Antenna pattern measurement method based on segmented gain variable threshold single-bit receiver

Abstract

The application discloses a kind of antenna directional pattern measurement methods based on segmented gain variable threshold single-bit receiver, it is related to synthetic aperture radar radiometric calibration technical field.The method steps are as follows: the monotonic relationship of quantitative signal power and STR is analyzed, and quantitative model is established;The receiving signal amplitude reconstruction strategy of all sampling rate and SAR bandwidth ratio is constructed;The maximum amplitude and minimum amplitude of received signal are determined, the received signal amplitude is segmented and threshold is set;Signal spectrum is analyzed and quantitative signal power is estimated;STR and actual received signal power are calculated;Azimuth and range antenna directional pattern sample value are calculated, and according to radar equation, radar antenna azimuth and pitch gain is calculated, and antenna directional pattern is reconstructed using polynomial fitting.The application simplifies the structure of radar directional pattern gain measurement receiver, reduces the cost, guarantees the measurement accuracy, and can meet the demand of large-scale deployment and high-precision estimation of SAR radiometric calibration active receiver.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of synthetic aperture radar radiation calibration, and in particular to an antenna pattern measurement method based on a segmented gain variable threshold single-bit receiver. BACKGROUND

[0002] Synthetic aperture radar (SAR) has been widely applied in key fields such as disaster dynamic monitoring, ecological environment evaluation and national defense security warning due to its stable observation ability and high-resolution imaging advantage. In recent years, with the continuous breakthrough of miniaturization technology, the rapid rise of commercial aerospace industry and low-altitude economy promotes the transformation of SAR remote sensing technology from traditional qualitative analysis to high-precision quantitative research, which puts forward higher requirements for the accuracy of SAR system performance calibration.

[0003] Radiation calibration, as the core technology to ensure the performance calibration of the SAR system, is an important basis for ensuring the consistency and comparability of SAR data at different times and in different regions, and the accurate measurement of the antenna pattern is an indispensable key link in the radiation calibration process, and the measurement accuracy directly affects the reliability of SAR quantitative application.

[0004] At present, the commonly used antenna pattern measurement methods in the industry mainly include corner reflector method, natural distribution target method, ground receiver method and model-based measurement method, but the existing technical solutions all have significant limitations: the corner reflector method has the characteristics of simple structure and convenient operation, but it can only effectively measure the antenna pattern in the distance direction and cannot realize full coverage; the natural distribution target method is limited by geographical environment, has a narrow application area, and is subject to the satellite revisit period, so the timeliness of measurement is poor, and it is difficult to meet the rapid measurement demand in emergency scenarios; the ground receiver method, as one of the current mainstream methods, has a complex structure design and needs to integrate a large number of high-precision components, resulting in high engineering implementation cost, which is not conducive to the large-scale popularization and application of the technology.

[0005] In summary, the existing antenna pattern measurement technology cannot balance the measurement accuracy, cost control and scene applicability, and a low-cost, high-precision and highly adaptable technical solution is needed to solve the current industry pain points. SUMMARY

[0006] The purpose of the application is to provide an antenna pattern measurement method based on a segmented gain variable threshold single-bit receiver, to solve the problems of complex structure, high cost of traditional ground receivers, poor applicability and insufficient timeliness of existing antenna pattern measurement methods, and to ensure accurate measurement of the antenna pattern and meet the measurement needs of various SAR systems.

[0007] In order to achieve the above object, the application provides an antenna pattern measurement method based on segmented gain variable threshold single-bit receiver, and the specific steps are as follows:

[0008] Step S1, different signal amplitude to threshold ratio STR is set, the signal is single-bit sampled, the sampling result is subjected to fast Fourier transform (FFT) and the quantized signal power is calculated;

[0009] Step S2, the quantized signal power-STR relationship curve is drawn, the monotonic relationship between the quantized signal power and STR is analyzed, and the quantitative model between the quantized signal power and STR is established;

[0010] Step S3, based on the energy conservation principle, the stability of the quantized signal power under undersampling and normal sampling is determined, and the radar antenna pattern reconstruction strategy applied to the ratio of different sampling rates and synthetic aperture radar (SAR) signal bandwidth is constructed;

[0011] Step S4, according to the measurement bandwidth and the theoretical antenna pattern gain, the maximum and minimum amplitudes of the received signal are determined, the amplitudes are segmented according to the interval of STR when the quantized signal power changes monotonously with STR, and the fixed threshold of each segment is calculated;

