Underwater weak echo signal detection method and system for airborne ocean laser radar

Abstract

The invention belongs to the technical field of marine surveying and mapping, and discloses an airborne marine laser radar underwater weak echo signal detection method and system. According to the method, a waveform rising edge is determined by constructing a standard deviation curve, a water surface echo moment is accurately extracted, and two water surface geometric elements of a water surface point and a refraction vector are calculated on the basis; constructing a time-space constrained waveform superposition model, taking the Euclidean distance between the two water surface geometric elements as a neighbor discrimination standard, and screening and superposing neighbor waveforms by using a K neighbor judgment method to obtain a superposed pseudo waveform with a high signal-to-noise ratio; and finally, an improved normalized saliency peak detection method is utilized to judge the underwater echo position step by step and obtain the final underwater echo moment. After the model constructed by the invention is used for processing, the underwater weak echo detection of the airborne ocean laser radar is effectively realized, the maximum sounding performance of the airborne ocean laser radar is improved, and an effective technical support and a solution are provided for sounding data processing and application.

Description

Technical Field

[0001] The present invention belongs to the technical field of ocean surveying and mapping, and in particular relates to a method and system for detecting underwater weak echo signals of an airborne ocean laser radar. Background Art

[0002] Airborne ocean LiDAR bathymetry technology has the characteristics of high measurement accuracy, high density of measurement points, strong mobility and amphibious use. It is particularly suitable for rapid detection of complex terrains such as coastal zones, shallow water areas such as islands and reefs, and surrounding waters. It can obtain integrated terrain and geomorphic data above and below the water, and provide data foundation and important basis for marine resource development, marine ecological protection, coastal zone surveys and water resource management. The airborne ocean LiDAR bathymetry system can emit a water-permeable 532nm wavelength green laser and receive waveform data information including surface, water body and bottom echoes. By extracting each echo component, the laser travel time can be obtained. Combined with positioning and attitude data, the three-dimensional coordinates of the seabed terrain points can be calculated. Among them, bottom echo detection is the basis for obtaining accurate laser travel time. The detection capability of weak bottom echoes directly determines the maximum bathymetric performance of the system and is the key content of airborne ocean LiDAR bathymetric data processing.

[0003] Existing methods for detecting weak underwater echoes using airborne ocean lidar mainly include two categories: one is a detection method based on a single waveform; the other is a detection method combining multiple neighboring waveforms. Among them, the former uses time domain, frequency domain or time-frequency combined analysis methods to extract the position of weak underwater echoes based on the characteristic differences between the target echo and the noise signal in a single waveform, but this type of method does not fully consider the correlation between spatial neighboring waveforms; the latter uses waveform superposition and other methods to extract weak underwater echoes based on the consideration of the correlation between neighboring waveforms, but the criteria for distinguishing neighbors between waveforms are mostly in the form of a grid, which limits the flexibility of the method. In response to the above problems, the present invention proposes a more universal method for detecting weak underwater echo signals using airborne ocean lidar to improve the maximum depth measurement performance of airborne ocean lidar. Summary of the invention

[0004] In order to overcome the problems existing in the related art, the disclosed embodiment of the present invention provides a method and system for detecting weak echo signals from the bottom of the water by an airborne ocean laser radar, aiming to solve the problems in the existing detection technology that the neighbor relationship between waveforms is not fully utilized, and the detection flexibility and universality need to be improved.

[0005] The technical solution is as follows: A method for detecting weak underwater echo signals using an airborne ocean laser radar comprises the following steps: S1, determine the water surface echo position based on the waveform standard deviation curve, and obtain two geometric elements: water surface point and refraction vector; S2, constructing a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio; S3, based on the improved normalized significant peak detection method, judge and obtain the position of weak bottom echo step by step.

[0006] In step S1, the water surface echo position is determined based on the waveform standard deviation curve, and two geometric elements, the water surface point and the refraction vector, are obtained, including: S101, constructing a waveform standard deviation curve using a sliding window according to the standard deviation difference between the noise in the sounding waveform and the rising edge of the echo, and determining the rising edge position as the water surface echo position; S102, calculating the water surface point cloud according to the determined water surface echo position and positioning and attitude information , and based on the principal component analysis method, obtain the water surface points The corresponding refraction vector .

