High-resolution long-acting-distance synthetic aperture radar signal preprocessing method

Abstract

The invention discloses a high-resolution long-acting-distance synthetic aperture radar signal preprocessing method, and relates to the technical field of radar signal processing. The method comprises the following steps: determining a sampling rate and a sampling point number; performing analog-to-digital conversion on the analog intermediate-frequency signal, and acquiring data to obtain a digital intermediate-frequency signal; performing dechirp processing to obtain a point target radar echo signal; performing filtering processing to obtain a radar echo signal of the region of interest; reducing the radar echo signal of the region of interest into a linear frequency modulation signal through frequency modulation reconstruction; sampling the signal after frequency modulation reconstruction to obtain a corresponding signal sample sequence; reducing the length of the signal sample sequence in a data accumulation mode; and performing pulse compression on the signals in the accumulated signal sample sequence by adopting a preset pulse compression method. According to the invention, the difficulty of pulse compression, the requirements on hardware storage space and computing resources, the implementation difficulty of an analog receiver, and the pressure and processing difficulty of subsequent recording are reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of radar signal processing, and in particular to a high-resolution long-range synthetic aperture radar signal preprocessing method. Background Art

[0002] Synthetic aperture radar (SAR) is a microwave remote sensing imaging device capable of acquiring high-resolution images of the two-dimensional backscatter coefficient distribution of terrain surfaces, with resolution independent of range. Its all-day, all-weather capability and high operational efficiency have led to its widespread application in battlefield reconnaissance, mapping, and emergency response. The significant economic, military, and social impact it has generated remains a hot topic of scientific research.

[0003] Synthetic aperture radar (SAR) resolution includes range resolution and azimuth resolution. Range resolution is proportional to the bandwidth of the transmitted signal: it increases as the bandwidth of the transmitted signal increases. Azimuth resolution is proportional to the range of view of the target exposed to the radar. Because the beamwidth is fixed due to the physical size of the antenna, the highest azimuth resolution in stripe mode is fixed. There are two ways to improve the azimuth resolution of the radar: one is to reduce the antenna's azimuth size and increase the antenna's beamwidth; the other is to focus the radar antenna on the target of interest, increasing the target's view range and thus improving the resolution. The first method reduces the antenna's gain, thereby reducing the radar's operational power, and is therefore generally not used. The other method is the well-known spotlight mode.

[0004] The range of a radar is related to three factors: 1) the bandwidth of the transmitted signal; 2) the peak power of the transmitted signal; and 3) the gain of the antenna. For a given radar, the way to increase its range is to increase its average transmit power.

[0005] With the rapid development of solid-state electronic component technology, transmitters are gradually transitioning from traveling wave tubes to solid-state power amplifiers (SPAs). The numerous advantages of SPAs have led to their widespread application since their introduction. To exploit these advantages, two methods are commonly used in the design of synthetic aperture radars (SARs) using SPAs to improve the transmitter's duty cycle: 1) increasing the pulse repetition frequency (PRF); and 2) increasing the pulse width of the transmitted signal. Doubling the PRF while maintaining the same pulse width doubles the duty cycle, but at the same time, the data volume doubles, placing significant pressure on subsequent data recording and processing. Furthermore, increasing the PRF limits the unambiguous range, leading to range ambiguity. Therefore, increasing the PRF needs to be appropriately adjusted based on system capabilities. Increasing the pulse width of the transmitted signal is the primary method for improving the transmitter's duty cycle. Increasing the pulse width increases the range blind spot. For example, a 10µs pulse width corresponds to a 1.5km range blind spot, while a 100µs pulse width corresponds to a 15km range blind spot. For high-performance SARs, adjusting the pulse width can address this issue. If the effective range is short, the pulse width is narrow, and the corresponding distance blind area is small; if the effective range is long, the pulse width is wide, and the corresponding distance blind area is large. This is consistent with reality, because short effective range requires less transmitter energy, narrow pulses, and low duty cycle; long effective range requires more transmitter energy, so the pulse width is wide and the duty cycle is high.

