Underwater laser time service system and method based on spread spectrum communication

Abstract

The invention relates to an underwater laser time service system and method based on spread spectrum communication. The underwater laser time service system comprises a time service end and a time-served end. The time-served end performs spread spectrum modulation on local time and drives a laser to send an optical signal to an underwater channel; after receiving the time signal of the time-served end, the time service end carries out de-spreading processing, compares the time signal with a local high-precision time signal, carries out spread spectrum modulation on the comparison result and the local high-precision time signal, and then sends the result to an underwater channel; and after receiving the time service optical signal of the time service end, the time service optical signal is compared with the local time, and the absolute time difference between the time service optical signal and the local time is solved, so that time synchronization is completed. The anti-interference performance and confidentiality of laser signals in the underwater environment can be improved through spread spectrum modulation, long-distance transmission is achieved by combining the high transmissivity of blue and green light bands in the underwater, and finally high-precision time synchronization of the time service end and the time-served end is achieved. The method can effectively solve the problems of low underwater sound time service precision, large delay and large underwater time service attenuation of radio waves, and is especially suitable for underwater cluster time service scenes such as underwater navigation, ocean exploration and collaborative operation.

Description

Technical Field

[0001] This invention belongs to the field of underwater high-precision time synchronization technology, and relates to an underwater laser timing system and method based on spread spectrum communication, especially an underwater laser timing system and method combined with direct sequence spread spectrum communication, which can be used to realize high-precision time synchronization between underwater distributed nodes. Background Technology

[0002] High-precision time synchronization is a fundamental supporting technology for underwater collaborative operations and scientific observation. Current underwater time synchronization solutions, such as acoustic communication, radio frequency communication, or wired communication, all have significant shortcomings: acoustic communication suffers from large delays and low accuracy; radio frequency communication experiences significant attenuation in water, limiting transmission distance; and wired communication is inflexible and costly.

[0003] In comparison, blue-green optical communication is considered an ideal technology for achieving high-precision underwater time synchronization due to its advantages such as high bandwidth, low latency, high security, and resistance to electromagnetic interference. Theoretically, laser pulse-based time synchronization methods can achieve synchronization accuracy at the nanosecond or even picosecond level. However, when applying laser time synchronization technology to complex underwater environments, a key technical bottleneck arises: the absorption and scattering effects of seawater on the laser cause pulse waveform broadening and random jitter in arrival time. This jitter introduces significant timing errors into traditional time synchronization methods that rely on precise detection of the pulse's rising edge (such as leading-edge detection), severely limiting the actual accuracy and effective range of underwater laser time synchronization.

[0004] Therefore, the core technical problem to be solved by this invention is: how to overcome the problem of optical pulse jitter caused by absorption and scattering effects in underwater laser channels, thereby improving the timing accuracy and stable transmission distance of the underwater laser timing system.

[0005] A search revealed no publicly available literature of the same or similar prior art as this invention. Summary of the Invention

[0006] This invention addresses the problem of limited timing accuracy and distance caused by pulse jitter in underwater laser channels, as mentioned in the background section, by proposing an underwater laser timing system and method based on spread spectrum communication.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution: An underwater laser timing system based on spread spectrum communication includes: a timing terminal and at least one timing receiver; The time-receiving end is responsible for sending a time synchronization request to the time-synchronizing end to maintain time synchronization with the time-synchronizing end; The time synchronization terminal, as a time source reference, is responsible for receiving time synchronization requests from the time synchronization recipient and sending standard time signals to the time synchronization recipient. The time-synchronized end includes: a second time synchronization control module, a second data supervision module, a second spread spectrum coding module, a second laser emission module, a second laser receiving module, a second spread spectrum decoding module, a second data error correction module, and a time synchronization module; The second time synchronization control module is used to generate a local high-precision 1PPS+TOD signal. When there is a calibration requirement, it triggers the synchronization process to generate a time synchronization request signal containing the local time signal. The second data supervision module has its input connected to the second time control module. It is used to receive the local time signal to be transmitted and add a supervision bit to the signal to realize error detection and error correction functions. The output of the second data supervision module is connected to the input of the second spread spectrum coding module. The second spread spectrum coding module has its input connected to the second data supervision module, and is used to receive signals with supervision bits and perform spread spectrum modulation.

[0008] The second laser emitting module has its input end connected to the second spread spectrum coding module, which is used to modulate the spread spectrum signal onto the optical carrier and, in conjunction with the emitting optical system, focus the optical signal and emit it.

