Satellite-borne bistatic synthetic aperture radar (SAR) imaging system and method

Abstract

The invention provides a satellite-borne bistatic SAR imaging system and method, and the method comprises the steps: employing a coding signal pulse sequence as a periodic transmission sequence for a transmission signal, and enabling any pulses to be in an orthogonal relation; the pulse repetition frequency is set based on the bistatic configuration, and the adopted pulse repetition frequency enables the inter-satellite direct wave and the bistatic SAR ground echo to be received at the same time, and the waveforms of radar emission signals corresponding to the inter-satellite direct wave and the bistatic SAR ground echo are different; on the basis of the setting of a coded signal pulse sequence and the setting of a pulse repetition frequency, inter-satellite direct waves and observation area bistatic SAR echoes which are aliasing together in time are obtained, and the inter-satellite direct waves and observation area bistatic SAR echo signals obtained at the same moment are orthogonal; based on the obtained inter-satellite direct wave and the observation area bistatic SAR echo which are mixed together in time, the inter-satellite direct wave and the observation area bistatic SAR echo are extracted respectively, and the spaceborne bistatic SAR imaging function under the suppression of the inter-satellite direct wave is achieved.

Description

Technical Field

[0001] The present invention relates to the field of spaceborne radar technology, and in particular to a spaceborne bistatic SAR imaging system and method. Background Art

[0002] Spaceborne bistatic SAR places the transmitter and receiver on different satellite platforms. By separating the transmitting and receiving antennas, it solves the forward and backward imaging problems that cannot be achieved by traditional spaceborne SAR systems. Although bistatic SAR has many advantages, the birth of the new system structure also causes many complex technical problems, among which synchronization technology is the most critical, and the use of direct waves to achieve time-frequency synchronization is the most common synchronization technology. However, it should be pointed out that for existing bistatic SAR systems, the use of direct waves to achieve time-frequency synchronization requires the establishment of an additional direct wave receiving channel between the transmitter and the receiver in addition to the imaging channel. This means that in addition to the imaging antenna, the transmitter also sends the same direct wave as the imaging signal to the receiver through a special antenna. The receiver uses the received direct wave to focus the echo signal and obtain the SAR image. This will increase the development cost of the spaceborne radar system, increase the power consumption and weight of the entire satellite, and bring certain difficulties to the satellite system design.

[0003] The paper "TanDEM-X: A satellite format for high-resolution SAR interferometry" (IEEE Transactions on Geoscience and Remote Sensing, vol. 45, no. 11, pp. 3317–3341) introduces a dual-base SAR phase synchronization method suitable for close-range formations. The two satellites transmit by interrupting pulses and transmit signals to each other through inter-satellite synchronization antennas to achieve dual-satellite phase synchronization. However, the method proposed in this paper will face problems such as large size and high power consumption of synchronization antennas when applied to dual-base SAR systems in long-range formations.

[0004] The paper "An Advanced Phase Synchronization Scheme for LT-1" (IEEE Transactions on Geoscience and Remote Sensing, VOL. 58, NO. 3, pp. 1735-1746) proposed an advanced non-interruptible phase synchronization scheme for a dual-station synthetic aperture radar system. This scheme avoids data acquisition interruption by exchanging synchronization pulses after radar signal transmission and before echo reception, improves synchronization accuracy and prevents data loss. However, the method in this paper requires a phase-synchronized antenna to transmit phase-synchronized pulses. The dual-station SAR system used in long-distance formations will face problems such as large synchronization antenna size and high power consumption.

[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) eliminates the lowest echo by performing waveform diversity and dual-focusing on the received signal after waveform coding. This technology not only improves the image quality, but can also be used for range ambiguity suppression in a similar manner. However, this document does not yet involve how to apply the coded signal to dual-base 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) is based on the German TerraSAR-X satellite's integrated reception and separation experiment of subsatellite point echo and imaging echo. Orthogonal coding is achieved through up and down linear frequency modulation, and the orthogonal and simultaneous reception of subsatellite point echo and imaging echo are further achieved. The matched filter and anti-matched filter are then used to separate and extract the two. However, this document has not yet involved how to apply the coded signal to dual-base SAR imaging synchronization.

