An online adaptive waveform parameter adjustment method for synthetic aperture radar imaging

Abstract

The present invention discloses an online adaptive waveform parameter adjustment method for synthetic aperture radar imaging, the method comprising: obtaining an initial value of an azimuth angle, an initial value of a ground-grazing angle, and an initial value of a waveform parameter based on a flight trajectory of a flight platform; determining relevant data of synthetic aperture radar imaging; judging whether the relevant data meets a preset synthetic aperture radar imaging index; if so, constructing an offline waveform library based on current waveform parameters; and adjusting the waveform parameters online based on the offline waveform library and in combination with the flight trajectory of the flight platform. The method enables the synthetic aperture radar imaging waveform to adapt to the flight trajectory error, can guarantee the imaging performance under certain error conditions, and is applicable under both positive side and front oblique geometric conditions, and can improve the synthetic aperture radar imaging performance.

Description

Technical Field

[0001] The invention relates to the field of radar imaging, and in particular to an online adaptive waveform parameter adjustment method for synthetic aperture radar imaging. Background Art

[0002] The waveform design of synthetic aperture radar imaging system is closely related to many factors such as imaging geometry and imaging performance. Usually, the imaging waveform is designed offline. However, the flight platform equipped with radar (such as aircraft) is affected by external factors during the actual flight process, and there is often a certain error compared to the designed flight trajectory. The use of offline waveform design may cause a certain loss in imaging performance, or even make imaging processing impossible. Summary of the invention

[0003] In view of this, the present invention provides an online adaptive waveform parameter adjustment method for synthetic aperture radar imaging, which can solve the technical problem that the synthetic aperture radar imaging waveform designed offline often has a certain error compared with the designed flight trajectory, and the synthetic aperture radar imaging waveform designed offline causes a certain loss in imaging performance, or even makes imaging processing impossible.

[0004] In order to solve the above technical problems, the present invention is implemented as follows.

[0005] A method for adjusting online adaptive waveform parameters of synthetic aperture radar imaging, comprising:

[0006] Step S1: Based on the flight trajectory of the flight platform, an initial value of the azimuth angle, an initial value of the ground-rubbing angle, and an initial value of the waveform parameter are obtained, wherein the azimuth angle is the angle between the radar line of sight direction and the velocity direction projected on the horizontal plane, and the ground-rubbing angle is the angle between the radar line of sight direction and the horizontal plane;

[0007] Step S2: determining relevant data of synthetic aperture radar imaging, wherein the relevant data includes azimuth spectrum, transmission pulse shielding area, altitude line shielding area, azimuth resolution in ground distance plane, range resolution, value of off-axis angle, noise equivalent backscatter coefficient, synthetic aperture radar imaging width, and geometric distortion index;

[0008] Step S3: determine whether the relevant data meets the preset synthetic aperture radar imaging index; if so, build an offline waveform library based on the current waveform parameters and proceed to step S4; otherwise, update the azimuth angle, ground-grazing angle and the waveform parameters and proceed to step S2;

[0009] Step S4: Based on the offline waveform library and in combination with the flight trajectory of the flight platform, the waveform parameters are adjusted online.

[0010] Preferably, step S21: determining whether the azimuth spectrum is aliased;

[0011] Step S22: determining a transmit pulse shielding area and a height line shielding area, and judging whether there is transmit pulse shielding and / or height line shielding;

[0012] Step S23: Determine the azimuth resolution ρ in the ground distance plane ga And the range resolution ρ gr ;

[0013] Step S24: determining a value of an off-axis angle, where the off-axis angle refers to an angle at which the beam deviates from the antenna normal;

[0014] Step S25: determining the noise equivalent backscatter coefficient;

[0015] Step S26: Determine the synthetic aperture radar imaging width;

[0016] Step S27: determining geometric distortion indicators, wherein the geometric distortion indicators include output image rotation angle error, range geometric distortion error, azimuth geometric distortion error, image center azimuth error, and image center point elevation error. The image is an imaging result image output by a synthetic aperture radar.

[0017] Preferably, the step S21, determining whether the azimuth spectrum is aliased, comprises:

[0018] If the pulse repetition frequency PRF is less than the Doppler bandwidth, spectrum aliasing will occur. That is, if the pulse repetition frequency PRF is less than the instantaneous Doppler bandwidth Bd in the azimuth direction, spectrum aliasing will occur.

[0019] PRF<PRF min =B d , PRF min is the minimum value of PRF.

[0020] Preferably, the step S22: determining the transmission pulse shielding area and the height line shielding area, and judging whether there is transmission pulse shielding and / or height line shielding, comprises:

[0021] The calculation method of the transmit pulse shielding area is:

[0022] Calculate the starting sampling distance R of the radar echo signal min and the maximum sampling distance R max :

[0023] R min =R-Nrnew / 2×c / 2 / f s

[0024] R max =R min +Nr×c / 2 / f s

[0025] Among them, R is the effective distance of the center point of the image, Nrnew is the number of imaging processing points, Nr is the number of acquisition points in the distance direction, c is the speed of light, and f s is the signal sampling rate;

[0026] The transmit pulse shielding area is calculated by the following formula:

[0027]

[0028] Among them, T b is the guard band time, which is a fixed value, n is the transmit pulse number, PRT is the pulse repetition period, T p is the pulse width of the transmitting signal, R T is the distance where pulse obstruction exists;

[0029] The calculation method of the height line occlusion area is:

[0030]

[0031] Among them, H is the distance of the height line echo, H e is the height error of the flight platform, which is used as a protection against the height line occlusion. H is the distance where there is height line occlusion.

