A Method for Imaging of SAR Satellite with Non-Track Curve

Abstract

The present invention proposes an imaging method applicable to the non-track-along curve scene observed by SAR satellites, which can overcome the problem that traditional imaging methods cannot efficiently observe the oblique curve scene due to the track-along constraint, solve the system design problem in the non-track-along curve imaging mode of SAR satellites, and can achieve high-efficiency and high-resolution observation of the non-track-along oblique curve scene; generate curve beam footprints by fitting with the maximum illumination number as the criterion, and at the same time comprehensively consider two input methods of non-track-along scene orientations, solving the problem of single beam footprint orientation in the traditional track-along mode observation of SAR satellites; an orbit selection method, screening the orbit in combination with the space-varying characteristics of the down-view angle of the non-track-along curve beam footprint, solving the orbit selection problem in the non-track-along observation of SAR satellites; a satellite configuration design method, which can solve the problem of uneven azimuth resolution in the non-track-along imaging of SAR satellites by controlling the moving speed of the center of the surface beam footprint.

Description

Technical Field

[0001] The present invention belongs to the technical field of Synthetic Aperture Radar (SAR), and particularly relates to a method for non-track-aligned curve imaging of SAR satellites. Background Art

[0002] Spaceborne Synthetic Aperture Radar (SAR) is an active microwave remote sensing radar imaging system that operates on a satellite platform and can perform high-resolution observations of the earth's surface all day and all weather. The synthetic aperture radar processes the echo accumulated by continuously irradiating the target, effectively utilizes the Doppler frequency bandwidth in the target echo, and equivalently increases the azimuth antenna length to achieve high azimuth resolution. Spaceborne synthetic aperture radar is not restricted by environmental factors such as sunlight intensity and weather, and can complete important tasks such as topographic mapping, resource exploration, and battlefield situation reconnaissance, playing a significant role in economic development and national defense construction.

[0003] The main imaging modes of traditional spaceborne synthetic aperture satellites include the stripmap mode, spotlight mode, sliding spotlight mode, scan mode, and TOPS mode. The stripmap mode is the most basic imaging mode of conventional spaceborne SAR. Its antenna does not scan, and the azimuth resolution is only determined by the azimuth size of the antenna, with a large azimuth imaging swath width. In the spotlight mode, the beam gazes at a fixed area, and the azimuth resolution can be very high, but the imaging swath width is small, only the size of the ground wave footprint. In the sliding spotlight mode, the beam scanning makes the moving speed of the ground beam footprint slower than that of the stripmap mode. The azimuth resolution is higher than that of the stripmap mode, and the azimuth imaging swath width is between the stripmap mode and the spotlight mode. In the scan mode, the azimuth resolution is lower than that of the stripmap mode, and the beam can switch between different sub-imaging swaths to achieve a larger range imaging bandwidth. In the TOPS mode, the beam can also switch between different sub-imaging swaths, but the moving speed of the ground beam footprint is faster than that of the scan mode. Therefore, it can achieve a larger azimuth imaging bandwidth compared to the scan mode. The imaging swaths of the above traditional modes are all distributed along the satellite track direction, that is, they are all track-aligned straight imaging swaths. Their extension direction is single, and the observation method is not flexible. The actual target scene often has a certain observation angle with the satellite track direction, and the geographical trend is variable and irregular, such as oblique curve scenes like coastlines, highways, and earthquake zones. The single imaging swath of the traditional imaging mode often cannot fully cover these target scenes.

[0004] If a single imaging swath cannot fully cover the target area, the conventional solution is to observe by stitching multiple track-aligned imaging swaths, and there are the following two implementation methods:

[0005] "Single-track time-sharing" observation: sacrificing azimuth resolution to expand the range imaging swath width, and switching beams between different imaging swaths within the single-track observation time, such as the scanning mode or the TOPS mode;

[0006] "Multi-track revisit" observation: sacrificing observation efficiency and image consistency to a great extent, imaging different imaging swaths separately for each satellite orbit revisit to expand the range imaging swath width.