[0009] The second laser receiving module is used to receive the time signal from the time synchronization terminal through the receiving optical system, and to converge and convert it into a time synchronization electrical signal.

[0010] The second spread spectrum decoding module has its input terminal connected to the second laser receiving module, which is used to receive the timing electrical signal and perform spread spectrum demodulation to recover the time signal containing the supervision bit from the noise; the output terminal of the second spread spectrum decoding module is connected to the input terminal of the second data error correction module, which is used to send the demodulated time signal to the second data error correction module.

[0011] The second data error correction module has its input connected to the second spread spectrum decoding module, which is used to decode the received signal and use the supervision bit to realize error detection and error correction.

[0012] The output of the second data error correction module is connected to the input of the second time synchronization control module, and is used to send the corrected and accurate time signal to the second time synchronization control module.

[0013] The second timing control module has its input connected to the second data error correction module. It is used to receive the corrected time signal and calculate the time difference of the 1PPS rising edge between the received timing terminal time signal and the local time signal.

[0014] One output of the second time synchronization control module is connected to the time synchronization module and is used to send the calculated time difference to the time synchronization module. The time synchronization module has its input connected to the second time control module, which receives the time difference and performs precise delay adjustment on the local 1PPS signal of the time-synchronized end to synchronize it with the 1PPS signal of the time-synchronized end.

[0015] The timing terminal includes: a first timing control module, a first data monitoring module, a first spread spectrum coding module, a first laser transmitting module, a first laser receiving module, a first spread spectrum decoding module, and a first data error correction module; The first timing control module is used to generate a local high-precision 1PPS+TOD signal, and its output is connected to the first data supervision module to send the generated time signal to the first data supervision module. The first data supervision module has its input end connected to the first time synchronization control module, which is used to receive the time signal and add data supervision bits to the signal to generate a time signal with supervision bits, thereby realizing subsequent error detection and error correction functions; its output end is connected to the first spread spectrum coding module, which is used to send the time signal with supervision bits to the first spread spectrum coding module. The first spreading encoding module has its input end connected to the first data supervision module, which is used to receive the time signal with supervision bits and perform direct sequence spread spectrum modulation on the signal to generate a spread spectrum signal; its output end is connected to the first laser emitting module, which is used to send the spread spectrum signal to the first laser emitting module. The first laser emitting module has its input end connected to the first spread spectrum coding module, which is used to receive the spread spectrum signal and modulate the signal onto a laser carrier in the green band; the laser emitting module also includes an emitting optical system, which is used to focus the modulated optical signal and transmit it to the underwater channel to improve the underwater transmission distance. The first laser receiving module is used to receive the time signal transmitted from the time-synchronized end through the receiving optical system, converge it and convert it into a time signal electrical signal; its output end is connected to the first spread spectrum decoding module and is used to send the time signal electrical signal to the first spread spectrum decoding module. The first spread spectrum decoding module has its input end connected to the first laser receiving module, which is used to receive the timing electrical signal and recover the original time signal containing the supervision bit from the noise through relevant despreading processing; its output end is connected to the first data error correction module, which is used to send the despread time signal to the first data error correction module. The first data error correction module has its input end connected to the first spread spectrum decoding module, which is used to decode the despread time signal, and use the supervisory bit to realize 2-bit error detection and 1-bit error correction to recover accurate time information; its output end is connected to the first time synchronization control module, which is used to feed back the corrected time signal to the first time synchronization control module. The first timing control module is also used to receive the corrected time signal fed back by the first data error correction module, compare it with the local time signal, and calculate the 1PPS rising edge time difference between the timing end and the timing received end.

[0016] Preferably, both the first and second spreading coding modules adopt direct sequence spreading, using an m-sequence as a pseudo-random code to spread the spectrum of the signal.

[0017] Preferably, both the first and second laser emitting modules use blue band light to emit laser signals; the emitting optical systems of the first and second laser emitting modules use aspherical lenses and place them at the light outlet of the laser; the optical signal output by the first laser emitting module is transmitted to the time-synchronized end via an underwater channel, and the optical signal output by the second laser emitting module is transmitted to the time-synchronized end via an underwater channel.

[0018] Preferably, the receiving optical systems of both the first and second laser receiving modules employ Fresnel lenses.

[0019] Preferably, both the first and second data error correction modules use (7,4) Hamming codes for error control, which can achieve 2-bit error detection and 1-bit error correction.