[0007] Patent document CN112799056A (application number: 202011584728.5) discloses a satellite-borne radar altimeter method, which 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 nadir echo and the interference side high area echo have different radar transmission signal waveforms, obtain the bottom-looking radar altimeter echo and the side-looking radar altimeter echo that are overlapped in time, extract the bottom-looking radar altimeter echo and the side-looking radar altimeter echo signals respectively, and realize the bottom-looking altimeter and side-looking altimeter height measurement functions. However, the patent has not yet involved how to apply coded signals to dual-base SAR imaging synchronization.

[0008] The paper "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 problems in bi-station / multi-station 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 radar signals and synchronization signals through frequency diversity (FD) waveforms and bandpass filters. However, this method also requires a synchronization antenna to transmit phase synchronization signals. As the distance between formation satellites increases, the synchronization antenna faces problems such as larger size and higher power consumption.

[0009] The paper "A Noninterrupted Phase Synchronization Scheme of Multi staticSAR Based on Short-term Shift-orthogonal Signal" (2024 PhotonIcs & Electromagnetics Research Symposium, Chengdu, China) proposes a new phase synchronization scheme for multi-station synthetic aperture radar (SAR), which solves the interruption and accuracy problems in traditional schemes. The scheme adopts short-term shifted orthogonal (STSO) signals, whose energy concentration characteristics improve the synchronization accuracy. The peak phase error is extracted by pulse compression, avoiding the interruption of the radar signal sequence. Experiments show that the STSO signal has a lower cross-correlation amplitude in multi-station SAR, can extract the phase error more accurately, meet the synchronization requirements, and is particularly suitable for multi-station SAR systems with known baseline lengths. However, this method also requires a synchronous antenna to transmit phase synchronization signals. As the distance between formation satellites increases, it faces problems such as larger size and higher power consumption of the synchronous antenna. Summary of the invention

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

[0011] A spaceborne bistatic SAR imaging method provided by the present invention comprises:

[0012] Step S1: The periodic transmission sequence used by the active radar satellite is set to be a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are orthogonal to each other;

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

[0014] Step S3: obtaining the inter-satellite direct wave and the bistatic SAR echo of the observation area which are overlapped 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 acquired temporally mixed intersatellite direct waves and observation area bistatic SAR echoes, the intersatellite direct waves and the observation area bistatic SAR echoes are extracted respectively through orthogonal signal processing to achieve spaceborne bistatic SAR imaging.

[0016] Furthermore, the step S4 comprises:

[0017] Step S4.1: Using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time are range-compressed and tracked to extract the intersatellite direct wave, and the intersatellite phase synchronization error is inverted by tracking the extracted intersatellite direct wave to realize the intersatellite synchronization function;

[0018] Step S4.2: Using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, the signal after distance compression of the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time is removed from the intersatellite direct wave to obtain the radar signal after the intersatellite direct wave is removed, and the radar signal after the intersatellite direct wave is removed is subjected to inverse matching filtering processing using the transmission signal corresponding to the intersatellite direct wave as an inverse matching filter function, and the transmission signal corresponding to the bistatic SAR echo in the observation area is used as a matched filter function to perform distance compression, and the intersatellite phase synchronization error obtained in step S4.1 is used to compensate for the bistatic SAR echo in the observation area and perform subsequent imaging processing to realize the bistatic SAR imaging function.

[0019] Furthermore, in the step S1, the coded signal pulse sequence is transmitted in a form of periodic repetitive transmission with the number of pulses of the pulse sequence as a period;

[0020] In the step S2, setting the pulse repetition frequency includes: setting an echo receiving window design constraint and an echo receiving orthogonality design constraint;

[0021] The echo receiving window design constraint is that the radar echo receiving 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 bistatic SAR echo in the observation area.

