# 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
However, this type of localization requires high computational loads [122] which can easily drain the UAV energy [123], thus causing shorter flight time for IMR operations. It can also add extra weight to the UAV [124], thus reducing the payload capacity for carrying IMR tools. Nonetheless, using collaborative strategies as discussed in the previous section, the UAV can make use of the USV as a nearby wireless data processing and storage system provider [60; 61], as well as a power hub to prolong the UAV operation time [50; 80]. In addition, the LiDAR-based SLAM technique, which is not affected by magnetic interference, can be operated while a WT is fully operational. However, most of the time, a WT shutdown is necessary during maintenance and repair operations. Due to this fact, a UWB positioning system might be preferred as it can perform better under the temporary shutdown of an OWT during IMR works. Using a real-time accurate solution, a localization system on an OWT can easily be associated with the UAV localization system. Moreover, future revisiting is also more straightforward than the LiDAR-based localization since the collaborative UWB localization technique (integrated with GNSS-PPP and local WT IMUs) can point directly to the point of interest within the WT absolute coordinate system, instead of needing to find the local origin in the initial step. Apart from addressing the need for high-accuracy localization and navigation with respect to IMR activities close to a WT, it is also needed for other collaborative purposes such as for landing, docking, or recovery operations. Since UAVs have limited energy capacity due to current battery technology, their batteries need to be recharged frequently to prolong their operating times at sea. Since there is no charging station at sea, implementing a USV as a mobile charging station is seen as the current most practical solution. However, landing and docking a UAV onto a USV is more challenging at sea compared to onshore due to the presence of waves, sea current and higher wind speed. For a UAV to land on a USV, the most critical part is the ability to detect the location of the landing platform on a USV. To solve this matter, a visual-based method has been proposed to detect and identify a properly designed fiducial marker placed on a USV [44]. This fiducial marker enables UAV landings in the absence of the global positioning system by providing relative pose estimates between UAVs and the landing platform. Using a specific 2D pattern, a UAV will be able to view and determine its own relative position to land on a USV from a reasonable distance of 3.74 m. Further improvement to this method [45] has been made by proposing a three-stage fiducial marker that provides more visual exposure while landing with the first detection at 4.6 m and further detection at 1.4 m. Subsequently, both methods adopted further control strategies to finally land a UAV using a PID controller for accurate landing. However, the visual-based platform detection method is ineffective under adverse weather conditions such as rain or fog due to poor visibility. Hence, an alternative method using an infrared beacon and an infrared detector is proposed for the UAV landing guide [49]. In this method, an infrared detector is placed on the bottom of a UAV whilst the infrared beacon is placed at the centre of the USV landing platform. Under this configuration, the UAV can localize the infrared beacon for precise landing. Nevertheless, offsets in the landing position can be observed from the experimental outcome due to the wind effect during the landing tests. Due to the dynamic sea environment, especially from high wind and sea waves, robust control strategies utilizing sensors and actuators have been proposed to ensure the safe landing and transportation of a UAV on a USV [48].