# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
###### Abstract  
The ability to observe the world has seen significant developments in the last few decades, alongside the techniques and methodologies to derive accurate digital replicas of observed environments. Underwater ecosystems present greater challenges and remain largely unexplored, but the need for reliable and up-to-date information motivated the birth of the Interreg Italy-Croatia SUSHI DROP Project (SUstainable fisHeries wth DROnes data Processing). The aim of the project is to map ecosystems for sustainable fishing and to achieve this goal a prototype of an Unmanned Underwater Vehicle (UUV), named Blucy, has been designed and developed. Blucy was deployed during project missions for surveying the benthic zone in deep waters of the Adriatic Sea with non-invasive techniques compared to the use of trawl nets. This article describes the strategies followed, the instruments applied and the challenges to be overcome to obtain an accurately georeferenced underwater survey with the goal of creating a marine digital twin. 202  
contribute to the creation of the respective DTs has increased in parallel. Some particularly versatile platforms have seen considerable success both commercially and in the world of surveying for scientific research: drones. The possibility of carrying out a survey at a safe distance using a vehicle without an operator on board and with reduced procedures and costs compared to the past has been particularly attractive and has allowed the development of a large number of Unmanned Aerial Vehicles (UAV). Similarly, other specialized usage scenarios have developed by implementing Unmanned Surface Vehicles (USV) and Unmanned Underwater Vehicles (UUV). Among the various projects underway for the development of DT, it is worth mentioning the recent initiative of the European Commission: Destination Earth (DestinE) [8]. The main objective of the project is the development of a digital model of the entire Earth system with high precision and resolution that can allow us to monitor natural and anthropogenic phenomena in order to support sustainable development. These systems include: land, marine, atmosphere and biosphere. Among the areas of study, the marine one is certainly fundamental for life on Earth and at the same time complex to investigate. Water covers more than 70 percent of the Earth's surface and in particular the oceans conserve almost all the available water. Compared to the exploration of terrestrial or aerial systems, the study of submerged environments involves greater challenges: economic, logistical and security. Exploration costs are particularly high because of the vehicles, instruments and personnel required; it is not possible to rely on permanent infrastructures that are available only in small numbers and finally the particularly harsh and hostile environmental conditions pose risks to the safety of human life during these survey activities. Consequently, due to these issues, only a small percentage of the ocean floor has been mapped to date [15]. Fortunately UUV are becoming more and more efficient in marine exploration activities and can therefore solve some of the problems highlighted [16]. In order to carry out exploration and survey activities of submerged environments, the use of appropriate subaqueous vehicle position estimation techniques is essential. Only by knowing the position of the vehicle and its sensors is it possible to safely carry out a survey and produce results that can be compared with other surveys and georeferenced in maps and DT. Increasingly precise instruments and sensors allow for accurate positioning and advanced navigation techniques now allow efficient use of UUVs for surveying [17]. In order to study in depth the complex underwater environment and map different habitats, survey methodologies from different platforms with optical and acoustic sensors have been successfully experimented in the past [18; 19; 20].