阅读说明：本技术 基于多普勒中心估计的机载sar预处理方法 (Airborne SAR preprocessing method based on Doppler center estimation ) 是由 曹蕊 王勇 于 2021-09-18 设计创作，主要内容包括：基于多普勒中心估计的机载SAR预处理方法,本发明涉及机载SAR预处理方法。本发明的目的是为了解决现有机载SAR成像方法存在的SAR图像二维散焦和处理效率慢的问题。过程为：一、获得距离压缩后的机载SAR回波数据；二、初始化子数据块个数；三、若k≤K,则执行四；否则,执行十；四、获取子数据块；五、求解共轭相乘矩阵；六、得到相关函数；七、得到自相关函数；八、求解回波的多普勒频率估计值；九、令k＝k+1,并返回三；十、求解SAR平台运动引起的多普勒频率理论值；十一、得到ISAR平台运动引起的多普勒频率估计值；十二、得到拟合曲线；十三、获得最优成像数据段。本发明用于机载SAR预处理领域。(The invention discloses an airborne SAR preprocessing method based on Doppler center estimation, and relates to an airborne SAR preprocessing method. The invention aims to solve the problems of two-dimensional defocusing and low processing efficiency of SAR images in the conventional airborne SAR imaging method. The process is as follows: firstly, acquiring airborne SAR echo data after distance compression; secondly, initializing the number of the subdata blocks; if K is less than or equal to K, executing a fourth step; otherwise, executing ten; fourthly, obtaining the subdata blocks; fifthly, solving a conjugate multiplication matrix; sixthly, obtaining a correlation function; seventhly, obtaining an autocorrelation function; eighthly, solving the Doppler frequency estimation value of the echo; ninthly, enabling k to be k +1, and returning to the step three; solving a Doppler frequency theoretical value caused by the SAR platform motion; eleven, obtaining a Doppler frequency estimated value caused by the movement of the ISAR platform; twelfth, obtaining a fitting curve; thirteen, obtaining the optimal imaging data section. The method is used in the field of airborne SAR pretreatment.)

1. The airborne SAR preprocessing method based on Doppler center estimation is characterized by comprising the following steps: the method comprises the following specific processes:

acquiring airborne SAR echo data, performing distance compression on the acquired airborne SAR echo data by adopting distance dimension matched filtering in a distance-Doppler algorithm to obtain distance-compressed airborne SAR echo data, and recording the distance-compressed airborne SAR echo data as s_rb(m,n)；

Wherein m is the pulse sequence number of the echo, and n is the fast time sequence number; m is 1,2, …, N_a，N_aIs the number of pulses, N is 1,2, …, N_r，N_rSampling points for a fast time;

step two, initializing the subdata block serial number k to be 1, and setting the subdata block pulse number N_a0，1＜N_a0＜N_a(ii) a The number of the subdata blocks is

In the formula (I), the compound is shown in the specification,is a rounded down function;

step three, comparing K with K, and if K is less than or equal to K, executing step four; otherwise, executing step ten;

step four, taking N along the azimuth direction_a0Pulse to obtain sub data block

Sub data blockContaining N_a0A pulse signal;

in the formula, m₀Is the pulse number, m, of the sub-data block₀＝1,2,…,N_a0；

Step five, sub data blocksOne pulse beforeThe conjugate of the signal is multiplied by the next pulse to solve the conjugate multiplication matrix s (m)₁,n)；

Step six, multiplying conjugate multiplication matrix s (m)₁N) summing along the azimuth direction, and averaging to obtain a correlation function;

step seven, summing the correlation functions along the distance direction, and taking an average value to obtain an autocorrelation function;

step eight, solving the Doppler frequency estimated value f of the echo based on the autocorrelation function_dc(k)；

Step nine, making k equal to k +1, and returning to the step three;

step ten, when the ship target is static, the whole imaging process is regarded as SAR imaging, and the Doppler frequency theoretical value caused by the SAR platform motion is solved

Step eleven, when the ship target moves, the whole imaging process has SAR imaging and ISAR imaging, and the Doppler frequency estimated value of the echo is f_dc(k) (ii) a Echo-based Doppler frequency estimate f_dc(k) And Doppler frequency theoretical value caused by SAR platform motionObtaining Doppler frequency estimated value caused by ISAR platform movement

Step twelve, toPerforming curve fitting to obtain a fitting curve

Thirteen step of remembering coincidenceNumber of start-stop pulse of m_sAnd m_eObtaining the optimal imaging data segmentNamely, the SAR pretreatment process is completed;

in the formula, m_opt＝1,2,…,m_e-m_s+1 is the pulse number of the optimal imaging data segment, and δ is the threshold.

