A frequency synchronization method, device and system

Abstract

The embodiment of the present application provides a frequency synchronization method, device and system, the method is applied to a frequency synchronization device, and comprises the following steps: obtaining a first propagation distance and a first propagation time of a first synchronization signal generated by a transmitter in a first transmission link, and a second propagation distance and a second propagation time of a second synchronization signal generated by a receiver in a second transmission link; wherein the transmitter and the receiver are any distributed synthetic aperture radars located on different platforms, and the first propagation distance is equal to the second propagation distance; determining the frequency of the first synchronization signal based on the light speed, the multiple relationship between the first propagation time and the minimum interval of a transmitter timer, and the first propagation distance; determining the frequency of the second synchronization signal based on the light speed, the multiple relationship between the second propagation time and the minimum interval of a receiver timer, and the second propagation distance; and determining the frequency deviation based on the frequency of the first synchronization signal and the frequency of the second synchronization signal.

Description

Technical Field

[0001] This application relates to, but is not limited to, the field of synthetic aperture radar (SAR) technology, and particularly to a frequency synchronization method, device, and system. Background Technology

[0002] In distributed SAR, the transmitter and receiver reside on different platforms and use different ultra-stable oscillators (USOs) to generate clock signals, leading to frequency (phase) asynchrony. Frequency asynchrony refers to the inconsistency between the transmitter's modulation frequency and the receiver's demodulation frequency, resulting in phase errors in the demodulated echo signal. In ping-pong, single-transmitter-multiple-receiver, or multiple-transmitter-multiple-receiver modes, phase errors in the secondary satellite echoes reduce image resolution and signal-to-noise ratio (SNR), and cause horizontal / vertical deviations in the digital elevation model. Therefore, frequency asynchrony is a significant challenge in the implementation of distributed SAR systems. Summary of the Invention

[0003] In view of this, embodiments of this application provide a frequency synchronization method, device, and system.

[0004] The technical solution of this application is implemented as follows:

[0005] In a first aspect, embodiments of this application provide a frequency synchronization method, the method comprising: acquiring a first propagation distance and a first propagation time of a first synchronization signal generated by a transmitter in a first transmission link, and a second propagation distance and a second propagation time of a second synchronization signal generated by a receiver in a second transmission link; wherein the transmitter and the receiver are arbitrary distributed synthetic aperture radars located on different platforms, and the first propagation distance and the second propagation distance are equal; determining the frequency of the first synchronization signal based on the speed of light, a multiple relationship between the first propagation time and the minimum interval of the transmitter timer, and the first propagation distance; determining the frequency of the second synchronization signal based on the speed of light, a multiple relationship between the second propagation time and the minimum interval of the receiver timer, and the second propagation distance; and determining a frequency deviation based on the frequency of the first synchronization signal and the frequency of the second synchronization signal; wherein the frequency deviation represents the difference between the frequency of the first synchronization signal and the frequency of the second synchronization signal.

[0006] Secondly, embodiments of this application provide a frequency synchronization device, the device comprising an ultra-stable crystal oscillator, a synchronous transmitter, a first cable, a first circulator, a second cable, a second circulator, a third cable, and a signal acquisition device, wherein:

[0007] The ultra-stable crystal oscillator is used to provide a first frequency signal; wherein the ultra-stable crystal oscillator is a first ultra-stable crystal oscillator or a second ultra-stable crystal oscillator;

[0008] The synchronous transmitter is used to generate a first synchronization signal carrying the first frequency signal and transmit the first synchronization signal to the first port of the first circulator through the first cable.

[0009] The second port of the first circulator is connected to the second port of the second circulator via the second cable;

[0010] The third port of the second circulator is connected to the first port of the second circulator via the third cable;

[0011] The signal acquisition device is connected to the third port of the first circulator and is used to receive the first synchronization signal based on the first frequency signal.

[0012] Thirdly, embodiments of this application provide a frequency synchronization system, the system comprising M transmitters and N receivers, wherein each transmitter and each receiver employs the aforementioned frequency synchronization device.

