# Review of Structural Modal Tracking in Operational Modal Analysis: Methods and Applications
## 6 Conclusions  
This paper commenced with a concise review of the procedural framework and inherent limitations of current automated modal estimation methods. Subsequently, it provided a comprehensive summary of methodological developments and practical applications of modal tracking. The standard modal tracking process comprises two sequential stages: reference mode determination and current mode linking. Furthermore, the determination of reference modes is systematically categorized into two primary strategies: (i) clustering based on multiple prior modes and (ii) mode estimation based on the most recent response set. Mode linking is implemented by predefined criterion thresholds or the closest similarity. The advantages and limitations of the two prevalent methods were assessed through a comparative analysis of field measurements from the Shanghai Tower. The critical conclusions and suggestions for future research are summarized below:  
1. The study of automatic modal estimation has been common, particularly focusing on automatic interpretation of stabilization diagrams via clustering algorithms. Future research should emphasize the development of adaptive clustering thresholds and the establishment of universal estimation frameworks adaptable to diverse structural typologies. 2. The primary objective of modal tracking lies in extracting long-term modal evolution and keeping consistent modal orders. Established modal tracking methods predominantly employ clustering algorithms for reference mode determination, with mode linking typically implemented through defined criteria that integrate the MAC and relative frequency differences. This demonstrates significant research potential for automatic modal tracking in the future. Building upon adaptive updating for both reference modes and modal linking criteria, a pivotal challenge for modal tracking methods is balancing the capacity between the removal of spurious modes and the tracking of rapidly changing physical modes. * While structural long-term modal evolution analysis has gained engineering interest, particularly in investigating temperature-frequency coupling models, the current immaturity of modal tracking methods constrains their popularization in various scenarios. Future research should prioritize enhanced automation in two modal tracking stages and complexity reduction in operational procedures to facilitate broader implementation across diverse structural systems. Resources, S.F. and J.W. ; investigation, S.F. and J.W. ; software, S.F. ; data curation, J.W. ; writing--original draft preparation, S.F. ; writing--review and editing, J.W. ; visualization, S.F. ; supervision, J.W. ; project administration, J.W. ; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript. This research was supported by National Key R&D Program of China [2023 YFC3805700], Shanghai Qi Zhi Institute Innovation Program [SQZ202310], and Tongji University Interdisciplinary Research Project [2023-2-YB-06]. Not applicable. No new data were created or analyzed in this study. The authors declare no conflicts of interest. The following abbreviations are used in this manuscript: