A method for calculating parameters of high-resolution SAR imaging

Abstract

The present invention proposes a method for calculating high-resolution SAR imaging parameters. The method takes parameters such as imaging scene position, imaging synthetic aperture duration, sliding factor, minimum repetition rate, maximum repetition rate and the like as input conditions, and respectively calculates the zero Doppler center moment of the imaging scene and the imaging start and end times through a satellite-ground geometric model, and selects and outputs a repetition rate that meets SAR imaging performance indicators within an imaging arc segment with a fixed time step, thereby reducing the number of calculation overheads and avoiding a return iterative search process, saving storage resources, being suitable for implementation in a satellite-borne environment under resource-constrained conditions, and being convenient for deployment and implementation of satellite-borne hardware. The present invention can realize sliding beam imaging modes with different imaging resolutions and imaging widths by modifying parameters such as imaging synthetic aperture duration, sliding factor, minimum repetition rate, maximum repetition rate and the like, and integrates performance indicator calculation, having the characteristics of integrated wave position design.

Description

Technical Field

[0001] The invention belongs to the technical field of radar system design, and in particular relates to a method for calculating resolution SAR imaging parameters, which is suitable for wave position design in a SAR satellite sliding beamforming mode for realizing high-resolution imaging through whole-satellite attitude maneuvers. Background Art

[0002] In order to achieve high-resolution imaging while ensuring a certain azimuth mapping width, high-resolution SAR satellites usually use a sliding beam mode, which controls the azimuth rotation angle of the antenna beam, slows down the sliding speed of the beam footprint on the ground, and increases the synthetic aperture time of the imaging point to achieve high-resolution imaging. During the satellite rotation, due to the large echo distance migration, the single repetition rate used in traditional spaceborne SAR to achieve imaging will no longer be effective in the wave position design. Variable repetition rate design must be performed throughout the imaging process to ensure effective reception of echoes by moving the echo window. Figure 1 The working diagram of sliding beamforming is given. At present, the sliding beamforming mode has been successfully applied to the German TerraSAR-X satellite and can achieve 0.25m resolution imaging in azimuth.

[0003] The existing wave position design of the sliding beam mode of spaceborne SAR is based on a high-precision satellite-ground geometric model, which is given by ground computers through repeated iterative simulation. For example, Xi'an Institute of Space Radio Technology 201910549798.8 discloses a segmented variable repetition frequency timing design method for ultra-high resolution spaceborne SAR. This method uses a fixed slant range span to segment the azimuth, designs the azimuth echo receiving window and the working repetition frequency, and verifies the variable repetition frequency design results by simulation. If the conditions are not met, the slant range span needs to be readjusted until the echo of the entire scene can be effectively received. However, this method has the problems of a large number of segments, resulting in a large number of repetition frequency calculations and a large amount of calculations. At the same time, in the process of repeated iterations, the intermediate calculation results need to be stored, which requires high storage resources. It is not suitable for implementation in a spaceborne environment under resource-constrained conditions. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a high-resolution SAR imaging parameter calculation method, which reduces the calculation time overhead, avoids the return iterative search process, saves storage resources, and is convenient for deployment and implementation of space-borne hardware.

[0005] A high-resolution SAR imaging parameter calculation method, comprising:

[0006] Step (1), receiving the image scene position through the satellite-to-ground link Calculation parameters of imaging synthetic aperture time T, sliding factor A, minimum repetition rate and maximum repetition rate;

[0007] Step (2): extrapolate the satellite orbital position at a set time step to obtain the satellite position at each time. and speed

[0008] Step (3), according to the imaging scene position and the extrapolated satellite positions and speed Calculate the satellite Doppler frequency at each time And select the moment when the Doppler frequency sign changes as the zero Doppler moment of the imaging scene

[0009] Step (4), take the zero Doppler moment of the scene As the center, according to the total imaging time T of the sliding beam mode, the imaging start T is calculated. s and end time T e ;

[0010] Step (5), according to the sliding factor of the sliding beam mode and the imaging target position Calculate the coordinates of the virtual center point of the sliding bunching mode

[0011] Step (6): the imaging start time and end time obtained in step (4) and the virtual center point coordinates obtained in step (5) At the set time step, from the start time to the end time, the antenna's near-point slant range and far-point slant range in the range direction are obtained according to the satellite position and antenna beam width;

[0012] Step (7), according to the minimum repetition rate and the maximum repetition rate in the imaging mode, select the PRF that meets the constraint conditions from small to large in sequence with the set time step as the system repetition rate parameter;

[0013] Step (8), for the system repetition frequency parameters outputted in step (7), firstly screen the SAR performance index for the first system repetition frequency parameter, if it does not meet the conditions, then return to step (7), obtain the second system repetition frequency parameter, and determine whether it meets the SAR performance index; and so on, until the first system repetition frequency parameter that meets the SAR performance index is found.

