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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Neurorobot.</journal-id>
<journal-title>Frontiers in Neurorobotics</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Neurorobot.</abbrev-journal-title>
<issn pub-type="epub">1662-5218</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.3389/fnbot.2021.785563</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Neuroscience</subject>
<subj-group>
<subject>Methods</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>A Correlation Analysis of Geomagnetic Field Characteristics in Geomagnetic Perceiving Navigation</article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name><surname>Li</surname> <given-names>Hong</given-names></name>
<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
<xref ref-type="aff" rid="aff2"><sup>2</sup></xref>
<xref ref-type="corresp" rid="c001"><sup>&#x0002A;</sup></xref>
</contrib>
<contrib contrib-type="author">
<name><surname>Qu</surname> <given-names>Junsuo</given-names></name>
<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
<xref ref-type="aff" rid="aff2"><sup>2</sup></xref>
</contrib>
<contrib contrib-type="author">
<name><surname>Jiang</surname> <given-names>Xiangkui</given-names></name>
<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
</contrib>
<contrib contrib-type="author">
<name><surname>Niu</surname> <given-names>Yun</given-names></name>
<xref ref-type="aff" rid="aff3"><sup>3</sup></xref>
</contrib>
</contrib-group>
<aff id="aff1"><sup>1</sup><institution>School of Automation, Xi&#x00027;an University of Posts and Telecommunications</institution>, <addr-line>Xi&#x00027;an</addr-line>, <country>China</country></aff>
<aff id="aff2"><sup>2</sup><institution>Xi&#x00027;an Key Laboratory of Advanced Control and Intelligent Process, School of Automation, Xi&#x00027;an University of Posts and Telecommunications</institution>, <addr-line>Xi&#x00027;an</addr-line>, <country>China</country></aff>
<aff id="aff3"><sup>3</sup><institution>School of Marine Science and Technology, Northwestern Polytechnical University</institution>, <addr-line>Xi&#x00027;an</addr-line>, <country>China</country></aff>
<author-notes>
<fn fn-type="edited-by"><p>Edited by: Xin Luo, Chongqing Institute of Green and Intelligent Technology (CAS), China</p></fn>
<fn fn-type="edited-by"><p>Reviewed by: Bin Zhi Li, Chongqing Institute of Green and Intelligent Technology (CAS), China; Lun Hu, Xinjiang Technical Institute of Physics &#x00026; Chemistry, Chinese Academy of Sciences (CAS), China</p></fn>
<corresp id="c001">&#x0002A;Correspondence: Hong Li <email>lihong&#x00040;xupt.edu.cn</email></corresp>
</author-notes>
<pub-date pub-type="epub">
<day>23</day>
<month>12</month>
<year>2021</year>
</pub-date>
<pub-date pub-type="collection">
<year>2021</year>
</pub-date>
<volume>15</volume>
<elocation-id>785563</elocation-id>
<history>
<date date-type="received">
<day>29</day>
<month>09</month>
<year>2021</year>
</date>
<date date-type="accepted">
<day>15</day>
<month>11</month>
<year>2021</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#x000A9; 2021 Li, Qu, Jiang and Niu.</copyright-statement>
<copyright-year>2021</copyright-year>
<copyright-holder>Li, Qu, Jiang and Niu</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/"><p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p></license> </permissions>
<abstract><p>It is well-known that geomagnetic fields have multiple components or parameters, and that these geomagnetic parameters are related to each other. In this paper, a parameter selection method is proposed, and this paper mainly discusses the correlation of geomagnetic field parameters for geomagnetic navigation technology. For the correlation analysis between geomagnetic parameters, the similarity calculation of the correlation coefficient is firstly introduced for geomagnetic navigation technology, and the grouped results are obtained by data analysis. At the same time, the search algorithm (Hex-path algorithm) is used to verify the correlation analysis results. The results show the same convergent state for the approximate correlation coefficient. In other words, the simulation results are in agreement with the similarity calculation results.</p></abstract>
<kwd-group>
<kwd>animal geomagnetic perception</kwd>
<kwd>geomagnetic navigation</kwd>
<kwd>correlation analysis</kwd>
<kwd>hexpath algorithm</kwd>
<kwd>word magnetic model</kwd>
<kwd>world magnetic model</kwd>
</kwd-group>
<counts>
<fig-count count="7"/>
<table-count count="2"/>
<equation-count count="12"/>
<ref-count count="31"/>
<page-count count="8"/>
<word-count count="4453"/>
</counts>
</article-meta>
</front>
<body>
<sec sec-type="intro" id="s1">
<title>Introduction</title>
<p>Lots of evidence has indicated that many kinds of animals can achieve long-distance and goal-oriented navigation without pinpoint accuracy (Walker et al., <xref ref-type="bibr" rid="B24">2002</xref>). This is due to the existence of the &#x00027;geomagnetic sense&#x00027; (Walker et al., <xref ref-type="bibr" rid="B25">1997</xref>). Kramer states that animals could firstly determine their position relative to the goal and set the course for their goal by the Earth&#x00027;s geomagnetic field (Kramer, <xref ref-type="bibr" rid="B12">1953</xref>). It is reported that pigeons and sea turtles can reach a destination by sensing geomagnetic information (Rodda, <xref ref-type="bibr" rid="B19">1984</xref>; Dennis et al., <xref ref-type="bibr" rid="B3">2007</xref>). These animals can locate homing and foraging areas depending on their perception of the geomagnetic field.</p>
<p>Geomagnetic fields are a very important cue for navigation by these animals (Zhang et al., <xref ref-type="bibr" rid="B30">2019</xref>). At any point on the Earth&#x00027;s surface, geomagnetic fields can be described as vectors in three-dimensional space (see <xref ref-type="fig" rid="F1">Figure 1</xref>). It is fairly well-known that the fields are derived from sources in the core and crust of the Earth, and the total geomagnetic field vectors have seven components, which can be resolved into the north component <bold><italic>B</italic></bold><sub><bold><italic>X</italic></bold></sub>, the east component <bold><italic>B</italic></bold><sub><bold><italic>Y</italic></bold></sub>, the vertical component <bold><italic>B</italic></bold><sub><bold><italic>Z</italic></bold></sub>, the horizontal component <bold><italic>B</italic></bold><sub><bold><italic>H</italic></bold></sub>, the total intensity component <bold><italic>B</italic></bold><sub><bold><italic>F</italic></bold></sub>, the declination angle component <bold><italic>B</italic></bold><sub><bold><italic>D</italic></bold></sub>, and the inclination angle component <bold><italic>B</italic></bold><sub><bold><italic>I</italic></bold></sub>. Thus, geomagnetic fields can provide very stable information about a location which animals can use to navigate.</p>
<fig id="F1" position="float">
<label>Figure 1</label>
<caption><p>The vectors of the geomagnetic fields.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0001.tif"/>
</fig>
<p>Geomagnetic navigation technology can provide a reliable navigation method by measuring geomagnetic fields for mobile robots, such as underwater vehicle navigation (AUV) (Kinsey et al., <xref ref-type="bibr" rid="B10">2006</xref>) and unmanned aircraft (UA) (Yuan et al., <xref ref-type="bibr" rid="B29">2011</xref>; Kebria et al., <xref ref-type="bibr" rid="B9">2020</xref>). Conventional geomagnetic navigation methods mainly focus on geomagnetic matching algorithms, which mainly include the Mean Absolute Difference algorithm (MAD) (Caifa et al., <xref ref-type="bibr" rid="B1">2011</xref>), Mean Square Difference algorithm (MSD) (Rong, <xref ref-type="bibr" rid="B20">2016</xref>), and Iterative Closest Contour Point algorithm (ICCP) (Lin et al., <xref ref-type="bibr" rid="B13">2007</xref>; Xiao et al., <xref ref-type="bibr" rid="B28">2019</xref>; Luo et al., <xref ref-type="bibr" rid="B15">2021a</xref>). However, the conventional matching methods mainly depend on a priori geomagnetic map. There is a problem with the mentioned methods that the geomagnetic map needs to be drawn in advance (Ge and Zhou, <xref ref-type="bibr" rid="B4">2007</xref>). To avoid any dependency on a priori geomagnetic map, the geomagnetic perceiving navigation method is proposed.</p>
<p>To satisfy the practical demand of geomagnetic navigation, the selection methods of characteristic components for geomagnetic matching were proposed based on statistical modeling (Qiao et al., <xref ref-type="bibr" rid="B18">2007</xref>; Wei et al., <xref ref-type="bibr" rid="B26">2010</xref>; Shang et al., <xref ref-type="bibr" rid="B23">2019</xref>). The correlation research on the geomagnetic field parameters is indispensable for geomagnetic perceiving navigation, which introduced a new selection method for geomagnetic field parameters. We know that getting the right geomagnetic parameters affects the navigation result, and it is still important for navigational efficiency. This paper mainly discusses the correlation of geomagnetic field parameters for geomagnetic perceiving navigation technology.</p>
