Moving target radial velocity estimation and relocation method for terahertz circular SAR

Abstract

The present application belongs to the technical field of synthetic aperture radar imaging, and particularly relates to a method for estimating and relocating radial velocity of a moving target in a terahertz circular SAR. The present application extracts the time-frequency line of the distance unit where the moving target is located to obtain the Doppler center frequency of the moving target, estimates the radial velocity of the moving target through the Doppler center frequency, constructs a distance walk compensation function according to the estimated radial velocity to compensate for the offset generated in the azimuth direction of the moving target, and accurately realizes the relocation of the moving target. In addition, the present application only involves simple arithmetic operations and does not involve a long parameter search process, and has higher efficiency.

Description

Technical Field

[0001] This invention belongs to the field of radar imaging technology, specifically relating to a method for estimating and relocating the radial velocity of a moving target for terahertz circular SAR. Background Technology

[0002] Synthetic Aperture Radar (SAR) utilizes the correlation of echo signals to synthesize a large equivalent aperture from smaller real apertures through the relative motion of the radar and the target, thereby achieving high-resolution imaging. It can operate in all weather conditions and has advantages such as good detection capability, strong anti-interference ability, and penetrating power. Circular SAR can perform long-term imaging and 360° observation of a scene, enabling real-time monitoring of both the scene and moving targets when they are present.

[0003] The velocity and acceleration of moving targets affect imaging results. Range velocity causes the imaging result to deviate from its actual position in the azimuth direction, while azimuth velocity and range acceleration cause defocusing. Azimuth acceleration also causes sidelobe asymmetry in the image. Most of these moving targets are non-cooperative, with unknown and unpredictable velocities and accelerations. Therefore, it is necessary to estimate the radial velocity of the moving target. The estimated velocity can then be used for phase compensation of the imaging result to relocate the moving target and enable real-time monitoring. However, existing velocity estimation methods involve exhaustive searches of the moving target's parameters, resulting in high computational burden and low efficiency. Furthermore, in circular SAR mode, the aircraft's trajectory is circular, the range equation between the radar platform and the ground target is complex, and there are intricate coupling relationships between various changing parameters, making moving target parameter estimation difficult. Therefore, finding a radial velocity estimation method that ensures estimation accuracy while minimizing long search times is of great significance. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned problems and shortcomings by providing a terahertz circular SAR method for estimating and relocating the radial velocity of a moving target. This method involves dividing the circular SAR into sub-apertures, imaging the echo data from each sub-aperture using the PFA algorithm, selecting the range cell containing the moving target, and obtaining the time-frequency line of the moving target in the range-frequency domain and azimuth-time domain through short-time Fourier transform (STFT). Then, frequency slices are extracted at the initial moment of the time-frequency line, and the Doppler center frequency of the range data is obtained from the frequency corresponding to the peak value of the slice. The radial velocity of the moving target is estimated based on the relationship between the Doppler center frequency and the radial velocity. Finally, a range travel compensation function is constructed using the estimated velocity to perform phase compensation on the moving target echo, enabling the moving target to return to its original position and achieving relocation of the moving target.

[0005] The technical solution of this invention is: a method for estimating and relocating the radial velocity of a moving target using terahertz circular SAR, comprising the following steps:

[0006] Step 1: Demodulate the original echo data from the terahertz circular SAR, i.e., demodulate the received echo pulses. With reference signal The intermediate frequency signal is obtained by frequency mixing and de-modulation. ,in, For the fast time of radar echo signals, The echo signal is in slow time; the de-modulated signal is then transformed into the range-frequency domain and azimuth-time domain using a range-to-Fourier transform, yielding... Then, with the phase compensation function Multiply the signals, remove the remaining video phase term and the echo envelope slant term, and obtain the range compressed signal. :

[0007] Step 2: Based on the obtained full-aperture circular SAR echo data, divide the data into sub-apertures according to the set number of sub-apertures;

[0008] Step 3: Use the PFA algorithm to image the echo data within a single sub-aperture, find the defocused moving target within the imaging result, locate the range cell where it is located, and perform a range-to-Fourier transform on it to obtain the range-frequency domain azimuth-time domain signal.

[0009] Step 4: Obtain the time-frequency line of the moving target in the range cell using short-time Fourier transform. Extract a frequency slice at the initial moment of the time-frequency line. The frequency corresponding to the peak value of the slice is the initial Doppler center frequency of the moving target. ;

[0010] Step 5: Estimate the Doppler ambiguity number N, and based on the ambiguity number and... Obtain the Doppler center frequency :

[0011] Step 6, utilize Estimate the radial velocity of the target. ,in, For radar transmission signal carrier frequency, The angle between the radar-carrying aircraft's flight plane and the ground plane;

[0012] Step 7: Based on the obtained radial velocity Construct a distance travel compensation function The location of the moving target is compensated in the range frequency domain to obtain the relocation result of the moving target.

