# Collaborative Unmanned Vehicles for Inspection, Maintenance, and Repairs of Offshore Wind Turbines
## 3 Implementing Collaborative UVs for the OWT IMR Operations
### Collaborative Localization and Navigation Strategies
Alternatively, using Post-Processed Kinematic (PPK) [100], the maximum operating distance can be up to 100 km; however, the PPK system is not suitable for control operations as it is not a real-time system. Another alternative is by using the Network RTK (NRTK) which can provide centimetre-level accuracy below the 100 km range from a reference station [101]. Unfortunately, the NRTK method depends on available inland network reference stations, which do not exist at sea. Since more OWT farms are moving further from the land, the prospect of using the GNSS-RTK method is diminishing. However, Precise Point Positioning (PPP) technique for correcting GNSS position in real time can be used to provide centimetre-level accuracy without depending on reference stations [102]. Several articles integrating the PPP technique with GNSS for UAV have been reported [103; 104; 105]. Nevertheless, the effectiveness and the accuracy of operating PPP techniques for using GNSS positioning close to tall  
Figure 3: Inverted LBL approach. Figure 2: Configuration for multiple USVs-UAV using mobile UWB anchors. structures or wind turbines have not been explored yet. It has been discovered that the PPP is vulnerable to poor GNSS observations, cycle slips, and data outages [105]. Therefore, it can be assumed that the PPP method is also susceptible to inaccurate measurement when working in an area prone to GNSS signal loss, such as working closely to a tall structure or a wind turbine. Since the IMR operations are more concerned with the localization of the OMTs instead of the UVs, a wind turbine coordinate as an absolute coordinate system should be used. Using the wind turbine coordinate, dependencies on the GNSS can be reduced to only necessary navigation while approaching the OMTs. However, for a swaying OWT [106], as in the case of a floating OWT, its platform is greatly influenced by the wind and waves. A flight path generated from the initial absolute coordinate might not be accurate, as the final position on the OWT might already sway. Thus, a positioning system for a UV needs to be constantly updated with respect to the absolute coordinate of the OWT. For a swaying WT, a virtual point to represent an origin for the absolute coordinate (0,0) can be initially marked by a UAV. Once the origin is set, the UAV can either be path-planned (autonomous system) or remotely navigated to the point of interest. Due to the swaying motion of the floating OWT, a correctional plan for the UAV localization system needs to be executed based on the current position and orientation of the floating OWT. A correctional approach for UAVs based on ground truth navigation can be devised utilizing UWB anchors that are placed at the OWT base. Equipped with IMUs, this UWB device can provide the floating OWT with its current orientation and motion displacement. The information from this device can then be transmitted to the UAV for real-time relative localization corrections. However, the localization system that depends on the GNSS or UWB positioning does not provide robust navigation since the wind turbine causes magnetic disturbance [89]. Alternatively, a localization system that is not susceptible to any radio interference is seen as the more robust solution. Methods such as visual-based [83; 85; 107] and simultaneous localization and mapping (SLAM) [108; 109; 110] are gaining interest as the more reliable localization approach when working near an OWT. In a visual-based localization, irrespective of the swaying motion of the OWT, a UAV can detect the origin, which is virtually set earlier. Therefore, it can then easily correct the localization of any point of interest on the OWT with respect to the origin. Since a wind turbine consists of moving components as well as stationary structures, the selection of the origin's location is crucial. To localize any point on the WTB, the centre hub of a wind turbine is the most suitable choice to be assigned as the origin since its position remains the same with respect to the moving blades.