<p>In this paper, the similarity calculation based on the geomagnetic characteristics is a data association technology. For the correlation analysis between the geomagnetic parameters, the similarity calculation of the correlation coefficient is firstly introduced for geomagnetic perceiving navigation technology. Therefore, the correlation coefficient is calculated between the geomagnetic parameters, at the same time, the search algorithm (Hex-path algorithm) is used to verify the correlation analysis results. Simulation results of three cases are analyzed, and we can conclude that the same convergent state for the approximate correlation coefficient is apparent. In other words, the simulation results are in agreement with the similarity calculation results. In the future, we will focus on the research of geomagnetic perceiving navigation methods, and the present study is the preliminary preparation and the theoretical foundation for the follow-up work.</p>
<p>The rest of this paper is structured as follows: in section Correlation analysis on geomagnetic field parameters, the correlation analysis on the geomagnetic field parameters is introduced, and the results are given. In section Problem formulation of geomagnetic perceiving navigation, the search problem of geomagnetic perceiving navigation is raised. Next, the Hex-path algorithm is adopted in section Algorithm verification. Then, the simulation is introduced in section Results. Finally, the conclusion is given in section Conclusion.</p></sec>
<sec id="s2">
<title>Correlation Analysis on Geomagnetic Field Parameters</title>
<p>Geomagnetic fields can be divided into two categories: geomagnetic intensity and geomagnetic angle. The geomagnetic intensity is mainly composed of the total geomagnetic intensity (<bold><italic>B</italic></bold><sub><bold><italic>F</italic></bold></sub>), the horizontal intensity <bold>(<italic>B</italic></bold><sub><bold><italic>H</italic></bold></sub><bold>)</bold>, the north component (<bold><italic>B</italic></bold><sub><bold><italic>X</italic></bold></sub><bold>)</bold>, the east component <bold>(<italic>B</italic></bold><sub><bold><italic>Y</italic></bold></sub><bold>)</bold>, and the vertical component <bold>(<italic>B</italic></bold><sub><bold><italic>Z</italic></bold></sub><bold>)</bold>. The geomagnetic field angle is mainly composed of the geomagnetic declination angle <bold><italic>B</italic></bold><sub><bold><italic>D</italic></bold></sub> and the geomagnetic inclination angle <bold>(<italic>B</italic></bold><sub><bold><italic>I</italic></bold></sub><bold>)</bold> (Zhao et al., <xref ref-type="bibr" rid="B31">2014</xref>). Choosing suitable geomagnetic field parameters is the key to realizing geomagnetic perceiving navigation. Therefore, the selected geomagnetic parameters must have distinct statistical characteristics in the navigation area with high recognition and rich feature information.</p>
<p>If the Earth&#x00027;s geomagnetic field is an ideal magnetic dipole field, there will be two intersections PA and PB (see <xref ref-type="fig" rid="F2">Figure 2</xref>) between the two geomagnetic contours in the whole Earth range, indicating that the geomagnetic components on these two points are the same (Jiang and Ran, <xref ref-type="bibr" rid="B8">2011</xref>). It can be seen that only two geomagnetic characteristics fail to determine a unique geographical location on the Earth surface. Therefore, several geomagnetic characteristics are needed as geomagnetic parameters to ensure the unique position of the Earth surface.</p>
<fig id="F2" position="float">
<label>Figure 2</label>
<caption><p>The uniqueness of geomagnetic fields.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0002.tif"/>
</fig>
<p>The geomagnetic fields have seven parameters. There is a correlation between geomagnetic field parameters. In order to uniquely determine a position, we need to consider the independence of the three parameters from the seven vectors of the geomagnetic fields. Therefore, it is necessary to research the similarity degree of the seven vectors.</p>
<p>In essence, the similarity calculation based on the geomagnetic characteristics is a data association technology. The similarity between the geomagnetic characteristics mainly reflects the correlation of their variation characteristics. Therefore, the correlation coefficient is calculated by the similarity between multiple geomagnetic parameters, which are needed to take into account the global variation characteristics of the geomagnetic fields.</p>
<p>The data similarity calculation methods, which are commonly used in data association technology, mainly include the cosine similarity, the modified cosine similarity, and the correlation coefficient similarity (Saito et al., <xref ref-type="bibr" rid="B22">1999</xref>; Luo et al., <xref ref-type="bibr" rid="B16">2021b</xref>).</p>
<p>The three data similarity methods are followed as:</p>
<p>(i) The cosine similarity method can be described as:</p>
<disp-formula id="E1"><label>(1)</label><mml:math id="M1"><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mi>j</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>c</mml:mi><mml:mi>o</mml:mi><mml:mi>s</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>R</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>R</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mfrac><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle><mml:mo>&#x02260;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>,</mml:mo><mml:mi>i</mml:mi><mml:mo>,</mml:mo><mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub></mml:mrow></mml:mstyle></mml:mrow><mml:mrow><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msubsup><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msubsup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msubsup><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msubsup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt></mml:mrow></mml:mfrac></mml:math></disp-formula>
<p>where <bold><italic>R</italic></bold><sub><bold><italic>i</italic></bold></sub> and <bold><italic>R</italic></bold><sub><bold><italic>j</italic></bold></sub> are the vector-evaluated parameters, respectively.</p>
<p>(ii) The modified cosine similarity method can be described as:</p>
<disp-formula id="E2"><label>(2)</label><mml:math id="M2"><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mi>j</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>A</mml:mi><mml:mi>d</mml:mi><mml:mi>j</mml:mi><mml:mi>u</mml:mi><mml:mi>s</mml:mi><mml:mi>t</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>c</mml:mi><mml:mi>o</mml:mi><mml:mi>s</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>R</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>R</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mfrac><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle><mml:mo>&#x02260;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>,</mml:mo><mml:mi>i</mml:mi><mml:mo>,</mml:mo><mml:mi>j</mml:mi></mml:mstyle><mml:mo>=</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:mstyle></mml:mrow><mml:mrow><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt></mml:mrow></mml:mfrac></mml:math></disp-formula>
<p>where <inline-formula><mml:math id="M3"><mml:mover accent="true"><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>r</mml:mi></mml:mstyle></mml:mrow><mml:mo>&#x00304;</mml:mo></mml:mover></mml:math></inline-formula> is the evaluation value of the vector-evaluated parameters.</p>
<p>(iii) The correlation coefficient similarity method can be described as:</p>
<disp-formula id="E3"><label>(3)</label><mml:math id="M4"><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mi>j</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>A</mml:mi><mml:mi>d</mml:mi><mml:mi>j</mml:mi><mml:mi>u</mml:mi><mml:mi>s</mml:mi><mml:mi>t</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>c</mml:mi><mml:mi>o</mml:mi><mml:mi>s</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>R</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>R</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mfrac><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle><mml:mo>&#x02260;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>,</mml:mo><mml:mi>i</mml:mi><mml:mo>,</mml:mo><mml:mi>j</mml:mi></mml:mstyle><mml:mo>=</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:mstyle></mml:mrow><mml:mrow><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub></mml:mrow><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt></mml:mrow></mml:mfrac></mml:math></disp-formula>
<p>where <inline-formula><mml:math id="M5"><mml:mover accent="true"><mml:mrow><mml:msub><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>r</mml:mi></mml:mstyle></mml:mrow><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>i</mml:mi></mml:mstyle></mml:mrow></mml:msub></mml:mrow><mml:mo>&#x00304;</mml:mo></mml:mover></mml:math></inline-formula> and <inline-formula><mml:math id="M6"><mml:mover accent="true"><mml:mrow><mml:msub><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>r</mml:mi></mml:mstyle></mml:mrow><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>j</mml:mi></mml:mstyle></mml:mrow></mml:msub></mml:mrow><mml:mo>&#x00304;</mml:mo></mml:mover></mml:math></inline-formula> are the evaluation values of the vector-evaluated parameters <bold><italic>r</italic></bold><sub><bold><italic>i</italic></bold></sub> and <bold><italic>r</italic></bold><sub><bold><italic>j</italic></bold></sub>, respectively.</p>
