Calibration method of frequency modulation rate agile synthetic aperture radar

Abstract

The invention belongs to the field of synthetic aperture radar digital signal processing, and particularly relates to a frequency modulation frequency agile synthetic aperture radar calibration method, which comprises the following steps of: firstly, acquiring a calibration signal of a specific waveform parameter, and solving an amplitude-phase error between the calibration signal and an ideal frequency domain signal; and then compensating the amplitude-phase error on the basis of frequency domain matched filtering of the frequency modulation frequency agility signal to complete real-time calibration of the frequency modulation frequency agility synthetic aperture radar. A large amount of calibration time and storage space can be saved.

Description

Technical Field

[0001] The invention belongs to the field of synthetic aperture radar digital signal processing, and in particular relates to a calibration method for a frequency-modulated agile synthetic aperture radar. Background Art

[0002] As the resolution of synthetic aperture radar continues to improve, the required signal bandwidth and system bandwidth are getting larger and larger to ensure high resolution in the range. The increase in radar signal bandwidth has put forward higher and higher requirements for the amplitude and phase consistency of the signal generator output signal and the amplitude and phase consistency of the receiving channel, which has caused difficulties in design, debugging and processing. In addition, the unique discreteness of analog circuits makes it difficult to ensure that the signal characteristics of each set of equipment and the characteristics of the receiving channel are the same, resulting in a decrease in pulse compression performance.

[0003] The amplitude and phase errors of the synthetic aperture radar system include the errors introduced by the linear frequency modulation signal source, low noise amplifier (LNA) and the components of the main receiving channel. Amplitude and phase distortion will cause the main lobe to widen and the side lobe level to increase after the signal pulse is compressed. It may also cause asymmetric distortion of the side lobe, which is mainly manifested in the image as range defocusing and side lobe level interference. Therefore, in order to obtain high-resolution and clear images, the amplitude and phase errors of the synthetic aperture radar system must be calibrated.

[0004] The conventional SAR calibration method is to store the transmitted signal generated by the SAR in the processing unit after down-conversion, demodulation and AD conversion by the receiver. The signal spectrum can be used as a matched filter for imaging processing pulse compression after conjugation or inverse. The conventional SAR calibration method is applied to frequency-agile SAR, which has the following disadvantages:

[0005] 1. Design a SAR calibration state during the power-on self-test process of the synthetic aperture radar system or the mode switching process. In this state, the linear frequency modulation signal source generates a broadband transmission pulse delayed on the time axis and feeds it into the receiving channel through the calibration port of the antenna. The processing unit needs to collect multiple transmission pulse signals (i.e., calibration signals) for coherent accumulation and calculating the average value of the transmission signal. For frequency-agile synthetic aperture radar, it is necessary to collect multiple transmission pulse signals of all agile waveforms in the calibration state. For tens of thousands of frequency-agile waveforms, the SAR calibration state will take a lot of time, affecting the execution of the normal tasks of the synthetic aperture radar.

[0006] 2. For frequency-modulated agile synthetic aperture radar, the frequency modulation of the transmitted signal is different. Therefore, different matched filter data needs to be stored according to the different modulation frequencies. Assuming that the number of sampling points in the range direction of the synthetic aperture radar is 128KB, and each frequency point of the matched filter is represented as a 32-bit floating-point complex number, 1MB of storage space is required to store one matched filter data, and 1GB of storage space is required for 1024 matched filters. Since the waveform parameters of the agile synthetic aperture radar are flexible and changeable, it is necessary to store the matched filter data of all waveforms. Due to the limited storage capacity of the processing unit, it is impossible to store the matched filter data of all waveforms, and the engineering implementation is difficult.

[0007] In the currently available literature, the SAR calibration methods are all for SAR calibration with fixed modulation frequency, pulse width and other waveform parameters, and there are few SAR calibration methods for modulation frequency agile waveforms. In SAR applications, time resources are tight and hardware storage space is limited, so developing more efficient and faster SAR calibration methods has practical application value. Summary of the invention

[0008] In view of this, the present invention provides a calibration method for a frequency-modulated agile synthetic aperture radar, which is a more efficient and faster synthetic aperture radar calibration method. First, a calibration signal with specific waveform parameters is collected, and the amplitude and phase errors between the calibration signal and the ideal frequency domain signal are calculated; then, the amplitude and phase errors are compensated on the basis of frequency-domain matched filtering of the frequency-modulated agile signal, so as to complete the real-time calibration of the frequency-modulated agile synthetic aperture radar.

