Target positioning method based on geomagnetic anomaly and axis frequency magnetic field

Abstract

The invention relates to the technical field of magnetic detection, in particular to a target positioning method based on geomagnetic anomaly and an axis frequency magnetic field. Comprising the following steps: S1, preparing before entering water; s2, approaching a target; s3, target detection: when no target appears, an observation signal is composed of an environment magnetic field, and the energy value of the part is very small; once the target appears, the energy of the observation signal can be continuously and obviously increased, when the energy value is greater than a preset judgment threshold, the target point is judged as a suspected target point, and if the suspected point is continuously detected for a specified time, the existence of the target is confirmed; s4, combined positioning: under a Kalman filtering framework, enabling the target to be equivalent to a magnetic dipole and an electric dipole, and correcting a position predicted by a target state equation by combining measurement equations of the two models, thereby realizing combined positioning of the target and obtaining more accurate target position information; and S5, verifying and outputting a positioning result. The method has the advantages of improving the positioning precision, enhancing the anti-interference capability and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnetic detection, in particular to a target positioning method based on geomagnetic anomaly and shaft frequency magnetic field. BACKGROUND

[0002] Water surface target positioning is a core technical difficulty in the field of underwater detection, and is widely used in military reconnaissance, ocean monitoring, shipping safety and other scenes. When global navigation satellite system (GNSS), radar and sonar positioning are all invalid, magnetic detection positioning as a cross-medium and anti-interference alternative technology, its importance is significantly improved. The water surface target such as ship and submarine forms a static magnetic field due to the magnetization of its own material, and the characteristic low-frequency alternating magnetic field (shaft frequency magnetic field) is generated under the modulation of the propeller rotation by the anticorrosion current. These significant electromagnetic characteristics provide a physical basis for magnetic detection.

[0003] However, the existing magnetic detection positioning technology has the limitation of single magnetic field positioning: when only using magnetic anomaly positioning, it is easy to be disturbed by the dynamic change of geomagnetic field; when only using shaft frequency magnetic field positioning, there is a problem that the signal strength is weak and easy to be submerged by noise. When the above two positioning methods are used alone, high-precision positioning cannot be achieved in complex environment. SUMMARY

[0004] The present application provides a target positioning method based on geomagnetic anomaly and shaft frequency magnetic field, which aims to solve the problems of single magnetic anomaly positioning being disturbed by geomagnetic field and single shaft frequency magnetic field positioning signal being easy to be submerged by noise through the cooperative detection of geomagnetic anomaly and shaft frequency magnetic field, break through the limitation of single magnetic field positioning, and realize high-precision positioning in complex environment.

[0005] To achieve the above purpose, the technical scheme of the present application is as follows:

[0006] The present application provides a target positioning method based on geomagnetic anomaly and shaft frequency magnetic field, which includes the following steps:

[0007] S1. Preparing before entering the water, obtaining the heading, speed and initial position of the target by means of external sensors, constructing a state equation based on the assumption that the target moves at a constant speed in a straight line for a short time, and performing track extrapolation and prediction of the position of the target at future time based on the state equation to provide an initial reference for the motion of the underwater platform;

[0008] S2. Approaching the target, the underwater motion platform approaches the predicted target position from far to near under the assistance of real-time attitude and acceleration information provided by the inertial navigation module, and the original magnetic field data collected by the magnetic sensor is preprocessed by magnetic calibration and noise suppression to eliminate environmental interference;

[0009] S3. Target detection, when no target appears, the observation signal is composed of the environmental magnetic field, the energy value of this part is very small; once a target appears, the energy of the observation signal will continuously and significantly increase, when the energy value is greater than the preset decision threshold, it is judged as a suspected target point, and when the suspected points are continuously detected for a specified time, it is confirmed that the target exists;

[0010] S4. Joint positioning, under the Kalman filtering framework, the target is equivalent to a magnetic dipole and an electric dipole respectively, the position predicted by the target state equation is corrected by combining the measurement equations of the two models, the joint positioning of the target is realized, and more accurate target position information is obtained;

[0011] S5. Positioning result verification and output, compare the positioning result with the target position information derived from the state equation, calculate the degree of coincidence; if not, set the confidence of the positioning result to zero, repeat S1-S5; if yes, output the positioning result.

