A cross-medium underwater pseudorange navigation method based on laser acoustic signals

Abstract

This invention discloses a cross-medium underwater pseudorange navigation method based on laser acoustic signals, belonging to the field of cross-medium navigation and positioning technology. It is used for cross-medium navigation and includes: confirming the target sea area, obtaining the coordinates of the space-air platform, the space-air platform emitting a laser pulse sequence towards the sea surface, calculating the coordinates of the laser spot and using it as the coordinates of a virtual sound source; based on the coordinates of the virtual sound source, performing XOR spread spectrum to generate an on / off control flow, and exciting the corresponding time-series underwater acoustic code division multiple access signal through photoacoustic effects; the underwater vehicle acquiring the signal, combining it with locally stored Gold code sequences to recover the navigation message; constructing a nonlinear pseudorange observation equation set, and performing linearized iterative solutions to obtain the three-dimensional coordinates of the underwater vehicle. This invention uses the laser spot instantly formed on the sea surface by the space-air platform as a virtual sound source, achieving real-time covert navigation. Utilizing the extremely narrow autocorrelation main lobe characteristic of the spreading code, it accurately locks onto the direct wave, significantly improving ranging accuracy.

Description

Technical Field

[0001] This invention discloses a cross-medium underwater pseudorange navigation method based on laser acoustic signals, belonging to the field of cross-medium navigation and positioning technology. Background Technology

[0002] Currently, underwater vehicle navigation and positioning mainly rely on inertial navigation systems (INS) and underwater acoustic positioning systems (such as LBL, SBL, and USBL). However, INS suffers from the problem of error accumulation over time, requiring periodic calibration; traditional underwater acoustic positioning systems require the pre-deployment of physical acoustic buoys or beacons on the sea surface or seabed. This is not only costly and time-consuming to deploy, but surface buoys are also highly susceptible to revealing the underwater vehicle's position and are easily destroyed by enemy forces or damaged by harsh sea conditions. To address the problem of covert navigation across media, optoacoustic communication navigation has gradually become a research hotspot. However, existing technologies typically employ single-pulse ranging (such as PPM modulation), which is highly susceptible to severe interference from multipath reflections from the sea surface and seabed in shallow water or complex sea conditions, making it impossible to accurately extract the arrival time of the direct wave and hindering the achievement of high-precision positioning. Summary of the Invention

[0003] The purpose of this invention is to provide a cross-medium underwater pseudorange navigation method based on laser acoustic signals, in order to solve the problems in the prior art, such as the heavy reliance on physical buoys which are easily exposed, and the poor anti-multipath interference capability of traditional photoacoustic single-pulse ranging.

[0004] A cross-medium underwater pseudorange navigation method based on laser acoustic signals includes: S1. Confirm the target sea area, obtain the coordinates of the aerospace platform, synchronize the aerospace platform to a unified global time reference, and the aerospace platform emits a laser pulse sequence to the sea surface to form a laser spot on the sea surface. Calculate the coordinates of the laser spot and use the coordinates of the laser spot as the coordinates of the virtual sound source point. S2. Encode the coordinates of the virtual sound source point and the laser emission time into baseband navigation data bits. Use an orthogonal pseudo-random noise code sequence to construct a unique coding sequence for the aerospace platform. Perform XOR spread spectrum on the baseband navigation data bits and the unique coding sequence for the aerospace platform to generate on / off control flow. Excite the corresponding underwater acoustic code division multiple access signal from the virtual sound source point through photoacoustic effect. S3. The underwater vehicle collects underwater acoustic code division multiple access signals through hydrophones, constructs a cross-correlation function by combining it with the orthogonal pseudo-random noise code sequence stored in the underwater vehicle, detects the peak value of the cross-correlation function, sets a detection threshold to demodulate the navigation data, and recovers the navigation message based on the navigation data. S4. Based on the underwater acoustic signal propagation time and the speed of sound in water, and combined with virtual sound source points, establish a set of nonlinear pseudorange observation equations that include the position of the underwater vehicle and the clock error of the receiver. Using observation data from at least four different virtual sound source points, linearize and iteratively solve the set of nonlinear pseudorange observation equations to obtain the three-dimensional coordinates of the underwater vehicle.

[0005] S1 includes an aerospace platform for unmanned aerial vehicles (UAVs), and is designed with... For the number of aerospace platforms, The drones hovered or circled over the target sea area. From different directions, the UAV is equipped with a GNSS system receiver, an inertial measurement unit, an atomic clock, and a downward-looking fixed laser transmitter. It synchronizes its time with GNSS timing signals and controls the laser to emit a sequence of laser pulses toward the sea surface, forming a UAV laser grid.

