# 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
As for the positioning of the USVs around the OWT, they can be placed as far as the UWB signals can reach to minimize the effect of operating within a GNSS-denied environment. Currently, the commercialized UWB positioning devices for an outdoor environment have accuracy smaller than 5 cm within the range of 50 m and 75 m [94]. Their signal may reach up to 200 m but with lower accuracy [95], thus they are deemed less practical to be implemented for the time being since the average height of OWT is 100 m [87]. Nevertheless, disruptive innovation in the UWB range measurement using microwave radar has been presented where a very long-range measurement up to 6 km with accuracy below 10 cm has been achieved [96]. It is therefore expected that a mid-range high-accuracy UWB positioning system with a measurement range beyond 100 m will be available in the near future. For UUV localization, the UWB localization method is not suitable since it is based on a radio frequency which cannot be used for underwater applications. Instead, centimetre-level accuracy for the UUV coordinate can be achieved using the long baseline (LBL) method [97]. However, due to the nature of the LBL which needs to be fixed to the ocean bed, the detection range becomes limited for the UUV. To solve this issue, the inverted LBL method can adopt the multilateration technique by implementing four USVs and an AUV [97] as depicted in Figure 3. Within this configuration, all the USVs are equipped with GNSS receivers and ultra-short baseline (USBL) modems for transmitting and receiving acoustic signals. Meanwhile, the AUV is also fitted with a USBL modem. A practical approach for a full OWT IMR package could easily be realized by integrating both the UWB and the inverted LBL localization methods on the USVs. \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Ref.** & **Localization Methods** & **Descriptions** \\ \hline \(\left[83\right]\) & Simultaneous model fitting and pose estimation. & Positional accuracy can be achieved through precise image measurements which are based on an optimization algorithm. \\ \hline \(\left[84\right]\) & Robust visual-inertial odometry with point and line features. & A faster line detector and purified strategies enable the detection of wind turbine blades by extracting sufficient features. Fusion of IMU and point-line features exhibit higher accuracy. \\ \hline \(\left[85\right]\) & Visual SLAM with a synthetic depth map. & Using a monocular SLAM system, a visual-based localization system to detect wind turbine blades is improved using a synthetic depth map. The depth map is based on a Line Segment Detector. \\ \hline \(\left[86\right]\) & Autonomous visual navigation algorithm using image processing methods and blade feature detection method. & Using Wavelets, Wiener filtering for a deblurring system, image enhancement using Retinex, and Hough transform for wind turbine edge detection. Blade feature detection is performed by comparing the extracted features (circle and linear features) with the features from a blade library. \\ \hline \(\left[87\right]\) & Visual SLAM. & Focussing on localization within the internal wind turbine blade, localization and navigation can be achieved using the Visual SLAM method. To build a map for localization and navigation, V-SLAM uses distinguished features such as edges, corners, or colours. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Visual localization methods for UAV working near a wind turbine. By integrating the GNSS with Real-Time Kinematic positioning (RTK) [98], a centimetre accuracy level can be achieved for the UV localization system. Basically, in the GNSS-RTK system, at least two receivers are used where one of them is stationary (base station) and fixed on a precisely known location [99]. For OWT farms located more than 60 km from a base station, the RTK approach cannot be used as it already reached the maximum operational distance.