[0023] Furthermore, in step S4.1, the matched filter function includes performing pulse compression on the intersatellite direct wave and the observation area bistatic SAR echo received at each azimuth moment and mixed in time, respectively using the transmission signal corresponding to the intersatellite direct wave as a matched filter function;

[0024] The method of tracking the extracted intersatellite direct wave includes: calculating the echo distance unit where the intersatellite direct wave is located according to the spatial positions of two satellites, detecting bright lines around the echo distance unit to detect the intersatellite direct wave, and analyzing the bright lines to obtain intersatellite phase synchronization information.

[0025] Furthermore, the intersatellite direct wave is removed by performing range compression on the intersatellite direct wave and the bistatic SAR echo of the observation area which are temporally overlapped by using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, including setting the echo signals in the range unit corresponding to the detected intersatellite direct wave and the surrounding several range units to zero in the echo signal after the range compression;

[0026] The inverse matching filter function includes performing pulse decompression on the echo signal after removing the intersatellite direct wave for each direction time by using the transmission signal corresponding to the intersatellite direct wave as the inverse matching function to obtain the bistatic SAR echo in the observation area;

[0027] The distance compression using the transmission signal corresponding to the echo signal of the bistatic SAR observation area as a matched filter function includes: the bistatic SAR echo of the observation area at each direction and time uses the transmission signal corresponding to the bistatic SAR echo of the observation area as a matching function to perform pulse compression.

[0028] A spaceborne bistatic SAR imaging system provided by the present invention comprises:

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

[0030] Module M2: The pulse repetition frequency is set based on the bistatic configuration. The pulse repetition frequency after setting controls the radar working sequence so that the intersatellite direct wave and the bistatic SAR echo in the observation area can be received by the receiving satellite radar at the same time, and the radar transmission signal waveforms corresponding to the intersatellite direct wave and the bistatic SAR echo in the observation area are different;

[0031] 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 intersatellite direct wave and the bistatic SAR echo of the observation area that are overlapped in time are obtained, and the intersatellite direct wave and the bistatic SAR echo of the observation area obtained at the same time are in an orthogonal relationship;

[0032] Module M4: Based on the acquired temporally mixed intersatellite direct waves and bistatic SAR echoes in the observation area, the intersatellite direct waves and bistatic SAR echoes in the observation area are extracted separately through orthogonal signal processing to realize spaceborne bistatic SAR imaging.

[0033] Furthermore, the module M4 includes:

[0034] Module M4.1: Using the transmission signal corresponding to the intersatellite direct wave as the matched filter function, the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time are range compressed and tracked to extract the intersatellite direct wave. By tracking the extracted intersatellite direct wave, the intersatellite phase synchronization error is inverted to realize the intersatellite synchronization function.

[0035] Module M4.2: The intersatellite direct wave is removed from the signal after the distance compression of the intersatellite direct wave and the bistatic SAR echo in the observation area that are temporally overlapped by using the transmission signal corresponding to the intersatellite direct wave as the matched filter function, so as to obtain the radar signal after the intersatellite direct wave is removed. The radar signal after the intersatellite direct wave is removed is subjected to inverse matching filtering processing using the transmission signal corresponding to the intersatellite direct wave as the inverse matching filter function, and the transmission signal corresponding to the bistatic SAR echo in the observation area is used as the matched filter function for distance compression, and the intersatellite phase synchronization error obtained in module M4.1 is used to compensate for the bistatic SAR echo in the observation area 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 to transmit periodically and repeatedly with the number of pulses of the pulse sequence as a period;

[0037] In the module M2, setting the pulse repetition frequency includes: setting an echo receiving window design constraint and an echo receiving orthogonality design constraint;

[0038] The echo receiving window design constraint is that the radar echo receiving 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 transmission signal corresponding to the inter-satellite direct wave is different from the transmission signal corresponding to the bistatic SAR echo in the observation area.