<p>For the correlation analysis between the geomagnetic characteristics, the similarity calculation method of the correlation coefficient is firstly adopted. This paper proposes a method to calculate the similarity between several parameters by Euclidean distance. Therefore, the similarity calculation between geomagnetic characteristics can be expressed as follows:</p>
<disp-formula id="E4"><label>(4)</label><mml:math id="M7"><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>X</mml:mi><mml:mi>Y</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mfrac><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle><mml:mo>&#x02260;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>,</mml:mo><mml:mi>i</mml:mi><mml:mo>,</mml:mo><mml:mi>j</mml:mi></mml:mstyle><mml:mo>=</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>x</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>x</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>y</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>y</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:mstyle></mml:mrow><mml:mrow><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>x</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>x</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt><mml:msqrt><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle displaystyle='true'><mml:msubsup><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msubsup><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>y</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02212;</mml:mo><mml:mover accent='true'><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>y</mml:mi></mml:mstyle><mml:mo stretchy='true'>&#x000AF;</mml:mo></mml:mover></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow></mml:msqrt></mml:mrow></mml:mfrac></mml:math></disp-formula>
<p>where <bold><italic>X</italic></bold><bold> &#x0003D; (<italic>x</italic></bold><sub><bold>1</bold></sub><bold>,<italic>x</italic></bold><sub><bold>2</bold></sub><bold>,&#x02026;,<italic>x</italic></bold><sub><bold><italic>n</italic></bold></sub><bold>)</bold> and <bold><italic>Y</italic></bold><bold> &#x0003D; (<italic>y</italic></bold><sub><bold>1</bold></sub><bold>,<italic>y</italic></bold><sub><bold>2</bold></sub><bold>,&#x02026;,<italic>y</italic></bold><sub><bold><italic>n</italic></bold></sub><bold>)</bold> are two parameter sequences, <inline-formula><mml:math id="M8"><mml:mover accent="true"><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>x</mml:mi></mml:mstyle></mml:mrow><mml:mo>&#x00304;</mml:mo></mml:mover></mml:math></inline-formula> and <inline-formula><mml:math id="M9"><mml:mover accent="true"><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>y</mml:mi></mml:mstyle></mml:mrow><mml:mo>&#x00304;</mml:mo></mml:mover></mml:math></inline-formula> are the evaluation values of the two parameter sequences, and <bold><italic>r</italic></bold><sub><bold><italic>XY</italic></bold></sub> is the correlation coefficient between two geomagnetic parameters, &#x02212;1 &#x02264; <bold><italic>r</italic></bold><sub><bold><italic>XY</italic></bold></sub> <bold> &#x02264;<italic> 1</italic></bold>.</p>
<p>According to the correlation coefficient calculation method of data association, the degree of correlation between parameters can be divided into the following categories (Kong et al., <xref ref-type="bibr" rid="B11">2012</xref>; Hu et al., <xref ref-type="bibr" rid="B5">2019</xref>, <xref ref-type="bibr" rid="B7">2020</xref>; Wu et al., <xref ref-type="bibr" rid="B27">2020</xref>; Luo et al., <xref ref-type="bibr" rid="B17">2021c</xref>):</p>
<list list-type="order">
<list-item><p>If <bold><italic>r</italic></bold><sub><bold><italic>XY</italic></bold></sub> <bold>&#x0003C;</bold> 0.3, the two parameters are irrelevant;</p></list-item>
<list-item><p>If &#x02212;0.3 &#x02264; <bold><italic>r</italic></bold><sub><bold><italic>XY</italic></bold></sub> <bold>&#x0003C;</bold> 0.5, the two parameters have a low degree of linear correlation;</p></list-item>
<list-item><p>If &#x02212;0.5 &#x02264; <bold><italic>r</italic></bold><sub><bold><italic>XY</italic></bold></sub> <bold>&#x0003C;</bold> 0.8, the two parameters are indicating significant linear correlation;</p></list-item>
<list-item><p>If 0.8 &#x02264; <bold><italic>r</italic></bold><sub><bold><italic>XY</italic></bold></sub>, the two parameters generally have highly linear correlation.</p></list-item>
</list>
<p>According to the distribution characteristics of geomagnetic parameters, the geomagnetic parameters <bold><italic>B</italic></bold><sub><bold><italic>X</italic></bold></sub>, <bold><italic>B</italic></bold><sub><bold><italic>Y</italic></bold></sub>, and <bold><italic>B</italic></bold><sub><bold><italic>Z</italic></bold></sub> are different and independent from each other, so the correlation coefficient between the three parameters is 0 in theory.</p>
<p>If the correlation coefficient of any two geomagnetic parameters among the seven geomagnetic parameters is much larger than the correlation coefficient of the above three (<inline-formula><mml:math id="M19"><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>X</mml:mi><mml:mi>Y</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x02009;</mml:mtext><mml:mo>&#x02265;</mml:mo><mml:mtext>&#x02009;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>X</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02194;</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>Y</mml:mi></mml:mstyle></mml:msub></mml:mrow></mml:msub><mml:mo>,</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>X</mml:mi><mml:mi>Y</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x02009;</mml:mtext><mml:mo>&#x02265;</mml:mo><mml:mtext>&#x02009;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>X</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02194;</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>Z</mml:mi></mml:mstyle></mml:msub></mml:mrow></mml:msub><mml:mo>,</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>X</mml:mi><mml:mi>Y</mml:mi></mml:mstyle></mml:mrow></mml:msub><mml:mtext>&#x02009;</mml:mtext><mml:mo>&#x02265;</mml:mo><mml:mtext>&#x02009;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>r</mml:mi></mml:mstyle><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>Y</mml:mi></mml:mstyle></mml:msub><mml:mo>&#x02194;</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>Z</mml:mi></mml:mstyle></mml:msub></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula>), then the two parameters have high similarity, and these parameters with high correlation coefficients are usually grouped into a class.</p>
<p>The data analysis from the Word Magnetic Model (WMM2015) (Russell, <xref ref-type="bibr" rid="B21">2004</xref>; Chulliat et al., <xref ref-type="bibr" rid="B2">2015</xref>; Hu et al., <xref ref-type="bibr" rid="B6">2021</xref>) pointed out the correlation of geomagnetic components on the Earth surface. For simplification of data statistics, the seven geomagnetic characteristics in the northern hemisphere of the Earth are taken as examples. The northern hemisphere is divided into the first quadrant I and the second quadrant II (see <xref ref-type="fig" rid="F3">Figure 3</xref>).</p>
<fig id="F3" position="float">
<label>Figure 3</label>
<caption><p>The map of the Earth&#x00027;s magnetic field.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0003.tif"/>
</fig>
<p>The correlation coefficient of any two geomagnetic parameters can be calculated from the Word Magnetic Model database, and the results indicate that the seven parameters of the first and second quadrants of the northern hemisphere are divided into three groups as shown in <xref ref-type="table" rid="T1">Table 1</xref>.</p>
<table-wrap position="float" id="T1">
<label>Table 1</label>
<caption><p>The correlation statistical results of geomagnetic parameters.</p></caption>
<table frame="hsides" rules="groups">
<thead><tr>
<th valign="top" align="left"><bold>Components</bold></th>
<th valign="top" align="center"><bold>Correlation</bold></th>
<th valign="top" align="center"><bold>Value</bold></th>
</tr>
</thead>
<tbody>
<tr>
<td valign="top" align="left"><bold>(<italic>B</italic><sub><italic>X</italic></sub> <italic>B</italic><sub><italic>H</italic></sub>)</bold></td>
<td valign="top" align="center"><bold><italic>r</italic><sub><italic>B</italic><sub><italic>X</italic></sub>&#x02194;<italic>B</italic><sub><italic>H</italic></sub></sub></bold></td>
<td valign="top" align="center">0.9998</td>
</tr>
<tr>
<td valign="top" align="left"><bold>(<italic>B</italic><sub><italic>Y</italic></sub> <italic>B</italic><sub><italic>D</italic></sub>)</bold></td>
<td valign="top" align="center"><bold><italic>r</italic><sub><italic>B</italic><sub><italic>Y</italic></sub>&#x02194;<italic>B</italic><sub><italic>D</italic></sub></sub></bold></td>
<td valign="top" align="center">0.8365</td>
</tr>
<tr>
<td valign="top" align="left"><bold>(<italic>B</italic><sub><italic>Z</italic></sub> <italic>B</italic><sub><italic>I</italic></sub> <italic>B</italic><sub><italic>F</italic></sub>)</bold></td>
