Space-borne bistatic sar imaging system and method

Abstract

The application provides a spaceborne bistatic SAR imaging system and method, comprising: a transmitting signal adopts a coded signal pulse sequence as a periodic transmitting sequence, and the relationship between any pulses is orthogonal; a pulse repetition frequency is set based on a bistatic configuration, and the adopted pulse repetition frequency should make the space-to-space direct wave and the bistatic SAR ground echo be received simultaneously, and the corresponding radar transmitting signal waveforms of the two are different; based on the setting of the coded signal pulse sequence and the setting of the pulse repetition frequency, the space-to-space direct wave and the bistatic SAR echo of an observation area which are mixed together in time are obtained, and the space-to-space direct wave and the bistatic radar echo signal obtained at the same time are orthogonal; based on the obtained space-to-space direct wave and the bistatic SAR echo of the observation area which are mixed together in time, the space-to-space direct wave and the bistatic SAR echo of the observation area are extracted respectively, and the function of spaceborne bistatic SAR imaging under space-to-space direct wave suppression is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of spaceborne radar, in particular to a spaceborne bistatic SAR imaging system and method. BACKGROUND

[0002] The spaceborne bistatic SAR separates the transmitter and receiver on different satellite platforms, and solves the problem of forward-looking and backward-looking imaging which cannot be realized by the traditional spaceborne SAR system through the separation of the transmitting and receiving antennas. Although the bistatic SAR has many advantages, the birth of the new system structure also causes many complex technical problems, among which the synchronization technology is the most critical, and the use of direct wave to realize time-frequency synchronization is the most common synchronization technology. It should be pointed out that for the existing bistatic SAR system, the use of direct wave to realize time-frequency synchronization needs to establish a receiving channel of direct wave between the transmitter and receiver outside the imaging channel, which means that the transmitter sends the same direct wave as the imaging signal to the receiver through a special antenna in addition to the imaging antenna, and the receiver uses the received direct wave to focus the echo signal to obtain the SAR image. This will increase the cost of developing the spaceborne radar system, and also increase the power consumption, weight, etc. of the whole satellite, and bring certain difficulties to the satellite system design.

[0003] The document "TanDEM-X: A satellite formation for high-resolution SAR interferometry" (IEEE Transactions on Geoscience and Remote Sensing, vol. 45, no. 11, pp. 3317-3341) introduces a bistatic SAR phase synchronization method suitable for close-range formation, and the two satellites realize phase synchronization through interrupting pulse transmission and transmitting the signal through the inter-satellite synchronization antenna. But the method proposed in the document will face problems such as large size of synchronization antenna and large power consumption when applied to bistatic SAR system in long-range formation.

[0004] The document "An Advanced Phase Synchronization Scheme for LT-1" (IEEE Transactions on Geoscience and Remote Sensing, VOL. 58, NO. 3, pp. 1735-1746) proposes an advanced non-interruptive phase synchronization scheme for bistatic synthetic aperture radar systems. The scheme avoids data acquisition interruption, improves synchronization accuracy, and prevents data loss by exchanging synchronization pulses before radar signal transmission and after echo reception. However, the method requires a phase synchronization antenna for phase synchronization pulse transmission, which poses challenges such as large antenna size and high power consumption for long-range formation bistatic SAR systems.

[0005] The document "Nadir Echo Removal in Synthetic Aperture Radar via Waveform Diversity and Dual-Focus Postprocessing" (M. Villano et al., IEEE Geoscience and Remote Sensing Letters, vol. 15, no. 5, pp. 719-723) achieves nadir echo removal by waveform coding and dual-focus post-processing of received signals. This technique not only improves image quality but also can be used for range ambiguity suppression in a similar manner. However, the document does not address how to apply coded signals to bistatic SAR imaging synchronization.

[0006] The document "Experimental Demonstration of Nadir Echo Removal in SAR Using Waveform Diversity and Dual-Focus Postprocessing" (Jeon SY et al., IEEE Geoscience and Remote Sensing Letters, 2021, 19) presents an experiment based on the TerraSAR-X satellite of Germany for integrated reception and separation of nadir echoes and imaging echoes. The experiment uses up-and-down linear frequency modulation for orthogonal coding, further achieving orthogonal and simultaneous reception of nadir echoes and imaging echoes. Matched filtering and inverse matched filtering are then used to separate and extract the two types of echoes. However, the document does not address how to apply coded signals to bistatic SAR imaging synchronization.

[0007] Patent document CN112799056A (application number: 202011584728.5) discloses a spaceborne radar altimeter method, sets the coded signal pulse sequence as a periodic transmission sequence, and controls the pulse repetition frequency so that the nadir echo and the interference side high area echo are received by the radar receiver at the same time, and the radar transmission signal waveforms corresponding to the nadir echo and the interference side high area echo are different, the bottom view radar altimeter echo and the side view radar altimeter echo overlapped in time are obtained, the bottom view radar altimeter echo and the side view radar altimeter echo signal are extracted respectively, and the bottom view altimeter and the side view altimeter height measurement function are realized. However, this patent has not yet involved how to apply the coded signal to the bistatic SAR imaging synchronization.

