An underwater short-range laser positioning method based on ultra-short baseline positioning principle

Abstract

This invention discloses an underwater near-range laser positioning method based on the principle of ultra-short baseline positioning, comprising: using a laser at the transmitting end to generate linearly frequency-modulated laser light and dividing it into... N One local oscillator beam and one measurement beam; the measurement beam scans the target water area; the receiver uses a lens array to receive reflected light from nearby underwater targets. The lens array is arranged with ultra-short baseline positioning elements, and contains a total of [number missing] elements. N One receiving lens; for N The echo signal from each receiving lens and N The local oscillator signal is processed to obtain N This invention uses a medium-frequency signal and calculates the three-dimensional coordinates of an underwater near-range target based on the relevant formulas for frequency-modulated continuous wave (FM-CW) and ultra-short baseline (USBR) positioning. Based on the USBR positioning principle, this invention uses FM-CW laser ranging to replace traditional underwater acoustic ranging, fully utilizing the high precision and real-time performance advantages of FM-CW laser ranging at close range underwater locations. This enables accurate and rapid positioning of underwater near-range targets.

Description

Technical Field

[0001] This invention relates to the field of underwater positioning, specifically to a laser positioning method, and more specifically to an underwater near-range laser positioning method based on the principle of ultra-short baseline positioning. Background Technology

[0002] Ultra-short baseline (USBL) positioning is an underwater acoustic positioning technology widely used in marine scientific research, marine exploration, and underwater engineering. Its principle is to locate the target based on its azimuth and slant range; its accuracy is determined by the estimation accuracy of both azimuth and slant range. The calculation of both azimuth and slant range relies on the distance measurements between different array elements and the target; therefore, the accuracy of underwater ranging is closely related to the accuracy of USBL positioning.

[0003] In underwater ranging technology, underwater acoustic ranging and laser ranging are the two most common methods. They utilize the propagation characteristics of underwater acoustic signals and laser signals in water, respectively, to measure distance. Because underwater acoustic signals can propagate over long distances in water and are less affected by water quality, underwater acoustic ranging features high accuracy, strong long-range detection capability, and suitability for complex underwater environments, making it suitable for detecting underwater targets at long distances. For close-range underwater targets, laser ranging offers higher accuracy and faster speed than underwater acoustic ranging due to its extremely high propagation speed and less susceptibility to multipath effects. Among laser ranging methods, frequency-modulated continuous wave (FMCW) offers significantly higher ranging accuracy, faster measurement speed, and the ability to simultaneously measure the distance and velocity of the target object compared to phase-based and pulse-based laser ranging methods, making it widely used for target localization and tracking.

[0004] Currently, ultra-short baseline positioning (UBS) is based on underwater acoustic ranging technology. It indirectly calculates the azimuth and slant range by measuring the distances between different array elements and the target, making it suitable for locating distant underwater targets. However, for close-range underwater targets, the accuracy of distance measurements between different array elements and the target using underwater acoustic ranging is far lower than that of laser ranging, resulting in lower accuracy for estimating the azimuth and slant range. Summary of the Invention

[0005] To address the above issues, this invention provides an underwater near-range laser positioning method based on the principle of ultra-short baseline positioning. This positioning method combines the advantages of ultra-short baseline positioning and frequency-modulated continuous wave laser ranging, making it suitable for precise positioning of underwater near-range targets.

[0006] The technical solution adopted by this invention to solve its technical problem is: an underwater near-range laser positioning method based on the principle of ultra-short baseline positioning, comprising the following steps:

[0007] Step 1): The transmitter uses a tunable laser in the blue-green light band to generate a linear frequency modulated laser with a frequency modulation bandwidth in the gigahertz (GHz) range to ensure that the frequency modulated continuous wave laser ranging has a ranging accuracy in the centimeter (cm) range; the linear frequency modulated laser is split into N local oscillator beams and 1 measurement beam by multiple optical splitters; the emission direction of the measurement beam is changed to scan the target water area;

[0008] Step 2): The receiver uses a lens array to receive the reflected light from the measurement light illuminating the underwater near-field target; the lens array is arranged in a distribution pattern of ultra-short baseline positioning array elements, containing a total of N receiving lenses; the echo light signals from the N receiving lenses are interfered with and photoelectrically converted with the corresponding N local oscillator light signals to obtain N intermediate frequency signals.

