A method and system for suppressing leakage signals of continuous wave SAR using subsampling

Abstract

A method and system for suppressing leakage signal of continuous wave SAR by using secondary sampling, according to the characteristics that the leakage signal is strongly superimposed on the weak echo signal in the SAR system, after the first digital sampling of the SAR echo signal, secondary sampling is performed after the original signal cancellation (suppression of leakage signal) of the digital-to-analog conversion, and finally the effective echo data is obtained. The method can suppress the leakage signal without improving the antenna isolation, and can lay the foundation for the application of continuous wave SAR on satellite platform.

Description

Technical Field

[0001] This invention belongs to the field of microwave remote sensing technology and relates to a signal processing method and system for a continuous wave SAR system. Background Technology

[0002] Compared to conventional pulsed SAR systems, continuous wave SAR systems are characterized by their small size, light weight, low cost, and low peak transmit power. Currently, continuous wave SAR has numerous applications in early warning and monitoring. For spaceborne SAR applications, continuous wave SAR also offers certain advantages: spaceborne radar has a longer operating range, and continuous wave SAR can reduce peak transmit power, adapting to the development needs of miniaturized, lightweight, and low-power satellite platforms.

[0003] Since the transmit and receive channels of continuous wave SAR operate simultaneously, suppressing leakage signals from the transmit channel is a critical issue. Transmitted leakage signals can cause several problems for continuous wave SAR systems. Firstly, when the transmitted signal power is high, the leakage signal power will also be high, affecting system sensitivity and potentially causing the system to malfunction. Secondly, due to multi-path leakage from the transmitter to the receiver, the different time delays along each path result in a Doppler signal that is not necessarily zero, leading to the generation of false Doppler signals. Therefore, effectively suppressing leakage signals is a key factor in improving the performance of continuous wave SAR.

[0004] Common measures to address signal leakage problems include transmit / receive waveform control isolation techniques, signal cancellation techniques, and leakage suppression techniques using digital signal processing. However, these measures all have some shortcomings. Isolation techniques based on transmit / receive antenna control can be categorized into two types: one uses a shared transmit / receive antenna, which is limited by the isolation of the circulator; the other uses separate transmit / receive antennas, but dual antennas occupy more space, restricting the application of continuous wave radar on satellite platforms. Cancellation techniques based on transmit / receive channels, whether RF cancellation or IF cancellation, require introducing a reference signal from the transmitted signal, inevitably introducing noise into the system and affecting receiver sensitivity. Leakage suppression techniques based on modern signal processing involve complex calculations and a large computational load, making it difficult for frequency-modulated continuous wave radar to meet the time and space requirements of real-time applications. Summary of the Invention

[0005] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a method and system for suppressing continuous wave SAR leakage signals using secondary sampling. Without increasing the difficulty of antenna system design and engineering implementation, it can achieve the separation of spaceborne continuous wave SAR echo and leakage signal.

[0006] The technical solution of this invention is: a method for suppressing continuous wave SAR leakage signals using secondary sampling, comprising the following steps:

[0007] (1) Perform an AD conversion on the original echo signal x(τ) containing the ground object echo signal x0(τ) and the leakage signal x1(τ) to obtain the echo sampling signal. And store it;

[0008] (2) Sample the echo signal Perform DA conversion to obtain the analog signal.

[0009] (3) Modulus signal Differential operation is performed with the original echo signal x(τ) to obtain the differential mode signal.

[0010] (4) After gain amplification, the differential signal d(τ) is subjected to a second A / D conversion to obtain the differential sampled signal. And store it;

[0011] (5) Repeat steps (1) to (4) to complete the sampling of the entire original echo signal and realize the complete extraction of the ground object echo signal;

[0012] (6) Extract differential sampling signal and echo sampling signal Obtain raw signal samples Using D / A converters to obtain echo estimation signals

[0013] Furthermore, the echo sampling signal Only the leakage signal x1(τ) remains.

