Time sequence design method for multi-scene synchronous imaging of spaceborne SAR (Synthetic Aperture Radar)

Abstract

The invention discloses a time sequence design method for satellite-borne SAR (synthetic aperture radar) multi-scene synchronous imaging. The time sequence design method comprises the following steps: setting a PRF (pulse repetition frequency) range; in the PRF range, acquiring a PRF set for imaging of each scene in the training set; judging whether a PRF intersection exists between the PRF sets, used for imaging, of each scene in the training set or not; if the PRF intersection exists, searching a PRF1 used for synchronous imaging of all scenes in the training set in the PRF intersection; if the PRF intersection does not exist, ending; and in the PRI corresponding to the searched PRF1, according to a set arrangement rule, orderly arranging corresponding transmission pulses and time periods corresponding to echo receiving windows when each scene in the training set is imaged, and obtaining a time sequence of satellite-borne SAR multi-scene synchronous imaging. According to the invention, the time sequence design can realize effective imaging of any multi-scene synchronous imaging mode and randomly distributed observation scene positions and surveying and mapping breadth, and ensures that the imaging is complete and echo signals of all scenes are received without crosstalk.

Description

Technical Field

[0001] The present invention relates to the technical field of SAR, and particularly to a timing design method for multi-scene synchronous imaging of spaceborne SAR. Background Art

[0002] Spaceborne SAR has the imaging capabilities of all-weather and all-day, so it has great application value both in military and civilian fields. Currently, as shown in Figures 1 - 4 , in the multi-scene synchronous imaging mode, due to the differences in the positions of the observed scenes, when transmitting different pulses or receiving echoes from different scenes within an equivalent PRI, it is necessary to switch the antenna beam to ensure the effective reception of all scene echoes. With the change of the relative positions between multiple scenes, the pulse transmission period and the echo reception period within an equivalent PRI will also be adjusted accordingly. In addition, the echo window length will also change with the change of the width of the observed scene to ensure the integrity of the scene echo. Therefore, it is necessary to carry out research on the system working timing algorithm design of the generalized multi-scene synchronous imaging mode to ensure that there is no crosstalk between the transmitted pulses and the received echoes for these scenes, and they can all be effectively arranged within an equivalent PRI.

[0003] The existing timing design method for the multi-strip synchronous imaging mode is as shown in Figure 5 , this method has strict constraint conditions on the observed scenes, and it is necessary to ensure that the transmitted pulses and the echo reception windows of each scene bisect a complete equivalent pulse repetition interval (PRI) in time. This constraint will greatly limit the positions and widths of the synchronously observed scenes. In actual multi-scene synchronous imaging applications, these constraint conditions are often not satisfied, resulting in the failure of the original timing design method. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a timing design method for multi-scene synchronous imaging of spaceborne SAR, so as to achieve effective imaging for any multi-scene synchronous imaging mode, as well as the positions of the observed scenes and the mapping widths distributed arbitrarily, and ensure that its imaging is complete and all scene echo signals can be received without crosstalk.

[0005] A timing design method for multi-scene synchronous imaging of spaceborne SAR, the timing design method includes the following steps:

[0006] S0. Randomly select multiple scenes as the training set;

[0007] S1. Set the pulse repetition frequency PRF range according to the imaging requirements;

[0008] S2. Within the pulse repetition frequency PRF range, obtain the pulse repetition frequency PRF set for imaging of each scene in the training set;

[0009] S3. Determine whether there is a pulse repetition frequency (PRF) intersection among the sets of PRFs used for imaging each scene in the training set;

[0010] If there is such a PRF intersection, search for a PRF1 for synchronous imaging of all the scenes in the training set within the PRF intersection;

[0011] If there is no such PRF intersection, end;

[0012] S4. Within the pulse repetition interval (PRI) corresponding to the searched PRF1, orderly arrange the corresponding transmission pulses and the time periods corresponding to the echo receiving windows during imaging of each scene in the training set according to the set arrangement rule to obtain the timing sequence for spaceborne SAR multi-scene synchronous imaging.

[0013] Optionally, in S1, the method for setting the PRF range includes:

[0014] Set the selection interval of the PRF range to be [PRF min , PRF max , where;

[0015] The lower limit value PRF min of the PRF is:

[0016] PRF min = α × B d ;

[0017]

[0018] where α is the oversampling factor, α ∈ [1.2 - 1.25], θ az is the antenna beam width, V s is the satellite flight speed, λ is the system carrier frequency wavelength, and B d is the instantaneous Doppler bandwidth of the echo signal received by the system;

[0019] When designing the upper limit value PRF max of the PRF, it is necessary to consider that the transmission pulses and echo windows of each scene in the training set can be accommodated within an equivalent pulse repetition interval PRI. Therefore, the equivalent pulse repetition interval PRI needs to satisfy the constraint condition PRI eff , and the constraint condition PRI eff is:

[0020]

[0021] Among them, N is the number of scenes that need to be synchronously imaged, and T n is the transmitted pulse width corresponding to the nth scene, and EWL n is the echo window length corresponding to the nth scene;

[0022] The upper limit value PRF of the pulse repetition frequency PRF max is:

[0023]

[0024] Among them, R nf is the near slant range of the nth scene, and R nn is the far slant range of the nth scene, and c is the electromagnetic wave propagation speed.

[0025] Optionally, in S2, the method for obtaining the set of pulse repetition frequencies PRF for imaging each scene in the training set includes:

[0026] To ensure avoiding pulse emission interference when receiving echo signals, the set of pulse repetition frequencies PRF obtained for imaging the nth scene needs to satisfy the first constraint condition, and the first constraint condition is:

[0027]

[0028] Among them, k = 0, 1, 2,..., +∞, and T g is the transceiver switching time;

[0029] To ensure that the echo signal is not affected by strong interference in the sub-satellite point area, the set of pulse repetition frequencies PRF obtained for imaging the nth scene needs to satisfy the second constraint condition, and the second constraint condition is:

[0030]

[0031] Among them, m = 0, 1, 2,..., +∞, H is the satellite flight orbit altitude, and θ nadir is the maximum viewing angle of the sub-satellite point area.

[0032] Optionally, in S3, the method for searching for the pulse repetition frequency PRF1 for synchronous imaging of all scenes in the training set in the intersection of the pulse repetition frequencies PRF includes:

[0033] S3-01. Randomly select 1 pulse repetition frequency PRF value from the high-frequency region in the intersection of the pulse repetition frequencies PRF, and for each scene, combine the echo window corresponding to the selected pulse repetition frequency PRF value with the closest transmitted pulse into an independent time block;

[0034] S3-02. Determine whether multiple independent time blocks can be merged;

[0035] If they can be merged, merge multiple independent time blocks into an overall time block, and then proceed to S3-03;

[0036] If they cannot be merged, directly proceed to S3-03;

[0037] S3-03. Arrange all the independent time blocks and the overall time block in sequence to form a concatenated time block;

[0038] S3-04. Determine whether the total length of the concatenated time block exceeds the pulse repetition interval (PRI) corresponding to the current pulse repetition frequency (PRF);

[0039] If it exceeds the pulse repetition interval (PRI), select a pulse repetition frequency (PRF) value from the intersection of the pulse repetition frequencies (PRF) that is less than the pulse repetition frequency (PRF) value selected in the current S3-01 and re-enter S3-01;

[0040] If it does not exceed the pulse repetition interval (PRI), it indicates that the pulse repetition interval (PRI) can accommodate all the transmitted pulses and echo windows, and use the concatenated time block as the pulse repetition frequency (PRF1).

[0041] Optionally, in S3-02, the constraint condition for multiple independent time blocks to be able to be merged is:

[0042] Time_block m <Int start_n ;

[0043] Time_block m <Int end_n ;

[0044] Among them, Time_block m is the time block length of the m-th scene, and Int start_n is the interval between the leading edge of the echo reception window of the n-th scene and the trailing edge of its previous transmitted pulse, and Int end_n is the time interval between the trailing edge of the echo reception window of the n-th scene and the leading edge of its subsequent transmitted pulse.

[0045] Optionally, in S3-02, the method of merging multiple independent time blocks into an overall time block includes:

[0046] When Time_block m is embedded in Int start_n it is necessary to mEmissions at the head position of the Int start_n ;

[0047] When the Time_block m is embedded in the Int end_n , the Time_block m needs to be arranged at the tail position of the Int end_n ;

[0048] When any two of the independent time blocks are merged, the total number of the independent time blocks is correspondingly reduced by 1, and the reserved time at the front end of the echo window of the nth scene will change from Int start_n to Int` start_n ;

[0049] Int` start_n = Int start_n - Time_block m - T g .

[0050] Optionally, in S3-03, when arranging any one of the time blocks in the series-connected time blocks, several time intervals T g are set between two adjacent independent time blocks and / or the overall time block.

[0051] Optionally, in S3-03, after the arrangement of the series-connected time blocks, in the first time block of the emission, if the echo reception window is before the transmit pulse, the corresponding echo window is shifted backward in time by a complete pulse repetition interval PRI to adjust the starting reference zero moment of a complete pulse repetition interval PRI to the leading edge moment of the transmit pulse.