[0009] Step 3) Perform analog-to-digital conversion on the N intermediate frequency signals to obtain N frequency values ​​corresponding to the N intermediate frequency signals; obtain N distance values ​​corresponding to the N frequency values ​​according to the distance calculation formula of frequency-modulated continuous wave laser ranging, that is, the distance measurement results between different array elements and underwater near-range targets; calculate the azimuth and slant range of the underwater near-range target according to the arrangement of the lens array and the N distance values, based on the principle of ultra-short baseline positioning; calculate the three-dimensional coordinate position of the underwater near-range target based on the azimuth and slant range.

[0010] Advantages or beneficial effects of the present invention:

[0011] This invention uses frequency-modulated continuous wave laser ranging to replace traditional underwater acoustic ranging, based on the principle of ultra-short baseline positioning. Compared with underwater acoustic ranging, the distance measurement results between different array elements and the target are more accurate, resulting in higher estimation accuracy for azimuth angle and slant range, ultimately achieving higher positioning accuracy and enabling fast and accurate positioning of underwater targets at close range. Attached Figure Description

[0012] Figure 1 This is a flowchart of the underwater near-field laser positioning method based on the ultra-short baseline positioning principle in the embodiment;

[0013] Figure 2 This is a schematic diagram illustrating the implementation principle of the underwater near-field laser positioning method based on the ultra-short baseline positioning principle in the embodiments.

[0014] Figure 3 This is a schematic diagram illustrating the receiving effect of the receiving array in the embodiment, which only contains lens 1 and lens 3.

[0015] Figure 4This is a schematic diagram of the receiving effect when the receiving array in the embodiment only contains lens 2 and lens 4. Detailed Implementation

[0016] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, examples of which are shown in the drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0017] Example:

[0018] like Figure 1 As shown, an underwater near-range laser positioning method based on the principle of ultra-short baseline positioning includes the following process steps:

[0019] Step (1): The transmitter uses a tunable laser in the blue-green light band to generate a linear frequency modulated laser with a frequency modulation bandwidth of gigahertz (GHz) to ensure that the frequency modulated continuous wave laser ranging has a ranging accuracy of centimeters (cm); the linear frequency modulated laser is split into N local oscillator beams and 1 measurement beam by multiple optical splitters; the emission direction of the measurement beam is changed by a rotating mirror structure to scan the target water area;

[0020] Step (1) includes the following steps:

[0021] Step (1-1): Establish a three-dimensional rectangular coordinate system, such as... Figure 2 As shown, the x-axis, y-axis, and z-axis are perpendicular to each other. Assume there is an underwater target S at close range in the target water area, with coordinates (X, Y, Z). X, Y, and Z are the projections of S onto the x-axis, y-axis, and z-axis, respectively. S' is the projection of S onto the water surface, with coordinates (X, Y, 0). According to trigonometric relationships, we have...

[0022]

[0023] Where R is the slant distance, cosθ x The angle θ between the underwater near-range target S and the x-axis x The cosine value, cosθ y The angle θ between the underwater near-range target S and the y-axis y The cosine value;

[0024] Step (1-2): The light source at the transmitting end is a wavelength-tunable laser, located at the origin of the coordinate system, such as... Figure 2 As shown, its center wavelength is 532nm, the tuning method is sawtooth linear frequency modulation, the tuning rate is 200nm / s, and the tuning time is 40μs.

