Parabolic body system space-borne SAR non-azimuth multi-sub-band splicing wide swath imaging method

Abstract

The application discloses a parabolic body system space-borne SAR non-along-track multi-sub-band splicing wide-width imaging method. The application designs an observation band configuration, divides a target scene along a distance direction into multiple sub-bands, connects adjacent sub-bands in a head-to-tail mode, and then plans a wave foot track in an observation process according to target scene observation requirements, a beam scanning range and a beam maneuvering capability limitation, so that the beam starts scanning at an edge of a non-along-track target scene, when the wave foot moves to an end of the scene in the azimuth direction, the antenna attitude is gradually adjusted under the constraint of the beam maneuvering capability to make the wave foot turn, the wave foot moves from the end of the scene in the azimuth direction to the beginning of the scene in the azimuth direction, and the process is repeated until the wave foot track completely covers the target scene, and imaging under a single-track multi-sub-band splicing mode is realized. The application solves the problem that the parabolic body system space-borne SAR has poor beam maneuvering capability and cannot realize wide-width imaging under a single-track time-sharing strategy.

Description

Technical Field

[0001] This invention relates to the fields of synthetic aperture radar and non-track imaging technology, specifically to a parabolic spaceborne SAR non-track multi-subband stitching wide-swath imaging method. Background Technology

[0002] Spaceborne SAR non-track imaging (NAT) mode matches the imaging band with the actual geographical trend of the scene to achieve more flexible and efficient observation. It breaks through the limitation of traditional spaceborne SAR, which can only generate imaging bands along satellite orbits, and is a unique working mode of spaceborne SAR. This mode can fundamentally reduce echo data redundancy and significantly improve the observation efficiency of spaceborne SAR for narrow scenes, especially for certain "non-track scenes" such as seismic zones and coastlines, offering unique advantages. In practical applications, some scenes cannot be observed by a single track due to their large swath width. Therefore, single-track time-sharing or multi-track revisit strategies can be used to achieve wider non-track observations. The multi-track revisit strategy stitches together observations from multiple flights to achieve a wider swath, but its practicality is limited due to long observation time and low efficiency. The single-track time-sharing strategy expands the range imaging band width at the cost of sacrificing azimuth resolution, requiring beam switching between different imaging bands during the observation time. Common single-track time-sharing strategies include TOPS, SCAN, and mosaic imaging modes.

[0003] Phased array spaceborne SAR antennas, with multiple independent elements, possess strong beam maneuverability and can meet the requirements of rapid beam switching through agile electronic scanning. However, parabolic SAR antennas, which control beam scanning by adjusting antenna attitude, have poor maneuverability and cannot meet the beam maneuver requirements of imaging modes such as TOPS, SCAN, and mosaic. Wide-swath imaging under a single-track time-division strategy is also difficult to achieve. Therefore, it is necessary to deeply analyze the impact of satellite-to-ground configuration on beam maneuverability and study the design method of non-track multi-subband stitching wide-swath imaging for parabolic SAR antennas to improve the observation capability of non-track wide-swath scenes. Summary of the Invention

[0004] In view of this, the present invention provides a parabolic satellite SAR non-track multi-subband stitching wide-swath imaging method, which can realize wide-swath imaging under a single-track time-division strategy with the beam maneuverability of parabolic antennas.

[0005] The present invention relates to a parabolic satellite SAR non-track multi-subband stitching wide-swath imaging method. The satellite SAR uses a single-track multi-subband stitching method for imaging. The subband is formed by imaging the target scene longitudinally from the beginning to the end, or from the end to the beginning. Adjacent subbands are connected end to end.

[0006] Preferably, adjacent sub-bands partially overlap along the distance direction, with the overlapping portion being 8% to 15% of the sub-band width.

[0007] The preferred method is to first consider the sub-band distance and width W. r 1. Scene width W, taking an overlap of 10% of the sub-band width as an example, determine the number of sub-bands n:

[0008]

[0009] Where n is an integer;

[0010] Then, the observation band configuration is determined based on the number of sub-bands n, where n = 2, the observation band configuration is U-shaped; n = 3, the observation band configuration is S-shaped; n = 4, the observation band configuration is M-shaped; and so on.

[0011] Once the observation zone configuration is determined, target points are set based on this configuration as the trajectory. The spacing between target points is related to the trajectory curvature; the greater the curvature, the denser the distribution of target points.

[0012] Ideally, when the wave foot of the spaceborne SAR moves to the end of the current subband, the antenna attitude is gradually adjusted under the constraint of beam maneuverability, so that the wave foot turns and enters the next subband.

[0013] A better approach is to select a suitable orbital observation arc based on the latitude and longitude coordinates of the input scene center point and the range of viewpoint values ​​under spaceborne SAR.

[0014] Ideally, based on the non-track curve observation configuration, the satellite power-on time T1 and power-off time T2 are calculated; the yaw angle, pitch angle and roll angle of the satellite at each time are calculated; based on the sequence of yaw angle, pitch angle and roll angle, attitude control commands are generated to obtain the attitude control design results of the satellite and parabolic antenna.

[0015] Preferably, a wave foot tracking algorithm is used to generate the wave foot trajectory of the spaceborne SAR; the wave foot tracking algorithm is specifically as follows:

[0016] S201, divide the target scene into multiple contiguous sub-bands; sort the target points set in all sub-bands according to the observation order; input the sequence of target points to be observed, satellite orbit, and radar power-on time;

[0017] S202, let P be the position of the lower wave foot of the Earth-solid system during the i-th step of tracking. foot.f (i) The wave foot velocity is V foot.f (i) , wave foot P foot.f (i) To the current tracking target point P T The direction vector of (j) is v ij ·2 is the L2 norm operator, in V foot.f (i) and vij n direction vectors v are evenly distributed within the included angle. i ′ j (n) is called the wave foot optional direction; calculate the position P under each optional direction. ij ′(n), velocity V ij ′(n), acceleration a i ′ j (n), as shown in equation (1), where dt is the time derivative;

[0018]

[0019] During the first line tracking, the wave foot origin P foot.f (1) The location of the first observation target point, wave foot velocity V foot.f (1) Based on the given azimuth resolution, satellite velocity, beam intercept along the azimuth direction, and other parameters, the tracking target point P is calculated according to equations (2) and (3). T (1) is the location of the second target point;

[0020] S203, Calculate the azimuth resolution ρ for each selectable direction. a (n), the specific steps are as follows:

[0021] The azimuth resolution of spaceborne SAR is ρ a The expression is shown in equation (2), where V Sat.f For satellite velocity in the Earth-fixed system, B a To accumulate Doppler bandwidth, R E Where is the Earth's radius, and H is the orbital altitude. B a The analytical expression is also given in equation (3), where R(t)″ is the second derivative of the slant range path, λ is the wavelength, and V foot.f For the wave foot velocity in the Earth-solid system, l res It is the intercept along the foot of the wave in the half-power projection ellipse on the ground.

[0022]

[0023]

[0024] To obtain the Doppler bandwidth B a The analytical expression for l is given by equations (4) to (8). res The solution method, k in equations (4) and (5) r and k aLet β be the intersection of the range and azimuth profiles of the antenna beam in the Earth's inertial frame with the Earth's tangent plane, γ be the downward angle, and γ be the projection of the oblique angle onto the Earth's tangent plane at the nadir point. In the satellite orbital coordinate system, the X-axis is the direction of the satellite's velocity; the Z-axis vector lies in the satellite orbital plane and points towards the Earth's center; the Y-axis is solved using the right-hand rule, and η is the angle formed by the Y-axis and the intersection of the antenna range plane and the XOZ plane. ec For the geocentric angle, β e ′ c Let β be the geocentric angle that includes left and right view information, when looking to the right. e ′ c =β ec When looking left, β e ′ c =-β ec H X (θ) represents rotating the coordinate axis by θ degrees along the positive direction of the right-hand rule, with the positive X-axis as the axis. Y (θ) and H Z The definition of (θ) is similar. Equations (6) and (7) give the projection ellipse along k r and k a The lengths of the two axes are r and a, where θ e For k r and k a The included angle, σ r and σ r For L3 and k r k a The included angles are respectively, and L3 is the position vector of the satellite to the ground wave foot in the Earth inertial frame. These represent the beam distance and azimuth beamwidth, respectively. In equation (8), ω represents the ground wave velocity V in the ground inertial frame. foot With k a The included angle. Combining equations (4) to (8), we can obtain l. res The analytical expression for is shown in equation (9).