[0032] Preferably, the step S23: determining the azimuth resolution ρ in the ground distance plane ga And the range resolution ρ gr ,include:

[0033] The slant range plane is the plane where the radar velocity direction and the line of sight are located. The range resolution in the slant range plane is ρ r , azimuth resolution ρ a The calculation formula is as follows:

[0034]

[0035]

[0036] Where c is the speed of light, B is the signal bandwidth, λ is the wavelength; R is the distance from the center of the image, V m is the velocity of the flight platform, T a is the accumulation time; θ is the front tilt angle, which is calculated by the ground-rubbing angle β, the azimuth angle α and the velocity dive angle γ. The image is the imaging result image output by the synthetic aperture radar;

[0037] According to the oblique projection relationship, the azimuth resolution ρ in the ground distance plane is obtained ga and range resolution ρ gr The calculation formula is as follows:

[0038]

[0039]

[0040] Among them, k wa , k wr are the window function broadening factors in azimuth and range, respectively, and k ia and k ir are the azimuth and range projection coefficients respectively.

[0041] Preferably, the step S24: determining a value of an off-axis angle, where the off-axis angle refers to an angle at which the beam deviates from the antenna normal, includes:

[0042] Off-axis angle θ LZ Calculated by the following formula:

[0043]

[0044] Among them, vector is the normal vector of the antenna array in the antenna system, and the vector is the beam pointing vector in the antenna system, which is the beam pointing vector in the north sky east coordinate system After coordinate transformation to the antenna system, the coordinate transformation matrix is ​​obtained from the antenna installation angle and the flight platform attitude angle.

[0045] Preferably, the step S25: determining the noise equivalent backscatter coefficient comprises:

[0046] The noise equivalent backscatter coefficient NEσ0 is calculated as follows:

[0047]

[0048] Among them, R is the effective distance of the center point of the image, and the duty cycle D is determined by the pulse width T of the transmitted signal. p And the pulse repetition frequency PRF is calculated, T a is the accumulation time, k is the Boltzmann constant, T0 is the standard noise temperature, F n is the noise factor, P t is the peak power, G is the antenna normal gain, λ is the wavelength, and M is the number of views, all of which are constants; ρ gr is the range resolution in the ground plane, ρ ga is the azimuth resolution in the ground-range plane;

[0049] Overall loss value L ∑a =Ls×Le×Ld, where Ls is the radar system loss, Le is the rain attenuation, and Ld is the atmospheric attenuation.

[0050] Preferably, the step S26: determining the synthetic aperture radar imaging width includes:

[0051] The width includes the azimuth imaging width and the elevation imaging width, wherein:

[0052] The azimuth imaging width is:

[0053] W RX =R×θ A

[0054] The imaging width in the pitch direction is:

[0055]

[0056] Among them, θ A is the azimuth beamwidth, θ E is the elevation beamwidth, and β is the ground-grazing angle.

[0057] Preferably, the step S27 determines a geometric distortion index, wherein the geometric distortion index includes an output image rotation angle error, a range geometric distortion error, an azimuth geometric distortion error, an image center azimuth error, and an image center elevation error. The image is an imaging result image output by a synthetic aperture radar, and includes:

[0058]

[0059]

[0060]

[0061]

[0062]

[0063] Among them, η Azi_RMSE is the azimuth geometric distortion error of the ground distance map, η Ran_RMSE is the geometric distortion error of the ground distance map, θ XZ_RMSE is the output image rotation angle error, φ Azi_RMSE is the azimuth error of the image center point, φ Ele_RMSE is the pitch angle error of the image center point; N is the number of Monte Carlo simulations, L Azi and are the true value of the distance between two target points in azimuth and the measured value calculated in the i-th Monte Carlo simulation; L Ran and are the true value of the distance between the two target points and the measured value calculated in the i-th Monte Carlo simulation; θ XZ and are the true value of the image rotation angle and the measured value calculated in the i-th Monte Carlo simulation; φ Azi and are the true value of the azimuth angle of the image center and the measured value calculated in the i-th Monte Carlo simulation; φ Ele and are respectively the true value of the pitch angle at the center of the image and the measured value calculated in the i-th Monte Carlo simulation.

