## 1 Introduction
It is important to underlay the key importance of monitoring the floating wind turbine 3D motion in a real-time regime with adequate accuracy, as well as frequency and latency, with a view to assisting a structural-health monitoring (SHM) system. The SHM is focused on detecting and preventing otherwise feasible and unexpected damages. To accomplish these requirements and determine the position with high precision, the global navigation satellite system (GNSS) is commonly used. In this system, a spatial infrastructure allows navigation users, with a compatible receiver, to determine their positions, velocities, and time (PVT) by processing the satellites signals. These signals are provided by constellations of satellites with global (e.g., GPS, GLONASS, GALILEO, and BeiDou) and regional (e.g., QZSS and IRNSS) coverages that are often complemented with satellite-based augmentation systems (e.g., EGNOS) [13]. The determination of the PVT solution in the simplest method can be implemented with a GNSS antenna and receiver that capture the data and send them to a processing unit that provides the solution. This technique is easy to use and fast to compute; however, the accuracy obtained through this positioning technique is usually around 10 m, which is not enough in some demanding applications. To overcome this limitation, differential or relative positioning techniques (e.g., GNSS differential or real-time kinematics (RTK)) are mainly used. In their implementation, at least two receivers and antennas are employed in different positions. One of them acts as the base station and, generally, its position is known with high accuracy. Both receivers track the same satellite signals and the relative position coordinates of the rover are determined. The main drawback of this implementation is the accuracy dependence with the baseline length that separates the base and rover stations. In the RTK case, the maximum baseline length is limited to a short-range distance (30-50 km), since in this area the satellite clock and the atmospheric conditions are similar in both receivers and errors can be successfully removed [14; 15]. Another promising solution used to improve the positioning accuracy is called the precise point positioning (PPP) technique, in which the absolute and differential positioning algorithms are combined [16; 17]. Unlike the RTK solution, the PPP algorithm does not require the existence of a nearby base station but instead needs pseudo-range and carrier phase measurements, as well as the precise knowledge of the satellites' clock offsets and orbits. These last data are provided by external organizations in real time (usually involving a cost) or in final product form, available after a period of time (usually 15 days) [18]. Some private companies have developed a worldwide network infrastructure to generate the needed data and serve them via satellite communication; however, this approach cannot be implemented by all the applications because of the high costs and the need to employ specific hardware [19; 20]. Another alternative is computing the PPP solution from the open-access real-time corrections provided by the International GNSS Service (IGS) following the RTCM (Radio Technical Commission for Maritime Services) standard via NTRIP (networked transport of RTCM via internet protocol) [19; 20; 21]. The principal disadvantage of this technique lies in the long convergence time (between 20 min and some hours) to achieve centimeter-accuracy solutions, a factor that limits its use in some real-time applications. To minimize this drawback, multi-constellation and multi-frequency receivers are normally employed. In this way, it is possible not only to reduce the convergence time but also to allow for better ionosphere errors resolution (thanks to ionosphere-free combinations) and, therefore, more successful resolution of ambiguities, which results in a highly accurate position solution.