# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
## 2 Materials and Methods
### IUIV Payload
#### 2.1.2 Surveying Instruments  
A number of scientific instruments have been provided on board the drone for multi-parameter remote sensing survey of the marine environment. These include several passive and active sensors. In the current configuration there are two optical cameras, PilotCAM and BottomCAM, and an MBES (Figure 3). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sensor** & **Parameters** & **Model** \\ \hline FOG & Position, Attitude & iXblue Fiber Optic Gyroscope Compact C3 \\ DVL & Linear Speed and Acceleration & Nortek Doppler Velocity Log DVL500-300 m \\ AHRS & Latitude, Longitude & LORD 3 DM-GX5-45 GNSS/INS \\ ALT & Altitude & Tritech PA200 \\ USBL & Relative Position & EvoLogics S2C M 18/34 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Instruments for positioning of the UUV Blucy. Figure 2: Setup for USBL positioning on surface vessel: (**a**) GNSS antennas and INS platform. (**b**) Structure for USBL. The first optical camera is positioned in the frontal part of the drone and the sensor coupled to its lens has been selected to obtain an image with wide Field of View (FOV). The typical output of this sensor is a video stream transmitted in real time thanks to the wide bandwidth available in ROV mode using the fiber optic cable that connects the UUV with the RS. The sequence of images immediately visible on the RS allows precision navigation at particularly close range to the seabed. The same images are used for inspection, visual census and the video stream can be reprocessed afterwards with different Computer Vision algorithms. PilotCAM is installed on a special custom support that allows us to change the inclination angle of the sensor according to the different operational scenarios of the mission. BottomCAM is installed at the bottom of the UUV with viewpoint towards the seabed (nadir) to allow use for photogrammetric purposes. Thanks to the high resolution images and to the presence of several LED illuminators it is possible to acquire sequences of images following a specific survey plan. Parameters such as overlap, shutter speed are then studied and the height above the seabed is selected according to the necessary Ground Sample Distance (GSD) and visibility conditions in the water column. The optics selected for the setup has a focal length of 24 mm, ensuring a wide FOV. Following a classical photogrammetric scheme, the UUV navigation in the BottomCAM survey operations will follow a path maintaining constant altitude to avoid distortions in the final model. In ROV mode the shooting command is automatically given by RS according to the survey parameters. The bidirectional communication takes place according to network protocols through the fiber optic cable. Inside the waterproof container where the BottomCAM is located it has been foreseen the installation of a signal conversion module suitable to remotely control the camera and receive in real time the images acquired by the UUV on the RS. This allows an immediate processing using a particularly high performance workstation. The MBES active acoustic sensor is used to detect information and produce maps related to more distant objects. It is possible to survey the seabed, acquire high quality bathymetric data, water column information and quantitative fisheries stock assessment for habitat mapping, which is one of the main goals of SUSHI DROP Project. MBES can be used in both ROV and AUV modes. In the second mode all acquired information is stored in a dedicated on-board computer present inside the UUV architecture. In this case the data are only subsequently downloaded to the RS at the completion of the mission, due to the reduced bandwidth available for acoustic communications. Other parameters concerning the water column are punctually acquired during the navigation by MiniCT and MiniSVS sensors: conductivity, temperature and sound velocity.