Single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold

Abstract

The invention discloses a single-bit interferometric synthetic aperture radar data processing method based on a joint channel variable threshold, and relates to the technical field of signal processing. First, a quantization threshold is set to separate noise and useful signals in a frequency spectrum; then a bandpass filter is designed to suppress the quantization noise; finally, according to the strong correlation of adjacent pixel points of a radar image and the weak correlation of residual quantization noise, the pixel energy is coherently accumulated to improve the signal-to-noise ratio; the single-bit interferometric synthetic aperture radar data processing method based on a joint channel variable threshold proposed by the invention can improve the estimation accuracy of the interference phase, thereby obtaining a high-quality digital elevation image of a ground object.

Description

Technical Field

[0001] The present invention relates to the technical field of signal processing, and in particular to a single-bit interferometric synthetic aperture radar data processing method based on a joint channel variable threshold. Background Art

[0002] Interferometry Synthetic Aperture Radar (InSAR) plays an important role in the field of remote sensing due to its ability to obtain terrain height information. In recent years, with the rapid development of the low-altitude economy, deploying radars on small platforms to perform remote sensing tasks has become a hot research direction. Therefore, combining InSAR with small platforms such as drones has broad application prospects. However, these platforms provide very limited resources for radar systems, which poses a major challenge to the efficient acquisition, storage, and transmission of echo data. In particular, InSAR needs to transmit broadband waveforms and observe ground scenes for a long time, which will generate a huge amount of echo data in the process. In addition, the presence of two interferometric channels doubles the amount of data, which further increases the difficulty of integrating InSAR systems with small radar platforms. In 1997, G. Fornaro et al. analyzed the feasibility of single-bit quantized InSAR in G. Fornaro, V. Pascazio and G. Schirinzi, “SyntheticAperture Radar Interferometry Using One Bit Coded Raw and Reference Signals,” IEEE Transactions on Geoscience and Remote Sensing, vol. 35, no. 5, pp. 1245-1253, Sep. 1997, and proved that although single-bit quantization leads to high distortion of data, the interferometric phase still retains enough information to extract digital elevation models (DEMs). Nevertheless, quantization noise still has an adverse effect on the accuracy of DEM recovery. To solve this problem, they studied the expression of these quantization noises and proposed a one-bit quantization strategy based on oversampling. The core idea is to reduce the energy aliasing between noise and echo signals by broadening the spectrum of the signal. However, since the quantization noise has the same frequency center as the echo signal, the main energy of the noise cannot be suppressed.To address this problem, G. Fornaro et al. further proposed a center frequency offset strategy in "Synthetic Aperture Radar Interferometry Using One BitCoded Raw and Reference Signals," IEEE Transactions on Geoscience and RemoteSensing, vol. 35, no. 5, pp. 1245-1253, Sep. 1997, which further suppressed quantization noise by distinguishing target signal components and noise in the frequency band. However, this strategy requires the radar system to achieve at least a 4-fold oversampling rate, which is a significant challenge for radar platforms with limited resources. Therefore, it is urgent to explore ways to simplify the echo data processing process in order to achieve the successful deployment of InSAR on resource-constrained platforms. Summary of the invention

[0003] The purpose of the present invention is to provide a single-bit interferometric synthetic aperture radar data processing method based on a joint channel variable threshold, so as to solve the problem that the existing single-bit InSAR technology in the prior art only performs an oversampling operation on the echo data, but does not further process the residual quantization noise in the subsequent frequency band.

[0004] To achieve the above object, the present invention provides a single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold, comprising the following steps:

[0005] Step 1: Set the quantization threshold to separate the noise and the useful signal in the spectrum;

[0006] Step 2: Design a bandpass filter to suppress the quantization noise in step 1;

[0007] Step 3: Based on the strong correlation between adjacent pixels of the radar image and the weak correlation of the residual quantization noise, the pixel energy is coherently accumulated to improve the signal-to-noise ratio.