[0064] Beneficial effects:

[0065] The present invention proposes an online adaptive waveform parameter adjustment method for synthetic aperture radar imaging. First, according to the characteristics of synthetic aperture radar imaging, a set of offline design methods for imaging angles and waveform libraries is constructed, and then an online adaptive waveform adjustment method for synthetic aperture radar imaging is designed, so that the synthetic aperture radar imaging waveform can adapt to the flight trajectory error, can guarantee the imaging performance under certain error conditions, and can be applicable under both positive side and front oblique geometric conditions, which can improve the synthetic aperture radar imaging performance.

[0066] (1) The present invention can adaptively design waveform parameters according to the actual flight trajectory and ensure imaging performance under certain flight trajectory error conditions.

[0067] (2) When the present invention actually performs synthetic aperture radar imaging, it will adaptively adjust the waveform according to the actual flight trajectory of the flight platform.

[0068] (3) According to the actual flight trajectory of the flight platform, if the appropriate waveform parameters cannot be found, the radar will not perform imaging processing, thereby avoiding the output of some invalid imaging results and reducing the burden of subsequent image processing links.

[0069] (4) The present invention designs a waveform library and imaging angles suitable for the flight trajectory offline, wherein the imaging angle can provide guidance for the beam pointing during actual flight.

[0070] (5) The present invention constructs an offline waveform library, which provides a basis for querying matching waveform parameters.

[0071] (6) The present invention calculates a variety of parameter data, takes the information into consideration more comprehensively, and the waveform parameters set adaptively are more reasonable. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 A schematic flow chart of the method for adjusting online adaptive waveform parameters of synthetic aperture radar imaging provided by the present invention;

[0073] Figure 2 A schematic diagram of the azimuth spectrum aliasing provided by the present invention;

[0074] Figure 3A schematic diagram of the transmission pulse shielding area provided by the present invention;

[0075] Figure 4 A schematic diagram of the height line shielding area provided by the present invention;

[0076] Figure 5 A schematic diagram of imaging width calculation provided by the present invention;

[0077] Figure 6 A schematic diagram of the geometric distortion principle provided by the present invention;

[0078] Figure 7 The zebra diagram corresponding to the initial PRF of the waveform library provided by the present invention;

[0079] Figure 8 This is a zebra diagram corresponding to the PRF after the adaptive waveform adjustment provided by the present invention. DETAILED DESCRIPTION

[0080] like Figure 1 As shown, the present invention proposes an online adaptive waveform parameter adjustment method for synthetic aperture radar imaging, wherein the waveform parameters are waveform parameters used when the synthetic aperture radar images the irradiated area, including the transmission pulse width, pulse repetition frequency, imaging accumulation time, and signal bandwidth.

[0081] A method for adjusting online adaptive waveform parameters of synthetic aperture radar imaging, the method comprising the following steps:

[0082] Step S1: Based on the pre-designed flight trajectory of the flight platform, the initial value of the azimuth angle, the initial value of the ground-rubbing angle and the initial value of the waveform parameter are obtained, wherein the azimuth angle is the angle between the radar line of sight direction and the velocity direction projected on the horizontal plane, and the ground-rubbing angle is the angle between the radar line of sight direction and the horizontal plane;

[0083] Step S2: determining relevant data of synthetic aperture radar imaging, wherein the relevant data includes azimuth spectrum, transmission pulse shielding area, altitude line shielding area, azimuth resolution in ground distance plane, range resolution, value of off-axis angle, noise equivalent backscatter coefficient, synthetic aperture radar imaging width, and geometric distortion index;

[0084] Step S3: determine whether the relevant data meets the preset synthetic aperture radar imaging indicators; if they are satisfied, build an offline waveform library based on the current waveform parameters and enter step S4; otherwise, update the azimuth angle, ground-grazing angle and the waveform parameters and enter step S2;

[0085] Step S4: Based on the offline waveform library and in combination with the flight trajectory of the flight platform, the waveform parameters are adjusted online.

[0086] The present invention relates to the joint design of the imaging angle of a preset flight trajectory and an offline waveform library. The present invention comprises two parts: an imaging angle and a waveform library offline design criterion and a waveform adaptive optimization adjustment method. The imaging angle mentioned here refers to an azimuth angle and a ground-grazing angle, and the online adaptive waveform design of synthetic aperture radar imaging is realized through waveform adaptive optimization adjustment.

[0087] The step S2 comprises:

[0088] Step S21: Determine whether the azimuth spectrum is aliased;

[0089] Step S22: determining a transmit pulse shielding area and a height line shielding area, and judging whether there is transmit pulse shielding and / or height line shielding;

[0090] Step S23: Determine the azimuth resolution ρ in the ground distance plane ga And the range resolution ρ gr ;

[0091] Step S24: determining a value of an off-axis angle, where the off-axis angle refers to an angle at which the beam deviates from the antenna normal;

[0092] Step S25: determining the noise equivalent backscatter coefficient;

[0093] Step S26: Determine the synthetic aperture radar imaging width;

[0094] Step S27: determining geometric distortion indicators, wherein the geometric distortion indicators include output image rotation angle error, range geometric distortion error, azimuth geometric distortion error, image center azimuth error, image center elevation error, and the image is an imaging result image output by a synthetic aperture radar.

