A multi-transmit multi-receive radar time synchronization method based on direct wave and differential GPS ranging

Abstract

The application discloses a kind of based on direct wave and differential GPS ranging multi-transmit multi-receive radar time synchronization method, in the airborne distributed radar system of multiple radar units, two radar units at baseline end as signal transmitting source, all radar units have signal receiving ability and mutually in visible range.Two transmitting radars emit carrier frequency different linear frequency modulation wave, all radars receive direct wave and the echo signal of target reflection.Direct wave contains the time synchronization error between corresponding receiving and transmitting radar platform and baseline distance information.Obtain positioning information by means of differential GPS module, eliminate the baseline distance information in direct wave, and only contain time synchronization error direct wave as reference signal.Then the echo signal and reference signal are conjugated in frequency domain multiplication, that is, the echo signal can be matched filtered and time synchronization error compensated in distance direction.Each of all echo signals is processed above, and finally the time synchronization of distributed radar system is realized.

Description

Technical Field

[0001] This invention belongs to the field of radar technology, and in particular relates to a time synchronization method for multi-transmitter and multi-receiver radars based on direct wave and differential GPS ranging. Background Technology

[0002] In modern radar systems, distributed radar technology forms a radar network covering a wide area by dispersing multiple radar receiving and transmitting stations, thereby achieving all-round and multi-layered detection of targets. However, due to the separate transmission and receiving platforms, each platform uses its own frequency source. The differences between these frequency sources cause radars to use different time bases, resulting in synchronization errors in the received signals and affecting the overall performance and detection accuracy of the radar system.

[0003] In distributed radar systems, time synchronization is crucial because it directly affects the accuracy of signal processing and the coherence of the radar system. Errors in time synchronization can lead to phase distortion during signal processing, thereby affecting the reliability of target detection and clutter suppression. To ensure that the radar system can effectively perform its tasks, such as target tracking, imaging, and environmental monitoring, achieving high-precision time synchronization is essential.

[0004] Existing time synchronization technologies can generally be divided into three categories. The first is that each radar platform is equipped with a high-stability, high-accuracy frequency source, relying on the inherent stability of the frequency source to maintain time synchronization. This places strict requirements on the quality of the frequency source. The second is to use the timing signals of the global satellite navigation system to control the frequency sources of the transmitter and receiver, thereby achieving time synchronization. Examples include common-view method, all-view method, and precise point positioning. These technologies can provide time accuracy at the microsecond or even nanosecond level, thereby ensuring the consistency of radar signals in time. The third is to establish a synchronization link between the transmitting radar and the receiving radar to propagate the synchronization signal. Ground-based radars can achieve stable wired synchronization through optical fibers. For airborne or spaceborne platforms, wireless synchronization technologies such as two-way link synchronization and direct wave synchronization can be used.

[0005] Among the methods described above, the direct-wave synchronization method is widely used in time synchronization technology due to its ease of implementation. The direct-wave synchronization method directly utilizes the radar's transmitted signal, exhibiting stronger resistance to multipath effects and clutter. This means that even in complex signal environments, this method can maintain good synchronization performance. By using direct-wave signals, radar systems can improve system autonomy and reliability without requiring additional synchronization equipment. This method can quickly respond to changes in the relative motion between the transmitter and receiver, making it suitable for dynamic environments of mobile platforms. Compared to other time synchronization methods, direct-wave time synchronization is potentially simpler to implement and has lower maintenance costs, making it easier to implement and maintain, and reducing system complexity.

