Multi-input multi-output radar time synchronization method based on direct wave and differential GPS ranging

Abstract

The invention discloses a multi-transmitting and multi-receiving radar time synchronization method based on direct waves and differential GPS ranging, in an airborne distributed radar system formed by a plurality of radar units, two radar units at two ends of a baseline serve as signal transmitting sources, and all the radar units have signal receiving capability and are in a visual range. The two transmitting radars transmit linear frequency modulation waves with different carrier frequencies, and all the radars receive direct waves and echo signals reflected by a target. The direct waves contain time synchronization errors and baseline distance information between the corresponding transmitting and receiving radar platforms. Positioning information is obtained by means of a differential GPS module, baseline distance information in direct waves is eliminated, and the direct waves only containing time synchronization errors serve as reference signals. And then conjugate multiplication is carried out on the echo signal and the reference signal in the frequency domain, so that range-direction matched filtering and time synchronization error compensation can be carried out on the echo signal. All echo signals are processed one by one, and finally time synchronization of the distributed radar system is achieved.

Description

Technical Field

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

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

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

[0004] Existing time synchronization technologies can generally be divided into three categories. First, each radar platform is equipped with a high-stability and high-accuracy frequency source, which relies on the inherent stability of the frequency source to maintain time synchronization, which has strict requirements on the quality of the frequency source; second, the timing signal of the global satellite navigation system is used to control the frequency source of the transmitter and receiver to achieve time synchronization, such as the common view method, full view method and precise point positioning. These technologies can provide microsecond or even nanosecond time accuracy, thereby ensuring the temporal consistency of radar signals; third, by establishing a synchronization link between the transmitting radar and the receiving radar to transmit the synchronization signal, ground-based radars can achieve stable wired synchronization through optical fiber. For airborne or satellite platforms, wireless synchronization technologies such as two-way link synchronization and direct wave synchronization can be used.

[0005] Among the above methods, the direct wave synchronization method is widely used in time synchronization technology due to its advantage of easy implementation. The direct wave synchronization method directly uses the radar transmission signal and has stronger resistance to multipath effects and clutter. This means that even in a complex signal environment, this method can maintain good synchronization performance. By using direct wave signals, the radar system can improve the autonomy and reliability of the system without the need for additional synchronization equipment. This method can quickly respond to changes in relative motion between the transmitter and the receiver and is suitable for dynamic environments of mobile platforms. Compared with other time synchronization methods, direct wave time synchronization may be simpler to implement and have lower maintenance costs. It is easy to implement and maintain and reduces system complexity.

[0006] Distributed radar time synchronization technology is the basis for ensuring the efficient operation of radar systems. With the continuous development of technology, in the future, radar systems will rely on more accurate time synchronization to achieve wider applications and achieve better performance standards. Based on this, the present invention proposes a multi-transmitter and multi-receiver radar time synchronization method based on direct wave and differential GPS ranging. Summary of the invention

[0007] The present invention belongs to the field of radar technology, and discloses a multi-transmitter and multi-receiver radar time synchronization method based on direct wave and differential GPS ranging, which is used for error compensation of airborne distributed radar echo signals, thereby realizing time synchronization. In this method, for an airborne distributed radar system composed of multiple radar units, two radar units at both ends of a baseline are used as signal transmission sources, and all radar units have signal receiving capabilities and are within the visual range of each other. Two transmitting radars transmit linear frequency modulation waves with different carrier frequencies, and all radars receive direct waves and echo signals reflected by targets. Based on the time synchronization error and baseline distance information between corresponding transceiver radar platforms contained in the direct wave, the distance between transceiver radar platforms is obtained by differential GPS ranging to eliminate the distance information in the direct wave, thereby obtaining a reference signal containing only the time synchronization error. Then, the echo signal is conjugate-multiplied with the reference signal in the frequency domain, and the distance-directed matched filtering and time synchronization error compensation can be completed for the echo signal. The above-mentioned processing is performed on all echo signals one by one, and finally the time synchronization of the distributed radar system is realized.