[0014] Furthermore, in step (7), the constraint condition for PRF selection is:

[0015]

[0016] In the formula, int() is a rounding function used to extract the integer part of the data; frac() is used to extract the decimal part; Rmin and R max are the slant distances of the near and far points respectively; T p is the width of the echo, T g It is the set protection time, which is determined by the SAR system design.

[0017] Preferably, in step (8), the PRF is selected based on the azimuth ambiguity and range ambiguity indicators.

[0018] Preferably, in step (8), the calculation formula for azimuth ambiguity screening is:

[0019]

[0020] Where B is the azimuth Doppler bandwidth, which is determined by the SAR payload design; f is the Doppler frequency, f de is the equivalent Doppler center frequency estimation deviation, m is the azimuth ambiguity zone number, which is given by the antenna pattern; is the two-way gain function of the antenna azimuth, expressed as:

[0021]

[0022] Where V is the satellite-to-ground equivalent velocity, D a is the antenna size in azimuth.

[0023] Preferably, in step (8), the calculation formula for distance ambiguity screening is:

[0024]

[0025] Where DWP represents the echo delay time, τ w is the echo data window width, n is the ambiguity zone number, which is given by the antenna pattern; τ is the echo delay time, and R(τ) is the slant range, which is the slant range R of the close point obtained in step (6) min and the slope distance R of the distant point max , σ 0 (τ) is the backscatter coefficient of the ground target; θ i (τ) represents the incident angle; G(τ) is the two-way gain function of the range antenna, expressed as:

[0026]

[0027] Where λ is the operating wavelength, D r is the antenna size in the range direction, and α(τ) is the viewing angle corresponding to R(τ).

[0028] Preferably, in step (3), the Doppler frequency The calculation formula is as follows:

[0029]

[0030] is the imaging scene velocity vector in the geocentric inertial coordinate system, λ is the carrier wavelength, R st is the distance between the satellite and the target located at the center of the imaging scene.

[0031] Preferably, the set time is 1 s.

[0032] Preferably, in step (6), the slant distance between the satellite position and the virtual center point coordinates at each moment is estimated, specifically:

[0033] Known satellite positions:

[0034]

[0035] According to the antenna beam width, the nearest and farthest beam pointing vectors of the antenna in the distance direction are:

[0036]

[0037] Construct the equation of a line in space:

[0038]

[0039] Among them, a is the equatorial radius of the earth, c is the polar radius of the earth, l, m, n are the direction numbers, and the specific expression is:

[0040]

[0041] When X, Y, Z are When , the coordinate position (x, y, z) of the nearest beam in the ground-fixed coordinate system can be obtained by solving the spatial straight line equation, which is recorded as (x1, y1, z1). When the coordinate position (x, y, z) of the far beam in the ground-fixed coordinate system can be obtained by solving the spatial straight line equation, it is recorded as (x2, y2, z2), and then compared with the satellite coordinates in the satellite ground-fixed coordinate system Combined, we get the slope distance R of the near point min and the slope distance R of the distant point max They are:

[0042]

[0043]

[0044] The present invention has the following beneficial effects:

[0045] The present invention proposes a method for calculating high-resolution SAR imaging parameters. The method takes parameters such as imaging scene position, imaging synthetic aperture duration, sliding factor, minimum repetition rate, maximum repetition rate and the like as input conditions, and respectively calculates the zero Doppler center moment of the imaging scene and the imaging start and end times through a satellite-ground geometric model, and selects a repetition rate that meets SAR imaging performance indicators as an imaging parameter within an imaging arc with a fixed time step and outputs the repetition rate, which reduces the calculation times overhead, avoids the search process of returning iteration, saves storage resources, is suitable for implementation in a satellite environment under resource-constrained conditions, and is convenient for deployment and implementation of satellite hardware.

