阅读说明：本技术 一种舰船垂向感应磁场垂向分量的预测方法 (Prediction method for vertical component of vertical induced magnetic field of ship ) 是由 王毅 周国华 孙兆龙 姜润翔 武晓康 罗康 于 2021-08-06 设计创作，主要内容包括：本发明公开了一种舰船垂向感应磁场垂向分量的预测方法,步骤包括：数据归一化处理、构建对应的BP神经网络并初始化权值和阈值、对权值和阈值进行实数编码并将每个编码字符串作为一个个体、采用随机方法生成包含N个个体的遗传算法初始种群、设置遗传算法的初始参数并计算遗传算法的适应度值、搜索出种群中适应度函数值最大的个体作为BP神经网络的最佳权值和最佳阈值、将最佳权值和最佳阈值代入BP神经网络计算出舰船垂向感应磁场的垂向分量Ziz。该舰船垂向感应磁场垂向分量的预测方法通过遗传算法优化的BP神经网络模型利用已有数据来实时动态预测垂向分量Z-(iz),确保了垂向分量Z-(iz)预测的可靠。(The invention discloses a method for predicting vertical components of a ship vertical induction magnetic field, which comprises the following steps: the method comprises the steps of carrying out data normalization processing, constructing a corresponding BP neural network, initializing a weight and a threshold, carrying out real number coding on the weight and the threshold, taking each coding character string as an individual, generating a genetic algorithm initial population comprising N individuals by adopting a random method, setting initial parameters of a genetic algorithm, calculating a fitness value of the genetic algorithm, searching out the individual with the maximum fitness function value in the population as the optimal weight and the optimal threshold of the BP neural network, and substituting the optimal weight and the optimal threshold into the BP neural network to calculate the vertical component Ziz of the vertical induction magnetic field of the ship. The prediction method for the vertical component of the ship vertical induction magnetic field dynamically predicts the vertical component Z in real time by using the existing data through a BP neural network model optimized by a genetic algorithm iz Ensuring a vertical component Z iz The prediction is reliable.)

1. A method for predicting the vertical component of a ship vertical induction magnetic field is characterized by comprising the following steps:

step 1, carrying out data normalization processing on ship sample data;

step 2, constructing a corresponding BP neural network according to the characteristics of the ship sample data, and initializing the weight and the threshold of the BP neural network;

step 3, carrying out real number coding on the initialized weight and the threshold value to obtain corresponding coded character strings, and taking each coded character string as an individual;

step 4, generating a genetic algorithm initial population containing N individuals by adopting a random method;

step 5, setting initial parameters of the genetic algorithm and calculating the fitness value of the genetic algorithm;

step 6, searching out an individual with the maximum fitness function value in the population according to the fitness value of the genetic algorithm as the optimal weight and the optimal threshold of the BP neural network;

and 7, substituting the optimal weight and the optimal threshold into the BP neural network, operating the BP neural network by using the ship parameters monitored in real time, and calculating the vertical component Ziz of the ship vertical induction magnetic field.

2. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, further comprising a step 8 of constructing an error function E of the BP neural network, reversely transmitting an error value obtained by calculating the error function E in each iteration based on a gradient descent strategy so as to adjust a weight value and a threshold value of the BP neural network until the maximum iteration number is reached, and outputting the current vertical component Ziz as a final predicted value when the maximum iteration number is judged to be reached.

3. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 2, wherein in the step 8, the error function E of the constructed BP neural network is as follows:

in the formula, y_lThe actual value is represented by the value of,the predicted value is shown, and N is the number of output layer nodes.

4. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, wherein in the step 1, when data normalization processing is performed on ship sample data, the specific steps are as follows:

step 1.1, obtaining the maximum value J of each type of sample data_maxAnd minimum value J_min；

Step 1.2, respectively carrying out data normalization processing according to types, wherein the specific calculation formula of the data normalization processing is as follows:

in the formula, J_iIndicating ship sample data of the ith type.

5. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, wherein in the step 2, the specific steps of constructing the corresponding BP neural network according to the characteristics of the ship sample data are as follows:

step 2.1, analyzing the sample data of the N sampling points of the ship to obtain six types of sample data of each sampling point, wherein the six types of sample data comprise a course angle of the ship, a vertical component Zix of a longitudinal induction magnetic field of the ship, a vertical component Ziy of a transverse induction magnetic field of the ship, a vertical component Ziz of a vertical induction magnetic field of the ship, a horizontal component of a geomagnetic field and a vertical component of the geomagnetic field;

step 2.2, determining the number of nodes of an input layer, an output layer and a hidden layer of the BP neural network, wherein the input layer has 5N nodes, namely five types of sample data in each sampling point of the ship are taken as each node of the input layer, and the five types of sample data are respectively a course angle of the ship, a vertical component Zix of a longitudinal induction magnetic field of the ship, a vertical component Ziy of a transverse induction magnetic field of the ship, a horizontal component of a geomagnetic field and a vertical component of the geomagnetic field; the output layer is provided with N nodes, namely one type of sample data in each sampling point of the ship is used as each node of the output layer, and one type of sample data is the vertical component Ziz of the vertical induction magnetic field of the ship; the hidden layer has S nodes and is composed of a plurality of nodes,a is a constant of 1-10;

step 2.3, setting network parameters of the BP neural network, including training times and learning rate;

step 2.4, setting the transfer function of the output layer as a linear transfer function, and setting the transfer function of the hidden layer as:

6. the method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, wherein in the step 3, when the initialized weight and the threshold are subjected to real number coding to obtain the corresponding coding character string, the real number coding weight comprises a connection weight from an input layer to a hidden layer and a connection weight from the hidden layer to an output layer; the thresholds for real number encoding include neuron thresholds of the hidden layer and neuron thresholds of the output layer.

7. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, wherein in the step 5, the initial parameters of the genetic algorithm are set to include a maximum evolutionary algebra, a cross probability and a variation probability.

8. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, wherein in the step 5, when the fitness value of the genetic algorithm is calculated, the used fitness function is recorded as:

in the formula, y_lThe actual value is represented by the value of,the predicted value is shown, and N is the number of output layer nodes.

9. The method for predicting the vertical component of the ship vertical induction magnetic field according to claim 1, wherein in the step 6, the specific step of searching the individual with the maximum fitness function value in the population according to the fitness value of the genetic algorithm comprises the following steps:

step 6.1, judging whether the maximum evolution algebra is reached, if the maximum evolution algebra is not reached, adopting a roulette method to select and execute selection, intersection and variation operations of a genetic algorithm to generate a next generation population, returning to the step 5 to calculate the fitness value of the genetic algorithm, and if the maximum evolution algebra is reached, entering the step 6.2;

and 6.2, searching out the individual with the maximum fitness function value in the population, and taking the maximum individual as the optimal weight and the optimal threshold of the BP neural network.

Technical Field

The invention relates to a method for predicting a vertical component of a magnetic field, in particular to a method for predicting a vertical component of a vertical induction magnetic field of a ship.

Background

Ship magnetism can be broadly divided into fixed magnetism and induced magnetism. Based on the existing magnetism of the ship, the ship can generate a ship magnetic field around the ship. One typically uses a fixed magnetic field signal and an induced magnetic field signal to describe the magnetic field distribution around the ship. The purpose of the magnetic field protection measures is therefore to eliminate or compensate for the stationary and induced signals of the ship. At present, the magnetic field protection technologies of various countries can be roughly divided into two categories, namely active protection technologies and passive protection technologies.

In general, a fixed magnetic field signal of a ship is relatively stable in a period of time after the ship is passively demagnetized, and an induced magnetic field signal of the ship changes along with the difference of the posture, the course and the moving area of the ship. For a ship provided with the active protection system, the induced magnetic field signal can be dynamically detected in real time, and the induced magnetic field signal is dynamically offset or compensated by adjusting the current in the active protection system. However, for a ship without an active protection system, since an induced magnetic field signal of the ship cannot be obtained, the ship itself becomes a magnetic field exposure source in an active area, and the risk of being detected by a magnetic anomaly device is increased. Therefore, for ships without active protection systems, accurate prediction of the induced magnetic field of the ships is very important.