[0040] Furthermore, in the module M4.1, the matched filter function includes performing pulse compression on the intersatellite direct wave and the observation area bistatic SAR echo received at each position and time, which are overlapped in time, using the transmission signal corresponding to the intersatellite direct wave as the matched filter function;

[0041] The method of tracking the extracted intersatellite direct wave includes: calculating the echo distance unit where the intersatellite direct wave is located according to the spatial positions of two satellites, detecting bright lines around the echo distance unit to detect the intersatellite direct wave, and analyzing the bright lines to obtain intersatellite phase synchronization information.

[0042] Furthermore, the intersatellite direct wave is removed by performing range compression on the intersatellite direct wave and the bistatic SAR echo of the observation area which are temporally overlapped by using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, including setting the echo signals in the range unit corresponding to the detected intersatellite direct wave and the surrounding several range units to zero in the echo signal after the range compression;

[0043] The inverse matching filter function includes performing pulse decompression on the echo signal after removing the intersatellite direct wave for each direction time by using the transmission signal corresponding to the intersatellite direct wave as the inverse matching function to obtain the bistatic SAR echo in the observation area;

[0044] The distance compression using the transmission signal corresponding to the echo signal of the bistatic SAR observation area as a matched filter function includes: the bistatic SAR echo of the observation area at each direction and time uses the transmission signal corresponding to the bistatic SAR echo of the observation area as a matching function to perform pulse compression.

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

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

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

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

[0049] Figure 1 It is a schematic diagram of the design and signal processing flow of the spaceborne bistatic SAR system of the present invention;

[0050] Figure 2 It is the intersatellite direct wave and the bistatic SAR echo signal diagram of the observation area;

[0051] Figure 3 It is a display diagram of intersatellite direct wave signal extracted by matched filtering;

[0052] Figure 4This is the echo signal display after the direct wave is removed by inverse matching filtering and then matched filtering.

[0053] Figure 5 This is the result of bistatic SAR point target imaging after two-dimensional focusing. DETAILED DESCRIPTION

[0054] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0055] A spaceborne bistatic SAR imaging method provided by the present invention comprises:

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

[0057] Step S2: The pulse repetition frequency is set based on the dual-base configuration. The pulse repetition frequency after setting controls the radar working sequence so that the inter-satellite direct wave and the observation area dual-base radar echo can be received by the receiving satellite radar at the same time, and the radar transmission signal waveforms corresponding to the inter-satellite direct wave and the observation area dual-base SAR echo are different.

[0058] Step S3: obtaining the inter-satellite direct wave and the observation area bistatic SAR system echo that are overlapped in time, and the inter-satellite direct wave and the observation area bistatic SAR echo signals obtained at the same time are orthogonal.

[0059] Step S4: Based on the acquired temporally mixed intersatellite direct waves and observation area bistatic SAR echoes, the intersatellite direct waves and observation area bistatic SAR echo signals are extracted respectively through orthogonal signal processing, thereby realizing direct wave extraction and bistatic SAR imaging functions.

[0060] Specifically, the periodic transmission sequence used by the transmitting satellite to transmit the radar in step S1 is a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are orthogonal. The signal coding form is an orthogonal coded pulse sequence, and any two pulses in the sequence are orthogonal in the time domain. The pulse sequence transmission form is periodic repeated transmission with the number of pulses of the pulse sequence as a period.

[0061] Specifically, setting the pulse repetition frequency in step S2 includes: 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 intersatellite 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 intersatellite direct wave is different from the transmission signal corresponding to the bistatic SAR echo in the observation area.

[0062] Specifically, step S4 includes:

[0063] Step S4.1: Use the transmission signal corresponding to the inter-satellite direct wave as the matched filter function to perform distance compression tracking on the inter-satellite direct wave and the bi-base SAR echo in the observation area that are overlapped in time to extract the inter-satellite direct wave signal. By tracking the extracted inter-satellite direct wave signal, the inter-satellite phase synchronization error is inverted to realize the inter-satellite synchronization function.

