CN117784129A - Moving target radial velocity estimation and repositioning method for terahertz circumference SAR - Google Patents

Abstract

The present invention belongs to the field of synthetic aperture radar imaging technology, and specifically relates to a method for estimating and relocating radial velocity of moving targets for terahertz circular SAR. The present invention 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 movement compensation function according to the estimated radial velocity to compensate for the displacement of the moving target in azimuth, and accurately realizes the relocation of the moving target. In addition, the present invention only involves simple arithmetic operations, does not involve a long parameter search process, and has higher efficiency.

Description

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

Technical field

The invention belongs to the field of radar imaging technology, and specifically relates to a moving target radial velocity estimation and relocation method for terahertz circular SAR.

Background technique

Synthetic Aperture Radar (SAR) uses the correlation of echo signals to synthesize a smaller real aperture into a larger equivalent aperture through the relative motion between the radar and the target, thereby achieving high-resolution imaging. It can work all day and all weather, and has the advantages of good detection, strong anti-interference, and penetration. Circular SAR can perform long-term imaging and 360° observation of the scene. When there are moving targets in the scene, it can achieve real-time monitoring of the scene and moving targets.

The speed and acceleration of the moving target will affect the imaging results. The range velocity will cause the imaging result to deviate from its own position in the azimuth direction. The azimuth velocity and range acceleration will defocus the moving target. The azimuth acceleration will cause the side lobes of the image. Asymmetrical. Most of these moving targets are non-cooperative targets, and their movement speed and acceleration are unknown and unpredictable. Therefore, it is necessary to estimate the radial speed of the moving target, and use the estimated speed to perform phase compensation on the imaging results to achieve moving targets. Target relocation enables real-time monitoring of moving targets. However, the existing commonly used speed estimation method is to conduct an exhaustive search for the parameters of the moving target, which has a large computational burden and low efficiency. In the circular SAR mode, due to the arc-shaped trajectory of the aircraft, there is a gap between the radar platform and the ground target. The distance equation of the target is complex, and there are complex coupling relationships between the changing parameters, making it difficult to estimate the parameters of the moving target. Therefore, it is of great significance to find a radial velocity estimation method that can avoid long-term searches while ensuring estimation accuracy.

Contents of the invention

The purpose of the present invention is to provide a terahertz circular SAR moving target radial velocity estimation and relocation method in view of the above-mentioned existing problems and deficiencies. This method divides the circular SAR into sub-apertures and analyzes the echoes of the sub-apertures. The data is imaged using the PFA algorithm, and then the range unit where the moving target is located is selected, and the time-frequency line of the moving target is obtained through the short-time Fourier transform (STFT) in the range, frequency, azimuth and time domain, and then extracted at the initial moment of the time-frequency line Frequency slicing: obtain the Doppler center frequency of the range data through the frequency corresponding to the slice peak, estimate the radial velocity of the moving target based on the relationship between the Doppler center frequency and the radial velocity, and then use the estimated velocity to construct range movement compensation The function performs phase compensation on the echo of the moving target, so that the moving target returns to its original position and realizes the relocation of the moving target.

The technical solution of the present invention is: a terahertz circular SAR moving target radial velocity estimation and relocation method, which includes the following steps:

Step 1. Perform de-frequency modulation processing on the original echo data of terahertz circular SAR, that is, mix the received echo pulse s _r (t _r , _ta ) with the reference signal s _ref (t _r , _ta ). The de-frequency modulated intermediate frequency signal s _i (t _r , t _a ) is obtained, where tr is the fast time of the radar echo signal and t _a is the slow time of the echo signal; the de-frequency modulated signal is sent to Fourier through the distance The leaf transform is converted to the range frequency domain and the azimuth time domain to obtain S _i (f _r , t _a ), which is then multiplied by the phase compensation function S _c (f _r ) to remove the remaining video phase term and echo envelope tilt term, Obtain the distance compressed signal S(f _r ,t _a ):

Step 2. Based on the obtained full-aperture circumferential SAR echo data, divide the sub-apertures according to the set number of sub-apertures;

Step 3. Use the PFA algorithm to image the echo data within a single sub-aperture, find the defocused moving target in the imaging result, find the range unit where it is located, perform a range Fourier transform on it, and obtain the range-frequency domain orientation. time domain signal;

Step 4. Use the short-time Fourier transform to obtain the time-frequency line of the moving target in the distance unit, and extract the frequency slice at the initial moment of the time-frequency line. The frequency corresponding to the slice peak is the initial Doppler center frequency f of the moving target. _dco ;

Step 5. Estimate the Doppler ambiguity number N, and obtain the Doppler center frequency f _dc according to the ambiguity number and f _dco :

Step 6. Utilize Estimate the radial velocity v _y of the target, where f _c is the carrier frequency of the radar transmission signal, and gA is the friction angle between the flight plane of the radar aircraft and the ground plane;

Step 7: construct a range movement compensation function _HRW according to the obtained radial velocity _vy , compensate the position of the moving target in the range frequency domain, and obtain the relocation result of the moving target.