[0012] Further, the specific steps of target detection in S3 include:

[0013] S31. From time n1, estimate and store the energy value of the input signal using the following formula:

[0014]

[0015] In the formula:

[0016] n1=1+L*K

[0017] n2=N+L*K

[0018] Where N is the data window length; L is the step length of the data window sliding; the coefficient K controls the sliding of the window, K=0, 1, 2…;

[0019] S32. Continuously compare the energy value E obtained in S31 with the threshold T, if the energy value E is greater than the threshold T, it is considered that the point is a suspected target point;

[0020] S33. If the suspected target point is continuously detected for M seconds, it is confirmed that the target exists.

[0021] Further, the specific steps of joint positioning in S4 include:

[0022] S41. Magnetic anomaly positioning based on magnetic dipole model:

[0023] The magnetic dipole model of three-component magnetometer measuring magnetic anomaly is:

[0024]

[0025] In the formula: is the sensor output; x kFor system status; e k To mitigate noise during the measurement process, the magnetic target is treated as a point magnetic dipole. A tracking reference coordinate system is established using the three-component magnetometer coordinate system. The magnetic field of the magnetic target located at r at the three-component magnetometer is calculated by equation (2):

[0026]

[0027] In the formula: μ0=4π×10 -7 H / m is the free permeability; r(x,y,z) T The position vector of the target relative to the three-component magnetometer; m = [m x ,m y ,m z ] T The target magnetic moment; the target's magnetic moment parameter m is derived from the inherent magnetic moment. and induced magnetic moment m s Composition; During the target's motion, the measurement model of the single fluxgate sensor located at the origin of the reference coordinate system at time k is as follows:

[0028]

[0029] In the formula: r k =[x k ,y k ,z k ] T Let k be the target position at time k; e is the target magnetic moment at time k; k For measuring noise at time k, e k ~N(0, σ 2 );r k Let k be the position vector of the target relative to the single fluxgate sensor at time k;

[0030] Assume the relative position difference between the two measurement points is d. r Then the measurement model of the single fluxgate sensor at the second point is:

[0031]

[0032] S42. Axis-frequency magnetic field localization based on the electric dipole model:

[0033] The target static magnetic source can be equivalent to an electric dipole. Assuming the electric dipole is... The radiated magnetic field is calculated using the following formula:

[0034]

[0035] in, It is the electromagnetic wave number, and μ and ε are the permeability and permittivity of the propagation medium, respectively. For the receiving point P(x, y, z) distance from the magnetic dipole, θ is the angle between the receiving vector and the z-axis; the above calculated magnetic field is the calculation result in the spherical coordinate system of the electric dipole, which is converted into the value in the straight coordinate system of the sensor:

[0036]

[0037] wherein is the conversion matrix from the electric dipole coordinate system to the magnetic sensor coordinate system, and the axial frequency magnetic field measurement model of the single magnetic flux gate sensor located at the origin of the reference coordinate system at time k is:

[0038]

[0039] In the formula: r k = [x k , y k , z k ] T is the target position at time k; p k is the target electric dipole distance at time k, is the axial component of the electric dipole distance at time k in its own coordinate system; n k is the measurement noise at time k;

[0040] The relative position difference between the two measurement points is d r , and the axial frequency measurement model of the magnetic sensor at the second measurement point is:

[0041]

[0042] S43. Target joint positioning based on Kalman filtering framework:

[0043] The speed of the underwater moving platform is much higher than the target speed, so the target is assumed to move at a constant speed in a short time, and the motion model of the target can be expressed as:

[0044]

[0045] In the formula, T s is the sampling time; is the motion speed of the platform relative to the target at time k, v k-1 is the motion speed of the platform relative to the target at time (k-1); during the process of the target moving at a constant speed through the sensor, it is assumed that the target does not deflect during the motion process, and the target magnetic moment and the electric dipole distance will not change in a short time, i.e.:

[0046]

[0047] According to the measurement equation, the target state variable at time k is defined as:

[0048]

[0049] The state equation for the target motion can be written as:

[0050] x k =Ax k-1 (12)

[0051] The state transition matrix A in the formula is:

[0052]

[0053] Equation (13) can also be expressed as:

[0054]

[0055] In the formula This represents the initial state of the target.