[0006] The process of calculating the coordinates of the UAV laser spot includes the UAV using a downward-looking fixed laser emitter to install vector... Calculate the absolute coordinates of the laser spot on the sea surface: ; In the formula, For laser spot indexing, , , and For the first The coordinates of a virtual sound source point Let be the three-dimensional coordinate vector of the laser spot in the absolute coordinate system of the sea surface. This represents the three-dimensional position coordinate vector of the space platform in the same absolute coordinate system. This represents the straight-line distance from the laser emitter to the laser spot on the sea surface. The rotation matrix from the space platform coordinate system to the navigation coordinate system, constructed using Euler angles: ; In the formula, Yaw angle The pitch angle, This refers to the roll angle; Will As a virtual sound source.

[0007] S1 includes a space-based platform consisting of space-based satellites. The number of aerospace platforms; when At that time, the space-based satellite is a low-orbit satellite equipped with a deflector. Using the navigation data bit period as the time interval standard, the laser beam is sequentially struck at least 4 nodes on the sea surface to form a space-based satellite laser grid. when At that time, the space-based satellite was a low-orbit satellite, which simultaneously emitted lasers towards the target sea area, forming... Each independent beam landing point constitutes a space-based satellite laser grid; The space-based satellite carries a GNSS receiver and a star sensor. It obtains the orbital coordinates and attitude matrix of the space-based satellite through the POD algorithm, maintains time synchronization through an onboard rubidium clock, and sequentially strikes the grid nodes on the sea surface using a beam scanning method.

[0008] The process of calculating the coordinates of space-based satellite laser spots includes establishing the sea level equation using the WGS-84 Earth reference ellipsoid model: ; In the formula, , and The coordinates are in the three-dimensional rectangular coordinate system of the Earth's center and Earth's solidity. For the semi-major axis of the ellipsoid, It is the minor semi-axis of the ellipsoid; By combining the satellite's absolute coordinates, attitude matrix, and deflection angle, a three-dimensional pointing unit direction vector of the space-based satellite laser beam in the WGS-84 geocentric-ground-fixed coordinate system is constructed. By constructing a spatial linear parametric equation and simultaneously solving it with the Earth ellipsoid equation, a quadratic equation in one variable concerning the distance parameter is obtained. After eliminating roots that represent penetration of the Earth to the opposite side and lack physical meaning, the obtained intersection point is the theoretical coordinate of the point where the laser hits the sea surface. ; Will As a virtual sound source.

[0009] S2 includes, will With laser emission time Add frame header synchronization code to obtain baseband navigation data bits. ; The orthogonal pseudo-random noise code sequence uses Gold code sequences to construct a unique coding sequence for each aerospace platform. , For platform indexing, This represents the position of the chip in the sequence; Through the and A bitwise XOR spread spectrum is performed to generate an on / off control flow. The on / off control flow is a binary chip stream that controls the on / off state of the laser. When the chip of the XOR result is 1, it corresponds to controlling the laser to emit a laser pulse. When the chip is 0, it corresponds to controlling the laser to remain silent. The underwater acoustic code division multiple access signal with corresponding timing is excited at the virtual sound source point through photoacoustic effect.

[0010] The underwater acoustic code division multiple access signal is excited using unipolar photoacoustic spread spectrum modulation, and the navigation data bits are set to... The generated laser control sequence : ; In the formula, For XOR operation, The waveform is a laser pulse. For continuous time variables, For chip serial number, This refers to the chip width; After the laser sequence strikes the water surface, the acoustic pressure response of the water is treated as a transient pulse response and converted into an acoustic pressure pulse sequence with the same frequency and phase as the laser's on / off envelope. This forms an underwater acoustic code division multiple access signal.

[0011] S3 includes the underwater vehicle's navigation data bit cycle. Inside, the underwater vehicle acquires underwater acoustic code division multiple access signals via hydrophones and converts them into digital sound pressure signals. The digital sound pressure signals are then cross-correlated and integrated with locally stored Gold codes, including the calculation of the received continuous sound pressure signals using a sliding correlator. , and locally stored by Continuous-time waveform replica obtained by pulse shaping cross-correlation function : ; In the formula, The integration time; Utilizing the autocorrelation properties of spread spectrum signals, in The maximum peak value is searched in the middle, and the time corresponding to the maximum peak value is determined to be the arrival time of the direct sound wave. The secondary peaks that appear later are determined to be multipath reflection waves from the sea surface and are eliminated. like Inside The peak value exceeded the set detection threshold. The current navigation data bit is determined to be 1. like Inside The peak value is less than or equal to the set detection threshold. The current navigation data bit is determined to be 0. Based on continuous sliding detection of navigation data bit periods, it will exceed the detection threshold. of The time delay of the occurrence of the maximum main peak is used as the TOA of the direct wave, and the binary array obtained by the periodic determination of each data bit is reassembled into a binary code stream. After frame synchronization verification, the navigation message is restored.

[0012] S4 includes, S4.1, based on the propagation time of underwater acoustic signals and the speed of sound in water, combined with... Establish a set of nonlinear pseudorange observation equations that include the position of the underwater vehicle and the clock bias of the receiver: ; ; In the formula, To observe pseudorange, For underwater acoustic signal propagation time, , and The three-dimensional coordinates of the underwater vehicle's position. For receiver clock bias, This is the speed of sound in water.