[0009] The technical solution of the present invention is:

[0010] A method for calibrating a frequency-modulated agile synthetic aperture radar comprises the following steps:

[0011] S101: Collecting calibration signals, collecting N echo pulse signals of the radar in a calibration state;

[0012] S102: Accumulate and average the calibration signal, perform accumulation and summation on the N echo pulse signals and divide by N to obtain an average calibration signal;

[0013] S103: transforming the calibration signal into the frequency domain, and transforming the average calibration signal into a frequency domain calibration signal by fast Fourier transform;

[0014] S104: Calculate the error between the frequency domain calibration signal and the ideal frequency domain signal to obtain a reference error;

[0015] S105: converting the radar echo signal in a normal state into a frequency domain echo signal through fast Fourier transform;

[0016] S106: Perform matched filtering on the frequency domain echo signal;

[0017] S107: Compensate the reference error into the frequency domain echo signal after matched filtering.

[0018] Furthermore, after S107, the following steps are also included:

[0019] S108: frequency domain windowing filtering, performing windowing to suppress side lobes on the error-compensated frequency domain echo signal.

[0020] Furthermore, after S108, the following steps are also included:

[0021] S109: Perform inverse fast Fourier transform on the frequency domain echo signal after windowing in S108 to obtain a range time domain signal.

[0022] Further, in S101, the bandwidth of the linear frequency modulation signal transmitted by the frequency modulation agile synthetic aperture radar is assumed to be B, and the frequency modulation rate is assumed to be K; during acquisition, the bandwidth of the radar transmission signal is consistent, and the frequency modulation rate and pulse width of the linear frequency modulation signal are both pulse-to-pulse agile;

[0023] One of the modulation frequencies K0 of the radar is selected, a calibration state is set during the system power-on self-test process or the mode switching process, and a plurality of echo pulse signals are collected.

[0024] Furthermore, the S102 is specifically as follows:

[0025] The following formula is used to calculate the collected multiple echo pulse signals Cal n (τ) is accumulated and then averaged to obtain the average calibration signal Cav(τ)

[0026]

[0027] Furthermore, in S103, a frequency domain calibration signal is obtained based on the following formula:

[0028] Cav(f)=FFT(Cav(τ))

[0029] Where f is the radar echo signal frequency, and Cav(f) is the frequency domain calibration signal.

[0030] Furthermore, in S104,

[0031] The expression of the ideal frequency domain signal with modulation frequency K0 is:

[0032]

[0033] Where |K0| represents the absolute value of the modulation frequency K0;

[0034]

[0035] Where Err is the reference error.

[0036] Furthermore, in S105,

[0037] The radar range echo signal is expressed as

[0038]

[0039] Among them, R m is the real-time distance between the target and the radar, T m is the real-time agile pulse width of the radar transmission signal, K m The real-time agile modulation frequency of the radar transmission signal;

[0040] Based on the following formula, the radar range echo signal is subjected to fast Fourier transform to obtain the frequency domain echo signal:

[0041] S m (f) = FFT(S m (τ))m=1,2,……,M

[0042] In S106,

[0043] Frequency modulation is K m The expression of the ideal linear frequency modulation signal frequency domain filter is

[0044]

[0045] The filter is used to perform frequency domain matched filtering on the frequency domain echo signal to obtain:

[0046] Sout m (f) = H m (f) S m (f)

[0047] Sout m (f) is the frequency domain echo signal after matched filtering.

[0048] Further, in S107, compensation is calculated according to the following formula:

[0049]

[0050] In the formula represents the conjugation of Err.

[0051] Further, in S108, the radar imaging adopts a Hanning window;

[0052] Sout m (f) = Soutm (f) hanning

[0053] Where hanning represents the Hanning window;

[0054] In S109, the distance time domain signal is obtained based on the following formula:

[0055] Sout m (τ) = FFT(Sout m (f)).

[0056] Beneficial effects of the present invention:

[0057] The present invention proposes an efficient frequency modulation agile synthetic aperture radar calibration method. Compared with the conventional synthetic aperture radar calibration method, the method has the following advantages:

[0058] 1. It can save a lot of calibration time. Assuming that the synthetic aperture radar system is designed with M frequency modulation rates, the time consumed by this method is 1 / M of the time consumed by conventional synthetic aperture radar calibration.