[0095] In this embodiment, in step S3, after multiple iterations, a set of imaging angles and waveform parameters suitable for the preset flight trajectory of the platform is obtained, and the waveform parameters can be used to construct an offline waveform library.

[0096] In this embodiment, the step S4 determines the transmission pulse shielding and altitude line pulse shielding results under actual conditions in combination with the geometric relationship determined by the actual flight trajectory of the flight platform, and adjusts the waveform online.

[0097] The step S21, determining whether the azimuth spectrum is aliased, comprises:

[0098] The azimuth resolution is mainly obtained by pulse compression of the azimuth Doppler bandwidth. Therefore, it is necessary to ensure the integrity of the Doppler bandwidth information to prevent azimuth spectrum aliasing.

[0099] like Figure 2As shown in the figure, if the pulse repetition frequency PRF is less than the Doppler bandwidth, spectrum aliasing will occur. In order to ensure that the azimuth spectrum is not aliased, the pulse repetition frequency should be greater than the instantaneous Doppler bandwidth Bd in the azimuth direction, that is,

[0100] PRF ≥ PRF min =B d , PRF min is the minimum value of PRF.

[0101] The step S22: determining the transmission pulse shielding area and the height line shielding area, and judging whether there is transmission pulse shielding and / or height line shielding, comprises:

[0102] Occlusion mainly refers to emission pulse occlusion and height line occlusion.

[0103] like Figure 3 As shown, the calculation method of the transmission pulse shielding area is:

[0104] Calculate the starting sampling distance R of the radar echo signal min and the maximum sampling distance R max :

[0105] R min =R-Nrnew / 2×c / 2 / f s

[0106] R max =R min +Nr×c / 2 / f s

[0107] Among them, R is the effective distance of the center point of the image, Nrnew is the number of imaging processing points, Nr is the number of acquisition points in the distance direction, c is the speed of light, and f s is the signal sampling rate.

[0108] The transmit pulse shielding area is calculated by the following formula:

[0109]

[0110] Among them, T b is the guard band time, which is a fixed value, n is the transmit pulse number, PRT is the pulse repetition period, T p is the pulse width of the transmitting signal, R T is the distance at which pulse obstruction occurs.

[0111] like Figure 4 As shown, the calculation method of the height line occlusion area is:

[0112]

[0113] Among them, H is the distance of the height line echo, He is the height error of the flight platform, which is used as a protection against the height line occlusion. H is the distance where there is height line occlusion.

[0114] Step S23: Determine the azimuth resolution ρ in the ground distance plane ga And the range resolution ρ gr ,include:

[0115] The slant range plane is the plane where the radar velocity direction and the line of sight are located. The range resolution in the slant range plane is ρ r , azimuth resolution ρ a The calculation formula is as follows:

[0116]

[0117]

[0118] Where c is the speed of light, B is the signal bandwidth, λ is the wavelength; R is the distance from the center of the image, V m is the velocity of the flight platform, T a is the accumulation time; θ is the front tilt angle, which is calculated by the ground-rubbing angle β, the azimuth angle α and the velocity dive angle γ. The image is the imaging result output by the synthetic aperture radar.

[0119] According to the oblique projection relationship, the azimuth resolution ρ in the ground distance plane is obtained ga and range resolution ρ gr The calculation formula is as follows:

[0120]

[0121]

[0122] Among them, k wa , k wr are the window function broadening factors in azimuth and range, respectively, and k ia and k ir are the azimuth and range projection coefficients respectively.

[0123] The step S24: determining the value of the off-axis angle, where the off-axis angle refers to the angle at which the beam deviates from the antenna normal, includes:

[0124] Off-axis angle θ LZ Calculated by the following formula:

[0125]

[0126] Among them, vector is the normal vector of the antenna array in the antenna system, and the vector is the beam pointing vector in the antenna system, which is the beam pointing vector in the north sky east coordinate system After the coordinates are transformed to the antenna system, the coordinate transformation matrix is ​​obtained by the antenna installation angle and the flight platform attitude angle. The calculation formula is as follows:

[0127]

[0128] Among them, T a2m is the transformation matrix from the antenna coordinate system to the platform coordinate system, with the superscript T represents transpose, T a2m The transpose of is its inverse matrix, that is, the transformation from the platform coordinate system to the antenna coordinate system; C g2p is the transformation matrix from the North Celestial East coordinate system to the platform coordinate system, is the beam pointing vector in the North Celestial East coordinate system.

[0129] The beam pointing vector in the North Sky East coordinate system It is calculated from the azimuth angle α and the ground-rubbing angle β. The calculation formula is as follows:

[0130]

[0131] The step S25: determining the noise equivalent backscatter coefficient comprises:

[0132] The noise equivalent backscatter coefficient NEσ0 is calculated as follows:

[0133]

[0134] Among them, R is the effective distance of the center point of the image, and the duty cycle D is determined by the pulse width T of the transmitted signal. p And the pulse repetition frequency PRF is calculated, T a is the accumulation time, k is the Boltzmann constant, T0 is the standard noise temperature, F n is the noise factor, P t is the peak power, G is the antenna normal gain, λ is the wavelength, and M is the number of views, all of which are constants. gr is the range resolution in the ground plane, ρ ga is the azimuth resolution in the ground plane.