2. The doppler center estimation based airborne SAR preprocessing method according to claim 1, wherein: the sub-data block in the fourth stepThe expression of (a) is:

sub data block

3. The doppler center estimation based airborne SAR preprocessing method according to claim 2, wherein: the conjugate multiplication matrix s (m) in the step five₁And n) is expressed as:

in the formula, m₁Is a conjugate multiplication matrix s (m)₁,t_r) Pulse number of (1), m₁＝1,2,…,N_a0-1，[·]^HIs a conjugate function.

4. The Doppler center estimation based airborne SAR pre-processing method according to claim 3, characterized in that: the specific expression of the correlation function in the sixth step is as follows:

5. the Doppler center estimation based airborne SAR pre-processing method according to claim 4, characterized in that: the specific expression of the autocorrelation function in the seventh step is as follows:

6. the Doppler center estimation based airborne SAR pre-processing method according to claim 5, characterized in that: the Doppler frequency estimated value f of the echo in the step eight_dc(k) The specific expression of (A) is as follows:

f_dc(k)＝angle(R_b)/(2πPRT)

where angle (-) is the phase function and PRT is the pulse repetition period.

7. The Doppler center estimation based airborne SAR pre-processing method according to claim 6, characterized in that: in the step ten, the calculation mode of the Doppler frequency theoretical value caused by the SAR platform motion is as follows:

in the formula, V_rAs the aircraft flight speed, sgn [. cndot]In sign function, θ is the squint angle and λ is the wavelength.

8. The doppler center estimation based airborne SAR preprocessing method of claim 7, wherein: the oblique angle

In the formula, R_sIs a mineAnd the slant distance between the target center and the radar when the target is viewed rightly.

9. The doppler center estimation based airborne SAR preprocessing method of claim 8, wherein: the echo-based Doppler frequency estimate f in step eleven_dc(k) And Doppler frequency theoretical value caused by SAR platform motionObtaining Doppler frequency estimated value caused by ISAR platform movement; the specific process is as follows:

in the formula (I), the compound is shown in the specification,is an estimate of the doppler frequency caused by motion of the ISAR platform.

10. The doppler center estimation based airborne SAR preprocessing method of claim 9, wherein: fitting the curve in step twelveThe expression is as follows:

in the formula, a₀、a_i、b_iFor the fitting coefficient, ω is the fundamental frequency and p is the fitting order.

Technical Field

The invention relates to an airborne SAR pretreatment method.

Background

An airborne Synthetic Aperture Radar (SAR) plays an important role in the field of sea area monitoring for sea surface ship target imaging. In the SAR imaging scene, usually radar moves and a target is still, and during the imaging of a sea surface ship target, the ship target usually has translation and swing motion, so that the image of the ship in a radar image is defocused in two dimensions, the image quality is reduced, and the subsequent target classification and identification precision is influenced.

The currently proposed airborne SAR imaging algorithm or Inverse Synthetic Aperture Radar (ISAR) processing technology can improve the image quality to a certain extent, but the algorithms process all echo data in the ship observed period, so that the following defects are caused: (1) echo data contains all movement of a ship in an observation period, so that an imaging projection plane is continuously changed, and image defocusing is caused; (2) all echo data are processed, the calculated amount is large, and the imaging efficiency is influenced. Therefore, preprocessing is needed to be implemented before SAR imaging, so that the influence of ship target motion on imaging is reduced, the image quality is improved, the data processing amount is reduced, and the imaging efficiency is improved. Considering that the Doppler center change of the echo can reflect the Doppler frequency change of a single scattering point, and further reflect the target motion condition, SAR preprocessing can be performed according to the echo Doppler center frequency estimation result.

