Satellite-borne monostatic / bistatic synthetic aperture radar (SAR) bunching imaging combined time sequence construction method and system

Abstract

The invention provides a satellite-borne monostatic / bistatic synthetic aperture radar (SAR) bunching imaging combined time sequence construction method and system. The method comprises the following steps: acquiring main and auxiliary satellite parameters; constructing the pulse repetition frequency of the monostatic / bistatic SAR system based on double-star observation geometry; constructing a bunching imaging primary satellite single-base transmitting / receiving time sequence; constructing a beaming imaging mode cosatellite bistatic receiving time sequence; and outputting main and auxiliary satellite transmitting and receiving time sequences. According to the method, a unistatic / bistatic SAR combined time sequence construction method is effectively broken through, multi-constraint PRF combined optimization, bistatic time sequence cooperative fault tolerance and space-variant characteristic source inhibition measures are provided, and the problems of bistatic transceiving separation, time-frequency synchronization, space-variant compensation and the like which cannot be dealt with by a traditional unistatic SAR time sequence construction method are solved; and a set of complete bistatic time sequence construction system is formed, and the engineering practical value is higher.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of spaceborne radar, in particular to a spaceborne single / dual base SAR spotlight imaging joint timing construction method and system. BACKGROUND

[0002] The spaceborne dual base SAR system solves the problem of forward and backward imaging that cannot be realized by the traditional spaceborne SAR system by placing the transmitter and receiver on different satellite platforms, and has great application value. However, the configuration of the spaceborne dual base SAR system with transceiver separation also causes many complex technical problems, among which the construction of the dual base imaging pulse transmission and reception timing is related to whether the primary and auxiliary satellites can normally image without interference, and is one of the core problems in the system construction link. Compared with the timing construction of the traditional SAR system, the spaceborne dual base SAR system has the transmitter and receiver separated, and the spatial motion characteristics and time synchronization accuracy of the two will directly affect the signal coherence, so the timing of the transmission and reception pulses needs to be strictly coordinated to avoid signal mismatch; secondly, the dual base geometry configuration leads to more complex constraints on the pulse repetition frequency (PRF), and the selection of PRF also needs to consider the influence of direct wave, mirror reflection point echo interference and other factors on dual base imaging. Therefore, the timing construction of the dual base SAR system must be comprehensively considered and optimized from multiple aspects to ensure the stable operation and good imaging effect of the system.

[0003] The document "Repeat-pass space-surface bistatic SAR tomography: accurate imaging and first experiment" (Chen ZY, et al. Sci China Inf Sci, 2024) based on the chirp-Z transform orbit error correction method, establishes an accurate tomographic imaging model, and verifies the effectiveness through the LT-1 satellite experiment; for the first time, L-band repeat-pass SS-BiSAR tomographic imaging is realized, and the three-dimensional reconstruction accuracy is improved by 12%. However, it does not propose systematic constraint rules for timing construction, only compensates for orbit errors through post-processing, and does not avoid transmission signal conflicts (such as nadir point echo interference).

[0004] The document "Star source illumination dual / multi base SAR imaging" (Wu Junjie et al. Journal of Radars, 2023) proposes a star-ground and star-machine dual base SAR cooperative imaging mode, verifies the feasibility of forward-looking imaging; and uses multi-channel low-rank sparse recovery to improve the imaging quality. However, this method relies on GNSS or direct wave signal estimation for transmission synchronization, does not construct dynamic PRF constraints, and cannot adapt to the primary satellite variable PRF mode (such as spotlight mode pulse timing change).

[0005] The document "Cluster Cooperative Detection and Guidance Based on Dual / Multi-Base SAR" (Lü Jinhu et al., Acta Aeronautica et Astronautica, 2025) proposes a dual / multi-base SAR cooperative guidance closed-loop link (perception-decision-execution) to realize passive forward-looking imaging and multi-view information fusion, but does not involve PRF timing cooperation, multi-satellite transmission conflict depends on formation optimization, and does not solve the contradiction between PRF and resolution caused by space-varying Doppler bandwidth.

[0006] The document "Star Missile Bi-Base Forward-Looking SAR Imaging Algorithm Based on Joint Range-Azimuth Two-Dimensional NCS" (Liu Yuzhou et al., Journal of Radars, 2024) proposes a joint range-azimuth two-dimensional NCS algorithm to solve the problem of range migration space variation in star missile high-speed platform forward-looking imaging; the two-dimensional spectrum is recovered through automatic gate adjustment. However, its PRF construction is based on the assumption of fixed Doppler bandwidth, and does not consider the bi-static space variation characteristics.

[0007] Patent document CN201510519358.X discloses a timing determination method for an ultra-high resolution space-borne SAR system, which proposes dynamic pulse interval adjustment (PRI): according to the scene echo range migration trend, the PRI is continuously adjusted with the minimum step (5ns) to avoid transmission pulse and echo interference, but only for single-base scene construction, without considering the direct wave interference and main and auxiliary star cooperation specific to bi-static.

[0008] Therefore, the market needs a space-borne single / dual-base SAR spotlight imaging joint timing construction method and system that can be comprehensively considered and optimized from multiple aspects to ensure stable operation and good imaging effect of the system. SUMMARY

[0009] In view of the defects in the prior art, the purpose of the present application is to provide a space-borne single / dual-base SAR spotlight imaging joint timing construction method and system.

[0010] According to the space-borne single / dual-base SAR spotlight imaging joint timing construction method provided by the present application, the following steps are included:

[0011] Step S1: obtaining main and auxiliary star parameters;

[0012] Step S2: single / dual-base SAR system pulse repetition frequency construction based on dual-star observation geometry;

[0013] Step S3: spotlight imaging main star single-base transmission / reception timing construction;

[0014] Step S4: spotlight imaging mode auxiliary star bi-static reception timing construction;

[0015] Step S5: outputting main and auxiliary star transmission and reception timing.

[0016] Preferably, the main and auxiliary satellite parameters include the positions and velocities of the main and auxiliary satellites in the earth-fixed coordinate system within the observation time, the two-dimensional antenna pattern of the main and auxiliary radar antennas, the target observation region including the latitude and longitude boundaries and the elevation range of the observation region, and the time synchronization error of the dual-satellite system.

[0017] The pulse repetition frequency used in the step S2 is free from interference of the sub-satellite point echo and the transmission pulse pair dual-satellite echo reception, and is greater than the Doppler bandwidth of the mono / dual base SAR.

[0018] Preferably, the step S2 comprises:

[0019] Step S2.1: constructing a pulse repetition frequency workable interval U1 according to the criterion that the main satellite sub-satellite point echo is received at a different time from the main satellite received observation region echo, and the formula is as follows:

[0020]

[0021] wherein H tx represents the altitude of the main satellite, n represents any positive integer, PRF represents the pulse repetition frequency, R tx,min , and R tx,max respectively represent the minimum and maximum values of the slant range from the main satellite to the target region, Δt represents the guard interval time, and c represents the speed of light.