[0007] When the scene is "fat" and has a large extension in both the range and azimuth directions, it is feasible to use multiple imaging swaths along the satellite track for stitching observation. However, when the scene is "thin", has a large geographical extension and is not along the satellite track direction, using multiple imaging swaths along the satellite track for stitching observation is inefficient. In addition to the problems of low azimuth resolution and long data acquisition cycle caused by the above "single-track time-sharing" and "multi-track revisit" respectively, there is also a common problem of large data redundancy: the target area only occupies a very small proportion of the imaging swath after band stitching, and the echoes of a large number of non-interested areas will also be stored and processed simultaneously, resulting in a large waste of on-board resources. The above two methods cannot meet the two observation requirements of high observation efficiency and high resolution at the same time, which becomes a problem to be solved in the traditional mode. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide a non-azimuth curve imaging method for SAR satellites, which can solve the problem of observing oblique curve scenes by traditional spaceborne synthetic aperture radar satellites.

[0009] A non-azimuth curve imaging method for SAR satellites includes:

[0010] Step 1: Generate beam footprints according to the input observation target points or the trend of the observation scene;

[0011] Step 2: Select a suitable satellite orbit observation arc according to the beam footprints;

[0012] Step 3: Based on the observation resolution requirements, the orbit parameters of the satellite orbit observation arc and the satellite platform capabilities, obtain the satellite observation configuration, the on / off times of the satellite, and the attitude control instructions;

[0013] Step 4: Design the wave positions of the SAR radar based on the time-varying relationship of the slant range of the beam center under the satellite observation configuration;

[0014] Step 5: Based on the satellite orbit observation arc, control the satellite to execute the attitude control instructions, and according to the wave position design, control the SAR radar to transmit signals; within the on / off times of the satellite, perform imaging processing on the received echo signals to obtain the imaging result.

[0015] Preferably, the step 2 includes:

[0016] Step 21: Deduce the satellite orbit;

[0017] Step 22: Based on the satellite orbit, calculate all orbital observation arcs where the non-track curve beam footprint can be observed, and the corresponding range of nadir angle change for each orbital observation arc;

[0018] Step 23: Based on the satellite orbit altitude, satellite beam width, and observation swath width, calculate the nadir angle ranges corresponding to the nearest and farthest ends of the beam, and set it as the first observable range threshold TH1; take the satellite observable nadir angle range as the second observable range threshold TH2, and screen the orbital observation arcs in Step 22 where the nadir angle change range is within the first observable range threshold TH1 and the second observable range threshold TH2.

[0019] Preferably, in Step 3, the method for establishing the satellite observation configuration includes:

[0020] Assume there is a target P at a certain position on the center of the beam footprint, and the intercept of the projection of the beam on the ground at the target P along the moving speed V of the center of the beam footprint is l g in the direction; res ;

[0021] Define the slant range history of the SAR satellite relative to the target P as R p， Then the Doppler centroid frequency of the target at the beam illumination center time is K a as:

[0022]

[0023] where λ is the wavelength corresponding to the working frequency of the SAR satellite, t is the azimuth time, and t c is the beam illumination center time;

[0024] Obtain the azimuth resolution ρ a and the relationship between the Doppler centroid frequency K a is:

[0025]

[0026] where l res is the satellite velocity, H is the orbit altitude, and R E is the radius of the Earth;

[0027] Take multiple position points at the same interval on the center of the beam footprint. Based on the geometric relationship between the satellite orbit and the beam footprint, obtain the intercept l g of the projection of the beam at different position points on the center of the beam footprint on the ground along the V res direction; calculate the Doppler centroid frequency K a of different positions on the center of the beam footprint at the beam illumination center time based on formula (1); The required azimuth resolution ρ of the input observation task a , calculate the moving speed V of the surface beam footprint center at different position points of the beam footprint center based on formula (2) g ; Based on the interval between adjacent position points, the moving speed V of the surface beam footprint center g and the central observation time T0, calculate the corresponding relationship between the movement of the beam footprint center position and time, that is, the relationship between the beam pointing of the SAR radar and time changes, so as to determine the satellite observation configuration of the non-track curve.

[0028] Preferably, the wave position design method in step four includes:

[0029] Determine the sub-satellite point echo occlusion area and the transmitted pulse occlusion area on the two-dimensional plane with the pulse repetition frequency as the horizontal axis and the slant range of the beam center as the vertical axis, find the area that avoids the sub-satellite point echo occlusion area and the transmitted pulse occlusion area, determine the pulse repetition frequency range of this area, and perform wave position design within this range.

