compression domain data that corresponds one-to-one to the scintillation phase error (SPE), solves the coupling problem between the scintillation phase error and the azimuth data, avoids tedious azimuth splicing, and improves processing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0030] Figure 1 The present invention is a flow chart of the azimuth decoupling method of the ionospheric scintillation phase error of the low-band spaceborne SAR.

[0031] Figure 2 It is a schematic diagram of a dot matrix scene in the simulation verification experiment of the present invention.

[0032] Figure 3 These are the imaging results of three point targets using P-band spaceborne SAR under the influence of the scintillation effect of the present invention.

[0033] Figure 4 The figure is a comparison between the distance compression domain data of the present invention and the distance compression domain data of the PS mapping domain.

[0034] Figure 5 The scintillation phase error estimation results of the traditional method and the method of the present invention are shown in FIG.

[0035] Figure 6 These are the imaging results of three point targets using P-band spaceborne SAR after scintillation effect correction in the present invention. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0037] The idea of ​​the present invention is to propose a PSP (Phase Screen Projection) method, which realizes the decoupling of the scintillation phase error and the azimuth data by constructing an azimuth inverse matched filter and a de-skewing filter corresponding to an equivalent ionosphere PS (Phase Screen), thereby avoiding tedious azimuth splicing processing and improving processing efficiency.

[0038] See also Figure 1 As shown, this embodiment discloses a method for azimuth decoupling of ionospheric scintillation phase error of low-band spaceborne SAR, comprising the following steps:

[0039] Step S1, constructing a phase screen projection function:

[0040]

[0041] Among them, K a is the Doppler modulation frequency, α i =H p / H s is the mapping coefficient, and α i <1,H p is the ionosphere PS height, H s is the satellite altitude, i represents the coefficient corresponding to the ionosphere altitude, and j is a complex number.

[0042] Step S2: multiply the phase screen projection function by the azimuth frequency domain of the spaceborne SAR image to obtain azimuth re-frequency modulation data. The frequency domain expression of the azimuth re-frequency modulation data is:

[0043]

[0044] The calculation method of the azimuth frequency domain of the spaceborne SAR image is as follows: input the spaceborne P-band SAR single-view complex image affected by the ionospheric scintillation effect. Assuming that it works in strip mode, the azimuth frequency domain of the spaceborne SAR is:

[0045]

[0046] Among them, f a is the Doppler frequency, PRF is the pulse repetition frequency, σ n is the backscatter coefficient of the nth target in the scene, is a standard rectangular window function, η n is the azimuth center moment corresponding to the nth target, Φ0 is the constant phase, ΔΦi,n SPE experienced for the nth target.

[0047] Step S3, perform inverse Fourier transform on the azimuth re-frequency modulation data to obtain azimuth re-frequency modulation time domain data. The expression of the azimuth re-frequency modulation time domain data is:

[0048]

[0049] Among them, η is the azimuth slow time, T a is the synthetic aperture time, K′ a =K a / α i is the Doppler modulation frequency after mapping, α i T a is the time width of the azimuth linear frequency modulation signal, and the Doppler bandwidth B a =α i T a K′ a Same as original bandwidth.

[0050] Step S4: construct a phase screen projection de-skewing function, the expression of which is as follows:

[0051] H deramp (η) = exp(-jπK a η 2 )

[0052] Step S5, multiply the azimuth re-modulated time domain data by the phase screen projection function to obtain the phase screen distance compression domain data after azimuth de-skewing. The expression of the phase screen distance compression domain data after azimuth de-skewing is:

[0053]

[0054] Through the implementation of the present invention, phase screen distance compression domain data corresponding to the scintillation phase error can be obtained, solving the coupling problem between the scintillation phase error and the azimuth data. The present invention avoids tedious azimuth splicing and improves processing efficiency.

[0055] In order to verify the technical effect of the present invention, the phase screen projection (PSP) is combined with the traditional PCA method, namely PSP-PCA. If PSP-PCA can accurately estimate and effectively compensate for the azimuth space-variant scintillation phase error (SPE) and realize the scintillation effect correction of space-borne SAR, then it is proved that the PSP method is effective.