[0052] Optionally, in S3-04, the calculation method of the total length T total of the series-connected time blocks is:

[0053]

[0054] where K is the sum of the independent time blocks and the overall time block in the series-connected time blocks;

[0055] The constraint condition for taking the series-connected time blocks as the pulse repetition frequency PRF1 is:

[0056]

[0057] Optionally, in S4, the calculation method of the working time range of the echo window of any one of the time blocks in the time sequence of the spaceborne SAR multi-scene synchronous imaging includes:

[0058]

[0059] Among them, T echo_m is the echo window of the m-th time block, m ∈ [1, k], ΔT echo_m is the time interval between the starting front edge of the echo reception window of the m-th time block and the reference zero moment, EWL m is the length of the echo reception window of the m-th time block, j = 0, 1, 2, ···, +∞.

[0060] The beneficial effects that the present invention can produce include:

[0061] A timing design method for spaceborne SAR multi-scene synchronous imaging provided by the present invention. Under the system timing design algorithm of this method, the transmit pulses and echo window lengths of different scenes are no longer strictly restricted to be only half of the equivalent pulse repetition interval PRI, but the transmit pulse moments and echo window ranges of each scene are reasonably designed based on the principle of optimizing the system duty cycle, ensuring that the signal-to-noise ratio of the spaceborne SAR image and the corresponding NESZ index are optimal; realizing effective imaging for any multi-scene synchronous imaging mode, as well as for the observation scene positions and mapping widths with arbitrary distributions, and ensuring that its imaging is complete and all echo signals of all scenes are received without crosstalk. Description of the Drawings

[0062] Figure 1 is the schematic diagram of the system timing architecture of the traditional spaceborne multi-polarization imaging mode;

[0063] Figure 2 is the observation schematic diagram of the traditional spaceborne SAR multi-strip synchronous imaging mode

[0064] Figure 3 is the observation schematic diagram of the traditional spaceborne SAR multi-burst synchronous imaging;

[0065] Figure 4 is the working schematic diagram of the traditional spaceborne SAR hybrid imaging mode;

[0066] Figure 5 is the schematic diagram of the timing design method of the traditional multi-strip synchronous imaging mode;

[0067] Figure 6 is the flowchart of the timing design method of the spaceborne SAR multi-scene synchronous imaging of the present invention;

[0068] Figure 7 is the flowchart of the multi-scene-based PRF set search for the PRF available for synchronous imaging of the present invention;

[0069] Figure 8 is the framework schematic diagram of the relative position of the echo window within a PRI in the present invention;

[0070] Figure 9 Schematic diagram of time block merging for synchronous imaging of two different scenes in the present invention;

[0071] Figure 10 Schematic diagram of time series design within an equivalent PRI in the present invention;

[0072] Figure 11 Schematic diagram of adjusting the starting reference zero moment of the PRI to the leading edge moment of the transmit pulse in the present invention;

[0073] Figure 12 Schematic diagram of the slant range change of the strip mode observation scene (red straight line) and the sliding spotlight mode observation scene (blue curve) in the present invention;

[0074] Figure 13 Schematic diagram of azimuth segmentation in sliding spotlight mode imaging and step - by - step jump of the echo receiving window in the present invention to adapt to ultra - large range migration;

[0075] Figure 14 Schematic diagram of the timing arrangement of the 29th azimuth segment in Table 3 of the present invention;

[0076] Figure 15 Schematic diagram of the timing arrangement of the 31st azimuth segment in Table 3 of the present invention;

[0077] Figure 16 Schematic diagram of the timing arrangement of the 53rd azimuth segment in Table 3 of the present invention. Detailed implementation manners

[0078] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0079] Please refer to Figure 6 As shown, the present invention provides a timing design method for space - borne SAR multi - scene synchronous imaging. The timing design method includes the following steps:

[0080] Step 0: Randomly select multiple scenes as the training set;

[0081] Step 1: Set the PRF (pulse repetition frequency) range according to the imaging requirements;

[0082] Step 2: Within the PRF range, obtain the PRF set for imaging each scene in the training set;

[0083] Step 3: Determine whether there is a PRF intersection among the PRF sets used for imaging each scene in the training set; if there is a PRF intersection, search for PRF1 for synchronous imaging of all scenes in the training set in the PRF intersection; if there is no PRF intersection, end;

[0084] Step 4: Within the PRI (pulse repetition interval) corresponding to the searched PRF1, arrange the corresponding transmit pulses and the time periods corresponding to the echo reception windows in an orderly manner according to the set arrangement rules when imaging each scene in the training set, so as to obtain the timing sequence of spaceborne SAR multi-scene synchronous imaging.

[0085] Under the above timing design algorithm, the transmit pulses and echo window lengths of different scenes are no longer strictly restricted to be half of the equivalent PRI, but are reasonably designed based on the principle of optimizing the system duty cycle to determine the transmit pulse moments and echo window ranges of each scene, ensuring the optimal signal-to-noise ratio of the spaceborne SAR image and the corresponding NESZ index; to achieve effective imaging for any multi-scene synchronous imaging mode, as well as for the observation scene positions and mapping widths with arbitrary distributions, and ensure that its imaging is complete and all echo signals of all scenes are received without crosstalk.