[0012] Step S5, the signal after single-bit sampling is subjected to spectrum analysis, the single-frequency harmonic component is removed, and the quantized signal power is accurately calculated;

[0013] Step S6, according to the quantitative model in step S2 and the quantized signal power in step S5, the STR is calculated, and the mean value of the original amplitude of the received signal is determined according to the segmented threshold in step S4, so that the actual received signal power is obtained;

[0014] Step S7, the single receiver data and the multiple receiver data are processed respectively to obtain the azimuth antenna pattern sampling value and the range antenna pattern sampling value;

[0015] Step S8, according to the radar equation, the azimuth antenna gain and the elevation antenna gain are calculated;

[0016] Step S9, according to the calculated azimuth antenna gain and elevation antenna gain, the azimuth antenna pattern is reconstructed by linear interpolation, and the range antenna pattern is reconstructed by cubic polynomial fitting.

[0017] Preferably, in step S2, the monotonic relationship between the quantized signal power and STR is analyzed, the quantitative relationship between the quantized signal power and STR is determined in segments, and segmented linear fitting is performed, and the formula is as follows:

[0018] ;

[0019] wherein, To quantize signal power, and The first The coefficients of the linear term and the constant term of a segmental linear function.

[0020] Preferably, in step S3, the antenna pattern reconstruction strategy is specifically as follows: when When the value is less than 1, it is in an undersampling state. In this case, the signal spectrum will aliasing. According to the law of conservation of energy, at a receiver sampling rate... The corresponding frequency range ( Within this range, the power of the quantized signal remains unchanged, and the actual amplitude of the received signal is recovered according to the quantitative model in step S2; when When the value is greater than or equal to 1, there is no aliasing in the signal spectrum, and the quantized signal power can be calculated normally; that is, regardless of the SAR signal bandwidth. Regardless of the size, the receiver only needs to use a fixed sampling rate. This allows us to recover the actual amplitude information of the received signal and reconstruct the radar antenna pattern.

[0021] Preferably, in step S4, the amplitude of the received signal is determined by the elevation dimension antenna pattern gain. Azimuth antenna pattern gain A joint decision, in which, This is the angle between the nearest slant range of the SAR to the mapping strip and the slant range of the SAR to the receiver. The instantaneous oblique angle in azimuth direction; the maximum amplitude of the received signal. The minimum amplitude of the received signal, obtained when the receiver is located at the center of the surveying strip. This was achieved when the receiver was simultaneously located at the edge of the azimuth beam and the edge of the mapping strip. This is the slant distance from the SAR to the center of the mapping zone. and These are the shortest and longest slant distances from the SAR to the mapping zone, respectively.

[0022] Preferably, the amplitude of the received signal is segmented according to the interval of STR when the quantized signal power changes monotonically with STR, as follows:

[0023] Step S41: Calculate the threshold of the first segment of the received signal amplitude using the following formula:

[0024] ;

[0025] in, The threshold value for the first segment of the received signal amplitude. The maximum value of STR within the monotonically changing interval;

[0026] Step S42, the minimum amplitude of the first segment is calculated, and the formula is as follows:

[0027] ;

[0028] Wherein, is the minimum amplitude of the first segment of the received signal amplitude, is the minimum value of the monotonic change interval STR;

[0029] Step S43, the minimum amplitude of the first segment of the received signal amplitude is taken as the maximum amplitude of the second segment of the received signal amplitude , and the threshold of the second segment of the received signal amplitude is calculated;

[0030] Step S44, steps S41-S43 are repeated to calculate the threshold and the minimum amplitude of each subsequent segment in turn until the minimum amplitude of the last segment of the received signal amplitude calculated is less than or equal to the minimum amplitude of the received signal .

[0031] Preferably, in step S6, the STR is calculated according to the piecewise linear fitting relationship, and the formula is as follows:

[0032] ;

[0033] Wherein, is the quantized signal power under the instantaneous squint angle , , is the number of sampling points in the azimuth direction, is the signal threshold ratio at the i-th azimuth point;

[0034] The formula for calculating the actual received signal power is as follows:

[0035] ;

[0036] ;

[0037] Wherein, is the actual received signal power under the instantaneous squint angle , is the mean value of the original amplitude of the received signal under the instantaneous squint angle , is the threshold corresponding to the instantaneous squint angle .

