An airborne SAR echo receiving system and method based on non-uniform equivalent bandpass sampling

Abstract

The present invention discloses an airborne SAR echo receiving system and method based on non-uniform equivalent bandpass sampling. After the broadband radar echo signal is down-converted in one stage, a high-speed ADC chip with a large broadband input is selected to perform non-uniform equivalent bandpass sampling on the echo after frequency conversion, and the real-time sampling rate of the ADC can be reduced to less than 40% of the bandwidth of the echo signal; finally, IQ demodulation and pulse compression are performed in the digital domain, thereby avoiding the problem of amplitude and phase inconsistency of the broadband echo signal in the radio frequency domain IQ demodulation. Simulation analysis shows that this technology will not bring a large disturbance to the phase of the airborne SAR echo, can meet the pulse compression sidelobe level requirements and SAR imaging performance, and greatly reduce the sampling rate requirements of the ADC. Compared with the traditional airborne high-resolution SAR broadband echo receiving technology, this non-uniform equivalent bandpass sampling receiving technology has the characteristics of high performance, low cost, low power consumption, and simple structure.

Description

Technical Field

[0001] The present invention belongs to the field of radar technology in the electronic information industry, and relates to an airborne SAR echo receiving system and method based on non-uniform equivalent bandpass sampling. Background Art

[0002] In recent years, high-resolution synthetic aperture radar (SAR), broadband and ultra-wideband radar technologies have attracted much attention in the scientific and engineering communities. The SAR system has a large signal bandwidth, so it has high-resolution imaging capabilities, which can provide more detailed information for target detection, classification and recognition, greatly improving the probability of target detection and recognition; at the same time, high-resolution interferometric SAR also provides high-quality data basic information for high-precision digital elevation models (DEM). However, due to the high-speed movement of the aircraft platform carrying the radar, the traditional equivalent sampling technology is difficult to meet the sampling and reception requirements for quickly obtaining broadband radar echoes. Therefore, expensive ultra-high-speed sampling receivers are required to quickly sample and receive radar echoes, which greatly increases the difficulty of large-scale promotion of airborne high-resolution SAR systems.

[0003] Spaceborne SAR systems have global Earth imaging observation capabilities, but their resolution is limited, generally 1m-50m, mainly because international spectrum laws limit the available bandwidth of spaceborne SAR signals. Airborne SAR mainly performs close-range high-resolution imaging of local areas, generally 0.1-0.5m, and its signal bandwidth is above 1GHz. According to the Nyquist sampling theorem, the sampling rate of the corresponding sampling receiver should be above 2Gsps, and the sampling quantization resolution needs to be 8-12 bits to meet the needs of later SAR imaging, which makes it difficult for airborne SAR systems to receive broadband echo signals.

[0004] At present, there are three main methods for receiving broadband echoes of airborne high-resolution SAR: first, according to the Nyquist sampling theorem, the broadband echo signal is directly sampled and received using an ultra-high-speed ADC (sampling rate of 2-10Gbps). This method can ensure the amplitude and phase characteristics of the echo signal, but it is expensive; second, the broadband echo signal is divided into multiple channels, each of which is demodulated into a narrowband signal of a different frequency band, and parallel sampling and reception are performed through multiple high-speed sampling receivers, and finally the broadband signal is synthesized in the digital domain. This method can better restore the amplitude and phase characteristics of the signal, but the structure of the multi-channel demodulation and sampling receiver is complex, and the real-time performance is affected by the computational complexity of the broadband signal synthesis algorithm; third, the broadband echo signal is de-skewed and then subjected to a lower-frequency Nyquist sampling method. This method is suitable for application scenarios with a smaller imaging width, and has very stringent requirements on the flatness of the RF circuit in a wide frequency band.

[0005] Equivalent sampling technology is widely used in ultra-wideband radars, which can significantly reduce the sampling rate requirements of sampling receivers, but its platform movement speed is slow, generally 1-10m / s. If equivalent sampling is directly used for airborne high-resolution SAR echo reception, the echo phase disturbance caused by the high-speed movement of the aircraft will occur, which will seriously affect the SAR imaging quality. Summary of the invention

[0006] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, and to propose an airborne SAR echo receiving system and method based on non-uniform equivalent bandpass sampling. This technology can significantly reduce the real-time sampling frequency of airborne high-resolution SAR broadband echo signal reception, and does not require ultra-high-speed ADC devices or complex multi-channel parallel receivers, but uses non-uniform equivalent bandpass sampling technology to reduce sampling reception requirements. After the broadband radar echo signal is down-converted by one level, a high-speed ADC chip with a large broadband input is selected to perform non-uniform equivalent bandpass sampling on the echo after frequency conversion, and the real-time sampling rate of the ADC can be reduced to less than 40% of the echo signal bandwidth; finally, IQ demodulation and pulse compression are performed in the digital domain, avoiding the problem of amplitude and phase inconsistency of the broadband echo signal in the radio frequency domain IQ demodulation. Through simulation analysis, it is shown that this technology will not bring large disturbances to the phase of the airborne SAR echo, can meet the pulse compression sidelobe level requirements and SAR imaging performance, and significantly reduce the sampling rate requirements of the ADC. Compared with the traditional airborne high-resolution SAR broadband echo receiving technology, this non-uniform equivalent bandpass sampling receiving technology has the characteristics of high performance, low cost, low power consumption and simple structure.