[0013] This invention estimates the radial velocity of a moving target by obtaining its Doppler center frequency, and constructs a range travel compensation function based on the estimated radial velocity to perform phase compensation on the echo, so that the moving target returns to its true position. This avoids a long parameter search process, ensures estimation accuracy, and greatly improves computational efficiency.

[0014] The beneficial results of this invention are that it improves the efficiency of radial velocity estimation of moving targets, optimizes the repositioning results of moving targets, and does not involve complex calculations in the implementation process. Attached Figure Description

[0015] Figure 1 This is a flowchart of the present invention;

[0016] Figure 2 To simulate the PFA imaging results of a moving target;

[0017] Figure 3 The results of the simulated moving target STFT;

[0018] Figure 4 To simulate the Doppler center frequency of a moving target;

[0019] Figure 5 The result is the result after distance compensation for a simulated moving target;

[0020] Figure 6 Measured data of moving targets and their shadows;

[0021] Figure 7 The measured data represents the STFT results of the moving target.

[0022] Figure 8 The measured Doppler center frequency;

[0023] Figure 9 The result is the measured distance after walking compensation. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and simulation examples to demonstrate its practicality.

[0025] As attached Figure 1 As shown, the terahertz circular SAR moving target radial velocity estimation method of the present invention can effectively relocate the moving target after imaging processing and radial velocity estimation of the input raw SAR echo. The specific implementation steps are as follows:

[0026] Step 1: Demodulate the original echo and process the received signal. With reference signal The intermediate frequency signal is obtained by frequency mixing and de-modulation. Its expression is:

[0027]

[0028] in, For faster radar signal processing, For radar signal slow time, The backscattering coefficient is... The instantaneous slant range between the radar and the target. The signal pulse width, At the speed of light, The frequency modulation slope of the LFM signal. For reference slope distance, The remaining slant range after frequency modulation removal;

[0029] Then, regarding the fast time... Performing a Fourier transform, the resulting range-frequency domain and azimuth-time domain signals are:

[0030]

[0031] The first exponential term is the echo envelope slant term, and the third exponential term is the residual video term (RVP), which needs to be removed, along with the corresponding phase compensation function. Multiplying them together, we get the signal after removing the skew. :

[0032]

[0033] Step 2: Based on the obtained full-aperture circular SAR echo data, select an appropriate number of sub-apertures for sub-aperture division;

[0034] Step 3: Use the PFA algorithm to image the sub-aperture data, extract the defocused moving target from the imaging results, find the range cell where it is located, and perform a range-to-Fourier transform on it to obtain the range-frequency domain azimuth-time domain signal.

[0035] Step 4: Obtain the time-frequency line of the moving target using short-time Fourier transform. Extract frequency slices at the initial time of the time-frequency line. The frequency corresponding to the peak value of the slice is the initial Doppler center frequency of the moving target. ;

[0036] Step 5: Estimate the Doppler ambiguity number N, and based on the ambiguity number and... Obtain the Doppler center frequency :

[0037]

[0038] Wherein, PRF is the pulse repetition frequency of the radar transmitted signal.

[0039] Step 6, utilize Estimate the radial velocity of the target. ,in, For radar transmission signal carrier frequency, The angle between the radar-carrying aircraft's flight plane and the ground plane;

[0040] Step 7: Based on the obtained radial velocity Construct a distance travel compensation function ,

[0041]

[0042] Compensation is performed in the range frequency domain to obtain the relocalization result of the moving target. This is the reference slant range between the radar and the ground scene.

[0043] Simulation Example

[0044] The above method is attached. Figure 1 The illustrated procedure performs velocity estimation and verification on both simulated and measured data. The specific simulation parameters are set as follows: terahertz circular SAR imaging mode is used; the radar distance from the scene center is 2000 meters; the imaging scene radius is 80 meters; the center carrier frequency of the transmitted signal is 220 GHz; the ground clearance angle is 60°; and the radar carrier's flight speed is... The original coordinates of the moving target in the scene are The distance-to-radial velocity is azimuth velocity is The measured parameters are as follows: terahertz circular video SAR imaging mode, radar distance from the center of the scene is 300 meters, flight altitude is 1000 meters, and average flight speed is... The center frequency is 216 GHz, the signal bandwidth is 900 MHz, and the pulse repetition frequency is 16000 Hz.