[0013] S4 includes, in S4.2, using observation data from at least four different virtual sound source points to perform a linearized iterative solution to the nonlinear pseudorange observation equations, including setting initial values ​​for the underwater vehicle. : ; In the formula, , and These are the initial values ​​for the three-dimensional coordinates of the underwater vehicle. Calculate the Jacobian matrix : ; ; In the formula, the constant 1 of the Jacobian matrix is... The partial derivatives; Calculate the residual vector : ; In the formula, To convert the current state matrix The first result obtained by substituting into the nonlinear observation equation Predicted pseudorange of a virtual sound source; The Gauss-Newton iterative method is used to solve the problem iteratively, and the correction amount of the state vector in each iteration is obtained. : ; based on Update status: ; In the formula, For the first The state matrix of the next iteration; when of When the L2 norm is less than the preset iteration threshold, the iteration stops and the three-dimensional coordinates of the underwater vehicle are output.

[0014] Compared with existing technologies, this invention has the following advantages: it completely eliminates the dependence on physical buoys on the sea surface, using the light spot instantly formed on the sea surface by the laser emitted by the space platform as a virtual sound source, thus achieving real-time covert navigation; it innovatively adopts the optical intensity on-off keying-direct sequence spread spectrum (OOK-DSSS) system to generate acoustic CDMA signals; by utilizing the extremely narrow autocorrelation main lobe characteristics of the spreading code, the underwater vehicle can filter out the hysteresis secondary peaks generated by multipath reflections from the sea surface, accurately lock onto the direct wave, and significantly improve ranging accuracy; by using orthogonal PRN codes to distinguish different spatial beams / platforms, the underwater vehicle only needs a single hydrophone to simultaneously receive signals from multiple virtual sound sources and perform parallel correlation calculations, realizing true underwater passive unidirectional broadcast pseudorange positioning, and supporting an unlimited number of users simultaneously. Attached Figure Description

[0015] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic diagram of data transmission in the method of the present invention; Figure 3 This is a schematic diagram of the cross-medium underwater pseudorange navigation of the UAV network according to the present invention; Figure 4 This is a diagram illustrating the verification of acoustic CDMA cross-correlation peak detection and anti-multipath principle at the receiver. Figure 5 This is a convergence curve of the 3D pseudorange positioning error with the number of iterations (UAV). Figure 6 This is a schematic diagram of the satellite single-satellite scanning transmedium underwater pseudorange navigation of the present invention; Figure 7 This is a convergence curve of the 3D pseudorange positioning error as a function of the number of iterations (satellite). Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0017] A cross-medium underwater pseudorange navigation method based on laser acoustic signals includes: S1. Confirm the target sea area, obtain the coordinates of the aerospace platform, synchronize the aerospace platform to a unified global time reference, and the aerospace platform emits a laser pulse sequence to the sea surface to form a laser spot on the sea surface. Calculate the coordinates of the laser spot and use the coordinates of the laser spot as the coordinates of the virtual sound source point. S2. Encode the coordinates of the virtual sound source point and the laser emission time into baseband navigation data bits. Use an orthogonal pseudo-random noise code sequence to construct a unique coding sequence for the aerospace platform. Perform XOR spread spectrum on the baseband navigation data bits and the unique coding sequence for the aerospace platform to generate on / off control flow. Excite the corresponding underwater acoustic code division multiple access signal from the virtual sound source point through photoacoustic effect. S3. The underwater vehicle collects underwater acoustic code division multiple access signals through hydrophones, constructs a cross-correlation function by combining it with the orthogonal pseudo-random noise code sequence stored in the underwater vehicle, detects the peak value of the cross-correlation function, sets a detection threshold to demodulate the navigation data, and recovers the navigation message based on the navigation data. S4. Based on the underwater acoustic signal propagation time and the speed of sound in water, and combined with virtual sound source points, establish a set of nonlinear pseudorange observation equations that include the position of the underwater vehicle and the clock error of the receiver. Using observation data from at least four different virtual sound source points, linearize and iteratively solve the set of nonlinear pseudorange observation equations to obtain the three-dimensional coordinates of the underwater vehicle.

[0018] S1 includes an aerospace platform for unmanned aerial vehicles (UAVs), and is designed with... For the number of aerospace platforms, The drones hovered or circled over the target sea area. From different directions, the UAV is equipped with a GNSS system receiver, an inertial measurement unit, an atomic clock, and a downward-looking fixed laser transmitter. It synchronizes its time with GNSS timing signals and controls the laser to emit a sequence of laser pulses toward the sea surface, forming a UAV laser grid.