[0030] Preferably, in step one, if the input is the observation target point, the target points are selected based on the principle that the SAR can irradiate the most target points in an observation task;

[0031] Preferably, in step one, if the input is the observation scene, target points are set at a set interval along the scene direction on the scene direction center line.

[0032] Preferably, according to the moments corresponding to the starting point and the ending point of the beam footprint center in the non-track curve observation configuration, the corresponding satellite power-on time T1 and power-off time T2 are obtained.

[0033] The present invention has the following beneficial effects:

[0034] The present invention proposes an imaging method applicable to the non-track curve scene of SAR satellite observation, which can overcome the problem that the traditional imaging method cannot efficiently observe the oblique curve scene due to the track constraint, solves the system design problem in the non-track curve imaging mode of the SAR satellite, and can achieve high-efficiency and high-resolution observation of the non-track oblique curve scene;

[0035] In step one, the curve beam footprint is generated by fitting with the maximum irradiation number as the criterion, and at the same time, two input methods of the non-track scene direction are comprehensively considered, which solves the problem of the single beam footprint direction in the traditional track mode observation of the SAR satellite; the orbit selection method in step two combines the down-view angle variation characteristics of the non-track curve beam footprint to screen the orbit, which solves the orbit selection problem in the non-track observation of the SAR satellite;

[0036] The configuration design method in Step 3 can solve the problem of uneven azimuth resolution in the non-track-aligned imaging of SAR satellites by controlling the moving speed of the center of the ground beam footprint.

[0037] The system design method in Step 4 can solve the serious problem of echo reception loss caused by the large change in the slant range of the satellite beam center in the non-track-aligned observation of SAR satellites by adopting the continuous variable pulse interval technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is the system design flow chart of the non-track-aligned curve imaging method for SAR satellites;

[0039] Figure 2 is the schematic diagram of the observation configuration of the non-track-aligned curve imaging method for SAR satellites

[0040] Figure 3 is the target distribution and beam footprint of the simulation of the non-track-aligned curve imaging method for SAR satellites;

[0041] Figure 4 (a) and Figure 4 (b) are respectively the simulation orbit and the selected result of the orbit arc segment of the non-track-aligned curve imaging method for SAR satellites;

[0042] Figure 5 is the design result of the observation configuration of the simulation of the non-track-aligned curve imaging method for SAR satellites;

[0043] Figure 6 is the design result of the attitude control of the simulation of the non-track-aligned curve imaging method for SAR satellites;

[0044] Figure 7 (a) and Figure 7 (b) are respectively the zebra diagram and the wave position design result of the simulation of the non-track-aligned curve imaging method for SAR satellites;

[0045] Figure 8 is the imaging result of point targets of the simulation of the non-track-aligned curve imaging method for SAR satellites;

[0046] Figure 9 (a), Figure 9 (b) and Figure 9 (c) are respectively the evaluation of the imaging result of the scene center point target, the range profile, and the azimuth profile of the simulation of the non-track-aligned curve imaging method for SAR satellites. DETAILED DESCRIPTION OF THE INVENTION

[0047] The present invention will be described in detail below with reference to the accompanying drawings and by way of examples.

[0048] A non-track-aligned curve imaging method for SAR satellites provided by the present invention, as Figure 1 shown, its specific steps include:

[0049] Step 1: Generate a non-track-aligned curve beam footprint according to the input observed target point or the input observed scene direction.

[0050] (1) If the input is an observed target point, select the target points based on the principle that the SAR can illuminate the most target points in an observation mission; if the input is an observed scene, set target points at a certain interval along the scene direction on the center line of the scene direction.

[0051] (2) Based on the longitude and latitude coordinates of the selected target points, fit and generate a non-track-aligned curve beam footprint.

[0052] Step 2: Perform orbit prediction based on the input satellite orbit parameters, and calculate the orbit observation arc segment and the nadir angle change range of the non-track-aligned curve beam footprint. Select a suitable orbit observation arc segment according to the observation swath requirement and the satellite observable nadir angle range.