[0135] Overall loss value L ∑a =Ls×Le×Ld, where Ls is the radar system loss, Le is the rain attenuation, and Ld is the atmospheric attenuation.

[0136] The step S26: determining the synthetic aperture radar imaging width comprises:

[0137] like Figure 5The geometric relationship shown allows the imageable width to be calculated based on the azimuth and elevation distances covered by the 3dB beamwidth.

[0138] The width includes the azimuth imaging width and the elevation imaging width, wherein:

[0139] The azimuth imaging width is:

[0140] W RX =R×θ A

[0141] The imaging width in the pitch direction is:

[0142]

[0143] Among them, θ A is the azimuth beamwidth, θ E is the elevation beamwidth, and β is the ground-grazing angle.

[0144] The step S27: determining geometric distortion indicators, wherein the geometric distortion indicators include output image rotation angle error, range geometric distortion error, azimuth geometric distortion error, image center azimuth error, image center elevation error, and the image is an imaging result image output by a synthetic aperture radar, including:

[0145] Schematic diagram of the principle of geometric distortion of synthetic aperture radar imaging Figure 6 As shown, Figure 6 The rectangular virtual frame on the ground is the distance map, the center of the distance map is marked as point 1, and the four corners of the distance map are points 2, 3, 4 and 5. Select two target points along the range direction and azimuth direction respectively, and calculate the distortion of their relative distances.

[0146] First, calculate the true value of each parameter. Calculate the rectangular coordinates [PointX, PointY] of the five points on the ground distance map based on the geometric relationship and imaging width.

[0147] Calculate the distance between the two points 4 and 5 in azimuth and the two points 2 and 5 in distance, and get the azimuth spacing and distance spacing:

[0148]

[0149]

[0150] Calculate the output image rotation angle:

[0151]

[0152] Calculate the azimuth angle and elevation angle of the image center point based on the geometric relationship:

[0153]

[0154]

[0155] Then calculate the measured values ​​of various parameters. According to the rectangular coordinates of each point [PointXPointY], height H and three-axis speed V x 、V y 、V z , calculate the slant range value PointR(k) and Doppler center value PointF of the five points on the ground respectively d (k).

[0156]

[0157]

[0158] Random errors are added to the altitude and flight speed according to their error levels. According to the slant range Doppler frequency formula, the coordinates of the five points are inferred based on the parameters with errors [PointX e ,PointY e ].

[0159] According to the method of calculating the true value, according to the coordinates of each point with error [PointX e (k)PointY e (k)] Calculate the distance L between points 4 and 5 in the azimuth direction Azi_e The distance L between points 2 and 5 Ran_e ; Calculate the image rotation angle θ based on points 2 and 5 XZ_e ; According to the image center point 1, calculate the image center point azimuth φ Azi_e and the pitch angle φ of the image center Ele_e .

[0160] Finally, the error values ​​of geometric distortion indicators are counted. Based on the above method of adding random errors and calculating various indicators, N Monte Carlo simulations are performed, and the distance between the two target points in the azimuth direction and the distance between the two target points in the range map, the output image rotation angle, the azimuth angle and pitch angle of the image center point of each Monte Carlo simulation are calculated. Combined with the true values ​​of the five variables, the root mean square error of multiple Monte Carlo simulations of the five variables is calculated, and the error values ​​of the five indicators can be obtained.

[0161]

[0162]

[0163]

[0164]

[0165]

[0166] Among them, η Azi_RMSE is the azimuth geometric distortion error of the ground distance map, η Ran_RMSE is the geometric distortion error of the ground distance map, θ XZ_RMSE is the output image rotation angle error, φ Azi_RMSE is the azimuth error of the image center point, φ Ele_RMSE is the pitch angle error of the image center point; N is the number of Monte Carlo simulations, is the distance between points 4 and 5 in the azimuth direction with errors in the i-th Monte Carlo simulation, L Azi is the true value of the distance between points 4 and 5 in the azimuth direction calculated when there is no error; is the distance between points 2 and 5 calculated in the case of error in the i-th Monte Carlo simulation, L Ran is the true value of the distance between points 2 and 5 calculated when there is no error; is the image rotation angle in the presence of error calculated by the i-th Monte Carlo simulation, θ XZ is the true value of the image rotation angle calculated when there is no error; is the azimuth of the image center point in the case of error calculated by the i-th Monte Carlo simulation, φ Azi is the true value of the azimuth angle of the image center point calculated when there is no error; is the pitch angle of the image center point in the case of error calculated by the i-th Monte Carlo simulation, φ Ele is the true value of the pitch angle of the image center point calculated when there is no error.