[0007] In step S101, the rising edge position is determined as the water surface echo position, and the expression is: (1) In the formula, is the water surface echo position, To find the data location that meets the conditions; is the standard deviation curve, , is the original echo signal Middle position The standard deviation of is the sliding window size, is the standard deviation function; The standard deviation curve The extreme point position of is the noise threshold; is the difference operation, is a symbolic function; In step S102, each water surface point is obtained The corresponding refraction vector , the expression is: (2) In the formula, are the refraction vector and the incident vector, respectively. are the refractive indices of laser light in air and water, respectively. denote the angle of incidence and the angle of refraction respectively, For water surface point Psi The normal vector at .

[0008] In step S2, a waveform superposition model with time and space constraints is constructed to obtain a superposition pseudo waveform with a high signal-to-noise ratio, including: S201, traverse all water surface points The corresponding refraction vector , construct a waveform superposition model with time and space constraints, including neighbor waveform selection and pseudo waveform acquisition; and calculate the Euclidean distance between all water surface points and refraction vectors to obtain the water surface point distance vector set Refraction distance vector collection ; in, , is the total number of sea surface points, is the two-norm operation, For the With The Euclidean distance between sea surface points is For the With The Euclidean distance between the refraction vectors, For the The coordinates of the sea surface points, For the The refraction vector corresponding to the sea surface point; S202, according to Nearest neighbor criterion, obtain the nearest neighbor criterion with each water surface point Neighbor Water surface points and corresponding refraction vectors ; From the selected The refraction vectors of the nearest neighbors In, again according to The nearest neighbor criterion is used to obtain the water surface points The corresponding refraction vector Neighbor refraction vectors and corresponding water surface points; the final determined neighbor waveform; S203, from the waveform The neighboring waveform data are superimposed, Gaussian white noise is filtered out, and a superimposed pseudo waveform with a higher signal-to-noise ratio is obtained.

[0009] In step S202, obtain Neighbor Water surface points and corresponding refraction vectors , the expression is: (3) In the formula, are the neighboring water surface points and refraction vectors screened for the first time, is the index of the neighboring water surface points screened for the first time, The sorted index of the array elements from small to large, To obtain values ​​from the sea surface point set, For the refraction vector set R Take the value in From 0 to interval vector; again, according to The nearest neighbor criterion is used to obtain the water surface points The corresponding refraction vector Neighbor The refraction vector and the corresponding water surface point are expressed as: (4) In the formula, are the neighboring water surface points and refraction vectors that are screened again, is the index of the neighboring water surface points screened again, For the collection Take the value in For the collection Take the value in is the refraction vector R i With Collection Subtract all corresponding vectors in , From 0 to An interval vector of ; Finally, the determined neighbor waveform set ,in, They are No. 1 to No. The waveform data of the nearest neighbors.

[0010] In step S203, a superimposed pseudo waveform with a higher signal-to-noise ratio is obtained, and the expression is: (5) In the formula, To superimpose a pseudo waveform, For the The water surface echo position of the neighboring waveform, is the length of the waveform taken, is the waveform data interval taken, The intercept vector A certain section of is the number of neighboring waveforms finally determined.

[0011] In step S3, based on the improved normalized significant peak detection method, the position of the weak echo at the bottom of the water is judged and obtained step by step, including: S301, obtaining the amplitudes of all peaks in the superimposed pseudo waveform , Separation and prominence , and normalize the three indicators respectively to calculate the significance of each peak; S302, superimposing a pseudo waveform In, by Start searching backwards from time. If the search value is greater than If the search fails, the search continues; otherwise, the search stops; according to the position at the time of stopping , obtain the weak bottom echo search range; S303, calculate each water surface point within the echo search interval The significance value of each peak in the corresponding waveform is used to determine the peak with the greatest significance as the final bottom echo position. .

[0012] In step S301, the significance of each peak is calculated, and the expression is: (6) In the formula, is the significance value; Represent the normalized amplitude, separation and protrusion values ​​respectively; the protrusion greater than the noise threshold The peak position with the largest significance value is taken as the approximate time of the bottom echo ; Noise Threshold It is expressed as: (7) In the formula, is the tail noise in the superimposed pseudo waveform; To obtain the tail of the superimposed pseudo waveform Values; is the standard deviation function; is the mean value function.

[0013] In step S302, the weak echo search interval is , is the starting time of the interval, is the position at the time of stopping.

[0014] Another object of the present invention is to provide a method for detecting weak echo signals from the bottom of water by an airborne ocean laser radar. The system implements the method for detecting weak echo signals from the bottom of water by an airborne ocean laser radar. The system comprises: A geometric element acquisition module is used to determine the water surface echo position based on the waveform standard deviation curve and obtain two geometric elements: the water surface point and the refraction vector; A superposition pseudo waveform acquisition module is used to construct a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio; The module for acquiring the position of weak bottom echoes is used to judge and acquire the position of weak bottom echoes step by step based on an improved normalized significant peak detection method.

[0015] Combined with all the above technical solutions, the beneficial effects of the present invention are as follows: the present invention constructs a standard deviation curve to determine the rising edge of the waveform, accurately extracts the water surface echo moment, and on this basis calculates the two water surface geometric elements of the water surface point and the refraction vector; constructs a waveform superposition model with time and space constraints, uses the Euclidean distance between the two water surface geometric elements as the neighbor discrimination criterion, and uses the K nearest neighbor judgment method to screen the neighbor waveforms and superimpose them to obtain a superimposed pseudo waveform with a high signal-to-noise ratio; finally, uses the improved normalized significant peak detection method to step by step determine the bottom echo position and obtain the final bottom echo moment. Through the method of the present invention, the weak bottom echo detection of the airborne ocean laser radar is effectively realized, the maximum sounding performance of the airborne ocean laser radar is improved, and effective technical support and solutions are provided for the processing and application of sounding data.

[0016] The present invention uses the K nearest neighbor judgment method to screen the nearest neighbor waveforms and superimpose them to obtain a superimposed pseudo waveform with a high signal-to-noise ratio. The purpose is to innovatively propose the acquisition of spatiotemporal elements, the construction of a waveform superposition model with spatiotemporal constraints, and the step-by-step detection of seafloor echoes. The present invention uses an improved normalized significance peak detection method to jointly define the significance score according to the prominence, separation, and amplitude of the signal.

[0017] The present invention can achieve effective detection of weak echo signals from the bottom of the water by airborne ocean laser radar, improve the maximum depth sounding performance of airborne ocean laser radar, and provide accurate and reliable terrain data for shallow water areas in the fields of ocean, mapping, transportation, and military. The present invention proposes a waveform superposition model based on time and space constraints to achieve accurate detection of weak echoes from the bottom of the water. Since the present invention makes full use of the time and space neighbor relationship of the waveform, compared with the existing technology, the present invention has stronger applicability and higher detection accuracy. The method of time and space constraint waveform superposition in the present invention eliminates noise interference and improves the recognizability of the bottom echo signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure; Figure 1 It is a schematic diagram of a method for detecting weak underwater echo signals by an airborne ocean laser radar provided in an embodiment of the present invention; Figure 2 The embodiment of the present invention provides a method for constructing a waveform standard deviation curve graph by using a sliding window in water surface echo position determination based on a waveform standard deviation curve; Figure 3 It is a water surface echo position map determined in water surface echo position determination based on a waveform standard deviation curve provided by an embodiment of the present invention; Figure 4 is a schematic diagram of neighbor waveform selection provided by an embodiment of the present invention; Figure 5 is a schematic diagram of constructing a pseudo waveform provided by an embodiment of the present invention; Figure 6 The invention provides a method for obtaining the amplitude of all peak values ​​of a waveform in determining a weak seabed echo based on normalized significant peak detection. , Separation and prominence Schematic diagram; Figure 7 The invention provides a method for determining a weak seabed echo based on normalized significant peak detection, wherein the prominence of the superimposed pseudo waveform is greater than the noise threshold. The peak position with the largest significance value is taken as the approximate time of the bottom echo Schematic diagram; Figure 8 The invention provides a method for obtaining a weak bottom echo search interval in weak bottom echo determination based on normalized significant peak detection. Schematic diagram; Fig. 9 The embodiment of the present invention provides a method for determining the weak seabed echo based on normalized significant peak detection, in which the peak position with the greatest significance is determined as the final seabed echo position. Schematic diagram; Fig.10 It is a schematic diagram of underwater topographic points solved by the method of the present invention; Fig.11 It is the underwater topographic point map solved by the SPD algorithm of the prior art; Fig.12 For the present invention Fig.10 A schematic diagram of the bathymetric profile corresponding to the solid line in the middle; Fig.13 For the present invention Fig.11 Schematic diagram of the second bathymetric profile corresponding to the solid line in the middle; Fig.14 For the present invention Fig.10 A schematic diagram of the bathymetric profile corresponding to the middle dotted line; Fig.15 For the present invention Fig.11 Schematic diagram of the second bathymetric profile corresponding to the middle dotted line. DETAILED DESCRIPTION