[0053] Step 21: Input the satellite orbit parameters, including but not limited to the six orbital elements shown in Table 1 and Greenwich Mean Time, and deduce the orbit.

[0054] Table 1 Satellite Orbit Parameters

[0055]

[0056] Step 22: Based on the deduced orbit, calculate all the orbit observation arc segments that can observe the non-track-aligned curve beam footprint, and the corresponding nadir angle change range for each orbit observation arc segment.

[0057] Step 23: Based on the satellite orbit altitude, satellite beam width and observation swath, calculate the nadir angle range corresponding to the nearest and farthest ends of the beam, and set it as the first observable range threshold TH1; take the satellite observable nadir angle range as the first observable range threshold TH2, and screen the orbit observation arc segments in Step 22 whose nadir angle change range is within the threshold TH1 and threshold TH2.

[0058] Step 3: Based on the observation resolution requirement, orbit parameters and satellite platform capabilities, conduct non-track-aligned observation configuration design to obtain the satellite's power-on and power-off times and the attitude control design results.

[0059] Step 31: Based on the observation resolution requirement, the orbit parameters of the orbit observation arc segments screened in Step 23 and the satellite platform capabilities, establish a non-track-aligned curve observation configuration, specifically:

[0060] The scene center is defined as the center position of the length of the beam footprint, and the position of the SAR satellite on the orbit observation arc segment with the minimum nadir angle relative to the scene center is defined as the satellite observation center position, and the time is recorded as T0. The observation configuration is as shown in Figure 2As shown in the figure, the satellite speed is Vr, the orbital altitude is H, and the central slant range is R. The moving speed of the center of the surface beam footprint is Vg, and the direction is the tangent direction of the beam footprint. The positive direction of the X-axis extends along the scene, from the starting point to the ending point of the beam footprint, which is defined as the azimuth direction of non-track imaging. The positive direction of the Y-axis is perpendicular to the scene extension direction and is defined as the range direction, and the Z-axis points to the center of the earth. α is the angle between the positive direction of the Y-axis and the ground projection vector Rg of the central slant range, which is defined as the "observation oblique angle" and represents the degree of beam squint. If the cross product of the positive direction of the Y-axis and Rg is the positive direction of the Z-axis, then α is positive, otherwise it is negative; θ is the angle between the scene direction and the satellite sub-track, which is defined as the "scene inclination angle" and represents the degree of inclination of the target scene relative to the satellite track. Wa and Wr respectively represent the azimuth and range imaging strip widths.

[0061] Assume that there is a target P at a certain position on the center of the beam footprint, and the intercept of the surface beam ellipse at the target P along the Vg direction is l res , as Figure 2 shown. At different positions on the center of the beam footprint, the intercept l res of the surface beam ellipse is different.

[0062] Define the slant range history of the SAR satellite relative to the target P as R p， Then the Doppler frequency modulation rate of the target at the beam illumination center time is K a as:

[0063]

[0064] where λ is the wavelength corresponding to the working frequency of the SAR satellite, t is the azimuth time, and t c is the beam illumination center time.

[0065] Define ρ a as the azimuth resolution, and the relationship between the azimuth resolution ρ a and the Doppler frequency modulation rate K a can be obtained as:

[0066]

[0067] where R E is the radius of the earth.

[0068] Set points P1, P2, P3,..., P at different positions on the center of the beam footprint at the same interval n , and based on the geometric relationship between the satellite orbit and the beam footprint, obtain the intercept l i of the surface beam ellipse along the Vg direction at different positions P res (i = 1,..n) of the beam footprint center. Calculate the beam footprint center at different positions P i(i = 1,..n) Doppler frequency modulation rate K at the beam illumination center time a The required azimuth resolution ρ of the input observation task a , Based on formula (2), the different positions P of the beam footprint center can be calculated i (i = 1,..n) Ground beam footprint center moving speed V g The size of. Based on adjacent P i (i = 1,..n) Position interval, ground beam footprint center moving speed V g With the central observation time T0, the corresponding relationship between the movement of the beam footprint center position and time is calculated, that is, the relationship between the beam pointing of the SAR radar and time changes, so as to determine the satellite observation configuration of the non-track curve.

[0069] Step 32: According to the moments corresponding to the starting point and the ending point of the beam footprint center in the non-track curve observation configuration, obtain the corresponding satellite power-on time T1 and power-off time T2.