[0014] Furthermore, the original echo signal x(τ) undergoes an AD conversion, including sampling and quantization, wherein the sampling rate is at least twice the frequency of the input signal, and the quantization method is as follows:

[0015] (31) Calculate N0 = P k (x1) / P k (x0), P k Let the peak-to-peak value be represented. Assume the quantization length is N = 0.5N0. Input the pre-sampled leakage signal x1(τ) into the Lloyds algorithm for training to obtain the quantization value cod with the minimum mean square quantization error and the decision boundary p, thereby obtaining the quantizer AD(x, N).

[0016] (32) Input the aliased signal training sequence x(τ)=x0(τ)+x1(τ) into the quantizer for quantization to obtain the quantization result x(n)=AD(x,N); perform DA transformation on x(n) to obtain Then perform differential analysis to obtain the original echo reconstruction signal.

[0017] (33) Calculate the signal-to-noise ratio (SNR) of the original echo signal. The noise introduced by quantization is N(τ) = d(τ) - x0(τ), then the quantization SNR is...

[0018]

[0019] Calculate the SNR(N) within the range of N = 0.5N0 to N0, and take the quantization length N corresponding to the maximum SNR value. 最优 The final quantizer AD(x, N) was designed. 最优 ).

[0020] Furthermore, the aforementioned echo sampling signal Perform DA conversion to obtain the analog signal. Specifically, the waveform storage direct reading method is adopted.

[0021] This invention also provides a system for suppressing continuous wave SAR leakage signals using secondary sampling, comprising an RF differential circuit, a signal memory, a differential signal memory, a digital-to-analog converter module, an analog-to-digital converter module, and a control module, wherein:

[0022] Analog-to-digital conversion module: Performs AD sampling on the original echo signal x(τ) containing ground feature echo signal x0(τ) and leakage signal x1(τ) to obtain the echo sample signal. The signal is then sent to a signal memory for storage; a differential sampling signal is obtained by performing a secondary AD sampler on the differential signal d(τ). And it is sent to the differential signal memory for storage;

[0023] Digital-to-analog converter module: converts the echo sampling signal Perform DA conversion to obtain the analog signal. Sample the original signal Perform DA conversion to obtain the echo estimation signal.

[0024] Radio frequency differential circuit: converts analog signals Differential operation is performed with the original echo signal x(τ) to obtain the differential mode signal.

[0025] Control module: Extracts differential sampled signals from differential signal memory Extract the echo sampling signal from the signal memory. Obtain raw signal samples And then sent to the digital-to-analog converter.

[0026] The advantages of this invention compared to the prior art are:

[0027] (1) This invention utilizes secondary sampling to suppress continuous wave SAR leakage signals, which can extract low-power echo signals without affecting receiver sensitivity. It has the characteristics of long operating distance and is suitable for satellite platforms and targets at long distances. At the same time, it has the characteristics of all-weather operation, and is not afraid of clouds and rain, and has strong penetration, so it can work under various harsh conditions.

[0028] (2) The method of the present invention has strong anti-interference ability. The secondary sampling continuous wave SAR adopts radio frequency differential circuit in the radio frequency front end, which can not only suppress leakage signal and noise signal, but also improve the anti-interference ability of the system.

[0029] (3) Compared with traditional methods, the present invention is simple in structure, low in cost, and unaffected by frequency band. The system implementation only requires radio frequency differential circuit, signal memory, differential signal memory, digital-to-analog conversion module and analog-to-digital conversion module. It does not require special design to improve the isolation of the antenna transceiver channel, nor does it require improving the data processing capability, which is very difficult from both the design and engineering implementation perspectives. Attached Figure Description

[0030] Figure 1 This is a flowchart of the method of the present invention;

[0031] Figure 2 This is a flowchart illustrating the original echo signal restoration process of this invention.

[0032] Figure 3 This is a schematic diagram of the echo signal (after pulse compression) of ground objects in an embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram of the leakage signal (after pulse compression) in an embodiment of the present invention;

[0034] Figure 5 This is a schematic diagram of the superimposed signal of leakage signal and echo signal (the ground target signal is submerged) in an embodiment of the present invention;

[0035] Figure 6 This is a schematic diagram of the AD conversion result of the aliased signal in an embodiment of the present invention;

[0036] Figure 7 This is a schematic diagram of the differential processing output signal in an embodiment of the present invention;

[0037] Figure 8 This is a schematic diagram of the signal pulse compression result of ground target reconstruction in an embodiment of the present invention. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0039] To suppress continuous wave SAR leakage signals using secondary sampling, this invention designs a continuous wave SAR system based on secondary sampling. The core idea of ​​this system design is to use sampling quantization to separate the leakage signal during signal reception, and then use radio frequency differential processing to cancel and suppress the leakage signal.