[0086] In the above Step 1, the method for setting the PRF range includes: setting the selection interval of the PRF range as [PRF min , PRF max , where; the lower limit value PRF min of the PRF is:

[0087] PRF min = α × B d ;

[0088]

[0089] where α is the oversampling factor, α ∈ [1.2 ~ 1.25], θ az is the antenna beam width, V s is the satellite flight speed, λ is the system carrier frequency wavelength, B d is the instantaneous Doppler bandwidth of the echo signal received by the system; it should be noted that in this embodiment, to satisfy the azimuth Nyquist sampling theorem, the PRF of the system must be greater than B d ; at the same time, when designing the upper limit value PRF max of the PRF, it is determined by the transmit pulse widths and echo window widths of each observation scene, and it is necessary to consider that the transmit pulses and echo windows of each scene in the training set can be accommodated within an equivalent PRI. Therefore, the PRI needs to satisfy the constraint condition PRI eff , and the constraint condition PRI eff is:

[0090]

[0091] Among them, N is the number of scenes to be synchronously imaged, and T n is the emission pulse width corresponding to the nth scene, and EWL n is the echo window length corresponding to the nth scene;

[0092] The upper limit value of PRF, PRF max is:

[0093]

[0094] Among them, R nf is the near slant range of the nth scene, and R nn is the far slant range of the nth scene, and c is the electromagnetic wave propagation speed.

[0095] In the above [PRF min , PRF max range interval, further select the set of PRFs available for imaging for each scene, and the constraint condition for determining the imable PRF is that the scene echo window cannot be crosstalked with the emission pulse and the sub-satellite point interference in time. Specifically, in step two, the method for obtaining the set of PRFs for imaging for each scene in the training set includes: To ensure avoiding pulse emission interference when receiving the echo signal, the set of PRFs for imaging of the nth scene obtained needs to satisfy the first constraint condition, and the first constraint condition is:

[0096]

[0097] Among them, k = 0, 1, 2,..., +∞, and T g is the transceiver switching time;

[0098] To ensure that the echo signal is not affected by strong sub-satellite point interference, the set of PRFs for imaging of the nth scene obtained needs to satisfy the second constraint condition, and the second constraint condition is:

[0099]

[0100] Among them, m = 0, 1, 2,..., +∞, H is the satellite flight orbit altitude, and θ nadir is the maximum viewing angle of the sub-satellite point area.

[0101] In the above step three, as Figure 7 shown, the method for searching for PRF1 for synchronous imaging of all scenes in the training set in the PRF intersection includes:

[0102] Step S3-01: Randomly select 1 PRF value from the high-frequency region in the PRF intersection, and for each scene, combine the echo window corresponding to the selected PRF value with the closest transmit pulse into an independent time block;

[0103] Step S3-02: Determine whether multiple independent time blocks can be merged; if they can be merged, merge the multiple independent time blocks into an overall time block, and then proceed to Step S3-03; if they cannot be merged, directly proceed to Step S3-03;

[0104] Step S3-03: Arrange all the independent time blocks and the overall time block in sequence to form a concatenated time block;

[0105] Step S3-04: Determine whether the total length of the concatenated time block exceeds the PRI corresponding to the current PRF; if it exceeds the PRI, select a PRF value from the PRF intersection that is less than the PRF value selected in the current Step S3-01 and re-enter Step S3-01; if it does not exceed the PRI, it indicates that the PRI can accommodate all the transmit pulses and echo windows, and use the concatenated time block as PRF1.

[0106] In the above Step S3-01, as Figure 8 shown, among the PRF intersections available for imaging in all scenes, first pick out the largest PRF value to evaluate its applicability to achieving multi-scene synchronous imaging. It should be noted that the PRF value in the high-frequency interval is selected first because the larger the PRF, the higher the signal-to-noise ratio of the corresponding SAR image and the relatively better the image quality. When the spaceborne SAR system individually images each scene at the PRF, the relative position of the scene echo within the PRI can be defined by using the two parameters Int start and Int end . Among them, Int start is the interval between the leading edge of the echo window and the trailing edge of the previous pulse, and Int start is the interval between the trailing edge of the echo window and the leading edge of the next pulse. By comparing Int start and Int end , we pick out the transmit pulse that is closer to the echo window in time and pack them into an independent time block. Taking the time series given in Figure 8 as an example, since Int start < Int end , the echo window will be packed and combined with the previous transmit pulse into an independent time block. The start time of this time block is the transmit leading edge of the previous transmit pulse, and the end time is the trailing edge of the echo window.