[0070] Step 33: According to the relationship between the beam pointing and time changes in the non-track curve observation configuration, calculate the yaw angle, pitch angle and roll angle of the satellite at each moment. Based on the yaw angle, pitch angle and roll angle sequences, generate attitude control commands to obtain the satellite attitude control design result.

[0071] Step Four: Based on the time-varying relationship of the slant range of the beam center under the non-track observation configuration, perform wave position design to obtain the system design result of the SAR satellite non-track curve imaging method.

[0072] Step 41: Determine the azimuth sampling frequency range according to the slant range change history and the observation instantaneous bandwidth of the non-track observation configuration.

[0073] Step 42: Based on the azimuth sampling frequency range and the slant range history, perform wave position design on the premise of maximizing echo reception to obtain the system design result of the SAR satellite non-track curve imaging method.

[0074] Step Five: Based on the observable orbital arc segment, control the satellite to execute the attitude control command and the system design command, transmit the linear frequency modulation signal and receive the echo signal. Perform imaging processing on the received echo signal to obtain the imaging result.

[0075] (1) Based on the available orbital observation arc segment obtained in Step Two, control the satellite to power on at the power-on time T1. Based on the satellite attitude control command obtained in Step Three, perform real-time control on the yaw angle, pitch angle and roll angle of the satellite to make the satellite beam center point to the expected beam footprint. At the same time, based on the system design result obtained in Step Four, transmit the linear frequency modulation signal and receive the echo signal until the satellite powers off at the power-off time T2.

[0076] (2) Perform imaging processing on the received echo signal to obtain an imaging result.

[0077] Example:

[0078] Table 2 shows the coordinate information parameters of the input target, Table 3 shows the orbit parameters, and Table 4 shows the satellite simulation parameters.

[0079] Table 2 Target Parameter List

[0080]

[0081]

[0082] Table 3 Satellite Orbit Parameter List

[0083]

[0084] Table 4 Key Parameter List of Radar Satellite

[0085] Parameter Name Value Unit Orbit Altitude 600 km Observation Oblique Angle 15 deg Scene Dip Angle 14.3543 deg Range Width 10 km Azimuth Width 120 km Carrier Frequency 10 GHz Pulse Width 15 us

[0086] To verify the non-track curve imaging method of the SAR satellite, the following simulation is carried out. The input target coordinates are shown in Table 2, and the parameters in Table 3 are used for simulation. The back-projection imaging algorithm is adopted to obtain the imaging result and observe its imaging ability.

[0087] Step 1: Generate a non-track curve beam footprint according to the input observation target point or the trend of the input observation scene.

[0088] <1> The distribution of target points is as shown by the triangle ("△") in Figure 3 , with a total of fifteen targets. The selected target positions are located along the east coast of the Malay Peninsula.

[0089] <2> Select five targets at the scene center for beam footprint fitting to obtain the beam footprint as shown by the curve in Figure 3 , and the starting and ending points of the beam footprint are as shown by the circles ("○") in Figure 3 .

[0090] Step 2: Based on the input satellite orbit parameters, perform orbit prediction, and calculate the orbital observation arc segments and the nadir angle change ranges corresponding to different positions of the non-track curve beam footprint. According to the observation swath requirement and the satellite's observable nadir angle range, select a suitable orbital observation arc segment.

[0091] <1> The input satellite orbit parameters are shown in Table 3. Based on the orbit parameters, orbit deduction is carried out to obtain orbit Q1, and the orbit deduction time is 2 hours. Figure 4 The background sphere in Figure 4 (a) is the full orbit map, Figure 4(b) is a partial enlarged view. The deduced orbit Q1 is Figure 4 the dark curve in (a) that circles the Earth once. The designed beam footprint in Step 1 is the light curve on the surface of the Earth model.

[0092] <2>According to the deduced orbit Q1, screen the orbital observation arc segments with the down-viewing angle change range within the first observable range threshold TH1 and the second observable range threshold TH2, that is, use the arc segment orbit Q2. The satellite position at the moment when the down-viewing angle is the smallest is Figure 4 the position of the circle in it.

