# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
## 2 Materials and Methods
### UUV Architecture  
The multi-purpose UUV developed for the project and named Blucy, is designed to operate in a dual hybrid navigation mode: ROV or AUV depending on the mission setup. In both cases, power is supplied to the UUV by an integrated battery, with no need for remote power transmission. The UUV is built for depths up to 250 m.  
In ROV navigation mode, Blucy is tethered to the surface ship with the help of a 600 m long fiber optic cable (Figure 4). The cable does not carry energy as this is supplied directly on board the drone to allow operation in dual mode. The selected fiber optic cable has a nominal overall diameter of 7.80 mm, specific gravity of 0.95 kg/dm\({}^{3}\), LCP fiber braid strength member, LDPE UV resistant sheath, hydrolysis UV resistant PUR outer sheath and a total breaking strength of 500 kg. The ROV mode is mainly used for close inspection of the seabed performed in hostile environments and also in particularly tight spaces at moderate cruising speed and when high position accuracy is required. In this mode all data are transmitted in real time and recorded on the RS rugged computer.  
The AUV navigation mode is suitable for surveying large areas at high cruising speed in safe conditions, considering that the maximum range of Blucy is 6 h of continuous navigation. A typical survey in AUV mode must necessarily follow a path previously planned through the use of way-points at a safe distance from the seabed and any other obstacle. Furthermore, to ensure the necessary performances and increase the reliability of Blucy during the AUV missions, a supervision scheme for early actuator Fault Detection and Isolation (FDI) is required [45; 46]. With the scientific sensors equipped on board, including MBES, it is possible to analyze elements more than 100 m away from the AUV position while maintaining an adequate resolution for the study of the seabed. In this mode the status of the AUV, its position and commands are received and transmitted with a dedicated acoustic channel via USBL, connecting the AUV to the RS present on the surface vessel. The underwater acoustic modem used provides a full-duplex digital communication, using self-adaptive algorithms to maintain a nominal bitrate up to 13.9 kbit/s with a bit error rate less than \(10^{-10}\), depending on marine conditions. Among other challenges to face for acoustic communication and submarine digital data transmission, we can mention some that also concern the AUV sector: attenuation, time synchronization, data transmission delay, underwater noises, stratification and multi-path effect [47]. These problems affect  
Figure 3: Main surveying instruments: (**a**) BottomCAM and its network data transmission router before insertion into the waterproof case. (**b**) BottomCAM 15 mm frontal glass. (**c**) MBES sonar head (top) and final BottomCAM canister (bottom) installed in UUV.  
the ability to control UUV navigation in real time. At this stage of development the large amount of data acquired by the drone cannot be transmitted in real time in AUV mode. All data are recorded on a solid-state drive (SSD) within dedicated subsystems and then downloaded at the end of the mission.  
All UUV subsystems and their functional connections are listed in Figure 5.