A method for predistortion processing of ultra-wideband spaceborne synthetic aperture radar

Abstract

This invention proposes a predistortion processing method for ultra-wideband spaceborne synthetic aperture radar (UWAR), solving the problem of severe degradation of the amplitude and phase characteristics of linear frequency modulated (LFM) signals within the signal band due to system errors. The method includes: constructing an ideal UWAR LFM signal based on actual system parameters; constructing a calibration closed-loop circuit within the UWAR system to obtain the actual UWAR LFM signal after distortion; calculating the amplitude and phase difference between the actual UWAR LFM signal and the ideal UWAR LFM signal to obtain the distortion error curve; fitting the error curve using a piecewise polynomial fitting method with the least squares approach; stitching together the amplitude and phase errors between adjacent fitted piecewise error curves using a sliding window weighted average method; constructing a time-domain predistortion compensation signal based on the extracted amplitude and phase errors; and inputting the predistortion compensation signal into the UWAR transmitter for compensation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of signal processing, and particularly relates to a pre-distortion processing method for an ultra-wideband space-borne synthetic aperture radar. BACKGROUND

[0002] Ultra-high resolution imaging is a hotspot of research and application of synthetic aperture radar, and higher resolution is more conducive to description of details of ground objects, and has great application potential in military reconnaissance, geological exploration and the like. High resolution in the azimuth direction depends on synthetic aperture time, and high resolution in the range direction depends on a large bandwidth linear frequency modulation signal, so the ultra-wideband linear frequency modulation signal is a key to realization of high resolution in the range direction of a SAR system. However, with increase of the signal bandwidth of the SAR system, nonlinear characteristics of analog components in the whole transceiver link of the SAR system will cause serious deterioration of amplitude-frequency characteristics and phase-frequency characteristics of the linear frequency modulation signal in the band, and the signal in the band has high-frequency fluctuation of amplitude and phase error, thereby causing distortion of a main lobe of the signal after pulse compression, lifting of an integrated sidelobe ratio, and decline of image quality after two-dimensional focusing processing.

[0003] Therefore, before transmission of the ultra-wideband linear frequency modulation signal of the space-borne SAR system, compensation of high-frequency fluctuation of amplitude and phase error in the transceiver link is necessary for realization of range direction high resolution imaging of the ultra-wideband space-borne SAR system. For ultra-high resolution space-borne SAR imaging, a pre-distortion processing method for the ultra-wideband synthetic aperture radar needs to be researched, the high-frequency fluctuation of amplitude and phase error of the ultra-wideband linear frequency modulation signal in the band caused by the non-ideal system needs to be accurately extracted on the basis of an internal calibration loop, and compensation is performed before signal transmission, so as to solve the problem of deterioration of amplitude and phase characteristics of the linear frequency modulation signal in the band caused by system error, and improve image quality of the ultra-high resolution space-borne SAR. SUMMARY

[0004] The application provides a pre-distortion processing method for an ultra-wideband space-borne synthetic aperture radar, and solves the problem of serious deterioration of amplitude and phase characteristics of a linear frequency modulation signal in a band caused by system error in a synthetic aperture radar system under a large bandwidth condition.

[0005] The application is implemented by the following technical scheme.

[0006] A pre-distortion processing method for an ultra-wideband space-borne synthetic aperture radar comprises the following steps.

[0007] Step 1: constructing an ideal ultra-wideband linear frequency modulation signal according to actual system parameters;

[0008] Step 2: constructing an internal calibration closed loop of the ultra-wideband synthetic aperture radar system, and obtaining an actual ultra-wideband linear frequency modulation signal after distortion;

[0009] Step 3: Calculate the amplitude and phase differences between the actual ultra-wideband linear frequency modulation signal after distortion as described in Step 2 and the ideal ultra-wideband linear frequency modulation signal as described in Step 1, and obtain the error curve after distortion;

[0010] Step 4: Fit the error curve described in Step 3 using a piecewise polynomial fitting method with the least squares method. Then, stitch the error curves together by using a sliding window weighted average method to measure the amplitude and phase errors between adjacent piecewise error curves.

[0011] Step 5: Construct a time-domain predistortion compensation signal based on the amplitude and phase errors extracted in Step 4;

[0012] Step 6: Input the pre-distortion compensation signal into the ultra-wideband synthetic aperture radar transmitter for compensation.

