# Underwater Drone Architecture for Marine Digital Twin: Lessons Learned from SUSHI DROP Project +
## 2 Materials and Methods
### IUIV Payload
#### 2.1.1 Positioning Instruments  
A multitude of dedicated positioning tools are provided on board the UUV, starting with the global positioning system that allows absolute localization of the drone during surface navigation. Internally to the Attitude and Heading Reference System (AHRS) instrument are a Global Navigation Satellite System (GNSS) receiver and an Inertial Navigation System (INS) platform. Additional instruments provided for relative positioning during submarine activities are: Fiber Optic Gyroscope (FOG), Doppler Velocity Log (DVL) and Ultra-Short Baseline (USBL). The information acquired by the instruments is processed in real time by Extended Kalman Filter (EKF) according to a Fossen model [43]. These positioning systems are used in real time for navigation operations and thus for piloting Blucy. They are also essential during data processing to provide an accurate georeferencing of submarine surveys and return of project outputs in a universal reference system.  
The data acquired by sensors such as MiniCT and MiniSVS are also essential for precise positioning: conductivity, temperature, pressure and speed of sound. These sensors are used to improve the depth estimation of the UUV. In addition, the data obtained from MiniSVS are used for sound velocity correction within the algorithm for USBL. The same data are crucial for MBES during beam forming and beam steering.  
Positioning instruments are used at different stages during the mission: the start of operations occurs above the water surface with the ability to acquire positioning using AHRS by identifying the position and attitude of the UUV in a geographic reference system. In the underwater navigation phase the GNSS signal is no longer acquired and the positioning is acquired in dead-reckoning using the mentioned procedure via EKF with data detected in combination with FOG, DVL and USBL. For the latter system, it is crucial to implement a correct reference on the surface vehicle by accurately measuring the offsets between the instruments on the boat. In the installation foreseen for the project operational scenario, two GNSS antennas are present to obtain an absolute positioning of the surface vessel and a heading angle, while the pitch and roll parameters are measured by an additional INS platform properly positioned and calibrated. The 3D offset (X, Y, Z)and rotation (rX, rY, rZ) between the GNSS surface vessel Antenna Reference Point (ARP) and the USBL transponder ARP, bound to the structure but appropriately positioned underwater (Figure 2), is measured on the boat and the resulting 3D vector is reported in the Remote Station (RS) software dedicated to USBL positioning.  
Using these positioning techniques it is possible to check in real time the position of the UUV during underwater navigation and to know its attitude. Positioning information is transmitted by cable in ROV mode and by acoustic channel in AUV mode. Thanks to the on-board instruments it is also possible to obtain redundancy on the position estimation.  
Positioning instruments are listed in Table 1. Regarding the choice of positioning tools, the first selection concerned the FOG platform, which is the main element for dead-reckoning navigation. Further components have been selected to ensure maximum compatibility and interoperability between the different subsystems.