# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
## 3 Results and Discussion  
### Prototype UUV  
One of the main results of the SUSHI DROP Project is the realization of the complete prototype UUV previously designed and studied in the different components: sensors and subsystems. With the scientific payload available, procedures were tested to survey the data necessary to create a marine DT. The development of the drone took place following an iterative process, starting from the theoretical specifications and verifying the specifications of the components available on the market to select the different subsystems. The goal was to create a hybrid shape optimized for both high and low cruising speeds with a functional set-up for both ROV and AUV modes. ROVs often turn out to be box-shaped, while AUVs are torpedo-shaped. Blucy initial design was then iteratively modified in four stages of design: concept, preliminary, functional and final (Figure 7). The final UUV has an approximate weight of 200 kg including the on-board battery and the following main body dimensions in millimeters: Length 2000, Width 350, Height 740. The final shape of the prototype UUV allows for a reduced roll during navigation and particularly during survey activities, compared to typical AUVs with a torpedo structure. This design choice improves both the navigation ability allowing a better NGC, and the possibility to perform a better optical and acoustic survey minimizing the need to compensate for the attitude of the vehicle. Each element of the Blucy scientific payload was carefully positioned to maximize its effectiveness during survey activities. The positioning and communication sensors are located at the top of the drone, above the buoyancy foam layer, to avoid any interference. In the barycentric part of the structure there is the on-board computer and under it the heaviest element: the battery. All survey sensors are located in the lower part and are coupled with LED illuminators. The PilotCAM is mounted at an angle of about 45 degrees with respect to the seabed, while the photogrammetric BottomCAM is mounted perpendicularly to allow a three-dimensional reconstruction and the realization of the Digital Elevation Model (DEM) and orthomosaics of the seabed. The MBES is also positioned at the front with a tilting head, while FOG, DVL and altimeter sensors are at the rear. All on-board acoustic instruments were carefully selected with appropriate technical specifications to avoid any interference  
Figure 7: Four stages of design for Blucy UUV. during mission activities. The different subsystems operate in different operating frequency bands: USBL 18-34 kHz, ALT 200 kHz, MBES 200-450 kHz and DVL 500 kHz. Regarding the propulsion the UUV is equipped with a pair of thrusters on each axis: two vertical, two longitudinal and two lateral for a total of six thrusters. UUV components are listed in Figure 8.  
The features of the designed drone allow it to be multipurpose and modular. Being able to rely on a precision navigation for detailed inspections and at the same time a propulsion sufficient to cover large areas it is possible to perform different operational scenarios within the same mission: MBES survey and close seabed survey. Finally, thanks to the modularity of the architecture, it is possible to operate in two complementary modes (ROV or AUV) with the additional possibility of exchanging the scientific payload and possibly each secondary subsystem according to mission requirements (Figure 9). Figure 8: Main prototype UUV components: (**a**) Wi-Fi communication. (**b**) USBL transponder. (**c**) Radio communication. (**d**) Hook. (**e**) AHRS: GNSS and INS. (**f**) Thruster: vertical. (**g**) Main buoyancy foam. (**h**) Lateral buoyancy foam panel. (**i**) HDPE structure. (**j**) Thruster: lateral. (**k**) Fiber optic cable. (**l**) Thruster: longitudinal. (**m**) Altimeter. (**n**) Frontal LED lights. (**o**) PilotCAM. (**p**) Adjustable LED lights. (**q**) MBES sonar head. (**r**) MiniSVS. (**s**) MiniCT. (**l**) BottomCAM. (**u**) MBES computer. (**v**) Bottom LED lights.