## 2 Materials and Methods  
### Materials  
As already mentioned, the main objective of the smart sensor is to monitor the floating wind turbine's 3D motion and register it in real time with the required accuracy, frequency, and latency to be usable by other modules in the platform system. The registered data help the digital twin in the estimation of loads at the mooring lines and, therefore, enables the SHM module to detect unexpected damage and use it as feedback to the closed-loop control algorithms to reduce loads. For that, the baseline design proposed was the strategic placement of three smart sensors on the platform and a concentrator unit that is responsible for receiving the data from each sensor, processing these data, packing them together, and delivering them to the digital twin. Figure 1 shows a block diagram of the proposed design. At the hardware level, each smart sensor node consists of three main blocks: (i) GNSS receiver and antennas, (ii) IMU, and (iii) processing unit. As the sensor must be based on cutting-edge mass-market technology, the GNSS receiver selected is an Ublox ZED F9P, a multi-band GNSS module with integrated multi-band RTK technology for centimeter-level accuracy. The inertial measurement unit chosen is the Xsens MTi-610, which is an industrial-grade, affordable MEMS-based orientation sensor with an easy-to-use open SDK [31]. It is a fully functional, self-contained module that contains a three-axis gyroscope, three-axis accelerometer, three-axis magnetometer, barometer, high-accuracy crystal, and low-power microcontroller unit. The main processing unit is based on the Raspberry Pi3 Model B+ because of its elevated efficiency, since it combines high computational power with low consumption, which makes it ideal for the concerned application. Furthermore, the system is equipped with two antennas: the first one is dedicated to GNSS signal reception (Tallysman TW3972, Kanata, ON, Canada), which is able to receive signals from multi-constellations and multi-frequencies, and its IP67 certification makes it suitable for maritime environments, while the second one is used for the wireless communication (@2.4 GHz) between smart sensor nodes and the concentrator unit and for receiving the real-time corrections. Finally, concerning the power supply of each node, a solar panel (SOLARA S320P41, Hamburg, Germany) is included to generate direct current and charge the battery integrated inside the smart sensor through the autonomous power supply subsystem. In this sense, a battery of 300 Wh and its corresponding power management system were selected to provide the system autonomy for more than 3 days even without any solar charge. The detailed block diagram is presented in Figure 2.  
Figure 3a presents the design proposed integrated inside a waterproof housing (machined to install the connectors needed) that corresponds to the international standard rating of IP68, withstanding dust, dirt, and sand and providing the necessary resistance to submersion up to a maximum depth of 1.5 m underwater, which makes it appropriate for the environment. For its part, in Figure 3b, the smart sensor components and placement inside the said enclosure are depicted. Figure 1: Mooring sense smart sensor system block. Figure 2: Smart sensor node block diagram. Figure 4 shows the software architecture of the smart sensor. The software has been developed under a Raspbian operating system. It manages the proper behavior and the correct communication between all parts that integrate it, in such a way that it sends the precise 3D motion to the concentrator. The main module oversees the coordination of the different parts of the system, handling its requests and responses and delivering the position or attitude solution to the concentrator when requested. It is also in charge of checking the sensors and power system alarms and reporting them and receiving the RTCM data that will be used by the real-time PPP software (RTKlib) for positioning estimation.