[0020] Preferably, the time synchronization module uses a programmable absolute delay unit inside the FPGA to achieve a time delay with picosecond-level precision.

[0021] An underwater laser timing method based on spread spectrum communication includes the following steps: Step 1: The time-controlled end generates a signal containing 1PPS+TOD information. The second data supervision module adds 3 supervision bits to every 4 bits of data in the time signal. The second spread spectrum coding module performs spread spectrum modulation. The second laser emission module converts the spread spectrum signal into an optical signal and sends it to the underwater channel. Step 2: The first laser receiving module of the timing end receives the optical signal sent by the timing end and converts it into an electrical signal. The first spread spectrum decoding module despreads the electrical signal to recover the timing signal containing the supervision bit. The first data error correction module detects and corrects errors in the timing signal. The first timing control module compares the time signal after passing through the first data error correction module with the local signal and merges and encodes the comparison result with the local time signal. The first data supervision module adds supervision bits to the signal. The first spread spectrum encoding module spreads and encodes the time signal after adding supervision bits. The spread spectrum signal is transmitted to the underwater channel by the first laser transmitting module. Step 3: The second laser receiving module of the time-receiving end receives the time signal sent by the time-synchronizing end and converts it into an electrical signal. The second spread spectrum decoding module despreads the electrical signal. The second data error correction module detects and corrects errors in the despread signal. The second time synchronization control module compares the time signal after passing through the second data error correction module with the local signal to accurately calculate the underwater laser link time delay and the absolute time difference between the rising edge of the local 1PPS and the time-synchronizing end 1PPS. The time synchronization module is used to accurately delay the local 1PPS to keep the time of the time-receiving end synchronized with the time of the time-synchronizing end.

[0022] The advantages and beneficial effects of this invention are as follows: 1. By introducing direct sequence spread spectrum technology, underwater optical pulse jitter and background noise are effectively suppressed, ensuring the reliability of the timing signal. Due to absorption and scattering effects in the underwater laser channel, optical pulse waveform broadening and arrival time jitter occur, causing significant errors in traditional methods based on pulse leading-edge detection. In this invention, both the timing end and the receiving end are equipped with spread spectrum coding and decoding modules. Before transmission, a pseudo-random code (such as an m-sequence) is used to spread the spectrum of the time signal, and corresponding despreading is performed at the receiving end. Therefore, this technique brings extremely high processing gain, giving the system strong anti-interference capabilities. It can extract weak signals submerged in noise and interference, effectively combating pulse waveform distortion and timing jitter caused by water scattering, while also giving the signal a low probability of interception, enhancing confidentiality.

[0023] 2. By combining a high-bandwidth laser link with precise time difference calculation, a balance between high precision and low latency is achieved. Traditional underwater acoustic timing suffers from large delays, while radio frequency timing has limited bandwidth and significant attenuation, making it difficult to simultaneously meet the requirements of high precision and low latency. This invention uses blue-green lasers as the information carrier and utilizes the first and second timing control modules to precisely calculate the 1PPS rise time difference. Simultaneously, the high bandwidth of the laser allows the use of spread spectrum signals without sacrificing transmission efficiency. Blue-green lasers exhibit low transmission loss underwater, providing a physical basis for "long-distance" transmission. Furthermore, laser signals propagate at the speed of light, and their "delay" is far lower than the speed of sound. Combined with the precise time difference calculation capabilities of the first and second timing control modules, the system can achieve synchronization accuracy at the nanosecond to picosecond level and low link latency at the nanosecond level, overcoming the bottlenecks of high delay in underwater acoustic timing and short distance in radio frequency timing.

[0024] 3. By adding data supervision and error correction mechanisms, accurate transmission of time information is ensured in harsh underwater channels. Due to the instability of underwater channels, errors may occur in the transmitted data bits, leading to time synchronization failure. In this invention, the system is equipped with first and second data supervision modules and first and second data error correction modules. At the transmitting end, a supervision bit (using (7,4) Hamming code) is added to the time data, and corresponding decoding and error correction are performed at the receiving end (achieving 2-bit error detection and 1-bit error correction). This technique provides redundant protection for critical time command data, enabling the detection and correction of random errors generated during transmission, greatly improving the robustness and reliability of the time synchronization system, and allowing it to adapt to complex underwater communication environments. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the system composition in this invention; Figure 2 This is a schematic diagram of the peak pulse power transmission technology in this invention; Figure 3 This is a schematic diagram of the receiving circuit in the laser receiving module of the present invention; Figure 4 This is a schematic diagram of the time synchronization process in this invention; Figure 5 This is a diagram of the overall system structure in this invention. Detailed Implementation

[0026] The structure of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that these embodiments are descriptive and not limiting.