[0107] In the above Step S3-02, the merging constraint condition for multiple independent time blocks to be able to be merged is:

[0108] Time_block m <Int start_n ;

[0109] Time_block m <Int end_n ;

[0110] Among them, Time_block m is the time block length of the m-th scene, and Int start_n is the interval between the leading edge of the echo reception window of the n-th scene and the trailing edge of its previous transmission pulse, and Int end_n is the time interval between the trailing edge of the echo reception window of the n-th scene and the leading edge of its next transmission pulse. As Figure 8 shown, if multiple different scenes can meet the above merging constraint conditions, select the time block with the shortest time, and embed Time_blockm into this time block. Specifically, the method of merging multiple independent time blocks into an overall time block includes: when Time_block m is embedded into Int start_n , Time_block m needs to be arranged at the head position of Int start_n ; when Time_block m is embedded into Int end_n , Time_block m needs to be arranged at the tail position of Int end_n ; as Figure 9 shown, when any two independent time blocks are merged, the total number of independent time blocks is correspondingly reduced by 1, and the reserved time at the front end of the echo window of the n-th scene will change from Int start_n to Int` start_n ; therefore, between two adjacent transmission pulses, a protection time of T g needs to be left to realize the switching of the transmission waveform;

[0111] Int` start_n = Int start_n - Time_block m - T g .

[0112] In the above step S3-03, as Figure 10 shown, when arranging any one time block in the series time blocks, several time intervals T g are set between two adjacent independent time blocks and / or overall time blocks., to ensure that there is no crosstalk between adjacent time blocks, and finally obtain a time series within an equivalent PRI. It should be noted that after arranging all the time blocks for multi-scene synchronous imaging, the starting moment of an equivalent PRI should be selected as the leading edge moment of the transmitted pulse. Therefore, as Figure 11 shown, after the arrangement of the series-connected time blocks, in the first time block of the arrangement, if the echo reception window is before the transmitted pulse, the corresponding echo window is shifted backward in time by a complete pulse repetition interval PRI to adjust the starting reference zero moment of a complete pulse repetition interval PRI to the leading edge moment of the transmitted pulse. At this time, based on Figure 11 the reference zero moment of the transmitted pulse 1 in, the time ranges where each event is located within a complete equivalent PRI can be strictly defined, and the spaceborne SAR system will work according to the corresponding time ranges to ensure synchronous imaging of multiple observation scenes.

[0113] In the above step S3-04, after arranging K independent time blocks, the total length T total of the series-connected time blocks is calculated as follows:

[0114]

[0115] where K is the sum of the independent time blocks and the overall time blocks in the series-connected time blocks; the constraint condition for taking the series-connected time blocks as the pulse repetition frequency PRF1 is:

[0116]

[0117] It should be noted that if the constraint condition is satisfied, it indicates that the PRF1 selected from the PRF intersection can accommodate all the transmitted pulses and echo reception windows, so it can be effectively applied to multi-scene synchronous imaging. Otherwise, a lower PRF value needs to be selected from the PRF intersection for further iteration to evaluate its effectiveness until finally a PRF1 that can achieve multi-scene synchronous imaging is selected.

[0118] Furthermore, in step four, after obtaining the PRF that can effectively achieve multi-scene synchronous imaging and the reference zero moment, we can establish the precise working timing for all pulse transmissions and echo receptions during the imaging period. Taking Figure 10 the echo window of the m-th time block in as an example, the calculation method for the working time range of the echo window of any time block in the timing of spaceborne SAR multi-scene synchronous imaging includes:

[0119]

[0120] where T echo_mis the echo window for the m-th time block, where m ∈ [1, k], and ΔT echo_m is the time interval between the starting front edge of the echo reception window for the m-th time block and the reference zero time, EWL m is the length of the echo reception window for the m-th time block, and j = 0, 1, 2, ···, +∞.

[0121] In this embodiment, taking a Figure 4 shown hybrid imaging mode as an example, the effectiveness of the time sequence design method for the spaceborne SAR multi-scene synchronous imaging system given by the present invention is verified. This hybrid imaging mode combines the strip mode and the sliding spotlight mode, and the corresponding system operating parameters are shown in Table 1:

[0122] Table 1

[0123] Serial number Item Parameter value 1 Orbit altitude 700 km 2 System carrier frequency 9.8 GHz 3 Earth radius 6371 km 4 SAR antenna length 6m 5 SAR antenna height 1.5m 6 Transmission pulse width 20 μs 7 Guard time interval 2 μs 8 Near - end view of the strip - mode observation scene 25.5° 9 Far - end view of the strip - mode observation scene 26.6° 10 Center view of the sliding spotlight - mode observation scene 26° 11 Sliding spotlight - mode scene size 8 km × 8 km 12 Sliding spotlight - mode resolution 0.1m