[0025]

[0026]

[0027]

[0028]

[0029]

[0030]

[0031] Equation (10) gives the analytical expression of R(t)″, where a foot.f Vfoot.f Let dL3 and dL4 represent the velocity and acceleration of the wave foot in the Earth-solid system, respectively. 2 L3 is the first and second derivative of L3 with respect to time.

[0032]

[0033] Then, the velocity in the selectable direction of the wave foot is updated using equation (11), and the process jumps to step S202 until the calculated azimuth resolution matches the desired resolution ρ. a0 The residuals between them are small enough;

[0034]

[0035] S204, based on yaw ψ, pitch θ, and roll angle Analyze its higher-order differential analytical expression, calculate the maneuverability in each feasible direction, and select the direction that satisfies the constraints of the spaceborne SAR platform and is closest to v. ij Feasible direction number n i ; then, n i The position and velocity are used as the wave foot position P in the (i+1)th step. foot.f (i+1) Wave foot velocity V foot.f (i+1) The acceleration is the wave foot acceleration a in the i-th step. foot.f (i+1) ;

[0036] S205, after each iteration, it is necessary to determine whether the tracking target needs to be switched. The criterion is shown in equation (12), where R set The set length threshold has a value range within a distance width; v foot.f (i) Let be the velocity direction vector of the i-th wave foot.

[0037]

[0038] Repeat steps S202 to S205 until the entire target point sequence has been traversed, and output the wave foot trajectory.

[0039] Preferably, in S204, n i The method of obtaining it is:

[0040]

[0041]

[0042]

[0043]

[0044] Where F(·) is the attitude angle range constraint, G(·) is the attitude angular velocity constraint, H(·) is the attitude angular acceleration constraint, and (●) is the attitude angular acceleration constraint. H Let be the conjugate transpose of the matrix, and ζ be the weighting factor; in equation (14), a < 1, b < 0.5, L foot L represents the distance between the wave foot and the previous tracking target. T The distance between the two tracking targets is denoted as .

[0045] Beneficial effects:

[0046] (1) This invention designs the observation band configuration, dividing the target scene into multiple sub-bands along the range direction, with adjacent sub-bands connected end-to-end. Then, based on the target scene observation requirements, beam scanning range, and beam maneuverability limitations, the beam foot trajectory is planned during the observation process, so that the beam begins scanning at the edge of the non-track target scene. When the beam foot moves to the end of the scene's azimuth direction, the antenna attitude is gradually adjusted to turn the beam foot, moving it from the end of the scene's azimuth direction to the beginning of the scene's azimuth direction, repeating this process until the beam foot trajectory completely covers the target scene, achieving imaging under a single-track multi-sub-band stitching method. This invention solves the problem that the beam maneuverability of parabolic spaceborne SAR is poor, making it impossible to achieve wide-swath imaging under a single-track time-division strategy.

[0047] (2) Adjacent sub-bands partially overlap along the distance direction, which can ensure that there is no blind spot in the distance direction and achieve complete imaging of the target area.

[0048] (3) The wave foot tracking algorithm used in this invention can solve the problems of difficulty in designing non-tracking wave feet and large deviation of wave foot trajectory under the beam maneuverability constraint of parabolic antenna, and can control the orientation resolution in real time.

[0049] (4) This invention makes full use of the limited beam maneuverability of the parabolic spaceborne SAR to break through the existing technology. Attached Figure Description

[0050] Figure 1 This is a flowchart of the non-track single-track multi-imaging-strip stitching imaging process of the spaceborne SAR satellite of this invention.