[0022] Step S2.2: constructing a pulse repetition frequency workable interval U2 according to the criterion that the main satellite received observation region echo does not conflict with the transmission pulse time, and the formula is as follows:

[0023]

[0024] wherein m represents any positive integer, PRF represents the pulse repetition frequency, R tx,min , and R tx,max respectively represent the minimum and maximum values of the slant range from the main satellite to the target region, Δt represents the guard interval time, and c represents the speed of light.

[0025] Step S2.3: constructing a pulse repetition frequency workable interval U3 according to the criterion that the main and auxiliary satellite mirror reflection point echo is received at a different time from the auxiliary satellite observation region echo, and the formula is as follows:

[0026]

[0027] wherein k represents any positive integer, R tx , and R rx respectively represent the slant ranges from the main and auxiliary satellites to the target region, min represents the minimum value, and max represents the maximum value; R t_reflect represents the distance from the main satellite to the reflection point, and R r_reflect represents the distance from the main satellite to the reflection point.

[0028] Step S2.4: Construct the pulse repetition frequency workable interval U4 according to the criterion that the direct wave from the primary star to the secondary star is received at a different time from the echo of the observation area of the secondary star, and the formula is as follows:

[0029]

[0030] wherein r tx and r rx respectively represent the positions of the primary and secondary stars in the earth fixed coordinate system, and l represents any positive integer;

[0031] Step S2.5: Construct the pulse repetition frequency workable interval U5 according to the criterion that the pulse repetition frequency is greater than a certain proportion of the imaging Doppler bandwidths of the primary and secondary stars, and the formula is as follows:

[0032] PRF>Q·max(B Doppler,mono , B Doppler,bis )

[0033] wherein B Doppler,mono and B Doppler,bis respectively represent the Doppler bandwidths of the primary and secondary stars, and Q represents an oversampling coefficient;

[0034] Step S2.6: Take the intersection of the above five pulse repetition frequency workable intervals to determine the pulse repetition frequency workable interval U of the simultaneous operation of the single / dual base SAR, and the formula is as follows:

[0035] U=U1∩U2∩U3∩U4∩U5.

[0036] Preferably, the step S2.5 comprises the following substep:

[0037] Step S2.5.1: Calculate the radar slant range of the observation area under the dual base geometry, and the formula is as follows:

[0038] R bi (x,y,z)=R tx (x,y,z)+R rx (x,y,z)

[0039] wherein (x,y,z) is the point target coordinate, (x tx ,y tx ,z tx ), (x rx ,y rx ,z rx ) are respectively the positions of the primary and secondary stars, and R bi (x,y,z) represents the radar slant range of the point target under the dual base geometry, and R tx (x,y,z), R rx (x,y,z) respectively represent the slant ranges of the point target to the primary and secondary stars;

[0040] Step S2.5.2: Set equal interval distance and different Doppler frequencies, carry out bistatic target positioning, calculate Doppler frequency of each point, formula as follows:

[0041]

[0042] Wherein, represents the Doppler frequency corresponding to the mth point target (x m ,y m ,z m ), λ represents wavelength, R tx , R rx respectively represent the line of sight vector of the primary and secondary stars to the point, v tx , v rx respectively represent the velocity vector of the primary and secondary stars relative to the point.

[0043] Step S2.5.3: Convert the radar line of sight from the double star to the target point to the antenna coordinate system, calculate the corresponding transmitting / receiving radar gain of the target point, formula as follows:

[0044]

[0045] Wherein, G tx and G rx respectively represent the transmitting gain and receiving gain, G tx0 , G rx0 respectively represent the peak gain of the transmitting antenna and the receiving antenna, Δφ tx , Δφ rx respectively represent the distance beam width of the transmitting antenna and the receiving antenna, Δθ tx , Δθ rx respectively represent the azimuth beam width of the transmitting antenna and the receiving antenna, (φ tx0 , θ tx0 ), (φ rx0 , θ rx0 ) respectively represent the beam pointing of the transmitting antenna and the receiving antenna; φ tx , φ rx respectively represent the azimuth angle of the primary star and the secondary star, θ tx , θ rx respectively represent the elevation angle of the primary star and the secondary star.

[0046] Step 2.5.4: Calculate the normalized intensity of the secondary star receiving power with Doppler frequency by using the radar equation, formula as follows:

[0047]

[0048] Wherein, represents the secondary star receiving power P rxwith the change of Doppler frequency

[0049] Step S2.5.5: Taking the Doppler frequency in the power spectrum range where -6dB is located as the secondary star Doppler bandwidth at a certain distance, solving the equation for all point targets:

[0050]

[0051] Let the high value of the solution be f d_upper , and the low value be f d_lower , then the slant range R bi_0 corresponding Doppler bandwidth B doppler_0 can be obtained, that is:

[0052]

[0053] Step S2.5.6: Traversing the slant range range calculated in S2.5.1, taking the maximum secondary star Doppler bandwidth corresponding to all distance units as the pulse repetition frequency construction.

[0054] Preferably, the step S4 comprises:

[0055] Step S4.1: According to the received observation area bistatic echo, determine the receiving window time range according to the main star pulse operation timing;

[0056] Step S4.2: According to the high tolerance of time synchronization error, expand the receiving window;

[0057] Step S4.3: Output the secondary star receiving timing.

[0058] According to the present application, a kind of spaceborne single / bistatic SAR spotlight imaging joint timing construction system is provided, comprising:

[0059] Module M1: obtain main and secondary star parameters;

[0060] Module M2: single / bistatic SAR system pulse repetition frequency construction based on double star observation geometry;

[0061] Module M3: spotlight imaging main star monostatic transmission / reception timing construction;

[0062] Module M4: spotlight imaging mode secondary star bistatic reception timing construction;

[0063] Module M5: output main and secondary star transmission and reception timing.

[0064] ​Preferably, the main and auxiliary satellite parameters include the positions and velocities of the main and auxiliary satellites in the earth-fixed coordinate system within the observation time, the two-dimensional antenna pattern of the main and auxiliary radar antennas, the target observation region including the latitude and longitude boundaries and the elevation range of the observation region, and the time synchronization error of the dual-satellite system.

[0065] The pulse repetition frequency used in the module M2 is such that the sub-satellite point echo, the transmission pulse pair and the dual-satellite echo reception are not interfered, and is greater than the Doppler bandwidth of the mono / dual base SAR.

[0066] Preferably, the module M2 comprises:

[0067] Module M2.1: According to the criterion that the main satellite sub-satellite point echo and the main satellite received observation region echo are not received at the same time, the working interval U1 of the pulse repetition frequency is constructed, and the formula is as follows:

[0068]

[0069] Wherein, H tx represents the altitude of the main satellite, n represents any positive integer, PRF represents the pulse repetition frequency, R tx,min , R tx,max respectively represent the minimum and maximum values of the slant range from the main satellite to the target region, Δt represents the guard interval time, and c represents the speed of light.