[0013] The beneficial effects of this invention are:

[0014] 1. Compared with existing predistortion processing techniques, this invention solves the problem of severe deterioration of in-band amplitude and phase characteristics caused by system errors under large bandwidth conditions;

[0015] 2. This invention can accurately extract and compensate for the amplitude and phase error components of high-frequency fluctuations within the ultra-wideband linear frequency modulated signal band, thereby improving the quality of ultra-high resolution SAR two-dimensional focusing images;

[0016] 3. This invention performs sliding window weighted summation on each segment of the fitted error curve to obtain the accurate fitting amplitude and phase value of the entire error curve.

[0017] 4. The method of the present invention can be used in ultra-wideband spaceborne synthetic aperture radar systems and has important application value. Attached Figure Description

[0018] Figure 1 This is a flowchart of the pre-distortion processing method for ultra-wideband spaceborne synthetic aperture radar of the present invention;

[0019] Figure 2 This is a schematic diagram of the ultra-wideband linear frequency modulation signal phase error estimation of the present invention;

[0020] Figure 3 This is a schematic diagram comparing the pulse compression results before and after predistortion compensation for the ultra-wideband linear frequency modulation signal according to the present invention. Detailed Implementation

[0021] Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be understood that the embodiments shown and described in the drawings are merely exemplary and are intended to illustrate the principles and spirit of the present invention, and are not intended to limit the scope of the present invention.

[0022] likeFigure 1 As shown, the predistortion processing method for ultra-wideband spaceborne synthetic aperture radar of the present invention specifically includes the following steps:

[0023] Step 1: Construct an ideal ultra-wideband linear frequency modulated signal based on the actual system parameters; the specific formula is as follows:

[0024]

[0025] Where A(τ) and φ(τ) represent the amplitude and phase of the ideal signal, and τ and T p These represent the distance, time, and pulse width of the transmitted signal, respectively.

[0026] Step 2: Construct a calibration closed-loop loop within the ultra-wideband synthetic aperture radar system to obtain the actual ultra-wideband linear frequency modulated signal after distortion;

[0027] In this embodiment, the internal calibration closed-loop circuit of the ultra-wideband synthetic aperture radar system is constructed in the following way: the output of the frequency-modulated signal source is directly down-converted to zero intermediate frequency by the receiver after passing through an attenuator and then filtered. After being digitized by the A / D converter of the data recorder, the signal is acquired and stored to obtain the actual ultra-wideband linear frequency-modulated signal after distortion.

[0028]

[0029] Where V(τ) and ψ(τ) represent the distortion components of the amplitude and phase of the actual ultra-wideband linear frequency modulated signal.

[0030] Step 3: Calculate the amplitude and phase differences between the actual ultra-wideband linear frequency modulated (UFM) signal after distortion as described in Step 2 and the ideal UFM signal as described in Step 1, and obtain the distortion error curve; the formulas for the amplitude and phase differences are as follows:

[0031]

[0032]

[0033] Among them, s d,i (τ) represents the i-th frame signal in the internal calibration closed loop, and phase(·) represents the corresponding phase taking operation.

[0034] Step 4: Fit the error curve described in Step 3 using a piecewise polynomial fitting method with the least squares approach. Then, stitch the adjacent piecewise error curves together using a sliding window weighted average to measure the amplitude and phase errors. The specific formula is as follows:

[0035]

[0036] in, and Let represent the phase errors of the i-th and j-th segment error curves fitted using the piecewise polynomial fitting method with least squares, respectively. and ω1 and ω2 represent the magnitude errors of the error curves of the i-th and j-th segments fitted by the piecewise polynomial fitting method using the least squares method, respectively, and the i-th and j-th segments are adjacent error curve segments with a half-overlapping region; ω1 and ω2 represent the weighting coefficients corresponding to the error curves of the i-th and j-th segments, respectively, and their values ​​are equally spaced discrete values ​​in the intervals [1,0] and [0,1], respectively. This is the result of splicing adjacent error curve segments after fitting.

[0037] In practice, for each segment of the fitted error curve, a sliding window weighted summation is performed segment by segment according to the above process to obtain the precise fitting amplitude and phase values ​​of the entire error curve. and

[0038] Step 5: Construct a time-domain predistortion compensation signal based on the amplitude and phase errors extracted in Step 4; the specific formula is as follows:

[0039]

[0040] Step 6: Input the pre-distortion compensation signal into the ultra-wideband synthetic aperture radar transmitter for compensation.

[0041] In specific implementation, the time-domain predistortion signal s constructed in step five will be used... pre (τ) is input to the ultra-wideband synthetic aperture radar transmitter to compensate for the amplitude and phase error components of the in-band high-frequency fluctuations caused by system errors in the ultra-wideband linear frequency modulated signal.