[0056] Figure 2 The schematic diagram of the dot matrix scene in the simulation verification experiment. Set up a dot matrix scene containing three point targets, such as Figure 2 As shown, the azimuth and range directions of two adjacent point targets are 2 kilometers apart.

[0057] Figure 3 The following are the imaging results of three point targets of P-band spaceborne SAR under the influence of scintillation effect. The simulated P-band SAR satellite operates at an orbital altitude of 700km, with a ground incidence angle of 30 degrees, an operating frequency of 500MHz, a system bandwidth of 56MHz, a sampling rate of 60MHz, a Doppler bandwidth of 1223Hz, and a ground range resolution and azimuth resolution better than 5m. Under moderate scintillation conditions, the influence of ionospheric scintillation effect on azimuth imaging of spaceborne P-band SAR is simulated. The PS altitude is set to 350km. Figure 3 Figures a, b, and c show the images of three point targets A, B, and C affected by the ionospheric scintillation effect, and the azimuth imaging performance is seriously deteriorated.

[0058] Table 1 shows the azimuth profile indicators of P-band spaceborne SAR under the influence of scintillation effect. Affected by moderate scintillation, although the main lobe performance of point targets in azimuth remains good, its side lobes are significantly enhanced, causing the Peak-to-Sidelobe Ratio (PSLR) to deteriorate to -4.20dB, -5.63dB, and -5.97dB, respectively, and the Integrated Sidelobe Ratio (ISLR) to deteriorate to -0.52dB, -0.25dB, and -0.71dB, respectively, and the peak energy is lost by about 2dB.

[0059] Table 1

[0060] Resolution(m) Broadening factor PSLR(dB) ISLR(dB) Peak power loss (dB) Point target A 4.76 0.96 -4.20 -0.52 1.87 Point target B 4.78 0.97 -5.63 -0.25 2.07 Click Target C 4.83 0.98 -5.97 -0.71 2.17

[0061] Figure 4 This is a comparison of the range compression domain data and the phase screen (PS) mapping domain range compression domain data. The P-band spaceborne SAR image data is mapped to the satellite orbit altitude and the phase screen altitude to obtain the range compression domain and phase screen projection mapping domain data respectively. The range compression domain data is the data after range compression and range migration correction and before azimuth matching filtering. The azimuth data width of the range compression domain data is also the synthetic aperture length, which is about 43km, see Figure 4 (a) The data width of the phase screen projection mapping domain is the mapping of the synthetic aperture length to the phase screen height. Since the simulated phase screen height is 350 km, α i = 0.5, the data width of the phase screen projection mapping domain is half of the synthetic aperture length, about 21.5 km, see Figure 4 (b) The results show that the phase screen projection achieves the decoupling of the scintillation phase error and the azimuth data at the phase screen height.

[0062] Figure 5The scintillation phase error (SPE) estimation results of the traditional PCA and PSP-PCA methods are shown in Figure 2. The traditional PGA, PCA and the proposed PSP-PCA methods are applied to P-band spaceborne SAR images affected by the scintillation effect. Figure 5 In the scintillation phase error estimation result of (a), because the traditional PCA does not take into account the azimuth and spatial variation characteristics of the scintillation phase error, the scintillation phase error estimation result is far from the scintillation phase error injected by the original simulation, which means that the traditional PCA is ineffective in correcting the ionospheric scintillation effect. Figure 5 (b) The residual phase error of PSP-PCA and its linear component and high-frequency component are given. It can be seen that the difference between the estimated flicker phase error result and the original flicker phase error is a linear phase and a high-frequency phase error component. Since the linear phase component does not affect the image focusing performance, it can be ignored, while the high-frequency phase error will cause random sidelobe lifting, and the standard deviation of the residual high-frequency phase error is only 8.49°. The above estimation results show that compared with the traditional PCA method, PSP-PCA obtains reliable flicker phase error estimation results, which indirectly proves that the phase screen projection (PSP) realizes the decoupling of the flicker phase error (SPE) and the azimuth data.

[0063] Figure 6 The P-band spaceborne SAR imaging results of three point targets after scintillation effect correction. The SPE estimated by the PSP-PCA method is further used for error compensation. The P-band spaceborne SAR point target images after scintillation effect correction are shown in Figure 6. Except for a few weak random sidelobes, the three point target images are very close to the ideal two-dimensional sinc function shape.