[0013] In this embodiment, the frequency synchronization method is implemented using the same frequency synchronization device. That is, the transmission links for synchronization signals of different frequencies within the same frequency synchronization device are identical. This results in extremely high accuracy of the frequency deviation obtained by subtracting the frequencies of the synchronization signals generated by transmitters and receivers from those of different platforms, thereby solving the problem of high-precision frequency synchronization in distributed SAR. It should be understood that the above general description and the following detailed description are merely exemplary and explanatory, and not intended to limit the technical solutions of this application. Attached Figure Description

[0014] In the accompanying drawings (which are not necessarily drawn to scale), similar reference numerals may describe similar parts in different views. Similar reference numerals with different letter suffixes may indicate different examples of similar parts. The drawings illustrate, by way of example and not limitation, the various embodiments discussed herein.

[0015] Figure 1 This is a schematic diagram of the transmitter and receiver in a distributed SAR system.

[0016] Figure 2 A schematic diagram illustrating the implementation process of a frequency synchronization method applied to a frequency synchronization device, provided in an embodiment of this application;

[0017] Figure 3 This is a schematic diagram of the composition structure of a frequency synchronization device provided in an embodiment of this application;

[0018] Figure 4 A schematic diagram of a frequency synchronization system applied to a transmitter and receiver is provided as an embodiment of this application;

[0019] Figure 5 A schematic diagram illustrating the frequency of the synchronization signal generated by the transmitter, the frequency of the synchronization signal generated by the receiver, and a preset frequency provided in the embodiments of this application;

[0020] Figure 6(a) is a schematic diagram of the simulation results showing the estimated frequency deviation Δf as a function of SNR, provided by the embodiments of this application. Figure 1 ;

[0021] Figure 6(b) is a schematic diagram of the simulation results showing the variation of residual frequency error Δf with SNR provided by the embodiment of this application. Figure 2 ;

[0022] Figure 7(a) is a schematic diagram of the simulation results provided by the embodiment of this application, showing the change of the estimated frequency deviation Δf as a function of the preset frequency deviation Δf. Figure 3 ;

[0023] Figure 7(b) is a schematic diagram of the simulation results showing the variation of residual frequency error Δf with preset frequency deviation Δf provided by the embodiment of this application. Figure 4 ;

[0024] Figure 8(a) is a schematic diagram of the simulation results showing the change of the estimated frequency deviation Δf with the number of experiments provided by the embodiment of this application. Figure 5 ;

[0025] Figure 8(b) is a schematic diagram of the simulation results showing the change of residual frequency error Δf with the number of experiments provided by the embodiment of this application;

[0026] Figure 9 Schematic diagram seven showing the simulation results of the residual frequency error Δf as a function of SNR, provided for embodiments of this application;

[0027] Figure 10(a) is a schematic diagram of the simulation results showing the variation of the estimated frequency deviation Δf with the cable length ΔL provided by the embodiment of this application;

[0028] Figure 10(b) is a schematic diagram of the simulation results showing the variation of residual frequency error Δf with cable length ΔL provided by the embodiment of this application. Figure 9 . Detailed Implementation

[0029] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the specific embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0030] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this disclosure. However, it will be apparent to those skilled in the art that this disclosure may be practiced without one or more of these details. In other instances, to avoid confusion with this disclosure, certain technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and structures described in detail.

[0031] In the accompanying drawings, for clarity, the dimensions of layers, areas, and elements, as well as their relative dimensions, may be exaggerated. The same reference numerals denote the same elements throughout.

[0032] It should be understood that when an element or layer is referred to as "on," "adjacent to," "connected to," or "coupled to" other elements or layers, it may be directly on, adjacent to, connected to, or coupled to other elements or layers, or there may be intervening elements or layers. Conversely, when an element is referred to as "directly on," "directly adjacent to," "directly connected to," or "directly coupled to" other elements or layers, there are no intervening elements or layers. It should be understood that although the terms first, second, third, etc., may be used to describe various elements, components, areas, layers, and / or portions, these elements, components, areas, layers, and / or portions should not be limited by these terms. These terms are only used to distinguish one element, component, area, layer, or portion from another element, component, area, layer, or portion. Therefore, without departing from the teachings of this disclosure, the first element, component, area, layer, or portion discussed below may be referred to as a second element, component, area, layer, or portion. And the discussion of a second element, component, area, layer, or portion does not imply that the first element, component, area, layer, or portion necessarily exists in this disclosure.

[0033] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprise” and / or “comprising,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0034] To facilitate understanding of the embodiments of this application, the concepts and technical solutions related to the embodiments of this application will be briefly introduced below.