[0167] In summary, the image rotation angle error, range geometric distortion error, azimuth geometric distortion error, image center azimuth error, and image center pitch angle error are respectively:

[0168]

[0169]

[0170]

[0171]

[0172]

[0173] Among them, η Azi_RMSE is the azimuth geometric distortion error of the ground distance map, η Ran_RMSE is the geometric distortion error of the ground distance map, θ XZ_RMSE is the output image rotation angle error, φ Azi_RMSEis the azimuth error of the image center point, φ Ele_RMSE is the pitch angle error of the image center point; N is the number of Monte Carlo simulations, L Azi and are the true value of the distance between two target points in azimuth and the measured value calculated in the i-th Monte Carlo simulation; L Ran and are the true value of the distance between the two target points and the measured value calculated in the i-th Monte Carlo simulation; θ XZ and are the true value of the image rotation angle and the measured value calculated in the i-th Monte Carlo simulation; φ Azi and are the true value of the azimuth angle of the image center and the measured value calculated in the i-th Monte Carlo simulation; φ Ele and are respectively the true value of the pitch angle at the center of the image and the measured value calculated in the i-th Monte Carlo simulation.

[0174] The step S3: determining whether the relevant data meets the preset synthetic aperture radar imaging index; if all meet, constructing an offline waveform library based on the current waveform parameters, and entering step S4; otherwise, updating the azimuth angle, the ground-grazing angle and the waveform parameters, and entering step S2, wherein:

[0175] Calculate the above-mentioned azimuth blur, occlusion judgment, resolution, noise equivalent backscatter coefficient, off-axis angle, imaging width, and geometric distortion indicators to determine whether the required imaging indicators are met. If the above-mentioned imaging performance indicators cannot be met, repeat the above-mentioned step S2, iteratively calculate the above-mentioned indicators by adjusting the imaging angle and waveform parameters, and find a set of optimal imaging angles and waveform parameters. The imaging angle can provide guidance for the beam pointing during actual imaging, and the waveform can be used as a waveform library under the geometric conditions. Through the waveform library, the imaging waveform parameters of the radar under each geometric relationship determined by the platform flight trajectory can be obtained.

[0176] The step S4: adjusting the waveform parameters online based on the offline waveform library and in combination with the flight trajectory of the flight platform, comprises:

[0177] According to the geometric relationship determined by the actual flight trajectory of the flight platform, the corresponding waveform parameters in the waveform library are found, and the transmission pulse occlusion and altitude line pulse occlusion results under this actual situation are determined. The pulse repetition frequency PRF in the waveform parameters is adjusted according to the occlusion results. After adjustment, the pulse repetition frequency PRF without transmission pulse occlusion and altitude line pulse occlusion is obtained to perform synthetic aperture imaging.

[0178] During the actual movement of the flight platform, the platform's trajectory is often affected by external factors and has certain errors, and it cannot move strictly according to the pre-set model. Whether the offline waveform can be applied to the actual platform geometry relationship, it is necessary to first determine whether there is a transmit pulse shielding and a height line shielding. In particular, transmit pulse shielding will cause the radar to freeze, which must be avoided.

[0179] First, determine whether the waveform library pulse repetition frequency PRF0 has transmission pulse obstruction and height line obstruction. If neither obstruction exists, select the waveform library pulse repetition frequency PRF0 as the pulse repetition frequency under the geometric relationship condition, and other waveform parameters remain unchanged.

[0180] If there is at least one type of occlusion within the sampling distance segment, the pulse repetition frequency PRF is selected between [(1-m)PRF0, (1+m)PRF0] with a certain step size to determine whether each pulse repetition frequency value does not have two types of occlusions, where m is a positive number less than 1 and can be adjusted according to the actual system. If there is a pulse repetition frequency value that satisfies the absence of both types of occlusions, the pulse repetition frequency value with the smallest difference from the waveform library pulse repetition frequency PRF0 is selected as the pulse repetition frequency under the current geometric relationship conditions. If there are two types of occlusions in the entire range of possible pulse repetition frequencies, it means that a suitable pulse repetition frequency value cannot be found, and the radar does not perform imaging processing at this time.

[0181] By performing online adaptive design of the pulse repetition frequency, when the platform motion trajectory deviates from the designed trajectory, the pulse repetition frequency PRF with no transmission pulse obstruction and height line obstruction can be selected to perform synthetic aperture imaging processing.

[0182] In order to better illustrate the purpose and advantages of the present invention, specific implementation methods are described, and further description is given below in conjunction with the accompanying drawings, experiments and data analysis.

[0183] (1) Offline waveform library design simulation experiment

[0184] Assuming that the radar altitude is 15km, the ground-grazing angle is fixed at 38°, and the azimuth angle is fixed at 27°, the design of the offline waveform library is analyzed and explained under this geometric relationship condition.

[0185] First, the waveform library is designed offline based on the image index requirements and the boundary conditions of various errors. When demonstrating the geometric distortion and image rotation angle error indicators, a fixed error is added to the height, and random values ​​that obey the Gaussian distribution are added to the platform attitude angle and speed. On this basis, Monte Carlo simulation is performed to calculate the index values. The waveform library is designed using the waveform library offline design criteria as shown in Table 1.