Disclosure of Invention

The invention aims to solve the problems of two-dimensional defocusing and low processing efficiency of an SAR image in the conventional airborne SAR imaging method, and provides an airborne SAR preprocessing method based on Doppler center estimation.

The airborne SAR preprocessing method based on Doppler center estimation comprises the following specific processes:

acquiring airborne SAR echo data, performing distance compression on the acquired airborne SAR echo data by adopting distance dimension matched filtering in a distance-Doppler algorithm to obtain distance-compressed airborne SAR echo data, and recording the distance-compressed airborne SAR echo data as s_rb(m,n)；

Wherein m is the pulse sequence number of the echo, and n is the fast time sequence number; m is 1,2, …, N_a，N_aIs the number of pulses, N is 1,2, …, N_r，N_rSampling points for a fast time;

step two, initializing the subdata block serial number k to be 1, and setting the subdata block pulse number N_a0，1＜N_a0＜N_a(ii) a The number of the subdata blocks is

In the formula (I), the compound is shown in the specification,is a rounded down function;

step three, comparing K with K, and if K is less than or equal to K, executing step four; otherwise, executing step ten;

step four, taking N along the azimuth direction_a0Pulse to obtain sub data block

Sub data blockContaining N_a0A pulse signal;

in the formula, m₀Is the pulse number, m, of the sub-data block₀＝1,2,…,N_a0；

Step five, sub data blocksMultiplying the conjugate of the previous pulse signal by the next pulse signal, and solving a conjugate multiplication matrix s (m)₁,n)；

Step six, multiplying conjugate multiplication matrix s (m)₁N) summing along the azimuth direction, and averaging to obtain a correlation function;

step seven, summing the correlation functions along the distance direction, and taking an average value to obtain an autocorrelation function;

step eight, solving the Doppler frequency estimated value f of the echo based on the autocorrelation function_dc(k)；

Step nine, making k equal to k +1, and returning to the step three;

step ten, when the ship target is static, the whole imaging process is regarded as SAR imaging, and the Doppler frequency theoretical value caused by the SAR platform motion is solved

Step eleven, when the ship target moves, the whole imaging process has SAR imaging and ISAR imaging, and the Doppler frequency estimated value of echoIs f_dc(k) (ii) a Echo-based Doppler frequency estimate f_dc(k) And Doppler frequency theoretical value caused by SAR platform motionObtaining Doppler frequency estimated value caused by ISAR platform movement

Step twelve, toPerforming curve fitting to obtain a fitting curve

Thirteen step of remembering coincidenceNumber of start-stop pulse of m_sAnd m_eObtaining the optimal imaging data segmentNamely, the SAR pretreatment process is completed;

in the formula, m_opt＝1,2,…,m_e-m_s+1 is the pulse number of the optimal imaging data segment, and δ is the threshold.

The invention has the beneficial effects that:

the invention provides an airborne SAR preprocessing method based on Doppler center estimation, which takes the phase influence and imaging projection plane change caused by ship target motion into consideration to cause the phenomenon of two-dimensional defocusing of ship images in imaging results. In the scene of imaging a moving ship target by an airborne SAR, the SAR preprocessing method provided by the invention obtains the Doppler frequency caused by the moving ship target by utilizing the difference between the echo Doppler frequency and the Doppler frequency caused by the movement of the SAR platform, and obtains an optimal imaging data section by selecting a part with small Doppler frequency caused by the ISAR platform, namely a part with small influence of the movement of the ship target on imaging, so as to finish the SAR preprocessing process. By SAR preprocessing, the influence of ship target motion on imaging can be effectively reduced, the radar image quality is improved, the problem of SAR image two-dimensional defocusing caused by the existing airborne SAR imaging method is solved, and the follow-up target classification and identification are guaranteed; meanwhile, the optimal imaging data section is selected by SAR preprocessing, and compared with the method of directly performing imaging processing, the method effectively reduces the calculated amount and improves the imaging processing efficiency.