[0056] Therefore, a single fluxgate sensor positioning model is established based on the measurement model of magnetic anomaly and shaft frequency magnetic field, and the state equation:

[0057]

[0058] This localization model locates the target within a Kalman filter framework, thereby obtaining the accurate target position.

[0059] Furthermore, the surface target positioning method is implemented based on a magnetic detection device, which is installed on an underwater motion platform; the magnetic detection device includes a magnetic sensor, a magnetic detection control unit, an inertial navigation module, a data transmission radio, and a secondary power supply.

[0060] Furthermore, there are two types of magnetic sensors: a three-component magnetometer and a total field magnetometer; the three-component magnetometer is specifically a fluxgate magnetometer, and the total field magnetometer is specifically an optically pumped magnetometer.

[0061] Furthermore, the magnetic detection control section includes a processor, a data storage unit, and a signal acquisition module.

[0062] The beneficial effects achieved by this invention are as follows:

[0063] 1) Improved positioning accuracy: By using a three-axis fluxgate magnetometer and an optical pump magnetometer to collect magnetic anomaly and axis frequency magnetic field data, and combining it with a Kalman filter fusion algorithm, dynamic correction of the target position is achieved, and the positioning error is significantly reduced compared with single magnetic field positioning.

[0064] 2) Enhanced anti-interference capability: By utilizing the complementary characteristics of strong magnetic anomaly signal intensity and the filterable axial frequency magnetic field, interference such as geomagnetic field fluctuations and environmental noise is effectively suppressed, significantly improving detection accuracy and success rate. Attached Figure Description

[0065] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description only represent some of the embodiments of the present application, and for those skilled in the art, other drawings can be obtained from the structures shown in the drawings without any creative effort.

[0066] Fig. 1 is a data processing flowchart of the present application.

[0067] Fig. 2 is a principle block diagram of the energy-based magnetic anomaly detection algorithm of the present application.

[0068] Fig. 3 is a positioning overall flowchart of the present application. DETAILED DESCRIPTION

[0069] The technical solutions in the embodiments of the present application will be described clearly and completely with reference to the accompanying drawings of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without any creative effort fall within the scope of protection of the present application.

[0070] It should be noted that if the embodiments of the present application involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement condition, etc. between the components in a certain posture (as shown in the drawings), and if the certain posture changes, the directional indications also change accordingly.

[0071] In addition, if the embodiments of the present application involve descriptions of "first", "second", etc., the descriptions of "first", "second", etc. are only for description purposes, and cannot be understood as indicating or implying the relative importance of the indicated technical features or implicitly indicating the number of the indicated technical features. Therefore, the features limited by "first", "second" can explicitly or implicitly include at least one of the features. In addition, the meaning of "and / or" appearing throughout the text is that it includes three parallel schemes, taking "A and / or B" as an example, which includes A scheme, or B scheme, or A and B schemes. In addition, the technical solutions of each embodiment can be combined with each other, but it must be based on the fact that a person skilled in the art can realize it, and when the combination of technical solutions contradicts each other or cannot be realized, it should be considered that the combination of technical solutions does not exist, and is not within the scope of protection claimed by the present application.

[0072] The water surface target exists static magnetic field (i.e. magnetic anomaly relative to the geomagnetic field) in the geomagnetic field background, and the anti-corrosion current of the water surface target and the like modulated through the propeller will form an axial frequency magnetic field, and the two kinds of magnetic fields can be used to realize the detection of the target.