[0008] The document "An Innovative Push-To-Talk (PTT) Synchronization Scheme for Distributed SAR" (IEEE Transactions on Geoscience and Remote Sensing, VOL. 60, 2022) proposes an innovative Push-To-Talk (PTT) synchronization scheme for the time and frequency synchronization problem in bistatic / multistatic synthetic aperture radar (Bi- / M-SAR) systems. This scheme uses a two-segment frequency rate (TSFR) waveform composed of two linear frequency modulation (LFM) signals to achieve high-precision synchronization without interrupting radar data acquisition, and effectively separates the radar signal and the synchronization signal through frequency diversity (FD) waveform and bandpass filter. However, this method still needs a synchronization antenna to transmit the phase synchronization signal, and as the distance between formation satellites increases, the size of the synchronization antenna becomes larger and the power consumption becomes higher.

[0009] A Noninterrupted Phase Synchronization Scheme of Multi staticSAR Based on Short-term Shift-orthogonal Signal (2024PhotonIcs&Electromagnetics Research Symposium, Chengdu, China,) proposes a new multi-station synthetic aperture radar (SAR) phase synchronization scheme, which solves the interruption and precision problems in the traditional scheme. The scheme uses short-term shift-orthogonal (STSO) signals, which have energy concentration characteristics that improve synchronization accuracy. Peak phase error is extracted through pulse compression, avoiding interruption of radar signal sequences. Experiments show that STSO signals have lower cross-correlation amplitude in multi-station SAR, can more accurately extract phase error, meet synchronization requirements, and are particularly suitable for multi-station SAR systems with known baseline length. However, this method requires a synchronization antenna to transmit the phase synchronization signal, and as the distance between formation satellites increases, the size of the synchronization antenna increases, and the power consumption is large. SUMMARY

[0010] In view of the defects in the prior art, the purpose of the present application is to provide a spaceborne bistatic SAR imaging system and method.

[0011] According to the spaceborne bistatic SAR imaging method provided by the present application, the following steps are included:

[0012] Step S1: The periodic transmission sequence used by the active radar satellite for transmission is a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are in an orthogonal relationship;

[0013] Step S2: Set the pulse repetition frequency based on the bistatic configuration, and control the radar timing sequence so that the inter-satellite direct wave and the bistatic SAR echo of the observation area can be received by the satellite radar at the same time, and the radar transmission signal waveforms corresponding to the inter-satellite direct wave and the bistatic SAR echo of the observation area are different;

[0014] Step S3: Obtain the inter-satellite direct wave and the bistatic SAR echo of the observation area that are mixed together in time, and the inter-satellite direct wave and the bistatic SAR echo of the observation area obtained at the same time are in an orthogonal relationship;

[0015] Step S4: Based on the inter-satellite direct wave and the bistatic SAR echo of the observation area obtained by mixing together in time, the inter-satellite direct wave and the bistatic SAR echo of the observation area are extracted through orthogonal signal processing, and spaceborne bistatic SAR imaging is realized.

[0016] Further, the step S4 includes:

[0017] Step S4.1: taking the transmission signal corresponding to the inter-satellite direct wave as a matched filter function to perform distance compression tracking on the inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time, extracting the inter-satellite direct wave, and inversely calculating the inter-satellite phase synchronization error through the extracted inter-satellite direct wave to realize the inter-satellite synchronization function;

[0018] Step S4.2: removing the inter-satellite direct wave from the signal after distance compression of the inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time by taking the transmission signal corresponding to the inter-satellite direct wave as a matched filter function, obtaining the radar signal after removing the inter-satellite direct wave, performing inverse matched filter processing on the radar signal after removing the inter-satellite direct wave by taking the transmission signal corresponding to the inter-satellite direct wave as an inverse matched filter function, performing distance compression by taking the transmission signal corresponding to the observation area bistatic SAR echo as a matched filter function, compensating the observation area bistatic SAR echo by using the inter-satellite phase synchronization error obtained in step S4.1, and performing subsequent imaging processing to realize the bistatic SAR imaging function.

[0019] Further, in the step S1, the transmission form of the coded signal pulse sequence is periodic repeated transmission with the number of pulses of the pulse sequence as a period.

[0020] In the step S2, the pulse repetition frequency setting includes setting echo reception window design constraints and echo reception orthogonality design constraints.

[0021] The echo reception window design constraint is that the radar echo reception window can simultaneously receive the complete inter-satellite direct wave and the observation area bistatic SAR echo.

[0022] The echo reception orthogonality design constraint is that the transmission signal corresponding to the inter-satellite direct wave is different from the transmission signal corresponding to the observation area bistatic SAR echo.

[0023] Further, in the step S4.1, the matched filter function includes pulse compression of the inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time received at each azimuth time by taking the transmission signal corresponding to the inter-satellite direct wave as a matched filter function.

[0024] The tracking extraction manner of the inter-satellite direct wave includes: calculating the echo distance unit where the inter-satellite direct wave is located according to the spatial positions of the two satellites, detecting the bright line around the echo distance unit to realize detection of the inter-satellite direct wave, and analyzing the inter-satellite phase synchronization information accordingly.

[0025] Further, the inter-satellite direct wave corresponding to the transmitted signal is taken as the matched filter function to remove the inter-satellite direct wave from the range-compressed signals of the inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time, including setting to zero the echo signals in the detected inter-satellite direct wave corresponding distance unit and surrounding several distance units in the range-compressed echo signals.

[0026] The inverse matched filter function includes taking the transmitted signal corresponding to the inter-satellite direct wave as the inverse matched function to pulse decompress the echo signals after removing the inter-satellite direct wave at each azimuth time, to obtain the observation area bistatic SAR echo.