[0006] Distributed radar time synchronization technology is fundamental to ensuring the efficient operation of radar systems. With continuous technological advancements, future radar systems will rely on more precise time synchronization to achieve wider applications and superior performance. Based on this, this invention proposes a time synchronization method for multi-transmitter, multi-receiver radars based on direct wave and differential GPS ranging. Summary of the Invention

[0007] This invention belongs to the field of radar technology and discloses a time synchronization method for multi-transmitter, multi-receiver radars based on direct wave and differential GPS ranging. This method is used to compensate for errors in the echo signals of airborne distributed radars, thereby achieving time synchronization. In this method, for an airborne distributed radar system composed of multiple radar units, two radar units at either end of the baseline serve as signal transmitters. All radar units have signal receiving capabilities and are within each other's line of sight. The two transmitting radars transmit linear frequency modulated waves with different carrier frequencies, and all radars receive the direct wave and the echo signals reflected from the target. Based on the time synchronization error between the corresponding transmitting and receiving radar platforms and the baseline distance information contained in the direct wave, differential GPS ranging is used to obtain the distance between the transmitting and receiving radar platforms and eliminate the distance information in the direct wave, thus obtaining a reference signal containing only the time synchronization error. Then, the echo signal and the reference signal are multiplied by their conjugate in the frequency domain to perform range-directed matched filtering and time synchronization error compensation on the echo signal. This process is performed on all echo signals one by one, ultimately achieving time synchronization of the distributed radar system.

[0008] To facilitate the description of the present invention, the following terms are defined first:

[0009] Definition 1. Distributed Radar

[0010] Distributed radar systems consist of multiple geographically dispersed radar units that acquire multi-channel observation data of targets in the spatial frequency domain through multiple transmission and reception. These radar units rely on precise time synchronization and phase modulation techniques to achieve collaborative operation, thereby optimizing overall detection efficiency. Distributed radar technology plays a crucial role in fields such as topographic mapping and environmental remote sensing. It overcomes the limitations of single radar systems in terms of antenna size and transmission power, achieving detection capabilities at longer distances and with higher resolution through the joint detection of multiple radar units. See "Ma Lun. Research on Wide-Area, High-Resolution Imaging Methods of Distributed Small Satellite SAR [D]. Xi'an University of Electronic Science and Technology, 2008."

[0011] Definition 2. Range-directed matched filter reference signal

[0012] Range-direction matched filtering is the range-direction imaging step in the SAR imaging algorithm. It compensates for the phase of the quadratic term in the echo signal by multiplying the spectrum of the reference signal with the spectrum of the echo signal, thereby enabling pulse compression of the echo signal and obtaining the range-direction imaging result. See "Lan G. Cumming Frank H. Wong. Synthetic Aperture Radar Imaging: Algorithms and Implementation [M]. Electronic Industry Press, 2012".

[0013] Definition 3: Principle of Stationary Phase

[0014] In synthetic aperture radar (SAR) data processing, it is usually necessary to transform the linear frequency modulated (LFM) signal to the frequency domain for analysis. However, since the phase of the LFM signal is a quadratic function of the fast time τ, directly and accurately deriving the analytical expression of the Fourier transform of the LFM signal is not easy. To overcome this difficulty, the principle of stationary phase (POSP) can be used to obtain an approximate analytical expression of the Fourier transform of the LFM signal. See "Zhang Xiaoling, Shi Jun, Wei Shunjun. Three-Dimensional Synthetic Aperture Radar [M]. Beijing: National Defense Industry Press, 2017". For an echo signal s0(τ) with time delay, after transformation to the frequency domain using the POSP principle, s0(f) can be obtained. τ The details are as follows:

[0015]

[0016] Definition 4. Inverse Fourier Transform of Range

[0017] The range-directed inverse Fourier transform (IRFT) is a mathematical transformation in signal processing, and it is the inverse process of the Fourier transform. In synthetic aperture radar (SAR) imaging technology, the range-directed IRFT is often used to convert pulse compression signals from the frequency domain back to the time domain to obtain radar images focused in the range direction. See "Zhang Xiaoling, Shi Jun, Wei Shunjun. Three-Dimensional Synthetic Aperture Radar [M]. Beijing: National Defense Industry Press, 2017".