[0038] Preferably, in step S8, the radar equation is calculated according to the formula as follows:

[0039] ; ​

[0040] in, and These are the received signal power and the transmitted signal power, respectively. For wavelength, This is the slant range between the SAR and the receiver. For azimuth antenna gain, For the elevation dimension antenna gain;

[0041] Azimuth antenna gain The calculation formula is as follows:

[0042] ;

[0043] in, For instantaneous oblique angle The azimuth antenna gain is below;

[0044] Angle The elevation dimension antenna gain is calculated using the following formula:

[0045] ;

[0046] in, The actual power of the received signal at the zero Doppler moment. The closest slant range from SAR to the mapping zone and the SAR to the first... The angle between the slant ranges of the receivers For the first One receiver, It is the instantaneous oblique angle of the SAR and the receiver at the zero Doppler moment.

[0047] Therefore, this invention proposes an antenna pattern measurement method based on a segmented gain variable threshold single-bit receiver, the advantages of which are as follows:

[0048] (1) The present invention adopts a single-frequency threshold single-bit quantization strategy, which reduces the amount of data storage and avoids the complex high-precision component design of traditional ground receivers, greatly simplifies the structure, reduces hardware costs and operating power consumption, solves the pain point of high engineering implementation cost of traditional ground receivers, and is more conducive to large-scale promotion and application.

[0049] (2) This invention optimizes the stability of the quantized signal power-STR relationship curve through Monte Carlo experiments and establishes a quantitative model in the monotonic range of STR. At the same time, in response to the problem of large dynamic range of signal amplitude, a segmented fixed threshold strategy is adopted to ensure that each segment of signal STR is in the monotonic range, effectively recovering the actual amplitude of the signal, ensuring the accuracy of antenna pattern measurement, and meeting the accuracy requirements of SAR radiation calibration.

[0050] (3) The application proposes an adaptive scheme of the ratio of different sampling rates to the bandwidth of a SAR signal based on the principle of energy conservation, so that the receiver only needs a fixed sampling rate to meet the antenna pattern measurement requirements of different bandwidth SAR systems, without the need to adjust the sampling rate according to the type of SAR, thereby breaking through the strict limitation of the sampling rate in the traditional method and being more adaptable.

[0051] The technical solutions of the application will be further described in detail below with reference to the drawings and embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1 A flowchart of the antenna pattern measurement method based on the segmented gain variable threshold single-bit receiver of the application is shown in the figure.

[0053] Figure 2 A geometric scene diagram of a synthetic aperture radar (SAR) in the embodiment of the application is shown in the figure. DETAILED DESCRIPTION

[0054] In order to make the technical solutions, advantages and purposes of the application clearer, the technical solutions of the embodiments of the application will be described clearly and completely below. The described embodiments are part of the embodiments of the application, rather than all the embodiments. Based on the described embodiments of the application, all other embodiments obtained by those skilled in the art without the need for creative labor are within the protection scope of the application.

[0055] Unless otherwise defined, the technical terms or scientific terms used in the application should be understood as the usual meanings understood by those skilled in the art in the field of the application.

[0056] EMBODIMENT

[0057] As shown in the figure, the application provides an antenna pattern measurement method based on a segmented gain variable threshold single-bit receiver, and the specific steps are as follows: Figure 1 S1, set different signal amplitude to threshold ratios (STR), perform single-bit sampling on the signal, perform fast Fourier transform (FFT) on the sampling results and calculate the quantized signal power, finally draw a quantized signal power-STR relationship curve, analyze the monotonic relationship between the quantized signal power and the STR, and establish a quantitative model between the quantized signal power and the STR;

[0058] During the single-frequency threshold single-bit sampling of the signal, signal harmonics and single-frequency threshold harmonics will be intermodulated. The amplitude of the intermodulation component of the first-order harmonic of the signal and the threshold changes monotonously with the STR in a large STR interval, and the amplitude of the intermodulation component of the first-order harmonic of the signal and the threshold changes monotonously with the STR in a large STR interval, which can be used to recover the amplitude information of the signal, and the specific steps are as follows:

[0059]

[0060] ​By setting different STRs, the signal is sampled bit by bit. The sampling results are then subjected to a Fast Fourier Transform (FFT) to calculate the quantized signal power, and finally, a quantized signal power-STR relationship curve is plotted. Given the randomness of the initial phase of the single-frequency threshold, which leads to unstable results, Monte Carlo experiments are conducted to obtain a more stable curve relationship. For example, when STR is within the range of -20dB to -5dB, the quantitative relationship between the quantized signal power and STR is determined piecewise, and piecewise linear fitting is performed, as shown in the following formula:

[0061] ;

[0062] in, To quantize signal power, and The first The coefficients of the linear term and the constant term of a segmental linear function.