[0051] Figure 2 This is a schematic diagram of the observation band configuration determined by the number of subbands. (a) Number of subbands n = 2; (b) Number of subbands n = 3; (c) Number of subbands n = 4.

[0052] Figure 3 This is a schematic diagram of the observation configuration for the two sub-bands.

[0053] Figure 4 Flowchart for designing the observation configuration.

[0054] Figure 5 The wave foot planning results and target coverage are shown. Among them, (a) wave foot planning results; (b) wave foot trajectory optical map; (c) azimuth resolution; (d) swath width.

[0055] Figure 6 Let be the beam attitude angle and its first and second derivatives. (a) Beam attitude angle; (b) Beam attitude angular velocity; (c) Beam attitude angular acceleration. Detailed Implementation

[0056] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0057] This invention provides a parabolic spaceborne SAR non-track multi-subband stitching wide-swath imaging method, the flowchart of which is shown below. Figure 1 As shown, it includes the following steps:

[0058] Step 1: Determine the orbital observation arc segment based on the input scene center point, and determine the number of imaging strip stitching times based on the observation scene width to determine the observation strip configuration.

[0059] S11 selects a suitable orbital observation arc based on the latitude and longitude coordinates of the input scene center point and the range of satellite downward angle values.

[0060] S12, based on the radar's single-track observation swath width Wr and the observation scene width W, determine the number of sub-bands n:

[0061]

[0062] Where n is an integer;

[0063] S13. The observation band configuration is determined based on the number of sub-bands, n. Where n = 2, the observation band configuration is U-shaped; n = 3, the observation band configuration is S-shaped; n = 4, the observation band configuration is M-shaped; and so on. For example... Figure 2 , Figure 3 As shown.

[0064] Once the observation zone configuration is determined, target points are set based on this configuration as the trajectory. The spacing between target points is related to the trajectory curvature; the greater the curvature, the denser the distribution of target points.

[0065] In addition, in order to ensure that there are no blind spots in the range direction and to achieve complete imaging of the target area, adjacent sub-bands partially overlap along the range direction, with the overlap portion being 8% to 15% of the sub-band width.

[0066] Step 2: Based on resolution requirements, orbital parameters, and the beam maneuverability of the parabolic antenna, a reference point for the wave foot on the ground is given, the wave foot trajectory is generated, and a non-track observation configuration design is performed to obtain the attitude control design results of the satellite and the parabolic antenna.

[0067] S21. Based on the number of image strip stitchings and the geographical trend of the non-track target scene, the ground reference point for the wave foot is given. The wave foot tracking algorithm for generating the wave foot trajectory can refer to the wave foot tracking algorithm in the Chinese patent application "Joint Design and Optimization Method for Spaceborne SAR Non-track Multi-target Imaging Satellite-Ground Configuration" (Application No.: CN202210692674.7), which is explained in detail below:

[0068] The wave foot tracking algorithm uses the analytical expression of yaw, pitch, roll angles and their higher-order derivatives, which describe the beam attitude, to "grow" the wave foot by iterating point by point on the wave foot trajectory. This results in a wave foot trajectory that satisfies the beam maneuverability constraints of the parabolic antenna and covers as many target points as possible. The algorithm consists of the following six steps:

[0069] a) Input the sequence of target points to be observed, satellite orbit, and radar power-on time t0, and set the first target point as the wave foot starting point P. foot.f (1) The second target is set as the current tracking target point P. T (1) Wave foot velocity V foot.f (1) The parameters such as azimuth resolution, satellite velocity, and beam intercept along the azimuth direction are calculated according to equations (2) and (3);

[0070] b) Let P be the position of the lower wave foot of the Earth-solid system during the i-th step of tracking. foot.f (i) The wave foot velocity is V foot.f (i) , wave foot P foot.f (i) To the current tracking target point P T The direction vector of (j) is v ij In V foot.f (i) and v ij n direction vectors v are evenly distributed within the included angle. i ′ j (n) is called the wave foot optional direction. Calculate the position P under each optional direction. ij ′(n), velocity V ij ′(n), acceleration a i ′ j (n), as shown in equation (1), where dt is the time derivative;