[0019] The process of calculating the coordinates of the UAV laser spot includes the UAV using a downward-looking fixed laser emitter to install vector... Calculate the absolute coordinates of the laser spot on the sea surface: ; In the formula, For laser spot indexing, , , and For the first The coordinates of a virtual sound source point Let be the three-dimensional coordinate vector of the laser spot in the absolute coordinate system of the sea surface. This represents the three-dimensional position coordinate vector of the space platform in the same absolute coordinate system. This represents the straight-line distance from the laser emitter to the laser spot on the sea surface. The rotation matrix from the space platform coordinate system to the navigation coordinate system, constructed using Euler angles: ; In the formula, Yaw angle The pitch angle, This refers to the roll angle; Will As a virtual sound source.

[0020] S1 includes a space-based platform consisting of space-based satellites. The number of aerospace platforms; when At that time, the space-based satellite is a low-orbit satellite equipped with a deflector. Using the navigation data bit period as the time interval standard, the laser beam is sequentially struck at least 4 nodes on the sea surface to form a space-based satellite laser grid. when At that time, the space-based satellite was a low-orbit satellite, which simultaneously emitted lasers towards the target sea area, forming... Each independent beam landing point constitutes a space-based satellite laser grid; The space-based satellite carries a GNSS receiver and a star sensor. It obtains the orbital coordinates and attitude matrix of the space-based satellite through the POD algorithm, maintains time synchronization through an onboard rubidium clock, and sequentially strikes the grid nodes on the sea surface using a beam scanning method.

[0021] The process of calculating the coordinates of space-based satellite laser spots includes establishing the sea level equation using the WGS-84 Earth reference ellipsoid model: ; In the formula, , and The coordinates are in the three-dimensional rectangular coordinate system of the Earth's center and Earth's solidity. For the semi-major axis of the ellipsoid, It is the minor semi-axis of the ellipsoid; By combining the satellite's absolute coordinates, attitude matrix, and deflection angle, a three-dimensional pointing unit direction vector of the space-based satellite laser beam in the WGS-84 geocentric-ground-fixed coordinate system is constructed. By constructing a spatial linear parametric equation and simultaneously solving it with the Earth ellipsoid equation, a quadratic equation in one variable concerning the distance parameter is obtained. After eliminating roots that represent penetration of the Earth to the opposite side and lack physical meaning, the obtained intersection point is the theoretical coordinate of the point where the laser hits the sea surface. ; Will As a virtual sound source.

[0022] S2 includes, will With laser emission time Add frame header synchronization code to obtain baseband navigation data bits. ; The orthogonal pseudo-random noise code sequence uses Gold code sequences to construct a unique coding sequence for each aerospace platform. , For platform indexing, This represents the position of the chip in the sequence; Through the and A bitwise XOR spread spectrum is performed to generate an on / off control flow. The on / off control flow is a binary chip stream that controls the on / off state of the laser. When the chip of the XOR result is 1, it corresponds to controlling the laser to emit a laser pulse. When the chip is 0, it corresponds to controlling the laser to remain silent. The underwater acoustic code division multiple access signal with corresponding timing is excited at the virtual sound source point through photoacoustic effect.

[0023] The underwater acoustic code division multiple access signal is excited using unipolar photoacoustic spread spectrum modulation, and the navigation data bits are set to... The generated laser control sequence : ; In the formula, For XOR operation, The waveform is a laser pulse. For continuous time variables, For chip serial number, This refers to the chip width; After the laser sequence strikes the water surface, the acoustic pressure response of the water is treated as a transient pulse response and converted into an acoustic pressure pulse sequence with the same frequency and phase as the laser's on / off envelope. This forms an underwater acoustic code division multiple access signal.

[0024] S3 includes the underwater vehicle's navigation data bit cycle. Inside, the underwater vehicle acquires underwater acoustic code division multiple access signals via hydrophones and converts them into digital sound pressure signals. The digital sound pressure signals are then cross-correlated and integrated with locally stored Gold codes, including the calculation of the received continuous sound pressure signals using a sliding correlator. , and locally stored by Continuous-time waveform replica obtained by pulse shaping cross-correlation function : ; In the formula, The integration time; Utilizing the autocorrelation properties of spread spectrum signals, in The maximum peak value is searched in the middle, and the time corresponding to the maximum peak value is determined to be the arrival time of the direct sound wave. The secondary peaks that appear later are determined to be multipath reflection waves from the sea surface and are eliminated. like Inside The peak value exceeded the set detection threshold. The current navigation data bit is determined to be 1. like Inside The peak value is less than or equal to the set detection threshold. The current navigation data bit is determined to be 0. Based on continuous sliding detection of navigation data bit periods, it will exceed the detection threshold. of The time delay of the occurrence of the maximum main peak is used as the TOA of the direct wave, and the binary array obtained by the periodic determination of each data bit is reassembled into a binary code stream. After frame synchronization verification, the navigation message is restored.

[0025] S4 includes, S4.1, based on the propagation time of underwater acoustic signals and the speed of sound in water, combined with... Establish a set of nonlinear pseudorange observation equations that include the position of the underwater vehicle and the clock bias of the receiver: ; ; In the formula, To observe pseudorange, For underwater acoustic signal propagation time, , and The three-dimensional coordinates of the underwater vehicle's position. For receiver clock bias, This is the speed of sound in water.