[0064] Table 2 shows the azimuth profile indicators of the P-band spaceborne SAR after scintillation effect correction. It can be seen that the sidelobe performance has been greatly improved. The PSLR deterioration after correction is less than 0.3dB, the ISLR deterioration is less than 1.5dB, and the peak power no longer suffers from the loss caused by the ionospheric scintillation effect. The correction results once again verify the effectiveness of PSP-PCA, and PSP decoupling is a key step in realizing the scintillation effect correction of spaceborne SAR.

[0065] Table 2

[0066] Resolution(m) Broadening factor PSLR(dB) ISLR(dB) Peak power loss (dB) Point target A 4.99 1.01 -13.05 -8.86 -0.14 Point target B 4.94 1.00 -13.29 -8.83 0.01 Click Target C 4.98 1.01 -13.08 -8.67 -0.15

[0067] The present invention can obtain phase screen distance compression domain data corresponding to the scintillation phase error (SPE) one by one, solves the coupling problem between the scintillation phase error and the azimuth data, avoids tedious azimuth splicing, and improves processing efficiency.

[0068] Although the embodiments of the present invention are described in conjunction with the accompanying drawings, the patent owner may make various variations or modifications within the scope of the appended claims. As long as they do not exceed the protection scope described in the claims of the present invention, they should be within the protection scope of the present invention.

Claims

1. A method for azimuth decoupling of ionospheric scintillation phase error of low-band spaceborne SAR, characterized in that: The following steps are involved: S1, construct phase screen projection function; S2, multiplying the phase screen projection function with the azimuth frequency domain of the spaceborne SAR image to obtain azimuth re-frequency modulation data; S3, performing inverse Fourier transform on the azimuth re-frequency modulation data to obtain azimuth re-frequency modulation time domain data; S4, constructing a phase screen projection de-skewing function; S5. Multiply the azimuth re-modulated time domain data by the phase screen projection de-skewing function to obtain the phase screen distance compression domain data after azimuth de-skewing.

2. The azimuth decoupling method for low-band spaceborne SAR ionospheric scintillation phase error according to claim 1, characterized in that: The formula of the phase screen projection function in step S1 is: Among them, K a is the Doppler modulation frequency, α i =H p / H s is the mapping coefficient, and α i <1,H p is the ionosphere PS height, H s is the satellite altitude, i represents the coefficient corresponding to the ionosphere altitude, and j is a complex number.

3. The azimuth decoupling method for low-band spaceborne SAR ionospheric scintillation phase error according to claim 1, characterized in that: The calculation method of the azimuth frequency domain of the spaceborne SAR image is: Input the single-view complex image of the spaceborne P-band SAR affected by the ionospheric scintillation effect. Assuming it works in strip mode, the azimuth frequency domain of the spaceborne SAR is: Among them, f a is the Doppler frequency, PRF is the pulse repetition frequency, σ n is the backscatter coefficient of the nth target in the scene, is a standard rectangular window function, η n is the azimuth center moment corresponding to the nth target, Φ0 is the constant phase, ΔΦ i,n SPE experienced for the nth target.

4. The azimuth decoupling method for low-band spaceborne SAR ionospheric scintillation phase error according to claim 1, characterized in that: The frequency domain expression of the azimuth re-modulation data in step S2 is:

5. The azimuth decoupling method for low-band spaceborne SAR ionospheric scintillation phase error according to claim 1, characterized in that: The expression of the azimuth re-frequency modulation time domain data is: Among them, η is the azimuth slow time, T a is the synthetic aperture time, K a ′=K a / α i is the Doppler modulation frequency after mapping, α i T a is the time width of the azimuth linear frequency modulation signal, and the Doppler bandwidth B a =α i T a K a ′ is the same as the original bandwidth.

6. The azimuth decoupling method for low-band spaceborne SAR ionospheric scintillation phase error according to claim 1, characterized in that: The expression of the phase screen projection de-skewing function is: H deramp (η)=exp(-jπK a 'or 2 )。 7. The azimuth decoupling method for low-band spaceborne SAR ionospheric scintillation phase error according to claim 1, characterized in that: The expression of the phase screen distance compression domain data after de-slanting in the azimuth is:

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