[0007] The technical solution of the present invention is: an airborne SAR echo receiving system based on non-uniform equivalent bandpass sampling, comprising a non-uniform equivalent bandpass sampling module, a frequency synthesizer and distributor, a broadband signal transmitter, a circulator, a SAR antenna, a broadband signal receiver and a radar echo data storage;

[0008] The non-uniform equivalent bandpass sampling module receives the radar working mode and working parameters, and generates the corresponding radar digital intermediate frequency signal to the broadband signal transmitter according to the working parameters; at the same time, the non-uniform equivalent bandpass sampling and reception is performed on the RF signal after the first-stage demodulation, and the digital domain signal recovery, IQ demodulation and pulse compression processing are performed, and finally the compressed echo signal is stored in the radar echo data storage;

[0009] The frequency synthesizer and distributor generates the working clock signal required by the broadband signal transmitter, broadband signal receiver, and non-uniform equivalent bandpass sampling module;

[0010] The broadband signal transmitter receives the radar digital intermediate frequency signal sent by the non-uniform equivalent bandpass sampling module, converts it into a radar intermediate frequency signal, modulates, multiplies the frequency and amplifies the power of the radar intermediate frequency signal, obtains the high-resolution SAR RF signal and outputs it to the circulator, and finally feeds the SAR antenna;

[0011] The circulator feeds the transmission signal of the broadband signal transmitter to the SAR antenna, and transmits the echo signal received by the SAR antenna to the broadband signal receiver;

[0012] The SAR antenna transmits and receives radar signals in a directionally controlled manner in the imaging detection area;

[0013] The broadband signal receiver performs switch conduction, first-stage bandpass filtering, variable gain low-noise amplification and first-stage mixing processing on the echo signal output by the circulator, as well as second-stage bandpass filtering, second-stage low-noise amplification and third-stage bandpass filtering processing to obtain a broadband echo signal for sampling and quantization in a non-uniform equivalent bandpass sampling module;

[0014] The radar echo data storage device stores the radar echo data output by the non-uniform equivalent bandpass sampling module.

[0015] The non-uniform equivalent bandpass sampling module includes: a field programmable gate array FPGA, an analog-to-digital converter and a programmable delay line;

[0016] The field programmable gate array FPGA transmits and feeds back the working mode and working parameters with the aircraft platform, generates radar digital intermediate frequency signals for the broadband signal transmitter, controls the delay amount of the programmable delay line, and performs signal recovery, IQ demodulation and pulse compression processing on the echo signal sampled by the analog-to-digital converter ADC, and finally stores the echo data in the radar echo data memory;

[0017] The analog-to-digital converter performs non-uniform equivalent bandpass sampling and reception on the echo signal after the first-stage mixing, and transmits the sampled and quantized echo data to the FPGA for caching and processing;

[0018] The programmable delay line delays the sampling clock of the analog-to-digital converter ADC.

[0019] The broadband signal receiver comprises: a radio frequency switch, a first-stage bandpass filter, a first-stage variable gain low noise amplifier, a first-stage mixer, a second-stage bandpass filter, a second-stage low noise amplifier and a third-stage bandpass filter;

[0020] The RF switch conducts the SAR broadband echo signal to achieve switch isolation of the strongly coupled signal of the broadband signal transmitter;

[0021] The first-stage bandpass filter suppresses the out-of-band frequencies of the broadband echo signal;

[0022] The first-stage variable gain low noise amplifier performs variable gain low noise power amplification on weak echo signals to improve the dynamic range of the SAR receiver;

[0023] The first-stage mixer performs a first-stage down-conversion on the echo signal to obtain a broadband echo signal with a lower frequency;

[0024] The second-stage bandpass filter suppresses the out-of-band frequencies of the down-converted echo signal;

[0025] The second-stage low-noise amplifier performs power amplification on the echo signal of the first-stage mixer;

[0026] The third-stage bandpass filter further suppresses the out-of-band frequencies of the echo signal.