[0040] The secondary sampling continuous wave SAR system includes the main control system, baseband signal generation system, modulator, transmitter, and receiver system of the traditional pulse SAR system, as well as the newly introduced secondary sampling receiving system, including the signal storage unit, radio frequency differential module, DA digital-to-analog converter module, and AD analog-to-digital converter module.

[0041] like Figure 1 As shown, the main implementation process of the method of the present invention is as follows:

[0042] (1) The original signal x(1,τ) received by SAR at azimuth time t=1 contains ground object echo signal x0(1,τ) and leakage signal x1(1,τ). After AD conversion, a discrete digital signal is obtained. And stored in the signal storage unit, The signal consists only of leakage signals.

[0043] x(1, τ) is the original echo signal or the intermediate frequency signal obtained by down-converting the original echo. It consists of the echo signal and the leakage signal, with the leakage signal having a signal strength approximately 70 dB higher than the echo signal. The echo signal reflects the changes in the backscattering coefficient of the scene, but due to its lower energy, it is submerged in the leakage signal. Currently, the dynamic range of the receiver input and the quantization range of the data shaper are limited, making it impossible to obtain a high-precision echo signal by directly sampling and quantizing the x(1, τ) signal.

[0044] Generally, SAR uses linear frequency modulated signals as the transmitted signal. Since the transmission power of spaceborne SAR is relatively large, the peak power is usually in the kilowatt range, so the energy of the leakage signal is relatively large, but it changes slowly over time and is relatively stable. x0(1,τ) is the radio frequency signal obtained by SAR from ground illumination. Due to the large path attenuation, the signal power is much smaller than the leakage signal, and it contains the scattering information of ground objects, which changes rapidly over time.

[0045] The function of the AD (Analog-to-Digital) converter module is to convert analog signals with continuous time and amplitude into digital signals with discrete time and amplitude. This involves two processes: sampling and quantization. The bandwidth of SAR transmitted signals is typically several hundred megabits per second. According to the Nyquist sampling theorem, the sampling rate of the AD converter must be at least twice the frequency of the input signal. The quantization process is equivalent to the filtering process. The quantization interval of the quantizer is designed to be approximately the magnitude of the amplitude of the ground feature echo x0(1, τ). Ground feature echoes with low power and rapid changes x0(1, τ) are filtered out. Only the leakage signal remains. Traditional uniform quantization is difficult to meet design requirements. If the quantization interval is smaller than the amplitude range of the ground object echo signal, the quantized signal will contain some echo signal, failing to achieve the filtering purpose. A large quantization interval will lead to too large quantization error. The power of the error signal introduced in subsequent differential processing is even greater than the power of the ground object echo signal itself, and the useful signal is submerged. Therefore, a non-uniform quantization method is required. For example, using the Lloyds-Max quantizer can obtain the minimum mean square quantization error, and the quantization length is fixed. At the same time, the quantization interval is set to 1 to 2 times the peak-to-peak value of x0(1,τ). k Within the range of (x0), effective filtering and reduction of quantization error can be achieved. The specific design steps of the quantizer are as follows:

[0046] a. Calculate N0 = P k (x1) / P k (x0), let the quantization length be N = 0.5N0, input the pre-sampled leakage signal x1(τ) into the Lloyds algorithm for training, obtain the quantization value cod with the minimum mean square quantization error and the decision boundary p, and thus obtain the quantizer AD(x, N);

[0047] b. Input the aliased signal training sequence x(τ) = x0(τ) + x1(τ) into the quantizer for quantization to obtain the quantized result x(n) = AD(x, N). Perform a DA transformation on x(n) to obtain... Then perform differential analysis to obtain the original echo reconstruction signal.

[0048] c. Calculate the signal-to-noise ratio (SNR) of the original echo signal. The noise introduced by quantization is N(τ) = d(τ) - x0(τ), then the quantization SNR is...