[0025] The relationship between laser linewidth and frequency:

[0026]

[0027] Where c represents the speed of light, λ represents the wavelength, Δλ represents the linewidth in nm, and Δv represents the linewidth in Hz; the relationship between distance resolution and bandwidth in frequency-modulated continuous wave laser ranging is as follows:

[0028]

[0029] Where c represents the speed of light, B represents the bandwidth of the transmitted signal, and ΔR represents the distance resolution;

[0030] According to formulas (2) and (3), the modulation bandwidth of the laser under the above modulation parameters is 8.5 GHz and the distance resolution is 1.76 cm, thus ensuring the high precision and high speed measurement of frequency-modulated continuous wave laser ranging.

[0031] Steps (1-3) involve passing the linear frequency-modulated laser output from the tunable laser through a 1×2 optical splitter, splitting it into local oscillator light and measurement light at a ratio of 90% to 10%; the local oscillator light is then passed through a 1×4 optical splitter, splitting it into four local oscillator lights at a ratio of 25% each; the four local oscillator lights enter four optical mixers, each serving as one of the input signals in the optical mixer; the measurement light is focused by a collimator, and then the emission direction of the measurement light is adjusted by a rotating mirror structure to scan the target water area;

[0032] Step (2): The receiver uses a lens array to receive the reflected light from the underwater near-field target illuminated by the measurement light; the lens array is arranged in the distribution of ultra-short baseline positioning array elements, and contains a total of N receiving lenses; the echo light signals of the N receiving lenses are interfered with and photoelectric converted with the corresponding N local oscillator light signals to obtain N intermediate frequency signals.

[0033] Step (2) includes the following steps:

[0034] Step (2-1): The receiver uses a lens array to receive the reflected light from the underwater near-field target illuminated by the measurement light. The lens array has four receiving lenses that focus the light signal, corresponding to the number of local oscillator beams: lens 1, lens 2, lens 3, and lens 4. Each lens can be analogous to an array element in the ultra-short baseline positioning principle, located on two mutually perpendicular coordinate axes, the x-axis and the y-axis, respectively. The distance between the lenses in each direction, i.e., the baseline, is dcm. The specific distribution of the lens array is as follows: Figure 2 As shown;

[0035] Step (2-2): When the measuring light shines on the underwater near-range target S, its reflected light is received by the four receiving lenses in the lens array.

[0036] First, the four receiving lenses generate four echo signals, namely echo signal 1, echo signal 2, echo signal 3 and echo signal 4.

[0037] Then, the four echo optical signals and the corresponding four local oscillator optical signals interfere with each other in four optical mixers to generate four intermediate frequency optical signals, namely intermediate frequency optical signal 1, intermediate frequency optical signal 2, intermediate frequency optical signal 3 and intermediate frequency optical signal 4.

[0038] The last four intermediate frequency optical signals are converted into four intermediate frequency signals by four avalanche photodiodes, namely intermediate frequency signal 1, intermediate frequency signal 2, intermediate frequency signal 3 and intermediate frequency signal 4.

[0039] Step (3): Perform analog-to-digital conversion on the N intermediate frequency signals, and calculate the N frequency values ​​corresponding to the N intermediate frequency signals according to the Fourier transform; obtain the N distance values ​​corresponding to the N frequency values ​​according to the distance calculation formula of frequency-modulated continuous wave laser ranging, that is, the distance measurement results between different array elements and underwater near-range targets; calculate the azimuth and slant range of the underwater near-range target according to the arrangement of the lens array and the N distance values, based on the principle of ultra-short baseline positioning; calculate the three-dimensional coordinate position of the underwater near-range target based on the azimuth and slant range.