[0073] The positioning by using the magnetic anomaly or the axial frequency magnetic field has advantages and disadvantages: the magnetic anomaly value generated by the target is much larger than the axial frequency magnetic field, but the magnetic anomaly value will be superimposed with the geomagnetic field, so the interference caused by the geomagnetic field needs to be considered in the motion condition; the value of the axial frequency magnetic field is smaller than the magnetic anomaly, but most of the low frequency interference can be removed through filtering, and the axial frequency magnetic field can be used as a supplement of the magnetic anomaly field. In order to solve the problems in the prior art, the advantages and disadvantages of the positioning by using the two kinds of magnetic fields are combined, the magnetic anomaly and the axial frequency magnetic field are measured through the three-axis flux gate and the magnetometer at the same time, and thus the positioning of the target is realized.

[0074] The application provides a target positioning method based on the geomagnetic anomaly and the axial frequency magnetic field, and the method is implemented based on a magnetic detection device arranged on an underwater motion platform. When the underwater motion platform approaches the target from far to near, the magnetic detection device collects relevant magnetic field data, and after being solved by a positioning algorithm, the position information of the target is given in real time.

[0075] The underwater motion platform can be an autonomous underwater vehicle (AUV), a tethered remote control underwater vehicle (ROV), a hybrid underwater vehicle (ARV) or an underwater glider, etc. The underwater motion platform generally comprises a shell, a buoyancy and stability system, a power system and an electric power system (including a storage battery and a primary power supply circuit, etc.). Since the underwater motion platform involved in the application adopts the prior art and is irrelevant to the innovation point of the application, the specific principle and technical details are not described herein.

[0076] The magnetic detection device comprises a magnetic sensor, a magnetic detection control part, an inertial navigation module, a data transmission radio station and a secondary power supply.

[0077] The magnetic sensor comprises two kinds of magnetic sensors, i.e. a three-component magnetometer and a total field magnetometer. The three-component magnetometer is specifically a flux gate (note: the three-component magnetometer is not limited to the flux gate, and can be other magnetic sensors with equivalent performance, and the flux gate is only a kind of concrete expression in this paper), which is used for detecting three-axis magnetic field data. The flux gate is a kind of magnetic field measuring element (also called a magnetometer) which works by using the nonlinear change characteristics of the magnetic permeability of ferromagnetic materials. The total field magnetometer is specifically an optical pumping magnetometer (note: the total field magnetometer is not limited to the optical pumping magnetometer, and can be other magnetic sensors with equivalent performance, and the optical pumping magnetometer is only a kind of concrete expression in this paper), which is used for detecting total field data of the magnetic field. The optical pumping magnetometer is a kind of high-sensitivity magnetic field measuring instrument based on the optical pumping magnetic resonance principle, and is mainly used for accurately detecting the total strength of the magnetic field (i.e. the absolute value of the magnetic field).

[0078] The magnetic detection control part is responsible for internal signal acquisition and control, and performs algorithm operation on the detection data to complete the magnetic anomaly target positioning function; specifically including:

[0079] The processor includes a DSP, FPGA or ARM chip, and is used as a core carrier for data processing and algorithm operation, and is responsible for executing positioning algorithms and coordinating the workflow of the entire system;

[0080] The data storage unit includes a memory (RAM) and a non-volatile storage (such as a Flash, a hard disk), and is used for real-time storage of raw magnetic data, platform state information (such as inertial navigation data), intermediate calculation results and final positioning calculation data;

[0081] The signal acquisition module includes an analog-to-digital converter (ADC) for converting analog signals output by the magnetic sensor (optical pump magnetometer, magnetic flux gate) into digital signals for processing by the processor.

[0082] The inertial navigation module is mainly used for measuring the real-time motion state parameters (including real-time attitude and acceleration information) of the underwater motion platform, and providing basic data support for target positioning; including a gyroscope, an accelerometer, a data processing unit and an interface circuit; the data processing unit includes an embedded processor, which integrates and calibrates the output data of the gyroscope and the accelerometer, and outputs the real-time attitude and acceleration information of the platform; the interface circuit communicates with the data storage and processing unit of the magnetic detection control part to transmit measurement data (such as through SPI, UART and other interfaces).

[0083] The secondary power supply is responsible for converting the 28V primary power supply provided by the underwater motion platform into the secondary power supply required by each circuit module inside the magnetic detection device and providing the on-off switch of the sensor.

