## 1 Introduction  
With the development of aerospace technology, large-scale flexible structures with ultralow-frequency modal characteristics [1; 2; 3; 4], such as solar panels and developable trusses [5], have been widely applied. In order to determine dynamics behavior of the ultralow-frequency flexible structure and ensure the safety in the space environment, it is necessary to conduct modal testing on the structure [6; 7; 8; 9; 10; 11]. Modal testing methods are divided into two categories: contact measurement and non-contact measurement. Traditional modal tests generally use contact measurement methods, using sensors, etc. to obtain structural response signals. The main problem of the contact measurement method is that it will bring additional mass and constraints to the lightweight and flexible structure, which will affect its dynamics performance and measurement results [12; 13; 14]. The influence of the additional quality of the sensors are always need to eliminate. [PERSON] et al. [15] proposed a method based on the Sherman-Morrison identity to eliminate the additional mass of the sensor. This method uses a virtual mass to eliminate the influence of the additional mass, which can be well applied to the vibrating test. [PERSON] [16] and others proposed a one-step elimination method based on the Sherman-Morrison-Woodbury formula, which directly eliminates the mass load effect of the transducer in the modal test. The non-contact measurement method has become an important research direction of experimental mechanics, among which the optical detection method is the main one. The non-contact measurement method does not need to attach the sensor to the test piece, fundamentally eliminates the influence of additional mass, and has the advantage of high measurement sensitivity. The main methods of optical measurement methods to measure strain include moire method [17; 18], holographic interferometry [19] and digital image correlation method [20]. Digital Image Correlation method (DIC) is an emerging vibration measurement method in recent years. It has the advantages of not affecting the dynamic performance of the structure under test and saving the cost of modal measurement. It is an ideal method for vibration measurement of flexible structures [21; 22; 23]. Many scholars have done a lot of research and application on the DIC method, and have obtained many research results. [PERSON], [PERSON] et al. [24] proposed a method to obtain the modal parameters of the artificial wings of the beetle's rear wing by using the discrete cosine transform technique based on the three-dimensional DIC method. The results obtained are in good agreement with the finite element analysis. [PERSON] [25] and others combined the DIC method with the ITD method for the first time to determine the modal parameters of the helicopter rotor blades under working conditions. For the vibration test of ultralow-frequency flexible structures, [PERSON] et al. [26] used the DIC method to excite the steel fan blades with white noise signals for modal analysis, and used the frequency domain decomposition method to determine the mode from the output power spectral density matrix. Regarding modal parameters; [PERSON] et al. [27] proposed a method that can blindly extract the modal frequency, damping ratio and full-field mode shape from the vibration video image of the structure, which improved the problem of difficulty in arranging speckle patterns or targets in structural modal testing with DIC method. For the study of spacecraft dynamic parameter identification [28; 29], frequency domain methods were mostly used in the early days, such as Frequency Domain Decomposition [30]. However, this method requires inverse Fourier transform in terms of damping identification, which is realized by exponential decay method in the time domain, and the accuracy is not high due to the influence of truncation error.