[0006] In the prior art, there are two common methods for receiving signals from high-resolution, long-range synthetic aperture radars:

[0007] The first type: analog de-slant reception, its working principle block diagram is as follows Figure 1 shown.

[0008] Please refer to Figure 1 , assuming that the pulse width of the transmitted signal is T, and the echo time corresponding to the width of the mapping band W is T0 ( c represents the speed of light), and the echo delay at the center of the surveying band is t c The working principle of analog de-skew reception is:

[0009] First, the radar system generates a frequency modulation signal with the same slope as the transmitted signal. The duration is equal to T+T0, with t c The linear frequency modulation signal centered at is used as the analog reference local oscillator; it is multiplied with the echo signal (equivalent to analog de-skewing) to cancel the quadratic phase component of the echo signal and realize the conversion of the linear frequency modulation signal to a point frequency signal. The frequency f1 of the point frequency signal is proportional to the distance r from the point target to the center of the mapping band:

[0010]

[0011] In formula (1): K r Represents the frequency modulation slope of the transmitted signal. Because the ground object can be regarded as a number of point targets with different r, the de-skewing signal is composed of a group of point frequency signals with different frequencies superimposed.

[0012] The de-skewed signal then undergoes a low-pass filter to remove invalid signals outside the survey area. This reduces the bandwidth of the received signal while retaining the useful signal within the observation area. Finally, a high-precision, low-speed ADC samples the signal, achieving analog de-skewed reception of the echo signal.

[0013] However, in actual application, the above Figure 1 The method for receiving radar echo signals with de-slanted scanning has the following technical defects:

[0014] (1) It is difficult to generate a broadband, high-quality analog de-skewed reference signal;

[0015] (2) In the digital domain, it is necessary to correct the space-varying error introduced by the nonlinearity of the received signal and the space-invariant error introduced by the nonlinearity of the reference signal. In engineering practice, it is difficult for the radar transmit signal and the reference local oscillator to be in an ideal state, so they will both have a nonlinearity error. The effect of the nonlinearity error on the signal after de-skewing is as follows: Figure 2 shown. Figure 2 Figure a shows the received signal before de-skewing. It is a superposition of different delayed versions of the transmitted signal. The signals from left to right correspond to distances from near to far. The swept frequency nonlinearity in the received signal is space-variant. Figure b shows the time-frequency distribution of the reference signal, which is space-invariant. Figure c shows the received signal and the reference LO after de-skewing.

[0016] (3) The hardware complexity of the simulation part is high.

[0017] The core of the above-mentioned problem of analog de-skewing is the frequency sweep nonlinearity caused by technical limitations, which becomes a key factor affecting the quality of the final synthetic aperture radar image.

[0018] The second type: digital intermediate frequency receiving technology, its working principle block diagram is as follows Figure 3 shown.

[0019] Please refer to Figure 3 The radar receives echo signals that are down-converted to an intermediate frequency using a fixed-frequency local oscillator. After filtering, amplification, and gain control, they are sampled by a high-speed ADC, and digital down-conversion and low-pass filtering are completed in the digital domain to obtain a digital baseband echo signal.

[0020] The biggest drawback of this approach is that the data volume of a single PRF is very large, making real-time pulse compression difficult. For example, if the transmit signal bandwidth is 1800 MHz, the sampling rate is 2160 MSPS, the mapping bandwidth is 2 km, and the pulse width is 150 µs, then the number of points in a single PRF sample is: 2160 * (2000 / 150 + 150) = 352,800. Implementing a Fourier transform of this large number of points in real time using an FPGA or DSP is difficult, if not impossible.

[0021] The core of the above-mentioned problem of digital intermediate frequency receiving mode is that long-point pulse compression cannot be achieved in real time using traditional methods due to technical limitations. Summary of the Invention

[0022] In order to overcome the above problems or at least partially solve the above problems, the present invention provides a high-resolution long-range synthetic aperture radar signal preprocessing method.