[0027] The range compression using the transmitted signal corresponding to the bistatic SAR observation area echo signal as the matched filter function includes that the observation area bistatic SAR echo at each azimuth time respectively uses the transmitted signal corresponding to the observation area bistatic SAR echo as the matched function to pulse compress.

[0028] According to the present application, a kind of spaceborne bistatic SAR imaging system is provided, including:

[0029] Module M1: make the periodic transmission sequence used by the transmitting satellite to transmit radar transmission be a coded signal pulse sequence, and any pulse in the coded signal pulse sequence is orthogonal relationship;

[0030] Module M2: set pulse repetition frequency based on bistatic configuration, and the pulse repetition frequency after setting controls radar operation timing so that inter-satellite direct wave and observation area bistatic SAR echo can be received by receiving star radar simultaneously, and the radar transmitted signal waveform corresponding to inter-satellite direct wave and observation area bistatic SAR echo respectively is different;

[0031] Module M3: based on the setting of coded signal pulse sequence and the setting of pulse repetition frequency based on bistatic configuration, obtain inter-satellite direct wave and observation area bistatic SAR echo mixed together in time, and the inter-satellite direct wave and observation area bistatic SAR echo obtained at the same time are orthogonal relationship;

[0032] Module M4: based on the inter-satellite direct wave and observation area bistatic SAR echo mixed together in time obtained, extract inter-satellite direct wave and observation area bistatic SAR echo respectively through orthogonal signal processing, to realize spaceborne bistatic SAR imaging.

[0033] Further, the module M4 includes:

[0034] Module M4.1: The transmitted signal corresponding to the inter-satellite direct wave is used as a matched filter function to perform distance compression tracking on the inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time, to extract the inter-satellite direct wave, and the inter-satellite phase synchronization error is inverted through the extracted inter-satellite direct wave, to realize the inter-satellite synchronization function.

[0035] Module M4.2: The inter-satellite direct wave is removed from the signal after distance compression of the inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time using the transmitted signal corresponding to the inter-satellite direct wave as a matched filter function, to obtain the radar signal after removing the inter-satellite direct wave, the radar signal after removing the inter-satellite direct wave is obtained is used as an anti-matched filter function to perform anti-matched filter processing on the radar signal after removing the inter-satellite direct wave using the transmitted signal corresponding to the inter-satellite direct wave, and the transmitted signal corresponding to the observation area bistatic SAR echo is used as a matched filter function to perform distance compression, and the inter-satellite phase synchronization error obtained by module M4.1 is used to compensate the observation area bistatic SAR echo and perform subsequent imaging processing, to realize the bistatic SAR imaging function.

[0036] Further, in the module M1, the transmission form of the coded signal pulse sequence is periodic repetition transmission with the number of pulses of the pulse sequence as a period.

[0037] In the module M2, the pulse repetition frequency is set to include setting an echo reception window design constraint and an echo reception orthogonality design constraint.

[0038] The echo reception window design constraint is that the radar echo reception window can simultaneously receive the complete inter-satellite direct wave and the observation area bistatic SAR echo.

[0039] The echo reception orthogonality design constraint is that the transmitted signal corresponding to the inter-satellite direct wave is different from the transmitted signal corresponding to the observation area bistatic SAR echo.

[0040] Further, in the module M4.1, the matched filter function includes pulse compression of the inter-satellite direct wave and the observation area bistatic SAR echo received at each azimuth time and mixed together in time using the transmitted signal corresponding to the inter-satellite direct wave as a matched filter function.

[0041] The way of tracking and extracting the inter-satellite direct wave includes calculating the echo distance unit where the inter-satellite direct wave is located according to the spatial positions of the two satellites, detecting the bright line around the echo distance unit to realize the detection of the inter-satellite direct wave, and analyzing the inter-satellite phase synchronization information accordingly.

[0042] Further, the inter-satellite direct wave corresponding to the transmitted signal is used as a matched filter function to remove the inter-satellite direct wave from the signals of the range-compressed inter-satellite direct wave and the observation area bistatic SAR echo mixed together in time, including setting the echo signals in the detected inter-satellite direct wave corresponding distance unit and surrounding several distance units to zero in the range-compressed echo signals.

[0043] The inverse matched filter function includes using the inter-satellite direct wave corresponding to the transmitted signal as an inverse matched function to pulse decompress the echo signals after removing the inter-satellite direct wave at each azimuth time, to obtain the observation area bistatic SAR echo.

[0044] The range compression using the transmitted signal corresponding to the bistatic SAR observation area echo signal as a matched filter function includes that the observation area bistatic SAR echo at each azimuth time uses the transmitted signal corresponding to the observation area bistatic SAR echo as a matched function for pulse compression.