Among all induced magnetic field components of the ship (including a vertical component Zix of a longitudinal induced magnetic field, a vertical component Ziy of a transverse induced magnetic field and a vertical component Ziz of a vertical induced magnetic field), Zix and Ziy can be directly measured by means of a magnetic sensor array arranged below the ship, the vertical component Ziz cannot be directly measured by the method, and the whole measuring process is more complicated. Generally, a geomagnetic simulation method is generally used to predict the vertical component Ziz of the ship magnetic field. Specifically, geomagnetic simulation coils are arranged around the ship, the current in the geomagnetic simulation coils is adjusted to change the background magnetic field around the ship, so that the change condition of the vertical component Ziz is obtained, and the ship magnetic field vertical component Ziz in the local geomagnetic environment is calculated. The experimental result shows that the prediction accuracy of the method is easily influenced by the unevenness of the magnetic field generated by the geomagnetic simulation coil, the requirement on the hardware condition of the magnetic field measuring station is high, and the application of the method in the practical degaussing practice is further influenced.

Disclosure of Invention

The purpose of the invention is as follows: the method for predicting the vertical component of the vertical induced magnetic field of the ship can predict the vertical component Z of the ship in real time by using known data_izThe method becomes an important alternative method of the traditional geomagnetic simulation model.

The technical scheme is as follows: the invention relates to a method for predicting vertical components of a ship vertical induction magnetic field, which comprises the following steps:

step 1, carrying out data normalization processing on ship sample data;

step 2, constructing a corresponding BP neural network according to the characteristics of the ship sample data, and initializing the weight and the threshold of the BP neural network;

step 3, carrying out real number coding on the initialized weight and the threshold value to obtain corresponding coded character strings, and taking each coded character string as an individual;

step 4, generating a genetic algorithm initial population containing N individuals by adopting a random method;

step 5, setting initial parameters of the genetic algorithm and calculating the fitness value of the genetic algorithm;

step 6, searching out an individual with the maximum fitness function value in the population according to the fitness value of the genetic algorithm as the optimal weight and the optimal threshold of the BP neural network;

and 7, substituting the optimal weight and the optimal threshold into the BP neural network, operating the BP neural network by using the ship parameters monitored in real time, and calculating the vertical component Ziz of the ship vertical induction magnetic field.

Further, the method comprises the step 8 of constructing an error function E of the BP neural network, reversely transmitting an error value obtained by calculating the error function E in each iteration based on a gradient descent strategy so as to adjust the weight and the threshold of the BP neural network until the maximum iteration number is reached, and outputting the current vertical component Ziz as a final predicted value when the maximum iteration number is judged to be reached.

Further, in step 8, the error function E of the constructed BP neural network is:

in the formula, y_lThe actual value is represented by the value of,the predicted value is shown, and N is the number of output layer nodes.

Further, in step 1, when data normalization processing is performed on ship sample data, the specific steps are as follows:

step 1.1, obtaining the maximum value J of each type of sample data_maxAnd minimum value J_min；

Step 1.2, respectively carrying out data normalization processing according to types, wherein the specific calculation formula of the data normalization processing is as follows:

in the formula, J_iIndicating ship sample data of the ith type.

Further, in step 2, the specific steps of constructing the corresponding BP neural network according to the characteristics of the ship sample data are as follows:

step 2.1, analyzing the sample data of the N sampling points of the ship to obtain six types of sample data of each sampling point, wherein the six types of sample data comprise a course angle of the ship, a vertical component Zix of a longitudinal induction magnetic field of the ship, a vertical component Ziy of a transverse induction magnetic field of the ship, a vertical component Ziz of a vertical induction magnetic field of the ship, a horizontal component of a geomagnetic field and a vertical component of the geomagnetic field;

step 2.2, determining the number of nodes of an input layer, an output layer and a hidden layer of the BP neural network, wherein the input layer has 5N nodes, namely the shipThe five types of sample data in each sampling point are used as each node of the input layer, and the five types of sample data are respectively a course angle of a ship, a vertical component Zix of a longitudinal induction magnetic field of the ship, a vertical component Ziy of a transverse induction magnetic field of the ship, a horizontal component of a geomagnetic field and a vertical component of the geomagnetic field; the output layer is provided with N nodes, namely one type of sample data in each sampling point of the ship is used as each node of the output layer, and one type of sample data is the vertical component Ziz of the vertical induction magnetic field of the ship; the hidden layer has S nodes and is composed of a plurality of nodes,a is a constant of 1-10;

step 2.3, setting network parameters of the BP neural network, including training times and learning rate;

step 2.4, setting the transfer function of the output layer as a linear transfer function, and setting the transfer function of the hidden layer as:

further, in step 3, when performing real number coding on the initialized weight and the threshold to obtain a corresponding coded character string, the real number coded weight includes a connection weight from the input layer to the hidden layer and a connection weight from the hidden layer to the output layer; the thresholds for real number encoding include neuron thresholds of the hidden layer and neuron thresholds of the output layer.

Further, in step 5, initial parameters of the genetic algorithm are set to include maximum evolution algebra, cross probability and mutation probability.

Further, in step 5, when calculating the fitness value of the genetic algorithm, the fitness function used is recorded as:

in the formula, y_lThe actual value is represented by the value of,the predicted value is shown, and N is the number of output layer nodes.

Further, in step 6, the specific step of searching out the individual with the maximum fitness function value in the population according to the fitness value of the genetic algorithm is as follows:

step 6.1, judging whether the maximum evolution algebra is reached, if the maximum evolution algebra is not reached, adopting a roulette method to select and execute selection, intersection and variation operations of a genetic algorithm to generate a next generation population, returning to the step 5 to calculate the fitness value of the genetic algorithm, and if the maximum evolution algebra is reached, entering the step 6.2;

and 6.2, searching out the individual with the maximum fitness function value in the population, and taking the maximum individual as the optimal weight and the optimal threshold of the BP neural network.

Compared with the prior art, the invention has the beneficial effects that: the invention dynamically predicts the vertical component Z in real time by using the existing data through a BP neural network model optimized by a genetic algorithm_izEnsuring a vertical component Z_izThe prediction is reliable, a large number of geomagnetic simulation coils do not need to be arranged, the measurement method is simple, the influence of the unevenness of the magnetic field generated by the geomagnetic simulation coils is not easy to occur, and the requirement on the hardware condition of the magnetic field measurement station is not high.

Drawings

FIG. 1 is a flow chart of a method of the present invention;

FIG. 2 is a diagram showing the number of neurons in the input layer, hidden layer and output layer of the BP neural network according to the present invention;

FIG. 3 is a schematic view of a ship simulation model of the present invention;

fig. 4 is a schematic position diagram of a ship model in a geomagnetic simulation environment according to the present invention;

FIG. 5 shows the vertical component Z of the ship model of the present invention in the first test city_izSchematic diagram of predicted value and simulation value of;

FIG. 6 shows the vertical component Z of the ship model of the present invention in a second test city_izSchematic diagram of predicted value and simulation value of;

FIG. 7 is a graph illustrating the maximum Relative Error (RE) for each test city of the present invention;

FIG. 8 is a graph showing the Relative Error (RE) averages for 21 test cities according to the present invention;

FIG. 9 is a schematic diagram of a longitudinal geomagnetic simulation coil and a ship model according to the present invention;

fig. 10 is a schematic diagram of a vertical geomagnetism model coil and a ship model according to the present invention;

FIG. 11 is a vertical component Z of a first test city of the present invention_izA schematic diagram of measured and predicted values of;

FIG. 12 is a vertical component Z of a second test city of the present invention_izSchematic representation of measured and predicted values.

Detailed Description

The technical solution of the present invention is described in detail below with reference to the accompanying drawings, but the scope of the present invention is not limited to the embodiments.

As shown in fig. 1, the method for predicting the vertical component of the vertical induced magnetic field of the ship comprises the following steps:

step 1, carrying out data normalization processing on ship sample data, and transforming the ship sample data into an interval of [ -1,1 ];

step 2, constructing a corresponding BP neural network according to the characteristics of the ship sample data, and initializing the weight and the threshold of the BP neural network;

step 3, carrying out real number coding on the initialized weight and the threshold value to obtain corresponding coded character strings, and taking each coded character string as an individual;

step 4, generating a genetic algorithm initial population containing N individuals by adopting a random method;

step 5, setting initial parameters of the genetic algorithm and calculating the fitness value of the genetic algorithm;

step 6, searching out an individual with the maximum fitness function value in the population according to the fitness value of the genetic algorithm as the optimal weight and the optimal threshold of the BP neural network;

and 7, substituting the optimal weight and the optimal threshold into the BP neural network, operating the BP neural network by using the ship parameters monitored in real time, and calculating the vertical component Ziz of the ship vertical induction magnetic field.