[0026] S4 includes, in S4.2, using observation data from at least four different virtual sound source points to perform a linearized iterative solution to the nonlinear pseudorange observation equations, including setting initial values ​​for the underwater vehicle. : ; In the formula, , and These are the initial values ​​for the three-dimensional coordinates of the underwater vehicle. Calculate the Jacobian matrix : ; ; In the formula, the constant 1 of the Jacobian matrix is... The partial derivatives; Calculate the residual vector : ; In the formula, To convert the current state matrix The first result obtained by substituting into the nonlinear observation equation Predicted pseudorange of a virtual sound source; The Gauss-Newton iterative method is used to solve the problem iteratively, and the correction amount of the state vector in each iteration is obtained. : ; based on Update status: ; In the formula, For the first The state matrix of the next iteration; when of When the L2 norm is less than the preset iteration threshold, the iteration stops and the three-dimensional coordinates of the underwater vehicle are output.

[0027] After the laser sequence of this invention strikes the water surface, the acoustic pressure response of the seawater is treated as a transient pulse response and converted into an acoustic pressure pulse sequence with the same frequency and phase as the laser's on / off envelope. The principle behind underwater acoustic code division multiple access (CDMA) signals is as follows: when a high-energy, narrow-pulse laser pulse sequence strikes the sea surface, the seawater surface strongly absorbs the light energy (photoacoustic-thermoelastic mechanism). The local water body heats up and undergoes transient thermal expansion in an extremely short time. Since the time for the water body to absorb light energy, expand locally, and radiate sound waves is on the order of nanoseconds to microseconds, this conversion process can be considered an ideal pulse response. Therefore, because the spreading chip controls the 'present' and 'absent' state of the laser, the envelope of the sound pressure pulse sequence excited in the water perfectly replicates the on / off timing of the laser pulse in a macroscopic time sequence, achieving the same frequency and phase across the physical medium. This allows for the direct printing of underwater acoustic CDMA signals across the medium without an underwater acoustic transducer.

[0028] The following description, in conjunction with the accompanying drawings, further illustrates the process of this invention. Figure 1 As shown, the system is divided into three stages: the first stage is the air-based / space-based launch system, the second stage is the cross-medium physical conversion, and the third stage is the underwater receiving and processing system. The first stage includes, in sequence, the networking and global time synchronization of the air-space platform, acquisition of the platform's real-time status (precise orbit determination / attitude / ephemeris), coordinate mapping and processing (calculating virtual sound source coordinates based on ray tracing or mesh models), generation of navigation messages and allocation of orthogonal PRN spreading codes, laser modulation (OOK-DSSS) mapping the spreading codes to laser pulse on / off sequences, and control of laser beam scanning or array transmission towards the sea surface. The system uses an intensity on / off keying-direct sequence spread spectrum (OOK-DSSS) mechanism to generate the on / off control flow. The specific generation logic is as follows: first, the navigation data... Upsampling is performed, followed by a discrete Gold code sequence assigned to a specific platform. Perform a bitwise XOR operation. The rule is: when the XOR result is 1, control the laser to emit a high-energy short pulse; when the result is 0, the laser remains silent (does not emit light) during the chip period.

[0029] The calculation of the coordinates of space-based satellite laser spots involves a rigorous spatial geometric calculation process, using the WGS-84 geocentric coordinate system as a reference, to construct the space ray parameter equations: ; ; ; in For distance parameters, It is the direction vector; Substituting this ray equation into the Earth ellipsoid equation simplifies to obtain the equation regarding... The quadratic equation of Solving this equation usually yields two real roots. , The smaller root corresponds to the point where the laser beam hits the sea surface on the side of the Earth facing the satellite; while the larger root corresponds to the point where the ray penetrates the Earth's interior and reaches the point on the Earth's back side. This intersection point is physically meaningless due to the physical obstruction of the Earth itself. After removing the larger root, the coordinates of the remaining intersection point are the absolute coordinates of the laser spot. When the laser beam hits the sea surface, the second stage begins, which includes photoacoustic effects (light energy transiently converted into heat energy to generate pressure waves), the excitation of broadband acoustic CDMA signals in the water, and signal propagation in the underwater channel (accompanied by multipath reverberation and noise superposition). When the sound wave reaches the submersible, the third stage begins. First, the hydrophone collects the mixed underwater acoustic signal, then performs parallel correlation detection, uses a local PRN code copy for matched filtering, and performs peak discrimination. During peak discrimination, a set detection threshold is used. It is usually taken as 60% of the theoretical maximum autocorrelation peak value (i.e. Alternatively, the setting can be adaptively configured based on the underwater environment dynamic noise variance according to the Neyman-Pearson criterion. Detect relevant peaks and identify whether they are direct waves: if not, return to the hydrophone acquisition step for underwater mixed acoustic signals; if it is a direct wave, identify the main peak and record the time delay of its appearance. This time is the precise time of arrival (TOA) of the direct wave. Simultaneously, based on the polarity or amplitude of the main peak within this integration period, it is determined whether the currently demodulated navigation data bit is 0 or 1. Through continuous sliding detection and recording over several cycles, the recovered binary code stream is spliced ​​and reassembled. After finding the frame header synchronization code to complete frame synchronization, the complete navigation message, including the space platform coordinates, ephemeris, and transmission timestamp, can be decoded and recovered. Subsequently, pseudorange observations are calculated to determine if at least four sets of pseudoranges from different sound sources have been acquired. If not, the process returns to the hydrophone to collect underwater mixed acoustic signals; if so, a linear navigation equation system is constructed and then iteratively solved to calculate the three-dimensional coordinates and clock error.