[0045] Compared with the prior art, the present application has the following beneficial effects:

[0046] 1. The present application effectively breaks through the limitation that the direct wave receiving antenna and the ground echo receiving antenna of the traditional bistatic radar need to be designed independently, simplifies the hardware design difficulty of the satellite radar load, reduces the power consumption, weight and cost demand of the satellite, and can effectively improve the observation performance of the bistatic SAR satellite;

[0047] 2. The present application solves the problem that the inter-satellite direct wave receiving system design and the observation area bistatic radar receiving system must be designed independently through the signal coding, PRF optimization design, inter-satellite direct wave signal extraction and processing, and observation area bistatic SAR echo signal extraction and processing technical features, realizes the technical effect of one design for two functions of the satellite-borne bistatic SAR inter-satellite synchronization and ground imaging, and same frequency band operation; BRIEF DESCRIPTION OF DRAWINGS

[0048] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments, made with reference to the accompanying drawings:

[0049] Figure 1 is a satellite-borne bistatic SAR system design and signal processing flowchart of the present application;

[0050] Figure 2 is a signal diagram of the inter-satellite direct wave and the observation area bistatic SAR echo;

[0051] Figure 3 is a display diagram of the inter-satellite direct wave signal extracted by the matched filter;

[0052] Figure 4It is an echo signal display figure after removing direct wave by anti-matching filter and then matching filter

[0053] Figure 5 It is a bistatic SAR point target imaging result figure after two-dimensional focusing. DETAILED DESCRIPTION

[0054] The application will be described in detail below with specific embodiments. The following examples 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 are within the scope of the application.

[0055] According to the satellite-borne bistatic SAR imaging method provided by the application, comprising:

[0056] Step S1: The periodic transmission sequence adopted by the active radar satellite for transmission is a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are in orthogonal relationship.

[0057] Step S2: Set the pulse repetition frequency based on the bistatic configuration, and the set pulse repetition frequency controls the radar timing to make the inter-satellite direct wave and the bistatic radar echo of the observation area can be received by the receiving star radar at the same time, and the radar signal waveforms corresponding to the inter-satellite direct wave and the bistatic SAR echo of the observation area are different.

[0058] Step S3: Obtain the inter-satellite direct wave and the bistatic SAR system echo of the observation area mixed together in time, and the inter-satellite direct wave and the bistatic SAR echo signal obtained at the same time are orthogonal.

[0059] Step S4: Based on the inter-satellite direct wave and the bistatic SAR echo of the observation area mixed together in time, the inter-satellite direct wave and the bistatic SAR echo signal are extracted by orthogonal signal processing, realizing the functions of direct wave extraction and bistatic SAR imaging.

[0060] Specifically, the periodic transmission sequence adopted by the transmitting star for radar transmission in step S1 is a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are in orthogonal relationship. The signal coding form is a quadrature coded pulse sequence, and any two pulses in the sequence are in time domain orthogonal relationship. The pulse sequence transmission form is periodic repetition transmission with the number of pulses in the pulse sequence as the period.

[0061] Specifically, the step S2 of setting the pulse repetition frequency comprises: setting an echo receiving window design constraint and an echo receiving orthogonality design constraint. The echo receiving window design constraint is that the radar echo receiving window can simultaneously receive the complete inter-satellite direct wave and the echo signal of the bistatic SAR observation area. The echo receiving orthogonality design constraint is that the transmission signal corresponding to the inter-satellite direct wave is different from the transmission signal corresponding to the bistatic SAR echo of the observation area.

[0062] Specifically, the step S4 comprises:

[0063] Step S4.1: The inter-satellite direct wave and the bistatic SAR echo of the observation area mixed together in time are subjected to distance compression tracking by taking the transmission signal corresponding to the inter-satellite direct wave as a matched filter function to extract the inter-satellite direct wave signal, the inter-satellite phase synchronization error is inverted by the extracted inter-satellite direct wave signal, and the inter-satellite synchronization function is realized.

[0064] Step S4.2: The inter-satellite direct wave is removed from the signal after distance compression of the inter-satellite direct wave and the bistatic SAR echo of the observation area mixed together in time by taking the transmission signal corresponding to the inter-satellite direct wave as a matched filter function, to obtain the radar signal after removing the inter-satellite direct wave, the obtained radar signal after removing the inter-satellite direct wave is subjected to inverse matched filter processing by taking the transmission signal corresponding to the inter-satellite direct wave as an inverse matched filter function, and distance compression is performed by taking the transmission signal corresponding to the bistatic SAR echo of the observation area as a matched filter function, and the bistatic SAR observation area echo signal is compensated and subjected to subsequent imaging processing by using the inter-satellite synchronization phase estimation value obtained in step S4.1, to realize the bistatic SAR imaging function.

[0065] Specifically, the matched filter function in the step S4.1 comprises: the inter-satellite direct wave and the bistatic SAR echo of the observation area received at each azimuth time and mixed together in time are respectively subjected to pulse compression by taking the transmission signal corresponding to the inter-satellite direct wave as a matched filter function. The tracked and extracted inter-satellite direct wave signal comprises: the echo distance unit where the inter-satellite direct wave is located is calculated according to the spatial positions of the two satellites, a bright line is detected around the echo distance unit to realize detection of the inter-satellite direct wave, and the inter-satellite phase synchronization information is analyzed accordingly.

[0066] Specifically, the distance compressed signal removes the inter-satellite direct wave, including setting to zero the echo signals in the distance unit corresponding to the detected inter-satellite direct wave signal and the surrounding several distance units in the distance compressed echo signal. The inverse matching filter function includes using the inter-satellite direct wave corresponding to the transmitted signal as an inverse matching function for pulse decompression of the echo signal after removing the inter-satellite direct wave at each azimuth time, to obtain a bistatic SAR echo signal in the observation region. The distance compression using the transmitted signal corresponding to the bistatic SAR observation region echo signal as a matching filter function includes that the bistatic SAR echo signals in the observation region at each azimuth time are respectively pulse compressed using the transmitted signal corresponding to the bistatic SAR echo signal in the observation region as a matching function.