[0070] Module M2.2: According to the criterion that the main satellite received observation region echo and the transmission pulse time do not conflict, the working interval U2 of the pulse repetition frequency is constructed, and the formula is as follows:

[0071]

[0072] Wherein, m represents any positive integer, PRF represents the pulse repetition frequency, R tx,min , R tx,max respectively represent the minimum and maximum values of the slant range from the main satellite to the target region, Δt represents the guard interval time, and c represents the speed of light.

[0073] Module M2.3: According to the criterion that the main and auxiliary satellite mirror reflection point echo and the auxiliary satellite observation region echo are not received at the same time, the working interval U3 of the pulse repetition frequency is constructed, and the formula is as follows:

[0074]

[0075] Wherein, k represents any positive integer, R tx , R rx respectively represent the slant range from the main and auxiliary satellites to the target region, min represents the minimum value, and max represents the maximum value; R t_reflect represents the distance from the main satellite to the reflection point, and R r_reflect represents the distance from the main satellite to the reflection point.

[0076] Module M2.4: Construct the pulse repetition frequency workable interval U4 with the criterion of receiving the direct wave from the primary star to the secondary star and the echo from the secondary star observation area at different times, and the formula is as follows:

[0077]

[0078] Wherein, r tx and r rx represent the positions of the primary and secondary stars in the earth fixed coordinate system, and l represents any positive integer;

[0079] Module M2.5: Construct the pulse repetition frequency workable interval U5 with the criterion of the pulse repetition frequency being greater than a certain proportion of the imaging Doppler bandwidths of the primary and secondary stars, and the formula is as follows:

[0080] PRF>Q·max(B Doppler,mono , B Doppler,bis )

[0081] Wherein, B Doppler,mono and B Doppler,bis represent the Doppler bandwidths of the primary and secondary stars, and Q represents the oversampling coefficient;

[0082] Module M2.6: Determine the pulse repetition frequency workable interval U of the simultaneous operation of the single / dual base SAR by taking the intersection of the above five pulse repetition frequency workable intervals, and the formula is as follows:

[0083] U=U1∩U2∩U3∩U4∩U5.

[0084] Preferably, the module M2.5 comprises the following sub-steps:

[0085] Module M2.5.1: Calculate the radar slant range of the observation area under the dual base geometry, and the formula is as follows:

[0086] R bi (x,y,z)=R tx (x,y,z)+R rx (x,y,z)

[0087] Wherein, (x,y,z) is the point target coordinate, (x tx ,y tx ,z tx ), (x rx ,y rx ,z rx ) are the positions of the primary and secondary stars respectively, R bi (x,y,z) represents the radar slant range of the point target under the dual base geometry, R tx (x,y,z), R rx (x,y,z) represent the slant ranges of the point target to the primary and secondary stars respectively;

[0088] Module M2.5.2: Set equal interval distance and different Doppler frequencies, carry out bistatic target positioning, calculate Doppler frequencies of each point, the formula is as follows:

[0089]

[0090] Wherein, represents the Doppler frequency corresponding to the mth point target (x m ,y m ,z m ), λ represents the wavelength, R tx , R rx respectively represent the line of sight vector of the primary and secondary stars to the point, v tx , v rx respectively represent the velocity vector of the primary and secondary stars relative to the point.

[0091] Module M2.5.3: Convert the radar line of sight from the double star to the target point to the antenna coordinate system, calculate the corresponding transmitting / receiving radar gain of the target point, the formula is as follows:

[0092]

[0093] Wherein, G tx and G rx respectively represent the transmitting gain and receiving gain, G tx0 , G rx0 respectively represent the peak gain of the transmitting antenna and the receiving antenna, Δφ tx , Δφ rx respectively represent the distance beam width of the transmitting antenna and the receiving antenna, Δθ tx , Δθ rx respectively represent the azimuth beam width of the transmitting antenna and the receiving antenna, (φ tx0 , θ tx0 ), (φ rx0 , θ rx0 ) respectively represent the beam pointing of the transmitting antenna and the receiving antenna; φ tx , φ rx respectively represent the azimuth angle of the primary star and the secondary star, θ tx , θ rx respectively represent the elevation angle of the primary star and the secondary star.

[0094] Step 2.5.4: Calculate the normalized intensity of the secondary star receiving power with Doppler frequency by using the radar equation, the formula is as follows:

[0095]

[0096] Wherein, represents the secondary star receiving power P rxwith the change of Doppler frequency

[0097] Module M2.5.5: for all point targets, solve the equation with Doppler frequency in the power spectrum range where -6dB is located as the auxiliary star Doppler bandwidth under a certain distance:

[0098]

[0099] Let the high value of the solution be f d_upper , and the low value be f d_lower , then the slant range R bi_0 corresponding Doppler bandwidth B doppler_0 can be obtained, that is:

[0100]

[0101] Module M2.5.6: traverse the slant range range calculated in S2.5.1, and take the maximum auxiliary star Doppler bandwidth corresponding to all distance units as the pulse repetition frequency.

[0102] Preferably, the module M4 comprises:

[0103] Module M4.1: according to the received observation area bistatic all echo, determine the receiving window time range according to the main star pulse working time sequence;

[0104] Module M4.2: according to the high tolerance of time synchronization error, expand the receiving window;

[0105] Module M4.3: output auxiliary star receiving time sequence.

[0106] Compared with the prior art, the present application has the following beneficial effects:

[0107] 1. The present application effectively breaks through the single / bistatic SAR joint time sequence construction method, proposes multi-constraint PRF joint optimization, bistatic time sequence collaborative fault tolerance, and space-variable characteristic source suppression measures, solves the problems such as bistatic transmission separation, time-frequency synchronization, and space-variable compensation that the traditional single bistatic SAR time sequence construction method cannot cope with, forms a complete bistatic time sequence construction system, and has higher engineering practical value.

[0108] 2. The present application proposes an auxiliary star dynamic receiving window expansion technology, which adjusts the auxiliary star receiving time sequence adaptively, is compatible with the main star variable PRF mode (such as the change of the pulse time sequence of the spotlight imaging), and has high tolerance to the double star synchronization error. Combined with the modeling of the bistatic geometric gain, the auxiliary star receiving power normalization intensity dynamically matches the main star transmitting signal, so that the system anti-interference ability can be improved.

[0109] ​3. The application actively avoids the conflict of downlink echo and direct wave interference of bistatic radar by five-dimensional PRF constraint interval, reduces error from the source and reduces the dependence on post-processing. The introduced five-dimensional PRF constraint actively avoids the risk of under-sampling through dynamic calculation of Doppler bandwidth, and is compatible with single / bistatic scenarios.