[0064] Step S4.2: Remove the intersatellite direct wave from the signal after distance compression of the intersatellite direct wave and the bistatic SAR echo in the observation area that are temporally overlapped by using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, and obtain the radar signal after the intersatellite direct wave is removed. Perform inverse matching filtering on the radar signal after the intersatellite direct wave is removed using the transmission signal corresponding to the intersatellite direct wave as an inverse matching filter function, and perform distance compression using the transmission signal corresponding to the echo signal in the bistatic SAR observation area as a matched filter function, and use the intersatellite synchronization phase estimation value obtained in step S4.1 to compensate the bistatic SAR observation area echo signal and perform subsequent imaging processing to realize the bistatic SAR imaging function.

[0065] Specifically, the matched filter function in step S4.1 includes pulse compression of the intersatellite direct wave and the bistatic SAR echo in the observation area which are received at each position and are mixed in time, respectively, using the transmission signal corresponding to the intersatellite direct wave as the matched filter function. The intersatellite direct wave signal extracted by tracking includes calculating the echo distance unit where the intersatellite direct wave is located according to the spatial positions of the two satellites, and detecting the bright line around the echo distance unit to realize the detection of the intersatellite direct wave, and analyzing and obtaining the intersatellite phase synchronization information accordingly.

[0066] Specifically, the distance compressed signal removes the intersatellite direct wave, including setting the echo signals in the distance unit corresponding to the detected intersatellite direct wave signal and the surrounding several distance units to zero in the echo signal after the distance compression. The inverse matching filter function includes using the transmission signal corresponding to the intersatellite direct wave as the inverse matching function to perform pulse decompression for the echo signal after the intersatellite direct wave is removed at each orientation time, so as to obtain the bistatic SAR echo signal in the observation area. The distance compression using the transmission signal corresponding to the bistatic SAR echo signal in the observation area as the matched filtering function includes: the bistatic SAR echo signal in the observation area at each orientation time uses the transmission signal corresponding to the bistatic SAR echo in the observation area as the matching function to perform pulse compression.

[0067] The present invention will be further described below in conjunction with the accompanying drawings.

[0068] Refer to the attached Figure 1 The theoretical analysis basis of the present invention is briefly introduced as follows:

[0069] A two-in-one system for inter-satellite synchronization and ground imaging of a spaceborne bistatic SAR based on signal coding and a method thereof, characterized in that it includes radar orthogonal coding signal sequence design, pulse repetition frequency design, direct wave signal extraction and processing, and target echo signal extraction and processing. Among them:

[0070] 1) Radar coded signal sequence design

[0071] The coding signal pulse sequence is transmitted periodically, and any pulses in the pulse signal sequence are orthogonal. Its 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 kth transmitted pulse, t r is the distance to 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 intersatellite direct wave and the observation area bistatic SAR echo are received simultaneously, but the radar transmission signal waveforms corresponding to the two have different pulse repetition frequencies (PRFs). The mathematical model is as follows:

[0077]

[0078]

[0079]

[0080]

[0081]

[0082] Where PRF is the pulse repetition period, R near and R far is the shortest and longest slant distance of the observation area within a synthetic aperture time, c is the speed of light, T proc To protect time, R s is the distance between the two stars, is the floor operator, and mod(·) is the remainder operator.

[0083] 3) Extraction and processing of intersatellite direct wave signals

[0084] The transmission signal corresponding to the intersatellite direct wave signal is used as the matched filter function to perform distance compression on the received echo signal, and the intersatellite direct wave signal is extracted by tracking and the intersatellite phase synchronization information is inverted to realize the intersatellite synchronization function;

[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] In the formula, s(t a ,t r ) is the radar echo, s dir (t a ,t r ) is the intersatellite direct wave, s obs (t a ,t r ) is the echo in the observation area, and ta is the azimuth time. Assuming that the intersatellite direct wave s dir (t a ,t r ) The corresponding transmitted signal is s k (t r ), using this signal to s(t a ,t r ) for pulse compression.

[0088]

[0089] In the formula, s dir,comp (t a ,t r ) is the signal after the intersatellite direct wave pulse is compressed, s' obs,comp (t a ,t r ) is the compressed signal of the echo in the observation area.