[0023] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0024] The present invention provides a high-resolution long-range synthetic aperture radar signal preprocessing method, comprising the following steps:

[0025] Determine the sampling rate f based on the transmitted signal bandwidth B, pulse width T and mapping bandwidth W s , number of sampling points N;

[0026] According to the sampling rate f s Perform analog-to-digital conversion on the analog intermediate frequency signal and collect N-point data to obtain a digital intermediate frequency signal in digital form, with the number of sampling points being N;

[0027] De-skewing is performed on the collected N-point digital intermediate frequency signals to obtain the corresponding point target radar echo signals;

[0028] Filter the point target radar echo signal to remove the radar echo signal of no interest to obtain the radar echo signal of the area of interest;

[0029] Through frequency modulation reconstruction, the radar echo signal of the area of interest is restored to a linear frequency modulation signal;

[0030] According to the preset sampling ratio M, the FM reconstructed signal is sampled to reduce the sampling rate and obtain the corresponding signal sample sequence; the sampling rate after sampling is The length of the signal sample sequence is:

[0031] Reduce the length of the signal sample sequence by data accumulation, and the length of the signal sample sequence after accumulation is P;

[0032] The preset pulse compression method is used to perform pulse compression on the signal in the accumulated signal sample sequence, and the final radar signal processing result is output.

[0033] The present invention combines digital down-conversion, low-pass filtering, sampling, error compensation, pulse compression and other processes and completes them all together; it reduces the difficulty of pulse compression, the requirements for hardware storage space and computing resources, the difficulty of implementing analog receivers, and the pressure and difficulty of subsequent recording and processing.

[0034] Furthermore, the above method for performing de-skewing processing on the collected N-point digital intermediate frequency signals includes the following steps:

[0035] The digital intermediate frequency signal is de-skewing according to a preset linear frequency modulation reference local oscillator with the same frequency modulation slope as the transmitted signal, thereby removing the secondary frequency modulation phase term in the point target radar echo signal and retaining the linear phase term whose frequency is proportional to the distance from the scene center.

[0036] Furthermore, the method of performing pulse compression on the signal in the accumulated signal sample sequence using a preset pulse compression method comprises the following steps:

[0037] Perform FFT transformation on the accumulated signal and transform it into the frequency domain;

[0038] The signal transformed into the frequency domain is multiplied point by point by a preset pulse pressure reference function to obtain a complex multiplication result;

[0039] Perform IFFT transformation on the complex multiplication result to complete the pulse compression processing.

[0040] Furthermore, the high-resolution long-range synthetic aperture radar signal preprocessing method further includes the following steps:

[0041] Sampling the system closed-loop signal to obtain a closed-loop signal;

[0042] Perform analog-to-digital conversion, digital de-skewing, low-pass filtering, re-frequency modulation, downsampling, and accumulation on the closed-loop signal to obtain a closed-loop reference signal of length P;

[0043] Perform FFT transformation on the closed-loop reference signal and take its conjugate to obtain the pulse pressure reference function.

[0044] Furthermore, the above sampling rate f s satisfy: Where: c represents the speed of electromagnetic wave propagation.

[0045] Furthermore, the number of sampling points N satisfies: Where: c represents the speed of electromagnetic wave propagation.

[0046] Furthermore, the point target radar echo signal is filtered by a low-pass filter.

[0047] Furthermore, the passband frequency f of the low-pass filter is lpf satisfy: Where: c represents the speed of electromagnetic wave propagation.

[0048] Furthermore, the reference signal used in the FM reconstruction has the same length and bandwidth as the reference signal used in the de-skewing process, but has an opposite FM slope.

[0049] Furthermore, the above pulse compression length P satisfies: Where: c represents the speed of electromagnetic wave propagation.

[0050] The present invention has at least the following advantages or beneficial effects:

[0051] 1. Combine digital down-conversion, low-pass filtering, sampling, error compensation, pulse compression and other processes and complete them together to improve signal processing efficiency.

[0052] 2. Reduce the difficulty of pulse compression: For high-resolution long-range synthetic aperture radar, the traditional pulse compression method is difficult to implement in hardware due to the long number of points. The method proposed in this invention reduces the data length of pulse compression from When it is reduced to P, the data length reaches 10 times, which greatly reduces the difficulty of pulse compression.

