## 1 Introduction  
With the development of aerospace industry in various countries, the number of space debris is increasing, and the safe operation of orbiting spacecraft is seriously threatened [1; 2; 3; 4; 5; 6]. Although a honeycomb panel is provided on the outer surface of the manned spacecraft capsule, the high-speed space debris can pass through the protective layer and strike the outer surface of the spacecraft's sealed cabin [7; 8; 9]. In order to ensure the air pressure balance and operational safety of the spacecraft, it is especially important to sense and locate the debris impact at the earliest possible moment [10; 11; 12; 13]. The impact signal exists in the form of a Lamb wave in the plate. At present, the research on the propagation law and impact location technology of the Lamb wave in the flat structure is relatively mature. However, periodic stiffeners are usually provided on the outer surface of the spacecraft capsule, to ensure the sufficient mechanical strength of the spacecraft, especially the large manned spacecraft. The Lamb wave will attenuate, transmit, reflect, scatter and modal change when passing through structures such as stiffeners or defects, which increases the difficulty of locating the impact source. The traditional methods for impact source localization in flat panels are difficult to apply to stiffened panels directly. In recent years, some researchers have conducted in-depth research on Lamb waves in such structures. [PERSON] et al. [14] utilized the frequency domain spectral element method to discretize the finite-sized surface mounted piezoelectric structure, and used the semi-analytical boundary integral method to evaluate the wave phenomenon in the host laminate structure. The dynamic interaction of perfectly bonded or damaged piezoelectric structures with layered elastic waveguides was simulated. [PERSON] et al. [15; 16] studied the propagation law of Lamb waves in two kinds of aspect ratio stiffened plates. The transmission energy of different frequencies of Lamb waves is different when passing through the stiffeners. This phenomenon is more obvious in the stiffened plate with a large aspect ratio. The accuracy of the leak location was improved by selecting the A0 mode Lamb wave of a specific frequency band. By simplifying the model, the conclusion that the signal attenuation is severe when the S0 mode Lamb wave frequency is near the resonant frequency of the stiffened plate was obtained. However, the research was mainly on the influence of single stiffeners on the Lamb wave signal, and there was no relevant research on the impact signal. [PERSON] et al. [17] studied the reflection characteristics of stiffeners on Lamb waves with different angles of incidence, [PERSON] et al. [18] studied the relationship between reflection coefficients of different modes Lamb waves at the end of the plate with incident angles and frequencies. In addition, the characteristics of the Lamb wave signal in the stiffened aluminum plate with cracks [19], the interaction between the Lamb wave and different types of notches [20], and the modal transformation phenomenon of Lamb wave when it passes through notches [21], there are also scholars to carry out related research. For the locating of the impact source in the panel, some scholars have carried out research based on a variety of methods [22; 23; 24; 25; 26; 27; 28; 29; 30; 31], such as the polyvinylidene fluoride (PVDF) thin film method, fiber grating method, acoustic emission method and so on. Some scholars have studied based on the PVDF film method. NASA [22; 23] has developed a two-dimensional PVDF film position sensing detector for detecting cosmic dust. [PERSON] et al. [32] improved on this basis and designed a detector that can detect space debris larger than 1 mm. The impact events in cement-based composites were identified by PVDF film, and the impact-induced crack evolution was studied by active sensing method [33].