[0018] Definition 5. Traditional Backward Projection Algorithm

[0019] Traditional backprojection algorithms utilize the radar platform's position information to calculate the historical distance between the platform and scene pixels. Then, by traversing the distance history, they find the corresponding echoes in the data after echo pulse compression interpolation, perform phase compensation for the corresponding distances, coherently accumulate the data, and project the accumulated result into the image space to complete the imaging process. The main steps of the backprojection algorithm include: range-direction matched filtering, range-direction zero-padding interpolation, range-direction echo indexing, azimuth-direction phase compensation, and azimuth coherent accumulation. For details on the traditional backprojection algorithm, please refer to "Shi Jun. Research on the Principles and Imaging Technology of Bistatic SAR and Linear Array SAR [D] Doctoral Dissertation, University of Electronic Science and Technology of China, 2009".

[0020] The technical solution of this invention is: a time synchronization method for multi-transmitter and multi-receiver radar based on direct wave and differential GPS ranging, the method comprising:

[0021] This method is applied to an airborne distributed radar system consisting of N radar elements (N>2), with two radar elements T1 and T2 located at opposite ends of the baseline configuration. N As a signal transmission source, all radar units T1, R2, ..., R in the system N-1 T N All have signal receiving capabilities;

[0022] Step 1: Based on the fact that the radar units are all within each other's line of sight, the two transmitting radar units transmit linear frequency modulated wave signals with different carrier frequencies to the other radar units and the target;

[0023] Step 2: Launch radar platforms T1 and T N When receiving the echo signal reflected from the target after its own transmission, it is considered self-transmission and self-reception, and is regarded as having no time error; radar platforms R2, ..., R N-1 T N Receives the direct wave transmitted by T1 and the corresponding echo signal reflected from the target. Radar platforms T1, R2, ..., R N-1 Receive T N The transmitted direct wave and the corresponding echo signal reflected by the target;

[0024] Step 3: Based on the fact that the direct wave and echo signals of the same transmitting and receiving radar platforms contain the same time synchronization error, the real-time distance between the transmitting and receiving radar platforms is obtained by using differential GPS positioning technology. The distance information contained in the corresponding direct wave is removed, and only the time error information is retained.

[0025] Step 4: Using the direct wave signal after removing the location information as the reference signal, multiply it with the echo signal in the frequency domain conjugate to perform precise time compensation and correct the time error, thereby realizing range-direction matched filtering of the echo signal;

[0026] Step 5: Repeat steps 1 to 4 for the direct wave and echo signals of each pair of transceiver radar units in the distributed radar system, obtain the range-direction matched filter reference signal, and perform time synchronization error compensation on the echo signal to ensure that all echo signals in the system have undergone time synchronization processing, and finally achieve time synchronization of the entire distributed radar system.

[0027] Furthermore, in step 1, the two transmitting radar units simultaneously transmit linear frequency modulated signals with different carrier frequencies, specifically as follows:

[0028]

[0029] Where τ is the fast time, η is the slow time, A0 is the signal amplitude, and T r The pulse width. f c For carrier frequencies, radar platforms T1 and T N The carrier frequencies are f c1 f cN K is the frequency modulation, and rect(.) represents the rectangular window function operation. For the signal transmitted by the transmitting radar T1 to the other radars and the observation target area, the transmitting radar T1 receives the echo signal that is self-transmitted and self-received and does not require time synchronization, while the other N-1 radars receive the direct wave and the radar echo signal reflected back from the target.

[0030] Furthermore, in step 3, based on the fact that the time synchronization errors contained in the direct wave signal and echo signal of the same transmitting and receiving radar platform are the same, specifically:

[0031]

[0032] Among them, s t_r (τ,η) represents the down-converted echo signal transmitted by radar element t and received by radar element r, where t takes values ​​T1 and T2. N r takes values ​​of R2, ..., R N-1 R t_r (η) represents the sum of the distances from the radar element to the target at slow time η (t, r), where c represents the speed of light, and Δτ t_r (η) represents the time synchronization error between radar units t and r at slow time η; g t_r (τ,η) represents the down-converted direct wave transmitted by radar element t and received by radar element r, where D t_r (η) represents the distance between radar units t and r at slow time η.