[0049]

[0050] d. Calculate the SNR(N) within the range of N = 0.5N0 to N0, and take the quantization length N corresponding to the maximum SNR. 最优 The final quantizer AD(x, N) was designed. 最优 ).

[0051] (2) Quantize the signal Perform a DA conversion to obtain the restored signal of the leaked signal x1(1,τ).

[0052] DA conversion is a digital-to-analog conversion that uses the traditional waveform storage direct reading method. It reads waveform samples stored in the signal storage unit as sampling data, performs digital-to-analog conversion according to the system's preset signal bandwidth, time width and other parameters, and then generates the corresponding analog signal through low-pass filtering.

[0053] (3) Restore the output signal of the DA converter The original signal x(1, τ) is input into the differential module, and after differential processing, a differential signal dominated by the echo signal is obtained. The difference signal d(1, τ) is obtained by performing an A / D transformation. Stored in the differential signal memory, d(1,τ) is the estimated signal of the ground target echo signal x0(1,τ), such as... Figure 2 As shown.

[0054] Differential circuits are widely used in radio frequency (RF) integrated circuits and microwave monolithic integrated circuits due to their advantages such as low noise, high common-mode rejection, high linearity, and large dynamic range. In this invention, an RF differential circuit is applied to the RF front end. The input is a dual-ended signal, consisting of the RF signal received by the radar at the current azimuth time and the RF signal recovered after DA conversion. The output is a single-ended signal representing the differential signal of the input signal.

[0055] Since x(1,τ) contains ground object echo signal x0(1,τ) and leakage signal x1(1,τ), It is the restored signal of the leaked signal x1(1,τ). After passing through the radio frequency differential module, the leaked signals cancel each other out, and only the ground object echo signal remains in the differential signal.

[0056] (4) Repeat steps (1) to (3) and process the signals x(t, τ) arriving at azimuth times t = 2, 3...M in sequence to obtain ground object echo samples. and leakage signal sampling

[0057] (5) The differential signal stored in the differential signal memory With stored in the signal memory Adding them together yields a discrete digital signal. It is the sampling result of the original aliased signal x(1,τ).

[0058] Example

[0059] To verify the method described above, simulation experiments were conducted to demonstrate the method of suppressing continuous wave SAR leakage signals using secondary sampling. The main simulation settings are as follows:

[0060] I. System Parameters:

[0061] Transmit power: 4800W

[0062] Radar range: 800 km;

[0063] Echo reception power: -30dBm

[0064] Leakage signal power: 10dBm

[0065] Quantization bits: 12 bits

[0066] Bandwidth: 48MHz

[0067] Pulse width: 20µs

[0068] Sampling rate: 96MHz

[0069] FM slope: 2.4 × 10 12 Hz / s

[0070] Slant distance from scene center: 800km

[0071] Equivalent slant range of leaked signal: 799km

[0072] II. Simulation Results:

[0073] Simulation results are as follows Figures 3-8 As shown, the received leakage signal in the simulation has a power 40 dB higher than the echo signal. Figure 3 This is the result of pulse compression of ground object echo signals. Figure 4 This is the result of leakage signal pulse compression. Figure 5 This is the result of pulse compression of the aliased signal between the system's received echo signal and the leakage signal. It can be seen that the leakage signal significantly drowns out the ground feature echo signal. Figure 6 It is the result of the AD conversion of the aliased signal, and the echo signal is filtered out. Figure 7 The differential processing output signal d(n) is also the ground target reconstruction signal. Comparing this signal d(n) with the lossless ground target signal x0(n), the waveform characteristics are basically the same. The signal-to-noise ratio is calculated as SNR=10log(∑|d(n)-x0(n)| / ∑|x0(n)|)=9.6623dB. Figure 8 It is the result of pulse compression of the ground target reconstruction signal, and the peak value is obtained at the true value of the target slant range.

[0074] The simulation results show that the method of using secondary sampling to suppress continuous wave SAR leakage signals can separate leakage signals from echo signals. The invention has both theoretical and engineering feasibility and has certain application prospects.

[0075] The contents not described in detail in this specification are common knowledge to those skilled in the art.