[0045] Figure 2 The imaging results of the simulation data processed by the PFA algorithm are presented. It can be seen that the moving target deviates from its original position in the azimuth direction, which is caused by its radial velocity.

[0046] Figure 3 The time-frequency diagram of the short-time Fourier transform of the signal of the distance cell where the moving target is located is given, and its intercept is the initial Doppler center frequency of the moving target.

[0047] Figure 4 Given the frequency slices corresponding to the initial moment in the time-frequency diagram, find the frequency corresponding to the peak value, which is the specific value of the initial Doppler center frequency. Based on the initial Doppler center frequency and the estimated ambiguity number... The accurate value of the Doppler center frequency was obtained. Then, based on the relationship between velocity and Doppler center frequency, the radial velocity of the target is calculated as follows: The target's actual radial velocity is The relative error is Within acceptable limits.

[0048] Figure 5 The estimated radial velocity is given. The imaging results of the moving target after distance walking compensation show that the moving target returned to its true position after compensation, thus realizing the relocalization of the moving target.

[0049] Figure 6 The results of the measured data on moving targets and their shadow imaging are presented. In video SAR, due to the obstruction of radar transmission signals by moving targets, shadows will be generated in the image. The position of the shadow corresponds to the actual position of the target. In the imaging results, it is a dark area that can be seen relatively clearly.

[0050] Figure 7 The short-time Fourier transform time-frequency plot of the distance cell where the moving target is located in the measured data is given;

[0051] Figure 8 Given the frequency slices corresponding to the initial moment in the time-frequency diagram, find the frequency corresponding to the peak value, which is the specific value of the initial Doppler center frequency. Based on the initial Doppler center frequency and the estimated ambiguity number... The accurate value of the Doppler center frequency was obtained. Then, the radial velocity of the target is calculated according to the formula. ;

[0052] Figure 9 The imaging results of the moving target are presented after compensation by the distance travel compensation function based on the estimated radial velocity. After compensation, the moving target and the shadow location coincide, indicating that the moving target returned to its true position through compensation, which proves the correctness of the estimated velocity and realizes the relocation of the moving target.

Claims

1. A method for estimating and relocating the radial velocity of a moving target using terahertz circular SAR, characterized in that, Includes the following steps: S1. Demodulate the raw echo data from the terahertz circular SAR, i.e., receive the echo pulse s. r (t r ,t a ) and reference signal s ref (t r ,t a The intermediate frequency signal s is obtained by frequency mixing and de-modulation. i (t r ,t a ), where t r For the fast time of radar echo signals, t a The echo signal is in slow time; the de-modulated signal is transformed into the range-frequency domain and azimuth-time domain using a range-to-Fourier transform, yielding S. i (f r ,t a Then, with the phase compensation function S c (f r Multiply by the given terms, remove the remaining video phase term and the echo envelope skew term, and obtain the range compressed signal S(f). r ,t a ): Among them, f r For fast time-frequency domain variables, σ is the target backscattering coefficient, and T p Where c is the signal pulse width, and K is the speed of light. γ R is the frequency modulation slope of the LFM signal. △ The remaining slant range after frequency modulation removal; S2. Based on the obtained full-aperture circular SAR echo data, select an appropriate number of sub-apertures for sub-aperture division; S3. The PFA algorithm is used to image the echo data within a single sub-aperture, find the moving target in the imaging result, locate the range cell where it is located, and perform a range-to-Fourier transform on it to obtain the range-frequency domain azimuth-time domain signal. S4. Obtain the time-frequency line of the moving target using the short-time Fourier transform. Extract a frequency slice at the initial moment of the time-frequency line. The frequency corresponding to the peak value of the slice is the initial Doppler center frequency f of the moving target. dco ; S5. Estimate the Doppler ambiguity number N, and based on the ambiguity number and f dco Obtain the Doppler center frequency f dc : f dc =f dco +N×PRF Wherein, PRF is the pulse repetition frequency of the radar transmitted signal; S6, Utilization Obtain the radial velocity v of the target y , where f c , where g is the carrier frequency of the radar transmission signal, and gA is the ground contact angle between the radar aircraft's flight plane and the ground plane; S7. Based on the obtained radial velocity v y Construct the distance travel compensation function H RW , Compensation is performed in the range frequency domain to obtain the relocalization result of the moving target, where R ref This is the reference slant range between the radar and the ground scene.

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