[0071]

[0072] c) Calculate the azimuth resolution ρ for each selectable direction. a (n), the specific steps are as follows:

[0073] The azimuth resolution of spaceborne SAR is ρa The expression is shown in equation (2), where V Sat.f For satellite velocity in the Earth-fixed system, B a To accumulate Doppler bandwidth, R E Where is the Earth's radius, and H is the orbital altitude. B a The analytical expression is also given in equation (3), where R(t)″ is the second derivative of the slant range path, λ is the wavelength, and V foot.f For the wave foot velocity in the Earth-solid system, l res It is the intercept along the foot of the wave in the half-power projection ellipse on the ground.

[0074]

[0075]

[0076] To obtain the Doppler bandwidth B a The analytical expression for l is given by equations (4) to (8). res The solution method, k in equations (4) and (5) r and k a Let β be the intersection of the range and azimuth profiles of the antenna beam in the Earth's inertial frame with the Earth's tangent plane, γ be the downward angle, and γ be the projection of the oblique angle onto the Earth's tangent plane at the nadir point. In the satellite orbital coordinate system, the X-axis is the direction of the satellite's velocity; the Z-axis vector lies in the satellite orbital plane and points towards the Earth's center; the Y-axis is solved using the right-hand rule, and η is the angle formed by the Y-axis and the intersection of the antenna range plane and the XOZ plane. ec For the geocentric angle, β e ′ c Let β be the geocentric angle that includes left and right view information, when looking to the right. e ′ c =β ec When looking left, β e ′ c =-β ec H X (θ) represents rotating the coordinate axis by θ degrees along the positive direction of the right-hand rule, with the positive X-axis as the axis. Y (θ) and H Z The definition of (θ) is similar. Equations (6) and (7) give the projection ellipse along k r and k a The lengths of the two axes are r and a, where θ e For k r and k a The included angle, σ r and σ r For L3 and k r k a The included angles are respectively, and L3 is the position vector of the satellite to the ground wave foot in the Earth inertial frame. These represent the beam distance and azimuth beamwidth, respectively. In equation (8), ω represents the ground wave velocity V in the ground inertial frame. foot With k a The included angle. Combining equations (4) to (8), we can obtain l. res The analytical expression for is shown in equation (9).

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083] Equation (10) gives the analytical expression of R(t)″, where a foot.f V foot.f Let dL3 and dL4 represent the velocity and acceleration of the wave foot in the Earth-solid system, respectively. 2 L3 is the first and second derivative of L3 with respect to time.

[0084]

[0085] Then, the velocity in the selectable direction of the wave foot is updated using equation (11), and the process jumps to step (b) until the calculated azimuth resolution matches the desired resolution ρ. a0 The residuals between them are small enough;

[0086]

[0087] d) Based on yaw ψ, pitch θ, and roll angle Using its analytical expression and higher-order differentials, the maneuverability in each feasible direction is calculated, and the direction that satisfies the platform constraints and is closest to v is selected. ij Feasible direction number n i As shown in equation (12), where F(·) is the attitude angle range constraint, G(·) is the attitude angular velocity constraint, H(·) is the attitude angular acceleration constraint, and ζ is the weighting factor; in equation (13), a < 1, b < 0.5, L foot L represents the distance between the wave foot and the previous tracking target. T Let n be the distance between the two tracking targets. Then, n... i The position and velocity are used as the wave foot position P in the (i+1)th step. foot.f (i+1) Wave foot velocity V foot.f(i+1) The acceleration is the wave foot acceleration a in the i-th step. foot.f (i+1) As in equation (15);

[0088]

[0089]

[0090]

[0091]

[0092] e) After each iteration, it is necessary to determine whether the tracking target needs to be switched. The criterion is shown in equation (16). The tracking target can be switched if any condition of the equation is met, that is, let j = j + 1, where R set The set length threshold, whose value ranges within a distance width, v foot.f (i) Let be the velocity direction vector of the i-th wave foot.