[0053] 3. Reduced requirements for hardware storage space and computing resources. The method provided by the present invention is specifically customized around FPGA. Every step in it can be implemented using the IP core provided by FPGA, and the real-time performance is extremely high. Compared with traditional methods, this method reduces the difficulty of hardware implementation.

[0054] 4. Reduce the difficulty of implementing analog receivers. An analog de-skew receiver needs to generate a linear frequency modulation signal at the center of the scene with the same frequency modulation slope as the transmitted signal, and with a time width and bandwidth greater than the time width and bandwidth of the transmitted signal, as the local oscillator of the de-skew receiver. Secondly, there are high requirements for signal linearity. If the signal linearity is poor, space-varying amplitude and phase errors will be generated, which will bring difficulties to subsequent signal processing and even affect the image quality of the synthetic aperture radar.

[0055] 5. Reduced subsequent recording pressure and processing difficulty. After processing digital IF data using the method proposed in this invention, the sampling rate is the same as that of traditional synthetic aperture radar. The data length is only related to the mapping bandwidth, not the pulse width. The reduction in range samples significantly reduces the pressure on subsequent imaging processing and data recorders. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0057] Figure 1 Flowchart of a radar echo signal de-skewing receiving method in the prior art;

[0058] Figure 2 A schematic diagram of a signal before de-skewing, a reference signal, and a signal after de-skewing in a radar echo signal de-skewing receiving method in the prior art;

[0059] Figure 3 This is a flow chart of a digital intermediate frequency receiving method for radar echo signals in the prior art;

[0060] Figure 4 This is a flow chart of a high-resolution, long-range synthetic aperture radar signal preprocessing method according to an embodiment of the present invention;

[0061] Figure 5 : is a time-frequency relationship diagram of a digital intermediate frequency echo signal in an embodiment of the present invention;

[0062] Figure 6 1 is a time-frequency relationship diagram of a digitally de-skewed reference local oscillator in an embodiment of the present invention;

[0063] Figure 7 1 is a time-frequency relationship diagram of an echo signal after digital de-skewing in an embodiment of the present invention;

[0064] Figure 8 is the amplitude-frequency response curve of the low-pass filter according to an embodiment of the present invention;

[0065] Figure 9 The time-frequency relationship of the signal after low-pass filtering in the embodiment of the present invention;

[0066] Figure 10 A time-frequency relationship diagram of a reference local oscillator for frequency modulation reconstruction according to an embodiment of the present invention;

[0067] Figure 11 This is a time-frequency relationship diagram of the echo signal after frequency modulation reconstruction according to an embodiment of the present invention;

[0068] Figure 12 The time domain waveforms before and after accumulation in the embodiment of the present invention;

[0069] Figure 13 This is a block diagram of the principle of implementing a pulse compression system using classic pulse compression in an embodiment of the present invention;

[0070] Figure 14 This is the result of using traditional pulse compression in an embodiment of the present invention;

[0071] Figure 15 Comparison of the pulse compression results provided by the embodiment of the present invention and the pulse compression results of the traditional method. DETAILED DESCRIPTION

[0072] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0073] Therefore, 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 as claimed, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0074] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0075] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprises" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further limitations, an element defined by the sentence "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device that includes the element.

[0076] Example:

[0077] The parameters in the embodiment of the present invention are shown in Table 1.

[0078] Table 1 Parameters used in the examples

[0079] Serial number Parameter name Parameter value 1. IF center frequency 3.6GHz 2. Signal bandwidth 1600MHz 3. Mapping bandwidth 1.5km 4. Pulse width 150us 5. Scene center distance 150km

[0080] like Figure 4 As shown, the present invention provides a high-resolution long-range synthetic aperture radar signal preprocessing method, comprising the following steps:

[0081] S1. Determine the sampling rate f based on the transmitted signal bandwidth B, pulse width T and mapping bandwidth W s , the number of sampling points N; the above sampling rate f s satisfy: Where c represents the speed of electromagnetic wave propagation. The intermediate frequency sampling frequency is determined to be 5400 MSPS, and the frequency modulation slope of the transmitted signal is 10.6667 MHz / us.