[0019] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0020] In order to solve the problem of low signal-to-noise ratio and difficulty in extracting weak echo signals from the bottom of the water by airborne ocean lidar, the present invention proposes a weak seabed echo detection method based on a time-space constrained waveform superposition model. First, the rising edge of the waveform is located by constructing a standard deviation curve to determine the moment of the sea surface echo, and the two elements of the sea surface point and the corresponding refraction vector are calculated based on this. Then, the Euclidean distance between the two elements is used as the K nearest neighbor judgment standard to construct a time-space constrained waveform superposition model to obtain a superimposed pseudo waveform with a higher signal-to-noise ratio. Finally, the normalized significant peak detection method is used to determine the final position of the weak seabed echo.

[0021] The research object of the present invention is weak echo of the bottom of the water from airborne ocean laser sounding. The present invention improves waveform superposition according to waveform characteristics and proposes a waveform superposition model with time and space constraints.

[0022] Embodiment 1, as Figure 1 As shown, the method for detecting weak underwater echo signals of an airborne ocean laser radar provided by an embodiment of the present invention includes: S1, determine the water surface echo position based on the waveform standard deviation curve, and obtain two geometric elements: water surface point and refraction vector; S2, constructing a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio; S3, based on the improved normalized significant peak detection method, judge and obtain the position of weak bottom echo step by step.

[0023] Exemplarily, in step S1, the water surface echo position is determined based on the waveform standard deviation curve, and the two geometric elements of the water surface point and the refraction vector are obtained, including: S101, based on the standard deviation difference between the noise in the sounding waveform and the rising edge of the echo, a waveform standard deviation curve is constructed using a sliding window (e.g. Figure 2 ), and determine the precise rising edge position as the water surface echo position ( Figure 3 ), the innovative proposal of the present invention is shown in formula (1): (1) In the formula, is the water surface echo position, To find the data location that meets the conditions; is the standard deviation curve, , is the original echo signal Middle position The standard deviation of is the sliding window size, is the standard deviation function; The standard deviation curve The extreme point position of is the noise threshold; is the difference operation, is a symbolic function; S102, calculating the water surface point cloud according to the determined water surface echo position and positioning and attitude information , and based on the principal component analysis method, obtain the water surface points The corresponding refraction vector .

[0024] Get each water surface point The corresponding refraction vector , the expression is: (2) In the formula, are the refraction vector and the incident vector, respectively. are the refractive indices of laser light in air and water, respectively. denote the angle of incidence and the angle of refraction respectively, For water surface point Psi The normal vector at .

[0025] Exemplarily, in step S2, constructing a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio includes: S201, traverse all water surface points and its corresponding refraction vector , and calculate the Euclidean distance between other water surface points and the refraction vector to obtain the water surface point distance vector set Refraction distance vector collection ;in, , represents the two-norm operation, , is the two-norm operation, , is the total number of sea surface points, is the two-norm operation, For the With The Euclidean distance between sea surface points is For the With The Euclidean distance between the refraction vectors, For the The coordinates of the sea surface points, For the The refraction vector corresponding to the sea surface point.