[0046] The present invention can realize sliding beam imaging modes with different imaging resolutions and imaging widths by modifying parameters such as imaging synthetic aperture time, sliding factor, minimum repetition rate, and maximum repetition rate; and integrates performance index calculation, with the characteristics of integrated wave position design. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is a schematic diagram of the method flow of the present invention;

[0048] Figure 2 It is the SAR satellite sliding beam mode imaging model;

[0049] Figure 3 is the slope distance variation range;

[0050] Figure 4 Select the result for variable repetition frequency parameter;

[0051] Figure 5 The target distribution for the sliding point;

[0052] Figure 6 This is the result of sliding point imaging. DETAILED DESCRIPTION

[0053] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0054] In order to achieve high-resolution imaging, existing SAR satellites that achieve high-resolution imaging through whole-satellite attitude maneuvers usually adopt a sliding beam imaging mode. In this mode, due to the large variation in the scene slant range span, the echo window determined by a single repetition rate (PRF) can no longer completely receive the scene echo, so it is necessary to adopt a variable repetition rate method to form receiving windows at different positions to ensure the effective reception of the echo. The present invention is based on the sliding beam working characteristics adopted by the satellite, and designs a high-resolution SAR imaging parameter on-orbit calculation method, which mainly solves the problem of adaptive adjustment of the system repetition rate due to slant range changes, and has the characteristics of low computing and storage resource overhead, and is convenient for deployment and implementation of on-board hardware.

[0055] The technical solution of the present invention is as follows:

[0056] A high-resolution SAR imaging parameter on-orbit calculation method mainly includes the following steps:

[0057] Step 1: Receive the imaging scene position via the satellite-to-ground link Calculation parameters such as imaging synthetic aperture time T, sliding factor A, minimum repetition rate, maximum repetition rate, etc.

[0058] Step 2: Extrapolate the satellite orbital position with a fixed time step (such as 1 second) to obtain the satellite position at each time and speed

[0059] Step 3: According to the imaging scene position and the extrapolated satellite positions and speed Calculate the satellite Doppler frequency f at each moment dc , and select the moment when the Doppler frequency symbol changes to determine the zero Doppler moment of the imaging scene The Doppler frequency calculation formula is as follows:

[0060]

[0061] In the formula is the satellite position vector in the geocentric inertial coordinate system, is the imaging scene position vector in the geocentric inertial coordinate system, is the velocity vector of the satellite in the geocentric inertial coordinate system, is the imaging scene velocity vector in the geocentric inertial coordinate system, λ is the carrier wavelength, R st is the distance between the satellite and the target located at the center of the imaging scene.

[0062] Step 4: Take the zero Doppler moment of the scene As the center, according to the total imaging time T of the sliding beam mode, the imaging start T is calculated. s and end time T e ,satisfy:

[0063]

[0064] Step 5: According to the sliding factor of the sliding beam mode and the imaging target position Calculate the coordinates of the virtual center point of the sliding bunching mode

[0065]

[0066] Where A is the sliding factor, which is determined by the above parameters. is the imaging target position vector in the geocentric inertial coordinate system, is the satellite position vector at zero Doppler time.

[0067] Step 6: According to the imaging start time, end time and virtual center point coordinates obtained in steps 3 and 4, calculate the near-point slant range and far-point slant range of the antenna in the range direction from the start time to the end time with a fixed time step (such as 1 second) according to the satellite position and antenna beam width. The calculation method is as follows:

[0068] Known satellite positions:

[0069]

[0070] According to the antenna beam width, the nearest and farthest beam pointing vectors of the antenna in the distance direction are:

[0071]

[0072] Construct the equation of a line in space:

[0073]

[0074] Among them, a = 6378140m is the equatorial radius of the earth, c = 6356755m is the polar radius of the earth, l, m, n are the direction numbers, and the specific expression is:

[0075]

[0076] When X, Y, Z are When , the coordinate position of the nearest beam in the ground-fixed coordinate system can be obtained by solving the spatial straight line equation, which is recorded as (x1, y1, z1). When the coordinate position of the far beam in the ground-fixed coordinate system can be obtained by solving the spatial straight line equation, recorded as (x2, y2, z2), and then compared with the satellite coordinates in the satellite ground-fixed coordinate system Combined, we get the slope distance R of the near point min and the slope distance R of the distant point max They are:

[0077]

[0078]

[0079] Step 7: According to the minimum repetition rate and the maximum repetition rate in the imaging mode, select the PRF that meets the constraint conditions from small to large as the system repetition rate parameter with a fixed step size (such as 1). The constraint conditions for PRF selection are:

[0080]

[0081] In the formula, C represents the speed of light, int() is a rounding function used to extract the integer part of the data; frac() is used to extract the decimal part; R min and R max is the slant distance of the near point and the slant distance of the far point, given by step 5; T p is the width of the echo, T g It is the set protection time, which is determined by the SAR system design.