[0082] In some embodiments of the present invention, the sampling rate given by the above formula is much higher than the sampling rate specified by the Nyquist sampling theorem. The purpose is to ensure that there is no aliasing in the signal spectrum after de-skewing. The bandwidth of the signal after digital de-skewing is It is larger than the transmitted signal bandwidth B. This is easily satisfied in a digital intermediate frequency sampling system.

[0083] S2, according to the sampling rate f s Perform analog-to-digital conversion on the analog intermediate frequency signal and collect N-point data to obtain a digital intermediate frequency signal in digital form. The number of sampling points is N. The above-mentioned number of sampling points N satisfies: Where: c represents the speed of electromagnetic wave propagation.

[0084] In some embodiments of the present invention, the high speed ADC uses a sampling rate f s Perform analog-to-digital conversion on the analog IF signal, collect N data points, and obtain a digital IF signal in digital form. The number of sampling points is selected as 864,000. Figure 5 A time-frequency relationship diagram of the digital echo signal is given. It can be seen from the figure that except for the linear frequency modulation signal of the echo signal in the surveying area, which is complete, the target echo signals in other distance ranges are incomplete and need to be filtered out from the echo signal.

[0085] S3, performing de-skewing processing on the collected N-point digital intermediate frequency signals to obtain corresponding point target radar echo signals;

[0086] Furthermore, it includes: de-skewing the digital intermediate frequency signal according to a preset linear frequency modulation reference local oscillator with the same frequency modulation slope as the transmitted signal, removing the secondary frequency modulation phase term in the point target radar echo signal, and retaining the linear phase term whose frequency is proportional to the distance from the scene center.

[0087] In some embodiments of the present invention, a pre-calculated linear frequency modulation reference oscillator with a length of N and the same frequency modulation slope as the transmitted signal is used to perform de-skewing on the digital intermediate frequency signal, removing the secondary frequency modulation phase term in the point target echo signal while retaining the linear phase term whose frequency is proportional to the distance from the scene center. The center frequency of the linear frequency modulation reference oscillator is the center frequency of the digital intermediate frequency. After digital de-skewing, the echo of the target at the center of the scene becomes a DC signal, and the signal frequency f of the targets at other locations is r It is proportional to the distance r between the target and the center of the scene, as shown in the following formula: Figure 6 The time-frequency relationship diagram of the digital de-skewed reference local oscillator is given. The center frequency of the reference local oscillator is 3600 MHz, the number of continuous points is 864,000, the minimum frequency is 2747 MHz, the maximum frequency is 4453 MHz, and the total bandwidth is 1706 MHz, which is the sum of the bandwidth of the transmitted signal and the bandwidth of the signal corresponding to the survey area.

[0088] Figure 7 A time-frequency plot of the echo signal after digital de-skewing is presented. After de-skewing, the echo signals of point targets are now point-frequency signals. The frequency of the point-frequency signal is proportional to the distance from the target to the scene center. The pulse duration of the point-frequency signal for targets in the center of the survey area is 150µs. As the distance from the scene center increases, the frequency increases and the duration decreases. This is due to the incompleteness of these target echoes. The highest frequency of the de-skewing target is 1654MHz, and the lowest frequency is -1654MHz. The total bandwidth of the de-skewing signal is 3308MHz, which is the sum of twice the bandwidth of the transmitted signal and the signal bandwidth corresponding to the survey bandwidth.

[0089] S4. Filtering the point target radar echo signal to remove radar echo signals of no interest to obtain radar echo signals of the area of interest;

[0090] Furthermore, the point target radar echo signal is filtered by a low-pass filter. The passband frequency f of the low-pass filter is lpf satisfy: Where: c represents the speed of electromagnetic wave propagation.

[0091] In some embodiments of the present invention, the maximum passband frequency of the low-pass filter needs to be greater than the maximum frequency corresponding to the target of interest, ensuring that the frequency components of the target of interest pass through while irrelevant signals are filtered out, and simultaneously filtering out the image frequency components of the useful signal. The digitally de-skewed signal includes signals inside and outside the surveying area. It is necessary to filter out the echo signals outside the area of no interest through low-pass filtering. The amplitude-frequency response curve of the designed FIR low-pass filter is shown as follows: Figure 8The filter has a passband cutoff frequency of 54 MHz and an out-of-band rejection of over 60 dB.

