## 1 Introduction
This methodology does not assume a straight-line wave propagation path and can predict the source location without any knowledge of the elastic properties of the material; however, this study reported only a theoretical approach without the development of a real-time system to validate the proposed technique. In [15], the authors of this work presented a guided-wave ultrasound SHM system based on linear arrays of interdigital piezopolymer transducers bonded to a composite pressure vessel for spacecrafts. In that system, interdigital transducers were adopted to perform both damage assessment and impact detection/localization using a simple fixed-threshold technique. In a related research development [16], the authors presented a novel approach to estimate the DToA between the impact response signals collected by a triplet of sensors, overcoming the limitations of classical methods that rely on amplitude thresholds calibrated for a specific sensor type. Finally, in [17] is proposed a simple laboratory procedure based on a set-up with a pair of sensors that are symmetrically placed with respect to the impact point to estimate the uncertainty of the DToA and the propagation velocity. In this work we have upgraded the real-time electronic system reported in [15] that allows for the accommodation of analog front-end electronic characteristics thanks to its modular design; different sensor types (piezoceramic, piezocomposites, and piezopolymers) need custom analog front-ends (AFE) to match their electrical impedance, sensitivity, and bandwidth. In this work, we designed and implemented 16 new receiver boards to collect information of the propagated extensional S\({}_{0}\) mode Lamb waves thanks to the improved bandwidth, adjustable gain, and signal-to-noise ratio. This new feature allowed us to overcome the limitation of using information for the localization carried by the higher level, but slower, A\({}_{0}\) mode. The detection of small amplitude (mV-10 mV) S\({}_{0}\) mode signal with threshold method simplifies the electronic design with benefits also to power consumption and rapid event detection. As observed in [18; 19], A\({}_{0}\) mode propagates on longer paths with low attenuation but at lower propagation velocity and exhibits dispersion phenomena in the low-frequency range. Such characteristics complicate the interpretation of the impact-generated signals, especially considering large structures and multiple reflections from boundary, where in many cases S\({}_{0}\) is preferred. Moreover, the importance of sensor networks for aircraft SHM was reported in [20], considering the large dimensions and the trade-off between system performance, reliability, and cost. The study presented in this paper differs from previously cited works because the aim was not to present the best method or technique for locating impact positions, but to report the analysis of errors on impact localization (see Section 2) using real-time electronics; this paper also evaluates the influence of the number of passive PZT sensors used (see Section 3) on the position error. The real-time electronics can also acquire multiple impacts on the same point for statistical analysis. We present the results of experimental tests with low-energy impact on a 1.4 mm thick aluminum plate using Physik Instrumente P-876.SP1 piezoceramic transducers, varying the number of channels from four to eight. The signals were then used to find the minimum of a cost function for the localization of the impact and, therefore, the evaluation of the positioning error relative to the size of the investigation area and the number of passive channels used. Finally, to help the reader in finding selected topics we include, a list of acronyms used in this paper.