[0008] Preferably, in step 1, the quantization threshold is set to separate the noise and the useful signal in the spectrum as follows:

[0009] S11. Set the quantization threshold. The expression is as follows:

[0010] ;

[0011] S12. Calculate the SAR echo signal at the radar receiver. The expression is as follows:

[0012] ;

[0013] S13. Obtain a quantized signal according to the SAR echo signal and the quantization threshold. The expression is as follows:

[0014] ;

[0015] S14, perform frequency domain analysis on the signal quantized in S13, and separate the noise and the useful signal in the spectrum. The expression is as follows:

[0016] ;

[0017] In the formula, Represents the amplitude of the echo signal, represents the exponential function, is an imaginary unit, is the phase of the echo signal, which is a function of the fast time variable, represents the magnitude of the quantization threshold, represents the frequency of the threshold, and They represent the radar fast time and slow time respectively. represents the symbolic function, and denote the extraction operators of the real and imaginary parts of the signal, respectively. represents the quantized time domain signal, represents the Fourier transform operator, It is about the series variable and The constant, is the frequency variable, is the original SAR echo signal spectrum.

[0018] Preferably, the bandpass filter designed in step 2 is specifically as follows: the passband width of the bandpass filter is larger than the bandwidth of the signal, the passband width of the bandpass filter is 1.2 times to 1.4 times the signal bandwidth, and the center frequency of the bandpass filter is aligned with the center frequency of the signal component of the baseband.

[0019] Preferably, the expression for suppressing the quantization noise by the designed bandpass filter in step 2 is as follows:

[0020] ;

[0021] In the formula, represents the signal spectrum after passing through the bandpass filter (i.e. after quantization noise suppression), is the transfer function of the bandpass filter.

[0022] Preferably, in step 3, the specific process of coherently accumulating pixel energy according to the strong correlation of adjacent pixels of the radar image and the weak correlation of the residual quantization noise is as follows: Perform inverse Fourier transform to obtain the filtered time domain signal , respectively, in the two receiving channels of the interferometric synthetic aperture radar to filter the filtered signal Two-dimensional pulse compression imaging is performed to obtain two radar images. According to the strong correlation of adjacent pixels of the radar image and the weak correlation of residual quantization noise, pixel energy coherent accumulation is performed on adjacent radar image pixels in the form of digital beamforming.

[0023] Preferably, the process of performing coherent accumulation of pixel energy for adjacent radar image pixels in the form of digital beamforming is as follows:

[0024] Pick a pixel at random from a radar image , extract the surrounding Pixels ( The value is usually 3 to 7), and the weighted vector is constructed by using the correlation between these pixels. To achieve energy coherent superposition, the expression is as follows:

[0025] ;

[0026] In the formula, It is the weighting vector used in digital beamforming. The weighting vector can be flexibly constructed in an adaptive or non-adaptive manner according to actual data or radar system parameters. The construction principle is generally the maximum signal-to-noise ratio of the pixel. After the above-mentioned energy coherent accumulation, the signal-to-noise ratio of the image is effectively improved, so the extracted pixel phase is closer to the real echo signal phase.

[0027] Preferably, the calculation expression of the signal-to-noise ratio SNR is as follows:

[0028] ;

[0029] In the formula, and Represent the 2-norm reflecting the signal and noise energy respectively.

[0030] Therefore, the present invention adopts the above-mentioned single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold, which has the following beneficial effects:

[0031] (1) The time-varying quantization threshold can effectively separate quantization noise from useful signals, alleviate image distortion and suppress quantization noise, thereby improving the accuracy of elevation images;

[0032] (2) By designing a frequency bandpass filter, the quantization noise can be effectively suppressed and the interference to subsequent processing can be reduced;

[0033] (3) By coherently accumulating pixel energy, the signal-to-noise ratio and phase estimation accuracy of the radar image are improved, thereby improving the quality of the final digital elevation image.