[0027] An airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling, the steps are as follows:

[0028] 1) The broadband signal receiver processes the SAR broadband echo signal through switch conduction, bandpass filtering, variable gain low noise amplification and first-stage mixing, and then performs two-stage bandpass filtering, two-stage low noise amplification and three-stage bandpass filtering, and finally outputs it to the analog-to-digital converter of the non-uniform equivalent bandpass sampling module;

[0029] 2) The FPGA in the non-uniform equivalent bandpass sampling module controls the programmable delay line to delay the sampling clock of the ADC, so as to implement non-uniform equivalent bandpass sampling of the broadband echo signal after the first-stage mixing, and transmit the sampled and quantized echo data to the FPGA;

[0030] 3) Implement signal recovery, IQ demodulation and pulse compression of the digital echo signal after non-uniform equivalent bandpass sampling within the FPGA;

[0031] 4) The frequency synthesizer and distributor provides the local oscillator frequency signal f for the broadband signal transmitter c1 , providing the local oscillator frequency signal f for the first mixer of the broadband signal receiver c2 , provides a sampling clock signal f for the ADC inside the non-uniform equivalent bandpass sampling module ADC , providing the FPGA with a working clock f FPGA , and the local oscillator frequency signal f of the broadband signal transmitter c1 Mixing local oscillator frequency signal f with broadband signal receiver c2 There is an adjustable frequency offset to increase the freedom of non-uniform equivalent bandpass sampling.

[0032] The broadband signal receiver performs bandpass filtering and variable gain low noise amplification on the SAR broadband echo signal to obtain a radio frequency echo signal S1; and then obtains the local oscillator frequency signal f1 output by the frequency synthesizer and distributor. c2After one-stage mixing, two-stage bandpass filtering and low-noise amplification, a broadband echo signal S2 in a lower radio frequency domain is obtained.

[0033] The analog-to-digital converter in the non-uniform equivalent bandpass sampling module directly performs non-uniform equivalent bandpass sampling on the broadband echo signal S2, the input signal bandwidth of the analog-to-digital converter is greater than the bandwidth of the echo signal S2, and the sampling frequency of the analog-to-digital converter is lower than 40% of the bandwidth of the echo signal S2; the programmable delay line delays the sampling clock of the analog-to-digital converter, and the delay amount is an integer multiple of the non-uniform equivalent bandpass sampling interval.

[0034] The specific process of step 3) is as follows:

[0035] 31) FPGA controls the delay amount of the programmable delay line to the analog-to-digital converter sampling clock;

[0036] 32) FPGA performs signal timing recovery, digital domain IQ demodulation and pulse compression processing on the echo signal received by non-uniform equivalent bandpass sampling.

[0037] The specific process of step 4) is as follows:

[0038] 41) The frequency synthesizer and distributor generates the local oscillator frequency signal f for the first-stage mixer of the broadband signal receiver c2 , and after multi-stage frequency division, the sampling frequency f of the analog-to-digital converter is generated ADC The operating frequency f of the FPGA FPGA ;

[0039] 42) For frequency f c2 After multi-level frequency division, the local oscillator frequency signal f of the broadband signal transmitter is generated. c1 .

[0040] The non-uniform equivalent bandpass sampling module includes: a field programmable gate array FPGA, an analog-to-digital converter and a programmable delay line;

[0041] The field programmable gate array FPGA transmits and feeds back the working mode and working parameters with the aircraft platform, generates radar digital intermediate frequency signals for the broadband signal transmitter, controls the delay amount of the programmable delay line, and performs signal recovery, IQ demodulation and pulse compression processing on the echo signal sampled by the analog-to-digital converter ADC, and finally stores the echo data in the radar echo data memory;

[0042] The analog-to-digital converter performs non-uniform equivalent bandpass sampling and reception on the echo signal after the first-stage mixing, and transmits the sampled and quantized echo data to the FPGA for caching and processing;

[0043] The programmable delay line delays the sampling clock of the analog-to-digital converter ADC.

[0044] The broadband signal receiver comprises: a radio frequency switch, a first-stage bandpass filter, a first-stage variable gain low noise amplifier, a first-stage mixer, a second-stage bandpass filter, a second-stage low noise amplifier and a third-stage bandpass filter;

[0045] The RF switch conducts the SAR broadband echo signal to achieve switch isolation of the strongly coupled signal of the broadband signal transmitter;

[0046] The first-stage bandpass filter suppresses the out-of-band frequencies of the broadband echo signal;

[0047] The first-stage variable gain low noise amplifier performs variable gain low noise power amplification on weak echo signals to improve the dynamic range of the SAR receiver;

[0048] The first-stage mixer performs a first-stage down-conversion on the echo signal to obtain a broadband echo signal with a lower frequency;

[0049] The second-stage bandpass filter suppresses the out-of-band frequencies of the down-converted echo signal;

[0050] The second-stage low-noise amplifier performs power amplification on the echo signal of the first-stage mixer;

[0051] The third-stage bandpass filter further suppresses the out-of-band frequencies of the echo signal.

[0052] Compared with the prior art, the present invention has the following advantages:

[0053] 1) The present invention innovatively proposes a broadband echo signal sampling and receiving technology based on non-uniform equivalent bandpass sampling, which jointly optimizes uniform equivalent sampling, bandpass sampling, and SAR azimuth oversampling, so that non-uniform equivalent bandpass sampling can be better applied to airborne SAR systems, greatly reducing the degree of echo signal phase disturbance caused by the high-speed movement of the aircraft.

