## 1 Introduction  
In order to address the challenges of climate change and accommodate rapidly growing demand of energy, solar energy, being a clean and safe renewable energy source, has been widely adopted for power generation in recent years [1]. However, with the rapid expansion in solar power plants, flatten terrain resources are becoming increasingly scarce. Although several types of flexible substrate photovoltaic (PV) modules, such as GaAs, perovskite, silicon, and CdTe-based modules, have been developed to improve adaptability, their widespread application remains limited [2]. Compared with conventional rigid PV modules, these flexible PV modules generally exhibit lower photoelectric conversion efficiency and higher production costs [3]. As a result, their use is largely restricted to small-scale, distributed, or portable applications, such as wearable devices, vehicles, and emergency systems. For large-scale solar power plants and installations in complex terrains, robust support structures with rigid PV panels remain essential for providing wide-area coverage, directional alignment, wind resistance [4]. Despite these advantages, the inherent low stiffness and low damping ratios of flexible PV support structures systems make them highly susceptible to wind-induced vibrations, which can further lead to PV modules being lifted, collisions, latent cracks, and even damage to the supporting structures. Therefore, wind-induced vibration must be taken into account during the design process. Current research methodologies for this issue primarily include wind tunnel aeroelastic tests as well as finite element (FE) simulations. However, the former lacks field-measured damping ratios for model calibration, while the latter suffers from insufficient validation against field-measured modal frequencies and mode shapes. Moreover, both methods seldom consider the changes in structural parameters of flexible PV supports during their service life. To address these limitations, field modal testing should be conducted to obtain accurate modal properties of flexible PV support structures. Meanwhile, FE model updating utilizing field-measured data is essential to develop a high-fidelity FE model that faithfully represents the actual structure. Previous studies of flexible PV support structures based on wind tunnel aeroelastic tests have revealed that critical wind speeds are low and cables may suddenly collapse due to intensive vibration [5; 6; 7; 8; 9; 10; 11; 12]. Furthermore, [PERSON] et al. [13] observed flexible PV support fluttering in aeroelastic tunnel testing, which was mainly characterized by coupled torsional and vertical motions. These findings underscore the high susceptibility of flexible PV support structures to flutter instability. The damping of flexible PV support structures is crucial in determining whether its vibration will diverge and lead to flutter. However, there is a notable lack of field studies providing accurate identification. [PERSON] et al. [14] measured the first three order modal parameters of a solar tracker, but the modal properties of solar trackers notably differ from those of flexible PV supports. [PERSON] et al. [15] conducted field testing on large-span flexible PV supports, obtaining only the vertical bending modal damping ratio and frequency. Therefore, more comprehensive field modal testing of flexible PV support structures is necessary to obtain other high-order modal properties. In field modal testing, it is difficult to obtain the full-frequency response information of a structure using a single type sensor. In 1973, the U.S. Department of Defense [16] first proposed data fusion from different types of sensors to obtain more accurate response estimates and improve system redundancy. Subsequently, this technology has been widely applied in the structural health monitoring of bridges, high-rise buildings, and transmission towers. In 2007, [PERSON] and [PERSON] [17] proposed a multi-rate Kalman filter for data fusion to address the issue of different sensor sampling rates.