[0042] The effectiveness of the above steps will be verified by conducting an experiment:

[0043] Following step two, a wired closed-loop circuit was constructed for the 3.6 GHz ultra-wideband linear frequency modulation signal, and the actual ultra-wideband linear frequency modulation signal after distortion was acquired.

[0044] According to the operation process in step four, for the distorted ultra-wideband linear frequency modulation signal in the wired closed loop, a piecewise polynomial fitting method based on the least squares method is adopted, and the error curves fitted between each segment are spliced ​​together by using a sliding window weighted summation method to ensure the continuity of the error curves.

[0045] After compensating for the distorted signal in step six, pulse compression processing is performed on the 3.6 GHz ultra-wideband linear frequency modulated signal, and a -35 dB Taylor window is added within its effective frequency band. The corresponding processing results are as follows: Figure 2 and Figure 3 As shown.

[0046] like Figure 2 , 3 As shown, the phase distortion results extracted by traditional methods clearly demonstrate that ultra-wideband signals exhibit severe distortion in the transmit and receive links, leading to main lobe distortion and asymmetrical sidelobe elevation after pulse compression. The predistortion processing scheme proposed in this invention, based on a piecewise polynomial fitting method using the least squares approach, employs a sliding window weighted summation to capture high-frequency fluctuations in the error curve more precisely. However, traditional predistortion processing schemes, using conventional polynomial fitting throughout the entire error curve, fail to capture high-frequency fluctuations, and even after compensation and pulse compression, elevated and asymmetrical sidelobes still exist.

[0047] Table 1 Simulation Parameters

[0048]

[0049] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A method for pre-distortion processing of an ultra-wideband space-borne synthetic aperture radar, characterized in that, The method comprises the following steps: Step one: constructing an ideal ultra-wideband linear frequency modulation signal according to actual system parameters; the ideal ultra-wideband linear frequency modulation signal is constructed in the following way: wherein, and denote the amplitude and phase of the ideal signal, and denote the range-to-time and transmit signal pulse width, respectively; Step two: constructing an internal calibration closed loop of an ultra-wideband synthetic aperture radar system to obtain a distorted actual ultra-wideband linear frequency modulation signal; the distorted actual ultra-wideband linear frequency modulation signal is constructed in the following way: wherein and denote the distorted part of the actual ultra-wideband chirp signal amplitude and phase; Step three: calculating the amplitude and phase difference between the distorted actual ultra-wideband linear frequency modulation signal in step two and the ideal ultra-wideband linear frequency modulation signal in step one to obtain an error curve after distortion; the amplitude and phase difference is calculated in the following way: wherein, is a predetermined scaling factor for the inner loop of the closed loop control system frame signal, represents a corresponding phase extraction operation; Step four: fitting the error curve in step three by using a segmented polynomial fitting method of least squares, and splicing the error curves by using a sliding window weighted average method for the amplitude and phase errors between adjacent segmented error curves after fitting; Step five: constructing a time domain pre-distortion compensation signal according to the amplitude and phase errors extracted in step four; Step six: inputting the pre-distortion compensation signal into an ultra-wideband synthetic aperture radar transmitter for compensation.

2. The pre-distortion processing method of an ultra-wideband space-borne synthetic aperture radar according to claim 1, characterized in that, The internal calibration closed loop of the ultra-wideband synthetic aperture radar system is constructed in the following way: the output of a frequency modulation signal source directly enters a receiver after passing through an attenuator, is down-converted to zero intermediate frequency, is filtered, is digitized by A / D conversion of a data recorder, and is collected and stored to obtain a distorted actual ultra-wideband linear frequency modulation signal.

3. The pre-distortion processing method of an ultra-wideband space-borne synthetic aperture radar according to claim 1, wherein, The error curve is spliced by using a sliding window weighted average method in the following way: wherein, and respectively represent the phase error of the first and second error curve segments fitted by the piecewise polynomial fitting method using the least square method, and respectively represent the amplitude error of the first and second error curve segments fitted by the piecewise polynomial fitting method using the least square method, and the first and second segments are adjacent error curve segments with a half overlap region; and respectively represent the weighted coefficients corresponding to the first and second error curve segments, and the values are equidistant scattered values in the interval [1, 0] and the interval [0, 1] respectively; is the result of splicing the adjacent error curve segments after fitting.

4. The pre-distortion processing method of an ultra-wideband space-borne synthetic aperture radar according to claim 3, characterized in that, The time domain pre-distortion compensation signal is constructed in the following way:

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