<td valign="top" align="center"><bold><italic>r</italic><sub><italic>B</italic><sub><italic>Z</italic></sub>&#x02194;<italic>B</italic><sub><italic>I</italic></sub></sub></bold>&#x00026;<bold><italic>r</italic><sub><italic>B</italic><sub><italic>Z</italic></sub>&#x02194;<italic>B</italic><sub><italic>F</italic></sub></sub></bold>&#x00026;<bold><italic>r</italic><sub><italic>B</italic><sub><italic>I</italic></sub>&#x02194;<italic>B</italic><sub><italic>F</italic></sub></sub></bold></td>
<td valign="top" align="center">0.9756 &#x00026; 0.9666 &#x00026; 0.8992</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>In geomagnetic perceiving navigation, the greater the correlation coefficient value between geomagnetic parameters, the higher the degree of the linear correlation between the two parameters, which can be equated to one category. As for the selection of the geomagnetic parameters, it is advisable to choose one with a small correlation coefficient.</p>
<p>According to the above analysis, the correlation of the geomagnetic parameters should be considered when selecting the geomagnetic parameters for navigation search. In other words, the geomagnetic components should be selected from the three groups, respectively, such as (<bold><italic>B</italic></bold><sub><bold><italic>X</italic></bold></sub><bold><italic>B</italic></bold><sub><bold><italic>Y</italic></bold></sub><bold><italic>B</italic></bold><sub><bold><italic>Z</italic></bold></sub>), (<bold><italic>B</italic></bold><sub><bold><italic>H</italic></bold></sub><bold><italic>B</italic></bold><sub><bold><italic>D</italic></bold></sub><bold><italic>B</italic></bold><sub><bold><italic>I</italic></bold></sub>), (<bold><italic>B</italic></bold><sub><bold><italic>F</italic></bold></sub><bold><italic>B</italic></bold><sub><bold><italic>D</italic></bold></sub><bold><italic>B</italic></bold><sub><bold><italic>H</italic></bold></sub>), etc.</p></sec>
<sec id="s3">
<title>Problem Formulation of Geomagnetic Perceiving Navigation</title>
<sec>
<title>Mathematical Model</title>
<p>Geomagnetic perceiving navigation is the search process of geomagnetic multi-parameters without a priori geomagnetic map. It indicates that an agent could only perceive the variation of the geomagnetic parameters to reach the destination by a geomagnetic sensor. The search behavior is the response to the geomagnetic environment stimuli, and its physical significance is that the geomagnetic multi-parameter could only determine geographic locations on the Earth (Liu et al., <xref ref-type="bibr" rid="B14">2014</xref>). The process of geomagnetic perceiving navigation is the convergence process of the geomagnetic parameters from the start location to the target location. When the geomagnetic parameters converge to zero, it indicates that the agent has finished the navigation task. Therefore, the multi-objective convergence process can be considered as:</p>
<disp-formula id="E5"><label>(5)</label><mml:math id="M10"><mml:mrow><mml:mo>{</mml:mo><mml:mrow><mml:mtable><mml:mtr><mml:mtd><mml:mrow><mml:mi>min</mml:mi><mml:mtext>&#x000A0;&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo><mml:mo stretchy='false'>(</mml:mo></mml:mstyle><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>f</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>k</mml:mi></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>f</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>k</mml:mi></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:mn>...</mml:mn><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>f</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>n</mml:mi></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>k</mml:mi></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo stretchy='false'>)</mml:mo></mml:mrow></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>s</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>.</mml:mo><mml:mi>t</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>.</mml:mo><mml:mo>:</mml:mo></mml:mstyle><mml:mtext>&#x000A0;&#x000A0;&#x000A0;&#x000A0;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>t</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>g</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo></mml:mstyle><mml:msup><mml:mi>&#x003B8;</mml:mi><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle></mml:msup></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo>&#x02264;</mml:mo><mml:mi>&#x003B4;</mml:mi></mml:mrow></mml:mtd></mml:mtr></mml:mtable></mml:mrow></mml:mrow></mml:math></disp-formula>
<p>where <bold><italic>k</italic></bold> is the number of iterations, <bold><italic>F</italic></bold>(<bold>&#x02981;</bold>) is the objective function, <bold><italic>f</italic></bold>(<bold>&#x02981;</bold>) is the sub-objective function, <bold>&#x003B8;<sup><italic>k</italic></sup></bold> is the movement direction of an agent, <bold><italic>g</italic></bold>(<bold>&#x02981;</bold>) is the constraint condition, and <bold>&#x003B4;</bold> is a preset value.</p></sec>
<sec>
<title>The Normalization of the Objective Function</title>
<p>The search process of geomagnetic perceiving navigation presents a posteriori and temporal characteristics, and the convergence of the geomagnetic multi-parameters has a strong incentive and a restriction relationship with the motion behavior of an agent. Considering that the above process is a multi-objective posterior search, the objective function needs to be established for the optimization. Based on the characteristics of geomagnetic fields, the sub-objective function of the geomagnetic parameter search is constructed as:</p>
<disp-formula id="E6"><label>(6)</label><mml:math id="M11"><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>f</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>k</mml:mi></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:msup><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo stretchy='false'>(</mml:mo><mml:mi>t</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo stretchy='false'>(</mml:mo><mml:mi>k</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>,</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle><mml:mo>&#x02264;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle><mml:mo>&#x02264;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>7</mml:mn></mml:mstyle></mml:math></disp-formula>
<p>where <bold><italic>B</italic></bold><sub><bold><italic>i</italic></bold></sub><bold>(<italic>t</italic>)</bold> is the <bold><italic>i</italic></bold>th geomagnetic parameter of the target location and <bold><italic>B</italic></bold><sub><bold><italic>i</italic></bold></sub><bold>(<italic>k</italic>)</bold> is the <bold><italic>i</italic></bold>th geomagnetic parameter of the current location. There are different magnitudes and units within the geomagnetic field parameters, the objective function should be normalized as:</p>
<disp-formula id="E7"><label>(7)</label><mml:math id="M12"><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle><mml:mo>)</mml:mo></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mfrac><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>N</mml:mi></mml:mstyle></mml:mfrac><mml:mstyle displaystyle='true'><mml:munderover><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>7</mml:mn></mml:mstyle></mml:munderover><mml:mrow><mml:mfrac><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>f</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mi>k</mml:mi></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>f</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi><mml:mo>,</mml:mo><mml:mn>0</mml:mn></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:mfrac></mml:mrow></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mfrac><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>N</mml:mi></mml:mstyle></mml:mfrac><mml:mstyle displaystyle='true'><mml:munderover><mml:mo>&#x02211;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>7</mml:mn></mml:mstyle></mml:munderover><mml:mrow><mml:mfrac><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo stretchy='false'>(</mml:mo><mml:mi>t</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo stretchy='false'>(</mml:mo><mml:mi>k</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow><mml:mrow><mml:msup><mml:mrow><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo stretchy='false'>(</mml:mo><mml:mi>t</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mo stretchy='false'>(</mml:mo><mml:mn>0</mml:mn><mml:mo stretchy='false'>)</mml:mo></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:msup></mml:mrow></mml:mfrac></mml:mrow></mml:mstyle></mml:math></disp-formula>
<p>The purpose of the geomagnetic perceiving navigation is that the objective function could converge to the optimal value in the search process, which can be expressed as:</p>
<disp-formula id="E8"><label>(8)</label><mml:math id="M13"><mml:mrow><mml:mo>&#x02016;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi><mml:mo stretchy='false'>(</mml:mo><mml:mi>k</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi><mml:mo stretchy='false'>(</mml:mo><mml:mi>k</mml:mi></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn><mml:mo stretchy='false'>)</mml:mo></mml:mstyle></mml:mrow><mml:mo>&#x02016;</mml:mo></mml:mrow><mml:mo>&#x02264;</mml:mo><mml:mi>&#x003B5;</mml:mi></mml:math></disp-formula>