[0035] A distributed SAR system consists of M spatially separated transmitters and N receivers, such as... Figure 1 As shown, T1 and T2 represent transmitters in the distributed SAR system, and R1 and R2 represent receivers in the distributed SAR system. Through high-precision collaborative work between multiple transmitters and receivers, various observation tasks can be achieved, such as high-resolution wide-swath imaging, multi-angle imaging, and interferometry. Distributed SAR systems possess advantages such as wide spatial distribution, flexible baseline adjustment capabilities, and strong concealment, making them a technological high ground that major spacefaring nations are vying to conquer. In the past two decades, the United States, Germany, Italy, China, and other countries have conducted spaceborne, spaceborne and airborne, and airborne distributed SAR Earth observation experiments and space missions. Among them, the German Aerospace Center (DLR) launched two spaceborne SAR satellites, TSX and TDX, in 2007 and 2010 respectively, forming the well-known TanDEM-X bistatic SAR system. In 2022, my country launched two L-band fully polarimetric LuTan-1 satellite-borne bistatic SAR systems, and in 2023 launched a macro-... Figure 1 The satellite-borne distributed SAR system.

[0036] Here, we assume the SAR platform transmitting radar signals is the primary satellite, and the SAR platform receiving echoes is the secondary satellite. Based on communication principles, phase synchronization methods for distributed SAR systems can currently be divided into three categories. The first category uses full-duplex communication methods, such as the pulse-switched phase synchronization scheme used in the TanDEM-X and LuTan-1 missions, and the Double-MirrorLink scheme planned for the German Aerospace Center's MirrorSAR mission. The second category uses simplex communication methods, such as the direct-wave synchronization scheme using the direct wave transmitted by the primary satellite as a matched filter, and the push-to-talk (PTT) time and frequency synchronization scheme proposed by scholars from the Aerospace Information Research Institute of the Chinese Academy of Sciences in 2021. The third category uses link-free methods, such as the GPS-disciplined atomic clock and post-processing schemes based on radar echoes. All of the above phase synchronization schemes have certain drawbacks, and their frequency stability is approximately 10... -11 ~10 -12 In this range, it is difficult to meet the requirements of higher precision interferometric measurement tasks.

[0037] Because the frequency stability of existing technologies such as pulse-switched phase synchronization, direct-wave synchronization, GPS-disciplined atomic clock synchronization, and echo-based post-processing synchronization is approximately 10... -11 ~10 -12 In the meantime, it is difficult to meet the requirements of higher precision interferometric measurements. Therefore, distributed SAR frequency synchronization is an important research direction.

[0038] Based on this, embodiments of this application provide a frequency synchronization method, referring to... Figure 2 This method, applied to a frequency synchronization device, may include the following steps S201 to S204, wherein:

[0039] Step S201: Obtain the first propagation distance and first propagation time of the first synchronization signal generated by the transmitter in the first transmission link, or the second propagation distance and second propagation time of the second synchronization signal generated by the receiver in the second transmission link; wherein the transmitter and the receiver are arbitrary distributed synthetic aperture radars located on different platforms, and the first propagation distance and the second propagation distance are equal;

[0040] In implementation, the transmitter and receiver use the same system framework to achieve frequency synchronization. This means that the transmission links for the first synchronization signal generated by the transmitter and the second synchronization signal generated by the receiver are identical during transmission; that is, the first transmission link for the first synchronization signal generated by the transmitter is equivalent to the second transmission link for the second synchronization signal generated by the receiver. Since the first transmission link for the first synchronization signal generated by the transmitter is equivalent to the second transmission link for the second synchronization signal generated by the receiver, the first propagation distance of the first synchronization signal generated by the transmitter in the first transmission link is equal to the second propagation distance of the second synchronization signal generated by the receiver in the second transmission link.

[0041] Step S202: Determine the frequency of the first synchronization signal based on the speed of light, the multiple relationship between the first propagation time and the minimum interval of the transmitter timer, and the first propagation distance;

[0042] Here, the frequency f of the first synchronization signal is determined by the following formula (1). Ti .

[0043]

[0044] Where L represents the first propagation distance, in meters (m);

[0045] C represents the constant speed of light, approximately equal to 3 × 10⁻⁶. 8 m / s (meters per second);

[0046] X Ti The propagation time is represented by the minimum interval Δt of the transmitter timer. Ti X Ti Times, X Ti ∈R.

[0047] Step S203: Determine the frequency of the second synchronization signal based on the speed of light, the multiple relationship between the second propagation time and the minimum interval of the receiver timer, and the second propagation distance;

[0048] Here, the frequency f of the second synchronization signal is determined by the following formula (2). Rj .