[0027] An underwater laser timing system based on spread spectrum communication, such as Figure 1 As shown, it includes: a time synchronization terminal and at least one time-synchronized terminal; The time-receiving end is responsible for sending a time synchronization request to the time-synchronizing end to maintain time synchronization with the time-synchronizing end; The time synchronization terminal, as a time source reference, is responsible for receiving time synchronization requests from the time synchronization recipient and sending standard time signals to the time synchronization recipient. In this embodiment, the time-synchronized end includes: a second time synchronization control module, a second data supervision module, a second spread spectrum coding module, a second laser emission module, a second laser reception module, a second spread spectrum decoding module, a second data error correction module, and a time synchronization module.

[0028] The connection relationships and signal flow between the modules are as follows: The second time synchronization control module is used to generate a local high-precision 1PPS+TOD signal. When there is a calibration requirement, it triggers a synchronization process to generate a time synchronization request signal containing the local time signal. The second data monitoring module, with its input connected to the second time synchronization control module, receives the local time signal to be transmitted and adds a monitoring bit to the signal to achieve error detection and correction functions. The output of the second data monitoring module is connected to the input of the second spread spectrum coding module.

[0029] The second spreading coding module has its input connected to the second data monitoring module. It receives the signal with monitoring bits and performs spreading modulation. Preferably, a direct sequence spreading method is used, employing an m-sequence as a pseudo-random code to spread the signal's spectrum. The output of the second spreading coding module is connected to the input of the second laser emitting module.

[0030] The second laser emitting module has its input connected to a second spread spectrum coding module, used to modulate the spread spectrum signal onto an optical carrier, and then focus the optical signal for emission using a transmitting optical system. Preferably, the second laser emitting module uses blue light to emit the laser signal; the transmitting optical system uses an aspherical lens and places it at the laser's optical output. The optical signal output by the second laser emitting module is transmitted to the timing terminal via an underwater channel.

[0031] The second laser receiving module is used to receive the timing optical signal from the timing terminal through a receiving optical system, and to converge and convert it into a timing electrical signal. Preferably, the receiving optical system is a Fresnel lens. The output terminal of the second laser receiving module is connected to the input terminal of the second spread spectrum decoding module, and is used to send the timing electrical signal to the second spread spectrum decoding module.

[0032] The second spread spectrum decoding module has its input connected to the second laser receiving module. It receives the timing signal and performs spread spectrum demodulation to recover the time signal containing the monitoring bits from the noise. The output of the second spread spectrum decoding module is connected to the input of the second data error correction module, and is used to send the demodulated time signal to the second data error correction module.

[0033] The second data error correction module, with its input connected to the second spread spectrum decoding module, is used to decode the received signal and implement error detection and correction using a check bit. Preferably, a (7,4) Hamming code is used for error control, which can achieve 2-bit error detection and 1-bit error correction. The output of the second data error correction module is connected to the input of the second time synchronization control module, and is used to send the corrected and accurate time signal to the second time synchronization control module.

[0034] The second time synchronization control module has its input connected to the second data error correction module. It receives the corrected time signal and calculates the 1PPS rising edge time difference between the received time signal and the local time signal. One output of the second time synchronization control module is connected to the time synchronization module, and it sends the calculated time difference (delay) to the time synchronization module. The time synchronization module, with its input connected to the second time control module, receives the time difference (delay) and precisely adjusts the local 1PPS signal of the time-synchronized end to synchronize it with the 1PPS signal of the time-synchronizing end. Preferably, the time synchronization module uses a programmable absolute delay unit inside the FPGA to achieve a time delay with ps-level precision.