[0093]

[0094] f) Repeat steps (b) to (e) until the entire target point sequence has been traversed and the wave foot trajectory is output.

[0095] The algorithm described above can solve the problems of difficulty in designing non-tracking foot and large deviation of foot trajectory under the beam maneuverability constraint of parabolic antennas, and can control the orientation resolution in real time.

[0096] S22, based on observation resolution requirements, orbital parameters, beam maneuverability of the parabolic antenna, and ground reference points, designs a non-track observation configuration, plans the wave foot motion path and satellite position, and determines the non-track observation configuration. The process is as follows: Figure 4 As shown.

[0097] S23. Based on the non-track curve observation configuration, calculate the satellite's power-on time T1 and power-off time T2. Calculate the satellite's yaw, pitch, and roll angles at each time point. Based on the yaw, pitch, and roll angle sequences, generate attitude control commands to obtain the satellite and parabolic antenna attitude control design results.

[0098] Step 3: Based on the time-varying relationship of beam center slant range under the non-track observation configuration, beam position design is carried out to obtain the system design results of the spaceborne SAR non-track multi-subband stitching mode.

[0099] S31. Determine the azimuth sampling frequency range based on the central slant distance variation history and instantaneous bandwidth of the non-track observation configuration.

[0100] S32, based on the azimuth sampling frequency range and center slant range history, conducts wave position design with the premise of maximizing echo reception, and obtains the system design results of the spaceborne SAR non-track multi-subband stitching mode.

[0101] Step 4: Based on the designed wave foot trajectory, transmit signals and receive data to obtain preliminary imaging results.

[0102] S41, based on the available orbital imaging range obtained in step one, control the satellite to power on at power-on time T1. Based on the satellite attitude control commands obtained in step two, control the satellite's yaw angle, pitch angle, and roll angle in real time to ensure that the satellite beam center points to the expected wave foot trajectory. Simultaneously, based on the system design results obtained in step three, transmit linear frequency modulated signals and receive echo signals until the satellite powers off at power-off time T2.

[0103] S42 performs imaging processing on the received echo signal to obtain preliminary imaging results.

[0104] Step 5: Select and stitch together the portion of the imaging results that falls within the orbital imaging range to obtain the final wide-swath imaging result.

[0105] Simulation Experiment: The simulation parameters for mapping strip planning in the non-track multi-subband stitching mode of spaceborne SAR are shown in Table 1.

[0106] Table 1. List of Simulation Parameters for Spaceborne SAR Non-Tracking Multi-Subband Stitching Wide-Switch Imaging Mapping Sector Planning

[0107]

[0108]

[0109] To verify that spaceborne SAR non-track multi-subband stitching wide-swath imaging can solve the problem that parabolic spaceborne SAR cannot achieve wide-swath imaging under a single-track time-division strategy, under the parameters in Table 1, the spaceborne SAR non-track multi-subband stitching wide-swath imaging method described in this invention is used to plan observation tasks for a set of scene center points.

[0110] The selected scene is the Odessa urban area, with a scene width of approximately 10km, exceeding the single-track observation swath width. Conventional non-track imaging modes cannot completely cover the target within a single track. Therefore, single-track multi-imaging-strip stitching imaging is required to completely cover the scene and achieve wide-swath imaging of non-track scenes. The following describes the results of the embodiment with accompanying figures:

[0111] Figure 5 The wave foot planning results and target coverage of the proposed method are given, with adjacent sub-bands having an overlap of 10% of the sub-band width in the distance direction. Figure 5(a) This is the wave foot planning result designed in this embodiment. There are 23 target points marked with "×" at the center of the scene. Beam footprint fitting is performed on all targets at the center of the scene to obtain the wave foot planning result as shown below. Figure 5 As shown by the curve in (a), the wave foot trajectory makes one turn, and the overall shape is U-shaped, with a distance of 5 km before and after the turn; Figure 5 (b) is the wave foot trajectory optical map of this embodiment. The planned survey zone is 20km long and 11km wide, and the target points are distributed as follows: Figure 5 (b) As shown by the asterisk, there are a total of 23 targets; the stitching result of the two imaging strips shows the same trend as... Figure 5 (a) The planning results are consistent; Figure 5 (c) The azimuth resolution designed in this embodiment is stable at around 0.5m, which meets the observation requirements and is consistent with the expected value; Figure 5 (d) is the swath width designed in this embodiment. The value is guaranteed to be ≥6km, which can ensure that there is a small amount of overlap between the observation zones before and after the wave foot turns, so as to achieve complete observation of the target scene. Figure 6 The range of attitude angles, angular velocities, and angular accelerations for this experiment are given. The satellite maneuver constraints for this simulation are: beam attitude angle ≤ 45°, resultant attitude angular velocity ≤ 0.8° / s, and resultant attitude angular acceleration ≤ 0.08° / s². 2 Based on the given attitude angles and their first and second derivatives, it can be seen that the maneuver constraints are satisfied. Therefore, this embodiment can achieve non-track imaging mode design under given platform maneuver constraints.

[0112] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A parabolic spaceborne SAR non-track multi-subband stitching wide-swath imaging method, characterized in that, The spaceborne SAR uses a single-track multi-subband stitching method for imaging. The subband is formed by imaging the target scene longitudinally from the beginning to the end, or from the end to the beginning. Adjacent subbands are connected end to end. Adjacent subbands partially overlap along the range direction, and the overlapping part is 8% to 15% of the subband width. When the wave foot of the spaceborne SAR moves to the end of the current subband, the antenna attitude is gradually adjusted under the constraint of beam maneuverability to turn the wave foot and enter the next subband. First, based on the latitude and longitude coordinates of the input scene center point and the range of viewpoint values ​​under spaceborne SAR, a suitable orbital observation arc is selected. Then, based on the sub-band distance width W r The scene width W determines the number of sub-bands n, where n is an integer; Next, the observation band configuration is determined based on the number of sub-bands n, where n = 2, the observation band configuration is U-shaped; n = 3, the observation band configuration is S-shaped; n = 4, the observation band configuration is M-shaped; and so on. Once the observation zone configuration is determined, target points are set based on this configuration as the trajectory. The spacing between target points is related to the trajectory curvature; the greater the curvature, the denser the distribution of target points. Based on the non-track curve observation configuration, the satellite power-on time T1 and power-off time T2 are calculated; the yaw angle, pitch angle and roll angle of the satellite at each time are calculated; based on the sequence of yaw angle, pitch angle and roll angle, attitude control commands are generated, and the attitude control design results of the satellite and parabolic antenna are obtained.

2. The method as described in claim 1, characterized in that, When the overlap is 10% of the subband width, the number of subbands n is:

3. The method according to any one of claims 1 to 2, characterized in that, Wave foot trajectories of spaceborne SAR are generated using a wave foot tracking algorithm; the wave foot tracking algorithm is specifically as follows: S201, divide the target scene into multiple contiguous sub-bands; sort the target points set in all sub-bands according to the observation order; input the sequence of target points to be observed, satellite orbit, and radar power-on time; S202, let P be the position of the lower wave foot of the Earth-solid system during the i-th step of tracking. foot.f (i) The wave foot velocity is V foot.f (i) , wave foot P foot.f (i) To the current tracking target point P T The direction vector of (j) is v ij , ||·||2 is the L2 norm operator, in V foot.f (i) and v ij n direction vectors v′ are uniformly set in the included angle. ij (n) is called the selectable direction of the wave foot; Calculate the position P′ in each possible direction. ij (n), velocity V′ ij (n), acceleration a′ ij (n), as shown in equation (1), where dt is the time derivative; During the first line tracking, the wave foot origin P foot.f (1) The location of the first observation target point, wave foot velocity V foot.f (1) Based on the given azimuth resolution, satellite velocity, and beam intercept parameters along the azimuth, the tracking target point P is calculated according to equations (2) and (3). T (1) is the location of the second target point; S203, Calculate the azimuth resolution ρ for each selectable direction. a (n), the specific steps are as follows: The azimuth resolution of spaceborne SAR is ρ a The expression is shown in equation (2), where V Sat.f R represents the satellite velocity in the Earth-fixed system. E H is the Earth's radius; H is the orbital altitude; B is the Earth's radius. a To accumulate Doppler bandwidth, B a The analytical expression is shown in equation (3), where R(t)″ is the second derivative of the slant range path, λ is the wavelength, and V foot.f For the wave foot velocity in the Earth-solid system, l res The intercept along the foot of the wave in the half-power projection ellipse on the ground; To obtain the Doppler bandwidth B a The analytical expression for l is given by equations (4) to (8). res The solution method, k in equations (4) and (5) r and k a Let β be the intersection of the range and azimuth profiles of the antenna beam in the Earth's inertial frame with the Earth's tangent plane, γ be the downward angle, and γ be the projection of the oblique angle onto the Earth's tangent plane at the nadir point. In the satellite orbital coordinate system, the X-axis is the direction of the satellite's velocity; the Z-axis vector lies in the satellite orbital plane and points towards the Earth's center; the Y-axis is solved using the right-hand rule, and η is the angle formed by the Y-axis and the intersection of the antenna range plane and the XOZ plane. ec For the geocentric angle, β′ ec Let β′ be the geocentric angle that includes left and right view information, when looking to the right. ec =β ec When looking left, β′ ec =-β ec H X (θ) represents rotating the coordinate axis by θ degrees along the positive direction of the right-hand rule, with the positive X-axis as the axis. Y (θ) and H Z The definition of (θ) is similar; equations (6) and (7) give the projection ellipse along k r and k a The lengths of the two axes are r and a, where θ e For k r and k a The included angle, σ r and σ r For L3 and k r k a The included angles are respectively, and L3 is the position vector of the satellite to the ground wave foot in the Earth inertial frame. These represent the beam distance and azimuth beamwidth, respectively; ω in equation (8) represents the ground wave foot velocity V in the ground inertial frame. foot With k a The included angle; combined with formulas (4) to (8), we can obtain l res The analytical expression for is shown in equation (9); Equation (10) gives the analytical expression of R(t)″, where a foot.f V foot.f Let dL3 and dL4 represent the velocity and acceleration of the wave foot in the Earth-solid system, respectively. 2 L3 is the first and second derivative of L3 with respect to time; Then, the velocity in the selectable direction of the wave foot is updated using equation (11), and the process jumps to step S202 until the calculated azimuth resolution matches the desired resolution ρ. a0 The residuals between them are small enough; S204, based on yaw ψ, pitch θ, and roll angle Analyze its higher-order differential analytical expression, calculate the maneuverability in each feasible direction, and select the direction that satisfies the constraints of the spaceborne SAR platform and is closest to v. ij Feasible direction number n i ; then, n i The position and velocity are used as the wave foot position P in the (i+1)th step. foot.f (i+1) Wave foot velocity V foot.f (i+1) The acceleration is the wave foot acceleration a in the i-th step. foot.f (i+1) ; S205, after each iteration, it is necessary to determine whether the tracking target needs to be switched. The criterion is shown in equation (12), where R set The set length threshold has a value range within a distance width; v foot.f (i) Let be the velocity direction vector of the i-th wave foot. Repeat steps S202 to S205 until the entire target point sequence has been traversed, and output the wave foot trajectory.

4. The method as described in claim 3, characterized in that, In S204, n i The method of obtaining it is: Where F(·) is the attitude angle range constraint, G(·) is the attitude angular velocity constraint, H(·) is the attitude angular acceleration constraint, and (●) is the attitude angular acceleration constraint. H Let be the conjugate transpose of the matrix, and ζ be the weighting factor; in equation (14), a < 1, b < 0.5, L foot L represents the distance between the wave foot and the previous tracking target. T The distance between the two tracking targets is denoted as .

Images