The BP neural network model is constructed by utilizing the BP neural network model optimized by the genetic algorithm through the existing ship sample data, so that the vertical component Z can be dynamically predicted in real time_izEnsuring a vertical component Z_izThe prediction is reliable, a large number of geomagnetic simulation coils do not need to be arranged, the measurement method is simple, the influence of the unevenness of the magnetic field generated by the geomagnetic simulation coils is not easy to occur, and the requirement on the hardware condition of the magnetic field measurement station is not high.

Further, the method comprises the step 8 of constructing an error function E of the BP neural network, reversely transmitting an error value obtained by calculating the error function E in each iteration based on a gradient descent strategy so as to adjust the weight and the threshold of the BP neural network until the maximum iteration number is reached, and outputting the current vertical component Ziz as a final predicted value when the maximum iteration number is judged to be reached.

Further, in step 8, the error function E of the constructed BP neural network is:

in the formula, y_lThe actual value is represented by the value of,the predicted value is shown, and N is the number of output layer nodes.

Further, in step 1, when data normalization processing is performed on ship sample data, the specific steps are as follows:

step 1.1, obtaining the maximum value J of each type of sample data_maxAnd minimum value J_min；

Step 1.2, respectively carrying out data normalization processing according to types, wherein the specific calculation formula of the data normalization processing is as follows:

in the formula, J_iIndicating ship sample data of the ith type.

Further, in step 2, the specific steps of constructing the corresponding BP neural network according to the characteristics of the ship sample data are as follows:

step 2.1, analyzing the sample data of the N sampling points of the ship to obtain six types of sample data of each sampling point, wherein the six types of sample data comprise six types of sample data of a course angle of the ship, a vertical component Zix of a longitudinal induction magnetic field of the ship, a vertical component Ziy of a transverse induction magnetic field of the ship, a vertical component Ziz of a vertical induction magnetic field of the ship, a horizontal component of a geomagnetic field and a vertical component of the geomagnetic field;

step 2.2, determining the number of nodes of an input layer, an output layer and a hidden layer of the BP neural network, wherein the input layer has 5N nodes, as shown in FIG. 2, it indicates that N sampling points are placed below keels of ships, that is, five types of sample data in each sampling point of the ships are used as each node of the input layer, and the five types of sample data are respectively a course angle of the ships, a vertical component Zix of a longitudinal induction magnetic field of the ships, a vertical component Ziy of a transverse induction magnetic field of the ships, a horizontal component of a geomagnetic field and a vertical component of the geomagnetic field; the output layer is provided with N nodes, namely one type of sample data in each sampling point of the ship is used as each node of the output layer, and one type of sample data is the vertical component Ziz of the vertical induction magnetic field of the ship; the hidden layer has S nodes and is composed of a plurality of nodes,a is a constant of 1-10;

step 2.3, setting network parameters of the BP neural network, including training times and learning rate;

step 2.4, setting the transfer function of the output layer as a linear transfer function, and setting the transfer function of the hidden layer as:

further, in step 3, when performing real number coding on the initialized weight and the threshold to obtain a corresponding coded character string, the real number coded weight includes a connection weight from the input layer to the hidden layer and a connection weight from the hidden layer to the output layer; the thresholds for real number encoding include neuron thresholds of the hidden layer and neuron thresholds of the output layer.

Further, in step 5, initial parameters of the genetic algorithm are set to include maximum evolution algebra, cross probability and mutation probability.

Further, in step 5, when calculating the fitness value of the genetic algorithm, the fitness function used is recorded as:

in the formula, y_lThe actual value is represented by the value of,the predicted value is shown, and N is the number of output layer nodes.