[0033] Furthermore, in step 4, since the direct wave contains the time error between the transmitting and receiving radar platforms, a reference signal is constructed by removing the range information from the direct wave and retaining only the time error information. First, the direct wave is converted to the frequency domain according to the stationary phase theorem, as follows:

[0034]

[0035] f τ Representing a fast time frequency, D is obtained from differential GPS ranging. t_r (η), and for g t_r (f τ ,η) Compensation phase 2π(f c +f τ )D t_r (η) / c, and thus the reference signal h is obtained. t_r (f τ,η), specifically:

[0036]

[0037] Furthermore, in step 4, the time synchronization error compensation involves transforming the echo signal to the frequency domain and multiplying it with the conjugate of the reference signal to complete range-directed matched filtering, thereby achieving time error compensation. The details are as follows:

[0038]

[0039] in, To achieve time-synchronized frequency domain echo signals, Indicates the reference signal h t_r (f τ Taking the conjugate of η, the signal is transformed into a time-domain signal via inverse Fourier transform in the range direction, as follows:

[0040]

[0041] This invention utilizes the signal emitted by the radar itself, eliminating the need for additional signal transmission systems and synchronization equipment. This simplifies system design and makes implementation easier, enhancing the system's autonomy and reliability. Direct waves exhibit stronger resistance to multipath effects and clutter. Even in complex and variable signal environments, the direct wave time synchronization method maintains high synchronization performance, ensuring stable operation of the radar system. Attached Figure Description

[0042] Figure 1 This is a flowchart of the present invention.

[0043] Figure 2 This is a schematic diagram of the distributed radar system of the present invention transmitting and receiving signals.

[0044] Figure 3 These are the simulation results of the present invention under a fixed time error condition; (a) is before compensation, and (b) is after compensation by the present method.

[0045] Figure 4 These are the simulation results of the present invention under linear time error conditions; (a) is before compensation, and (b) is after compensation by the present method. Detailed Implementation

[0046] This invention was supported by the National Key Research and Development Program of China (G072022YFB3901603).

[0047] To further illustrate the above technical solutions, a case study is provided below for reference. The specific implementation plan is summarized as follows:

[0048] A time synchronization method for multi-transmitter, multi-receiver radar based on direct wave and differential GPS ranging, characterized in that the method includes:

[0049] In the airborne distributed radar system defined in Definition 1, which consists of four radar units, the two radar units T1 and T4 located at both ends of the baseline configuration serve as signal transmission sources, while all radar units T1, R2, R3, and T4 in the system have signal reception capabilities.

[0050] Since the radar units are all within each other's line of sight, the two transmitting radar units transmit linear frequency modulated (LFM) wave signals with different carrier frequencies to the other radar units and the target. When transmitting radar platforms T1 and T4 receive the echo signals reflected from the target after their own transmission, it is a self-transmitting and self-receiving process, which can be considered as having no time error. Radar platforms R2, R3, and T4 receive the direct wave transmitted by T1 and the corresponding echo signals reflected from the target, while radar platforms T1, R2, and R3 receive the direct wave transmitted by T4 and the corresponding echo signals reflected from the target.

[0051] The direct wave and echo signals from the same transmitting and receiving radar platforms contain the same time synchronization error. By utilizing differential GPS positioning technology to obtain the real-time distance between the transmitting and receiving radar platforms, the distance information contained in the corresponding direct wave is removed, thus retaining only the time error information. Using the direct wave signal after removing the location information as a reference signal, range-direction matched filtering is applied to the echo signal in the frequency domain to perform precise time compensation and correct the time error. This process is repeated for the direct wave and echo signals of each pair of transmitting and receiving radar units in the distributed radar system. The range-direction matched filtering reference signal defined in Definition 2 is obtained, and time synchronization error compensation is performed on the echo signal to ensure that all echo signals in the system undergo time synchronization processing, ultimately achieving time synchronization of the entire distributed radar system.