[0067] The application will be further described below with reference to the accompanying drawings.

[0068] Referring to the drawings Figure 1 The theoretical analysis basis of the application is briefly introduced as follows:

[0069] A satellite-borne bistatic SAR inter-satellite synchronization and earth imaging two-in-one system and method based on signal coding, characterized in that it includes radar orthogonal coded signal sequence design, pulse repetition frequency design, direct wave signal extraction and processing, and target echo signal extraction and processing. Wherein:

[0070] 1) Radar coded signal sequence design

[0071] The coded signal pulse sequence is periodically transmitted, and any pulses in the pulse signal sequence are in an orthogonal relationship, and the mathematical model is as follows:

[0072] s k (t r )=s k+N (t r )

[0073]

[0074] In the formula, s k (t r ) is the azimuth kth transmitted pulse, t r is the distance time, N is the pulse sequence length (repetition period), T p is the pulse width. Typical signal forms include signals.

[0075] 2) Pulse repetition frequency design

[0076] Based on the bistatic configuration, the inter-satellite direct wave and the bistatic SAR echo in the observation region are received at the same time, but the pulse repetition frequencies (PRF) of the radar transmitted signals corresponding to the two are different, and the mathematical model is as follows:

[0077]

[0078]

[0079]

[0080]

[0081]

[0082] where PRF is the pulse repetition period, R near and R far are the shortest and longest slant ranges of the observation region within one synthetic aperture time, c is the speed of light, T proc is the protection time, R s is the distance between two satellites, is the floor operator, and mod(·) is the modulo operator.

[0083] 3) Inter-satellite direct wave signal extraction and processing

[0084] The received echo signal is range-compressed using the transmitted signal corresponding to the inter-satellite direct wave signal as the matched filter function, the inter-satellite direct wave signal is extracted by tracking, and inter-satellite phase synchronization information is inverted to realize inter-satellite synchronization.

[0085] The echo signal received by the radar can be expressed as

[0086] s(t a ,t r ) = s dir (t a ,t r ) + s obs (t a ,t r )

[0087] where s(t a ,t r ) is the radar echo, s dir (t a ,t r ) is the inter-satellite direct wave, s obs (t a ,t r ) is the echo of the observation region, and ta is the azimuth time. It is assumed that the transmitted signal corresponding to the inter-satellite direct wave s dir (t a ,t r ) is s k (t r ), and this signal is used to pulse-compress s(t a ,t r ).

[0088]

[0089] where s dir,comp (t a ,t r ) is the compressed signal of inter-satellite direct wave, s' obs,comp (t a ,t r ) is the compressed signal of the echo in the observation region.

[0090] After pulse compression, the inter-satellite direct wave will appear as a bright line in the image, and the signal of the observation region is still defocused due to the mismatch of the matched filter. At this time, the echo distance unit where the inter-satellite direct wave is located can be calculated according to the distance information of the double satellites, and the bright line around the unit is detected to realize the detection and extraction of the inter-satellite direct wave signal. Subsequently, the existing mature direct wave phase synchronization method can be used to realize the extraction of inter-satellite phase synchronization information and complete the inter-satellite synchronization function.

[0091] 4) Extraction and processing of ground observation region echo signal of bistatic SAR

[0092] The inter-satellite direct wave signal is removed from the echo signal after range compression, and the transmission signal corresponding to the inter-satellite direct wave signal is used as the inverse matched filter function for inverse matched filter processing. The transmission signal corresponding to the echo signal of the observation region is used as the matched filter function for range compression, the inter-satellite phase synchronization information is compensated, and then imaging processing operation is performed to realize the bistatic SAR imaging function.

[0093] The echo signal after removing the inter-satellite direct wave can be expressed as

[0094] s' obs,comp (t a ,t r ) = s comp (t a ,t r ) - s dir,comp (t a ,t r )

[0095] The transmission signal corresponding to the inter-satellite direct wave is used as the inverse matched filter function to perform inverse matched filter processing, and the echo signal of the observation region can be obtained.

[0096] s' obs (t a ,t r ) = s' obs,comp (t a ,t r ) * s k (t r )

[0097] Suppose that the transmitting signal corresponding to the echo of the radar observation area at this moment is s j (t r ), after distance compression, the SAR image of the observation area can be obtained by using mature bistatic SAR imaging processing technology, and the bistatic ground imaging function is completed, as shown in Figure 5 .

[0098] Here, the effectiveness of the application is verified by simulation data. The simulation parameters are shown in Table 1.

[0099] Table 1 Simulation parameters of spaceborne bistatic SAR system

[0100]

[0101] Suppose that the encoding signal adopts the following short offset quadrature waveform:

[0102]

[0103] In the formula, K r is the frequency modulation of the transmitting signal,

[0104]

[0105] The pulse repetition frequency (PRF) designed by the patent is 3545Hz. As can be seen from Figure 2 , Figure 3 and Figure 4 , the application can simultaneously realize the inter-satellite direct wave and the observation area echo signal, and realize the integrated design of the two.

[0106] The method of the application realizes integrated reception of bistatic SAR echo data and direct wave data by setting the orthogonal encoding signal pulse sequence and controlling the pulse repetition frequency, and realizes signal separation and extraction by using the orthogonality of the two, effectively solving the problems of phase synchronization and echo reception timing.