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

[0009] Definition 1. Distributed radar

[0010] A distributed radar system consists of multiple radar units dispersed in different locations. They capture multi-channel observation data of the target in the spatial frequency domain by means of multiple transmissions and multiple receptions. These radar units rely on precise time synchronization and phase control technology to achieve collaborative operation, thereby optimizing the overall detection efficiency. Distributed radar technology plays a key role in the fields of topographic mapping, environmental remote sensing, etc. It breaks through the limitations of a single radar system in antenna size and transmission power. Through the joint detection of multiple radar units, it has the ability to detect at a longer distance and with finer resolution. For details, see "Ma Lun. Research on wide-area and high-resolution imaging methods of distributed small satellite SAR [D]. Xidian University, 2008."

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

[0012] Range matched filtering is the range imaging step in the SAR imaging algorithm. It multiplies the spectrum of the reference signal with the spectrum of the echo signal to compensate for the phase of the quadratic term in the echo signal so that the echo signal can achieve pulse compression and obtain the range imaging result. For details, see "Lan G. Cumming Frank H. Wong. Synthetic Aperture Radar Imaging: Algorithm and Implementation [M]. Electronic Industry Press, 2012".

[0013] Definition 3: Stationary phase principle

[0014] In the process of synthetic aperture radar data processing, it is usually necessary to transform the linear frequency modulation signal into the frequency domain for analysis. However, since the phase of the linear frequency modulation signal is a quadratic function of the fast time τ, it is not easy to directly and accurately derive the analytical expression of the Fourier transform of the linear frequency modulation signal. In order to overcome this problem, the principle of stationary phase (POSP) can be used to obtain an approximate analytical expression of the Fourier transform of the linear frequency modulation signal. For details, see "Zhang Xiaoling, Shi Jun, Wei Shunjun. Three-dimensional Synthetic Aperture Radar [M]. Beijing: National Defense Industry Press, 2017". For the echo signal s0(τ) with time delay, after transforming it to the frequency domain according to the principle of stationary phase, we can get s0(f τ ), as follows:

[0015]

[0016] Definition 4. Distance Inverse Fourier Transform

[0017] The range inverse Fourier transform is a mathematical transformation in signal processing, which is the inverse process of the Fourier transform. In synthetic aperture radar (SAR) imaging technology, the range inverse Fourier transform is usually used to convert the pulse pressure signal from the frequency domain back to the time domain to obtain a radar image focused upward in range. For details, see "Zhang Xiaoling, Shi Jun, Wei Shunjun. Three-dimensional Synthetic Aperture Radar [M]. Beijing: National Defense Industry Press, 2017".

[0018] Definition 5. Traditional Backprojection Algorithm

[0019] The traditional back-projection algorithm uses the position information of the radar platform to calculate the distance history between the platform and the scene pixel points, and then finds the corresponding echo in the echo pulse pressure interpolation data by traversing the distance history, performs phase compensation for the corresponding distance, and then coherently accumulates them, and projects the accumulated results into the image space to complete the imaging processing. The back-projection algorithm mainly includes the following steps: range matching filtering, range zero-filling interpolation, range echo indexing, azimuth phase compensation and azimuth coherent accumulation. For details of the traditional back-projection algorithm, please refer to "Shi Jun. Research on the Principles and Imaging Technology of Dual-Base SAR and Linear Array SAR [D] Doctoral Dissertation of University of Electronic Science and Technology of China. 2009"

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

[0021] The method is applied to an airborne distributed radar system composed of N radar units (N>2), where two radar units T1 and T2 located at the two ends of the baseline configuration are N As the signal transmission source, all radar units T1, R2, ..., R N-1 、T N All have the ability to receive signals;

[0022] Step 1: Based on the fact that the radar units are within the visual range of each other, the two transmitting radar units transmit linear frequency modulation wave signals with different carrier frequencies to the remaining radar units and the target;

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

[0024] Step 3: Based on the fact that the same direct wave and echo signal of the 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, and the distance information contained in the corresponding direct wave is removed, leaving only the time error information.