Further, in step 6, the specific step of searching out the individual with the maximum fitness function value in the population according to the fitness value of the genetic algorithm is as follows:

step 6.1, judging whether the maximum evolution algebra is reached, if the maximum evolution algebra is not reached, adopting a roulette method to select and execute selection, intersection and variation operations of a genetic algorithm to generate a next generation population, returning to the step 5 to calculate the fitness value of the genetic algorithm, and if the maximum evolution algebra is reached, entering the step 6.2;

and 6.2, searching out the individual with the maximum fitness function value in the population, and taking the maximum individual as the optimal weight and the optimal threshold of the BP neural network.

Simulation verification:

in order to verify the reliability of the BP neural network model of the invention, enough sampling points are collected firstly. Therefore, the invention utilizes finite element software COMSOL to establish a ship model with the length of 3.8m, the width of 0.5m and the height of 0.65m, as shown in figure 3. This model consists of 47050 cells and 10897 nodes. The required ship induction magnetic field signals Zix, Ziy and Ziz can be obtained by inputting the horizontal component of the geomagnetic field, the vertical component of the geomagnetic field and the course angle of the ship in finite element software. The relative position of the ship model in the geomagnetic field simulator is shown in fig. 4.

In order to reduce the workload, the invention selects the geomagnetic field data of 21 port cities worldwide and inputs the data into COMSOL simulation software for generating a required data set. The horizontal and vertical components of these harbor urban geomagnetic fields are shown in table 1 according to the world geomagnetic model (WMM). Meanwhile, under each geomagnetic field environment, the invention also considers 8 heading angles of the ship model, including a north heading, a north heading 45 degrees, an east heading, a south heading 45 degrees, a west heading and a north heading 45 degrees. Therefore, the invention can obtain a data set containing 8 × 21 samples for training the BP neural network model of the invention.

TABLE 1 geomagnetic field of different cities

Geomagnetic field	Horizontal component (nT)	Vertical component (nT)	Geomagnetic field	Horizontal component (nT)	Vertical component (nT)
						1	33467	35815	12	30651	42050
2	37636	26248	13	22300	44496
						3	26727	45558	14	24083	39956
4	30435	35404	15	19099	49702
						5	19151	45353	16	31138	38258
6	23106	41997	17	30594	31699
						7	34137	28047	18	18479	50509
8	24085	40158	19	24783	40890
						9	31372	35158	20	37473	23593
10	19515	44868	21	20892	46808
						11	24090	39852

The horizontal component of the geomagnetic field, the vertical component of the geomagnetic field and the course of the ship are input into finite element software COMSOL of 21 cities, so that the vertical component Z of the vertical induction magnetic field of the ship can be obtained_ix,Z_iyAnd Z_izAll these parameters were acquired by means of 23 measurement points set 0.7m below the ship model.

On the basis of completing the data acquisition work, the whole process is implemented according to the steps shown in fig. 1. The input layer has 5 × 23 neurons, comprising 23 measurement points, each of which is represented by 5 parameters (vertical component Z)_ixA vertical component Z_iyHorizontal component of the geomagnetic field, vertical component of the geomagnetic field, course angle of the ship). The output layer has 1 × 23 neurons including 23 measurement points, each measurement point having vertical component Z of vertical induced magnetic field of ship_izAs an output.

The invention divides the obtained sample data into a training set and a testing set. The parameters of the first 19 cities (8 × 19 samples) are used for training and the parameters of the remaining 2 cities (8 × 2 samples) are used for prediction. According to the flow shown in fig. 1, a comparison result between the predicted value and the simulated value can be obtained, as shown in fig. 5 and fig. 6. The present invention utilizes the relative error RE to compare the difference between the predicted value and the simulated or measured value. The relative error RE is calculated by the formula:

in the formula, b_preRepresenting a vertical component Z_izPredicted value of (b)_tRepresenting a vertical component Z_izMeasured or simulated values of. From the simulation results, for each test city, the vertical component Z of the ship model_izThe error between the simulated value of (c) and the predicted value is very small. In the test results of 8 course angles of each test city, the maximum relative error RE is less than 1%.