[0124] Based on the system observation parameters given in Table 1, the ground range of the strip mode observation scene can be calculated to be 338.33 km to 354.09 km, and the corresponding mapping swath width is 15.66 km. The slant range of this observation scene is 785.50 km to 793.17 km. For the high-resolution sliding spotlight mode, the antenna beam needs to rotate from 13.64° to -13.64° in the azimuth direction to increase the beam dwell time and the corresponding azimuth Doppler bandwidth. The entire imaging time T a = 55.43 s. Referring to Figure 12 shown, the present invention gives the slant range change method of the spaceborne SAR and two synchronous observation scenes. Among them, the two red lines represent the near slant range and the far slant range of the strip mode observation scene, and it can be seen that they do not change during the entire imaging process. The two blue curves show the change of the near slant range and the far slant range of the sliding spotlight mode observation scene with the azimuth slant angle during imaging.

[0125] From Figure 12 the shown results, it can be seen that the range migration effect of the sliding spotlight mode is very serious. Among them, the range migration amount of the sliding spotlight mode echo has reached 27.3 km. The PRF in the traditional spaceborne SAR mode needs to be reduced to a very low value to fully adapt to this migration amount, which will cause serious azimuth ambiguity deterioration. To solve this problem, first, the entire imaging time needs to be divided into several independent sub-blocks in the azimuth direction, and the range migration amount within each sub-block is restricted. At the same time, the system PRF and the echo reception window will also be adaptively adjusted according to the range of the echo slant range in the azimuth sub-block to ensure that the scene echo within each sub-block is completely received. By analyzing Figure 12 the interval between the two blue curves in, the maximum slant range span R of the sliding spotlight mode observation scene can be obtained maxThe slant range is 4.9 km, and this slant range span will be used as a reference for azimuth sub-block division. In this embodiment, a slant range margin of 0.5 km is reserved, and 0.5 km is reserved to accommodate the slant range migration amount in each azimuth sub-block. Based on this, the echo reception window within each azimuth sub-block will be able to accommodate a slant range span of 5.9 km. As Figure 13 shown, where the pink broken line represents the step jump mode of the echo reception window during imaging, and the scene echoes represented by the two blue curves are always enclosed within the echo reception window represented by the pink broken line, indicating that the echoes can be effectively received.

[0126] In this embodiment, referring to Figure 13 and the azimuth segmentation method for sliding spotlight mode imaging given in Table 2, when the echo of the sliding spotlight mode imaging area gradually moves from the far end to the near end and then returns to the far end, the echo reception window also shows a similar stepped change mode. Based on this result, the corresponding system timing design can be carried out for this multi-scene synchronous imaging mode. Among them, for each azimuth segmentation interval in the sliding spotlight mode, the slant range span ΔW1 of the imaging area is:

[0127] ΔW1 = ΔR max + 1 km = 5.9 km;

[0128] For the stripmap mode, the slant range span ΔW2 of the echo is:

[0129] ΔW2 = 7.67 km;

[0130] Therefore, the upper limit value PRF max of the PRF interval is:

[0131]

[0132] According to the azimuth length of the antenna and the platform speed, the instantaneous Doppler bandwidth B dop of the echo can be obtained as 2220 Hz. Therefore, the lower limit value PRF min of the PRF needs to meet the constraint condition: PRF min ≥ B dop = 2220 Hz; combined with the maximum duty cycle dc max that the spaceborne SAR system power amplifier can work with, and the azimuth oversampling factor, the final synchronous imaging PRF range is limited to [3000 Hz, 6500 Hz].

[0133] Table 2

[0134]

[0135]

[0136]

[0137]

[0138] In this embodiment, based on the above PRF range: [3000Hz, 6500Hz], the timing design method of the present invention is used to design the corresponding timing for synchronous imaging of two scenarios. Since the slant ranges of the sliding spotlight imaging regions in each segment in the azimuth direction are different, the synchronous imaging timing design needs to be carried out independently for each segment. The final design results are shown in Table 3.

[0139] Table 3

[0140]

[0141]

[0142]

[0143]

[0144] In the above, the 29th, 31st, and 53rd segments in the azimuth direction are selected from Table 2 as three representative azimuth segment intervals to analyze and verify the effectiveness of the synchronous imaging timing design method given by the present invention. It can be seen from Table 3 that the equivalent PRF of the 29th segment is 4610Hz, and the corresponding PRI is 216.92 μs. Within this time range, the relative time range intervals between two transmit pulses and the echo reception window are as Figure 14 shown, where transmit pulse 1 and echo window 1 are the transmit pulse and echo window in the strip mode, and transmit pulse 2 and echo window 2 are the transmit pulse and echo window in the sliding spotlight mode. Similarly, Figure 15 gives the timing arrangement within the PRI of the 31st segment in the azimuth direction, Figure 16 and gives the timing arrangement within the PRI of the 53rd segment in the azimuth direction.