[0030] A schematic diagram of the method of the present invention is shown below. Figure 2 As shown, the space-based / space-based transmitter generates a digital CDMA signal from the pseudo-random code (PRN) and data (OOK), then inputs it into a laser modulator to emit a modulated laser beam to the sea surface; the signal is converted from photoacoustic signal to optical signal at the sea surface and propagates in the underwater channel to form an underwater acoustic CDMA signal (a mixture of multi-source signals and noise); the underwater receiver hydrophone receives the underwater acoustic CDMA signal and processes the received mixed signal and a copy of the local pseudo-random code.

[0031] Embodiment 1 of the present invention Figure 3 As shown, a network of four drones was used to transmit signals from the air, hovering directly above a virtual sound source on the sea surface. Due to the low flight altitude of the drones (around 100 meters), the energy loss of the laser penetrating the atmosphere was minimal, resulting in good quality acoustic signals generated underwater. The actual coordinates of the underwater vehicle were (200m, 150m, -100m), and the speed of sound in water was set to 1500 m / s. The aerospace platform... High altitude to the sea surface ( )Activate 4 virtual sound source arrays. Figure 3 This invention visually demonstrates that it completely eliminates the reliance on physical buoys on the water surface, effectively overcoming the problems of easy exposure and difficult deployment of traditional underwater acoustic long baseline systems.

[0032] At the signal receiver, under Gaussian white noise with a detected signal-to-noise ratio of -5dB, the multipath reflected waves from the sea surface / seabed lagged by approximately 3ms and attenuated by 50%. Combined with... Figure 4 It can be seen that even in harsh underwater channels with strong noise and strong multipath interference, the correlation function output by the sliding correlator at the receiver can still exhibit an extremely sharp autocorrelation main lobe due to the use of the optical-acoustic direct sequence spread spectrum (OA-DSSS) system. Figure 4 The system clearly demonstrates its ability to accurately pinpoint the main peak of the direct wave (red dot, corresponding to a physical propagation time of approximately 180ms) and physically remove the lagging multipath reflection secondary peaks, fundamentally solving the problem of traditional single-pulse photoacoustic ranging being unable to resist multipath propagation.

[0033] Finally, the extracted pseudorange observations are input into a system of nonlinear equations containing clock error unknowns for iterative optimization. The initial dive prediction coordinates of the submersible are set as follows: The initial clock bias value is 0. The nonlinear iterative solution process data for the UAV's cross-medium underwater navigation is shown in Table 1: Table 1. Data Table of Nonlinear Iterative Solution Process for Cross-Media Underwater Navigation (UAV) ; Combined with Table 1 and Figure 5 As can be seen from the convergence curve, the damped nonlinear iterative algorithm used in this invention exhibits extremely strong stability and convergence speed. Even with a deviation of 265 meters from the actual position, the algorithm only requires 3 iterations for the 3D positioning error to significantly converge, rapidly decreasing to the 0.3-meter level; by the 5th iteration, not only the 3D spatial coordinates... The system clock error almost perfectly coincided with the actual location, and a precise calculation of 1.003 ms was obtained. This demonstrates that under favorable observation conditions, the nonlinear pseudorange calculation algorithm proposed in this invention can quickly lock onto the target, requiring only 3 to 5 iterations to rapidly converge the 3D positioning error from an initial blind zone of several hundred meters to a sub-meter level accuracy of 0.3 meters. This proves the high efficiency of this invention in covert operations in shallow seas.