Drawings

FIG. 1 is a flow chart of an airborne SAR pre-processing method based on Doppler center estimation according to the present invention;

fig. 2a is a front view of a scattering point model of a ship used in the first and second embodiments, where a rectangular coordinate system O-XYZ is established with a ship center as an origin O, a true east direction as an X axis, a true north direction as a Y axis, and a vertical sea level direction as a Z axis;

FIG. 2b is a side view of a scattering point model of a ship used in the first and second embodiments;

FIG. 2c is a top view of a scattering point model of the ship used in the first and second embodiments;

FIG. 2d is a three-dimensional view of a scattering point model of the ship used in the first and second embodiments;

fig. 3 is a CS image before SAR preprocessing in the first embodiment, where CS is chirp scaling;

FIG. 4 is a diagram illustrating the result of estimating the Doppler frequency of the echo according to the second embodiment of the present invention;

FIG. 5 is a diagram illustrating the result of estimating Doppler frequency caused by the ISAR platform according to the second embodiment of the present invention;

fig. 6 is a CS image after SAR preprocessing obtained by the method of the present invention in the second embodiment;

FIG. 7 is a CS image before SAR pre-processing in the third embodiment;

FIG. 8 is a diagram illustrating the result of estimating the Doppler frequency of the echo according to the fourth embodiment of the present invention;

FIG. 9 shows the result of estimating Doppler frequency caused by the ISAR platform according to the fourth embodiment of the present invention;

fig. 10 is a CS image after SAR preprocessing obtained by the method of the present invention in the fourth embodiment.

Detailed Description

The first embodiment is as follows: the embodiment is described with reference to fig. 1, and the specific process of the airborne SAR preprocessing method based on doppler center estimation in the embodiment is as follows:

the method comprises the steps of firstly, obtaining airborne SAR echo data, adopting distance dimension matching filtering in a distance-Doppler (RD) algorithm to carry out distance compression on the obtained airborne SAR echo data, obtaining airborne SAR echo data after distance compression, and recording the distance-compressed airborne SAR echo data as s_rb(m,n)；

Wherein m is the pulse sequence number of the echo, and n is the fast time sequence number; m is 1,2, …, N_a，N_aIs the number of pulses, N is 1,2, …, N_r，N_rSampling points for a fast time;

step two, initializing the subdata block serial number k to be 1, and setting the subdata block pulse number N_a0，1＜N_a0＜N_a(ii) a The number of the subdata blocks is

In the formula (I), the compound is shown in the specification,is a rounded down function;

step three, comparing K with K, and if K is less than or equal to K, executing step four; otherwise, executing step ten;

step four, taking N along the azimuth direction_a0Pulse to obtain sub data block

Sub data blockContaining N_a0A pulse signal;

in the formula, m₀Is the pulse number, m, of the sub-data block₀＝1,2,…,N_a0；

Step five, mixing the seedsData blockMultiplying the conjugate of the previous pulse signal by the next pulse signal, and solving a conjugate multiplication matrix s (m)₁,n)；

Step six, multiplying conjugate multiplication matrix s (m)₁N) summing along the azimuth direction, and averaging to obtain a correlation function;

step seven, summing the correlation functions along the distance direction, and taking an average value to obtain an autocorrelation function;

step eight, solving the Doppler frequency estimated value f of the echo based on the autocorrelation function_dc(k)；

Step nine, making k equal to k +1, and returning to the step three;

step ten, when the ship target is static, the whole imaging process can be regarded as SAR imaging, and the Doppler frequency theoretical value caused by the SAR platform motion is solved

Step eleven, when the ship target moves, the whole imaging process has SAR imaging and ISAR imaging, and the Doppler frequency estimated value of the echo is f_dc(k) (ii) a Echo-based Doppler frequency estimate f_dc(k) And Doppler frequency theoretical value caused by SAR platform motionObtaining Doppler frequency estimated value caused by ISAR platform movement

The Doppler frequency of the echo is the total Doppler frequency and is the Doppler frequency estimated value caused by the SAR and ISAR platforms;

step twelve, toPerforming curve fitting, wherein Fourier fitting can be adopted to obtain fitting curve

Thirteen step of takingPart, the part characterizes that the target motion has little influence on imaging, can improve the image quality and is consistent withNumber of start-stop pulse of m_sAnd m_eObtaining the optimal imaging data segmentNamely, the SAR pretreatment process is completed, and other imaging processes can be carried out subsequently;

in the formula, m_opt＝1,2,…,m_e-m_s+1 is the pulse number of the optimal imaging data segment, and δ is the threshold.