[0090] After pulse compression, the intersatellite direct wave will appear as a bright line in the image, but due to the mismatch of the matched filter, the signal in the observation area is still in a defocused state. At this time, the echo distance unit where the intersatellite direct wave is located can be calculated based on the distance information of the binary stars, and the bright line can be detected around the unit to realize the detection and extraction of the intersatellite direct wave signal. Subsequently, the existing mature direct wave phase synchronization method can be used to realize the extraction of intersatellite phase synchronization information and complete the intersatellite synchronization function.

[0091] 4) Extraction and processing of echo signals in the ground observation area of ​​dual-base SAR

[0092] The intersatellite direct wave signal is removed from the echo signal after range compression, and then the transmission signal corresponding to the intersatellite direct wave signal is used as the inverse matching filter function for inverse matching filter processing, and the transmission signal corresponding to the echo signal in the observation area is used as the matching filter function for range compression, and the intersatellite phase synchronization information is compensated. Then, the imaging processing operation is performed to realize the dual-base SAR imaging function.

[0093] The echo signal after removing the intersatellite 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] By using the transmission signal corresponding to the intersatellite direct wave as the inverse matching filter function and performing inverse matching filter processing on it, the echo signal of the observation area can be obtained.

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

[0097] Assume that the transmitted signal corresponding to the echo in the radar observation area at this moment is s j (t r ), after distance compression, the mature dual-base SAR imaging processing technology can be used to obtain the SAR image of the observation area and complete the dual-base ground imaging function, such as Figure 5 shown.

[0098] Here, simulation data is used to verify the effectiveness of the present invention. The simulation parameters are shown in Table 1.

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

[0100]

[0101] Assume that the coded signal uses the following short offset quadrature waveform:

[0102]

[0103] In the formula, K r To adjust the frequency of the transmitted signal,

[0104]

[0105] The pulse repetition frequency (PRF) of this patented design is 3545Hz. Figure 2 , Figure 3 and Figure 4 It can be seen that the present invention can simultaneously realize inter-satellite direct waves and observation area echo signals, realizing an integrated design of the two.

[0106] The method of the present invention receives dual-base SAR echo data and direct wave data in an integrated manner by setting an orthogonal coding signal pulse sequence and controlling the pulse repetition frequency, and realizes signal separation and extraction by utilizing the orthogonality of the two, thereby effectively solving the problems of phase synchronization and echo reception timing.

[0107] The present invention utilizes signal coding technology to simultaneously receive direct waves and bistatic SAR echo data, realizing the integration of inter-satellite phase synchronization information extraction and satellite-borne bistatic SAR earth imaging, which is the first of its kind in China and abroad.

[0108] The present invention also provides a spaceborne bistatic SAR imaging system, which can be implemented by executing the process steps of the spaceborne bistatic SAR imaging method, that is, those skilled in the art can understand the spaceborne bistatic SAR imaging method as a preferred implementation of the spaceborne bistatic SAR imaging system. The system includes:

[0109] Module M1: Let the periodic transmission sequence adopted by the transmitting satellite to transmit the radar be a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are orthogonal.

[0110] Module M2: The pulse repetition frequency is set based on the dual-base configuration. The pulse repetition frequency after setting controls the radar working sequence so that the inter-satellite direct wave and the observation area dual-base SAR echo can be received by the receiving satellite radar at the same time, and the radar transmission signal waveforms corresponding to the inter-satellite direct wave and the observation area dual-base SAR echo 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, the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time are obtained, and the intersatellite direct wave and the bistatic SAR echo in the observation area obtained at the same time are in an orthogonal relationship.

[0112] Module M4: Based on the acquired temporally mixed intersatellite direct waves and bistatic SAR echoes in the observation area, the intersatellite direct waves and bistatic SAR echoes in the observation area are extracted separately through orthogonal signal processing to realize spaceborne bistatic SAR imaging.