[0026] On this basis, a waveform superposition model with time and space constraints is constructed. The model essentially considers the correlation of neighboring waveforms to improve the signal-to-noise ratio of waveforms to achieve detection of weak echo signals on the bottom of the water. The model mainly includes S202 neighbor waveform selection (such as Figure 4 ) and S203 pseudo waveform acquisition (such as Figure 5 ) two parts.

[0027] S202, according to the K nearest neighbor criterion, first obtain the Neighbor Water surface points and corresponding refraction vectors , the innovative proposal of the present invention is shown in formula (3): (3) In the formula, are the neighboring water surface points and refraction vectors screened for the first time, is the index of the neighboring water surface points screened for the first time, The sorted index of the array elements from small to large, To obtain values ​​from the sea surface point set, For the refraction vector set R Take the value in From 0 to An interval vector of ; From the filtered The refraction vectors of the nearest neighbors In, again according to The nearest neighbor criterion is used to obtain the water surface points The corresponding refraction vector Neighbor The refraction vector and the corresponding water surface point are innovatively proposed in the present invention as shown in formula (4): (4) In the formula, are the neighboring water surface points and refraction vectors that are screened again, is the index of the neighboring water surface points screened again, For the collection Take the value in For the collection Take the value in is the refraction vector R i With Collection Subtract all corresponding vectors in , From 0 to An interval vector of ; Through the neighbor constraints of the above two elements, the final neighbor waveform is determined .in, They are No. 1 to No. The waveform data of the nearest neighbors.

[0028] S203, from the waveform The neighboring waveform data are superimposed, Gaussian white noise is filtered out, and a superimposed pseudo waveform with a higher signal-to-noise ratio is obtained. The innovative proposal of the present invention is shown in formula (5): (5) In the formula, To superimpose a pseudo waveform, For the The water surface echo position of the neighboring waveform, is the length of the waveform taken, is the waveform data interval taken, The intercept vector A certain section of is the number of neighboring waveforms finally determined.

[0029] Exemplarily, in step S3, based on the improved normalized significant peak detection method, step-by-step judging and obtaining the position of the weak bottom echo includes: S301, obtaining the amplitudes of all peaks in the superimposed pseudo waveform , Separation and prominence ,(like Figure 6 ), and normalize the three indicators respectively to calculate the significance of each peak. The present invention innovatively proposes the following formula (6): (6) In the formula, is the significance value; Represent the normalized amplitude, separation and protrusion values ​​respectively; the protrusion greater than the noise threshold The peak position with the largest significance value is taken as the approximate time of the bottom echo ,(like Figure 7 ); where the noise threshold It is expressed as: (7) In the formula, is the tail noise in the superimposed pseudo waveform; To obtain the tail of the superimposed pseudo waveform Values; is the standard deviation function; is the mean value function.

[0030] S302, superimposing a pseudo waveform In, by Start searching backwards from time. If the search value is greater than If the search fails, the search will continue. If the search fails, the search will stop. The present invention innovatively obtains the weak echo search interval under the water ,in, is the starting time of the interval, is the position at the time of stopping (such as Figure 8 ).

[0031] S303, calculate each water surface point within the echo search interval The significance value of each peak in the corresponding waveform is used to determine the peak with the greatest significance as the final bottom echo position. ,like Fig. 9 shown.

[0032] Through the above embodiments, it can be known that the present invention proposes a method for detecting weak echo signals from the bottom of the water by an airborne ocean laser radar. Compared with the prior art, the position of the water surface echo is determined based on the waveform standard deviation curve, and two geometric elements, namely, the water surface point and the refraction vector, are obtained; a waveform superposition model with time and space constraints is constructed, and the neighboring waveforms are screened and superimposed according to the Euclidean distance between the two geometric elements and the K nearest neighbor judgment method to obtain a superimposed pseudo waveform with a high signal-to-noise ratio; based on the improved normalized significant peak detection method, the final position of the weak echo from the bottom of the water is judged and determined step by step. After being processed by the model constructed by the present invention, the detection of weak echoes from the bottom of the water by the airborne ocean laser radar is effectively realized, the maximum sounding performance of the airborne ocean laser is improved, and effective technical support and solutions are provided for the processing and application of sounding data.