[0110] 4. The application dynamically determines the lower limit of PRF by pre-calculating the space-varying Doppler bandwidth, models the bistatic geometric gain, optimizes the matching of Doppler bandwidth and PRF, and dynamically adjusts PRF by using radar equation and gain model based on real-time calculation of bistatic geometric auxiliary star Doppler bandwidth to avoid spectral aliasing caused by under-sampling and improve the adaptability of high-speed platforms. BRIEF DESCRIPTION OF DRAWINGS

[0111] Other features, objects, and advantages of the application will become more apparent from the following detailed description of non-limiting embodiments with reference to the following drawings:

[0112] Figure 1 The application is a flowchart for joint timing construction of spaceborne single / bistatic SAR spotlight imaging;

[0113] Figure 2 The application is a schematic diagram of the interference of inter-satellite direct wave and main / auxiliary star mirror reflection point echo on auxiliary star imaging;

[0114] Figure 3 The application is a diagram of the interference of inter-satellite direct wave and main / auxiliary star mirror reflection point echo on auxiliary star signal;

[0115] Figure 4 The application is a zebra map for bistatic timing construction. DETAILED DESCRIPTION

[0116] The application will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the application, but do not limit the application in any form. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the application. These all belong to the protection scope of the application.

[0117] The application inputs the position and velocity of the main / auxiliary star in the earth-fixed coordinate system, the antenna pattern, and the target observation area, etc. parameters; based on the system pulse repetition frequency construction of the bistatic observation geometry, the pulse repetition frequency adopted makes the downlink echo and the transmission pulse have no interference on the bistatic echo reception, and is greater than the Doppler bandwidth of single / bistatic SAR; the main star (single base) transmission / reception timing construction of spotlight imaging; the auxiliary star (bistatic) reception timing construction of spotlight imaging mode, the adopted reception window can adapt to the variable pulse repetition frequency of the main star, and has tolerance to the synchronization error of the bistatic radar; the main / auxiliary star transmission and reception timing is output.

[0118] Embodiment 1

[0119] According to the application, a kind of spaceborne single / dual base SAR bunching imaging joint timing construction method is provided, as shown in Figure 1 Including:

[0120] Step S1: obtaining primary and secondary star parameters. The primary and secondary star parameters include the position and velocity of the primary and secondary star in the earth-fixed coordinate system within the observation time, the two-dimensional antenna pattern of the primary and secondary radar antenna, and the target observation region including the latitude and longitude boundary and the elevation range of the observation region, and the double-star time synchronization error.

[0121] Step S2: single / dual base SAR system pulse repetition frequency construction based on double-star observation geometry. The pulse repetition frequency used in the step S2 makes the sub-satellite point echo, transmission pulse pair double-star echo reception without interference, and is greater than single / dual base SAR Doppler bandwidth. The step S2 includes:

[0122] Step S2.1: according to the criterion that the primary star sub-satellite point echo and the primary star received observation region echo are not received at the same time, the pulse repetition frequency working interval U1 is constructed, and the formula is as follows:

[0123]

[0124] Wherein, H tx Indicates the height of the primary star satellite, n indicates any positive integer, PRF indicates the pulse repetition frequency, R tx,min , R tx,max Respectively represent the minimum and maximum of the slant range of the primary star to the target region, Δt represents the guard interval time (determined by radar signal pulse width τ pulse And switching protection time), c represents the speed of light.

[0125] Step S2.2: according to the criterion that the primary star received observation region echo and transmission pulse time do not conflict, the pulse repetition frequency working interval U2 is constructed, and the formula is as follows:

[0126]

[0127] Wherein, m indicates any positive integer, PRF indicates the pulse repetition frequency, R tx,min , R tx,max Respectively represent the minimum and maximum of the slant range of the primary star to the target region, Δt represents the guard interval time (determined by radar signal pulse width τ pulse And switching protection time), c represents the speed of light.

[0128] Step S2.3: according to the criterion that the primary and secondary star mirror image reflection point echo and the secondary star observation region echo are not received at the same time, the pulse repetition frequency working interval U3 is constructed, and the formula is as follows:

[0129]

[0130] where k represents an arbitrary positive integer, PRF represents the pulse repetition frequency, R tx , R rx respectively represent the slant ranges of the primary and secondary satellites to the target area, min represents the minimum value, max represents the maximum value, and c represents the speed of light. t_reflect R r_reflect represents the distance from the primary satellite to the reflection point, and R tx represents the distance from the primary satellite to the reflection point, and it can be known from the geometric relationship that:

[0131]

[0132] where H rx represents the satellite height of the primary satellite, H ground represents the satellite height of the secondary satellite, and d tx represents the ground distance between the two satellites. For the bistatic SAR system of the translation-invariant configuration, the pulse repetition frequency operating interval U3 is relatively stable due to the relatively fixed relative position of the two satellites, but for other bistatic SAR systems, U3 is dynamically changed, and the timing construction must be calculated multiple times according to the motion of the two bases under this configuration.

[0133] Specifically, for the secondary satellite to receive the echo, the influence of the mirror reflection point echo of the primary satellite needs to be considered. The principle of this influence is the same as the ground point echo interference received by the primary satellite, which is the interference signal of the transmitted signal reflected by the ground to the secondary satellite.

[0134] Step S2.4: According to the criterion that the direct wave from the primary satellite to the secondary satellite is different from the echo of the observation area of the secondary satellite, the pulse repetition frequency operating interval U4 is constructed, and the formula is as follows:

[0135]

[0136] where r tx and r rx respectively represent the positions of the primary and secondary satellites in the earth-fixed coordinate system, l represents an arbitrary positive integer, PRF represents the pulse repetition frequency, R tx , R rx respectively represent the slant ranges of the primary and secondary satellites to the target area, min represents the minimum value, max represents the maximum value, and c represents the speed of light.

[0137] Step S2.5: According to the criterion that the pulse repetition frequency is greater than a certain proportion of the imaging Doppler bandwidths of the primary and secondary satellites, the pulse repetition frequency operating interval U5 is constructed, and the formula is as follows:

[0138] PRF>Q·max(B Doppler,mono , B Doppler,bis )

[0139] where B Doppler,mono and B Doppler,bis represent the primary / secondary satellite Doppler bandwidth, and Q represents the oversampling factor.

[0140] Step S2.6: Take the intersection of the above five pulse repetition frequency workable intervals, determine the pulse repetition frequency workable interval U of the simultaneous operation of the mono- / bi-static SAR, and the formula is as follows:

[0141] U = U1∩U2∩U3∩U4∩U5.

[0142] The calculation method of the secondary satellite Doppler imaging bandwidth in the step S2.5 includes the following sub-steps:

[0143] Step S2.5.1: Calculate the radar slant range of the observation area under the bi-static geometry; let (x, y, z) be the point target coordinates, (x tx ,y tx ,z tx ), and (x rx ,y rx ,z rx ) be the positions of the primary and secondary satellites, respectively, to obtain the radar slant range R bi (x, y, z) of the point target under the bi-static geometry, and the formula is as follows:

[0144] R bi (x, y, z) = R tx (x, y, z) + R rx (x, y, z)

[0145] where R tx (x, y, z) and R rx (x, y, z) represent the slant ranges of the point target to the primary and secondary satellites, respectively, i.e.