[0035] The timing terminal includes modules with similar functions: a first timing control module, a first data monitoring module, a first spread spectrum coding module, a first laser emission module, a first laser receiving module, a first spread spectrum decoding module, and a first data error correction module; The first timing control module is used to generate a local high-precision 1PPS+TOD signal, and its output is connected to the first data supervision module to send the generated time signal to the first data supervision module. The first data supervision module has its input end connected to the first time synchronization control module, which is used to receive the time signal and add data supervision bits to the signal to generate a time signal with supervision bits, thereby realizing subsequent error detection and error correction functions; its output end is connected to the first spread spectrum coding module, which is used to send the time signal with supervision bits to the first spread spectrum coding module. The first spreading encoding module has its input end connected to the first data supervision module, which is used to receive the time signal with supervision bits and perform direct sequence spread spectrum modulation on the signal to generate a spread spectrum signal; its output end is connected to the first laser emitting module, which is used to send the spread spectrum signal to the first laser emitting module. The first laser emitting module has its input end connected to the first spread spectrum coding module, which is used to receive the spread spectrum signal and modulate the signal onto the green laser carrier; the first laser emitting module also includes an emitting optical system, which is used to focus the modulated optical signal and transmit it to the underwater channel to improve the underwater transmission distance; The first laser receiving module is used to receive the time signal transmitted from the time-synchronized end through the receiving optical system, converge it and convert it into a time signal electrical signal; its output end is connected to the first spread spectrum decoding module and is used to send the time signal electrical signal to the first spread spectrum decoding module. The first spread spectrum decoding module has its input end connected to the first laser receiving module, which is used to receive the timing electrical signal and recover the original time signal containing the supervision bit from the noise through relevant despreading processing; its output end is connected to the first data error correction module, which is used to send the despread time signal to the first data error correction module. The first data error correction module has its input end connected to the first spread spectrum decoding module, which is used to decode the despread time signal, and use the supervisory bit to realize 2-bit error detection and 1-bit error correction to recover accurate time information; its output end is connected to the first time synchronization control module, which is used to feed back the corrected time signal to the first time synchronization control module. The first timing control module is also used to receive the corrected time signal fed back by the first data error correction module, compare it with the local time signal, and calculate the 1PPS rising edge time difference between the timing end and the timing received end.

[0036] The functions and roles of each module within the system are further explained below: The second time synchronization control module at the time-receiving end: combines a high-precision atomic clock chip to generate a local time signal, uses FPGA to coordinate the work of internal modules to ensure normal system operation; adopts 1PPS+TOD encoding method to serially output the time signal containing day, hour, minute and second; and starts timing at the rising edge of the local 1PPS. The second data monitoring module at the time-receiving end adds a monitoring bit in the middle of the time signal to monitor and correct erroneous data. In this system, a (7,4) Hamming code design is adopted. For every 4 bits of original information sent, 3 monitoring bits are added, which enables error correction of 1 bit error and detection of 2 bit error at the receiving end.

[0037] The second spreading coding module of the time-receiving end: adopts direct sequence spreading technology, multiplying the local time signal after Hamming code encoding with a pseudo-random noise sequence m-sequence to achieve spreading coding; the pseudo-random noise sequence adopts an m-sequence, which is easy to implement in hardware; The second laser emitting module at the time-receiving end is responsible for converting the modulated electrical signal into a laser signal. This module consists of a high-power pulsed laser diode and an optical system. A laser diode with a center wavelength of 455nm is used as the laser emitter, and the optical system focuses and emits the laser into the underwater environment. The laser diode's driving circuit employs peak pulsed power emission technology to enhance the laser signal's propagation distance and anti-interference capability.

[0038] The aforementioned peak pulse power emission technology achieves high peak power output in an extremely short time by compressing the time scale. Simultaneously, it increases the bandwidth of the emission system and enhances the steepness of the laser pulse rise edge by connecting a capacitor in parallel across the resistor connected in series with the laser diode. This achieves... Figure 2 As shown.

[0039] The first laser receiving module of the timing terminal consists of a receiving optical system and a receiving circuit. The receiving optical system focuses the timing optical signal onto the receiving surface of a photodiode. The receiving circuit consists of a photodiode and a transimpedance amplifier. The photodiode converts the weak optical signal into an electrical signal, and the transimpedance amplifier amplifies and outputs the weak electrical signal. The receiving circuit implements the following... Figure 3 As shown.

