# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
## 3 Results and Discussion
### UUV Deployment  
The sea missions under the SUSHI DROP project took place in the year 2021 near the Italian and Croatian coast. The sites covered by the mission in Italian territory include the areas near the town of Fano, Pedaso and Ortona in the Costa dei Trabocchi. In Croatian territory several missions were carried out in the portion of the sea in the waters of Split. With a wide selection of case studies it was possible to evaluate different characteristics of biodiversity by sampling significant areas in the Adriatic Sea basin. The areas where project missions were carried out are shown in the Figure 10.  
During in-water activities, it was critical to consider the conditions of the different mission sites in terms of currents and water turbidity. In particular, when the UUVs are configured in ROV mode (Figure 11) for close seabed survey, they are affected by the presence of strong currents also due to the presence of the cable and related management challenges [53].  
The unambiguous use of a geographic reference system was essential for the missions performed, both during navigation and data processing. In fact, only through the use of a correct coordinate system is it possible to ensure the repeatability of the survey, generating maps and models as DT accompanied by the necessary metadata for the correct scale and georeferencing of spatial information.  
The result provided in Figure 12 is an example of a transect performed by the UUV, partially above sea water and partially submerged, shows the effectiveness of the positioning sensors after rigorous calibration. The sequence starts from the southeast with the drone on the surface positioned by the on-board GPS receiver shown by the points highlighted on the map with blue color. When the drone submerges, it can no longer receive the satellite signal and relative positioning is activated via USBL, respectively, to the surface research vessel, shown on the map with yellow points. The drone performs the optical and acoustic survey remaining submerged and at the end of the transect, when it re-emerges in the north-west part of the image. There is a substantial coincidence of positioning, excluding some outliers especially at the air-water interface, where neither GNSS signals nor acoustic ones are stably received. The map shows a reduced drift of positioning and allows an accurate georeferencing of the acquired data.  
Figure 9: Exploded view technical drawing of Blucy UUV structure and hardware subsystems.  
Figure 11: Blucy deployment from surface vessel in ROV mode during SUSHI DROP project mission.  
Figure 10: Area of SUSHI DROP marine missions. Coordinates WGS84, bathymetric lines at 10 m interval from GEBCO Bathymetric Compilation Group 2021 (2021). The GEBCO 2021 Grid—a continuous terrain model of the global oceans and land. NERC EDS British Oceanographic Data. Centre NOC [[https://doi.org/10/gnóh](https://doi.org/10/gnóh)]([https://doi.org/10/gnóh](https://doi.org/10/gnóh)) (accessed on 15 January 2022).  
Regarding submarine navigation with the absence of fiber optic cable, data transmission is particularly low in bandwidth and subject to the critical issues described in Section 2.2. The architecture of the UUV ensures sufficient bandwidth to transmit the acquired data when the vehicle is on the water surface through the Wi-Fi antenna. At this stage of development further testing needs to be performed to optimize the data transmission during underwater acquisition through the USBL acoustic channel. Future developments, which go beyond the topics discussed in this paper, will involve the application of signal compression algorithms to minimize the size of the acquired data and allow it to be transmitted over the acoustic channel.