[0145] In the above, by comparing the timing arrangements within one PRI of these three segment intervals, it can be seen that due to the range migration caused by the change of the azimuth oblique angle, the relative positions between the transmit pulses and the echo windows in the two modes will also change accordingly. In Figure 16 , the interval between the two transmit pulses is very small, and similarly, the interval between the echo windows is also very small. In this case, there are either two echo reception windows between the two transmit pulses or no reception window. Compared with the case of Figure 16 , there must be an echo reception window between two adjacent transmit pulses in Figure 14 and Figure 15 , which also means that in the latter two cases, the interval between the transmit pulses will be relatively larger than that in the case shown in Figure 16 .

[0146] In this embodiment, taking the timing result of the 29th paragraph in Table 3 as an example, the effectiveness of the above timing design result is analyzed. Specifically, within 1 PRI, the transmit window of the strip mode is [0, 20 μs], and the echo reception window is [34.22 μs, 105.41 μs]. Therefore, the range of the scene slant range where the corresponding echo window can receive echoes is:

[0147] R strip = {n × PRI eff + [34.22 μs ~ (105.41 μs - T p )]} × c / 2;

[0148] where n = 0, 1, 2, ···, +∞; when n = 24, the slant range interval covered by the echo window of the strip mode can be obtained as [785.50 km, 793.17 km]. This result is consistent with the slant range span of the strip mode, thus verifying the effectiveness of strip mode imaging. Similarly, the transmit window of the sliding spotlight mode is [107.41 μs, 127.41 μs], the echo reception window is [150.99 μs, 210.32 μs], and the range of the scene slant range where the echo window can receive echoes is:

[0149] R slip = {m × PRI eff + [150.99 μs ~ (210.32 μs - T p )] - 107.41 μs} × c / 2;

[0150] where m = 0, 1, 2, ···, +∞; when m = 24, the slant range of the scene covered by the echo window is [786.90 km, 792.80 km]. This result is completely consistent with the slant range of the sliding spotlight imaging scene in the 29th paragraph of Table 2, verifying the effectiveness of the timing result for sliding spotlight imaging.

Claims

1. A timing design method for multi-scene synchronous imaging of spaceborne SAR, characterized in that, The timing design method includes the following steps: S0. Randomly select multiple scenes as the training set; S1. Set the pulse repetition frequency (PRF) range according to the imaging requirements; S2. Within the pulse repetition frequency (PRF) range, obtain the set of pulse repetition frequencies (PRFs) for imaging each scene in the training set; S3. Determine whether there is a pulse repetition frequency (PRF) intersection among the sets of pulse repetition frequencies (PRFs) for imaging each scene in the training set; If there is a pulse repetition frequency (PRF) intersection, search for the pulse repetition frequency (PRF1) for synchronous imaging of all scenes in the training set within the pulse repetition frequency (PRF) intersection; If there is no pulse repetition frequency (PRF) intersection, end; S4. Within the pulse repetition interval (PRI) corresponding to the searched pulse repetition frequency (PRF1), orderly arrange the corresponding transmission pulses and the time periods of the echo reception windows for imaging each scene in the training set according to the set arrangement rule to obtain the timing of multi-scene synchronous imaging of spaceborne SAR.

2. The timing design method for spaceborne SAR multi-scene synchronous imaging according to claim 1, characterized in that, In S1, the method for setting the pulse repetition frequency (PRF) range includes: Set the selection range for the pulse repetition frequency PRF to be [PRF min , PRF max , where; The lower limit value PRF of the pulse repetition frequency PRF min is: PRF min = α × B d ; where α is the oversampling factor, α ∈ [1.2 - 1.25], θ az is the antenna beamwidth, V s is the satellite flight speed, λ is the system carrier frequency wavelength, B d is the instantaneous Doppler bandwidth of the echo signal received by the system; When designing the upper limit value PRF of the pulse repetition frequency PRF max it is necessary to consider that the transmitted pulses and echo windows of each scene in the training set can be accommodated within an equivalent pulse repetition interval PRI. Therefore, the equivalent pulse repetition interval PRI needs to satisfy the constraint condition PRI eff The constraint condition PRI eff is as follows: Where N is the number of scenes to be synchronously imaged, and T n is the emission pulse width corresponding to the nth scene, and EWL n is the echo window length corresponding to the nth scene; The upper limit value PRF of the pulse repetition frequency PRF max is as follows: where R nf is the near-range slant range of the nth scene, and R nn is the far-range slant range of the nth scene, and c is the electromagnetic wave propagation speed.