[0040] The first spread spectrum decoding module of the timing terminal: adopts direct sequence spread spectrum technology, performs correlation operation between the electrical signal output by the laser receiving module of the timing terminal and the locally generated pseudo-random noise sequence to realize spread spectrum decoding; The first data error correction module of the time synchronization end performs Hamming encoding inverse operation on the despread time signal to perform 2-bit error detection and 1-bit error correction; The first timing control module of the timing terminal: extracts 1PPS and TOD information from the received time signal from the time-synchronized terminal; The first timing control module at the timing terminal generates a local time signal by combining a high-precision atomic clock chip, and uses an FPGA to coordinate the operation of various internal modules to ensure normal system operation; simultaneously, it calculates the time delay between the next local 1PPS and the 1PPS sent by the timing terminal. Then, at the next rising edge of the local 1PPS, the local 1PPS + TOD and latency will be calculated. Serial output, output to the first data monitoring module of the timing terminal; The first data monitoring module of the timing terminal adds a monitoring bit in the middle of the timing signal to monitor and correct erroneous data. It adopts a (7,4) Hamming code design, adding 3 monitoring bits for every 4 bits of original information sent. The first spreading encoding module of the time synchronization terminal: multiplies the time signal after Hamming code encoding with the m sequence to achieve spreading encoding; The first laser emission module of the timing terminal converts the spread-spectrum encoded electrical signal into a laser signal. A laser diode with a center wavelength of 525nm is used as the laser emitter, and the optics department focuses and emits the laser into the underwater environment. Simultaneously, the laser diode's driving circuit employs peak pulse power emission technology to enhance the laser signal's propagation distance and anti-interference capability.

[0041] The second laser receiver module at the time-synchronized end consists of a photodiode and a transimpedance amplifier. The photodiode is used to convert weak optical signals into electrical signals, and the transimpedance amplifier is used to amplify and output the weak electrical signals.

[0042] The second spread spectrum decoding module of the time-receiving end: adopts direct sequence spread spectrum technology, performs correlation operation between the electrical signal output by the second laser receiving module of the time-receiving end and the locally generated pseudo-random noise sequence to realize spread spectrum decoding; The second data error correction module at the time-controlled end performs Hamming encoding inverse operation on the despread time signal to perform 2-bit error detection and 1-bit error correction. The second time synchronization control module at the receiving end extracts 1PPS+TOD and time difference information from the received time signal from the time synchronization end. Simultaneously calculate the time difference between the next local 1PPS and the 1PPS sent from the time synchronization terminal. Since the distance between the transmitting and receiving links is equal, that is... The time difference between the time receiver and the time transmitter can be obtained by combining the following formula. : The time synchronization module at the receiving end: After obtaining the time difference, the locally generated 1PPS is delayed through the time synchronization module, thereby achieving time synchronization with the time-sensing end every second. The time synchronization flowchart is as follows. Figure 4 As shown.

[0043] The overall system structure is as follows Figure 5 As shown.

[0044] An underwater laser timing system based on spread spectrum communication includes the following steps: Step 1: The time-receiving end generates a time signal containing 1PPS+TOD information. The second data supervision module adds 3 supervision bits to every 4 bits of data in the time signal. The second spread spectrum coding module performs spread spectrum modulation. The second laser emission module converts the spread spectrum signal into an optical signal and sends it to the underwater channel. Step 2: The first laser receiving module of the timing end receives the optical signal sent by the timing end and converts it into an electrical signal. The first spread spectrum decoding module despreads the electrical signal to recover the timing signal containing the supervision bit. The first data error correction module detects and corrects errors in the timing signal. The first timing control module compares the time signal after passing through the first data error correction module with the local signal and merges and encodes the comparison result with the local time signal. The first data supervision module adds supervision bits to the signal. The first spread spectrum encoding module spreads and encodes the time signal after adding supervision bits. The spread spectrum signal is transmitted to the underwater channel by the first laser transmitting module. Step 3: The second laser receiving module of the time-receiving end receives the time signal sent by the time-synchronizing end and converts it into an electrical signal. The second spread spectrum decoding module despreads the electrical signal. The second data error correction module detects and corrects errors in the despread signal. The second time synchronization control module compares the time signal after passing through the second data error correction module with the local signal to accurately calculate the underwater laser link time delay and the absolute time difference between the rising edge of the local 1PPS and the time-synchronizing end 1PPS. The time synchronization module is used to accurately delay the local 1PPS to keep the time of the time-receiving end synchronized with the time of the time-synchronizing end.