This invention estimates the radial velocity of the target by obtaining the Doppler center frequency of the moving target, and constructs a distance movement compensation function through the estimated radial velocity, and performs phase compensation on the echo, so that the moving target returns to its true position. , avoiding the long parameter search process, ensuring the estimation accuracy and greatly improving the computing efficiency.

The beneficial results of the present invention are to improve the radial velocity estimation efficiency of the moving target and optimize the relocation result of the moving target, and no complex calculations are involved in the implementation process.

Description of drawings

Figure 1 is a flow chart of the present invention;

Figure 2 shows the PFA imaging results of simulated moving targets;

Figure 3 is the STFT result of the simulated moving target;

Figure 4 shows the Doppler center frequency of the simulated moving target;

Figure 5 shows the result after distance movement compensation of the simulated moving target;

Figure 6 Actual measured data moving target and its shadow;

Figure 7 shows the STFT results of moving targets on measured data;

Figure 8 shows the Doppler center frequency of the measured data;

Figure 9 shows the measured data after distance movement compensation.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and simulation examples to prove the practicability of the present invention.

As shown in Figure 1, through a terahertz circular SAR moving target radial velocity estimation method of the present invention, the input SAR original echo can be processed through imaging processing and radial velocity estimation to effectively realize the relocation of the moving target. Positioning, the specific implementation steps are as follows:

Step 1: De-frequency-modulate the original echo, mix the received signal s _r (t _r , _ta ) with the reference signal s _ref (t _r , _ta ) to obtain a de-frequency-modulated intermediate frequency signal _si (t _r , _ta ), which is expressed as:

Among them, t _r is the radar signal fast time, _ta is the radar signal slow time, s is the backscattering coefficient, R(t _a ) is the instantaneous slant range between the radar and the target, T _p is the signal pulse width, and c is the speed of light, K _γ is the frequency modulation slope of the LFM signal, R _ref is the reference slope distance, R _Δ =R(t _a )-R _ref is the remaining slope distance after de-frequency modulation;

Then Fourier transform is performed on the fast time t _r , and the obtained range frequency domain azimuth time domain signal is:

The first exponential term is the echo envelope tilt term, and the third exponential term is the residual video term (RVP), which needs to be removed and multiplied with the corresponding phase compensation function _Sc ( _fr ) to obtain the de-skewed signal S( _fr , _ta ):

Step 2: Based on the obtained full-aperture circumferential SAR echo data, select the appropriate number of sub-apertures to divide the sub-apertures;

Step 3: Use the PFA algorithm to image the sub-aperture data, extract the defocused moving target in the imaging result, find the range unit where it is located, perform a range Fourier transform on it, and obtain the range frequency domain azimuth time domain signal;

Step 4: Use the short-time Fourier transform to obtain the time-frequency line of the moving target, and extract the 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 _dco of the moving target;

Step 5: Estimate the Doppler ambiguity number N, and obtain the Doppler center frequency f _dc according to the ambiguity number and f _dco :

_fdc ＝ _fdco +N×PRF

Among them, PRF is the pulse repetition frequency of the radar transmission signal.

Step 6. Utilize Estimate the radial velocity v _y of the target, where f _c is the carrier frequency of the radar transmission signal, and gA is the friction angle between the flight plane of the radar aircraft and the ground plane;

Step 7. According to the obtained radial velocity v _y , construct the distance walking compensation function H _RW ,

Compensation is performed in the range frequency domain to obtain the relocation result of the moving target. R _ref is the reference slant distance between the radar and the ground scene.

Simulation Example

According to the aforementioned method, that is, the process operation shown in Figure 1, the speed is estimated and verified on the simulated data and the measured data respectively. The specific settings of the simulation parameters are as follows: using the imaging mode of terahertz circular SAR, the distance of the radar from the center of the scene is 2000 meters, the radius of the imaging scene is 80 meters, the central carrier frequency of the transmitted signal is 220GHz, the ground-grazing angle is 60°, and the radar The flight speed of the carrier aircraft is 30m/s, the original coordinates of the moving target in the scene are (2m, 3m), the range (radial) speed is 5m/s, and the azimuth speed is 10m/s; the parameters of the measured data are as follows: Terahertz circular video SAR imaging mode, the distance of the radar from the center of the scene is 300 meters, the flight altitude is 1000 meters, the average flight speed is 62.1m/s, the center frequency is 216GHz, the signal bandwidth is 900MHz, and the pulse repetition frequency is 16000Hz.