[0033] Another object of the present invention is to provide an airborne ocean laser radar underwater weak echo signal detection system, the system comprising: A geometric element acquisition module is used to determine the water surface echo position based on the waveform standard deviation curve and obtain two geometric elements: the water surface point and the refraction vector; A superposition pseudo waveform acquisition module is used to construct a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio; The module for acquiring the position of weak bottom echoes is used to judge and acquire the position of weak bottom echoes step by step based on an improved normalized significant peak detection method.

[0034] In order to verify the effectiveness of the proposed algorithm, the present invention conducts experiments and analysis based on the airborne ocean lidar bathymetric data measured in a certain area of ​​the sea. The traditional Significance-based Peak Detection (SPD) algorithm is selected as a comparison method to analyze the improvement of the maximum bathymetric performance of the proposed method. Fig.10 and Fig.11The bottom terrain points solved by the method of the present invention and the SPD algorithm are shown respectively. It can be seen intuitively that the bathymetric points solved by the algorithm of the present invention extend to deeper water depths and can better restore the bottom feature landforms at deeper depths. In addition, in order to further analyze the performance of the method, two bathymetric profiles parallel to the navigation direction and bathymetric profile 2 in the navigation direction are intercepted, corresponding to Fig.10 The dashed and solid lines in the figure. Fig.10 The solid lines in the middle correspond to the bathymetric profiles 1 and 2 respectively. Fig.12 and Fig.13 It can be seen that the topography of the profile is undulating, which is caused by underwater reefs. The underwater topography obtained by the method of the present invention has a continuous trend, and the maximum depth is 6.93m relative to the average water surface. The underwater topography obtained by the SPD algorithm is discontinuous, and there are topographic discontinuities around the reefs and in areas with large water depths. In addition, Fig.10 The dashed lines in the middle correspond to the bathymetric profiles 1 and 2. Fig.14 and Fig.15 The underwater terrain of this section is relatively flat, and the maximum sounding depth obtained by the method of the present invention is 8.67m, while the maximum sounding depth obtained by the SPD algorithm is 5.68m. Therefore, the present invention can effectively realize the detection of weak echoes on the bottom of the water and significantly improve the maximum sounding capability of airborne ocean lidar.

[0035] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for detecting weak underwater echo signals by airborne ocean laser radar, characterized in that: The method comprises the following steps: S1, determine the water surface echo position based on the waveform standard deviation curve, and obtain two geometric elements: water surface point and refraction vector; S2, constructing a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio; S3, based on the improved normalized significant peak detection method, judge and obtain the position of weak bottom echo step by step.

2. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 1, characterized in that: In step S1, the water surface echo position is determined based on the waveform standard deviation curve, and two geometric elements, the water surface point and the refraction vector, are obtained, including: S101, constructing a waveform standard deviation curve using a sliding window according to the standard deviation difference between the noise in the sounding waveform and the rising edge of the echo, and determining the rising edge position as the water surface echo position; S102, calculating the water surface point cloud according to the determined water surface echo position and positioning and attitude information , and based on the principal component analysis method, obtain the water surface points The corresponding refraction vector .

3. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 2, characterized in that: In step S101, the rising edge position is determined as the water surface echo position, and the expression is: （1） In the formula, is the water surface echo position, To find the data location that meets the conditions; is the standard deviation curve, , is the original echo signal Middle position The standard deviation of is the sliding window size, is the standard deviation function; The standard deviation curve The extreme point position of is the noise threshold; is the difference operation, is a symbolic function; In step S102, each water surface point is obtained The corresponding refraction vector , the expression is: （2） In the formula, are the refraction vector and the incident vector, respectively. are the refractive indices of laser light in air and water, respectively. denote the angle of incidence and the angle of refraction respectively, For water surface point Psi The normal vector at .

4. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 1, characterized in that: In step S2, a waveform superposition model with time and space constraints is constructed to obtain a superposition pseudo waveform with a high signal-to-noise ratio, including: S201, traverse all water surface points The corresponding refraction vector , construct a waveform superposition model with time and space constraints, including neighbor waveform selection and pseudo waveform acquisition; calculate the Euclidean distance between all water surface points and refraction vectors, and obtain the water surface point distance vector set Refraction distance vector collection ; in, , is the total number of sea surface points, is the two-norm operation, For the With The Euclidean distance between sea surface points is For the With The Euclidean distance between the refraction vectors, For the The coordinates of the sea surface points, For the The refraction vector corresponding to the sea surface point; S202, according to Nearest neighbor criterion, obtain the nearest neighbor criterion with each water surface point Neighbor Water surface points and corresponding refraction vectors ; From the selected The refraction vectors of the nearest neighbors In, again according to The nearest neighbor criterion is used to obtain the water surface points The corresponding refraction vector Neighbor refraction vectors and corresponding water surface points; the final determined neighbor waveform; S203, from the waveform The neighboring waveform data are superimposed, Gaussian white noise is filtered out, and a superimposed pseudo waveform with a higher signal-to-noise ratio is obtained.

5. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 4, characterized in that: In step S202, obtain Neighbor Water surface points and corresponding refraction vectors , the expression is: （3） In the formula, are the neighboring water surface points and refraction vectors screened for the first time, is the index of the neighboring water surface points screened for the first time, The sorted index of the array elements from small to large, To obtain values ​​from the sea surface point set, For the refraction vector set R Take the value in From 0 to An interval vector of ; Again, according to The nearest neighbor criterion is used to obtain the water surface points The corresponding refraction vector Neighbor The refraction vector and the corresponding water surface point are expressed as: （4） In the formula, are the neighboring water surface points and refraction vectors that are screened again, is the index of the neighboring water surface points screened again, For the collection Take the value in For the collection Take the value in is the refraction vector R i With Collection Subtract all corresponding vectors in , From 0 to An interval vector of ; Finally, the determined neighbor waveform set ,in, They are No. 1 to No. The waveform data of the nearest neighbors.

6. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 4, characterized in that: In step S203, a superimposed pseudo waveform with a higher signal-to-noise ratio is obtained, and the expression is: （5） In the formula, To superimpose a pseudo waveform, For the The water surface echo position of the neighboring waveform, is the length of the waveform taken, is the waveform data interval taken, The intercept vector A certain section of is the number of neighboring waveforms finally determined.

7. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 1, characterized in that: In step S3, based on the improved normalized significant peak detection method, the position of the weak bottom echo is judged and obtained step by step, including: S301, obtaining the amplitudes of all peaks in the superimposed pseudo waveform , Separation and prominence , and normalize the three indicators respectively to calculate the significance of each peak; S302, superimposing a pseudo waveform In, by Start searching backwards from time. If the search value is greater than If the search fails, the search continues; otherwise, the search stops; according to the position at the time of stopping , obtain the weak bottom echo search range; S303, calculate each water surface point within the echo search interval The significance value of each peak in the corresponding waveform is used to determine the peak with the greatest significance as the final bottom echo position. .

8. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 7, characterized in that: In step S301, the significance of each peak is calculated, and the expression is: （6） In the formula, is the significance value; Represent the normalized amplitude, separation and protrusion values ​​respectively; the protrusion greater than the noise threshold The peak position with the largest significance value is taken as the approximate time of the bottom echo ; Noise Threshold It is expressed as: （7） In the formula, is the tail noise in the superimposed pseudo waveform; To obtain the tail of the superimposed pseudo waveform values; is the standard deviation function; is the mean value function.

9. The method for detecting weak underwater echo signals of an airborne ocean laser radar according to claim 7, characterized in that: In step S302, the weak bottom echo search interval is , is the starting time of the interval, is the position at the time of stopping.

10. An airborne ocean laser radar underwater weak echo signal detection system, characterized in that: The system implements the method for detecting weak underwater echo signals of an airborne ocean laser radar as claimed in any one of claims 1 to 9, and the system comprises: A geometric element acquisition module is used to determine the water surface echo position based on the waveform standard deviation curve and obtain two geometric elements: the water surface point and the refraction vector; A superposition pseudo waveform acquisition module is used to construct a waveform superposition model with time and space constraints to obtain a superposition pseudo waveform with a high signal-to-noise ratio; The module for acquiring the position of weak bottom echoes is used to judge and acquire the position of weak bottom echoes step by step based on an improved normalized significant peak detection method.

Images