[0052] The distributed radar system comprises four radar units. Two radar units, T1 and T4, located at either end of the baseline configuration, are responsible for transmitting signals. All radar units are capable of receiving signals and ensure they are within each other's effective illumination area to achieve effective signal exchange. The two transmitting radar units simultaneously transmit linear frequency modulated (LFM) signals with different carrier frequencies, specifically:

[0053]

[0054] in, To save time, For slow time, A0 = 1 represents the signal amplitude, and T... r =10μs is the pulse width. f c The carrier frequencies for radar platforms T1 and T4 are f1 and f2, respectively. c1 =77GHz, f c4=60GHz, K=1×10 14 Hz / s represents the frequency modulation. For signals transmitted by transmitting radar T1 to other radars and the target area, transmitting radar T1 receives the echo signals that are self-transmitted and self-received, requiring no time synchronization. The other three radars, R2, R3, and T4, receive the direct wave and the radar echo signals reflected back from the target. The reception of signals transmitted by transmitting radar T4 is similar.

[0055] Assuming that the time synchronization error in the direct wave and echo signals of the same transmitting and receiving radar platform is the same, taking radar unit T1 transmitting and radar unit R2 receiving as an example, in three-dimensional spatial coordinates (range-azimuth-altitude), both radar units T1 and R2 fly along the azimuth direction. The flight trajectory of radar unit T1 is as follows: The flight trajectory of radar unit R2 is The target's position is P0(η) = (0m, 0m, 0m). 1_2 (τ,η) represents the down-converted echo signal transmitted by radar unit T1 and received by radar unit R2. This represents the sum of the distances from radar units T1 and R2 to the target at slow time η, where c = 3 × 10⁻⁶. 8 m / s represents the speed of light, Δτ 1_2 (η) = 10ns represents the time synchronization error between radar units T1 and R2 at slow time η; g 1_ (2τ,η) represents the down-converted direct wave transmitted by radar unit T1 and received by radar receiver R2. The distance between radar units T1 and R2 at slow time η is specifically:

[0056]

[0057] The reference signal defined in Definition 2 is based on the fact that the direct wave contains time error and range information between the transmitting and receiving radar platforms. The reference signal is constructed by removing the range information from the direct wave and retaining only the time error information. First, the direct wave is transformed into the frequency domain according to the stationary phase theorem defined in Definition 3, as follows:

[0058]

[0059] in, D is obtained based on differential GPS ranging. 1_2 (η) = 10m, and for g 1_2 (f τ ,η) Compensation phase This leads to the obtaining of the reference signal h. 1_2 (f τ ,η), specifically:

[0060]

[0061] Time synchronization error compensation involves transforming the echo signal to the frequency domain and multiplying it with the conjugate of the reference signal to perform range-directed matched filtering as defined in Definition 4, thereby achieving time error compensation. The details are as follows:

[0062]

[0063] in, To achieve time synchronization of the frequency domain echo signal, it is converted into a time domain signal by inverse Fourier transform in the range direction, as follows:

[0064]

[0065] At this point, time synchronization has been achieved between radar unit T1's transmission and radar unit R2's reception. Echo signal s 1_2 (τ,η) and The imaging result after processing by the traditional back projection algorithm is as follows Figure 3 As shown. With scene parameters remaining constant, the time error is a linear error. At that time, the imaging results of the echo signals before and after compensation after processing by the traditional back projection algorithm are as follows: Figure 4 As shown. Simulation results demonstrate that this method corrects the range and azimuth position shifts caused by time errors, achieving time synchronization between radar unit T1 and radar unit R2. Similarly, for the remaining s 1_3 (τ,η), s 1_4 (τ,η), s 4_1 (τ,η), s 4_2 (τ,η), s 4_3 The above operations are performed on all (τ,η) to ultimately achieve time synchronization of the distributed radar.