[0084] The data transmission module sends the real-time calculated position data to the terminal computer for real-time viewing by the test personnel. In the underwater environment, the data transmission module usually uses underwater acoustic communication technology (such as sound wave modulation and demodulation) to transmit data through water medium, which has strong anti-interference ability but low transmission rate. If the platform is close to the water surface, the data can be transmitted through the radio frequency module (such as WiFi, 4G), which is suitable for short-distance high-speed scenarios. Since the present application uses existing technology, and this module is not the focus of the present application, the specific technical details and principles are not described again.

[0085] As shown in Figs. 1-3 The water surface target positioning method specifically includes the following steps:

[0086] S1. Preparing for entering water, obtaining the target's heading, speed and initial position information by means of radar, GNSS and other external sensors, constructing state equation based on the assumption that the target moves at a constant speed in a straight line in a short time, and making track prediction and prediction of the target's position at future time to provide initial reference for the underwater platform movement;

[0087] S2. Approaching the target, the underwater moving platform approaches the predicted target position from far to near with the assistance of real-time attitude and acceleration information provided by the inertial navigation module, and pre-processes the original magnetic field data collected by the magnetic sensor to eliminate environmental interference through magnetic calibration and noise suppression; wherein the magnetic calibration and noise suppression adopt existing technologies, which will not be described herein.

[0088] S3. Target detection, when there is no target, the observation signal is composed of the environmental magnetic field, and the energy value of this part is very small; once the target appears, the energy of the observation signal will continuously and significantly increase, and when the energy value is greater than the preset decision threshold, it is judged as a suspected target point, and when the suspected points are continuously detected for a specified time, it is confirmed that the target exists;

[0089] S4. Joint positioning, under the Kalman filtering framework, the target is equivalent to a magnetic dipole (for magnetic anomaly positioning) and an electric dipole (for axis frequency magnetic field positioning) respectively, and the position predicted by the target state equation is corrected by combining the measurement equations of the two models to realize joint positioning of the target and obtain more accurate target position information;

[0090] S5. Positioning result verification and output, comparing the positioning result with the target position information derived from the state equation, calculating the degree of coincidence; if not, the positioning result confidence is set to zero, and S1-S5 are repeatedly executed; if yes, the positioning result is output. Wherein, the coincidence degree is given based on the distance between the positioning result and the derived position, if the distance is too large (a threshold value is given according to experience) and does not conform to the actual situation, the confidence is set to zero.

[0091] Further, S3 adopts a signal detection method based on magnetic anomaly detection technology, as shown in Fig. 2 The specific steps of the energy-based ship magnetic anomaly signal detection method are as follows:

[0092] S31. From time n1, the energy value of the input signal is estimated and stored using the following formula:

[0093]

[0094] In the formula:

[0095] n1=1+L*K

[0096] n2=N+L*K

[0097] Wherein, x tE is the energy value based on the OBF algorithm, representing the square sum of the target signal in the orthogonal basis space; N is the data window length; L is the step length of the data window sliding; the coefficient K controls the sliding of the window, K=0, 1, 2, ….

[0098] In addition, in order to reduce the estimation variance of the high-order distance, the value of N should not be too small; the value of N should not be too large, otherwise part of the energy of the noise will be added, thereby weakening the signal energy value and reducing the detection performance.

[0099] S32. Continuously compare the energy value E obtained in S31 with the threshold T, if the energy value E is greater than the threshold T, it is considered that the point is a suspected target point;

[0100] S33. If the suspected target point is detected for M consecutive seconds, it is confirmed that the target exists.

[0101] The values of M and the threshold T depend on the required false alarm probability and the missed alarm probability of the detection, and the false alarm probability decreases and the missed alarm probability increases as M and the threshold T increase.