[0092] Figure 9 The time-frequency relationship of the signal after low-pass filtering is shown. Low-pass filtering removes signals outside the survey area, retaining the target echo signals within the survey area. The upper line in the figure represents the echo signal of the target in the survey area; the middle line represents the echo signal of the target at the center of the scene; and the lower line represents the echo signal of the target in the survey area.

[0093] S5. Restore the radar echo signal of the region of interest to a linear frequency modulation signal through frequency modulation reconstruction. The reference signal used in the frequency modulation reconstruction has the same length and bandwidth as the reference signal of the de-skewing process, but has an opposite frequency modulation slope.

[0094] In some embodiments of the present invention, the echo signal of the region of interest is restored to a linear frequency modulation signal through frequency modulation reconstruction, so that the echo signal of each point target is exactly the same as the signal before de-skewing; the length of the reference signal used in the frequency modulation reconstruction is the same as the length of the reference signal of the digital de-skewing, the same bandwidth, the opposite frequency modulation slope, and the center frequency is 0Hz. The baseband digital echo signal is obtained through frequency modulation reconstruction, and digital down-conversion and low-pass filtering are realized synchronously. In the process of actual engineering implementation, digital de-skewing reception cannot compensate for the nonlinear error of the signal. For this reason, the de-skewing signal is re-frequency modulated and reconstructed into a linear frequency modulation signal. The time-frequency relationship of the reference local oscillator of the frequency modulation reconstruction is as follows: Figure 10 As shown. The slope of this local oscillator is opposite to that of the de-slanted local oscillator, with a center frequency of 0Hz, a minimum frequency of -885MHz, a maximum frequency of 885MHz, and a total bandwidth of 1770MHz, which is the sum of the transmission signal bandwidth and the signal bandwidth corresponding to the mapping bandwidth. Here, the center frequency is 0, which means that the reconstructed linear frequency modulation signal is a baseband signal, no longer a digital intermediate frequency signal. The time-frequency relationship of the reconstructed echo signal is as follows Figure 11 As shown in the figure, only the target echo within the survey area is shown, and its duration is the same as the pulse width of the transmitted signal, 150µs. The minimum frequency of the signal is -800MHz, the maximum frequency is 800MHz, and the total bandwidth is 1600MHz, the same as the bandwidth of the transmitted signal.

[0095] S6. According to the preset sampling ratio M, the signal after FM reconstruction is sampled to reduce the sampling rate to obtain the corresponding signal sample sequence; the sampling rate after sampling is The length of the signal sample sequence is: The sampling rate of the sampled sequence is greater than the bandwidth of the transmitted signal:

[0096] In some embodiments of the present invention, since the sampling rate of 5400MHz is much higher than the actual signal bandwidth of 1600MHz, the sampling rate is reduced by decimation. The sampling ratio is 3, and the sampling rate after decimation is 1800MSPS. The length of the sample sequence is 288000, and the sampling rate and the length of the sample sequence are both reduced by 3 times. The mapping bandwidth is 1.5km, the corresponding time length is 10us, and the corresponding number of points is 18000 points. With a certain margin, the number of points after accumulation is set to 19200 points, which is 15 times the number of points before accumulation. Therefore, the original sequence is accumulated, and the signal waveforms before and after accumulation are as follows: Figure 12 The upper figure is the time domain waveform before accumulation, and the lower figure is the time domain waveform after accumulation. After accumulation, the amplitude of the signal is greater than the amplitude of the signal before accumulation.

[0097] S7. Reduce the length of the signal sample sequence by data accumulation. The length of the signal sample sequence after accumulation is P. The pulse compression length P satisfies: Where: c represents the speed of electromagnetic wave propagation.

[0098] S8. Use a preset pulse compression method to perform pulse compression on the signal in the accumulated signal sample sequence, and output the final radar signal processing result.