[0063] S2. Based on the principle of energy conservation, the stability of the quantized signal power under undersampling and normal sampling conditions is determined, and an antenna pattern reconstruction strategy is constructed for different sampling rates and the ratio of synthetic aperture radar SAR signal bandwidth, as follows:

[0064] Receiver sampling rate It is fixed; however, there are many types of SAR, and their signal bandwidth varies. They vary. In order to meet the measurement requirements of various SAR antenna patterns, a system suitable for different sampling rates and signal bandwidth ratios is constructed. Antenna pattern reconstruction strategy. When When the value is less than 1, it is in an undersampling state. In this case, the signal spectrum will aliasing. According to the law of conservation of energy, at a receiver sampling rate... The corresponding frequency range ( Within this range, the estimated quantized signal power remains essentially unchanged, therefore the actual amplitude of the received signal can still be recovered based on the quantitative model in step 1; when When the value is greater than or equal to 1, there is no aliasing in the signal spectrum, and the quantized signal power can be directly estimated normally. Therefore, regardless of the SAR signal bandwidth, the receiver only needs to use a fixed sampling rate to recover the actual amplitude information of the received signal, thereby reconstructing the radar antenna pattern.

[0065] S3. Based on the survey band width and theoretical antenna pattern gain, determine the maximum and minimum amplitude of the received signal, and divide the amplitude into segments according to the interval of STR when the quantized signal power changes monotonically with STR, and calculate the fixed threshold for each segment.

[0066] like Figure 2 As shown, the amplitude of the received signal is determined by the gain of the antenna pattern in the elevation dimension. and the gain of the azimuth antenna pattern are determined jointly, wherein, is the included angle between the slant range from the SAR to the swath and the slant range from the SAR to the receiver, is the instantaneous slant angle in the azimuth direction; the maximum amplitude of the received signal is the minimum amplitude of the received signal, obtained when the receiver is located at the center of the swath is obtained when the receiver is located at the edge of the azimuth beam and the edge of the swath simultaneously, wherein, is the slant range from the SAR to the center of the swath, and are the shortest and the longest slant ranges from the SAR to the swath, respectively.

[0067] Due to the different placement positions of the receiver and the motion of the SAR platform, the amplitude of the received signal has a large dynamic range. In order to control the STR within the interval of the monotonic relationship, it is necessary to segment the amplitude of the received signal and set different thresholds for each segment, and the specific steps are as follows:

[0068] S31, calculate the threshold of the first segment of the amplitude of the received signal, the formula is as follows:

[0069] ;

[0070] wherein, is the threshold of the first segment of the amplitude of the received signal, is the maximum value of the monotonic change interval STR;

[0071] S32, calculate the minimum amplitude of the first segment, the formula is as follows:

[0072] ;

[0073] wherein, is the minimum amplitude of the first segment of the amplitude of the received signal, is the minimum value of the monotonic change interval STR;

[0074] S33, take the minimum amplitude of the first segment of the amplitude of the received signal as the maximum amplitude of the second segment of the amplitude of the received signal , calculate the threshold of the second segment of the amplitude of the received signal ;

[0075] S34, repeat steps S31-S33, sequentially calculate the threshold and the minimum amplitude of each subsequent segment, until the minimum amplitude calculated for the last segment of the amplitude of the received signal is less than or equal to the minimum amplitude of the received signal .

[0076] S4, after the signal is single-frequency threshold single-bit sampled, the signal spectrum will introduce each harmonic component of the single-frequency threshold, and the existence of these harmonic components will lead to unstable quantization signal power estimation. In order to more accurately estimate the quantization signal power, the signal after single-bit sampling is subjected to spectrum analysis, and the single-frequency harmonic component is removed;

[0077] S5, according to the quantitative model in step S1 and the quantization signal power in step S4, the STR is calculated, the original amplitude of the received signal is determined according to the segment threshold in step S3, and the actual received signal power is obtained;

[0078] When the azimuth beam coverage area of the SAR is completely scanned through a receiver, the receiver records the received signals at different instantaneous squint angles , wherein, , is the number of sampling points in the azimuth direction. Since the receiver adopts single-frequency threshold single-bit sampling, the recorded data is a quantization signal, and the actual amplitude of the received signal needs to be recovered in order to calculate the actual received power. When the instantaneous squint angle of the SAR and the receiver is , the average of the original amplitude of the received signal received by the receiver is , and according to the step S1, there is a monotonic relationship between the known quantization signal power and the STR, and the quantization signal power estimated in step S4 is combined , the STR is calculated according to the piecewise linear fitting relationship, and the formula is as follows:

[0079] ;

[0080] wherein, is the quantization signal power at the instantaneous squint angle , is the signal threshold ratio at the th azimuth point;

[0081] Then, according to the segment in step S3, the threshold corresponding to the current instantaneous squint angle is determined, and the average of the original amplitude of the received signal is calculated, and the formula is as follows:

[0082] ;

[0083] wherein, is the average of the original amplitude of the received signal at the instantaneous squint angle , is the threshold corresponding to the instantaneous squint angle .