<p>where <bold>&#x003B5;</bold> is a preset value.</p>
<p>Based on the above description, the geomagnetic perceiving navigation problem could be generalized as the multi-objective posterior search problem, by calculating the objective function to find the optimal solution.</p></sec></sec>
<sec id="s4">
<title>Algorithm Verification</title>
<p>To analyze the correlation of the geomagnetic data, the Hex-path algorithm (Russell, <xref ref-type="bibr" rid="B21">2004</xref>) will be adopted in the searching process of geomagnetic perceiving navigation. We know that the Hex-path algorithm has been used before, but only in odor source searching. The Hex-path odor searching algorithm could guide a mobile robot with a single gas sensor to search for an underground odor source. Here, the Hex-path algorithm is used to guide toward the target of lower objective function. Although the Hex-path algorithm is applied in different fields, the essence is the same, which means are all based on a change in the objective function. The major difference between the two is the fact that the Hex-path odor searching algorithm is guided toward regions of higher objective function. The reason for using this algorithm is the fact that it is simple and easy to implement step-by-step.</p>
<p>The Hex-path algorithm is implemented in the following steps:</p>
<p><bold>Step 1:</bold> <italic>Heading initialization</italic>. Randomly generate a number of <bold><italic>N</italic></bold> initial individuals in the heading space <bold><italic>Q</italic></bold>, which can be defined as <bold><italic>Q</italic></bold> <bold>&#x0003D;</bold> {<bold>&#x003B8;<italic><sub>i</sub></italic>|<italic>i</italic> &#x0003D; 1, 2,&#x022EF;, <italic>N</italic></bold> }. <bold>&#x003B8;<italic><sub>i</sub></italic></bold> can be expressed as:</p>
<disp-formula id="E9"><label>(9)</label><mml:math id="M14"><mml:msub><mml:mi>&#x003B8;</mml:mi><mml:mi>i</mml:mi></mml:msub><mml:mtext>&#x000A0;</mml:mtext><mml:mo>=</mml:mo><mml:mtext>&#x000A0;</mml:mtext><mml:mo>&#x00394;</mml:mo><mml:mi>&#x003B8;</mml:mi><mml:mo>&#x000D7;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>j</mml:mi><mml:mo>,</mml:mo><mml:mi>j</mml:mi></mml:mstyle><mml:mo>&#x02208;</mml:mo><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn><mml:mo>,</mml:mo><mml:mi>m</mml:mi></mml:mstyle></mml:mrow><mml:mo>]</mml:mo></mml:mrow></mml:math></disp-formula>
<p>where <inline-formula><mml:math id="M15"><mml:mstyle mathvariant="bold-italic"><mml:mi>m</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant="bold-italic"><mml:mo>=</mml:mo></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mfrac><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mn>2</mml:mn></mml:mstyle><mml:mstyle mathvariant="bold-italic"><mml:mi>&#x003C0;</mml:mi></mml:mstyle></mml:mrow><mml:mrow><mml:mstyle mathvariant="bold-italic"><mml:mi>&#x003B8;</mml:mi></mml:mstyle></mml:mrow></mml:mfrac></mml:math></inline-formula> and <bold>&#x025B3;</bold><italic>&#x003B8;</italic> is the sampling interval.</p>
<p><bold>Step 2:</bold> <italic>Heading selection</italic>. Randomly select the sample &#x003B8;<italic><sub>i</sub></italic>, and the probability of each selection is given by:</p>
<disp-formula id="E10"><label>(10)</label><mml:math id="M16"><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>p</mml:mi></mml:mstyle><mml:mo stretchy='false'>(</mml:mo><mml:msub><mml:mi>&#x003B8;</mml:mi><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mo stretchy='false'>)</mml:mo><mml:mo>=</mml:mo><mml:mtext>&#x000A0;</mml:mtext><mml:mfrac><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>N</mml:mi></mml:mstyle></mml:mfrac></mml:math></disp-formula>
<p><bold>Step 3:</bold> <italic>Heading updating rule</italic>. The objective function has been calculated at points (<bold><italic>k</italic>&#x02212;2</bold>) and (<bold><italic>k</italic>&#x02212;1</bold>). If the objective function at (<bold><italic>k</italic>&#x02212;1</bold>) decreases, it means that the target position is close to the right, and turning &#x025B3;<italic>&#x003B8;</italic> to the clockwise (CW) direction could be performed. Otherwise, turning &#x025B3;<italic>&#x003B8;</italic> to the counterclockwise (CCW) direction could be performed. The heading updating rule is shown in <xref ref-type="fig" rid="F4">Figure 4</xref>.</p>
<disp-formula id="E11"><label>(11)</label><mml:math id="M17"><mml:mrow><mml:mo>{</mml:mo><mml:mrow><mml:mtable><mml:mtr><mml:mtd><mml:mrow><mml:msup><mml:mi>&#x003B8;</mml:mi><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow></mml:msup><mml:mtext>&#x000A0;</mml:mtext><mml:mo>=</mml:mo><mml:mtext>&#x000A0;</mml:mtext><mml:msup><mml:mi>&#x003B8;</mml:mi><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle></mml:msup><mml:mo>&#x02212;</mml:mo><mml:mo>&#x00394;</mml:mo><mml:mi>&#x003B8;</mml:mi><mml:mo>,</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mi>f</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo>&#x02264;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>a</mml:mi><mml:mi>n</mml:mi><mml:mi>d</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>d</mml:mi></mml:mstyle><mml:mtext>&#x003B8;</mml:mtext><mml:mo>&#x0003C;</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mrow><mml:msup><mml:mi>&#x003B8;</mml:mi><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mstyle></mml:mrow></mml:msup><mml:mtext>&#x000A0;</mml:mtext><mml:mo>=</mml:mo><mml:mtext>&#x000A0;</mml:mtext><mml:msup><mml:mi>&#x003B8;</mml:mi><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle></mml:msup><mml:mo>+</mml:mo><mml:mo>&#x00394;</mml:mo><mml:mi>&#x003B8;</mml:mi><mml:mo>,</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi><mml:mi>f</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi></mml:mstyle><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo>&#x0003E;</mml:mo><mml:mi>F</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>2</mml:mn></mml:mstyle></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>a</mml:mi><mml:mi>n</mml:mi><mml:mi>d</mml:mi></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>d</mml:mi></mml:mstyle><mml:mtext>&#x003B8;</mml:mtext><mml:mo>&#x0003E;</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:mtd></mml:mtr></mml:mtable></mml:mrow></mml:mrow></mml:math></disp-formula>
<fig id="F4" position="float">
<label>Figure 4</label>
<caption><p>The turning selection diagram.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0004.tif"/>
</fig>
<p>where <bold><italic>d</italic></bold>&#x003B8; &#x0003D; &#x003B8;<bold><italic><sup>k</sup></italic></bold>&#x02212;&#x003B8;<bold><italic><sup>k&#x02212;1</sup></italic></bold>.</p>
<p><bold>Step 4:</bold> <italic>Terminate condition</italic>. If the search algorithm meets the termination condition (12), the search algorithm will terminate; otherwise go to step (3) above.</p>
<disp-formula id="E12"><label>(12)</label><mml:math id="M18"><mml:mrow><mml:mo>{</mml:mo><mml:mrow><mml:mtable><mml:mtr><mml:mtd><mml:mrow><mml:mrow><mml:mo>&#x02016;</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi><mml:mo stretchy='false'>(</mml:mo><mml:mi>k</mml:mi><mml:mo stretchy='false'>)</mml:mo></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>F</mml:mi><mml:mo stretchy='false'>(</mml:mo><mml:mi>k</mml:mi></mml:mstyle><mml:mo>&#x02212;</mml:mo><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>1</mml:mn><mml:mo stretchy='false'>)</mml:mo></mml:mstyle></mml:mrow><mml:mo>&#x02016;</mml:mo></mml:mrow><mml:mo>&#x02264;</mml:mo><mml:mi>&#x003B5;</mml:mi></mml:mrow></mml:mtd></mml:mtr><mml:mtr><mml:mtd><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mn>0</mml:mn></mml:mstyle><mml:mtext>&#x000A0;</mml:mtext><mml:mo>&#x02264;</mml:mo><mml:mtext>&#x000A0;</mml:mtext><mml:msub><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>g</mml:mi></mml:mstyle><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>i</mml:mi></mml:mstyle></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>B</mml:mi></mml:mstyle><mml:mo>,</mml:mo><mml:msup><mml:mi>&#x003B8;</mml:mi><mml:mstyle mathvariant='bold-italic' mathsize='normal'><mml:mi>k</mml:mi></mml:mstyle></mml:msup></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo>&#x02264;</mml:mo><mml:mi>&#x003B4;</mml:mi></mml:mrow></mml:mtd></mml:mtr></mml:mtable></mml:mrow></mml:mrow></mml:math></disp-formula>
</sec>
<sec sec-type="results" id="s5">
<title>Results</title>
<p>To show the correlation of geomagnetic field characteristics, numerical simulations are implemented.</p>
<sec>
<title>Simulation Setup</title>
<p>The Word Magnetic Model (WMM2015) is used to provide real-time geomagnetic data. Simulations have been carried out based on the seven physical fields.</p>
<p>In simulations, we choose a rectangular area from 20&#x000B0; north latitude and 85&#x000B0; west longitude (20N, 85W) to 45&#x000B0; north latitude and 125&#x000B0; west longitude (45N, 125W). In this scenario, the starting position is the red square &#x0201C;&#x025A1;,&#x0201D; and the target position is the red triangle &#x0201C;&#x025BD;,&#x0201D; which are depicted in <xref ref-type="fig" rid="F5">Figure 5</xref>. The simulation parameter are listed in <xref ref-type="table" rid="T2">Table 2</xref>.</p>
<fig id="F5" position="float">
<label>Figure 5</label>
<caption><p>Geomagnetic perceiving navigation within the Hex-path algorithm.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0005.tif"/>
</fig>
<table-wrap position="float" id="T2">
<label>Table 2</label>
<caption><p>Setting navigation parameters.</p></caption>
<table frame="hsides" rules="groups">
<thead><tr>
<th valign="top" align="left"><bold>No</bold></th>
<th valign="top" align="left"><bold>Parameters</bold></th>
<th valign="top" align="center"><bold>Size</bold></th>
</tr>
</thead>
<tbody>
<tr>
<td valign="top" align="left">1</td>
<td valign="top" align="left">&#x025B3;&#x003B8;</td>
<td valign="top" align="center">60&#x000B0;</td>
</tr>
<tr>
<td valign="top" align="left">2</td>
<td valign="top" align="left"> &#x003B5;</td>
<td valign="top" align="center">0.001</td>
</tr>
<tr>
<td valign="top" align="left">3</td>
<td valign="top" align="left">&#x003B4;</td>
<td valign="top" align="center">0.05</td>
</tr>
<tr>
<td valign="top" align="left">4</td>
<td valign="top" align="left"><italic>d</italic><sub><italic>k</italic></sub></td>
<td valign="top" align="center">10 km</td>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
<sec>
<title>Simulation Results</title>
<p>To demonstrate the effectiveness of the Hex-path algorithm, several simulation results are given in this paper. As presented in <xref ref-type="fig" rid="F5">Figure 5</xref>, three different starting positions and target positions are randomly selected.</p>
<p><xref ref-type="fig" rid="F5">Figure 5</xref> illustrates the searching trajectory of the Hex-path algorithm, where the heading is updated continuously depending on the objective function at <bold><italic>k</italic></bold> and <bold><italic>k</italic>&#x02212;1</bold>. The searching results of the Hex-path algorithm present a good global search capability.</p>
<p><xref ref-type="fig" rid="F6">Figure 6</xref> illustrates the convergence property of the normalized objective function in three cases, from which we can see that the number of iterations is 286, 292, and 215 for the Hex-path algorithm, respectively. The results show that the Hex-path algorithm could converge to a stable state.</p>
<fig id="F6" position="float">
<label>Figure 6</label>
<caption><p>The convergence curves of the normalized objective function.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0006.tif"/>
</fig>
<p>The similarity in the number patterns of the normalized objective function shows the robustness of the Hex-path algorithm mentioned above. <xref ref-type="fig" rid="F6">Figure 6</xref> illustrates the steady states of the seven geomagnetic parameters.</p></sec>
<sec>
<title>Analysis of the Results</title>
<p><xref ref-type="fig" rid="F7">Figure 7</xref> shows the convergence property of the geomagnetic multi-parameter, wherein <xref ref-type="fig" rid="F7">Figure 7A</xref> represents the above Case 1, <xref ref-type="fig" rid="F7">Figure 7B</xref> represents the above Case 2, and <xref ref-type="fig" rid="F7">Figure 7C</xref> represents the above Case 3. The convergence curves of the sub-objective function show a significant difference between the multi-parameters.</p>
<fig id="F7" position="float">
<label>Figure 7</label>
<caption><p>The geomagnetic multi-parameter convergence curves. <bold>(A)</bold> Case 1, <bold>(B)</bold> Case 2, and <bold>(C)</bold> Case 3.</p></caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fnbot-15-785563-g0007.tif"/>
</fig>
<p>As can be seen in <xref ref-type="fig" rid="F7">Figure 7</xref>, the variation rules of the geomagnetic multi-parameters reflect the correlation on the geomagnetic field parameters, where the north geomagnetic field component <bold><italic>B</italic></bold><sub><bold><italic>X</italic></bold></sub> and the horizontal magnetic field component <bold><italic>B</italic></bold><sub><bold><italic>H</italic></bold></sub> show the same convergent state of change, the east geomagnetic field component <bold><italic>B</italic></bold><sub><bold><italic>Y</italic></bold></sub> and the declination angle <bold><italic>B</italic></bold><sub><bold><italic>D</italic></bold></sub> show the same convergent state of change, and the vertical geomagnetic field component <bold><italic>B</italic></bold><sub><bold><italic>Z</italic></bold></sub>, the inclination angle <bold><italic>B</italic></bold><sub><bold><italic>I</italic></bold></sub>, and the total intensity <bold><italic>B</italic></bold><sub><bold><italic>F</italic></bold></sub> show the same convergent state of change. Meanwhile, the same results can be seen in <xref ref-type="table" rid="T1">Table 1</xref>.</p>
<p>From the above analyses, we clearly see that the Hex-path algorithm ensures the desired target position depends on geomagnetic environment stimuli during the training iteration, and the learning task of each iteration also depends on those of the completed ones. The similarity in the number patterns of the objective function shows the consistency of the convergence trend on the correlation analysis results mentioned above.</p></sec></sec>
<sec sec-type="conclusions" id="s6">
<title>Conclusion</title>
<p>This paper discusses the correlation of geomagnetic field parameters, and the simulations show the same results as in <xref ref-type="table" rid="T1">Table 1</xref>. The results show the same convergent state for the approximate correlation coefficient. In other words, the approximate correlation coefficient between the geomagnetic parameters can be regarded as one class. The correlation study on geomagnetic field parameters is crucially important for geomagnetic navigation technology and introducing the selection and application of geomagnetic field parameters.</p>
<p>In the future, we will focus on the research of geomagnetic perceiving navigation methods, but the study of the correlation of geomagnetic field parameters is very necessary for geomagnetic perceiving navigation technology. We know that getting the right geomagnetic parameters might affect the navigation result, and it is still important for navigational efficiency. Therefore, this paper mainly discusses the correlation of geomagnetic field parameters for geomagnetic perceiving navigation technology. The present study is the preliminary preparation and the theoretical foundation for the follow-up work.</p></sec>
<sec sec-type="data-availability" id="s7">
<title>Data Availability Statement</title>
<p>The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.</p></sec>
<sec id="s8">
<title>Author Contributions</title>
<p>HL, JQ, XJ, and YN designed and established the theoretical model. HL wrote the paper and performed the experiments. JQ provided some ideas to improve and perfect the paper. YN analyzed the data. XJ reviewed and edited the manuscript. All authors read and approved the manuscript.</p></sec>
<sec sec-type="funding-information" id="s9">
<title>Funding</title>
<p>This work was supported by the Scientific Research Project of Education Department of Shaanxi Province Fund under grant 20JK0915, the General Special Project of Shaanxi Science and Technology Department under Grant No. 2021JQ-714, the XI&#x00027;AN Key Laboratory of Advanced Control and Intelligent Process under Grant No. 2019220714SYS022CG044, and the Laboratory of Science and Technology on Marine Navigation and Control under Grant No. 2021010106.</p></sec>
<sec sec-type="COI-statement" id="conf1">
<title>Conflict of Interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p></sec>
<sec sec-type="disclaimer" id="s10">
<title>Publisher&#x00027;s Note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p></sec>
</body>
<back>
<ack><p>The authors are grateful for the constructive suggestions from reviewers that significantly enhanced the presentation of this paper.</p>
</ack>
<ref-list>
<title>References</title>
<ref id="B1">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Caifa</surname> <given-names>G.</given-names></name> <name><surname>Li</surname> <given-names>A.</given-names></name> <name><surname>Hong</surname> <given-names>C.</given-names></name> <name><surname>Yang</surname> <given-names>H.</given-names></name></person-group> (<year>2011</year>). <article-title>&#x0201C;Algorithm for geomagnetic navigation and its validity evaluation,&#x0201D;</article-title> in <source>International Conference on Computer Science and Automation Engineering (CSAE)</source>, <volume>Shanghai</volume>, <fpage>573</fpage>&#x02013;<lpage>577</lpage>.</citation>
</ref>
<ref id="B2">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Chulliat</surname> <given-names>A.</given-names></name> <name><surname>Macmillan</surname> <given-names>S.</given-names></name> <name><surname>Alken</surname> <given-names>P.</given-names></name> <name><surname>Beggan</surname> <given-names>C.</given-names></name> <name><surname>Nair</surname> <given-names>M.</given-names></name> <name><surname>Hamilton</surname> <given-names>B.</given-names></name></person-group> (<year>2015</year>). <source>The US/UK World Magnetic Model for 2015-2020.</source> Available online at: http:no-ra.nerc.ac.uk/18737/</citation>
</ref>