[0093] Step 3: Based on the observation resolution requirements, orbital parameters, and satellite platform capabilities, conduct non-track-along observation configuration design to obtain the satellite's power-on and power-off times and the attitude control design results.

[0094] <1>Design the non-track-along observation configuration to obtain the observation configuration design results, such as Figure 5 shown. Figure 5 In (a) of it is the curve of the center slant range changing with time for the non-track-along curve observation configuration, Figure 5 In (b) of it is the curve of the down-viewing angle changing with time for the non-track-along curve observation configuration, Figure 5 In (c) of it is the curve of the slant viewing angle changing with time for the non-track-along curve observation configuration.

[0095] <2>According to the non-track-along observation configuration, the satellite's power-on time T1 is 21:33:15 seconds on April 24, 2022, the power-off time T2 is 21:33:27 seconds on April 24, 2022, and the observation time is 12 seconds. The satellite's attitude control design results are as Figure 6 shown. Figure 6 In (a) of it is the curve of the yaw angle of the satellite changing with time, Figure 6 In (b) of it is the curve of the pitch angle of the satellite changing with time, Figure 6 In (c) of it is the curve of the roll angle of the satellite changing with time.

[0096] Step 4: Based on the time-varying relationship of the beam center slant range under the non-track-along observation configuration, conduct wave position design to obtain the system design results of the SAR satellite non-track-along curve imaging method.

[0097] <1>According to the satellite observation requirements and the slant range history, the upper limit of the designed azimuth sampling frequency is 7200 Hz, and the lower limit of the azimuth sampling frequency is 5600 Hz.

[0098] <2>With the pulse repetition frequency (unit: Hz) as the horizontal axis and the beam center slant range (unit: km) as the vertical axis, determine the sub-satellite point echo occlusion area (black part) and the transmitted pulse occlusion area (dark gray part in the figure) on this two-dimensional plane, and draw to obtain as Figure 7(a) The zebra diagram shown. In the zebra diagram, find the area that avoids the echo occlusion area and the transmitted pulse occlusion area under the sub-satellite point, and determine the pulse repetition frequency range of this area. The wave position design area corresponding to the light gray part in Fig. 7(a). Perform wave position design within this range to obtain a pulse repetition frequency that can adapt to the time-varying center slant range to achieve complete echo transceiver. The curve of the pulse repetition frequency changing with time is as Figure 7 (b) shown, and the design of the non-track-aligned curve imaging method system is completed.

[0099] Step 5: Based on the observable orbital arc segment, control the satellite to execute the attitude control instruction and the system design instruction, transmit the chirp signal and receive the echo signal. Perform imaging processing on the received echo signal to obtain the imaging result.

[0100] <1> Based on the available orbital observation arc segment obtained in Step 2, control the start and end times of the simulation. Based on the satellite attitude control instruction obtained in Step 3, control the yaw angle, pitch angle, and roll angle of the satellite so that the satellite beam center points to the expected beam footprint. Based on the system design result obtained in Step 4, perform echo simulation to obtain the echo signal.

[0101] <2> The imaging algorithm used in the simulation is the back-projection time-domain imaging algorithm, and the definitions of the azimuth and range directions of the imaging grid are the same as those in Step 3. Figure 8 The simulation results of 15 point targets are given, and the point target results are all well focused. Figure 9 And Table 5 gives the evaluation results of the 8th target at the scene center.

[0102] Table 5 Imaging evaluation results of the scene center point

[0103] Theoretical Resolution Actual Resolution Peak Sidelobe Ratio Integrated Sidelobe Ratio Azimuth 2.4576m 2.4533m -13.6645 -11.93 Ground Range 2.2500m 2.2431m -13.3969 -10.90

[0104] After evaluation, the theoretical resolution and the actual resolution of the scene center point target are consistent. The two-dimensional peak sidelobe ratio is less than -13 dB, and the two-dimensional integrated sidelobe ratio is less than -10 dB, meeting the index requirements.

[0105] In this embodiment, multi-point target simulation is used, and the imaging results meet the index requirements, verifying the feasibility of the non-track-aligned curve imaging method for SAR satellites.