[0049]

[0050] Where L represents the second propagation distance, in meters (m);

[0051] X Rj The propagation time is represented by the minimum interval Δt of the receiver timer. Rj X Rj Times, X Rj ∈R.

[0052] Step S204: Determine the frequency deviation based on the frequency of the first synchronization signal and the frequency of the second synchronization signal; wherein the frequency deviation represents the difference between the frequency of the first synchronization signal and the frequency of the second synchronization signal.

[0053] Here, the frequency deviation Δf is determined using the following formula (3). TiRj .

[0054]

[0055] In this embodiment, the frequency synchronization method is implemented using the same frequency synchronization device. This means that the transmission links for synchronization signals of different frequencies within the same frequency synchronization device are identical. Consequently, the frequency deviation obtained by subtracting the frequencies of the synchronization signals generated by transmitters and receivers from those of different platforms has extremely high accuracy, thus solving the problem of high-precision frequency synchronization in distributed SAR.

[0056] In some embodiments, the method further includes: compensating for the phase error corresponding to the frequency deviation based on the frequency deviation, so as to achieve distributed SAR phase synchronization.

[0057] In some embodiments, the transmitter is configured to generate a first synchronization signal carrying the first frequency signal based on a first frequency signal generated by a first ultra-stable crystal oscillator in itself, and send the first synchronization signal to a signal acquisition unit in the transmitter; the receiver is configured to generate a second synchronization signal carrying the second frequency signal based on a second frequency signal generated by a second ultra-stable crystal oscillator in itself, and send the second synchronization signal to a signal acquisition unit in the receiver.

[0058] Here, because the frequency of the first frequency signal generated by the first ultra-stable crystal oscillator in the transmitter is different from the frequency of the second frequency signal generated by the second ultra-stable crystal oscillator in the receiver, the frequency of the first synchronization signal carrying the first frequency signal generated by the transmitter is different from the frequency of the second synchronization signal carrying the second frequency signal generated by the receiver.

[0059] In some embodiments, the method further includes: determining, based on the speed of light and the first transmission distance, the first propagation time of the first synchronization signal in the first transmission link; and the second propagation time of the second synchronization signal in the second transmission link; wherein the first propagation time is equal to the second propagation time.

[0060] Here, the first propagation time T of the first synchronization signal in the first transmission link is determined by the following formula (4). Ti .

[0061]

[0062] Among them, T Ti The value in seconds (s) represents the time required for the first synchronization signal generated by the transmitter to travel through a cable of length L.

[0063] The second propagation time T of the second synchronization signal in the second transmission link is determined by the following formula (5). Rj .

[0064]

[0065] Among them, T Rj The value in seconds (s) represents the time required for the first synchronization signal generated by the transmitter to travel through a cable of length L.

[0066] This application provides a frequency synchronization device, referencing... Figure 3 The device includes an ultra-stable crystal oscillator 301, a synchronous transmitter 302, a first cable 303, a first circulator, a second cable 307, a second circulator, a third cable 311, and a signal acquisition unit 312, wherein:

[0067] The ultra-stable crystal oscillator 301 is used to provide a first frequency signal; wherein the ultra-stable crystal oscillator is a first ultra-stable crystal oscillator or a second ultra-stable crystal oscillator;

[0068] The synchronous transmitter 302 is used to generate a first synchronization signal carrying the first frequency signal and transmit the first synchronization signal to the first port 304 of the first circulator through the first cable 303.

[0069] The second port 305 of the first circulator is connected to the second port 308 of the second circulator via the second cable 307;

[0070] The third port 309 of the second circulator is connected to the first port 310 of the second circulator via the third cable 311;

[0071] The signal acquisition unit 312 is connected to the third port 306 of the first circulator and is used to receive the first synchronization signal based on the first frequency signal.

[0072] In some embodiments, the first propagation distance of the first synchronization signal in the device is a preset value; wherein the first propagation distance is the sum of the length of the first cable, the length of the third cable, and twice the length of the second cable.

[0073] Here, the first propagation distance is the most accurate value of the frequency deviation obtained from the experimental results. The preset value can be a specific value or a value within a preset range.