<ref id="B3">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Dennis</surname> <given-names>T. E.</given-names></name> <name><surname>Rayner</surname> <given-names>M. J.</given-names></name> <name><surname>Walker</surname> <given-names>M. M.</given-names></name></person-group> (<year>2007</year>). <article-title>Evidence that pigeons orient to geomagnetic intensity during homing</article-title>. <source>Proc. R. Soc. B Biol. Sci.</source> <volume>274</volume>, <fpage>1153</fpage>&#x02013;<lpage>1158</lpage>. <pub-id pub-id-type="doi">10.1098/rspb.2007.3768</pub-id><pub-id pub-id-type="pmid">17301015</pub-id></citation></ref>
<ref id="B4">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Ge</surname> <given-names>Z.</given-names></name> <name><surname>Zhou</surname> <given-names>J.</given-names></name></person-group> (<year>2007</year>). <article-title>&#x0201C;A new approach to geomagnetic matching navigation,&#x0201D;</article-title> in <source>International Conference on Spatial Information Technology</source>, <volume>Wuhan</volume>, <fpage>67952</fpage>&#x02013;<lpage>67956</lpage>. <pub-id pub-id-type="doi">10.1117/12.774829</pub-id><pub-id pub-id-type="pmid">17645476</pub-id></citation></ref>
<ref id="B5">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Hu</surname> <given-names>L.</given-names></name> <name><surname>Chan</surname> <given-names>K.</given-names></name> <name><surname>Yuan</surname> <given-names>X.</given-names></name> <name><surname>Xiong</surname> <given-names>S.</given-names></name></person-group> (<year>2019</year>). <article-title>&#x0201C;A variational bayesian framework for cluster analysis in a complex network,&#x0201D;</article-title> in <source>IEEE Transactions on Knowledge and Data Engineering</source>. <pub-id pub-id-type="doi">10.1109/TKDE.2019.2914200</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B6">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Hu</surname> <given-names>L.</given-names></name> <name><surname>Pan</surname> <given-names>X.</given-names></name> <name><surname>Tan</surname> <given-names>Z.</given-names></name> <name><surname>Luo</surname> <given-names>X.</given-names></name></person-group> (<year>2021</year>). <article-title>A fast fuzzy clustering algorithm for complex networks via a generalized momentum method</article-title>. <source>IEEE Trans. Fuzzy Syst.</source> <volume>14</volume>, <fpage>1</fpage>&#x02013;<lpage>13</lpage>. <pub-id pub-id-type="doi">10.1109/TFUZZ.2021.3117442</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B7">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Hu</surname> <given-names>L.</given-names></name> <name><surname>Zhang</surname> <given-names>J.</given-names></name> <name><surname>Pan</surname> <given-names>X.</given-names></name> <name><surname>Yan</surname> <given-names>H.</given-names></name> <name><surname>You</surname> <given-names>Z.</given-names></name></person-group> (<year>2020</year>). <article-title>HiSCF: leveraging higher-order structures for clustering analysis in biological networks</article-title>. <source>Bioinformatics</source> <volume>37</volume>, <fpage>542</fpage>&#x02013;<lpage>550</lpage>. <pub-id pub-id-type="doi">10.1093/bioinformatics/btaa775</pub-id><pub-id pub-id-type="pmid">32931549</pub-id></citation></ref>
<ref id="B8">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Jiang</surname> <given-names>L.</given-names></name> <name><surname>Ran</surname> <given-names>L.</given-names></name></person-group> (<year>2011</year>). <article-title>&#x0201C;Pure geomagnetic homing navigation on earth surface,&#x0201D;</article-title> in <source>International Conference on Electronics, Communications and Control (ICECC)</source>, <volume>Ningbo</volume>, <fpage>971</fpage>&#x02013;<lpage>974</lpage>. <pub-id pub-id-type="doi">10.1109/ICECC.2011.6066720</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B9">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Kebria</surname> <given-names>P. M.</given-names></name> <name><surname>Khosravi</surname> <given-names>A.</given-names></name> <name><surname>Salaken</surname> <given-names>S. M.</given-names></name> <name><surname>Nahavandi</surname> <given-names>S.</given-names></name></person-group> (<year>2020</year>). <article-title>Deep imitation learning for autonomous vehicles based on convolutional neural networks</article-title>. <source>IEEE/CAA J. Autom. Sinica</source> <volume>1</volume>, <fpage>82</fpage>&#x02013;<lpage>95</lpage>. <pub-id pub-id-type="doi">10.1109/JAS.2019.1911825</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B10">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Kinsey</surname> <given-names>J. C.</given-names></name> <name><surname>Eustice</surname> <given-names>R. M.</given-names></name> <name><surname>Whitcomb</surname> <given-names>L. L.</given-names></name></person-group> (<year>2006</year>). <article-title>A survey of underwater vehicle navigation: Recent advances and new challenges</article-title>. <source>IFAC Conf. Manoeuver. Contr. Mar. Craft</source> <volume>88</volume>, <fpage>20090</fpage>&#x02013;<lpage>20102</lpage>. <pub-id pub-id-type="doi">10.1.1/134.5601</pub-id></citation>
</ref>
<ref id="B11">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Kong</surname> <given-names>L.</given-names></name> <name><surname>Qin</surname> <given-names>K.</given-names></name> <name><surname>Long</surname> <given-names>T.</given-names></name></person-group> (<year>2012</year>). <article-title>Global SST data mining based on fuzzy clustering</article-title>. <source>Geomat. Inform. Sci. Wuhan Univer.</source> <volume>37</volume>, <fpage>215</fpage>&#x02013;<lpage>219</lpage>. <pub-id pub-id-type="doi">10.13203/j.whugis2012.02.027</pub-id></citation>
</ref>
<ref id="B12">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Kramer</surname> <given-names>G..</given-names></name></person-group> (<year>1953</year>). <article-title>Wird die Sonnenhhe bei der Heimfinderorientierung verwertet?</article-title> <source>J. Ornithol.</source> <volume>94</volume>, <fpage>201</fpage>&#x02013;<lpage>219</lpage>. <pub-id pub-id-type="doi">10.1007/BF01922508</pub-id></citation>
</ref>
<ref id="B13">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Lin</surname> <given-names>Y.</given-names></name> <name><surname>Yan</surname> <given-names>L.</given-names></name> <name><surname>Tong</surname> <given-names>Q.</given-names></name></person-group> (<year>2007</year>). <article-title>&#x0201C;Underwater geomagnetic navigation based on ICP algorithm,&#x0201D;</article-title> in <source>IEEE International Conference on Robotics and Biomimetics</source>, <volume>Sanya</volume>, <fpage>2115</fpage>&#x02013;<lpage>2120</lpage>.</citation>
</ref>
<ref id="B14">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Liu</surname> <given-names>M.</given-names></name> <name><surname>Liu</surname> <given-names>K.</given-names></name> <name><surname>Peng</surname> <given-names>X.</given-names></name> <name><surname>Hong</surname> <given-names>L.</given-names></name></person-group> (<year>2014</year>). <article-title>&#x0201C;Bio-inspired navigation based on geomagnetic for the autonomous underwater vehicle,&#x0201D;</article-title> in <source>Oceans 2014 &#x02013; Taipei</source>, <volume>Taipei</volume>, <fpage>1</fpage>&#x02013;<lpage>5</lpage>. <pub-id pub-id-type="doi">10.1109/OCEANS-TAIPEI.2014.6964446</pub-id><pub-id pub-id-type="pmid">28747884</pub-id></citation></ref>
<ref id="B15">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Luo</surname> <given-names>X.</given-names></name> <name><surname>Liu</surname> <given-names>Z. L.</given-names></name> <name><surname>Jin</surname> <given-names>Z. Y.</given-names></name> <name><surname>Zhou</surname> <given-names>M.</given-names></name></person-group> (<year>2021a</year>). <article-title>Symmetric nonnegative matrix factorization-based community detection models and their convergence analysis</article-title>. <source>IEEE Trans. Neural Netw. Learn. Syst.</source> <volume>10</volume>, <fpage>1</fpage>&#x02013;<lpage>13</lpage>. <pub-id pub-id-type="doi">10.1109/TNNLS.2020.3041360</pub-id><pub-id pub-id-type="pmid">33513110</pub-id></citation></ref>
<ref id="B16">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Luo</surname> <given-names>X.</given-names></name> <name><surname>Qin</surname> <given-names>W.</given-names></name> <name><surname>Dong</surname> <given-names>A.</given-names></name> <name><surname>Sedraoui</surname> <given-names>K.</given-names></name> <name><surname>Zhou</surname> <given-names>M. C.</given-names></name></person-group> (<year>2021b</year>). <article-title>Efficient and high-quality recommendations via momentum-incorporated parallel stochastic gradient descent-based learning</article-title>. <source>IEEE/CAA J. Automat. Sinica</source> <volume>8</volume>, <fpage>402</fpage>&#x02013;<lpage>411</lpage>. <pub-id pub-id-type="doi">10.1109/JAS.2020.1003396</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B17">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Luo</surname> <given-names>X.</given-names></name> <name><surname>Zhou</surname> <given-names>Y.</given-names></name> <name><surname>Liu</surname> <given-names>Z.</given-names></name> <name><surname>Hu</surname> <given-names>L.</given-names></name> <name><surname>Zhou</surname> <given-names>M.</given-names></name></person-group> (<year>2021c</year>). <article-title>Generalized nesterov&#x00027;s acceleration-incorporated non-negative and adaptive latent factor analysis</article-title>. <source>IEEE Trans. Serv. Comput.</source> <volume>99</volume>, <fpage>1</fpage>&#x02013;<lpage>14</lpage>. <pub-id pub-id-type="doi">10.1109/TSC.2021.3069108</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B18">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Qiao</surname> <given-names>Y.</given-names></name> <name><surname>Wang</surname> <given-names>S.</given-names></name> <name><surname>Qi</surname> <given-names>Z.</given-names></name></person-group> (<year>2007</year>). <article-title>Selection of the characteristic variable of geomagnetism for matching</article-title>. <source>Seismol. Geomagnet. Observ. Res.</source> <volume>28</volume>, <fpage>42</fpage>&#x02013;<lpage>47</lpage>. <pub-id pub-id-type="doi">10.3969/j.issn.1003-3246.2007.01.007</pub-id></citation>