[0040] Step (3) includes the following steps:

[0041] Step (3-1) involves performing a 4-channel analog-to-digital conversion on the four intermediate frequency (IF) signals and using Fourier transform to calculate the frequency values ​​of the IF signals, which are f1, f2, f3, f4, f5, f6, f7, f8, f9 b1 ,f b2 ,f b3 and f b4 ;

[0042] The distance calculation formula for frequency-modulated continuous wave laser ranging is:

[0043]

[0044] Where c represents the speed of light, B represents the modulation bandwidth, T represents the modulation time width, and f b Indicates the frequency of the intermediate frequency signal;

[0045] According to formula (4), the four measurement distance values ​​corresponding to the four frequency values ​​are obtained, namely R1, R2, R3 and R4;

[0046] Step (3-2): Because the lens array is arranged in a cross shape with extremely small spacing between elements, and the origin of the coordinate system is located in the middle of the lens array, in most cases, the slant range, i.e., the distance between the origin of the coordinate system and the underwater near-range target, lies in the middle of the measured distances between the four elements and the underwater near-range target S. Therefore, based on the measured distance values ​​R1, R2, R3, and R4, their average value can be calculated and used as an approximation of the slant range R.

[0047]

[0048] Step (3-3): When the light signal reflected back from the underwater near-range target S reaches the lens array, since the distance between the array elements in the lens array is extremely small, it can be assumed that the light rays reflected to the lens array are parallel and make an angle θ with the x-axis. x The angle between the y-axis and the x-axis is θ. y ;

[0049] The light reception of lenses 1 and 3 on the x-axis is as follows: Figure 3 As shown, lens 1 first receives the reflected light, and then after passing through a distance d×cosθ x The reflected light received by rear lens 3 is R3-R1=d×cosθ x ,but

[0050]

[0051] The light reception of lenses 2 and 4 on the y-axis is as follows: Figure 4 As shown, lens 4 first receives the reflected light, and then after passing through a distance d×cosθ y The reflected light received by rear lens 2 is R2-R4=d×cosθ y ,but

[0052]

[0053] Step (3-4) involves setting the slant distance R and cosθ. x cosθ y Substituting the three values ​​into formula (1) yields the coordinates of the underwater near-field target S.

[0054] Through the above steps, the precise location of the underwater target S at close range can be achieved.