[0074] Continue to refer to Figure 3 Assuming the length of the first cable 303 is L1, the length of the second cable 307 is L2, and the length of the third cable 311 is L3, the first propagation distance of the first synchronization signal in the frequency synchronization device is L = L1 + 2 × L2 + L3. The function of the first cable 303 is that when the input end (transmitter) receives a signal, the output end (receiver) can also output a signal. The function of the second cable 307 is that the first synchronization signal can pass through twice, saving cable length. The function of the third cable 311 is that the first synchronization signal must be output from the output end to the signal acquisition device; it can only be output after the entire first synchronization signal has entered the third cable 311. Furthermore, for a circulator, it is impossible to simultaneously input and output the first synchronization signal, so a separate circulator and cable (i.e.,...) are required. Figure 3 (The second circulator and the third cable).

[0075] In some embodiments, the lengths of the first cable and the third cable are both related to the pulse width of the first synchronization signal.

[0076] In practice, the lengths of the first cable and the third cable are determined based on the pulse width of the first synchronization signal input from the input terminal to the first link. In other words, the lengths of the first cable and the third cable are determined by the pulse width of the first synchronization signal.

[0077] In some embodiments, the frequency synchronization device is a receiver or a transmitter.

[0078] This application provides a frequency synchronization system, referencing... Figure 4 The system includes M transmitters and N receivers, and each transmitter and each receiver uses the aforementioned frequency synchronization device.

[0079] Here, a distributed SAR system is formed by M transmitters and N receivers located on different platforms. This system can accurately estimate frequency deviations, thereby enabling tasks such as high-precision phase synchronization and high-precision measurement and mapping of distributed SAR.

[0080] To address the issue of high-precision frequency synchronization in distributed SAR, this application proposes a frequency synchronization method for distributed synthetic aperture radar without inter-satellite links. Here, it is assumed that the frequencies generated by the USOs (ultra-stable crystal oscillators) of the M transmitters are f0 and f1 respectively. Ti (i.e., the frequency of the first synchronization signal mentioned above), i∈{1,2,…,M}, and the frequencies generated by the USOs of the N receivers are respectively f Rj (i.e., the frequency of the second synchronization signal mentioned above), j∈{1,2,…,N} and the preset frequency is f0, where f Ti f RjThe relationship with f0 is as follows Figure 5 As shown.

[0081] according to Figure 3 and Figure 5 It can be seen that carrying USO Ti The frequency is f Ti =f0+Δf Ti The time required for the synchronization signal generated by Ti to travel through a cable of length L (i.e., the first propagation time mentioned above) is:

[0082]

[0083] Where, assuming

[0084] Similarly, carrying USO Rj The frequency is f Rj =f0+Δf Rj The time required for the synchronization signal generated by Rj to travel through a cable of length L (i.e., the second propagation time mentioned above) is:

[0085]

[0086] Where, assuming

[0087] Based on (5) and (6), the frequencies of Ti and Rj can be obtained as follows:

[0088]

[0089]

[0090] Furthermore, the frequency deviation between Ti and Rj can be expressed as:

[0091]

[0092] According to the above formula (3), by accurately measuring the time required for the synchronization signals generated by the Ti transmitter and Rj receiver to travel through a cable of constant length L, the frequency deviation Δf between the first synchronization signal generated by the transmitter and the second synchronization signal generated by the receiver of different platforms can be estimated. TiRj Finally, based on the estimated frequency deviations, the phase errors corresponding to these deviations are compensated in post-processing to achieve distributed SAR phase synchronization.

[0093] To realize a distributed SAR frequency synchronization method without inter-satellite links, this application proposes a synchronization system framework (i.e., the aforementioned frequency synchronization device), such as... Figure 3 As shown. The i-th transmitter Ti and the j-th receiver R jAll systems employ the same system framework for frequency synchronization. Without loss of generality, the workflow of this system framework will be described using transmitter Ti as an example.

[0094] Transmitter Ti transmits a linear frequency modulated synchronization signal with bandwidth Br and pulse width Tr. This linear frequency modulated synchronization signal passes sequentially through cable L1 (the first cable mentioned above), left circulator (the first circulator mentioned above), cable L2 (the second cable mentioned above), right circulator (the second circulator mentioned above), cable L3 (the third cable mentioned above), right circulator, cable L2 again, and left circulator. Finally, the synchronization signal is received by the signal acquisition unit. Figure 3 In the above, assuming the propagation delay of the synchronization signal within the circulator is 0, the total signal propagation distance is L = L1 + 2 × L2 + L3.

[0095] Similar to transmitter Ti, other transmitters and receivers in a distributed SAR system employ... Figure 3 The system framework shown performs frequency synchronization.