[0106] In summary, the above is only a preferred embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for SAR satellite non-track curve imaging, characterized in that, Including: Step 1: Generate beam footprints according to the input observed target points or the direction of the observed scene; Step 2: Select appropriate satellite orbit observation arc segments according to the beam footprints; Step 3: Based on the observation resolution requirements, the orbit parameters of the satellite orbit observation arc segments, and the satellite platform capabilities, obtain the satellite observation configuration, the on / off times of the satellite, and the attitude control commands; Step 4: Based on the time-varying relationship of the slant range of the beam center in the satellite observation configuration, perform waveform design for the SAR radar; Step 5: Based on the satellite orbit observation arc segments, control the satellite to execute the attitude control commands, and according to the waveform design, control the SAR radar to transmit signals; within the on / off times of the satellite, perform imaging processing on the received echo signals to obtain the imaging results; The said Step 2 includes: Step 21: Deduce the satellite orbit; Step 22: Based on the satellite orbit, calculate all the orbit observation arc segments that can observe the non-track-along curve beam footprints, and the corresponding nadir angle change range of each orbit observation arc segment; Step 23: Based on the satellite orbit altitude, the satellite beam width, and the observation swath, calculate the nadir angle ranges corresponding to the nearest and farthest ends of the beam, and set it as the first observable range threshold TH1; take the satellite observable nadir angle range as the second observable range threshold TH2, and screen the orbit observation arc segments in Step 22 whose nadir angle change ranges are within the first observable range threshold TH1 and the second observable range threshold TH2.

2. The method for imaging of a SAR satellite non-track curve as claimed in claim 1, wherein In the said Step 3, the method for establishing the satellite observation configuration includes: Assume that there is a target P at a certain position on the center of the beam footprint, and the projection of the beam on the ground moves at the target P along the center of the beam footprint at a speed V g The intercept in the direction is l res ; Define the slant range history of the SAR satellite relative to the target P as R p， Then the Doppler centroid frequency of the target at the beam illumination center time is K a where: where λ is the wavelength corresponding to the operating frequency of the SAR satellite, t is the azimuth time, and t c is the center time of beam illumination; Obtain the azimuth resolution ρ a and the Doppler frequency modulation rate is K a The relationship between them is as follows: where l res is the satellite velocity, H is the orbital altitude, and R E is the radius of the Earth, and the satellite velocity is Vr; Take multiple positions at the center of the beam footprint at the same interval, and based on the geometric relationship between the satellite orbit and the beam footprint, obtain the projection of the beam on the ground along V at different positions of the beam footprint center. g The intercept in the direction l res ; Based on formula (1), calculate the Doppler modulation rate K at different positions of the beam footprint center at the moment of beam illumination center a ; Input the required azimuth resolution ρ of the observation task a , based on formula (2), calculate the moving speed V of the beam footprint center at different positions of the beam footprint center g The size of the beam footprint is based on the interval between adjacent locations and the moving speed V of the center of the surface beam footprint. g As well as the central observation time T0, the corresponding relationship between the movement of the center position of the beam footprint and time is calculated, that is, the relationship between the beam pointing of the SAR radar and time, so as to determine the satellite observation configuration of the non-along-track curve.

3. A method for SAR satellite non-track curve imaging as claimed in claim 1, characterized in that, The waveform design method in the said Step 4 includes: Determine the sub-satellite point echo occlusion area and the transmitted pulse occlusion area on the two-dimensional plane with the pulse repetition frequency as the horizontal axis and the slant range of the beam center as the vertical axis, find the area that avoids the sub-satellite point echo occlusion area and the transmitted pulse occlusion area, determine the pulse repetition frequency range of this area, and perform waveform design within this range.

4. The method for off-track curve imaging of a SAR satellite according to claim 1, characterized in that, In the said Step 1, if the input is an observed target point, the target points are selected based on the principle that the SAR can irradiate the most target points in one observation task.

5. A method for SAR satellite non-track curve imaging according to claim 1, characterized in that In the said Step 1, if the input is an observed scene, target points are set at set intervals along the scene direction on the center line of the scene direction.

6. The method for off-track curve imaging of a SAR satellite according to claim 1, wherein According to the moments corresponding to the starting point and the ending point of the beam footprint center in the non-track-along curve observation configuration, obtain the corresponding satellite turn-on time T1 and turn-off time T2.

Images