## 1 Introduction
Consequently, the use of the PPP technique and multi-frequency receivers provide an advantageous option for numerous commercial applications, such as precision agriculture, geodetic surveying, airborne mapping, etc., where the operability is unaffected by initial convergence periods. In the specific case of offshore implementations, especially scenarios on high sea (>50 km from the coast) installations, where the RTK algorithm is not suitable, the PPP technique is the unique solution and, currently, there is intense research to improve its performance [19; 22]. To facilitate the PPP solution computation, the IGS service provides different precise products; among them are the IGS ultra-rapid products (IGU) and the multi-GNSS experiment (MGEX). The IGU products are the IGS predicted products that are available for real-time and near real-time use. The IGU products are only available for GPS and GLONASS. The MGEX has been set up by the IGS for tracking and analyzing all available GNSS signals and any space-based augmentation system (e.g., SBAS); these products are available after some days or weeks [23; 24; 25]. GNSS solutions, and especially techniques such as PPP and RTK, provide precise position and velocity information. However, GNSS methods also have some weaknesses. They typically provide low data output rates (typically 1-10 Hz), and signals are vulnerable to jamming (even unintentional interference), multipath, receiver noise in low-cost receivers, etc. For this reason, usually, other technologies are used to complement this algorithm and ensure constantly accurate position measurements even when GNSS data are unreliable [26; 27; 28]. Among the integration approaches, the inertial navigation system (INS) is a promising solution because of it being cost-effective and not requiring additional infrastructure. This system uses rotation and acceleration information from an inertial measurement unit (IMU) to calculate relative position over time. Once this last parameter is estimated, it is fused with the GNSS measurements to improve the accuracy and to serve positioning data with high frequency [29]. In this paper, the results obtained to validate the developed low-cost smart sensor GNSS system for real-time positioning and orientation for floating offshore wind platforms are presented. The proposed system takes advantage of the GNSS system and uses innovative techniques that combine multi-constellation and multi-frequency. Furthermore, for a high and accurate positioning, the GNSS solution is fused with the data acquired from an IMU, reaching final resolutions of 20 cm. Furthermore, besides the precision (<20 cm), the developed system has to comply with some technical restrictions such as velocity accuracy < 3 mm/s, attitude accuracy < 0.2 deg, data output rate of at least 1 Hz, a delay in the real-time output of 0.5 s, and a maximum bandwidth of 50 Kbps. The total cost of the Smart Sensor system can be around EUR 3000 and may vary slightly depending on the components included (receiver, antenna, inertial measurement unit, etc.). This represents a cost savings of approximately 90%, over a 25-year lifespan, in comparison with other monitoring technologies. When the smart sensor technology reaches high technology readiness levels (industrialization), a medium-scaled batch could save an additional 10-20% in material costs [30]. The validation of the smart motion sensor proposed was accomplished following a step-wise approach, starting from simple algorithm testing and verification of basic functionalities, and incrementally adding complexity to better represent real operation conditions. For this reason, three sets of tests were carried out: first, in a rotary table at the laboratory to check the sensor performance in the early stages of its development. After that, the registered motion of a floating platform in a water tank was used as input into the GNSS simulator, emulating similar conditions in platform locations. Finally, the low-cost smart sensor was validated in a real sea scenario.