[0059] 2. It can save a lot of storage space. Assuming that the synthetic aperture radar system is designed with M modulation rates, the storage space of the matched filter in this method is 1 / M of the storage space of the conventional synthetic aperture radar matched filter.

[0060] The invention can be used for synthetic aperture radar based on frequency modulation agile waveform. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0062] Figure 1 A time domain diagram of a calibration signal of a wideband agile SAR provided by an embodiment of the present invention;

[0063] Figure 2 This is the time domain diagram of 8.96us pulse width signal;

[0064] Figure 3 It is the ideal pulse pressure distance profile of 8.96us pulse width signal;

[0065] Figure 4 The pulse pressure distance profile after calibration of the 8.96us pulse width signal;

[0066] Figure 5 This is the time domain diagram of the 10.88us pulse width signal;

[0067] Figure 6 It is the ideal pulse pressure distance profile of 10.88us pulse width signal;

[0068] Figure 7 The pulse pressure distance profile after calibration of the 10.88us pulse width signal;

[0069] Figure 8 It is a frequency-modulated agile SAR image;

[0070] Fig. 9 The ideal pulse pressure range profile of a point target in a SAR image;

[0071] Fig.10 Pulse pressure range profile after calibration of point targets in SAR images. DETAILED DESCRIPTION

[0072] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0073] The following describes the embodiments of the present disclosure through specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all of the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present disclosure.

[0074] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described herein can be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on the present disclosure, it should be understood by those skilled in the art that an aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects described herein can be used to implement the device and / or practice the method. In addition, other structures and / or functionalities other than one or more of the aspects described herein can be used to implement this device and / or practice this method.

[0075] It should also be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present disclosure. The illustrations only show components related to the present disclosure rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0076] Additionally, in the following description, specific details are provided to facilitate a thorough understanding of the examples. However, it will be understood by those skilled in the art that the aspects described may be practiced without these specific details.

[0077] In one embodiment of the present invention, a calibration method for a frequency-modulated agile synthetic aperture radar is provided, comprising the following steps:

[0078] S101: Collecting calibration signals, collecting N echo pulse signals of the radar in a calibration state;

[0079] S102: Accumulate and average the calibration signal, perform accumulation and summation on the N echo pulse signals and divide by N to obtain an average calibration signal;

[0080] S103: transforming the calibration signal into the frequency domain, and transforming the average calibration signal into a frequency domain calibration signal by fast Fourier transform;

[0081] S104: Calculate the error between the frequency domain calibration signal and the ideal frequency domain signal to obtain a reference error;

[0082] S105: converting the radar echo signal in a normal state into a frequency domain echo signal through fast Fourier transform;

[0083] S106: Perform matched filtering on the frequency domain echo signal;

[0084] S107: Compensate the reference error into the frequency domain echo signal after matched filtering.

[0085] In this embodiment, the step S107 further includes the following steps:

[0086] S108: frequency domain windowing filtering, performing windowing to suppress side lobes on the error-compensated frequency domain echo signal.

[0087] In this embodiment, the step S108 further includes the following steps:

[0088] S109: Perform inverse fast Fourier transform on the frequency domain echo signal after windowing in S108 to obtain a range time domain signal.

[0089] In this embodiment, in S101, it is assumed that the bandwidth of the linear frequency modulation signal emitted by the frequency agile synthetic aperture radar is B, and the frequency modulation rate is K; during acquisition, the bandwidth of the radar emission signal is consistent, and the frequency modulation rate and pulse width of the linear frequency modulation signal are both pulse-to-pulse agile;

[0090] One of the modulation frequencies K0 of the radar is selected, a calibration state is set during the system power-on self-test process or the mode switching process, and a plurality of echo pulse signals are collected.

[0091] In this embodiment, the S102 is specifically:

[0092] The following formula is used to calculate the collected multiple echo pulse signals Cal n (τ) is accumulated and then averaged to obtain the average calibration signal Cav(τ)

[0093]

[0094] In S103, a frequency domain calibration signal is obtained based on the following formula:

[0095] Cav(f)=FFT(Cav(τ))

[0096] Where f is the radar echo signal frequency, and Cav(f) is the frequency domain calibration signal.

[0097] In this embodiment, in S104,

[0098] The expression of the ideal frequency domain signal with modulation frequency K0 is:

[0099]

[0100] Where |K0| represents the absolute value of the modulation frequency K0;

[0101]

[0102] Where Err is the reference error.