[0102] Further, the positioning scheme based on the Kalman filter framework in S4 uses two positioning methods, i.e., a target magnetic anomaly-based positioning method and a target axis frequency magnetic field-based positioning method to jointly position the target; as shown in Fig. 3 , the method specifically comprises the following steps:

[0103] S41. Magnetic anomaly positioning based on a magnetic dipole model:

[0104] The magnetic dipole model of the three-component magnetometer measuring the magnetic anomaly is:

[0105]

[0106] In the formula: is the sensor output; x k is the system state; e k is the noise in the measurement process; h is the target magnetic moment; generally, a magnetic target can be regarded as a point magnetic dipole, a tracking reference coordinate system is established in the three-component magnetometer coordinate system, and the magnetic field of the magnetic target located at r at the three-component magnetometer can be calculated by formula (2):

[0107]

[0108] In the formula: μ0=4π×10 -7 H / m is the vacuum permeability; r(x, y, z) T is the position vector of the target relative to the three-component magnetometer; m=[m x ,m y ,m z ] Tis the target magnetic moment. The magnetic moment parameter m of the target usually consists of inherent magnetic moment and induced magnetic moment m s ; the inherent magnetic moment is the magnetization formed during the construction of the target, which is an invariant vector in the body coordinate system of the target; the induced magnetic moment is generated by the magnetization of the target in the process of motion under the external magnetic field, and the size of this part of the magnetic moment is related to the size of the external magnetic field; during the motion of the target, the measurement model of the single fluxgate sensor located at the origin of the reference coordinate system at time k is

[0109]

[0110] wherein r k = [x k , y k , z k ] T is the position of the target at time k; is the magnetic moment of the target at time k; e k is the measurement noise at time k, which is usually independent Gaussian noise, e k ~ N(0, σ 2 ); r k is the position vector of the target relative to the single fluxgate sensor at time k.

[0111] Assuming that the relative position difference between two measurement points is d r , the measurement model of the single fluxgate sensor at the second point is:

[0112]

[0113] S42. Axial frequency magnetic field positioning based on electric dipole model:

[0114] Through the study of actual measurement data, it is found that the static magnetic source of the target can be equivalent to an electric dipole, and the distribution field thereof can be calculated by the electric dipole distribution field. Assuming that the electric dipole is The radiated magnetic field can be calculated by the following formula:

[0115]

[0116] wherein k is the wave number of electromagnetic wave, μ and ε are the magnetic permeability and dielectric constant of the propagation medium respectively, is the distance from the magnetic dipole to the receiving point P(x, y, z), θ is the included angle between the receiving vector and the z axis, and j is a unit complex number (representing a unit vector in a complex space). The magnetic field calculated above is the calculation result in the spherical coordinate system of the electric dipole, which can be converted into the value in the rectangular coordinate system of the sensor:

[0117]

[0118] wherein is the transformation matrix from the electric dipole coordinate system to the magnetic sensor coordinate system, and the axial frequency magnetic field measurement model of the single fluxgate sensor at the origin of the reference coordinate system at time k is

[0119]

[0120] where r k = [x k , y k , z k ] T is the target position at time k; p k is the target electric dipole distance at time k, is the axial component of the electric dipole distance at time k in its own coordinate system; n k is the measurement noise at time k, which is usually independent Gaussian noise, n k ~ N(0, σ 2 ).

[0121] The relative position difference between the two measurement points is d r , and the axial frequency measurement model of the magnetic sensor at the second measurement point is

[0122]

[0123] S43. Target joint positioning based on the Kalman filtering framework, specifically using the magnetic anomaly and axial frequency magnetic field model to form the measurement equation, and correcting the target position predicted by the target state equation:

[0124] The speed of the underwater moving platform is much higher than the target speed, so within a short time, the target can be assumed to move at a constant speed in a straight line, and the motion model of the target can be represented as:

[0125]

[0126] where T s is the sampling time; is the motion speed of the platform relative to the target at time k, v k-1 is the motion speed of the platform relative to the target at the previous sampling point (k-1). The speed of the underwater moving platform is given by the inertial navigation system on the platform carrier, and the speed and initial position information of the surface target are given by other sensors such as radar before the underwater moving platform enters the water. During the process of the target moving at a constant speed through the sensor, it is assumed that the target does not deflect during the movement, and the target magnetic moment and the electric dipole distance will not change within a short time, i.e.

