# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
###### Abstract
Due to the complexity of the underwater environment, UUVs are the only instruments capable of investigating the hostile underwater environment in some particular contexts. Several authors have approached the problem of controlling UUV vehicles with different methods to stabilize their attitude during navigation [21; 22; 23; 24]. UUVs are particularly critical when observations must necessarily be made with high spatial as well as temporal resolution [25]. For example, using acoustic techniques such as Multibeam Echosounder (MBES) [20] it is already possible to survey high-resolution environmental data to accurately map and provide an automatic unsupervised or supervised classification of benthic communities without the use of invasive techniques [18; 26]. Hopefully, thanks to ongoing technological and knowledge advances, UUVs could already in the next decade reach a level of reliability, performance, efficiency and economy of use similar to that which today belongs to the UAV domain: extremely versatile and economical to produce maps and digital models of numerous contexts related to the marine environment [27; 28; 29; 30; 31; 32]. Moreover, it will be very useful to be able to draw up a list of strategies and best practices for underwater monitoring as is already happening in the terrestrial field [33]. In this paper we describe the challenges and lessons learned from the multipurpose UUV prototype, expanding on the work published in [34], developed within the Interreg Italy-Croatia SUSHI DROP Project (SUstainable fiSHeries wIth DROnes data Processing) [[https://www.italy-croatia.eu/sushidrop](https://www.italy-croatia.eu/sushidrop)]([https://www.italy-croatia.eu/sushidrop](https://www.italy-croatia.eu/sushidrop)) (accessed on 15 January 2022). The objective of the project is the complete development of a UUV for surveying the benthic zone in the deep waters of the Adriatic Sea that is able to produce digital models of the underwater environment. We had the opportunity to develop a UUV prototype from scratch that combined the best possible features in terms of positioning and surveying while maintaining a low total cost and with multipurpose and modularity features. The developed prototype is innovative considering the entirety of its capabilities and the design choices that have identified operational solutions for the project objectives. The result of this work is a platform suitable for gathering information and providing efficient environmental investigation using non-invasive techniques in remote underwater environments. As UUVs are by definition unmanned, they also eliminate the risk of human operators and are therefore suitable vehicles for the exploration of a particularly hostile environment such as the marine one. Until today, the principal instrument to collect information about fish populations and communities is from the analysis of catch collected during surveys performed with research vessels. Traditional survey methods that rely on the capture of living organisms are employed aboard these vessels. Capture is typically accomplished through the use of bottom trawl gear. These traditional methods are invasive to the environment, have high operational costs and are also prohibited in particular marine protected areas. The captured fish is then manually sorted by a group of specially trained operators. For each species, a record is made of the biological variables: length, weight and other key characteristics. This information is useful to understand the specificities of marine populations in the area under investigation. To date, only in some monitoring applications can be found that the data collection phase is instrumentally automated through the use of computer databases at the end of the manual selection process [35].