[0025] Step 4: The direct wave signal after removing the position information is used as the reference signal, and is conjugate-multiplied with the echo signal in the frequency domain to perform accurate time compensation and correct the time error, thereby realizing the distance-directed matched filtering of the echo signal.

[0026] Step 5: Repeat steps 1 to 4 for the direct wave and echo signal of each pair of transmitting and receiving radar units in the distributed radar system, obtain the range matched filter reference signal and perform time synchronization error compensation on the echo signal to ensure that all echo signals in the system are time synchronized, 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 modulation signals with different carrier frequencies, specifically:

[0028]

[0029] Among them, τ is the fast time, η is the slow time, A0 is the signal amplitude, T r is the pulse width, f c is the carrier frequency, radar platform T1, T N The carrier frequencies are f c1 、f cN , K is the modulation frequency, 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 which is self-transmitted and self-received and does not require time synchronization, and the other N-1 radars receive the direct wave and the radar echo signal reflected by the target.

[0030] Further, in step 3, according to the same time synchronization error contained in the direct wave signal and the echo signal of the transmitting and receiving radar platform, it is specifically as follows:

[0031]

[0032] Among them, s t_r (τ,η) represents the down-converted echo signal transmitted by radar unit t and received by radar receiver r, and t is T1, T N , r takes the values ​​of R2, ..., R N-1 , R t_r (η) represents the sum of the distances from radar units t and r to the target at slow time η, c represents the speed of light, 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 unit t and received by radar receiver r, D t_r (η) represents the distance between radar units t and r at slow time η.

[0033] Furthermore, in step 4, based on the time error between the transmitting and receiving radar platforms contained in the direct wave, the reference signal is constructed by removing the distance information of the direct wave and retaining only the time error information; first, the direct wave is converted into the frequency domain according to the stationary phase theorem method, as follows:

[0034]

[0035] f τ Indicates the fast time frequency, 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 then obtain the reference signal h t_r (f τ,η), specifically:

[0036]

[0037] Furthermore, in step 4, the time synchronization error compensation is to transform the echo signal into the frequency domain, and conjugate and multiply it with the reference signal to complete the range matching filter to achieve time error compensation, as follows:

[0038]

[0039] in, To achieve time-synchronized frequency domain echo signals, Represents the reference signal h t_r (f τ ,η) takes the conjugate, and transforms it into the time domain signal in the distance direction, as follows:

[0040]

[0041] The present invention utilizes the signal emitted by the radar itself, eliminating the need for an additional signal transmission system and synchronization equipment, simplifying the system design and facilitating implementation, and improving the autonomy and reliability of the system. The direct wave has a stronger resistance to multipath effects and clutter. Even in a complex and changeable signal environment, the direct wave time synchronization method can still maintain a high synchronization performance, ensuring the stable operation of the radar system. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a flow chart of the present invention.

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

[0044] Figure 3 This is the simulation experiment verification result of the present invention under the condition of fixed time error; (a) is before compensation, and (b) is after compensation by this method.

[0045] Figure 4 This is the simulation experiment verification result of the present invention under the condition of linear time error; (a) is before compensation, and (b) is after compensation by this method. DETAILED DESCRIPTION

[0046] This invention was funded by the National Key R&D Program (G072022YFB3901603).

[0047] In order to further illustrate the above technical solution, a case is provided below for reference. The specific implementation plan is outlined as follows:

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

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

[0050] Based on the fact that the radar units are within the visual range of each other, the two transmitting radar units transmit linear frequency modulation wave signals with different carrier frequencies to the other radar units and targets. When the transmitting radar platforms T1 and T4 receive the echo signals reflected by the target after their own transmission, they are self-transmitting and self-receiving, which can be regarded as having no time error. Radar platforms R2, R3, and T4 receive the direct wave transmitted by T1 and the echo signals reflected by the corresponding targets, and radar platforms T1, R2, and R3 receive the direct wave transmitted by T4 and the echo signals reflected by the corresponding targets.