Claims

1. A method for suppressing continuous wave SAR leakage signals using secondary sampling, characterized in that... Includes the following steps: (1) For signals containing ground object echoes and leakage signals The original echo signal Perform an A / D conversion to obtain the echo sampling signal. And store it; (2) Sample the echo signal Perform DA conversion to obtain the analog signal. ; (3) Modulus signal Compared with the original echo signal Perform differential operations to obtain the differential signal. ; (4) Convert the differential signal After gain amplification, a second A / D conversion is performed to obtain the differential sampling signal. And store it; (5) Repeat steps (1) to (4) to complete the sampling of the entire original echo signal and realize the complete extraction of the ground object echo signal; (6) Extract differential sampling signal and echo sampling signal Obtain the original signal sample Echo estimation signal obtained using D / A converter ; The echo sampling signal Only the leakage signal remains. ; The original echo signal A / D conversion is performed, including sampling and quantization. The sampling rate is at least twice the frequency of the input signal. The quantization method is as follows: (31) Calculate , Represents the peak-to-peak value, and the quantization length is . The pre-sampled leakage signal The Lloyds algorithm is used for training to obtain the quantization value cod with the minimum mean square quantization error and the decision boundary p, thereby obtaining the quantizer. ; (32) Training sequence of aliased signals The input is quantized using a quantizer to obtain the quantization result. ;Will Obtain by performing DA transformation Then, differential processing is performed to obtain the original echo reconstruction signal. ; (33) Calculate the signal-to-noise ratio of the original echo signal, and the noise introduced by quantization is... Then the quantization signal-to-noise ratio is calculate within ,Pick Quantization length corresponding to the maximum value The final quantizer was designed. .

2. The method for suppressing continuous wave SAR leakage signals using secondary sampling according to claim 1, characterized in that: The echo sampling signal Perform DA conversion to obtain the analog signal. Specifically, the waveform storage direct reading method is adopted.

3. A system for suppressing continuous wave SAR leakage signals using secondary sampling, characterized in that... include: The system includes an RF differential circuit, a signal memory, a differential signal memory, a digital-to-analog converter module, an analog-to-digital converter module, and a control module, among which: Analog-to-digital conversion module: for signals containing ground object echoes. and leakage signals The original echo signal Perform AD sampling to obtain the echo sampling signal. And send it to the signal memory for storage; for differential mode signals A differential sampling signal is obtained by performing a secondary AD sampler. And it is sent to the differential signal memory for storage; Digital-to-analog converter module: converts the echo sampling signal Perform DA conversion to obtain the analog signal. Sample the original signal Perform DA conversion to obtain the echo estimation signal. ; Radio frequency differential circuit: converts analog signals Compared with the original echo signal Perform differential operations to obtain the differential signal. ; Control module: Extracts differential sampled signals from differential signal memory Extract the echo sampling signal from the signal memory. Obtain the original signal sample And send it to the digital-to-analog converter; The echo sampling signal Only the leakage signal remains. ; The analog-to-digital conversion module is used for signals containing ground object echoes. and leakage signals The original echo signal A / D conversion is performed, including sampling and quantization. The sampling rate is at least twice the frequency of the input signal. The quantization method is as follows: (71) Calculate , Represents the peak-to-peak value, and the quantization length is . The pre-sampled leakage signal The Lloyds algorithm is used for training to obtain the quantization value cod with the minimum mean square quantization error and the decision boundary p, thereby obtaining the quantizer. ; (72) Training sequence of aliased signals The input is quantized using a quantizer to obtain the quantization result. ;Will Obtain by performing DA transformation Then, differential processing is performed to obtain the original echo reconstruction signal. ; (73) Calculate the signal-to-noise ratio of the original echo signal, and the noise introduced by quantization is: Then the quantization signal-to-noise ratio is calculate within ,Pick Quantization length corresponding to the maximum value The final quantizer was designed. .

4. The system for suppressing continuous wave SAR leakage signals using secondary sampling according to claim 3, characterized in that: The aforementioned digital-to-analog converter module converts the echo sampling signal ADA conversion is performed, and the analog signal is obtained using the waveform storage direct reading method. .

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