[0034] In Embodiment 2 of this invention, taking a single LEO low-orbit satellite as an example, a fast two-dimensional deflector (FSM) onboard the satellite is used to sequentially scan laser beams toward pre-defined grid nodes on the sea surface in a time-division manner, thereby constructing four virtual sound source beam landing points on the sea surface through a "single-point divergence" method. Figure 6 As shown in the physical configuration, considering the satellite's actual orbit is as high as 500km, its height has been proportionally compressed in the image to allow it to be displayed alongside the underwater vehicle. The time interval for time-division scanning is based on the "navigation data bit cycle (i.e., one complete pseudo-random code cycle)". That is, the satellite continuously strikes a grid node until it has transmitted a complete data bit, then the reflector deflects to print the next data bit on another grid node. The data from the satellite's cross-medium underwater navigation nonlinear iterative solution process are shown in Table 2. Table 2. Data Table of Nonlinear Iterative Solution Process for Cross-Media Underwater Navigation (Satellite) ; Combination Figure 7As shown in Table 2, due to the extreme attenuation causing some jitter in the pseudorange measurement, the algorithm adaptively activated a "damped anti-oscillation strategy." Despite the increase in the number of iterations, the algorithm did not exhibit any divergence or tail oscillation throughout the process. By the 7th iteration, the positioning error had robustly decreased to 0.76 meters (sub-meter level); by the 10th iteration, the three-dimensional spatial coordinates almost perfectly coincided with the true value (error of only 0.0035m), and the clock difference of 2.004ms was accurately recovered.

[0035] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for cross-medium underwater pseudo-range navigation based on laser acoustic signals, characterized in that, include: S1. Confirm the target sea area, obtain the coordinates of the aerospace platform, synchronize the aerospace platform to a unified global time reference, and the aerospace platform emits a laser pulse sequence to the sea surface to form a laser spot on the sea surface. Calculate the coordinates of the laser spot and use the coordinates of the laser spot as the coordinates of the virtual sound source point. S2. Encode the coordinates of the virtual sound source point and the laser emission time into baseband navigation data bits. Use an orthogonal pseudo-random noise code sequence to construct a unique coding sequence for the aerospace platform. Perform XOR spread spectrum on the baseband navigation data bits and the unique coding sequence for the aerospace platform to generate on / off control flow. Excite the corresponding underwater acoustic code division multiple access signal from the virtual sound source point through photoacoustic effect. S3. The underwater vehicle collects underwater acoustic code division multiple access signals through hydrophones, constructs a cross-correlation function by combining it with the orthogonal pseudo-random noise code sequence stored in the underwater vehicle, detects the peak value of the cross-correlation function, sets a detection threshold to demodulate the navigation data, and recovers the navigation message based on the navigation data. S4. Based on the underwater acoustic signal propagation time and the speed of sound in water, and combined with virtual sound source points, establish a set of nonlinear pseudorange observation equations that include the position of the underwater vehicle and the clock error of the receiver. Using the observation data of at least four different virtual sound source points, linearize and iteratively solve the set of nonlinear pseudorange observation equations to obtain the three-dimensional coordinates of the underwater vehicle. S3 includes the underwater vehicle's navigation data bit cycle. Inside, the underwater vehicle acquires underwater acoustic code division multiple access signals via hydrophones and converts them into digital sound pressure signals. The digital sound pressure signals are then cross-correlated and integrated with locally stored Gold codes, including the calculation of the received continuous sound pressure signals using a sliding correlator. , and locally stored by Continuous-time waveform replica obtained by pulse shaping cross-correlation function : ； In the formula, The integration time; Utilizing the autocorrelation properties of spread spectrum signals, in The maximum peak value is searched in the middle, and the time corresponding to the maximum peak value is determined to be the arrival time of the direct sound wave. The secondary peaks that appear later are determined to be multipath reflection waves from the sea surface and are eliminated. like Inside The peak value exceeded the set detection threshold. The current navigation data bit is determined to be 1; like Inside The peak value is less than or equal to the set detection threshold. The current navigation data bit is determined to be 0. Based on continuous sliding detection of navigation data bit periods, it will exceed the detection threshold. of The time delay of the occurrence of the maximum main peak is used as the TOA of the direct wave, and the binary array obtained by the periodic determination of each data bit is reassembled into a binary code stream. After frame synchronization verification, the navigation message is restored.

2. The cross-medium underwater pseudorange navigation method based on laser acoustic signals according to claim 1, characterized in that, S1 includes an aerospace platform for unmanned aerial vehicles (UAVs), and is designed with... For the number of aerospace platforms, The drones hovered or circled over the target sea area. From different directions, the UAV is equipped with a GNSS system receiver, an inertial measurement unit, an atomic clock, and a downward-looking fixed laser transmitter. It synchronizes its time with GNSS timing signals and controls the laser to emit a sequence of laser pulses toward the sea surface, forming a UAV laser grid.

3. The cross-medium underwater pseudorange navigation method based on laser-acoustic signals according to claim 2, characterized in that, The process of calculating the coordinates of the UAV laser spot includes the UAV using a downward-looking fixed laser emitter to install vector... Calculate the absolute coordinates of the laser spot on the sea surface: ； In the formula, For laser spot indexing, , , and For the first The coordinates of a virtual sound source point Let be the three-dimensional coordinate vector of the laser spot in the absolute coordinate system of the sea surface. This represents the three-dimensional position coordinate vector of the space platform in the same absolute coordinate system. This represents the straight-line distance from the laser emitter to the laser spot on the sea surface. The rotation matrix from the space platform coordinate system to the navigation coordinate system, constructed using Euler angles: ； In the formula, Yaw angle The pitch angle, This refers to the roll angle; Will As a virtual sound source.