[0051] The same direct wave and echo signal of the transmitting and receiving radar platform 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, thereby removing the distance information contained in the corresponding direct wave, and only retaining the time error information. The direct wave signal after removing the position information is used as the reference signal, and the echo signal frequency domain is matched filtered in the distance direction to perform accurate time compensation and correct the time error. The above process is repeated for the direct wave and echo signal of each pair of transmitting and receiving radar units in the distributed radar system, and the distance-direction matched filtering reference signal defined in Definition 2 is obtained and the echo signal is compensated for the time synchronization error, ensuring that the echo signals in the system are all processed by time synchronization, and finally achieving time synchronization of the entire distributed radar system.

[0052] The distributed radar system consists of four radar units. The two radar units T1 and T4 at the two ends of the baseline configuration are responsible for transmitting signals, while all radar units have the function of receiving signals and ensure that they are within the effective illumination area to achieve effective signal interaction. The two transmitting radar units simultaneously transmit linear frequency modulation signals (LFM) with different carrier frequencies, specifically:

[0053]

[0054] in, For quick time, is the slow time, A0=1 is the signal amplitude, T r =10μs is the pulse width, f c is the carrier frequency, and the carrier frequencies of radar platforms T1 and T4 are f c1 =77GHz, f c4=60GHz, K = 1×10 14 Hz / s is the modulation frequency. 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, and the other three radars R2, R3, and T4 receive the direct wave and the radar echo signal reflected by the target. For the signal transmitted by the transmitting radar T4, the reception situation is similar.

[0055] According to the same direct wave signal and echo signal of the transmitting and receiving radar platform, the time synchronization error is the same. Take radar unit T1 transmitting and radar unit R2 receiving as an example. In the three-dimensional spatial coordinates (distance-azimuth-altitude), radar units T1 and R2 both fly along the azimuth direction, and the flight trajectory of radar unit T1 is The flight trajectory of radar unit R2 is The position of the target 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, represents the sum of the distances from radar units T1 and R2 to the target at the slow time η, c = 3 × 10 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 the slow time η; g 1_ (2τ,η) represents the down-converted direct wave transmitted by radar unit T1 and received by radar receiver R2. represents the distance between radar units T1 and R2 at slow time η, specifically:

[0056]

[0057] The reference signal defined in Definition 2 is based on the time error and distance information between the transmitting and receiving radar platforms contained in the direct wave. The reference signal is constructed by removing the distance 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 defined in Definition 3, as follows:

[0058]

[0059] in, D is obtained based on differential GPS ranging 1_2 (η) = 10m, and g 1_2 (f τ ,η) compensation phase Then we get the reference signal h 1_2 (f τ ,η), specifically:

[0060]

[0061] Time synchronization error compensation is to transform the echo signal into the frequency domain and conjugate it with the reference signal to complete the range-direction matched filtering defined in Definition 4 to achieve time error compensation, as follows:

[0062]

[0063] in, In order to realize the time-synchronized frequency domain echo signal, the inverse Fourier transform is performed into the time domain signal in the distance direction, as follows:

[0064]

[0065] So far, the time synchronization has been completed for radar unit T1 transmission and radar unit R2 reception. 1_2 (τ,η) and The imaging result after the traditional back-projection algorithm is as follows: Figure 3 As shown. When the scene parameters remain unchanged, the time error is a linear error When the echo signals before and after compensation are processed by the traditional back-projection algorithm, the imaging results are as follows: Figure 4 As shown in the simulation results, the method corrects the position offset in the range and azimuth directions caused by the time error and realizes the time synchronization of radar unit T1 and radar unit R2. 1_3 (τ,η),s 1_4 (τ,η),s 4_1 (τ,η),s 4_2 (τ,η),s 4_3 (τ,η) all perform the above operations, and finally achieve the time synchronization of distributed radar.