[0107] The application simultaneously receives direct wave and bistatic SAR echo data by using signal encoding technology, realizes the combination of inter-satellite phase synchronization information extraction and spaceborne bistatic SAR ground imaging, and is the first in the world.

[0108] The application also provides a spaceborne bistatic SAR imaging system, which can be realized by executing the flow steps of the spaceborne bistatic SAR imaging method, that is, the spaceborne bistatic SAR imaging method can be understood by those skilled in the art as the preferred embodiment of the spaceborne bistatic SAR imaging system. The system comprises:

[0109] Module M1: make the transmitting satellite transmit a periodic transmission sequence as a coded signal pulse sequence, and any pulse in the coded signal pulse sequence is in an orthogonal relationship.

[0110] Module M2: set the pulse repetition frequency based on the bistatic configuration, and the set pulse repetition frequency controls the radar timing to make the inter-satellite direct wave and the bistatic SAR echo of the observation area be received by the receiving satellite radar at the same time, and the radar signal waveforms corresponding to the inter-satellite direct wave and the bistatic SAR echo of the observation area are different.

[0111] Module M3: based on the setting of the coded signal pulse sequence and the setting of the pulse repetition frequency based on the bistatic configuration, obtain the inter-satellite direct wave and the bistatic SAR echo of the observation area that are time-overlapped, and the inter-satellite direct wave and the bistatic SAR echo of the observation area obtained at the same time are in an orthogonal relationship.

[0112] Module M4: based on the obtained inter-satellite direct wave and the bistatic SAR echo of the observation area that are time-overlapped, extract the inter-satellite direct wave and the bistatic SAR echo of the observation area through orthogonal signal processing, and realize the spaceborne bistatic SAR imaging.

[0113] Those skilled in the art know that in addition to implementing the system provided by the present application and each device, module and unit thereof in a pure computer readable program code manner, the system provided by the present application and each device, module and unit thereof can also be implemented in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers and embedded microcontrollers to achieve the same function by logically programming the method steps. Therefore, the system provided by the present application and each device, module and unit thereof can be considered as a hardware component, and the devices, modules and units included therein for achieving various functions can also be considered as structures within the hardware component; the devices, modules and units for achieving various functions can also be considered as both software modules for implementing methods and structures within hardware components.

[0114] The specific embodiments of the present application are described above. It should 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 arbitrarily combined with each other without conflict.