3. A timing design method for spaceborne SAR multi-scene synchronous imaging according to claim 1, characterized in that In S2, the method for obtaining the set of pulse repetition frequencies (PRFs) for imaging each scene in the training set includes: To ensure avoiding pulse emission interference when receiving echo signals, the set of pulse repetition frequencies (PRFs) for imaging the nth scene obtained needs to satisfy the first constraint condition, and the first constraint condition is: where k = 0, 1, 2, …, +∞, and T g is the transceiver switching time; To ensure that the echo signals are not affected by strong interference at the sub-satellite point, the set of pulse repetition frequencies (PRFs) for imaging the nth scene obtained needs to satisfy the second constraint condition, and the second constraint condition is: where m = 0, 1, 2, …, +∞, H is the satellite flight orbit altitude, and θ nadir is the maximum viewing angle of the sub-satellite point area.

4. A timing design method for on-orbit SAR multi-scene synchronous imaging according to claim 1, characterized in that, In S3, the method for searching for the pulse repetition frequency (PRF1) for synchronous imaging of all scenes in the training set within the pulse repetition frequency (PRF) intersection includes: S3-01. Randomly select 1 pulse repetition frequency (PRF) value from the high-frequency region of the pulse repetition frequency (PRF) intersection, and for each scene, combine the echo window corresponding to the selected pulse repetition frequency (PRF) value with the closest transmission pulse into an independent time block; S3-02. Determine whether multiple independent time blocks can be merged; If they can be merged, merge multiple independent time blocks into an overall time block, and then enter S3-03; If they cannot be merged, directly enter S3-03; S3-03. Arrange all the independent time blocks and the overall time block in sequence to form a series time block; S3-04. Determine whether the total length of the series time block exceeds the pulse repetition interval (PRI) corresponding to the current pulse repetition frequency (PRF); If it exceeds the pulse repetition interval (PRI), select a pulse repetition frequency (PRF) value smaller than the pulse repetition frequency (PRF) value selected in the current S3-01 from the pulse repetition frequency (PRF) intersection and re-enter S3-01; If it does not exceed the pulse repetition interval PRI, it indicates that the pulse repetition interval PRI can accommodate all the transmitted pulses and echo windows, and the serial time block is taken as the pulse repetition frequency PRF1.

5. A timing design method for on-orbit SAR multi-scene synchronous imaging according to claim 4, characterized in that, In S3-02, the constraint condition for merging between multiple independent time blocks is: Time_block m <Int start_n ; Time_block m <Int end_n ; Among them, Time_block m is the time block length of the m-th scene, Int start_n is the interval between the leading edge of the echo reception window of the n-th scene and the trailing edge of its previous transmit pulse, Int end_n is the time interval between the trailing edge of the echo reception window of the n-th scene and the leading edge of its next transmit pulse.

6. A timing design method for on-orbit SAR multi-scene synchronous imaging according to claim 5, characterized in that In S3-02, the method for merging multiple independent time blocks into an overall time block includes: When Time_block m is embedded into Int start_n , the Time_block m needs to be placed at the head position of the Int start_n ; When Time_block m is embedded into Int end_n it is necessary to place the said Time_block m at the tail position of the said Int end_n ; When any two of the independent time blocks are merged, the total number of the independent time blocks is correspondingly reduced by 1, and the reserved time at the front end of the echo window of the nth scene will change from Int start_n to Int s , tart_n ; Int s 、 tart_n =Int start_n -Time_block m -T g 。 7. A timing design method for on-orbit SAR multi-scene synchronous imaging according to claim 5, characterized in that, In S3-03, when arranging any one of the time blocks in the series time block, a plurality of time intervals T are set between two adjacent independent time blocks and / or the overall time block g .

8. A timing design method for spaceborne SAR multi-scene synchronous imaging according to claim 5, characterized in that In S3-03, after the arrangement of the serial time blocks, in the first time block of the emission, if the echo reception window is before the transmitted pulse, the corresponding echo window is shifted backward in time by a complete pulse repetition interval PRI to adjust the starting reference zero moment of a complete pulse repetition interval PRI to the leading edge moment of the transmitted pulse.

9. The timing design method for spaceborne SAR multi-scene synchronous imaging according to claim 5, characterized in that In S3-04, the total length T of the series time blocks total is calculated as follows: Where K is the sum of the independent time blocks and the overall time block in the serial time block; The constraint condition for taking the serial time block as the pulse repetition frequency PRF1 is:

10. A timing design method for on-orbit SAR multi-scene synchronous imaging according to claim 1, characterized in that, In S4, the calculation method for the working time range of the echo window of any time block in the timing of the spaceborne SAR multi-scene synchronous imaging includes: Among them, T echo_m is the echo window of the m-th time block, m ∈ [1, k], ΔT echo_m is the time interval between the start front edge of the echo receiving window of the m-th time block and the reference zero time, EWL m is the length of the echo receiving window of the m-th time block, j = 0, 1, 2, …, +∞.

Images