[0096] Next, a series of simulation experiments will be conducted to verify the frequency deviation of the high-precision synchronization signal obtained through the above-mentioned frequency synchronization equipment and frequency synchronization method.

[0097] Ideally, as shown in formula (3) above, processing the synchronization signals generated by multiple platforms can yield the frequency deviation Δf between USOs. TiRj However, the actual measured synchronization signal is

[0098] s real =s ideal (Δf)+n noise

[0099] Among them, s ideal (Δf) represents the ideal synchronization signal containing frequency deviation, n noise This refers to receiver noise. Here, the SNR of the synchronization signal is the ratio of the ideal synchronization signal power to the receiver noise power.

[0100] Based on the frequency synchronization system framework, basic principles, error model, and system parameters used for simulation in Table I, five sets of simulations were conducted to evaluate the accuracy of the frequency deviation of the aforementioned synchronization signal.

[0101] Table I. System parameters used for simulation

[0102]

[0103] In the first simulation experiment, the preset frequency deviation Δf is assumed to be 2Hz. The influence of the synchronization signal's SNR on the estimation accuracy of the frequency deviation Δf can then be obtained, as shown in Figures 6(a) and 6(b). In Figure 6(a), the dashed line and the curve represent the preset frequency deviation Δf and the estimated frequency deviation Δf, respectively. The curve in Figure 6(b) represents the residual frequency error Δf after compensation (i.e., the residual Δf in Figure 6(b)) as a function of SNR. When the SNR is greater than 50dB, the estimated frequency deviation Δf is stable. Therefore, in subsequent simulation experiments, it is assumed that the signal SNR is not less than 50dB.

[0104] In the second simulation experiment, it is assumed that SNR = 50dB and the preset frequency deviation is Δf ∈ [0, 10] Hz. The estimation results of the estimated frequency deviation Δf can be obtained under different preset frequency deviations Δf, as shown in Figures 7(a) and 7(b). Figure 7(a) shows the change of the estimated frequency deviation Δf with the preset frequency deviation Δf, and Figure 7(b) shows the change of the residual frequency error Δf with the preset frequency deviation Δf. It can be seen that when the preset frequency error Δf ∈ [0, 10] Hz, the residual frequency error Δf can be controlled within 4E-4 Hz. This indicates that the synchronization method has strong robustness in estimating the frequency deviation.

[0105] In the third simulation experiment, assuming a preset frequency deviation Δf of 2Hz and SNR = 50dB, 2000 sets of simulation experiments were conducted to estimate the frequency deviation Δf, and the results are shown as the curves in Figure 8(a). Subsequently, Kalman filtering was used to smooth the estimated frequency deviation Δf, and the results obtained are shown as the dashed lines in Figure 8(a), and their magnified versions are shown in Figure 8(b). It can be seen that the compensated residual frequency error Δf is 8E-6Hz. That is to say, the frequency stability of the distributed SAR system using the described frequency synchronization method can reach 8E-15.

[0106] In the fourth simulation experiment, the preset frequency deviation Δf is assumed to be 2Hz. Similar to the third simulation, 2000 simulations are performed under different SNRs to estimate the frequency deviation Δf, and the residual frequency error Δf after Kalman filtering is as follows: Figure 9 As shown in Figure 6(b), the residual frequency error Δf decreases with the increase of SNR, which is consistent with the results in Figure 6(b).

[0107] In the fifth simulation experiment, it is assumed that SNR = 50dB and the preset cable length variation is ΔL∈

[10] -6 10 -4[m]. For different cable length variations, the estimated frequency deviation Δf and residual frequency deviation Δf can be obtained, as shown in Figures 10(a) and 10(b). Figure 10(a) shows the variation of the estimated frequency deviation Δf with ΔL, and Figure 10(b) shows the variation of the residual frequency error Δf with ΔL. It can be seen that when the preset cable length ΔL < 10m... -5 When the distance is measured in meters, the residual frequency error Δf is less than 1E-3Hz (frequency stability reaches 1E-12). Therefore, other technical means (such as the Chinese remainder theorem ranging method) can be used to accurately estimate the actual cable length of the synchronization system and achieve higher precision frequency synchronization.