[0099] Furthermore, it includes: performing FFT transformation on the accumulated signal to transform it into the frequency domain; performing point-by-point complex multiplication on the signal transformed into the frequency domain and a preset pulse pressure reference function to obtain a complex multiplication result; performing IFFT transformation on the complex multiplication result to complete the pulse compression processing.

[0100] Furthermore, it also includes: sampling the system closed-loop signal to obtain a closed-loop signal; performing analog-to-digital conversion and digital de-skewing, low-pass filtering, re-frequency modulation, downsampling, and accumulation on the closed-loop signal to obtain a closed-loop reference signal of length P; performing FFT transformation on the closed-loop reference signal and taking conjugate to obtain a pulse pressure reference function.

[0101] In some embodiments of the present invention, a classic pulse compression method is used to perform pulse compression on the accumulated signal. The block diagram of the classic pulse compression implementation principle is as follows: Figure 13 The specific implementation steps are as follows:

[0102] (1) Perform FFT transformation on the accumulated signal to transform it into the frequency domain;

[0103] (2) Multiply the signal transformed into the frequency domain by the pre-stored pulse pressure reference function point by point to obtain the multiplication result; the process of obtaining the pulse pressure reference function is as follows:

[0104] The system's closed-loop signal is sampled to obtain a closed-loop signal. This closed-loop signal undergoes analog-to-digital conversion, digital de-skewing, low-pass filtering, frequency re-modulation, and accumulation to obtain a closed-loop reference signal of length P (19200). Because the closed-loop reference function is the actual closed-loop signal being acquired, it also includes the channel and signal errors of the entire system. Using it for pulse compression also compensates for system errors. The closed-loop reference signal undergoes an FFT transform and takes its conjugate to obtain the pulse compression reference function.

[0105] (3) Perform IFFT transformation on the complex multiplication result to complete the pulse compression processing.

[0106] Figure 14 The results after traditional pulse compression are shown. The distance between each target is 750m, corresponding to a two-way delay of 5us. Traditional pulse compression not only compresses the target echo signals inside the survey area, but also the target echo signals outside the survey area.

[0107] In order to test the effectiveness of the method proposed in the present invention, the results of pulse compression of the present invention are compared with those of classical pulse compression. Figure 15 The pulse compression results for point targets inside the survey area are exactly the same.

[0108] First, the present invention combines digital down-conversion, low-pass filtering, sampling, error compensation, pulse compression and other processes into one process, thus improving the signal processing efficiency. Second, the present invention reduces the difficulty of pulse compression: for high-resolution long-range synthetic aperture radar, the method proposed by the present invention reduces the data length of pulse compression from The data length is reduced to P, which is ten times the size, significantly reducing the difficulty of pulse compression. Furthermore, the present invention reduces the requirements for hardware storage space and computing resources: the method provided by the present invention is specifically customized for FPGAs, and each step can be implemented using the IP core provided by the FPGA. This method also offers extremely high real-time performance, significantly reducing the difficulty of hardware implementation compared to traditional methods. Furthermore, the present invention reduces the implementation difficulty of analog receivers: an analog de-skewed receiver must generate a linear frequency modulation signal at the center of the scene with the same frequency modulation slope as the transmitted signal, but with a time width and bandwidth greater than the transmitted signal's time width and bandwidth, as the de-skewed receiver's local oscillator. Furthermore, high requirements are placed on signal linearity. Poor signal linearity can result in space-varying amplitude and phase errors, complicating subsequent signal processing and even affecting synthetic aperture radar image quality. Finally, the present invention reduces the burden and difficulty of subsequent recording. After processing the digital intermediate frequency data using the method proposed by the present invention, the sampling rate is the same as that of traditional synthetic aperture radars, and the data length is dependent only on the surveying bandwidth, not the pulse width. The reduction in range samples significantly alleviates the burden on subsequent imaging processing and data recorders.