[0054] 2) The present invention performs timing recovery, IQ modulation and pulse compression processing algorithms for echo signals received by non-uniform equivalent bandpass sampling in the digital domain, which greatly reduces the difficulty of digital domain processing and enables the echo signal reception processing of non-uniform equivalent bandpass sampling to meet real-time requirements.

[0055] 3) The non-uniform equivalent bandpass sampling receiving technology of the present invention reduces the ADC sampling rate in the receiver to 40% of the echo signal bandwidth without introducing serious echo signal phase disturbance, greatly simplifies the receiver complexity of the high-resolution SAR system, and reduces the system hardware cost.

[0056] 4) The echo signal received by the non-uniform equivalent bandpass sampling of the present invention is matched with the corresponding fast processing algorithms such as digital domain signal recovery, IQ demodulation and pulse compression, so that the entire sampling reception and signal timing recovery can be achieved in real time, providing a technical basis for airborne real-time SAR imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is composed of airborne SAR systems based on non-uniform equivalent bandpass sampling;

[0058] Figure 2 It is the non-uniform equivalent bandpass sampling reception timing;

[0059] Figure 3 It is the FPGA digital domain echo signal processing flow;

[0060] Figure 4 It is the signal pulse compression result obtained by different sampling methods;

[0061] Figure 5 It is a near-area comparison of signal pulse compression results obtained by different sampling methods;

[0062] Figure 6 It is the simulation result of airborne SAR imaging based on non-uniform equivalent bandpass sampling; DETAILED DESCRIPTION

[0063] In an exemplary embodiment of the present invention, an airborne SAR echo receiving technology based on non-uniform equivalent bandpass sampling is provided.

[0064] Figure 1 The schematic diagram of the composition of an airborne SAR system based on non-uniform equivalent bandpass sampling according to an embodiment of the present invention comprises:

[0065] The non-uniform equivalent bandpass sampling module 10 receives the radar working mode and working parameters, and generates a corresponding radar digital intermediate frequency signal to the broadband signal transmitter 30 according to the working parameters; at the same time, the non-uniform equivalent bandpass sampling and reception is performed on the RF signal after the first-stage demodulation, and digital domain signal recovery, IQ demodulation and pulse compression processing are performed, and finally the compressed echo signal is stored in the radar echo data storage 70;

[0066] The frequency synthesizer and distributor 20 generates the working clock signals required by the broadband signal transmitter 30, the primary mixer 604 of the broadband signal receiver, the programmable delay line 103 of the non-uniform equivalent bandpass sampling module, and the FPGA 101;

[0067] The broadband signal transmitter 30 receives the radar digital intermediate frequency signal sent by the non-uniform equivalent bandpass sampling module 10, converts it into a radar intermediate frequency signal, modulates, multiplies the frequency and performs power amplification processing on the radar intermediate frequency signal, obtains a high-resolution SAR radio frequency signal and outputs it to the circulator 40, and finally feeds the SAR antenna 50;

[0068] The circulator 40 feeds the transmission signal of the broadband signal transmitter 30 to the SAR antenna 50, and transmits the echo signal received by the SAR antenna 50 to the broadband signal receiver 60;

[0069] SAR antenna 50, which transmits and receives radar signals in a directionally-directed manner in the imaging detection area;

[0070] The broadband signal receiver 60 performs switch conduction, first-stage bandpass filtering, variable gain low noise amplification and first-stage mixing processing, as well as second-stage bandpass filtering, second-stage low noise amplification and third-stage bandpass filtering processing on the echo signal output by the circulator 40, and obtains a broadband echo signal to the non-uniform equivalent bandpass sampling module 10 for sampling and quantization;

[0071] The radar echo data storage device 70 stores the radar echo data output by the non-uniform equivalent bandpass sampling module 10 .

[0072] 1. Reference Figure 1 The non-uniform equivalent bandpass sampling module 10 includes: a field programmable gate array FPGA101, which transmits and feeds back the working mode and working parameters with the aircraft platform, generates a radar digital intermediate frequency signal for the broadband signal transmitter 30, controls the delay amount of the programmable delay line 103 of the non-uniform equivalent bandpass sampling module, and performs signal recovery, IQ demodulation and pulse compression processing on the echo signal sampled by the analog-to-digital converter ADC102, and finally stores the echo data in the radar echo data memory 70; the analog-to-digital converter 102 performs non-uniform equivalent bandpass sampling and reception on the echo signal after the first-stage mixing, and transmits the sampled and quantized echo data to the FPGA101 for caching and processing; the programmable delay line 103 delays the sampling clock of the analog-to-digital converter ADC.