The second embodiment is as follows: the difference between this embodiment and the first embodiment is that the sub-data block in the fourth stepThe expression of (a) is:

sub data block

Other steps and parameters are the same as those in the first embodiment.

The third concrete implementation mode: in this embodiment, the difference between the first embodiment and the second embodiment is that the conjugate multiplication matrix s (m) in the fifth step₁And n) is expressed as:

in the formula, m₁Is a conjugate multiplication matrix s (m)₁,t_r) Pulse sequence ofNumber m₁＝1,2,…,N_a0-1，[·]^HIs a conjugate function.

Other steps and parameters are the same as those in the first or second embodiment.

The fourth concrete implementation mode: the difference between this embodiment and one of the first to third embodiments is that, in the sixth step, the specific expression of the correlation function is:

other steps and parameters are the same as those in one of the first to third embodiments.

The fifth concrete implementation mode: the difference between this embodiment and one of the first to the fourth embodiments is that, in the seventh step, the specific expression of the autocorrelation function is:

other steps and parameters are the same as in one of the first to fourth embodiments.

The sixth specific implementation mode: the difference between this embodiment and one of the first to fifth embodiments is that the doppler frequency estimated value f of the echo in the step eight_dc(k) The specific expression of (A) is as follows:

f_dc(k)＝angle(R_b)/(2πPRT)

in the formula, angle () is a phase function, and PRT (pulse repetition time) is a pulse repetition period.

Other steps and parameters are the same as those in one of the first to fifth embodiments.

The seventh embodiment: the difference between this embodiment and one of the first to sixth embodiments is that the calculation method of the doppler frequency theoretical value caused by the motion of the SAR platform in the step ten is as follows:

in the formula, V_rAs the aircraft flight speed, sgn [. cndot]In sign function, θ is the squint angle and λ is the wavelength.

Other steps and parameters are the same as those in one of the first to sixth embodiments.

The specific implementation mode is eight: this embodiment is different from one of the first to seventh embodiments in that the oblique angle

In the formula, R_sThe target center is the slant distance between the radar and the target when the radar is looking at the target.

Other steps and parameters are the same as those in one of the first to seventh embodiments.

The specific implementation method nine: this embodiment is different from the first to eighth embodiments in that the echo-based doppler frequency estimated value f in the eleventh step_dc(k) And Doppler frequency theoretical value caused by SAR platform motionObtaining Doppler frequency estimated value caused by ISAR platform movement; the specific process is as follows:

the Doppler frequency due to the object motion can be obtained by subtracting the Doppler frequency due to the SAR platform from the Doppler frequency of the echo, the Doppler frequency due to the ISAR platform being

In the formula (I), the compound is shown in the specification,is an estimate of the doppler frequency caused by motion of the ISAR platform.

The Doppler frequency of the echo is the total Doppler frequency, and is an estimated value of the Doppler frequency caused by the SAR and ISAR platforms.

Other steps and parameters are the same as those in one to eight of the embodiments.

The detailed implementation mode is ten: this embodiment is different from the first to ninth embodiments in that a curve is fitted in the twelfth stepThe expression is as follows:

in the formula, a₀、a_i、b_iFor the fitting coefficient, ω is the fundamental frequency and p is the fitting order.

ω、p、a₀、a_i、b_iIs based on the sum of the values m involved in the fittingAnd (4) determining.

Other steps and parameters are the same as those in one of the first to ninth embodiments.

The following examples were used to demonstrate the beneficial effects of the present invention:

the first embodiment is as follows:

in this embodiment, a Chirp Scaling (CS) algorithm is combined to image airborne SAR simulation data, and the obtained radar image is a result of no SAR preprocessing and is used to compare the effect after SAR preprocessing.

The scattering point model used for simulation is a ship model, the front view, the side view, the top view and the three-dimensional view of the ship model are shown in fig. 2a, 2b, 2c and 2d, the radar system parameters used for simulation are shown in table 1, and the three-dimensional swing parameters of a ship target are shown in table 2.

