## 3 System Specification  
The steering system specification for the unit (Table 1) is based on a combination of skid steering and a traditional rudder (Figure 5). \begin{table}
\begin{tabular}{l l} \hline \hline
**Specification** & **Data** \\ \hline Dimensions (L \(\times\) W \(\times\) H) & 4230 \(\times\) 2080 \(\times\) 1390 mm \\ Draft & 500 mm \\ Weight & 360 kg \\  & 48 V 200 Ah lithium iron phosphate battery (LiFePO\({}_{4}\)) (16 cells) for \\ Power supply & propulsion, 24 V lead-acid battery for electronics \\ Endurance & 12 h (ruise speed) \\ Motors & 2 \(\times\) Torqeed Cruise 4.0 \\ Remote control range & 40 km \\ Telemetry data range & 50 km \\ Payload data range & 6 km \\ Max speed & 14 knots \\ \hline \hline \end{tabular}
\end{table} 
Table 1: USV technical specification. Figure 4: Tracking areas for USV hydrographic survey using L1 controller. Skid-Steering is a type of vehicle steering where rotation (yaw) is obtained by a difference in the speeds of the left and right propellers (wheel) and is typical for vehicles with non-orientable propellers (wheels). Electric motors installed on the platform and situated in the right hull are turned using a linear actuator, therefore, maneuverability is enhanced, and the unit is more responsive to steering, when compared to steering using only pure skid steering. Additionally, this USV uses pivot turns when the angle of turn is greater than a specific angle (can be set in the Pivot Turn Angle parameter). The pivot turn angle was selected as a result of extensive trials and was equal to 45\({}^{\circ}\). The USV is equipped with an autopilot and the navigation position is calculated based on the autopilot's internal sensors: three magnetic compasses (nine MEMS magnetometers), an inertial navigation system (INS) based on simple MEMS (microelectromechanical system) sensors consisting of nine gyroscopes and nine accelerometers (compasses and INS embedded in autopilot) and an external GNSS receiver based on an UBlox M8N module. In that configuration, the USV position is calculated using an EKF (extended Kalman filter). The EKF is a 24-state extended Kalman filter and the autopilot's filter estimates the following states: altitude, velocity, position, gyro bias offsets, gyro scale factors, Z accel. bias, Earth's magnetic field, platform body magnetic field, and wind velocity. For calculations in this study, the navigation autopilot used the EKF output position, which means that this was not a pure GPS reading, but filtered and estimated based on other internal sensors. Only one GNSS module based on the UBlox M8N module was used (Table 2). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter Name** & **Specification** \\ \hline Channels & 72 \\ \hline \multirow{5}{*}{Signal tracking:} & GPS: L1C/A \\  & SBAS: L1C/A \\  & QZSS: L1C/A \\  & GLONASS: L1 OF \\  & BeiDou: B1 \\  & Galileo: E1B/C2 \\ \hline \multirow{2}{*}{Horizontal position accuracy 1} & GPS and GLONASS: 2.5 m \\  & SBAS: 2.0 m \\ \hline Velocity accuracy 2 & 0.05 m/s \\ \hline True heading accuracy 2 & 0.3\({}^{\circ}\) \\ \hline \multirow{3}{*}{Operating limits} & Altitude: 50,000 m \\  & Velocity: 500 m/s \\  & Acceleration: 4 g \\ \hline \multirow{2}{*}{Time to first fix} & Cold start: \textless{}26 s \\  & Warm start: \textless{}1 s \\ \hline Max output frequency & 5 Hz \\ \hline \hline \end{tabular}  
* CEP, 50%, 24 h static, \textless{}130 dBm, \textgreater{}6 SVs; 250% at 30 m/s.  
\end{table} 
Table 2: Navigation GPS specification. Figure 5: Combined skid and traditional steering system—motors and rudder design plan (**a**), photo of USV HydroDron during experiments (**b**). The GPS RTK module readings are not used for navigation; the module is part of the hydrographic equipment and additionally, the acquired RTK readings were used as a reference and as an independent UV position registration.