[0073] Among them, FPGA, ADC and delay line jointly realize the non-uniform equivalent bandpass sampling and receiving function. Its working sequence is as follows: Figure 2 Assume the aircraft speed is v a The radar antenna has an aperture of D in azimuth. a , SAR imaging width is w, incident angle is θ in , the time width of the radar signal is t p , the center frequency of the RF echo signal after the first level of demodulation is f c2 , the signal bandwidth is f BW, the sampling rate and sampling period of ADC are f ADC With T ADC , the delay step of the delay line is t int , the sampling and receiving time window of the radar echo signal is t w The period and number of mixed sampling are t hs and N, the number of ADC sampling cycles in each mixed sampling cycle is M, and the pulse repetition period and pulse repetition frequency of the radar system are T E-PRP With f E-PRP In order to reduce the phase disturbance of the radar echo signal and meet the SAR system's oversampling rate k in the range direction r and the azimuth oversampling rate k a Requirements, the above variables satisfy the following relationship:

[0074]

[0075]

[0076]

[0077] f E-PRP =2k a ·v a / D a (4)

[0078] From the above relationship, it can be seen that when the flight platform and radar working sampling settings are set and the SAR signal bandwidth is constant, the smaller the imaging width, the more times the non-uniform equivalent bandpass sampling is performed, and the lower the sampling rate requirement for the ADC.

[0079] 2. Reference Figure 1 , the broadband signal receiver 60 comprises:

[0080] The radio frequency switch 601 performs radio frequency switch conduction on the SAR broadband echo signal to realize switch isolation of the strongly coupled signal of the broadband signal transmitter; the first-stage bandpass filter 602 suppresses the out-of-band frequency of the broadband echo signal; the first-stage variable gain low noise amplifier 603 performs variable gain low noise power amplification on the weak echo signal to improve the dynamic range of the SAR receiver; the first-stage mixer 604 performs a first-stage down-conversion on the echo signal to obtain a broadband echo signal with a lower frequency; the second-stage bandpass filter 605 suppresses the out-of-band frequency of the down-converted echo signal; the second-stage low noise amplifier 606 performs power amplification on the first-stage mixed echo signal; the third-stage bandpass filter 607 further suppresses the out-of-band frequency of the echo signal.

[0081] Among them, the center frequency of the echo signal received by the antenna is f c , the local oscillator frequency of the first-stage demodulation is fc2 , the center frequency of the RF echo signal after the first level demodulation is f c3 , the bandwidth of the demodulated signal remains unchanged.

[0082] a. Real-time processing technology in digital domain

[0083] In this embodiment, the processing algorithms such as timing recovery, IQ demodulation and pulse compression of the non-uniform equivalent bandpass sampling received echo signal are processed by an FPGA chip. Figure 3 A schematic diagram of a timing recovery, IQ demodulation and pulse compression processing algorithm for a non-uniform equivalent passband sampling received signal according to an embodiment of the present invention, including:

[0084] The primary memory 1101 is used to cache the echo data of each mixed sampling;

[0085] The secondary memory 1102 is used for performing time sequence recovery on the echo data of multiple mixed samples;

[0086] A P-fold interpolator 1103 is used to perform a P-fold interpolation operation on the digital signal sampled by the non-uniform equivalent bandpass;

[0087] The bandpass filter 1104 is used to select the frequency components of the echo signal of the non-uniform equivalent bandpass sampling and suppress the interference of the out-of-band signal;

[0088] The secondary demodulation local oscillator signal 1105 is used to provide a secondary local oscillator signal for the digital domain secondary down-conversion of the echo signal;

[0089] An IQ demodulator 1106 is used to perform a digital domain two-level down-conversion process on the echo signal of the non-uniform equivalent bandpass sampling to obtain a corresponding baseband wideband echo signal;

[0090] A bandpass filter 1107, used for suppressing out-of-band frequencies of the baseband echo signal after IQ demodulation;

[0091] A Q-times extraction module 1108 is used to perform Q-times extraction on the baseband wideband echo signal with a high oversampling rate, so as to reduce the sampling rate of the baseband echo signal;

[0092] A reference signal 1109, used to provide a reference complex signal for pulse compression;

[0093] A pulse compressor 1110, configured to perform pulse compression processing on the baseband echo signal after the sampling rate is reduced;

[0094] The logic controller 1111 is used to control the timing of the pulse compressor and to communicate interactively with the logic controller 1112;

[0095] The logic controller 1112 is used to control the timing of IQ demodulation, bandpass filtering and Q-time extraction, and to communicate interactively with the logic controller 1113 and the logic controller 1111;

[0096] The logic controller 1113 is used to control the timing of the primary memory, the secondary memory, the P-fold interpolator, and the bandpass filter, and to communicate interactively with the logic controller 1112.

[0097] Among them, the P-times interpolator can use the sinc function interpolation, but in order to reduce the amount of calculation and save FPGA computing resources, the P-times interpolation here uses the zero interpolation method. The digital domain echo signal after non-uniform equivalent bandpass sampling is denoted as S1(n), n=0,1,2,...,MN-1, and the sequence n corresponds to the non-uniform equivalent bandpass sampling interval t int , the echo signal after P-multiplier interpolation is S2(p), p=0,1,2,...,PMN-1, and the relationship can be expressed as:

[0098]

[0099] This method can save a lot of DSP core resources inside the FPGA and can be implemented with only appropriate on-chip storage resources to meet real-time processing requirements.