[0127]

[0128] According to the measurement equation, the target state variable at time k is defined as:

[0129]

[0130] The state equation for the target motion can be written as:

[0131] x k =Ax k-1 (12)

[0132] The state transition matrix A in the formula is:

[0133]

[0134] Equation (13) can also be expressed as:

[0135]

[0136] In the formula This represents the initial state of the target.

[0137] Therefore, a single fluxgate sensor positioning model can be established based on the measurement model of magnetic anomaly and shaft frequency magnetic field, and the state equation:

[0138]

[0139] The positioning model locates the target within the Kalman filter framework. Equation (14) serves as the target's state equation, which can be used to calculate the target's trajectory and predict its position at a future time. Then, two positioning methods, magnetic anomaly positioning and axis frequency magnetic field positioning, are used to jointly locate the target based on equations (3) and (7), forming a measurement equation. This equation corrects the target position predicted by the target's state equation, thereby obtaining the accurate target position.

[0140] High-precision magnetometers and fluxgate magnetometers are used to detect magnetic anomalies and axis-frequency magnetic fields generated by ships. Combined with real-time attitude and acceleration information provided by inertial navigation, the target position is calculated using two positioning algorithms: one based on the magnetic anomaly and the other on the axis-frequency magnetic field. Simultaneously, prior information about the target before it enters the water, such as heading and speed, is obtained through other means. This information is used to construct a target state equation, estimate the initial target position, and correct the real-time calculation results to obtain more accurate target position information.

[0141] The above description is merely an optional embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A target localization method based on geomagnetic anomalies and axial frequency magnetic fields, characterized in that, Includes the following steps: S1. Pre-entry preparation: Use external sensors to obtain the target's heading, speed and initial position. Based on the assumption that the target moves in uniform linear motion for a short period of time, construct a state equation to calculate and predict the target's future position and provide an initial reference for the underwater platform's movement. S2. Approaching the target: With the assistance of real-time attitude and acceleration information provided by the inertial navigation module, the underwater motion platform approaches the predicted target position from a distance, and performs magnetic calibration and noise suppression preprocessing on the raw magnetic field data collected by the magnetic sensor to eliminate environmental interference. S3. Target detection: When no target is present, the observation signal consists of the ambient magnetic field, and the energy value of this part is very small. Once a target appears, the energy of the observation signal will continuously and significantly increase. When the energy value exceeds the preset decision threshold, it is judged as a suspected target point. If suspected points are continuously detected for a specified time, the existence of the target is confirmed. S4. Joint localization: Under the Kalman filter framework, the target is equivalent to a magnetic dipole and an electric dipole respectively. The position predicted by the target state equation is corrected by combining the measurement equations of the two models, so as to achieve joint localization of the target and obtain more accurate target position information. S5. Verify and output the positioning results. Compare the positioning results with the target location information derived from the state equation and calculate the degree of consistency between the two. If they do not match, set the confidence level of the positioning results to zero and repeat S1 to S5. If they match, output the positioning results.

2. The target positioning method based on geomagnetic anomaly and axial frequency magnetic field according to claim 1, characterized in that, The specific steps for target detection described in S3 include: S31. Starting from time n1, estimate and store the energy value of the input signal using the following formula; In the formula: n1=1+L*K n2=N+L*K Where N is the length of the data window; L is the step size of the data window sliding; and the coefficient K controls the sliding of the window, K = 0, 1, 2, ... S32. Continuously compare the energy value E obtained in S31 with the threshold T. If the energy value E is greater than the threshold T, then the point is considered a suspected target point. S33. If a suspected target point is detected for M consecutive seconds, the existence of the target is confirmed.