Claims

1. A time synchronization method for multiple-transmitter and multiple-receiver radars based on direct wave and differential GPS ranging, the method comprising: The method is applied to an airborne distributed radar system composed of N radar units (N>2), where two radar units T1 and T2 located at the two ends of the baseline configuration are N As the signal transmission source, all radar units T1, R2, ..., R N-1 、T N All have the ability to receive signals; Step 1: Based on the radar units being within visual range of each other, two transmitting radar units transmit linear frequency modulation wave signals with different carrier frequencies to the remaining radar units and the target; Step 2: Launch radar platform T1, T N When receiving the echo signal reflected by the target after its own transmission, it is considered as self-transmission and self-reception, and is considered to have no time error; radar platform R2, ..., R N-1 , T N Receive the direct wave emitted by T1 and the echo signal reflected by the corresponding target. Radar platforms T1, R2, ..., R N-1 Receive T N The direct wave emitted and the echo signal reflected by the corresponding target; Step 3: Based on the fact that the same direct wave and echo signal of the 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, and the distance information contained in the corresponding direct wave is removed, leaving only the time error information. Step 4: The direct wave signal after removing the position information is used as the reference signal, and is conjugate-multiplied with the echo signal in the frequency domain to perform accurate time compensation and correct the time error, thereby realizing the distance-directed matched filtering of the echo signal. Step 5: Repeat steps 1 to 4 for the direct wave and echo signal of each pair of transmitting and receiving radar units in the distributed radar system, obtain the range matched filter reference signal and perform time synchronization error compensation on the echo signal to ensure that all echo signals in the system are time synchronized, and finally achieve time synchronization of the entire distributed radar system.

2. A method for time synchronization of multiple transmission and multiple reception radars based on direct wave and differential GPS ranging as claimed in claim 1, characterized in that: In step 1, the two transmitting radar units simultaneously transmit linear frequency modulation signals with different carrier frequencies, specifically: Among them, τ is the fast time, η is the slow time, A0 is the signal amplitude, T r is the pulse width, f c is the carrier frequency, radar platform T1, T N The carrier frequencies are f c1 、f cN , K is the modulation frequency, 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 which is self-transmitted and self-received and does not require time synchronization, and the other N-1 radars receive the direct wave and the radar echo signal reflected by the target.

3. The method for time synchronization of multiple-transmit and multiple-receive radars based on direct wave and differential GPS ranging as claimed in claim 1, characterized in that: In step 3, the time synchronization error contained in the direct wave signal and the echo signal of the same transmitting and receiving radar platform is the same, specifically: Among them, s t_r (τ,η) represents the down-converted echo signal transmitted by radar unit t and received by radar receiver r, and t is T1, T N , r takes the values ​​of R2, ..., R N-1 , R t_r (η) represents the sum of the distances from radar units t and r to the target at slow time η, c represents the speed of light, 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 unit t and received by radar receiver r, D t_r (η) represents the distance between radar units t and r at slow time η.

4. The method for time synchronization of multiple transmission and multiple reception radars based on direct wave and differential GPS ranging as claimed in claim 1, characterized in that: In step 4, based on the time error between the transmitting and receiving radar platforms contained in the direct wave, the reference signal is constructed by removing the distance information of the direct wave and retaining only the time error information; first, the direct wave is converted into the frequency domain according to the stationary phase theorem method, as follows: f τ Indicates the fast time frequency, 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 then obtain the reference signal h t_r (f τ ,η), specifically:

5. The method for time synchronization of multiple transmission and multiple reception radars based on direct wave and differential GPS ranging as claimed in claim 1, characterized in that: In step 4, the time synchronization error compensation is to transform the echo signal into the frequency domain, and conjugate and multiply it with the reference signal to complete the range matching filter to achieve time error compensation, as follows: in, To achieve time-synchronized frequency domain echo signals, Represents the reference signal h t_r (f τ ,η) takes the conjugate, and transforms it into the time domain signal in the distance direction, as follows:

Images