[0100] Digital domain secondary demodulation local oscillator signal f c3 It is pre-stored in the on-chip ROM of the FPGA. The time series length can be selected as needed and can be expressed as:

[0101] S c3 (m) = -sin(2πf c3 ·m) (6)

[0102] C c3 (m) = cos(2πf c3 ·m) (7)

[0103] The length of the time series m is the same as the length of the echo signal output by the bandpass filter, and the time series m is the same as the time interval t. int / P corresponds to. The signal after IQ demodulation of the echo signal S3(m) output by the bandpass filter is:

[0104] I(m)=S3(m)·C c3 (m) (8)

[0105] Q(m)=S3(m)·S c3 (m) (9)

[0106] The above operations can be calculated in parallel using multiple DSP cores, and real-time processing capabilities can be achieved with fewer resources. The baseband IQ echo signals after bandpass filtering and Q-fold decimation are I1(q) and Q1(q), respectively, q = 0, 1, 2, ..., PMN / Q-1. The decimation operation is directly implemented using the read step and depth changes of the storage resources, without consuming computing resources, and is a real-time processing process. The reference complex signal R(l) for pulse compression of the sampled baseband echo signal is pre-stored in the on-chip ROM of the FPGA, and the result of pulse compression S(k) can be expressed as:

[0107] S(k)=IFFT{FFT(I1(q)+i·Q1(q))·conj(FFT(R(l)))} (10)

[0108] Among them, conj(·), FFT(·), and IFFT(·) are conjugate operations, fast Fourier transform, and inverse fast Fourier transform. Both FFT(·) and IFFT(·) are implemented by using multiple DSP cores inside the FPGA to perform semi-parallel butterfly operations.

[0109] b. Simulation results

[0110] In this embodiment, a set of airborne high-resolution SAR system parameters is used as an example for simulation verification, where the center frequency of the radar signal is f c =13.5GHz, bandwidth is f BW =1.2GHz, imaging resolution is better than 0.3m, aircraft flying altitude is H=3.0km, flying speed is v a =80m / s, the aperture length of the antenna in azimuth is D a =0.5m, the incident angle is θ in =35°, the antenna beam width in the distance direction is 9°, and the local oscillator frequency of the first-stage down-conversion is f c2 =15.5GHz, the sampling rate of the high-speed ADC is f ADC =450MHz, the equivalent PRF of the system is f E-PRP =600Hz, non-uniform equivalent bandpass sampling interval is t int =370ps, the non-uniform equivalent bandpass sampling period is t hs = 36us, the number of non-uniform equivalent bandpass sampling is N = 6, and the real-time processing time of FPGA for an echo signal is about 0.96ms, which is less than T E-PRP =1.67ms, which meets the requirements of onboard real-time processing. Bandpass sampling, traditional uniform equivalent bandpass sampling and the non-uniform equivalent bandpass sampling proposed in this invention are used for comparative analysis. The ADC sampling rate of bandpass sampling is 2.7GHz, and the signal pulse compression obtained by sampling is as follows: Figure 4As shown in (a), high-energy sidelobes will not appear in the distance; the echo signal pulse compression result obtained by traditional uniform equivalent bandpass sampling is as follows Figure 4 As shown in (b), the equivalent sampling period is t hs = 277.8us, the sampling rate of the high-speed ADC is 450MHz, it can be seen that the sidelobe level in the distance is relatively high, about -13.17dB, which will deteriorate the SAR imaging quality; the pulse compression result of the echo signal obtained by the non-uniform equivalent bandpass sampling proposed by the present invention is as follows Figure 4 As shown in (c), the sidelobe level in the distance is controlled below -31dB, which will not deteriorate the SAR imaging quality. The equivalent sampling period is t hs = 36us, the sampling rate of the high-speed ADC is 450MHz. From the comparison results, it can be seen that the non-uniform equivalent bandpass sampling receiving technology can better meet the sidelobe level of pulse compression (lower than -31dB). At the same time, the signal pulse compression results obtained by bandpass sampling and traditional uniform equivalent bandpass sampling are compared. Figure 5 As shown in (a), the conventional uniform equivalent bandpass sampling will deteriorate the near-side lobe; the signal pulse compression results obtained by the bandpass sampling and the non-uniform equivalent bandpass sampling proposed in the present invention are compared. Figure 5 As shown in (b), it can be seen that non-uniform equivalent bandpass sampling is almost the same as bandpass sampling and will not deteriorate the near-field sidelobes. The SAR imaging results of the dot matrix target echo signal received using non-uniform equivalent bandpass sampling are shown in Figure 6 As shown in the figure, the spacing of the uniform 4×4 dot matrix target is 0.4m. The SAR image does not have high side lobes and virtual images, and can better distinguish the dot matrix targets.