[0186] Table 1 Waveform library design results

[0187]

[0188] Based on the above waveform library, the synthetic aperture radar imaging indicators are shown in Table 2.

[0189] Table 2 Synthetic aperture radar imaging demonstration indicators based on waveform library

[0190]

[0191] Then, based on the above typical geometric conditions and waveform library parameters, the radar echo data is simulated, and the synthetic aperture radar imaging simulation is performed on the echo data. When simulating the imaging, the error values ​​are also added to the height and platform attitude velocity according to the design process. The synthetic aperture radar imaging is performed on the echo simulation data, and the image is evaluated. The specific results are shown in Table 3.

[0192] Table 3 Synthetic aperture radar imaging evaluation indicators based on simulation data

[0193]

[0194] It can be seen from the imaging indicators based on the waveform library and the synthetic aperture radar imaging evaluation indicators based on simulation data that although there are certain errors between the two, their orders of magnitude are comparable. Therefore, the offline waveform library design criteria proposed in the present invention can guide the design of synthetic aperture radar imaging waveforms.

[0195] (2) Adaptive waveform design simulation experiment

[0196] Add errors to the typical geometric relationship above to simulate the flight trajectory error of the platform during actual flight. Increase the altitude by 1km and reduce the ground-grazing angle by 6°, and get a geometric relationship with errors of 16km altitude, 32° ground-grazing angle, and 27° azimuth.

[0197] When the initial PRF in the waveform library is selected, the zebra pattern is as follows: Figure 7 As shown, the transmit pulse shielding area and the height line shielding area are determined. If the initial PRF is selected under this geometric condition, both the transmit pulse shielding and the height line shielding will exist, so the radar cannot be imaged.

[0198] The waveform parameters are adjusted by the waveform online adaptive adjustment method proposed in the present invention, and the new waveform parameters are shown in Table 4. It can be seen that the pulse repetition frequency PRF is adjusted from the initial 9.5kHz to 8.7kHz, and the accumulation time is also adjusted accordingly, while the signal pulse width and signal bandwidth remain unchanged. The corresponding zebra diagram is drawn using the adjusted PRF, as shown in Table 4. Figure 8It can be seen that under this geometric relationship, there is no transmission pulse shielding and height line shielding, so the radar can perform synthetic aperture radar imaging.

[0199] Table 4 Online adaptive waveform design results

[0200]

[0201] The imaging indexes are calculated by using the above-mentioned adaptively adjusted waveform parameters, as shown in Table 5. It can be seen that after the adaptive waveform design, except for the deterioration of the noise equivalent backscatter coefficient, the other synthetic aperture radar imaging indexes remain basically unchanged. The reason for the deterioration of the noise equivalent backscatter coefficient is that the geometric relationship has changed significantly, resulting in a significant increase in the radar range, so the target echo energy is reduced.

[0202] Table 5 Imaging indicators after adaptive waveform adjustment

[0203]

[0204] The above specific embodiments only describe the design principle of the present invention. The shapes and names of the components in the description may be different and are not limited. Therefore, those skilled in the art in the field of the present invention may modify or replace the technical solutions recorded in the above embodiments; and these modifications and replacements do not deviate from the creative purpose and technical solutions of the present invention and should all fall within the protection scope of the present invention.

Claims

1. A method for adjusting online adaptive waveform parameters of synthetic aperture radar imaging, characterized in that: include: Step S1: Based on the flight trajectory of the flight platform, an initial value of the azimuth angle, an initial value of the ground-rubbing angle, and an initial value of the waveform parameter are obtained, wherein the azimuth angle is the angle between the radar line of sight direction and the velocity direction projected on the horizontal plane, and the ground-rubbing angle is the angle between the radar line of sight direction and the horizontal plane; Step S2: determining relevant data of synthetic aperture radar imaging, wherein the relevant data includes azimuth spectrum, transmission pulse shielding area, altitude line shielding area, azimuth resolution in ground distance plane, range resolution, value of off-axis angle, noise equivalent backscatter coefficient, synthetic aperture radar imaging width and geometric distortion index; Step S3: determine whether the relevant data meets the preset synthetic aperture radar imaging index; if so, build an offline waveform library based on the current waveform parameters and proceed to step S4; otherwise, update the azimuth angle, ground-grazing angle and the waveform parameters and proceed to step S2; Step S4: Based on the offline waveform library and in combination with the flight trajectory of the flight platform, adjusting the waveform parameters online; The step S2, determining the relevant data of synthetic aperture radar imaging, includes: Step S21: Determine whether the azimuth spectrum is aliased; Step S22: determining a transmit pulse shielding area and a height line shielding area, and judging whether there is transmit pulse shielding and / or height line shielding; Step S23: Determine the azimuth resolution ρ in the ground distance plane ga And the range resolution ρ gr ; Step S24: determining a value of an off-axis angle, where the off-axis angle refers to an angle at which the beam deviates from the antenna normal; Step S25: determining the noise equivalent backscatter coefficient; Step S26: Determine the synthetic aperture radar imaging width; Step S27: determining geometric distortion indicators, wherein the geometric distortion indicators include output image rotation angle error, range geometric distortion error, azimuth geometric distortion error, image center azimuth error, image center elevation error, and the image is an imaging result image output by a synthetic aperture radar; The step S22: determining the transmission pulse shielding area and the height line shielding area, and judging whether there is transmission pulse shielding and / or height line shielding, comprises: The calculation method of the transmit pulse shielding area is: Calculate the starting sampling distance R of the radar echo signal min and the maximum sampling distance R max : R min =R-Nrnew / 2×c / 2 / f s R max =R min +Nr×c / 2 / f s Among them, R is the effective distance of the center point of the image, Nrnew is the number of imaging processing points, Nr is the number of acquisition points in the distance direction, c is the speed of light, and f s is the signal sampling rate; The transmit pulse shielding area is calculated by the following formula: Among them, T b is the guard band time, which is a fixed value, n is the transmit pulse number, PRT is the pulse repetition period, T p is the pulse width of the transmitting signal, R T is the distance where pulse obstruction exists; The calculation method of the height line occlusion area is: Among them, H is the distance of the height line echo, H e is the height error of the flight platform, which is used as a protection against the height line occlusion. H is the distance where there is height line occlusion.