[0082] Step 8: Perform SAR performance index screening on each repetition frequency parameter output in step 7. If it does not meet the conditions, repeat step 7. Specifically:

[0083] (1) Azimuth ambiguity screening, the calculation method is:

[0084]

[0085] In the formula, f prf represents the frequency corresponding to the wave position, which is the inverse of PRF; B is the azimuth Doppler bandwidth, which is determined by the SAR payload design; f is the Doppler frequency, which is calculated in step 3, f de is the equivalent Doppler center frequency estimation deviation, m is the azimuth ambiguity zone number, which is given by the antenna pattern; is the two-way gain function of the antenna azimuth, expressed as:

[0086]

[0087] Where V is the satellite-to-ground equivalent velocity, D a is the antenna size in azimuth;

[0088] (2) Distance ambiguity screening, the calculation method is:

[0089]

[0090] Where DWP represents the echo delay time, τ w is the echo data window width, n is the ambiguity zone number, which is given by the antenna pattern; τ is the echo delay time, and R(τ) is the slant range, which is the slant range R of the close point obtained in step (6). min and the slope distance R of the distant point max , σ 0 (τ) is the backscatter coefficient of the ground target; θ i (τ) represents the incident angle; G(τ) is the two-way gain function of the range antenna, expressed as:

[0091]

[0092] In the formula, α n and α fRepresent the near-end viewing angle and the far-end viewing angle respectively; λ is the working wavelength, D r is the antenna size in the range direction, and α(τ) is the viewing angle corresponding to R(τ).

[0093] Example:

[0094] This section designs a set of high-resolution SAR imaging parameters according to the given input system requirement parameters and the method proposed in the present invention, and verifies the effectiveness of the method of the present invention by analyzing the obtained system performance.

[0095] According to the method proposed in the present invention, the imaging parameters are calculated and the system input parameters are shown in the following table.

[0096] Appendix 1 Input parameter table in the embodiment

[0097]

[0098]

[0099] According to the satellite orbit parameters given in this paper, the zero Doppler time calculated by step 3 is 56 seconds. According to the synthetic aperture time input parameters and the calculation method of step 4, the start time is 48 seconds and the end time is 64 seconds.

[0100] The slope distances of the near and far points calculated according to steps 5-6 are as follows: Figure 3 As shown, according to steps 7-8, the variable repetition frequency selection result is obtained as follows Figure 4 shown.

[0101] Import the variable repetition frequency parameter information generated above into the echo simulation software, and set the simulation scene point target as follows Figure 5 As shown, the width of the target scene is about 25km×25km, and each target can be completely covered by the beam, ensuring the accumulation time of each point target.

[0102] Imaging uses Deramping + range NCS to achieve sliding beam imaging. The imaging algorithm includes azimuth deramping preprocessing and range NCS algorithm processing. At the same time, according to the characteristics of variable repetition frequency, a high-resolution variable repetition frequency imaging processing algorithm is designed. The main process of the algorithm is to interpolate the variable repetition frequency signal into a uniform signal, then perform azimuth signal upsampling, then use the NCS algorithm to perform range compression and range migration correction, and finally perform azimuth compression.

[0103] The final imaging result is shown in the figure below. It can be seen that the point target is well focused. The imaging resolution, integrated sidelobe ratio, and peak sidelobe ratio meet the requirements.