3. The target positioning method based on geomagnetic anomalies and axial frequency magnetic fields according to claim 1, characterized in that, The specific steps of joint positioning as described in S4 include: S41. Magnetic anomaly localization based on the magnetic dipole model: The magnetic dipole model for measuring magnetic anomalies using a three-component magnetometer is as follows: In the formula: For sensor output; x k For system status; e k To mitigate noise during the measurement process, the magnetic target is treated as a point magnetic dipole. A tracking reference coordinate system is established using the three-component magnetometer coordinate system. The magnetic field of the magnetic target located at r at the three-component magnetometer is calculated by equation (2): In the formula: μ0=4π×10 -7 H / m is the free permeability; r(x,y,z) T The position vector of the target relative to the three-component magnetometer; m = [m x ,m y ,m z ] T The target magnetic moment; the target's magnetic moment parameter m is derived from the inherent magnetic moment. and induced magnetic moment m s Composition; During the target's motion, the measurement model of the single fluxgate sensor located at the origin of the reference coordinate system at time k is as follows: In the formula: r k =[x k ,y k ,z k ] T Let k be the target position at time k; e is the target magnetic moment at time k; k For measuring noise at time k, e k ~N(0, σ 2 );r k Let k be the position vector of the target relative to the single fluxgate sensor at time k; Assume the relative position difference between the two measurement points is d. r Then the measurement model of the single fluxgate sensor at the second point is: S42. Axis-frequency magnetic field localization based on the electric dipole model: The target static magnetic source can be equivalent to an electric dipole. Assuming the electric dipole is... The radiated magnetic field is calculated using the following formula: in, It is the electromagnetic wave number, and μ and ε are the permeability and permittivity of the propagation medium, respectively. Let P(x,y,z) be the distance from the receiving point P(x,y,z) to the magnetic dipole, and θ be the angle between the receiving radius vector and the z-axis. The magnetic field calculated above is the result in the electric dipole spherical coordinate system. We will convert it to the sensor's Cartesian coordinate system. in Let be the transformation matrix from the electric dipole coordinate system to the magnetic sensor coordinate system. The axis frequency magnetic field measurement model of the single fluxgate sensor located at the origin of the reference coordinate system at time k is: In the formula: r k =[x k ,y k ,z k ] T p is the target position at time k; k Let k be the target electric dipole moment. Let n be the axial component of the electric dipole moment at time k in its own coordinate system; k Noise is measured at time k; The relative position difference between the two measurement points is d r Under the premise that, the shaft frequency measurement model of the magnetic sensor at the second measurement point is: S43. Joint target localization based on Kalman filter framework: Since the speed of the underwater platform far exceeds that of the target, it is assumed that the target moves at a constant linear velocity for a short period of time. Therefore, the target's motion model can be represented as follows: In the formula, T s Sampling time; It is the platform's velocity relative to the target, v k-1 Let be the platform's velocity relative to the target at the previous sampling point. Assuming the target does not deflect during its uniform passage past the sensor, the target's magnetic moment and electric dipole moment will remain almost unchanged in a short time, i.e.: Based on the measurement equation, the target state variable at time k is defined as: The state equation for the target motion can be written as: x k =Ax k-1 (12) The state transition matrix A in the formula is: Equation (13) can also be expressed as: In the formula This represents the initial state of the target. Therefore, a single fluxgate sensor positioning model is established based on the measurement model of magnetic anomaly and shaft frequency magnetic field, and the state equation: This localization model locates the target within a Kalman filter framework, thereby obtaining the accurate target position.

4. A target positioning method based on geomagnetic anomalies and axial frequency magnetic fields according to any one of claims 1 to 3, characterized in that: The surface target positioning method is implemented based on a magnetic detection device, which is installed on an underwater moving platform. The magnetic detection device includes a magnetic sensor, a magnetic detection control unit, an inertial navigation module, a data transmission radio, and a secondary power supply.

5. The target positioning method based on geomagnetic anomalies and axial frequency magnetic fields according to claim 4, characterized in that: There are two types of magnetic sensors: a three-component magnetometer and a total field magnetometer; the three-component magnetometer is specifically a fluxgate magnetometer, and the total field magnetometer is specifically an optically pumped magnetometer.

6. The target positioning method based on geomagnetic anomaly and axial frequency magnetic field according to claim 4, characterized in that: The magnetic detection control unit includes a processor, a data storage unit, and a signal acquisition module.

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