TABLE 1 Radar System simulation parameters

TABLE 2 Ship target three-dimensional swing parameters

Fig. 3 shows a CS image without SAR preprocessing, and it can be seen that the distance and orientation dimensions of the SAR image are heavily defocused, and the image entropy is 10.4735. In the case where SAR preprocessing is not performed, the CS algorithm needs to process 4664 as the number of pulses, 0s to 23.3150s as the imaging time period, and the calculation amount is large.

Example two:

the following examples were used to demonstrate the beneficial effects of the present invention:

in the embodiment, the onboard SAR simulation data is imaged by combining a Chirp Scaling (CS) algorithm, and the obtained radar image is a result after SAR pretreatment and is used for comparing the effect before SAR pretreatment.

The ship scattering point model, radar system parameters and ship motion parameters used for the simulation are consistent with fig. 2a, 2b, 2c, 2d, table 1 and table 2.

FIG. 4 shows the echo Doppler frequency f_dcEstimated value of (3), fitting result and Doppler frequency theoretical value caused by SAR platformIt can be seen that the rectangular area has f_dcIs very close toThe characteristic (b) indicates that the influence caused by the object motion is small during the imaging period. FIG. 5 shows the Doppler frequency estimation, fitting result and optimal start and end imaging moments caused by the ISAR platform, wherein the shaded areas representSelectingThe fitting result meets the conditionThe optimal start and end imaging times are shown by the dotted lines in the figure.

Fig. 6 is a CS image after SAR preprocessing, and it can be seen that the distance and orientation dimensions of the SAR image are well focused, and the image quality is improved. From the viewpoint of the image entropy, the preprocessed image entropy is 10.3203, which is smaller than that before preprocessing, and the effectiveness of the proposed SAR preprocessing algorithm can also be illustrated. After SAR preprocessing, the number of pulses to be processed by the CS algorithm is 1606, the imaging time period is 7.4210s to 15.4500s, and the calculated amount is obviously reduced.

Example three:

in this embodiment, a Chirp Scaling (CS) algorithm is combined to image airborne SAR measured data, and the obtained radar image is a result of no SAR preprocessing and is used to compare the effect after SAR preprocessing.

The data used are echo data recorded by the airborne SAR on the sea surface ship target, and the radar system parameters are shown in Table 3.

TABLE 3 Radar System simulation parameters

Fig. 7 shows the CS image without SAR preprocessing, and it can be seen that the distance and orientation dimensions of the SAR image have a certain degree of defocus, and the image entropy value is 8.6847. In the case where SAR preprocessing is not performed, the CS algorithm needs to process the pulse number of 10240, the imaging time period of 0s to 20.478s, and the calculation amount is large.

Example four:

the following examples were used to demonstrate the beneficial effects of the present invention:

in this embodiment, the method of the present invention is used in combination with a Chirp Scaling (CS) algorithm to image airborne SAR measured data, and the obtained radar image is a result after SAR preprocessing and is used to compare the effect before SAR preprocessing.

The data used is echo data recorded by the airborne SAR to the sea surface ship target, and the radar system parameters are consistent with the table.

FIG. 8 shows the echo Doppler frequencyf_dcEstimated value of (3), fitting result and Doppler frequency theoretical value caused by SAR platformIt can be seen that the rectangular area has f_dcIs very close toThe characteristic (b) indicates that the influence caused by the object motion is small during the imaging period. FIG. 9 shows the Doppler frequency estimates, the fitting results and the optimal start and end imaging moments caused by the ISAR platform, where the shaded areas representSelectingThe fitting result meets the conditionThe optimal start and end imaging times are shown by the dotted lines in the figure.

Fig. 10 is a CS image after SAR preprocessing, and it can be seen that the SAR image has good distance and azimuth dimension focusing effects, the hull detail part is clearer, and the image quality is improved. From the viewpoint of the image entropy, the preprocessed image entropy is 8.3891, which is smaller than that before preprocessing, and the effectiveness of the proposed SAR preprocessing algorithm can also be illustrated. After SAR preprocessing, the number of pulses to be processed by the CS algorithm is 1916, the imaging time period is 2.420s to 6.253s, and the calculated amount is obviously reduced.

The present invention is capable of other embodiments and its several details are capable of modifications in various obvious respects, all without departing from the spirit and scope of the present invention.