Figure 2 shows the imaging results of the simulation data through the PFA algorithm. It can be seen that the moving target deviates from its original position in the azimuth direction, which is caused by its radial velocity;

Figure 3 shows the time-frequency diagram of the short-time Fourier transformed signal of the distance unit where the moving target is located, and its intercept is the initial Doppler center frequency of the moving target;

Figure 4 shows the frequency slice at the initial moment corresponding to the time-frequency diagram. The frequency corresponding to the peak is the specific value of the initial Doppler center frequency, which is -448.8 Hz. According to the initial Doppler center frequency and the estimated fuzzy number N = 1, the exact value of the Doppler center frequency is f _dc = -448.8 Hz + N × PRF = 3775 Hz. According to the relationship between the speed and the Doppler center frequency, the radial velocity of the target is calculated to be _vy = 5.14 m/s, while the actual radial velocity of the target is 5 m/s, with a relative error of 2.8%, which is within the acceptable range.

Figure 5 shows the imaging results of the moving target after distance movement compensation using the estimated radial velocity v _y . After compensation, the moving target returns to its true position, realizing the relocation of the moving target;

Figure 6 shows the moving target and its shadow imaging results of the measured data. In video SAR, due to the obstruction of the moving target radar signal, shadows will be generated in the image. The position of the shadow corresponds to the true position of the target. In the imaging results The middle is a dark area, which can be seen visually;

Figure 7 shows the short-time Fourier transform time-frequency diagram of the distance unit where the moving target is located in the measured data;

Figure 8 shows the frequency slice corresponding to the initial moment of the time-frequency diagram. Find the frequency corresponding to the peak value, which is the specific value of the initial Doppler center frequency, which is -942.7Hz. According to the initial Doppler center frequency and the estimated ambiguity number N=0, the accurate value of the Doppler center frequency is f _dc =-942.7Hz. Then according to the formula, the radial velocity of the target is calculated as v _y =0.703m/ s;

Figure 9 shows the imaging result of the moving target after the estimated radial velocity is compensated by the distance movement compensation function. After compensation, the moving target coincides with the position of the shadow, indicating that the moving target has returned to its true position through compensation, proving the correctness of the estimated velocity and realizing the relocation of the moving target.

Claims (1)

1. The moving target radial velocity estimation and repositioning method for the terahertz circumference SAR is characterized by comprising the following steps of:

s1, performing de-frequency modulation processing on the original echo data of the terahertz circumference SAR, namely receiving echo pulse S _r (t _r ,t _a ) And reference signal s _ref (t _r ,t _a ) Mixing to obtain intermediate frequency signal s subjected to frequency modulation removal _i (t _r ,t _a ) Wherein t is _r Fast time, t, of radar echo signal _a Slow time for echo signal; converting the signal subjected to frequency removal processing into a distance frequency domain azimuth time domain through distance Fourier transform to obtain S _i (f _r ,t _a ) Then with phase compensation function S _c (f _r ) Multiplying to remove residual video phase term and echo envelope diagonal term to obtain distance compressed signal S (f) _r ,t _a )：

Wherein f _r For fast time-frequency domain variation, σ is the target backscatter coefficient, T _p Is the pulse width of the signal, c is the speed of light, K _γ For the frequency modulation slope of LFM signals, R _△ The residual slant distance after frequency modulation is removed;

s2, selecting proper sub-aperture numbers to divide sub-apertures according to the obtained full-aperture circumference SAR echo data;

s3, imaging echo data in a single sub-aperture by utilizing a PFA algorithm, searching a moving target in an imaging result, finding a distance unit where the moving target is located, and performing distance Fourier transform on the distance unit to obtain a distance frequency domain azimuth time domain signal;

s4, acquiring a time-frequency line of the moving target by utilizing short-time Fourier transform, and extracting a frequency slice at the initial time of the time-frequency line, wherein the frequency corresponding to the slice peak value is the initial Doppler center frequency f of the moving target _dco ；

S5, estimating Doppler fuzzy number N, and according to fuzzy number sum f _dco Obtaining Doppler center frequency f _dc ：

f _dc ＝f _dco +N×PRF

Where PRF is the pulse repetition frequency of the radar transmit signal.

S6, utilizingObtaining the radial velocity v of the target _y Wherein f _c gA is the ground wiping angle of the flight plane and the ground plane of the radar carrier;

s7, according to the obtained radial velocity v _y Constructing a distance walking compensation function H _RW ,

Compensating in a distance frequency domain to obtain a repositioning result of the moving target, wherein R _ref Is the reference slant range between the radar and the ground scene.