[0113] Those skilled in the art know that, in addition to realizing the system and its various devices, modules, and units provided by the present invention in a purely computer-readable program code, it is entirely possible to realize the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for realizing various functions can also be regarded as structures within the hardware component; the devices, modules, and units for realizing various functions can also be regarded as both software modules for realizing the method and structures within the hardware component.

[0114] The above describes the specific embodiments of the present invention. It should be understood that the present invention 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 essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A spaceborne bistatic SAR imaging method, characterized in that: include: Step S1: The periodic transmission sequence used by the active radar satellite is set to be a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are orthogonal to each other; Step S2: setting a pulse repetition frequency based on a bistatic configuration, wherein the pulse repetition frequency after setting controls the radar working sequence so that the inter-satellite direct wave and the observation area bistatic SAR echo can be received by the receiving satellite radar at the same time, and the radar transmission signal waveforms corresponding to the inter-satellite direct wave and the observation area bistatic SAR echo are different; Step S3: obtaining the inter-satellite direct wave and the bistatic SAR echo of the observation area which are overlapped 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; Step S4: Based on the acquired temporally mixed intersatellite direct waves and observation area bistatic SAR echoes, the intersatellite direct waves and the observation area bistatic SAR echoes are extracted respectively through orthogonal signal processing to achieve spaceborne bistatic SAR imaging.

2. The spaceborne bistatic SAR imaging method according to claim 1, characterized in that: The step S4 comprises: Step S4.1: Using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time are range-compressed and tracked to extract the intersatellite direct wave, and the intersatellite phase synchronization error is inverted by tracking the extracted intersatellite direct wave to realize the intersatellite synchronization function; Step S4.2: Using the transmission signal corresponding to the intersatellite direct wave as a matched filter function, the signal after distance compression of the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time is removed from the intersatellite direct wave to obtain the radar signal after the intersatellite direct wave is removed, and the radar signal after the intersatellite direct wave is removed is subjected to inverse matching filtering processing using the transmission signal corresponding to the intersatellite direct wave as an inverse matching filter function, and the transmission signal corresponding to the bistatic SAR echo in the observation area is used as a matched filter function to perform distance compression, and the intersatellite phase synchronization error obtained in step S4.1 is used to compensate for the bistatic SAR echo in the observation area and perform subsequent imaging processing to realize the bistatic SAR imaging function.

3. The spaceborne bistatic SAR imaging method according to claim 1, characterized in that: In the step S1, the coded signal pulse sequence is transmitted in a form of periodic repetitive transmission with the number of pulses of the pulse sequence as a period; In the step S2, setting the pulse repetition frequency includes: 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 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 bistatic SAR echo in the observation area.

4. The spaceborne bistatic SAR imaging method according to claim 2, characterized in that: In step S4.1, the matched filter function includes performing pulse compression on the intersatellite direct wave and the observation area bistatic SAR echo received at each position and time, respectively using the transmission signal corresponding to the intersatellite direct wave as the matched filter function; The method of tracking the extracted intersatellite direct wave includes: calculating the echo distance unit where the intersatellite direct wave is located according to the spatial positions of two satellites, detecting bright lines around the echo distance unit to detect the intersatellite direct wave, and analyzing the bright lines to obtain intersatellite phase synchronization information.

5. The spaceborne bistatic SAR imaging method according to claim 2, characterized in that: The intersatellite direct wave is removed by using the transmission signal corresponding to the intersatellite direct wave as a matched filter function to perform range compression on the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time, including setting the echo signals in the range unit corresponding to the detected intersatellite direct wave and the surrounding several range units to zero in the echo signal after the range compression; The inverse matching filter function includes performing pulse decompression on the echo signal after removing the intersatellite direct wave for each direction time by using the transmission signal corresponding to the intersatellite direct wave as the inverse matching function to obtain the bistatic SAR echo in the observation area; The distance compression using the transmission signal corresponding to the echo signal of the bistatic SAR observation area as a matched filter function includes: the bistatic SAR echo of the observation area at each direction and time uses the transmission signal corresponding to the bistatic SAR echo of the observation area as a matching function to perform pulse compression.