[0084] The actual received signal power is obtained from the average of the original amplitude of the received signal, and the formula is as follows:

[0085] ;

[0086] wherein, is the actual received signal power at the instantaneous squint angle ;

[0087] S6, processing the single receiver data and the multiple receiver data respectively to obtain the azimuth antenna pattern sample value and the range antenna pattern sample value, and reconstructing the azimuth antenna pattern and the range antenna pattern;

[0088] At the zero Doppler moment, each receiver records the received signal at the moment. According to the method in step S5, the actual power of the received signal of each receiver at the zero Doppler moment is calculated , corresponding to the actual received signal power at different included angles ; wherein, is the included angle between the nearest slant range of the SAR to the mapping band and the slant range (after zero Doppler calibration) of the SAR to the th receiver, is the th receiver.

[0089] S7, calculating the azimuth antenna gain and the elevation antenna gain according to the radar equation;

[0090] The radar equation calculation formula is as follows:

[0091] ;

[0092] wherein, and are the received signal power and the transmitted signal power respectively, is the wavelength, is the slant range between the SAR and the receiver, is the azimuth antenna gain, is the elevation antenna gain;

[0093] The azimuth antenna gain calculation formula is as follows:

[0094] ;

[0095] wherein, is the azimuth antenna gain at the instantaneous squint angle ;

[0096] is the range antenna gain at the included angle , and the calculation formula is as follows:

[0097] ;

[0098] wherein, is the instantaneous squint angle of the SAR and the receiver at the zero Doppler moment.

[0099] S8, according to the calculated azimuth antenna gain and elevation antenna gain, the azimuth antenna pattern is reconstructed by linear interpolation, and the range antenna pattern is reconstructed by cubic polynomial fitting.

[0100] It is worth noting that the contents not elaborated in the present application are all prior art and are well known to those skilled in the art.

[0101] Therefore, the present application provides an antenna pattern measurement method based on a segmented gain variable threshold single-bit receiver, which reduces data storage, simplifies the receiver structure, reduces hardware cost and power consumption through a single-frequency threshold single-bit quantization strategy, solves the problem of high cost of traditional ground receivers, and at the same time, combines a segmented fixed threshold strategy to recover the signal amplitude, guarantees the measurement accuracy to meet the SAR radiation calibration requirements, and also adapts the receiver to different bandwidth SARs at a fixed sampling rate based on the principle of energy conservation, breaks through the limitation of traditional sampling rate, and has stronger adaptability.