[0103] In this embodiment, in S105,

[0104] The radar range echo signal is expressed as

[0105]

[0106] Among them, R m is the real-time distance between the target and the radar, T m is the real-time agile pulse width of the radar transmission signal, K m The real-time agile modulation frequency of the radar transmission signal;

[0107] Based on the following formula, the radar range echo signal is subjected to fast Fourier transform to obtain the frequency domain echo signal:

[0108] S m (f) = FFT(S m (τ))m=1,2,……,MIn this embodiment, in S106,

[0109] Frequency modulation is K m The expression of the ideal linear frequency modulation signal frequency domain filter is

[0110] The filter is used to perform frequency domain matched filtering on the frequency domain echo signal to obtain: Sout m (f) = H m (f) S m (f)

[0111] Sout m (f) is the frequency domain echo signal after matched filtering.

[0112] In this embodiment, in S107, compensation is calculated according to the following formula:

[0113]

[0114] In the formula represents the conjugation of Err.

[0115] In this embodiment, in S108, the radar imaging adopts a Hanning window;

[0116] Sout m (f) = Sout m (f) hanning

[0117] Where hanning represents the Hanning window.

[0118] In this embodiment, the distance time domain signal is obtained in S109 based on the following formula:

[0119] Sout m (τ) = FFT(Sout m (f)).

[0120] Further explanation

[0121] 1. Collect calibration signal

[0122] Assume that the bandwidth of the linear frequency modulation signal emitted by the frequency agile synthetic aperture radar is B, and the frequency modulation rate is K. To ensure the same resolution in the range direction, the bandwidth of the transmitted signal should be kept the same. In order to improve the low interception and anti-interference performance of the signal, the frequency modulation rate and pulse width of the linear frequency modulation signal are generally agilely varied between pulses at the same time.

[0123] Assuming that the synthetic aperture radar system is designed with multiple modulation frequencies, one of the modulation frequencies is arbitrarily selected as the modulation frequency K0 of the calibration signal. A SAR calibration state is designed during the system power-on self-test process or mode switching process to collect multiple calibration signal echo pulses.

[0124] 2. Accumulate and average the calibration signal

[0125] The collected multiple calibration pulse signals Cal n (τ) are accumulated and summed, and then averaged to obtain the average calibration signal Cav(τ).

[0126]

[0127] 3. Transform the calibration signal into the frequency domain

[0128] Perform fast Fourier transform on the average calibration signal Cav(τ) to obtain the frequency domain calibration signal Cav(f):

[0129] Cav(f)=FFT(Cav(τ)) (2)

[0130] Where, f is the SAR range baseband signal frequency.

[0131] 4. Find the error between the frequency domain calibration signal and the ideal frequency domain signal

[0132] The frequency domain expression of the ideal linear frequency modulation signal with a modulation frequency of K0 is:

[0133]

[0134] Where |K0| represents the absolute value of the modulation frequency K0.

[0135]

[0136] 5. Transform the range baseband echo signal to the frequency domain

[0137] The range echo signal with frequency agility is

[0138]

[0139] Among them, R m is the real-time distance between the target and the radar, T m is the real-time agile pulse width of the transmitted signal, K mIt is the real-time agile modulation frequency of the transmitted signal.

[0140] Perform fast Fourier transform on the baseband echo signal of each pulse range, and get the frequency domain signal as follows:

[0141] S m (f) = FFT(S m (τ)) m=1,2,……,M (6)

[0142] 6. Frequency domain matched filtering of range-direction baseband echo signal

[0143] Frequency modulation is K m The ideal linear frequency modulation signal frequency domain filter expression is:

[0144]

[0145] An ideal matched filter is used to perform frequency domain matched filtering on the range baseband echo signal.

[0146] Sout m (f) = H m (f) S m (f) (8)

[0147] 7. Baseband echo signal error compensation

[0148]

[0149] In the formula represents the conjugation of Err.

[0150] 8. Frequency domain windowing filtering

[0151] The error-compensated baseband echo signal is windowed for sidelobe suppression. SAR imaging can use the Hanning window.

[0152] Sout m (f) = Sout m (f) · hanning (10)

[0153] Where hanning represents the Hanning window.

[0154] 9. Perform inverse fast Fourier transform on the signal

[0155] Sout m (τ) = FFT(Sout m (f)) (11)

[0156] Perform inverse fast Fourier transform on the windowed signal to obtain the distance time domain signal Sout m(τ). So far, the calibration of frequency-agile synthetic aperture radar is completed. After calibration, a relatively ideal range pulse pressure effect can be obtained.