[0111] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. An airborne SAR echo receiving system based on non-uniform equivalent bandpass sampling, characterized in that: It comprises a non-uniform equivalent bandpass sampling module (10), a frequency synthesizer and distributor (20), a broadband signal transmitter (30), a circulator (40), a SAR antenna (50), a broadband signal receiver (60) and a radar echo data storage device (70); The non-uniform equivalent bandpass sampling module (10) receives the radar working mode and working parameters, and generates a corresponding radar digital intermediate frequency signal according to the working parameters to send to the broadband signal transmitter (30); at the same time, the non-uniform equivalent bandpass sampling and reception is performed on the radio frequency signal after the first-stage demodulation, and digital domain signal recovery, IQ demodulation and pulse compression processing are performed, and finally the compressed echo signal is stored in the radar echo data storage (70); The frequency synthesizer and distributor (20) generates working clock signals required by the broadband signal transmitter (30), the broadband signal receiver (60), and the non-uniform equivalent bandpass sampling module (10); The broadband signal transmitter (30) receives the radar digital intermediate frequency signal sent by the non-uniform equivalent bandpass sampling module (10), converts it into a radar intermediate frequency signal, modulates, multiplies the frequency and performs power amplification processing on the radar intermediate frequency signal, obtains a high-resolution SAR radio frequency signal, outputs it to the circulator (40), and finally feeds the SAR antenna (50); The circulator (40) feeds the transmission signal of the broadband signal transmitter (30) to the SAR antenna (50), and transmits the echo signal received by the SAR antenna (50) to the broadband signal receiver (60); The SAR antenna (50) transmits and receives radar signals in a directionally-directed manner in the imaging detection area; The broadband signal receiver (60) performs switch conduction, first-stage bandpass filtering, variable gain low noise amplification and first-stage mixing processing, as well as second-stage bandpass filtering, second-stage low noise amplification and third-stage bandpass filtering processing on the echo signal output by the circulator (40), and obtains a broadband echo signal for sampling and quantization by the non-uniform equivalent bandpass sampling module (10); The radar echo data storage device (70) stores the radar echo data output by the non-uniform equivalent bandpass sampling module (10).

2. The airborne SAR echo receiving system based on non-uniform equivalent bandpass sampling according to claim 1, characterized in that: The non-uniform equivalent bandpass sampling module (10) comprises: a field programmable gate array FPGA (101), an analog-to-digital converter (102) and a programmable delay line (103); The field programmable gate array FPGA (101) transmits and feeds back the working mode and working parameters to the aircraft platform, generates a radar digital intermediate frequency signal for the broadband signal transmitter (30), controls the delay amount of the programmable delay line (103), performs signal recovery, IQ demodulation and pulse compression processing on the echo signal sampled by the analog-to-digital converter ADC (102), and finally stores the echo data in the radar echo data memory (70); The analog-to-digital converter (102) performs non-uniform equivalent bandpass sampling reception on the echo signal after the first-stage mixing, and transmits the sampled and quantized echo data to the FPGA (101) for buffering and processing; The programmable delay line (103) delays the sampling clock of the analog-to-digital converter ADC.

3. The airborne SAR echo receiving system based on non-uniform equivalent bandpass sampling according to claim 1, characterized in that: The broadband signal receiver (60) comprises: a radio frequency switch (601), a first-stage bandpass filter (602), a first-stage variable gain low noise amplifier (603), a first-stage mixer (604), a second-stage bandpass filter (605), a second-stage low noise amplifier (606) and a third-stage bandpass filter (607); The radio frequency switch (601) performs radio frequency switch conduction on the SAR broadband echo signal, thereby realizing switch isolation of the strongly coupled signal of the broadband signal transmitter; The first-stage bandpass filter (602) suppresses out-of-band frequencies of the broadband echo signal; The first-stage variable gain low noise amplifier (603) performs variable gain low noise power amplification on the weak echo signal to improve the dynamic range of the SAR receiver; The first-stage mixer (604) performs a first-stage down-conversion on the echo signal to obtain a broadband echo signal with a lower frequency; The second-stage bandpass filter (605) suppresses the out-of-band frequency of the down-converted echo signal; The second-stage low noise amplifier (606) performs power amplification on the echo signal of the first-stage mixer; The third-stage bandpass filter (607) further suppresses the out-of-band frequencies of the echo signal.