Claims

1. A time synchronization method for multi-transmitter, multi-receiver radar based on direct wave and differential GPS ranging, the method comprising: This method is applied to an airborne distributed radar system consisting of N radar elements, where N>2, with two radar elements T1 and T2 located at opposite ends of the baseline configuration. N As a signal transmission source, all radar units T1, R2, ..., R in the system N-1 T N All have signal receiving capabilities; Step 1: Based on the fact that the radar units are all within each other's line of sight, the two transmitting radar units transmit linear frequency modulated wave signals with different carrier frequencies to the other radar units and the target; Step 2: Launch radar platforms T1 and T N When receiving the echo signal reflected from the target after its own transmission, it is considered self-transmission and self-reception, and is regarded as having no time error; radar platforms R2, ..., R N-1 T N Receives the direct wave transmitted by T1 and the corresponding echo signal reflected from the target. Radar platforms T1, R2, ..., R N-1 Receive T N The transmitted direct wave and the corresponding echo signal reflected by the target; Step 3: Based on the fact that the direct wave and echo signals of the same transmitting and receiving radar platforms contain the same time synchronization error, the real-time distance between the transmitting and receiving radar platforms is obtained by using differential GPS positioning technology. The distance information contained in the corresponding direct wave is removed, and only the time error information is retained. Step 4: Using the direct wave signal after removing the location information as the reference signal, multiply it with the echo signal in the frequency domain conjugate to perform precise time compensation and correct the time error, thereby realizing range-direction matched filtering of the echo signal; Step 5: Repeat steps 1 to 4 for the direct wave and echo signals of each pair of transceiver radar units in the distributed radar system, obtain the range-direction matched filter reference signal, and perform time synchronization error compensation on the echo signal to ensure that all echo signals in the system have undergone time synchronization processing, and finally achieve time synchronization of the entire distributed radar system.

2. The time synchronization method for multi-transmitter and multi-receiver radar based on direct wave and differential GPS ranging as described in claim 1, characterized in that, In step 1, the two transmitting radar units simultaneously transmit linear frequency modulated signals with different carrier frequencies, specifically as follows: Where τ is the fast time, η is the slow time, A0 is the signal amplitude, and T r The pulse width. f c For carrier frequencies, radar platforms T1 and T N The carrier frequencies are f c1 f cN K is the frequency modulation, and rect(.) represents the rectangular window function operation. For the signal transmitted by the transmitting radar T1 to the other radars and the observation target area, the transmitting radar T1 receives the echo signal that is self-transmitted and self-received and does not require time synchronization, while the other N-1 radars receive the direct wave and the radar echo signal reflected back from the target.

3. The time synchronization method for multi-transmitter and multi-receiver radar based on direct wave and differential GPS ranging as described in claim 2, characterized in that, In step 3, based on the fact that the time synchronization errors contained in the direct wave signal and echo signal of the same transmitting and receiving radar platform are the same, specifically: Among them, s t_r (τ,η) represents the down-converted echo signal transmitted by radar element t and received by radar element r, where t takes values ​​T1 and T2. N r takes values ​​of R2, ..., R N-1 R t_r (η) represents the sum of the distances from the radar element to the target at slow time η (t, r), where c represents the speed of light, and Δτ t_r (η) represents the time synchronization error between radar units t and r at slow time η; g t_r (τ,η) represents the down-converted direct wave transmitted by radar element t and received by radar element r, where D t_r (η) represents the distance between radar units t and r at slow time η.

4. The time synchronization method for multi-transmitter and multi-receiver radar based on direct wave and differential GPS ranging as described in claim 3, characterized in that, Step 4 specifically addresses the time error between the transmitting and receiving radar platforms contained in the direct wave. A reference signal is constructed by removing range information from the direct wave and retaining only the time error information. First, the direct wave is converted to the frequency domain using the stationary phase theorem, as follows: f τ Representing a fast time frequency, D is obtained from differential GPS ranging. t_r (η), and for g t_r (f τ ,η) Compensation phase 2π(f c +f τ )D t_r (η) / c, and thus the reference signal h is obtained. t_r (f τ ,η), specifically:

5. The time synchronization method for multi-transmitter and multi-receiver radar based on direct wave and differential GPS ranging as described in claim 4, characterized in that, Step 4 specifically involves time synchronization error compensation by transforming the echo signal to the frequency domain and multiplying it with the conjugate of the reference signal to complete range-direction matched filtering and achieve time error compensation, as detailed below: in, To achieve time-synchronized frequency domain echo signals, Indicates the reference signal h t_r (f τ Taking the conjugate of η, the signal is transformed into a time-domain signal via inverse Fourier transform in the range direction, as follows:

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