[0157] This embodiment overcomes the problem of difficult engineering implementation, saves a lot of calibration time and storage space, and completes the correction of amplitude and phase errors of the synthetic aperture radar system.

[0158] The above is only a specific implementation of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present disclosure should be included in the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure should be based on the protection scope of the claims.

Claims

1. A method for calibrating a frequency-agile synthetic aperture radar, characterized in that: The following steps are involved: S101: Collecting calibration signals, collecting N echo pulse signals of the radar in a calibration state; S102: Accumulate and average the calibration signal, perform accumulation and summation on the N echo pulse signals and divide by N to obtain an average calibration signal; S103: transforming the calibration signal into the frequency domain, and transforming the average calibration signal into a frequency domain calibration signal by fast Fourier transform; S104: Calculate the error between the frequency domain calibration signal and the ideal frequency domain signal to obtain a reference error; S105: converting the radar echo signal in a normal state into a frequency domain echo signal through fast Fourier transform; S106: Perform matched filtering on the frequency domain echo signal; S107: Compensate the reference error into the frequency domain echo signal after matched filtering.

2. The calibration method of frequency modulation agile synthetic aperture radar according to claim 1, characterized in that: After S107, the following steps are also included: S108: frequency domain windowing filtering, performing windowing to suppress side lobes on the error-compensated frequency domain echo signal.

3. The calibration method of frequency modulation agile synthetic aperture radar according to claim 2, characterized in that: After S108, the following steps are also included: S109: Perform inverse fast Fourier transform on the frequency domain echo signal after windowing in S108 to obtain a range time domain signal.

4. The calibration method of frequency modulation agile synthetic aperture radar according to claim 3, characterized in that: In S101, it is assumed that the bandwidth of the linear frequency modulation signal emitted by the frequency agile synthetic aperture radar is B, and the frequency modulation rate is K; during acquisition, the bandwidth of the radar emission signal is consistent, and the frequency modulation rate and pulse width of the linear frequency modulation signal are both pulse-to-pulse agile; One of the modulation frequencies K0 of the radar is selected, a calibration state is set during the system power-on self-test process or the mode switching process, and a plurality of echo pulse signals are collected.

5. The calibration method of frequency modulation agile synthetic aperture radar according to claim 4, characterized in that: The S102 is specifically: The following formula is used to calculate the collected multiple echo pulse signals Cal n (τ) is accumulated and then averaged to obtain the average calibration signal Cav(τ) 6. The calibration method of frequency modulation agile synthetic aperture radar according to claim 5, characterized in that: In S103, a frequency domain calibration signal is obtained based on the following formula: Cav(f)=FFT(Cav(τ)) Where f is the radar echo signal frequency, and Cav(f) is the frequency domain calibration signal.

7. The calibration method of frequency modulation agile synthetic aperture radar according to claim 6, characterized in that: In S104, The expression of the ideal frequency domain signal with modulation frequency K0 is: Where |K0| represents the absolute value of the modulation frequency K0; Where Err is the reference error.

8. The calibration method of frequency modulation agile synthetic aperture radar according to claim 7, characterized in that: In S105, The radar range echo signal is expressed as Among them, R m is the real-time distance between the target and the radar, T m is the real-time agile pulse width of the radar transmission signal, K m The real-time agile modulation frequency of the radar transmission signal; Based on the following formula, the radar range echo signal is subjected to fast Fourier transform to obtain the frequency domain echo signal: S m (f)=FFT(S m (τ)) m=1,2,……,M In S106, Frequency modulation is K m The expression of the ideal linear frequency modulation signal frequency domain filter is The filter is used to perform frequency domain matched filtering on the frequency domain echo signal to obtain: Sout m (f)=H m (f)·S m (f) Sout m (f) is the frequency domain echo signal after matched filtering.

9. The calibration method of frequency modulation agile synthetic aperture radar according to claim 8, characterized in that: In S107, compensation is calculated according to the following formula: In the formula represents the conjugation of Err.

10. The calibration method of frequency modulation agile synthetic aperture radar according to claim 9, characterized in that: In S108, the radar imaging adopts a Hanning window; Sout m (f)=Sout m (f)·hanning Where hanning represents the Hanning window; In S109, the distance time domain signal is obtained based on the following formula: Sout m (τ)=FFT(Sout m (f))。

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