In order to analyze the influence of different sample sizes on the prediction accuracy of the BP neural network model, the next test is carried out. The relevant parameters of the ship model under each urban geomagnetic environment are used as a test set (8 multiplied by 1 samples), the relevant parameters of the ship model under the rest 20 urban geomagnetic environments are used as a training set, and the number of cities for training is increased from 2 to 20(2 multiplied by 8-20 multiplied by 8 samples). The maximum Relative Error (RE) of the ship model in each of the urban geomagnetic environments is recorded in consideration of 8 heading angles of the ship model, as shown in fig. 7 and 8.

As can be seen from FIG. 7, when the number of cities used for training exceeds 5, the vertical component Z is_izThe maximum Relative Error (RE) between the simulated value and the predicted value of (RE) is less than 10%. The maximum relative error will also continue to decrease as the sample city increases. The same conclusion can be drawn in fig. 8, which shows the maximum Relative Error (RE) average for 21 cities as a function of the number of samples.

Real ship verification:

the method selects a real ship model (4.3 m in length, 0.28m in width and 0.8m in height) of a magnetic field laboratory, the real ship model is parked on a north-south course guide rail, 9 groups of longitudinal geomagnetic simulation coils and 4 groups of vertical geomagnetic simulation coils are distributed around the real ship model, and each group of longitudinal or vertical coils comprises a certain number of turns. By varying the current in the geomagnetic simulation coil, the magnetic field laboratory can simulate any magnetic field environment in the range of the earth's magnetic field, and the overall schematic diagram is shown in fig. 9 and 10. 25 magnetic field sensors are arranged 0.7m below the real ship model and used for acquiring required test data.

TABLE 2 horizontal and vertical components of geomagnetic field in different cities

Geomagnetic field	Horizontal component (nT)	Vertical component (nT)
			1	33467	35815
2	37473	23593
			3	26727	45558
4	30435	35404
			5	34137	28047
6	24090	39852
			7	30651	42050
8	31138	38258
			9	30594	31699
10	24783	40890
			11	37636	26248

Data results of simulation analysis are considered, namely: when the number of sample cities exceeds 5, the prediction accuracy of the BP neural network model can be lower than 10%. Therefore, 11 cities of geomagnetic fields were selected for simulation, and corresponding magnetic field data were collected, as shown in table 2. Meanwhile, because of the limitation of laboratory conditions, the course angle of the ship cannot be adjusted, and only the real ship model magnetic field signal data in the north course is measured in the experimental process.

The experimental procedure was as follows:

step a: before the ship model enters, adjusting the current in the longitudinal and vertical geomagnetic simulation coils, and sequentially simulating different geomagnetic environments as shown in table 2;

step b: pushing the ship model into a designated area, measuring a magnetic field when the ship model is in the north course and a magnetic field when the ship model is in the south course, and recording magnetic field data;

step c: pushing the ship model out of the designated area, changing the current in the vertical working coil to change the vertical component of the external magnetic field in the area, and recording the magnetic field data;

step d: pushing the ship model to a specified position, measuring a magnetic field when the ship model is in the north course, and recording magnetic field data;

step e: and summarizing and sorting all the magnetic field values, calculating the induced magnetic fields Zix and Ziz of the ship model in different magnetic environments, and establishing a database for later use.

According to the above experimental procedure, 11 sets of sample data were obtained. Similarly, the first 9 groups of samples are used as training set, and the second 2 groups of samples are used as test set. According to the operation flow of the genetic neural network shown in FIG. 1, the vertical component Z can be obtained_izAs a result of the comparison between the predicted value and the measured value of (c),as shown in fig. 11 and 12. The relative error of the two test cities is respectively 4.81 percent and 9.26 percent, which is lower than 20 percent, and can meet the requirements of engineering application.

Therefore, the invention dynamically predicts the vertical component Z in real time by using the existing data through the BP neural network model optimized by the genetic algorithm_izThe simulation analysis and the experimental result verify that the BP neural network model of the invention is used for inducing the magnetic field signal Z_izAccuracy and validity of the assessment. For a full-size ship without an active demagnetization system, the BP neural network model is successfully applied to real-time evaluation of the induced magnetic field signal Z_izProvides an important path.

As noted above, while the present invention has been shown and described with reference to certain preferred embodiments, it is not to be construed as limited thereto. Various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.