Claims

1. A space-borne bistatic SAR imaging method, characterized in that, The method comprises the following steps: Step S1: the periodic transmission sequence used by the active radar satellite is a coded signal pulse sequence, and any two pulses in the coded signal pulse sequence are in orthogonal relationship; Step S2: set the pulse repetition frequency based on the bistatic configuration, and the set pulse repetition frequency controls the radar timing to enable the inter-satellite direct wave and the bistatic SAR echo in the observation area to be received by the satellite simultaneously, and the radar signal waveforms corresponding to the inter-satellite direct wave and the bistatic SAR echo in the observation area are different; Step S3: obtain the inter-satellite direct wave and the bistatic SAR echo in the observation area that are mixed together in time, and the inter-satellite direct wave and the bistatic SAR echo in the observation area obtained at the same time are in orthogonal relationship; Step S4: based on the inter-satellite direct wave and the bistatic SAR echo in the observation area that are mixed together in time, extract the inter-satellite direct wave and the bistatic SAR echo in the observation area through orthogonal signal processing, and realize satellite-borne bistatic SAR imaging; The step S4 comprises: Step S4.1: use the transmission signal corresponding to the inter-satellite direct wave as a matched filter function to perform distance compression tracking on the inter-satellite direct wave and the bistatic SAR echo in the observation area that are mixed together in time to extract the inter-satellite direct wave, use the extracted inter-satellite direct wave to invert the inter-satellite phase synchronization error, and realize inter-satellite synchronization function; Step S4.2: remove the inter-satellite direct wave from the signal after distance compression of the inter-satellite direct wave and the bistatic SAR echo in the observation area that are mixed together in time using the transmission signal corresponding to the inter-satellite direct wave as a matched filter function, obtain the radar signal after removal of the inter-satellite direct wave, use the transmission signal corresponding to the inter-satellite direct wave as an inverse matched filter function to perform inverse matched filter processing on the radar signal after removal of the inter-satellite direct wave, use the transmission signal corresponding to the bistatic SAR echo in the observation area as a matched filter function to perform distance compression, use the inter-satellite phase synchronization error obtained in step S4.1 to compensate the bistatic SAR echo in the observation area and perform subsequent imaging processing, and realize bistatic SAR imaging function; In the step S4.1, the matched filter function comprises pulse compression of the inter-satellite direct wave and the bistatic SAR echo in the observation area that are mixed together in time received at each azimuth time using the transmission signal corresponding to the inter-satellite direct wave as a matched filter function; The manner of tracking and extracting the inter-satellite direct wave comprises: calculating the echo distance unit where the inter-satellite direct wave is located according to the spatial positions of the two satellites, detecting the bright line around the echo distance unit to realize detection of the inter-satellite direct wave, and analyzing the inter-satellite phase synchronization information accordingly; The removal of the inter-satellite direct wave from the signal after distance compression of the inter-satellite direct wave and the bistatic SAR echo in the observation area that are mixed together in time using the transmission signal corresponding to the inter-satellite direct wave as a matched filter function comprises: setting the echo signals in the distance unit corresponding to the detected inter-satellite direct wave and the surrounding distance units to zero in the echo signal after distance compression. The anti-matching filter function comprises: using the transmission signal corresponding to the inter-satellite direct wave as an anti-matching function for pulse decompression of the echo signal after the inter-satellite direct wave is removed at each azimuth time, to obtain the bistatic SAR echo of the observation area; The distance compression of the echo signal corresponding to the transmission signal of the bistatic SAR observation area comprises: the bistatic SAR echo of the observation area at each azimuth time is pulse compressed by using the transmission signal corresponding to the bistatic SAR echo of the observation area as a matching function; In the step S1, the transmission form of the coded signal pulse sequence is periodic transmission with the pulse number of the pulse sequence as a period; In the step S2, the setting of the pulse repetition frequency comprises: setting an echo reception window design constraint and an echo reception orthogonality design constraint; The echo reception window design constraint is that the radar echo reception window can simultaneously receive the complete inter-satellite direct wave and the bistatic SAR echo of the observation area; The echo reception orthogonality design constraint is that the transmission signal corresponding to the inter-satellite direct wave is different from the transmission signal corresponding to the bistatic SAR echo of the observation area; Based on the bistatic configuration, the inter-satellite direct wave and the bistatic SAR echo of the observation area are simultaneously received, but the radar transmission signal waveforms corresponding to the two are different, and the mathematical model is as follows: where PRF is the pulse repetition period, R near and R far are the shortest and longest slant ranges of the observation region within one synthetic aperture time, c is the speed of light, T proc is the protection time, R s is the distance between two stars, is the floor operator, and mod(·) is the modulo operator. The distance compression of the received echo signal is performed by using the transmission signal corresponding to the inter-satellite direct wave signal as a matching filter function, the inter-satellite direct wave signal is extracted by tracking, and the inter-satellite phase synchronization information is inverted, to realize the inter-satellite synchronization function; The echo signal received by the radar is represented as s(t a ,t r ) = s dir (t a ,t r ) + s obs (t a ,t r ) where s(t a ,t r ) is the radar echo, s dir (t a ,t r ) is the inter-satellite direct wave, s obs (t a ,t r ) is the echo in the observation area, t a is the azimuth time, and it is assumed that the transmitted signal corresponding to the inter-satellite direct wave s dir (t a ,t r ) is s k (t r ), and the signal s(t a ,t r ) is pulse compressed using the signal In the formula, s dir,comp (t a ,t r ) is the compressed signal of the inter-satellite direct wave, s' obs,comp (t a ,t r ) is the compressed signal of the echo in the observation area; The inter-satellite direct wave signal is removed from the distance-compressed echo signal, and then the transmission signal corresponding to the inter-satellite direct wave signal is used as an anti-matching filter function for anti-matching filter processing, the transmission signal corresponding to the bistatic SAR echo of the observation area is used as a matching filter function for distance compression, the inter-satellite phase synchronization information is compensated, and then imaging processing operation is performed, to realize the bistatic SAR imaging function; The echo signal after the inter-satellite direct wave is removed is represented as s' obs,comp (t a ,t r )=s comp (t a ,t r )-s dir,comp (t a ,t r ) The transmission signal corresponding to the inter-satellite direct wave is used as an anti-matching filter function for anti-matching filter processing, to obtain the echo signal of the observation area; s' obs (t a ,t r )=s' obs,comp (t a ,t r )*s k (t r ) Suppose the transmitting signal corresponding to the echo of the radar observation area at this moment is s j (t r ), after distance compression, the SAR image of the observation area can be obtained by using mature bistatic SAR imaging processing technology, and the bistatic ground imaging function is completed.

