# Review of Structural Modal Tracking in Operational Modal Analysis: Methods and Applications
## 1 Introduction
Notably, modal damping ratios are generally excluded from reference modes due to their inherent uncertainty. The seminal tracking framework proposed by [PERSON] et al. [16] in 2009 remains influential, combining a preselected reference mode with a set of static thresholds to define which modes share the same modal properties. Specifically, hierarchical clustering is used to determine the reference mode, and the current mode is only linked to the reference mode if their modal assurance criterion (MAC) values exceed 0.8 and their relative frequency differences are below 15%. This framework has been extensively embraced by the scientific community and effectively implemented in diverse structures [38; 39; 40]. Many subsequent studies have essentially built upon and improved this process [41]. However, the numbers and contents of estimated modes vary over time under intricate operating conditions, necessitating regular manual threshold recalibration to avert tracking failures [42]. Traditional methods often struggle to ensure accurate matching and effective tracking of modal parameters. [PERSON] et al. [37] alleviated the limitations of these similarity criteria to a degree through dynamic updating strategies. [PERSON] et al. [43] developed a multistage subspace correlation technique that recursively updates the baseline list based on modal observability vector analysis. These innovations demonstrate progressive improvements in tracking methods in the presence of environmental uncertainty. It is noteworthy that rational sensor placement plays a crucial role in modal tracking implementation, particularly due to the prevalent use of mode shape-based criteria for distinguishing closely spaced or time-varying structural modes. While a comprehensive sensor array enhances tracking accuracy, it typically incurs higher costs for data acquisition and system maintenance. Conversely, reduced sensor configurations can lower both economic expenditures and computational time, albeit at the potential expense of information fidelity [44]. An optimal sensor placement (OSP) strategy should maximize structural measurement information while maintaining sensitivity to modal parameter variations and considering cost-effectiveness [45]. Currently, OSP methodologies have reached a mature stage of development, such as established techniques based on weighted standard deviation norm (WSDN) index [46], genetic algorithm [47], and firefly algorithm implementations [48]. In terms of engineering applications, modal tracking plays a critical role in capturing the long-term evolution of dynamic characteristics across diverse infrastructure, including bridges [49, 50], historic heritage construction [51, 52], super high-rise buildings [53, 54], wind turbines [55, 56, 57], stadiums [58], and concrete dams [59, 60]. [PERSON] et al. [61] utilized a Gaussian mixture model (GMM) to obtain clusters representing baseline modal properties and successfully tracked the modal evolution of a long-span bridge. [PERSON] et al. [62] and [PERSON] et al. [63] assessed the impacts of temperature variations on the natural frequencies of a monumental masonry bell tower and a church, respectively. Modal tracking has developed into a crucial study area over the past decades, holding significant potential for both theoretical progress and engineering practice. The existing literature lacks a comprehensive review of structural modal tracking. Therefore, it is necessary to summarize the principal findings and applications of existing research to delineate its development and provide recommendations for possible future work. As a prerequisite for modal tracking, Section 2 succinctly reviews the core content of AMPE. Section 3 offers a comprehensive overview of the steps and development of modal tracking methods, comparing the merits and drawbacks of two prevalent approaches. Section 4 delineates the application areas of modal tracking and corresponding results. Section 5 presents a discussion and recommendations for future research endeavors.