4. The cross-medium underwater pseudorange navigation method based on laser acoustic signals according to claim 1, characterized in that, S1 includes a space-based platform consisting of space-based satellites. The number of aerospace platforms; when At that time, the space-based satellite is a low-orbit satellite equipped with a deflector. Using the navigation data bit period as the time interval standard, the laser beam is sequentially struck at least 4 nodes on the sea surface to form a space-based satellite laser grid. when At that time, the space-based satellite was a low-orbit satellite, which simultaneously emitted lasers towards the target sea area, forming... Each independent beam landing point constitutes a space-based satellite laser grid; The space-based satellite carries a GNSS receiver and a star sensor. It obtains the orbital coordinates and attitude matrix of the space-based satellite through the POD algorithm, maintains time synchronization through an onboard rubidium clock, and sequentially strikes the grid nodes on the sea surface using a beam scanning method.

5. The cross-medium underwater pseudorange navigation method based on laser acoustic signals according to claim 4, characterized in that, The process of calculating the coordinates of space-based satellite laser spots includes establishing the sea level equation using the WGS-84 Earth reference ellipsoid model: ； In the formula, , and The coordinates are in the three-dimensional rectangular coordinate system of the Earth's center and Earth's solidity. For the semi-major axis of the ellipsoid, It is the minor semi-axis of the ellipsoid; By combining the satellite's absolute coordinates, attitude matrix, and deflection angle, a three-dimensional pointing unit direction vector of the space-based satellite laser beam in the WGS-84 geocentric-ground-fixed coordinate system is constructed. By constructing a spatial linear parametric equation and simultaneously solving it with the Earth ellipsoid equation, a quadratic equation in one variable concerning the distance parameter is obtained. After eliminating roots that represent penetration of the Earth to the opposite side and lack physical meaning, the obtained intersection point is the theoretical coordinate of the point where the laser hits the sea surface. ; Will As a virtual sound source.

6. The cross-medium underwater pseudorange navigation method based on laser acoustic signals according to any one of claims 3 or 5, characterized in that, S2 includes, will With laser emission time Add frame header synchronization code to obtain baseband navigation data bits. ; The orthogonal pseudo-random noise code sequence uses Gold code sequences to construct a unique coding sequence for each aerospace platform. , For platform indexing, This represents the position of the chip in the sequence; Through the and A bitwise XOR spread spectrum is performed to generate an on / off control flow. The on / off control flow is a binary chip stream that controls the on / off state of the laser. When the chip of the XOR result is 1, it corresponds to controlling the laser to emit a laser pulse. When the chip is 0, it corresponds to controlling the laser to remain silent. The underwater acoustic code division multiple access signal with corresponding timing is excited at the virtual sound source point through photoacoustic effect.

7. The cross-medium underwater pseudorange navigation method based on laser-acoustic signals according to claim 6, characterized in that, The underwater acoustic code division multiple access signal is excited using unipolar photoacoustic spread spectrum modulation, and the navigation data bits are set to... The generated laser control sequence : ； In the formula, For XOR operation, The waveform is a laser pulse. For continuous time variables, For chip serial number, This refers to the chip width; After the laser sequence strikes the water surface, the acoustic pressure response of the water is treated as a transient pulse response and converted into an acoustic pressure pulse sequence with the same frequency and phase as the laser's on / off envelope. This forms an underwater acoustic code division multiple access signal.

8. The cross-medium underwater pseudorange navigation method based on laser acoustic signals according to claim 7, characterized in that, S4 includes, S4.1, based on the propagation time of underwater acoustic signals and the speed of sound in water, combined with... Establish a set of nonlinear pseudorange observation equations that include the position of the underwater vehicle and the clock bias of the receiver: ； ； In the formula, To observe pseudorange, For underwater acoustic signal propagation time, , and The three-dimensional coordinates of the underwater vehicle's position. For receiver clock bias, This is the speed of sound in water.

9. The cross-medium underwater pseudorange navigation method based on laser acoustic signals according to claim 8, characterized in that, S4 includes, in S4.2, using observation data from at least four different virtual sound source points to perform a linearized iterative solution to the nonlinear pseudorange observation equations, including setting initial values ​​for the underwater vehicle. : ； In the formula, , and These are the initial values ​​for the three-dimensional coordinates of the underwater vehicle. Calculate the Jacobian matrix : ； ； In the formula, the constant 1 of the Jacobian matrix is... The partial derivatives; Calculate the residual vector : ； In the formula, To convert the current state matrix The first result obtained by substituting into the nonlinear observation equation Predicted pseudorange of a virtual sound source; The Gauss-Newton iterative method is used to solve the problem iteratively, and the correction amount of the state vector in each iteration is obtained. : ； based on Update status: ； In the formula, For the first The state matrix of the next iteration; when of When the L2 norm is less than the preset iteration threshold, the iteration stops and the three-dimensional coordinates of the underwater vehicle are output.

Images