[0104] Appendix 2 Point target imaging result indicators

[0105]

[0106] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-resolution SAR imaging parameter calculation method, characterized in that: include: Step (1), receiving the image scene position through the satellite-to-ground link Calculation parameters of imaging synthetic aperture time T, sliding factor A, minimum repetition rate and maximum repetition rate; Step (2): extrapolate the satellite orbital position at a set time step to obtain the satellite position at each time. and speed Step (3), according to the imaging scene position and the extrapolated satellite positions and speed Calculate the satellite Doppler frequency at each time And select the moment when the Doppler frequency sign changes as the zero Doppler moment of the imaging scene Step (4), take the zero Doppler moment of the scene As the center, according to the total imaging time T of the sliding beam mode, the imaging start T is calculated. s and end time T e ; Step (5), according to the sliding factor of the sliding beam mode and the imaging target position Calculate the coordinates of the virtual center point of the sliding bunching mode Step (6): the imaging start time and end time obtained in step (4) and the virtual center point coordinates obtained in step (5) At the set time step, from the start time to the end time, the antenna's near-point slant range and far-point slant range in the range direction are obtained according to the satellite position and antenna beam width; Step (7), according to the minimum repetition rate and the maximum repetition rate in the imaging mode, select the PRF that meets the constraint conditions from small to large in sequence with the set time step as the system repetition rate parameter; Step (8), for the system repetition frequency parameter outputted in step (7), firstly screen the SAR performance index for the first system repetition frequency parameter, if it does not meet the condition, then return to step (7), obtain the second system repetition frequency parameter, and determine whether it meets the SAR performance index; and so on, until the first system repetition frequency parameter that meets the SAR performance index is found; In step (7), the constraints for PRF selection are: In the formula, C represents the speed of light; int() is a rounding function used to extract the integer part of the data; frac() is used to extract the decimal part; R min and R max are the slant distances of the near and far points respectively; T p is the width of the echo, T g It is the set protection time, which is determined by the SAR system design; In step (6), the slant distance between the satellite position and the virtual center point coordinates at each time is estimated, specifically: Known satellite positions: According to the antenna beam width, the nearest and farthest beam pointing vectors of the antenna in the distance direction are: and Construct the equation of a line in space: Among them, a is the equatorial radius of the earth, c is the polar radius of the earth, l, m, n are the direction numbers, and the specific expression is: When X, Y, Z are When , the coordinate position (x, y, z) of the nearest beam in the ground-fixed coordinate system can be obtained by solving the spatial straight line equation, which is recorded as (x1, y1, z1). When the coordinate position (x, y, z) of the far beam in the ground-fixed coordinate system can be obtained by solving the spatial straight line equation, it is recorded as (x2, y2, z2), and then compared with the satellite coordinates in the satellite ground-fixed coordinate system Combined, we get the slope distance R of the near point min and the slope distance R of the distant point max They are:

2. A high-resolution SAR imaging parameter calculation method as claimed in claim 1, characterized in that: In the step (8), the PRF is selected according to the azimuth ambiguity and range ambiguity indicators.

3. A high-resolution SAR imaging parameter calculation method as claimed in claim 2, characterized in that: In step (8), the calculation formula for azimuth ambiguity screening is: In the formula, f prf It represents the frequency corresponding to the wave position, which is the inverse of PRF; B is the azimuth Doppler bandwidth, which is determined by the SAR payload design; f is the Doppler frequency, f de is the equivalent Doppler center frequency estimation deviation, m is the azimuth ambiguity zone number, which is given by the antenna pattern; is the two-way gain function of the antenna azimuth, expressed as: Where V is the satellite-to-ground equivalent velocity, D a is the antenna size in azimuth.

4. A high-resolution SAR imaging parameter calculation method as claimed in claim 2, characterized in that: In step (8), the calculation formula for distance ambiguity screening is: Where DWP represents the echo delay time, τ w is the echo data window width, n is the ambiguity zone number, which is given by the antenna pattern; τ is the echo delay time, and R(τ) is the slant range, which is the slant range R of the close point obtained in step (6) min and the slope distance R of the distant point max , σ 0 (τ) is the backscatter coefficient of the ground target; θ i (τ) represents the incident angle; G(τ) is the two-way gain function of the range antenna, expressed as: In the formula, α n and α f They represent the near and far perspectives respectively; λ is the operating wavelength, D r is the antenna size in the range direction, and α(τ) is the viewing angle corresponding to R(τ).

5. A high-resolution SAR imaging parameter calculation method as claimed in claim 1, characterized in that: In step (3), the Doppler frequency The calculation formula is as follows: is the imaging scene velocity vector in the geocentric inertial coordinate system, λ is the carrier wavelength, R st is the distance between the satellite and the target located at the center of the imaging scene.

6. A high-resolution SAR imaging parameter calculation method as claimed in claim 1, characterized in that: The setting time is 1 s.

Images