[0055] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An underwater near-range laser positioning method based on the principle of ultra-short baseline positioning, characterized in that, The steps include the following: Step 1): The transmitter uses a tunable laser in the blue-green light band to generate a linear frequency modulated laser with a frequency modulation bandwidth in the gigahertz-GHz range; the linear frequency modulated laser is split into multiple optical splitters. One local oscillator beam and one measurement beam; the emission direction of the measurement beam is changed to scan the target water area; Step 2), the receiver uses a lens array to receive the reflected light from the underwater near-field target illuminated by the measurement light; the lens array is arranged in a distribution pattern of ultra-short baseline positioning elements, and contains a total of One receiving lens; The echo light signal from each receiving lens and the corresponding Each local oscillator optical signal is subjected to interference and photoelectric conversion to obtain... One intermediate frequency signal; Step 3), for The intermediate frequency signal is converted from analog to digital to obtain The corresponding intermediate frequency signal A frequency value; obtained from the distance calculation formula of frequency-modulated continuous wave laser ranging. Each frequency value corresponds to These are distance values, representing the distance measurements between different array elements and nearby underwater targets; based on the arrangement of the lens array and... Based on the distance values, the azimuth and slant range of the underwater near-range target are calculated according to the ultra-short baseline positioning principle; the three-dimensional coordinate position of the underwater near-range target is calculated based on the azimuth and slant range. Step 1) includes the following steps: Step (1-1): Establish a three-dimensional rectangular coordinate system with the x-axis, y-axis, and z-axis perpendicular to each other. Assume there is an underwater target S at close range in the target water area, with coordinates (X, Y, Z), where X, Y, and Z are the projections of S onto the x-axis, y-axis, and z-axis, respectively. Let S be the projection of the object on the water surface, with coordinates (X, Y, 0). According to trigonometric relations, we have... （1） Where R is the slant distance. The angle between the underwater near-range target S and the x-axis The cosine value, The angle between the S-axis and the y-axis for a close-range underwater target. The cosine value; Steps (1-2): The light source at the transmitting end is a wavelength-tunable laser located at the origin of the coordinate system, with a center wavelength of 532 nm. The tuning method is sawtooth linear frequency modulation, with a tuning rate of 200 nm / s and a tuning time of 40 seconds. ; The relationship between laser linewidth and frequency: （2） Where c represents the speed of light. Indicates wavelength. This represents the linewidth in nm. Indicates the line width in Hz; The relationship between distance resolution and bandwidth in frequency-modulated continuous wave laser ranging: （3） Where c represents the speed of light, and B represents the bandwidth of the transmitted signal. Indicates distance resolution; According to formulas (2) and (3), the modulation bandwidth of the laser under the above modulation parameters is 8.5 GHz and the distance resolution is 1.76 cm. Steps (1-3) involve passing the linear frequency modulated laser output from the tunable laser through a 1×2 optical splitter, splitting it into local oscillator light and measurement light at a ratio of 90% to 10%; passing the local oscillator light through a 1×4 optical splitter, splitting it into four local oscillator lights at a ratio of 25% each; and then sending the four local oscillator lights into four optical mixers, each serving as one of the input signals in the optical mixer. The measurement light is focused by a collimator, and then the emission direction of the measurement light is adjusted by a rotating mirror structure to scan the target water area. Step 2) includes the following steps: Step (2-1): The receiver uses a lens array to receive the reflected light from the underwater near-field target illuminated by the measurement light. The lens array has four lenses for focusing the light signal, corresponding to the number of local oscillator beams: lens 1, lens 2, lens 3, and lens 4. Each lens can be analogous to an array element in the ultra-short baseline positioning principle, located on two mutually perpendicular coordinate axes, the x-axis and the y-axis, respectively, and the distance between the lenses in each direction, i.e., the baseline, is constant. cm; Step (2-2): When the measuring light shines on the underwater near-range target S, its reflected light is received by the four receiving lenses in the lens array. First, the four receiving lenses generate four echo signals, namely echo signal 1, echo signal 2, echo signal 3 and echo signal 4. Then, the four echo optical signals and the corresponding four local oscillator optical signals interfere with each other in four optical mixers to generate four intermediate frequency optical signals, namely intermediate frequency optical signal 1, intermediate frequency optical signal 2, intermediate frequency optical signal 3 and intermediate frequency optical signal 4. The last four intermediate frequency optical signals are converted into four intermediate frequency signals by four avalanche photodiodes, namely intermediate frequency signal 1, intermediate frequency signal 2, intermediate frequency signal 3 and intermediate frequency signal 4. Step 3) includes the following steps: Step (3-1) involves performing a 4-channel analog-to-digital conversion on the four intermediate frequency (IF) signals and using Fourier transform to calculate the frequency values ​​of the IF signals, which are respectively... and ; The distance calculation formula for frequency-modulated continuous wave laser ranging is: （4） Where c represents the speed of light, B represents the modulation bandwidth, and T represents the modulation duration. Indicates the frequency of the intermediate frequency signal; According to formula (4), the four measurement distance values ​​corresponding to the four frequency values ​​are obtained, that is... and ; Step (3-2): The lens array is arranged in a cross shape with minimal spacing between elements. The origin of the coordinate system is located in the middle of the lens array. The slant range, i.e., the distance between the origin of the coordinate system and the near-underwater target, is located in the middle of the measured distance values ​​between the four array elements and the near-underwater target S. Based on the measured distance value... Calculate their average and use it as an approximation of the slope distance R, i.e. （5） Step (3-3): When the light signal reflected back from the underwater near-range target S reaches the lens array, the light rays reflected to the lens array are parallel and make an angle with the x-axis of . The angle between the y-axis and the x-axis is ; Lens 1 receives the reflected light, and then after passing through a distance The rear lens 3 receives the reflected light, that is ,but （6） Lens 4 receives the reflected light, and then after a distance... The rear lens 2 receives the reflected light, that is ,but （7） Step (3-4), set the slope distance R, , Substituting the three values ​​into formula (1) yields the coordinates of the underwater near-field target S.

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