[0034] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is an overall flow chart of the single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold of the present invention;

[0036] Figure 2 This is a digital elevation model error analysis diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] See also Figure 1-Figure 2 , a single-bit interferometric synthetic aperture radar data processing method based on a joint channel variable threshold comprises the following steps:

[0039] Step 1: Set the quantization threshold to separate the noise and the useful signal in the spectrum. The specific process is as follows:

[0040] S11. Set the quantization threshold. The expression is as follows:

[0041] ;

[0042] S12. Calculate the SAR echo signal at the radar receiver. The expression is as follows:

[0043] ;

[0044] S13. Obtain a quantized signal according to the SAR echo signal and the quantization threshold. The expression is as follows:

[0045] ;

[0046] S14, perform frequency domain analysis on the signal quantized in S13, and separate the noise and the useful signal in the spectrum. The expression is as follows:

[0047] ;

[0048] In the formula, Represents the amplitude of the echo signal, represents the exponential function, is an imaginary unit, is the phase of the echo signal, which is a function of the fast time variable, represents the magnitude of the quantization threshold, represents the frequency of the threshold, and Respectively represent the radar fast time and slow time, represents the symbolic function, and denote the extraction operators of the real and imaginary parts of the signal, respectively. represents the quantized time domain signal, represents the Fourier transform operator, It is about the series variable and The constant, is the frequency variable, is the original SAR echo signal It can be seen that after using the quantization threshold designed by the present invention in the quantization process, the quantization noise is moved in the spectrum. (Hz), while the useful signal ( The spectrum of the signal component) has not changed. Therefore, the quantization noise and the useful signal are effectively separated in the spectrum.

[0049] Step 2, design a bandpass filter to suppress the quantization noise in step 1; in step 1, through the quantization threshold designed by the present invention, a large part of the noise energy is separated from the signal in the spectrum, and at this time a bandpass filter can be designed to suppress the quantization noise; wherein, the designed bandpass filter is specifically: the passband width of the bandpass filter is larger than the bandwidth of the signal, the passband width of the bandpass filter is 1.2 times to 1.4 times the signal bandwidth, and the center frequency of the bandpass filter is aligned with the center frequency of the signal component of the baseband; the expression for suppressing the quantization noise by the designed bandpass filter is as follows:

[0050] ;

[0051] In the formula, represents the signal spectrum after passing through the bandpass filter (i.e. after quantization noise suppression), is the transfer function of the bandpass filter.

[0052] Step 3: Based on the strong correlation of adjacent pixels of the radar image and the weak correlation of the residual quantization noise, the pixel energy is coherently accumulated to improve the signal-to-noise ratio. The specific process is: Perform inverse Fourier transform to obtain the filtered time domain signal , respectively, in the two receiving channels of the interferometric synthetic aperture radar to filter the filtered signal Two-dimensional pulse compression imaging is performed to obtain two radar images. According to the strong correlation of adjacent pixels of the radar image and the weak correlation of the residual quantization noise, pixel energy coherent accumulation is performed on adjacent radar image pixels in the form of digital beamforming. The process of performing pixel energy coherent accumulation on adjacent radar image pixels in the form of digital beamforming is as follows:

[0053] Pick a pixel at random from a radar image , extract the surrounding Pixels ( The value is usually 3 to 7), and the weighted vector is constructed by using the correlation between these pixels. To achieve energy coherent superposition, the expression is as follows:

[0054] ;

[0055] In the formula, It is the weighting vector used in digital beamforming. The weighting vector can be flexibly constructed in an adaptive or non-adaptive manner according to actual data or radar system parameters. The construction principle is generally the maximum signal-to-noise ratio of the pixel. After the above-mentioned energy coherent accumulation, the signal-to-noise ratio of the image is effectively improved, so the extracted pixel phase is closer to the real echo signal phase.

[0056] The calculation expression of signal-to-noise ratio SNR is as follows:

[0057] ;

[0058] In the formula, and Represent the 2-norm reflecting the signal and noise energy respectively.

[0059] The present invention makes an error analysis on the proposed method and the existing method, such as Figure 2As shown in the figure, the horizontal axis of the figure represents the radar distance unit, and the vertical axis represents the difference between the actual measured elevation value and the ideal elevation at a certain distance unit, that is, the elevation error; the elevation error curve of the existing method (no threshold method) is represented by "o", and the error curve of the proposed method is represented by "*". It can be clearly seen that the error of the proposed method is smaller than that of the existing no threshold method. In addition, the statistical average of the error points in the figure shows that the average elevation difference between the DEM obtained by the present invention and the DEM obtained by high-precision quantization is 1.175mm, which is better than the 3.765mm of the existing method. Figure 1 The dashed line and the mark at the dashed line are shown. It can be calculated that the relative error of the proposed method is reduced by (3.765-1.175) / 3.765×100%=68.79% compared with the existing method, which proves the effectiveness of the method proposed in the present invention.