[0045] The working principle of this invention is: The second timing control module inside the timing receiver generates a time signal. The second data monitoring module adds a monitoring bit in the middle of the time signal to monitor and correct erroneous data. The second spreading encoding module spreads and encodes the time signal to be transmitted to enhance the signal's anti-interference capability in the underwater laser link. The spread time signal is modulated onto the optical carrier by the second laser transmitting module, and the optical signal is then transmitted to the underwater laser link through the transmitting optical system. The timing terminal receives the optical signal from the time-receiving terminal through the first laser receiving module and converts it into an electrical signal. The first spread spectrum decoding module despreads the electrical signal, and the first data error correction module detects and corrects errors in the despread time signal to restore the original time signal. The first timing control module compares and calculates the time signal sent by the time-receiving terminal, and then spreads and encodes the calculation result and local time information. The spread time signal is modulated onto the optical carrier by the first laser transmitting module, and this optical signal is sent to the time-receiving terminal through the underwater laser link. The time-receiving end receives the optical signal from the time-synchronizing end through the second laser receiving module and converts it into an electrical signal. The second spread spectrum decoding module despreads the electrical signal, and the second data error correction module detects and corrects errors in the despread time signal to restore the original time signal. The second time synchronization control module calculates the time signal and, based on the calculation result and in conjunction with the time synchronization module, adjusts the local time to achieve high-precision time synchronization with the time-synchronizing end.

[0046] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. An underwater laser timing system based on spread spectrum communication, characterized in that: include: A time signaling end and at least one time-receiving end; The time-receiving end is responsible for sending a time synchronization request to the time-synchronizing end in order to maintain time synchronization with the time-synchronizing end; The time synchronization terminal, as a time source reference, is responsible for receiving time synchronization requests from the time synchronization recipient and sending standard time signals to the time synchronization recipient. The time-synchronized end includes: a second time synchronization control module, a second data supervision module, a second spread spectrum coding module, a second laser emission module, a second laser receiving module, a second spread spectrum decoding module, a second data error correction module, and a time synchronization module; The second time synchronization control module is used to generate a local high-precision 1PPS+TOD signal. When there is a calibration requirement, it triggers the synchronization process to generate a time synchronization request signal containing the local time signal. The second data supervision module has its input connected to the second time control module. It is used to receive the local time signal to be transmitted and add a supervision bit to the signal to realize error detection and error correction functions. The output of the second data supervision module is connected to the input of the second spread spectrum coding module. The second spread spectrum coding module has its input connected to the second data supervision module, and is used to receive the signal with supervision bits and perform spread spectrum modulation. The second laser emitting module has its input end connected to the second spread spectrum coding module, which is used to modulate the spread spectrum signal onto the optical carrier and, in conjunction with the emitting optical system, focus the optical signal and emit it. The second laser receiving module is used to receive the time signal from the time synchronization terminal through the receiving optical system, and to converge and convert it into a time synchronization electrical signal; The second spread spectrum decoding module has its input terminal connected to the second laser receiving module, which is used to receive the timing electrical signal and perform spread spectrum demodulation to recover the time signal containing the supervision bit from the noise; the output terminal of the second spread spectrum decoding module is connected to the input terminal of the second data error correction module, which is used to send the demodulated time signal to the second data error correction module. The second data error correction module has its input connected to the second spread spectrum decoding module, which is used to decode the received signal and use the supervision bit to realize error detection and error correction. The output of the second data error correction module is connected to the input of the second time synchronization control module, and is used to send the corrected and accurate time signal to the second time synchronization control module. The second timing control module has its input connected to the second data error correction module. It is used to receive the corrected time signal and calculate the time difference of the 1PPS rising edge between the received timing terminal time signal and the local time signal. One output of the second time synchronization control module is connected to the time synchronization module and is used to send the calculated time difference to the time synchronization module. The time synchronization module has its input connected to the second time control module, which is used to receive the time difference and perform precise delay adjustment on the local 1PPS signal of the time-synchronized end to synchronize it with the 1PPS signal of the time-synchronized end. The timing terminal includes: a first timing control module, a first data monitoring module, a first spread spectrum coding module, a first laser transmitting module, a first laser receiving module, a first spread spectrum decoding module, and a first data error correction module; The first timing control module is used to generate a local high-precision 1PPS+TOD signal, and its output is connected to the first data supervision module to send the generated time signal to the first data supervision module. The first data supervision module has its input end connected to the first time synchronization control module, which is used to receive the time signal and add data supervision bits to the signal to generate a time signal with supervision bits, thereby realizing subsequent error detection and error correction functions; its output end is connected to the first spread spectrum coding module, which is used to send the time signal with supervision bits to the first spread spectrum coding module. The first spreading encoding module has its input end connected to the first data supervision module, which is used to receive the time signal with supervision bits and perform direct sequence spread spectrum modulation on the signal to generate a spread spectrum signal; its output end is connected to the first laser emitting module, which is used to send the spread spectrum signal to the first laser emitting module. The first laser emitting module has its input end connected to a first spread spectrum coding module, which is used to receive the spread spectrum signal and modulate the signal onto a laser carrier in the green band; the first laser emitting module also includes an emitting optical system, which is used to focus the modulated optical signal and transmit it to the underwater channel to improve the underwater transmission distance. The first laser receiving module is used to receive the time signal transmitted from the time-synchronized end through the receiving optical system, converge it and convert it into a time signal electrical signal; its output end is connected to the first spread spectrum decoding module and is used to send the time signal electrical signal to the first spread spectrum decoding module. The first spread spectrum decoding module has its input end connected to the first laser receiving module, which is used to receive the timing electrical signal and recover the original time signal containing the supervision bit from the noise through relevant despreading processing; its output end is connected to the first data error correction module, which is used to send the despread time signal to the first data error correction module. The first data error correction module has its input end connected to the first spread spectrum decoding module, which is used to decode the despread time signal, and use the supervisory bit to realize 2-bit error detection and 1-bit error correction to recover accurate time information; its output end is connected to the first time synchronization control module, which is used to feed back the corrected time signal to the first time synchronization control module. The first timing control module is also used to receive the corrected time signal fed back by the first data error correction module, compare it with the local time signal, and calculate the 1PPS rising edge time difference between the timing end and the timing received end.