[0146]

[0147] Step S2.5.2: Set equal-interval distances and different Doppler frequencies, perform bi-static target positioning, and calculate the Doppler frequencies of the points. Let k equal-interval point targets be set on an equal-slant range line in the observation area, and the slant range is R bi_0 , then the Doppler frequency corresponding to the mth (0 < m ≤ k, m ∈ N + ) point target (x m ,y m ,z m ) can be obtained, i.e.

[0148]

[0149] where R tx , R rxrespectively denote the line-of-sight vectors of the primary and secondary satellites to the point, v tx , rx respectively denote the velocity vectors of the primary and secondary satellites relative to the point, and λ denotes the wavelength.

[0150] Step S2.5.3: Convert the radar line-of-sight of the binary star to the target point to the antenna coordinate system, and calculate the corresponding transmitting / receiving radar gain of the target point. Taking the point target in step S2.5.2 as an example, convert the radar line-of-sight vectors R tx , R rx to the antenna coordinate system, and then the azimuth angles φ tx , φ rx and the elevation angles θ tx , θ rx of the primary and secondary satellites can be obtained, that is,

[0151]

[0152] Further, the transmitting gain G tx and the receiving gain G rx can be obtained, that is,

[0153]

[0154] where G tx0 , G rx0 denote the peak gains of the transmitting antenna and the receiving antenna, Δφ tx , Δφ rx denote the distance beam widths of the transmitting antenna and the receiving antenna, Δθ tx , Δθ rx denote the azimuth beam widths of the transmitting antenna and the receiving antenna, and (φ tx0 , θ tx0 ), (φ rx0 , θ rx0 ) denote the beam pointing directions of the transmitting antenna and the receiving antenna.

[0155] Step S2.5.4: Calculate the normalized intensity of the secondary satellite receiving power with respect to the Doppler frequency by using the radar equation. Taking the point target in step S2.5.2 as an example, the change of the secondary satellite receiving power P rx with respect to the Doppler frequency can be obtained by the radar equation as

[0156]

[0157] where P tx is the transmitting power, σ bi is the bistatic ground scattering coefficient, and L sys is the system loss. After calculating the receiving powers of all k target points, the normalized intensity I normi.e.

[0158]

[0159] Step S2.5.5: Doppler frequency in the power spectrum range where -6dB is located is taken as the secondary star Doppler bandwidth at a certain distance; for all k point targets, solve the equation:

[0160]

[0161] Let the high value of the solution be f d_upper , and the low value be f d_lower , then the slant range R bi_0 corresponding Doppler bandwidth B doppler_0 can be obtained, i.e.

[0162]

[0163] Step S2.5.6: Traverse the slant range range calculated in S2.5.1 to take the maximum secondary star Doppler bandwidth corresponding to all distance units as the pulse repetition frequency construction. The traversal process can be represented as:

[0164]

[0165] Wherein, R bi_min , R bi_max represent the minimum slant range and the maximum slant range respectively.

[0166] Step S3: Construction of the transmit / receive timing of the main star (monostatic) in the beamforming imaging mode. The construction of the transmit / receive timing of the main star in the beamforming imaging mode in the step S3 is the same as that of the conventional monostatic SAR system, mainly considering the echo interference of the main star ground point and the conflict of the main star transmit / receive signals.

[0167] Step S4: Construction of the receive timing of the secondary star (bistatic) in the beamforming imaging mode. The receive window adopted can adapt to the variable pulse repetition frequency of the main star and has tolerance to the synchronization error of the dual stars. The construction of the receive timing of the secondary star (bistatic) in the beamforming imaging mode in the step S4, while taking into account the construction of the main star timing, also needs to consider the influence of direct wave signals and echo interference of main-secondary star mirror reflection points, including:

[0168] Step S4.1: According to the main star pulse operation timing, determine the time range of the receive window based on the criterion of receiving all echoes in the bistatic observation area.

[0169] Step S4.2: Extend the receive window based on the criterion of high tolerance to time synchronization error.

[0170] Step S4.3: Output the receive timing of the secondary star.

[0171] Step S5: output the main and auxiliary satellite transmission and reception timing. The main and auxiliary satellite transmission and reception timing output in the step S5 needs to meet the main satellite signal transmission and reception without conflict and the double satellite echo reception without interference.

[0172] Further, the spaceborne single / double base SAR spotlight imaging joint timing construction method is described in detail in combination with the drawings and simulation results, and the effectiveness of the present application is verified by using simulation data.

[0173] The simulation parameters are shown in Table 1, τ pulse represents the radar signal pulse width. It is assumed that the double satellite system is a shift-invariant configuration (i.e. the relative position of the double satellites remains unchanged) and works on the same orbital plane.

[0174] Table 1

[0175] Parameter name Value Satellite altitude 627 km Satellite spacing 250 km Duty cycle 5％ Guard time 0.05τ pulse ]]

[0176] Figure 2 The direct wave interference and mirror reflection point echo interference that the auxiliary satellite may suffer from are shown. When the simulation data used is a shift-invariant configuration, and the PRF is 3500 Hz without considering the auxiliary satellite timing selection, obvious interference bright lines can be seen in the auxiliary image after range compression (such as Figure 3 ). For single base SAR imaging, the selection of PRF can use the interlaced strip chart diagram, i.e. zebra chart or diamond chart, which is represented by the intersection of the sub-satellite point echo band and the transmission pulse shadow band in the two-dimensional plane of the ground distance from the sub-satellite point-pulse repetition frequency. When the transmission pulse, the sub-satellite point pulse does not fall within the echo window, i.e. the selection of PRF and the ground distance from the sub-satellite point is in the blank "large diamond area" in the zebra chart, the SAR can work normally. For the selection and construction of the transmission and reception timing of the double base SAR system, in addition to considering the sub-satellite point echo band (corresponding to U1) and the transmission pulse shadow band (corresponding to U2) that affect the main satellite imaging, the mirror reflection point echo band (corresponding to U3) and the direct wave band (corresponding to U4) that affect the auxiliary satellite imaging also need to be considered. The PRF is selected in the white diamond area shown in Figure 4 where all the four groups of bands do not affect, while meeting the criterion that the pulse repetition frequency is greater than a certain proportion of the Doppler bandwidth of the main and auxiliary satellite imaging (corresponding to U5).

[0177] Embodiment 2

[0178] The present application also provides a spaceborne single / double base SAR spotlight imaging joint timing construction system, which can be realized by executing the flow steps of the spaceborne single / double base SAR spotlight imaging joint timing construction method, i.e. the spaceborne single / double base SAR spotlight imaging joint timing construction method can be understood by those skilled in the art as the preferred embodiment of the spaceborne single / double base SAR spotlight imaging joint timing construction system.

[0179] According to the application, a spaceborne monostatic / bistatic SAR spotlight imaging joint timing construction system is provided, comprising:

[0180] Module M1: obtaining primary and secondary satellite parameters. The primary and secondary satellite parameters include the positions and velocities of the primary and secondary satellites in the earth-fixed coordinate system within the observation time, the two-dimensional antenna pattern of the primary and secondary radar antennas, the target observation region including the latitude and longitude boundaries and the elevation range of the observation region, and the time synchronization error of the dual satellites.

[0181] Module M2: constructing the pulse repetition frequency of the monostatic / bistatic SAR system based on the observation geometry of the dual satellites. The pulse repetition frequency used in the module M2 makes the subspace point echo, the transmission pulse, and the dual satellite echo reception interference-free, and is greater than the Doppler bandwidth of the monostatic / bistatic SAR. The module M2 comprises:

[0182] Module M2.1: constructing the pulse repetition frequency workable interval U1 based on the criterion that the primary satellite subspace point echo and the primary satellite received observation region echo are received at different times, and the formula is as follows:

[0183]

[0184] wherein H tx represents the altitude of the primary satellite, n represents any positive integer, PRF represents the pulse repetition frequency, R tx,min , and R tx,max respectively represent the minimum and maximum values of the slant range from the primary satellite to the target region, Δt represents the guard interval time, and c represents the speed of light.

[0185] Module M2.2: constructing the pulse repetition frequency workable interval U2 based on the criterion that the primary satellite received observation region echo and the transmission pulse time do not conflict, and the formula is as follows:

[0186]

[0187] wherein m represents any positive integer, PRF represents the pulse repetition frequency, R tx,min , and R tx,max respectively represent the minimum and maximum values of the slant range from the primary satellite to the target region, Δt represents the guard interval time, and c represents the speed of light.

[0188] Module M2.3: constructing the pulse repetition frequency workable interval U3 based on the criterion that the primary and secondary satellite mirror reflection point echo and the secondary satellite observation region echo are received at different times, and the formula is as follows:

[0189]

[0190] wherein k represents any positive integer, R tx , and R rxrespectively represent the slant ranges from the primary and secondary satellites to the target region, min represents the minimum value, and max represents the maximum value. R t_reflect represents the distance from the primary satellite to the reflection point, R r_reflect represents the distance from the primary satellite to the reflection point.

[0191] Module M2.4: based on the criterion that the direct wave from the primary satellite to the secondary satellite is received at a different time from the echo of the observation region of the secondary satellite, a workable interval U4 of the pulse repetition frequency is constructed, and the formula is as follows:

[0192]

[0193] wherein r tx and r rx respectively represent the positions of the primary and secondary satellites in the earth-fixed coordinate system, and I represents an arbitrary positive integer.

[0194] Module M2.5: based on the criterion that the pulse repetition frequency is greater than a certain proportion of the imaging Doppler bandwidths of the primary and secondary satellites, a workable interval U5 of the pulse repetition frequency is constructed, and the formula is as follows:

[0195] PRF>Q·max(B Doppler,mono , B Doppler,bis )

[0196] wherein B Doppler,mono and B Doppler,bis respectively represent the Doppler bandwidths of the primary and secondary satellites, and Q represents an oversampling coefficient. The module M2.5 includes the following sub-steps: Module M2.5.1: the slant range range of the observation region radar under the bistatic geometry is calculated, and the formula is as follows: R bi (x,y,z) = R tx (x,y,z) + R rx (x,y,z), wherein (x,y,z) is the point target coordinate, (x tx ,y tx ,z tx ), and (x rx ,y rx ,z rx ) are the positions of the primary and secondary satellites, respectively, R bi (x,y,z) represents the radar slant range of the point target under the bistatic geometry, and R tx (x,y,z), R rx (x,y,z) respectively represent the slant ranges of the point target to the primary and secondary satellites. Module M2.5.2: an equal-interval distance and different Doppler frequencies are set, the bistatic target positioning is performed, and the Doppler frequency of each point is calculated, and the formula is as follows:

[0197] wherein, represents the Doppler frequency of the mth point target (x m ,y mz m ) corresponding Doppler frequency, λ represents wavelength, R tx , R rx respectively represent the line-of-sight vector of the primary and secondary stars to the point, v tx , v rx respectively represent the velocity vector of the primary and secondary stars relative to the point. Module M2.5.3: Convert the radar line-of-sight of the binary star to the target point to the antenna coordinate system, calculate the corresponding transmit / receive radar gain of the target point, the formula is as follows:

[0198]

[0199] Wherein, G tx and G rx respectively represent the transmit gain and receive gain, G tx0 , G rx0 respectively represent the peak gain of the transmit antenna and the receive antenna, Δφ tx , Δφ rx respectively represent the distance beam width of the transmit antenna and the receive antenna, Δθ tx , Δθ rx respectively represent the azimuth beam width of the transmit antenna and the receive antenna, (φ tx0 , θ tx0 ), (φ rx0 , θ rx0 ) respectively represent the beam pointing of the transmit antenna and the receive antenna. φ tx , φ rx respectively represent the azimuth angle of the primary star and the secondary star, θ tx , θ rx respectively represent the elevation angle of the primary star and the secondary star. Step 2.5.4: Use the radar equation to calculate the normalized intensity of the secondary star receive power with Doppler frequency, the formula is as follows: Wherein, represents the change of the secondary star receive power P rx with Doppler frequency obtained by the radar equation. Module M2.5.5: Take the Doppler frequency within the power spectrum range of-6dB as the secondary star Doppler bandwidth under a certain distance, solve the equation for all point targets: Let the high value of the solution be f d_upper , and the low value be f d_lower , then the corresponding Doppler bandwidth B doppler_0 of the slant range R bi_0 can be obtained, that is: Module M2.5.6: Traverse the slant range calculated in S2.5.1, take the maximum secondary star Doppler bandwidth corresponding to all distance units as the pulse repetition frequency.

[0200] Module M2.6: Taking the intersection of the above five pulse repetition frequency workable intervals, determine the pulse repetition frequency workable interval U of the simultaneous operation of the mono- / bi-static SAR, the formula is as follows:

[0201] U=U1∩U2∩U3∩U4∩U5.

[0202] Module M3: Construct the mono-static transmission / reception time sequence of the main star of the spotlight imaging mode.

[0203] Module M4: Construct the bi-static reception time sequence of the auxiliary star of the spotlight imaging mode. The module M4 comprises: Module M4.1: According to the mono-static transmission time sequence of the main star, determine the time range of the reception window. Module M4.2: Extend the reception window according to the high tolerance of the time synchronization error. Module M4.3: Output the bi-static reception time sequence of the auxiliary star.

[0204] Module M5: Output the transmission and reception time sequence of the main and auxiliary stars.

[0205] Those skilled in the art know that, in addition to implementing the system provided by the present application and each device, module, unit thereof in the form of pure computer readable program code, the same function can also be realized by logically programming the method steps in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers and embedded microcontrollers. Therefore, the system provided by the present application and each device, module, unit thereof can be considered as a hardware component, and the devices, modules, units included therein for realizing various functions can also be considered as structures within the hardware component; the devices, modules, units for realizing various functions can also be considered as both software modules realizing the method and structures within the hardware component.

[0206] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essential content of the present application. The embodiments of the present application and the features in the embodiments can be combined with each other in any way without conflict.