[0060] Therefore, the present invention adopts the above-mentioned single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold, utilizes the special signal structure of quantization noise, and sets a quantization threshold that varies with SAR distance at the quantization end from the perspective of channel combination. The time-varying characteristics of the threshold can not only effectively alleviate the radar image distortion problem caused by single-bit quantization, but also effectively suppress the quantization noise introduced in the single-bit quantization process, thereby improving the accuracy of the terrain elevation image; at the same time, the present invention fully considers the advantage that single-bit quantization can simplify the radar sampling process, and proposes a radar image pixel energy accumulation strategy, thereby improving the signal-to-noise ratio of the radar image, thereby improving the estimation accuracy of the interference phase, and finally obtaining a high-quality terrain digital elevation image.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold, characterized in that: The following steps are involved: Step 1: Set the quantization threshold to separate the noise and the useful signal in the spectrum; Step 2: Design a bandpass filter to suppress the quantization noise in step 1; Step 3: Based on the strong correlation between adjacent pixels of the radar image and the weak correlation between the residual quantization noise, the pixel energy is coherently accumulated to improve the signal-to-noise ratio; In step 1, the quantization threshold is set to separate the noise and the useful signal in the spectrum as follows: S11. Set the quantization threshold. The expression is as follows: ； S12. Calculate the SAR echo signal at the radar receiver. The expression is as follows: ； S13. Obtain a quantized signal according to the SAR echo signal and the quantization threshold. The expression is as follows: ； S14, perform frequency domain analysis on the signal quantized in S13, and separate the noise and the useful signal in the spectrum. The expression is as follows: ； In the formula, Represents the amplitude of the echo signal, represents the exponential function, is an imaginary unit, is the phase of the echo signal, represents the magnitude of the quantization threshold, represents the frequency of the threshold, and They represent the radar fast time and slow time respectively. represents the symbolic function, and denote the extraction operators of the real and imaginary parts of the signal, respectively. represents the quantized time domain signal, represents the Fourier transform operator, It is about the series variable and The constant, is the frequency variable, is the original SAR echo signal Spectrum of The bandpass filter designed in step 2 is specifically as follows: the passband width of the bandpass filter is larger than the bandwidth of the signal, the passband width of the bandpass filter is 1.2 to 1.4 times the signal bandwidth, and the center frequency of the bandpass filter is aligned with the center frequency of the signal component of the baseband.

2. The single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold according to claim 1 is characterized in that: The expression for suppressing the quantization noise by the designed bandpass filter in step 2 is as follows: ； In the formula, represents the signal spectrum after passing through the bandpass filter, is the transfer function of the bandpass filter.

3. The single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold according to claim 2 is characterized in that: In step 3, the specific process of coherently accumulating pixel energy according to the strong correlation of adjacent pixels of the radar image and the weak correlation of the residual quantization noise is as follows: Perform inverse Fourier transform to obtain the filtered time domain signal , respectively, in the two receiving channels of the interferometric synthetic aperture radar to filter the filtered signal Two-dimensional pulse compression imaging is performed to obtain two radar images. According to the strong correlation of adjacent pixels of the radar image and the weak correlation of residual quantization noise, pixel energy coherent accumulation is performed on adjacent radar image pixels in the form of digital beamforming.

4. The single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold according to claim 3 is characterized in that: The process of coherently accumulating pixel energy for adjacent radar image pixels in the form of digital beamforming is as follows: Pick a pixel at random from a radar image , extract the surrounding Pixels , using the correlation between these pixels, construct a weighted vector To achieve energy coherent superposition, the expression is as follows: ； In the formula, is the weight vector used in digital beamforming.

5. The single-bit interferometric synthetic aperture radar data processing method based on joint channel variable threshold according to claim 4 is characterized in that: The calculation expression of signal-to-noise ratio SNR is as follows: ； In the formula, and Represent the 2-norm reflecting the signal and noise energy respectively.

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