[0108] The simulation results above show that: 1) the frequency synchronization method described above has extremely high accuracy in estimating the frequency deviation of USOs, and its frequency stability is within 10... -15 1) The method achieves high-precision frequency synchronization; 2) The phase synchronization accuracy increases with the increase of SNR; 3) The method is robust and can accurately estimate frequency deviations within different ranges; 4) Simulation results strongly demonstrate the above-mentioned invention. In summary, this method can solve the problem of high-precision frequency synchronization for spaceborne distributed SAR.

[0109] In the several embodiments provided in this disclosure, it should be understood that the disclosed structures and methods can be implemented in a non-target manner. The structural embodiments described above are merely illustrative. For example, the division of units is only a logical functional division, and in actual implementation, there may be other division methods, such as: multiple units or components can be combined, or integrated into another system, or some features can be ignored or not executed. In addition, the components shown or discussed are coupled or directly coupled to each other. The units described above as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units; some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs. The features disclosed in the several method or structural embodiments provided in this disclosure can be arbitrarily combined without conflict to obtain new method embodiments or structural embodiments.

[0110] The above descriptions are merely some embodiments of this disclosure, but the protection scope of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this disclosure should be included within the protection scope of this disclosure. Therefore, the protection scope of this disclosure should be determined by the scope of the claims.

Claims

1. A frequency synchronization method, characterized in that, Applied to frequency synchronization equipment, the method includes: The system acquires a first propagation distance and a first propagation time of a first synchronization signal generated by a transmitter in a first transmission link, and a second propagation distance and a second propagation time of a second synchronization signal generated by a receiver in a second transmission link; wherein the transmitter and the receiver are arbitrary distributed synthetic aperture radars located on different platforms, and the first propagation distance is equal to the second propagation distance; The frequency of the first synchronization signal is determined based on the speed of light, the multiple relationship between the first propagation time and the minimum interval of the transmitter timer, and the first propagation distance; The frequency of the second synchronization signal is determined based on the speed of light, the multiple relationship between the second propagation time and the minimum interval of the receiver timer, and the second propagation distance; A frequency deviation is determined based on the frequency of the first synchronization signal and the frequency of the second synchronization signal; wherein the frequency deviation represents the difference between the frequency of the first synchronization signal and the frequency of the second synchronization signal.

2. The method according to claim 1, characterized in that, The method further includes: Based on the frequency deviation, the phase error corresponding to the frequency deviation is compensated to achieve distributed SAR phase synchronization.

3. The method according to claim 2, characterized in that, The transmitter is used to generate a first synchronization signal carrying the first frequency signal based on the first frequency signal generated by the first ultra-stable crystal oscillator in itself, and send the first synchronization signal to the signal acquisition unit in the transmitter; The receiver is configured to generate a second synchronization signal carrying the second frequency signal based on the second frequency signal generated by the second ultra-stable crystal oscillator in itself, and send the second synchronization signal to the signal acquisition unit in the receiver.

4. The method according to any one of claims 1 to 3, characterized in that, The method further includes: Based on the speed of light and the first propagation distance, the first propagation time of the first synchronization signal in the first transmission link is determined; And the second propagation time of the second synchronization signal in the second transmission link; wherein the first propagation time is equal to the second propagation time.

5. A frequency synchronization device, characterized in that, The device includes an ultra-stable crystal oscillator, a synchronous transmitter, a first cable, a first circulator, a second cable, a second circulator, a third cable, and a signal acquisition unit, wherein: The ultra-stable crystal oscillator is used to provide a first frequency signal or a second frequency signal; wherein the ultra-stable crystal oscillator is a first ultra-stable crystal oscillator or a second ultra-stable crystal oscillator; The synchronous transmitter is used to generate a first synchronization signal carrying the first frequency signal and transmit the first synchronization signal to the first port of the first circulator through the first cable. The second port of the first circulator is connected to the second port of the second circulator via the second cable; The third port of the second circulator is connected to the first port of the second circulator via the third cable; The signal acquisition device is connected to the third port of the first circulator and is used to receive the first synchronization signal with the first frequency signal as a reference. Wherein, the first propagation distance of the first synchronization signal in the device is a preset value; wherein, the first propagation distance is the sum of the length of the first cable, the length of the third cable, and twice the length of the second cable; the length of the first cable and the length of the third cable are both related to the pulse width of the first synchronization signal.

6. The device according to claim 5, characterized in that, The frequency synchronization device is a receiver or a transmitter.

7. A frequency synchronization system, characterized in that, The system includes M transmitters and N receivers, each of the transmitters and receivers being implemented using any one of claims 5 or 6.

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