</ref>
<ref id="B19">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Rodda</surname> <given-names>G. H..</given-names></name></person-group> (<year>1984</year>). <article-title>The orientation and navigation of juvenile alligators: evidence of magnetic sensitivity</article-title>. <source>J. Comp. Physiol.</source> <volume>154</volume>, <fpage>649</fpage>&#x02013;<lpage>658</lpage>. <pub-id pub-id-type="doi">10.1007/BF01350218</pub-id></citation>
</ref>
<ref id="B20">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Rong</surname> <given-names>M. O..</given-names></name></person-group> (<year>2016</year>). <article-title>&#x0201C;Studying on comparison of different geomagnetic matching navigation algorithms,&#x0201D;</article-title> in <source>Geomatics and Spatial Information Technology</source>, <fpage>1</fpage>&#x02013;<lpage>5</lpage>.</citation>
</ref>
<ref id="B21">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Russell</surname> <given-names>R. A..</given-names></name></person-group> (<year>2004</year>). <article-title>&#x0201C;Chemical source location and the RoboMole project,&#x0201D;</article-title> in <source>Proceedings of the Australasian Conference on Robotics and Automation</source>, Canberra.</citation>
</ref>
<ref id="B22">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Saito</surname> <given-names>H.</given-names></name> <name><surname>Jin</surname> <given-names>C.</given-names></name> <name><surname>Ishio</surname> <given-names>S.</given-names></name></person-group> (<year>1999</year>). <article-title>Principle of magnetic field analysis by MFM signal transformation and its application to magnetic</article-title>. <source>IEEE Trans. Magnet.</source> <volume>35</volume>, <fpage>3992</fpage>&#x02013;<lpage>3992</lpage>. <pub-id pub-id-type="doi">10.1109/20.800732</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B23">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Shang</surname> <given-names>M. S.</given-names></name> <name><surname>Luo</surname> <given-names>X.</given-names></name> <name><surname>Liu</surname> <given-names>Z.</given-names></name> <name><surname>Chen</surname> <given-names>J.</given-names></name> <name><surname>Yuan</surname> <given-names>Y.</given-names></name> <name><surname>Zhou</surname> <given-names>M. C.</given-names></name></person-group> (<year>2019</year>). <article-title>Randomized latent factor model for high-dimensional and sparse matrices from industrial applications</article-title>. <source>IEEE/CAA J. Automat. Sinica</source> <volume>6</volume>, <fpage>131</fpage>&#x02013;<lpage>141</lpage>. <pub-id pub-id-type="doi">10.1109/JAS.2018.7511189</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B24">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Walker</surname> <given-names>M. M.</given-names></name> <name><surname>Dennis</surname> <given-names>T. E.</given-names></name> <name><surname>Kirschvink</surname> <given-names>J. L.</given-names></name></person-group> (<year>2002</year>). <article-title>The magnetic sense and its use in long-distance navigation by animals</article-title>. <source>Curr. Opin. Neurobiol.</source> <volume>12</volume>, <fpage>735</fpage>&#x02013;<lpage>744</lpage>. <pub-id pub-id-type="doi">10.1016/S0959-4388(02)00389-6</pub-id><pub-id pub-id-type="pmid">12490267</pub-id></citation></ref>
<ref id="B25">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Walker</surname> <given-names>M. M.</given-names></name> <name><surname>Diebel</surname> <given-names>C. E.</given-names></name> <name><surname>Haugh</surname> <given-names>C. V.</given-names></name> <name><surname>Pankhurst</surname> <given-names>P. M.</given-names></name> <name><surname>Montgomery</surname> <given-names>J. C.</given-names></name> <name><surname>Green</surname> <given-names>C. R.</given-names></name></person-group> (<year>1997</year>). <article-title>Structure and function of the vertebrate magnetic sense</article-title>. <source>Nature</source> <volume>390</volume>, <fpage>371</fpage>&#x02013;<lpage>376</lpage>. <pub-id pub-id-type="doi">10.1038/37057</pub-id><pub-id pub-id-type="pmid">20358649</pub-id></citation></ref>
<ref id="B26">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Wei</surname> <given-names>Q. I.</given-names></name> <name><surname>Wang</surname> <given-names>X. F.</given-names></name> <name><surname>Xi-Hai</surname> <given-names>L. I.</given-names></name> <name><surname>Liu</surname> <given-names>D. Z.</given-names></name></person-group> (<year>2010</year>). <article-title>Selection of characteristic components for geomagnetic matching based on statistical modeling</article-title>. <source>Progr. Geophys.</source> <volume>25</volume>, <fpage>324</fpage>&#x02013;<lpage>330</lpage>. <pub-id pub-id-type="doi">10.1017/S0004972710001772</pub-id><pub-id pub-id-type="pmid">30886898</pub-id></citation></ref>
<ref id="B27">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Wu</surname> <given-names>H.</given-names></name> <name><surname>Luo</surname> <given-names>X.</given-names></name> <name><surname>Zhou</surname> <given-names>M. C.</given-names></name></person-group> (<year>2020</year>). <article-title>Advancing non-negative latent factorization of tensors with diversified regularizations</article-title>. <source>IEEE Trans. Serv. Comput.</source> <volume>8</volume>, <fpage>1</fpage>&#x02013;<lpage>13</lpage>. <pub-id pub-id-type="doi">10.1109/TSC.2020.2988760</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
<ref id="B28">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Xiao</surname> <given-names>J.</given-names></name> <name><surname>Duan</surname> <given-names>X.</given-names></name> <name><surname>Qi</surname> <given-names>X.</given-names></name> <name><surname>Liu</surname> <given-names>Y.</given-names></name></person-group> (<year>2019</year>). <article-title>An improved ICCP matching algorithm for use in an interference environment during geomagnetic navigation</article-title>. <source>J. Navigat.</source> <volume>73</volume>, <fpage>1</fpage>&#x02013;<lpage>19</lpage>. <pub-id pub-id-type="doi">10.1017/S0373463319000535</pub-id><pub-id pub-id-type="pmid">30886898</pub-id></citation></ref>
<ref id="B29">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Yuan</surname> <given-names>S.</given-names></name> <name><surname>Zhang</surname> <given-names>J.</given-names></name> <name><surname>Wang</surname> <given-names>S.</given-names></name> <name><surname>Wei</surname> <given-names>J.</given-names></name></person-group> (<year>2011</year>). <article-title>&#x0201C;Research on real-time route planning for unmanned aircraft in geomagnetic matching guidance,&#x0201D;</article-title> in <source>IEEE</source>. International Conference on Mechatronics and Automation, <volume>Beijing</volume>, <fpage>197</fpage>&#x02013;<lpage>202</lpage>.</citation>
</ref>
<ref id="B30">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Zhang</surname> <given-names>Y.</given-names></name> <name><surname>Liu</surname> <given-names>X.</given-names></name> <name><surname>Luo</surname> <given-names>M.</given-names></name> <name><surname>Yang</surname> <given-names>C.</given-names></name></person-group> (<year>2019</year>). <article-title>Bio-Inspired approach for long-range underwater navigation using model predictive control</article-title>. <source>IEEE transactions on cybernetics</source>. <volume>2019</volume>, <fpage>2933</fpage>&#x02013;<lpage>2937</lpage>. <pub-id pub-id-type="doi">10.1109/TCYB.2019.2933397</pub-id><pub-id pub-id-type="pmid">31449042</pub-id></citation></ref>
<ref id="B31">
<citation citation-type="journal"><person-group person-group-type="author"><name><surname>Zhao</surname> <given-names>Z.</given-names></name> <name><surname>Hu</surname> <given-names>T.</given-names></name> <name><surname>Cui</surname> <given-names>W.</given-names></name> <name><surname>Huangfu</surname> <given-names>J.</given-names></name> <name><surname>Li</surname> <given-names>C.</given-names></name></person-group> (<year>2014</year>). <article-title>Long-Distance geomagnetic navigation: imitations of animal migration based on a new assumption</article-title>. <source>IEEE Trans. Geosci. Remote Sens.</source> <volume>52</volume>, <fpage>6715</fpage>&#x02013;<lpage>6723</lpage>. <pub-id pub-id-type="doi">10.1109/TGRS.2014.2301441</pub-id><pub-id pub-id-type="pmid">27295638</pub-id></citation></ref>
</ref-list> 
</back>
</article>