2. The method according to claim 1, characterized in that: The step S21, determining whether the azimuth spectrum is aliased, comprises: When the pulse repetition frequency PRF is less than the Doppler bandwidth, spectrum aliasing will occur, that is, the pulse repetition frequency PRF is less than the instantaneous Doppler bandwidth B in the azimuth direction. d Then spectrum aliasing will occur, that is, PRF<PRF min =B d , PRF min is the minimum value of PRF.

3. The method according to claim 1, characterized in that Step S23: Determine the azimuth resolution ρ in the ground distance plane ga And the range resolution ρ gr ,include: The slant range plane is the plane where the radar velocity direction and the line of sight are located. The range resolution in the slant range plane is ρ r , azimuth resolution ρ a The calculation formula is as follows: Where c is the speed of light, B is the signal bandwidth, λ is the wavelength; R is the distance from the center of the image, V m is the velocity of the flight platform, T a is the accumulation time; θ is the front tilt angle, which is calculated by the ground-rubbing angle β, the azimuth angle α and the velocity dive angle γ. The image is the imaging result image output by the synthetic aperture radar; According to the oblique projection relationship, the azimuth resolution ρ in the ground distance plane is obtained ga and range resolution ρ gr The calculation formula is as follows: Among them, k wa , k wr are the window function broadening factors in azimuth and range, respectively, and k ia and k ir are the azimuth and range projection coefficients respectively.

4. The method according to claim 1, characterized in that The step S24: determining the value of the off-axis angle, where the off-axis angle refers to the angle at which the beam deviates from the antenna normal, includes: Off-axis angle θ LZ Calculated by the following formula: Among them, vector is the normal vector of the antenna array in the antenna system, and the vector is the beam pointing vector in the antenna system, which is the beam pointing vector in the north sky east coordinate system After coordinate transformation to the antenna system, the coordinate transformation matrix is ​​obtained from the antenna installation angle and the flight platform attitude angle.

5. The method according to claim 1, characterized in that The step S25: determining the noise equivalent backscatter coefficient comprises: The noise equivalent backscatter coefficient NEσ0 is calculated as follows: Among them, R is the effective distance of the center point of the image, and the duty cycle D is determined by the pulse width T of the transmitted signal. p And the pulse repetition frequency PRF is calculated, T a is the accumulation time, k is the Boltzmann constant, T0 is the standard noise temperature, F n is the noise factor, P t is the peak power, G is the antenna normal gain, λ is the wavelength, and M is the number of views, all of which are constants; ρ gr is the range resolution in the ground plane, ρ ga is the azimuth resolution in the ground-range plane; Overall loss value L ∑a =Ls×Le×Ld, where Ls is the radar system loss, Le is the rain attenuation, and Ld is the atmospheric attenuation.

6. The method according to claim 1, characterized in that The step S26: determining the synthetic aperture radar imaging width comprises: The width includes the azimuth imaging width and the elevation imaging width, wherein: The azimuth imaging width is: IN RX =R×θ A The imaging width in the pitch direction is: Among them, θ A is the azimuth beamwidth, θ E is the beam width in elevation, β is the ground-grazing angle, and R is the effective distance of the image center point.

7. The method according to any one of claims 2 to 6, characterized in that The step S4 comprises: Based on the geometric relationship determined by the flight trajectory of the flight platform, the corresponding waveform parameters in the offline waveform library are obtained, and the transmission pulse occlusion and altitude line pulse occlusion results corresponding to the corresponding waveform parameters are determined. The pulse repetition frequency PRF in the waveform parameters is adjusted according to the occlusion results. After adjustment, the pulse repetition frequency PRF without transmission pulse occlusion and altitude line pulse occlusion is obtained to perform synthetic aperture imaging.

Images