# 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  
The accuracy, precision, and reliability of the UV localization and navigation system are important within the IMR activities of an OWT. Without localization accuracy within centimetre or millimetre range, a UV will still be capable of navigating without suffering collision with other objects. In addition, the implementation of multiple onboard sensors such as LIDAR, infrared obstacle detection sensor, Time-of-Flight sensors, ultrasonic sensor, or stereo vision [81] may prevent collision effectively. However, for a UV to work within the IMR activities, localization accuracy within the centimetre or millimetre range is important to accurately identify and report inspection data that need further attention. Working near a WT that is susceptible to losing signal or suffering measurement error related to GNSS, a coordinate for inspection data provided by a UAV might not be reliable. Even worse, for a UUV working underwater in a GNSS-denied environment coupled with low visibility, obtaining reliable and accurate IMR coordinates is even more challenging. As a result, operators or UVs might face difficulties in locating the exact position for future revisiting due to the measurement inconsistency. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Collaborative UVs** & **Strategies** & **Benefits** & **Challenges** \\ \hline \multirow{8}{*}{USV-UAV} & A USV carrying a fixed-tool UAV &  
For a UAV operating at a low height near a wind turbine structure, a GNSS is prone to fail due to multipath errors, where the receiver might read a reflecting signal instead of a direct signal [82]. To solve this problem, the visual localization and navigation method [83; 84; 85; 86; 87] is seen as a viable solution which is summarized in Table 4.  
Another method that can be used to achieve reliable coordinates for IMR data is by anchoring the UAV localization system to radio transmitters or specific points near an OWT structure. Having accuracy within centimetres resolution, ultra-wide band technology (UWB) is suitable to be applied for UAV localization in a GNSS-denied environment [88]. Using four UWB modules as fixed anchors to a UAV carrying a UWB transceiver, the coordinate of the UAV can be obtained [89; 90]. However, for OWT operation, the use of fixed anchors is not suitable, as the sea environment is always in motion. Nevertheless,  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Collaborative UVs** & **Strategies** & **Benefits** & **Challenges** \\ \hline \multirow{8}{*}{USV-UUV} & \multirow{8}{*}{A USV carrying a tool-changing UUV} & \multirow{8}{*}{\(\bullet\)} & Can perform multi-task operations. & \multirow{8}{*}{*} & Requires tools cartridge system beneath the USV. \\  & & & Different sets of tools can be stored on a USV. & \multirow{8}{*}{*} & Requires several trips to USV for tool changing. \\  & & & Option for using tethered UUV. & \multirow{8}{*}{*} & Requires a UUV recovery mechanism. \\ \cline{1-1}  & & & More flexible to perform high-dexterity tasks. & \multirow{8}{*}{*} & Difficult for a fully autonomous system. \\  & & & Can carry and work with several tools per trip. & \multirow{8}{*}{*} & Requires uninterrupted communication with the GC station through the USV. \\  & & & Different sets of tools can be stored on a USV. & \multirow{8}{*}{*} & Requires a UVMS recovery mechanism. \\ \cline{1-1}  & & & &  
anchors with auto-calibrated capability [91] have enabled mobile platforms to be used as anchors' placement for UAV localization. A possible collaborative solution that can be implemented using mobile UWB anchors is by utilizing multiple USVs as platforms for UAV localization. In this configuration, at least four USVs can be deployed to float around an OWT as shown in Figure 2. Three-dimensional positioning of a UAV from the UWB modules can be achieved using the multilateration method [92; 93] with a GNSS receiver installed on each of the USV.