4. A method for receiving airborne SAR echoes based on non-uniform equivalent bandpass sampling, characterized in that Here are the steps: 1) The broadband signal receiver (60) performs switch conduction, bandpass filtering, variable gain low noise amplification and first-stage mixing on the SAR broadband echo signal, and then performs two-stage bandpass filtering, two-stage low noise amplification and three-stage bandpass filtering, and finally outputs the signal to the analog-to-digital converter of the non-uniform equivalent bandpass sampling module (10); 2) the FPGA in the non-uniform equivalent bandpass sampling module (10) controls the programmable delay line (103) to delay the sampling clock of the ADC (102) so as to implement non-uniform equivalent bandpass sampling of the broadband echo signal after the first-stage mixing, and transmit the sampled quantized echo data to the FPGA; 3) Implement signal recovery, IQ demodulation and pulse compression of the digital echo signal after non-uniform equivalent bandpass sampling within the FPGA; 4) The frequency synthesizer and distributor (20) provides the local oscillator frequency signal f for the broadband signal transmitter c1 , providing the local oscillator frequency signal f for the first mixer of the broadband signal receiver c2 , providing a sampling clock signal f for the ADC (102) inside the non-uniform equivalent bandpass sampling module (10) ADC , providing the FPGA with a working clock f FPGA , and the local oscillator frequency signal f of the broadband signal transmitter c1 Mixing local oscillator frequency signal f with broadband signal receiver c2 There is an adjustable frequency offset to increase the freedom of non-uniform equivalent bandpass sampling.

5. The airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling according to claim 4, characterized in that: The broadband signal receiver (60) performs bandpass filtering and variable gain low noise amplification on the SAR broadband echo signal to obtain a radio frequency echo signal S1; and then obtains the radio frequency echo signal S1 together with the local oscillator frequency signal f output by the frequency synthesizer and distributor (20). c2 After one-stage mixing, two-stage bandpass filtering and low-noise amplification, a broadband echo signal S2 in a lower radio frequency domain is obtained.

6. The airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling according to claim 5, characterized in that: The analog-to-digital converter in the non-uniform equivalent bandpass sampling module directly performs non-uniform equivalent bandpass sampling on the broadband echo signal S2, the input signal bandwidth of the analog-to-digital converter is greater than the bandwidth of the echo signal S2, and the sampling frequency of the analog-to-digital converter is lower than 40% of the bandwidth of the echo signal S2; the programmable delay line delays the sampling clock of the analog-to-digital converter, and the delay amount is an integer multiple of the non-uniform equivalent bandpass sampling interval.

7. The airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling according to claim 6, characterized in that: The specific process of step 3) is as follows: 31) FPGA controls the delay amount of the programmable delay line to the analog-to-digital converter sampling clock; 32) FPGA performs signal timing recovery, digital domain IQ demodulation and pulse compression processing on the echo signal received by non-uniform equivalent bandpass sampling.

8. The airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling according to claim 7, characterized in that: The specific process of step 4) is as follows: 41) The frequency synthesizer and distributor (20) generates the local oscillator frequency signal f for the first-stage mixer of the broadband signal receiver c2 , and after multi-stage frequency division, the sampling frequency f of the analog-to-digital converter is generated ADC The operating frequency f of the FPGA FPGA ; 42) For frequency f c2 After multi-level frequency division, the local oscillator frequency signal f of the broadband signal transmitter is generated. c1 .

9. The airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling according to claim 4, characterized in that: The non-uniform equivalent bandpass sampling module (10) comprises: a field programmable gate array FPGA (101), an analog-to-digital converter (102) and a programmable delay line (103); The field programmable gate array FPGA (101) transmits and feeds back the working mode and working parameters to the aircraft platform, generates a radar digital intermediate frequency signal for the broadband signal transmitter (30), controls the delay amount of the programmable delay line (103), performs signal recovery, IQ demodulation and pulse compression processing on the echo signal sampled by the analog-to-digital converter ADC (102), and finally stores the echo data in the radar echo data memory (70); The analog-to-digital converter (102) performs non-uniform equivalent bandpass sampling reception on the echo signal after the first-stage mixing, and transmits the sampled and quantized echo data to the FPGA (101) for buffering and processing; The programmable delay line (103) delays the sampling clock of the analog-to-digital converter ADC.

10. The airborne SAR echo receiving method based on non-uniform equivalent bandpass sampling according to claim 4, characterized in that: The broadband signal receiver (60) comprises: a radio frequency switch (601), a first-stage bandpass filter (602), a first-stage variable gain low noise amplifier (603), a first-stage mixer (604), a second-stage bandpass filter (605), a second-stage low noise amplifier (606) and a third-stage bandpass filter (607); The radio frequency switch (601) performs radio frequency switch conduction on the SAR broadband echo signal, thereby realizing switch isolation of the strongly coupled signal of the broadband signal transmitter; The first-stage bandpass filter (602) suppresses out-of-band frequencies of the broadband echo signal; The first-stage variable gain low noise amplifier (603) performs variable gain low noise power amplification on the weak echo signal to improve the dynamic range of the SAR receiver; The first-stage mixer (604) performs a first-stage down-conversion on the echo signal to obtain a broadband echo signal with a lower frequency; The second-stage bandpass filter (605) suppresses the out-of-band frequency of the down-converted echo signal; The second-stage low noise amplifier (606) performs power amplification on the echo signal of the first-stage mixer; The third-stage bandpass filter (607) further suppresses the out-of-band frequencies of the echo signal.

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