Claims

1. A method for joint time series construction of spaceborne monostatic / bistatic SAR spotlight imaging, characterized in that: include: Step S1: Obtain the parameters of the main and auxiliary satellites; Step S2: constructing the pulse repetition frequency of the monostatic / bistatic SAR system based on the dual-satellite observation geometry; Step S3: constructing the transmission / reception timing of the single-base primary satellite for spotlight imaging; Step S4: constructing the auxiliary satellite dual-base receiving timing in the spotlight imaging mode; Step S5: Output the transmission and reception timing of the main and auxiliary satellites.

2. The method for joint time series construction of spaceborne monostatic / bistatic SAR spotlight imaging according to claim 1, characterized in that: The primary and secondary satellite parameters include the position and velocity of the primary and secondary satellites in the Earth-fixed coordinate system during the observation time, the two-dimensional antenna patterns of the primary and secondary radar antennas, the target observation area including the latitude and longitude boundaries and elevation range of the observation area, and the dual-satellite time synchronization error; The pulse repetition frequency used in step S2 ensures that the sub-satellite point echo and the transmitted pulse do not interfere with the reception of the dual-satellite echo, and is greater than the Doppler bandwidth of the monostatic / bistatic SAR.

3. The method for joint time series construction of spaceborne monostatic / bistatic SAR spotlight imaging according to claim 1, characterized in that: The step S2 comprises: Step S2.1: Based on the principle that the echo from the subsatellite point and the echo from the main satellite receiving observation area are not received simultaneously, the pulse repetition frequency operating range U1 is constructed, and the formula is as follows: Among them, H tx represents the altitude of the primary satellite, n represents any positive integer, PRF represents the pulse repetition frequency, R tx,min 、R tx,max They represent the minimum and maximum slant distances from the primary satellite to the target area, Δt represents the protection interval time, and c represents the speed of light; Step S2.2: Based on the principle that the time of receiving the echo in the observation area by the primary satellite does not conflict with the time of transmitting the pulse, the pulse repetition frequency working range U2 is constructed. The formula is as follows: Where m represents any positive integer, PRF represents the pulse repetition frequency, R tx,min 、R tx,max They represent the minimum and maximum slant distances from the primary satellite to the target area, Δt represents the protection interval time, and c represents the speed of light; Step S2.3: Based on the principle that the echoes from the main and auxiliary satellites’ mirror reflection points and the echoes from the auxiliary satellites’ observation area are not received simultaneously, the pulse repetition frequency operating range U3 is constructed. The formula is as follows: Where k represents any positive integer, R tx 、R rx Respectively represent the slant distances from the main and auxiliary satellites to the target area, min represents the minimum value, and max represents the maximum value; R t_reflect Indicates the distance from the main star to the reflection point, R r_reflect Indicates the distance from the main star to the reflection point; Step S2.4: Based on the principle that the direct wave from the primary satellite to the auxiliary satellite and the echo from the auxiliary satellite observation area are not received at the same time, the pulse repetition rate working range U4 is constructed, and the formula is as follows: Among them, r tx and r rx They represent the positions of the primary and secondary satellites in the Earth's fixed coordinate system, and l represents any positive integer; Step S2.5: Based on the pulse repetition frequency being greater than a certain ratio of the primary and secondary satellite imaging Doppler bandwidths, construct the pulse repetition frequency operating range U5, as follows: PRF>Q·max(B Doppler,mono ,B Doppler,bis ) Among them, B Doppler,mono and B Doppler,bis They represent the Doppler bandwidths of the primary and secondary satellites respectively, and Q represents the oversampling coefficient; Step S2.6: Take the intersection of the five pulse repetition frequency operable ranges to determine the pulse repetition frequency operable range U for simultaneous monostatic / bistatic SAR operation. The formula is as follows: U=U1∩U2∩U3∩U4∩U5.

4. The method for joint time series construction of spaceborne monostatic / bistatic SAR spotlight imaging according to claim 1, characterized in that: The step S2.5 includes the following sub-steps: Step S2.5.1: Calculate the radar slant range of the observation area under bistatic geometry using the following formula: R bi (x,y,z)=R tx (x,y,z)+R rx (x,y,z) Among them, (x, y, z) is the coordinate of the point target, (x tx ,y tx ,z tx )、(x rx ,y rx ,z rx ) are the positions of the main and auxiliary stars, R bi (x, y, z) represents the radar slant range of the point target under bistatic geometry, R tx (x,y,z), R rx (x, y, z) represent the slant distances from the point target to the primary and auxiliary satellites respectively; Step S2.5.2: Set equal intervals and different Doppler frequencies to perform dual-base target positioning. Calculate the Doppler frequency at each point using the following formula: in, Represents the mth point target (x m ,y m ,z m ) corresponds to the Doppler frequency, λ represents the wavelength, R tx 、R rx Respectively represent the sight vectors from the main satellite and the auxiliary satellite to the point, v tx 、v rx Respectively represent the velocity vectors of the main and auxiliary satellites relative to the point; Step S2.5.3: Convert the radar line of sight from the dual satellites to the target point to the antenna coordinate system and calculate the transmit / receive radar gain corresponding to the target point using the following formula: Among them, G tx and G rx Represents the transmit gain and receive gain respectively, G tx0 , G rx0 denote the peak gains of the transmitting antenna and the receiving antenna, Δφ tx , Δφ rx Denote the distance to the transmitting antenna and the receiving antenna, respectively, and the beam width, Δθ tx , Δθ rx represent the azimuth beamwidths of the transmitting and receiving antennas, respectively, (φ tx0 ,θ tx0 )、(φ rx0 ,θ rx0 ) represent the beam pointing of the transmitting antenna and the receiving antenna respectively; φ tx ,φ rx Represent the azimuths of the primary and secondary stars, θ tx ,θ rx Respectively represent the pitch angles of the primary and auxiliary satellites; Step 2.5.4: Use the radar equation to calculate the normalized intensity of the auxiliary satellite's received power versus Doppler frequency. The formula is as follows: in, represents the auxiliary satellite receiving power P obtained by the radar equation rx With Doppler frequency changes; Step S2.5.5: Take the Doppler frequency within the power spectrum range of -6 dB as the auxiliary satellite Doppler bandwidth at a specific distance, and solve the equation for all point targets: Let the high value of the solution be f d_upper , the low value is f d_lower , then the slope distance R can be obtained bi_0 The corresponding Doppler bandwidth B doppler_0 ,Right now: Step S2.5.6: Traverse the slant range calculated in S2.5.1 and use the maximum auxiliary satellite Doppler bandwidth corresponding to all range cells as the pulse repetition frequency.

5. The method for joint time series construction of spaceborne monostatic / bistatic SAR spotlight imaging according to claim 1, characterized in that: The step S4 comprises: Step S4.1: Based on the principle of receiving all echoes from the two bases in the observation area and according to the pulse operation sequence of the primary satellite, determine the receiving window time range; Step S4.2: Expand the receiving window based on the high tolerance of time synchronization error; Step S4.3: Output auxiliary satellite reception timing.