2. A space-borne bistatic SAR imaging system, characterized in that Comprise: Module M1: the periodic transmission sequence used by the transmitting satellite for radar transmission is a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are in an orthogonal relationship; Module M2: based on the bistatic configuration, the pulse repetition frequency is set, and the set pulse repetition frequency controls the radar working time sequence so that the inter-satellite direct wave and the bistatic SAR echo of the observation area can be simultaneously received by the receiving radar, and the radar transmission signal waveforms corresponding to the inter-satellite direct wave and the bistatic SAR echo of the observation area are different; Module M3: based on the setting of the coded signal pulse sequence and the setting of the pulse repetition frequency based on the bistatic configuration, the inter-satellite direct wave and the bistatic SAR echo of the observation area that are time-mixed together are obtained, and the inter-satellite direct wave and the bistatic SAR echo of the observation area obtained at the same time are in an orthogonal relationship; Module M4: based on the obtained time mixed together inter-satellite direct wave and observation area bistatic SAR echo, through orthogonal signal processing respectively extract inter-satellite direct wave and observation area bistatic SAR echo, realize satellite-borne bistatic SAR imaging; The module M4 comprises: Module M4.1: the corresponding transmission signal of inter-satellite direct wave is used as a matched filter function to distance-compress and track the time mixed together inter-satellite direct wave and observation area bistatic SAR echo to extract inter-satellite direct wave, the inter-satellite phase synchronization error is inverted through the tracked and extracted inter-satellite direct wave, and the inter-satellite synchronization function is realized; Module M4.2: the signal after distance compression of the time mixed together inter-satellite direct wave and observation area bistatic SAR echo is removed from the inter-satellite direct wave by using the corresponding transmission signal of the inter-satellite direct wave as a matched filter function, the radar signal after removing the inter-satellite direct wave is obtained, the obtained radar signal after removing the inter-satellite direct wave is processed by using the corresponding transmission signal of the inter-satellite direct wave as an inverse matched filter function, the observation area bistatic SAR echo is distance-compressed by using the corresponding transmission signal of the observation area bistatic SAR echo as a matched filter function, the observation area bistatic SAR echo is compensated and subsequent imaging processing is performed by using the inter-satellite phase synchronization error obtained by the module M4.1, and the bistatic SAR imaging function is realized; In the module M1, the transmission form of the coded signal pulse sequence is periodic repeated transmission with the number of pulses of the pulse sequence as a period; In the module M2, setting the pulse repetition frequency comprises: setting echo reception window design constraints and echo reception orthogonality design constraints; The echo reception window design constraint is that the radar echo reception window can simultaneously receive complete inter-satellite direct wave and observation area bistatic SAR echo; The echo reception orthogonality design constraint is that the transmission signal corresponding to the inter-satellite direct wave is different from the transmission signal corresponding to the observation area bistatic SAR echo; In the module M4.1, the matched filter function comprises pulse compression of the time mixed together inter-satellite direct wave and observation area bistatic SAR echo received at each azimuth time by using the transmission signal corresponding to the inter-satellite direct wave as a matched filter function; The manner of tracking and extracting the inter-satellite direct wave comprises: calculating the echo distance unit where the inter-satellite direct wave is located according to the spatial positions of the two satellites, detecting the bright line around the echo distance unit to realize detection of the inter-satellite direct wave, and analyzing the inter-satellite phase synchronization information according to the detection result; The signal after distance compression of the time mixed together inter-satellite direct wave and observation area bistatic SAR echo is removed from the inter-satellite direct wave by using the corresponding transmission signal of the inter-satellite direct wave as a matched filter function, which comprises zeroing the echo signal in the distance unit corresponding to the detected inter-satellite direct wave and the echo signal in the surrounding distance units in the distance-compressed echo signal; The inverse matched filter function comprises pulse decompression of the echo signal after removing the inter-satellite direct wave at each azimuth time by using the transmission signal corresponding to the inter-satellite direct wave as an inverse matched function, to obtain the observation area bistatic SAR echo. The transmission signal corresponding to the echo signal of the observation area of the bistatic SAR is taken as a matching filter function for distance compression, including: the echo of the observation area of the bistatic SAR at each azimuth time is respectively pulse compressed by taking the transmission signal corresponding to the echo of the observation area of the bistatic SAR as a matching function; The inter-satellite direct wave and the echo of the observation area of the bistatic SAR are simultaneously received based on the bistatic configuration, but the pulse repetition frequencies of the radar transmission signals corresponding to the two are different, and the mathematical model is as follows: where PRF is the pulse repetition period, R near and R far are the shortest and longest slant ranges of the observation region within one synthetic aperture time, c is the speed of light, T proc is the protection time, R s is the distance between two stars, is the floor operator, and mod(·) is the modulo operator. The transmission signal corresponding to the echo signal of the observation area of the bistatic SAR is taken as a matching filter function for distance compression, through tracking and extraction of the inter-satellite direct wave signal, and inversion of the inter-satellite phase synchronization information, the inter-satellite synchronization function is realized; The echo signal received by the radar is represented as s(t a ,t r ) = s dir (t a ,t r ) + s obs (t a ,t r ) In the formula, s(t a ,t r ) is a radar echo, s dir (t a ,t r ) is an inter-satellite direct wave, s obs (t a ,t r ) is an observation area echo, t a is an azimuth time, and it is assumed that a transmission signal corresponding to the inter-satellite direct wave s dir (t a ,t r ) is s k (t r ), and the signal s(t a ,t r ) is pulse compressed using the signal. where s dir,comp (t a ,t r ) is the compressed signal of the direct wave between satellites, and s' obs,comp (t a ,t r ) is the compressed signal of the echo in the observation area. The echo signal of the observation area is obtained by taking the transmission signal corresponding to the inter-satellite direct wave as a reverse matching filter function for reverse matching filter processing, taking the transmission signal corresponding to the echo signal of the observation area as a matching filter function for distance compression, compensating the inter-satellite phase synchronization information, and then performing imaging processing operation, so as to realize the bistatic SAR imaging function; The echo signal of the observation area is obtained by taking the transmission signal corresponding to the inter-satellite direct wave as a reverse matching filter function for reverse matching filter processing, taking the transmission signal corresponding to the echo signal of the observation area as a matching filter function for distance compression, compensating the inter-satellite phase synchronization information, and then performing imaging processing operation, so as to realize the bistatic SAR imaging function; s' obs,comp (t a ,t r )=s comp (t a ,t r )-s dir,comp (t a ,t r ) The echo signal of the observation area is obtained by taking the transmission signal corresponding to the inter-satellite direct wave as a reverse matching filter function for reverse matching filter processing, taking the transmission signal corresponding to the echo signal of the observation area as a matching filter function for distance compression, compensating the inter-satellite phase synchronization information, and then performing imaging processing operation, so as to realize the bistatic SAR imaging function; s' obs (t a ,t r )=s' obs,comp (t a ,t r )*s k (t r ) Suppose the transmitting signal corresponding to the echo of the radar observation area at this moment is s j (t r ), after distance compression, the SAR image of the observation area can be obtained by using mature bistatic SAR imaging processing technology, and the bistatic ground imaging function is completed.

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