[0109] In the embodiments provided in this application, it should be understood that the disclosed methods and systems can also be implemented in other ways. The method and system embodiments described above are merely illustrative. For example, the flowcharts and block diagrams in the accompanying drawings show the possible architectures, functions, and operations of the methods and systems, methods, and computer program products according to multiple embodiments of the present application. In this regard, each box in the flowchart or block diagram can represent a module, a program segment, or a portion of code, and the module, program segment, or a portion of code contains one or more executable instructions for implementing the specified logical functions. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in an order different from that marked in the accompanying drawings. For example, two consecutive boxes can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flowchart, and the combination of boxes in the block diagram and / or flowchart, can be implemented using a dedicated hardware-based system that performs the specified function or action, or can be implemented using a combination of dedicated hardware and computer instructions.

[0110] In addition, the functional modules in each embodiment of the present application can be integrated together to form an independent part, or each module can exist independently, or two or more modules can be integrated to form an independent part.

[0111] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

[0112] It will be apparent to those skilled in the art that the present application is not limited to the details of the exemplary embodiments described above and that the present application can be implemented in other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the present application is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

Claims

1. A high-resolution long-range synthetic aperture radar signal preprocessing method, characterized in that: The following steps are involved: Determine the sampling rate f based on the transmitted signal bandwidth B, pulse width T and mapping bandwidth W s , number of sampling points N; According to the sampling rate f s Perform analog-to-digital conversion on the analog intermediate frequency signal and collect N-point data to obtain a digital intermediate frequency signal in digital form, with the number of sampling points being N; De-skewing is performed on the collected N-point digital intermediate frequency signals to obtain the corresponding point target radar echo signals; Filter the point target radar echo signal to remove the radar echo signal of no interest to obtain the radar echo signal of the area of interest; Through frequency modulation reconstruction, the radar echo signal of the area of interest is restored to a linear frequency modulation signal; According to the preset sampling ratio M, the FM reconstructed signal is sampled to reduce the sampling rate and obtain the corresponding signal sample sequence; the sampling rate after sampling is The length of the signal sample sequence is: Reduce the length of the signal sample sequence by data accumulation, and the length of the signal sample sequence after accumulation is P; The preset pulse compression method is used to perform pulse compression on the signal in the accumulated signal sample sequence, and the final radar signal processing result is output.

2. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The method for performing de-skewing processing on the collected N-point digital intermediate frequency signals comprises the following steps: The digital intermediate frequency signal is de-skewing according to a preset linear frequency modulation reference local oscillator with the same frequency modulation slope as the transmitted signal, thereby removing the secondary frequency modulation phase term in the point target radar echo signal and retaining the linear phase term whose frequency is proportional to the distance from the scene center.

3. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The method of performing pulse compression on the signal in the accumulated signal sample sequence using a preset pulse compression method comprises the following steps: Perform FFT transformation on the accumulated signal and transform it into the frequency domain; The signal transformed into the frequency domain is multiplied point by point by a preset pulse pressure reference function to obtain a complex multiplication result; Perform IFFT transformation on the complex multiplication result to complete the pulse compression processing.

4. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 3, characterized in that: The following steps are also included: Sampling the system closed-loop signal to obtain a closed-loop signal; Perform analog-to-digital conversion, digital de-skewing, low-pass filtering, re-frequency modulation, downsampling, and accumulation on the closed-loop signal to obtain a closed-loop reference signal of length P; Perform FFT transformation on the closed-loop reference signal and take its conjugate to obtain the pulse pressure reference function.

5. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The sampling rate f s satisfy: Where: c represents the speed of electromagnetic wave propagation.

6. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The number of sampling points N satisfies: Where: c represents the speed of electromagnetic wave propagation.

7. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The point target radar echo signal is filtered through a low-pass filter.

8. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 7, characterized in that: The passband frequency f of the low-pass filter lpf satisfy: Where: c represents the speed of electromagnetic wave propagation.

9. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The reference signal used in the frequency modulation reconstruction has the same length and bandwidth as the reference signal used in the de-skewing process, but has an opposite frequency modulation slope.

10. The high-resolution long-range synthetic aperture radar signal preprocessing method according to claim 1, characterized in that: The pulse compression length P satisfies: Where: c represents the speed of electromagnetic wave propagation.

Images