2. The underwater laser timing system based on spread spectrum communication according to claim 1, characterized in that: Both the first and second spreading coding modules adopt the direct sequence spreading method, using an m-sequence as a pseudo-random code to spread the spectrum of the signal.

3. The underwater laser timing system based on spread spectrum communication according to claim 1, characterized in that: Both the first and second laser emitting modules use blue band light to emit laser signals; the emitting optical systems of both the first and second laser emitting modules use aspherical lenses and place them at the light outlet of the laser; the optical signal output by the first laser emitting module is transmitted to the time-synchronized end via an underwater channel, and the optical signal output by the second laser emitting module is transmitted to the time-synchronized end via an underwater channel.

4. The underwater laser timing system based on spread spectrum communication according to claim 1, characterized in that: The receiving optical systems of both the first and second laser receiving modules use Fresnel lenses.

5. The underwater laser timing system based on spread spectrum communication according to claim 1, characterized in that: Both the first and second data error correction modules use (7,4) Hamming codes for error control, which can achieve 2-bit error detection and 1-bit error correction.

6. The underwater laser timing system based on spread spectrum communication according to claim 1, characterized in that: The time synchronization module uses a programmable absolute delay unit inside the FPGA to achieve a time delay with picosecond-level precision.

7. An underwater laser timing method based on spread spectrum communication, characterized in that: Includes the following steps: Step 1: The time-controlled end generates a signal containing 1PPS+TOD information. The second data supervision module adds 3 supervision bits to every 4 bits of data in the time signal. The second spread spectrum coding module performs spread spectrum modulation. The second laser emission module converts the spread spectrum signal into an optical signal and sends it to the underwater channel. Step 2: The first laser receiving module of the timing end receives the optical signal sent by the timing end and converts it into an electrical signal. The first spread spectrum decoding module despreads the electrical signal to recover the timing signal containing the supervision bit. The first data error correction module detects and corrects errors in the timing signal. The first timing control module compares the time signal after the data error correction module with the local signal and merges and encodes the comparison result with the local time signal. The first data supervision module adds supervision bits to the signal. The first spread spectrum coding module spreads and encodes the time signal after adding supervision bits. The spread spectrum signal is transmitted to the underwater channel by the first laser transmitting module. Step 3: The second laser receiving module of the time-receiving end receives the time signal sent by the time-synchronizing end and converts it into an electrical signal. The second spread spectrum decoding module despreads the electrical signal. The second data error correction module detects and corrects errors in the despread signal. The second time synchronization control module compares the time signal after the data error correction module with the local signal to accurately calculate the underwater laser link time delay and the absolute time difference between the rising edge of the local 1PPS and the time-synchronizing end 1PPS. The time synchronization module is used to accurately delay the local 1PPS to keep the time of the time-receiving end synchronized with the time of the time-synchronizing end.

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