[0102] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application and not to limit them, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that: it can still modify or equivalently replace the technical solutions of the present application, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A method for measuring antenna pattern based on a segmented gain-variant threshold single-bit receiver, characterized in that, The specific steps are as follows: Step S1, set different signal amplitude and threshold ratio STR, sample the signal with single bit, perform fast Fourier transform FFT on the sampling result and calculate the quantized signal power; Step S2, draw a quantized signal power-STR relationship curve, analyze the monotonic relationship between the quantized signal power and STR, and establish a quantitative model between the quantized signal power and STR; Step S3, based on the principle of energy conservation, determine the stability of the quantized signal power under undersampling and normal sampling, and construct a radar antenna pattern reconstruction strategy applied to the ratio of all sampling rates to synthetic aperture radar SAR signal bandwidth; Step S4, according to the measured bandwidth and the theoretical antenna pattern gain, determine the maximum and minimum amplitudes of the received signal, and segment the amplitudes according to the interval of STR when the quantized signal power changes monotonously with STR, and calculate the fixed threshold of each segment; Step S5, perform spectrum analysis on the single-bit sampled signal, remove the single-frequency harmonic component, and accurately calculate the quantized signal power; Step S6, according to the quantitative model in step S2 and the quantized signal power in step S5, calculate STR, and determine the mean value of the original amplitude of the received signal according to the segmented threshold in step S4, to obtain the actual received signal power; Step S7, process single receiver data and comprehensive processing of multiple receiver data to obtain azimuth antenna pattern sampling value and range antenna pattern sampling value; Step S8, according to the radar equation, calculate the azimuth antenna gain and the elevation antenna gain; Step S9, according to the calculated azimuth antenna gain and elevation antenna gain, reconstruct the azimuth antenna pattern by linear interpolation, and reconstruct the range antenna pattern by cubic polynomial fitting; In step S3, the antenna pattern reconstruction strategy is as follows: when is less than 1, the signal is under-sampled, and the signal spectrum will be aliased. According to the energy conservation theorem, the quantized signal power remains unchanged within the frequency range corresponding to the frequency range , and the actual amplitude of the received signal is recovered according to the quantitative model in step S2; when is greater than or equal to 1, the signal spectrum is not aliased, and the quantized signal power is calculated normally; that is, regardless of the size of the SAR signal bandwidth , the receiver only needs to use a fixed sampling rate to recover the actual amplitude information of the received signal and reconstruct the radar antenna pattern. In step S4, the amplitudes of the received signal are segmented according to the interval of STR when the quantized signal power changes monotonously with STR, and the steps are as follows: Step S41, calculate the threshold of the first segment of the received signal amplitude, and the formula is as follows: ； wherein, is the maximum amplitude of the received signal, is the slant range of the SAR to the center of the swath, is a threshold value for the first segment of the received signal amplitude, is the maximum value of the monotonic change interval STR; Step S42, calculate the minimum amplitude of the first segment, and the formula is as follows: ； wherein is the minimum amplitude of the first section of the amplitude of the received signal, is the minimum value of the monotonic change interval STR; Step S43, calculating the minimum amplitude of the first section of the received signal amplitude as the maximum amplitude of the second section of the received signal amplitude , calculating the threshold value of the second section of the received signal amplitude ; Step S44, repeat steps S41-S43 to calculate the threshold value and the minimum amplitude of each subsequent segment, until the last segment of the received signal amplitude calculated minimum amplitude is less than or equal to the minimum amplitude of the received signal , and are the shortest and longest slant ranges from SAR to the mapping band, respectively.

2. The method of claim 1, wherein, In step S2, the monotonic relationship between the quantized signal power and STR is analyzed, the quantitative relationship between the quantized signal power and STR is determined by segmentation, and segmented linear fitting is performed, and the formula is as follows: ； wherein is the quantized signal power, and are the first order coefficient and the constant term coefficient of the linear function, respectively. are the first order coefficient and the constant term coefficient of the linear function, respectively.

3. The method of claim 2, wherein the threshold is varied based on a segment gain. 3 The amplitude of the received signal in step S4 is jointly determined by the elevation antenna pattern gain and the azimuth antenna pattern gain where, is the angle between the slant range from the SAR to the swath center and the slant range from the SAR to the receiver, is the instantaneous slant look angle in azimuth; the maximum amplitude of the received signal is the minimum amplitude of the received signal, which is obtained when the receiver is located at the center of the swath, is obtained when the receiver is simultaneously located at the edge of the azimuth beam and the edge of the swath, where, is the slant range from the SAR to the swath center, and are the shortest and longest slant ranges from the SAR to the swath, respectively.

4. The method of claim 2, wherein the threshold is varied based on the segment gain. In step S6, STR is calculated according to the segmented linear fitting relationship, and the formula is as follows: ； wherein is the instantaneous squint angle quantized signal power, , is the number of sampling points in the azimuth direction, is the signal threshold ratio at the th azimuth point. The formula for calculating the actual received signal power is as follows: ； ； wherein is the actual received signal power at the instantaneous squint angle , is the mean of the received signal raw amplitude at the instantaneous squint angle , is the corresponding threshold value at the instantaneous squint angle .

5. The method of claim 4, wherein the threshold is varied based on the segment gain. 5 In step S8, the formula of the radar equation is as follows: ； wherein, and are the received signal power and the transmitted signal power, respectively, is the wavelength, is the slant range between the SAR and the receiver, is the azimuth dimension antenna gain, is the elevation dimension antenna gain; Azimuthal antenna gain The calculation formula is as follows: ； wherein is the instantaneous squint angle in azimuth antenna gain; for the angle The antenna gain in the elevation direction is calculated as follows: ； wherein the received signal actual power at zero Doppler, the angle between the slant range from the SAR to the swath and the slant range from the SAR to the th receiver, the th receiver, th receiver, the instantaneous slant look angle of the SAR to the receiver at zero Doppler.

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