6. A spaceborne monostatic / bistatic SAR spotlight imaging joint time sequence construction system, characterized by: include: Module M1: Get the parameters of main and auxiliary satellites; Module M2: Pulse repetition frequency construction of monostatic / bistatic SAR system based on dual-satellite observation geometry; Module M3: Construction of the transmission / reception timing of the single-base primary satellite for spotlight imaging; Module M4: Construction of auxiliary satellite dual-base receiving timing in spotlight imaging mode; Module M5: Outputs the transmission and reception timing of the main and auxiliary satellites.

7. The spaceborne monostatic / bistatic SAR spotlight imaging joint time sequence construction system according to claim 6, characterized in that: The primary and secondary satellite parameters include the position and velocity of the primary and secondary satellites in the Earth-fixed coordinate system during the observation time, the two-dimensional antenna patterns of the primary and secondary radar antennas, the target observation area including the latitude and longitude boundaries and elevation range of the observation area, and the dual-satellite time synchronization error; The pulse repetition frequency used in the module M2 ensures that the sub-satellite point echo and the transmitted pulse do not interfere with the reception of the dual-satellite echo, and is greater than the Doppler bandwidth of the monostatic / bistatic SAR.

8. The spaceborne monostatic / bistatic SAR spotlight imaging joint time sequence construction system according to claim 6, characterized in that: The module M2 includes: Module M2.1: Based on the principle that the echo from the subsatellite point and the echo from the observation area of ​​the main satellite are not received simultaneously, the pulse repetition frequency operating range U1 is constructed. The formula is as follows: Among them, H tx represents the altitude of the primary satellite, n represents any positive integer, PRF represents the pulse repetition frequency, R tx,min 、R tx,max They represent the minimum and maximum slant distances from the primary satellite to the target area, Δt represents the protection interval time, and c represents the speed of light; Module M2.2: Based on the principle that the time of receiving the echo in the observation area by the primary satellite does not conflict with the time of transmitting the pulse, the pulse repetition frequency working range U2 is constructed. The formula is as follows: Wherein, m represents any positive integer; Module M2.3: Based on the principle that the echoes from the mirror reflection points of the main and auxiliary satellites and the echoes from the observation area of ​​the auxiliary satellites are not received simultaneously, the pulse repetition frequency operating range U3 is constructed. The formula is as follows: Where k represents any positive integer, R tx 、R rx Respectively represent the slant distances from the main and auxiliary satellites to the target area, min represents the minimum value, and max represents the maximum value; R t_reflect Indicates the distance from the main star to the reflection point, R r_reflect Indicates the distance from the main star to the reflection point; Module M2.4: Based on the principle that the direct wave from the primary satellite to the auxiliary satellite and the echo from the auxiliary satellite observation area are not received at the same time, the pulse repetition rate working range U4 is constructed. The formula is as follows: Among them, r tx and r rx They represent the positions of the primary and secondary satellites in the Earth's fixed coordinate system, and l represents any positive integer; Module M2.5: Based on the pulse repetition frequency being greater than a certain ratio of the primary and secondary satellite imaging Doppler bandwidths, the pulse repetition frequency operating range U5 is constructed. The formula is as follows: PRF>Q·max(B Doppler,mono ,B Doppler,bis ) Among them, B Doppler,mono and B Doppler,bis They represent the Doppler bandwidths of the primary and secondary satellites respectively, and Q represents the oversampling coefficient; Module M2.6: Take the intersection of the five pulse repetition frequency operating ranges mentioned above and determine the pulse repetition frequency operating range U for simultaneous monostatic / bistatic SAR operation. The formula is as follows: U=U1∩U2∩U3∩U4∩U5.

9. The spaceborne monostatic / bistatic SAR spotlight imaging joint time sequence construction system according to claim 6, characterized in that: The module M2.5 includes the following sub-steps: Module M2.5.1: Calculate the radar slant range of the observation area in bistatic geometry using the following formula: R bi (x,y,z)=R tx (x,y,z)+R rx (x,y,z) Among them, (x, y, z) is the coordinate of the point target, (x tx ,y tx ,z tx )、(x rx ,y rx ,z rx ) are the positions of the main and auxiliary stars, R bi (x, y, z) represents the radar slant range of the point target under bistatic geometry, R tx (x,y,z), R rx (x, y, z) represent the slant distances from the point target to the primary and auxiliary satellites respectively; Module M2.5.2: Set equal intervals and different Doppler frequencies to perform dual-base target positioning and calculate the Doppler frequency at each point. The formula is as follows: in, Represents the mth point target (x m ,y m ,z m ) corresponds to the Doppler frequency, λ represents the wavelength, R tx 、R rx Respectively represent the sight vectors from the main satellite and the auxiliary satellite to the point, v tx 、v rx Respectively represent the velocity vectors of the main and auxiliary satellites relative to the point; Module M2.5.3: Convert the radar line of sight from the dual satellites to the target point to the antenna coordinate system and calculate the transmit / receive radar gain corresponding to the target point using the following formula: Among them, G tx and G rx Represents the transmit gain and receive gain respectively, G tx0 , G rx0 denote the peak gains of the transmitting antenna and the receiving antenna, Δφ tx , Δφ rx Denote the distance to the transmitting antenna and the receiving antenna, respectively, and the beam width, Δθ tx , Δθ rx represent the azimuth beamwidths of the transmitting and receiving antennas, respectively, (φ tx0 ,θ tx0 )、(φ rx0 ,θ rx0 ) represent the beam pointing of the transmitting antenna and the receiving antenna respectively; φ tx ,φ rx Represent the azimuths of the primary and secondary stars, θ tx ,θ rx Respectively represent the pitch angles of the primary and auxiliary satellites; Step 2.5.4: Use the radar equation to calculate the normalized intensity of the auxiliary satellite's received power versus Doppler frequency. The formula is as follows: in, represents the auxiliary satellite receiving power P obtained by the radar equation rx With Doppler frequency changes; Module M2.5.5: Take the Doppler frequency within the power spectrum of -6dB as the auxiliary satellite Doppler bandwidth at a specific distance and solve the equation for all point targets: Let the high value of the solution be f d_upper , the low value is f d_lower , then the slope distance R can be obtained bi_0 The corresponding Doppler bandwidth B doppler_0 ,Right now: Module M2.5.6: Traverse the slant range calculated in S2.5.1 and construct the pulse repetition frequency using the maximum auxiliary satellite Doppler bandwidth corresponding to all range units.

10. The spaceborne monostatic / bistatic SAR spotlight imaging joint time sequence construction system according to claim 6, characterized in that: The module M4 includes: Module M4.1: Based on the principle of receiving all echoes from the two bases in the observation area and according to the pulse operation sequence of the primary satellite, the receiving window time range is determined; Module M4.2: Expand the receiving window based on the high tolerance of time synchronization error; Module M4.3: Output auxiliary satellite reception timing.

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