6. A spaceborne bistatic SAR imaging system, characterized in that: include: Module M1: Let the periodic transmission sequence used by the transmitting satellite to transmit the radar be a coded signal pulse sequence, and any pulses in the coded signal pulse sequence are orthogonal; Module M2: The pulse repetition frequency is set based on the bistatic configuration. The pulse repetition frequency after setting controls the radar working sequence so that the intersatellite direct wave and the bistatic SAR echo in the observation area can be received by the receiving satellite radar at the same time, and the radar transmission signal waveforms corresponding to the intersatellite direct wave and the bistatic SAR echo in 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 intersatellite direct wave and the bistatic SAR echo of the observation area that are overlapped in time are obtained, and the intersatellite 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 acquired temporally mixed intersatellite direct waves and bistatic SAR echoes in the observation area, the intersatellite direct waves and bistatic SAR echoes in the observation area are extracted separately through orthogonal signal processing to realize spaceborne bistatic SAR imaging.

7. The spaceborne bistatic SAR imaging system according to claim 6, characterized in that: The module M4 comprises: Module M4.1: Using the transmission signal corresponding to the intersatellite direct wave as the matched filter function, the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time are range compressed and tracked to extract the intersatellite direct wave. By tracking the extracted intersatellite direct wave, the intersatellite phase synchronization error is inverted to realize the intersatellite synchronization function. Module M4.2: The intersatellite direct wave is removed from the signal after the distance compression of the intersatellite direct wave and the bistatic SAR echo in the observation area that are temporally overlapped by using the transmission signal corresponding to the intersatellite direct wave as the matched filter function, so as to obtain the radar signal after the intersatellite direct wave is removed. The radar signal after the intersatellite direct wave is removed is subjected to inverse matching filtering processing using the transmission signal corresponding to the intersatellite direct wave as the inverse matching filter function, and the transmission signal corresponding to the bistatic SAR echo in the observation area is used as the matched filter function for distance compression, and the intersatellite phase synchronization error obtained in module M4.1 is used to compensate for the bistatic SAR echo in the observation area and perform subsequent imaging processing to realize the bistatic SAR imaging function.

8. The spaceborne bistatic SAR imaging system according to claim 6, characterized in that: In the module M1, the coded signal pulse sequence is transmitted in a form of periodic repetitive transmission with the number of pulses of the pulse sequence as a period; In the module M2, setting the pulse repetition frequency includes: 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 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 bistatic SAR echo in the observation area.

9. The spaceborne bistatic SAR imaging system according to claim 7, characterized in that: In the module M4.1, the matched filter function includes pulse compression of the intersatellite direct wave and the observation area bistatic SAR echo received at each position and time, respectively using the transmission signal corresponding to the intersatellite direct wave as the matched filter function; The method of tracking the extracted intersatellite direct wave includes: calculating the echo distance unit where the intersatellite direct wave is located according to the spatial positions of two satellites, detecting bright lines around the echo distance unit to detect the intersatellite direct wave, and analyzing the bright lines to obtain intersatellite phase synchronization information.

10. The spaceborne bistatic SAR imaging system according to claim 7, characterized in that: The intersatellite direct wave is removed by using the transmission signal corresponding to the intersatellite direct wave as a matched filter function to perform range compression on the intersatellite direct wave and the bistatic SAR echo in the observation area that are overlapped in time, including setting the echo signals in the range unit corresponding to the detected intersatellite direct wave and the surrounding several range units to zero in the echo signal after the range compression; The inverse matching filter function includes performing pulse decompression on the echo signal after removing the intersatellite direct wave for each direction time by using the transmission signal corresponding to the intersatellite direct wave as the inverse matching function to obtain the bistatic SAR echo in the observation area; The distance compression using the transmission signal corresponding to the echo signal of the bistatic SAR observation area as a matched filter function includes: the bistatic SAR echo of the observation area at each direction and time uses the transmission signal corresponding to the bistatic SAR echo of the observation area as a matching function to perform pulse compression.

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