## 2 Materials and Methods
### Concerning the Probes  
First and foremost, it should be emphasized that the primary objective behind the conception and design of the Smart Drifter Cluster is to gather data on the dynamics of surface currents. This is achieved by delivering GPS data that can be transmitted remotely, enabling the tracking of sea navigation routes. The inclusion of other marine monitoring instruments (such as oxygen, salinity, temperature, etc.) on board is considered optional and secondary to the core focus of the project.  
However, the project envisages the possibility of installing these instruments, but the constraints of cost (which underlies the idea of the Smart Drifter Cluster) and the small size of the floating elements clearly affect the choice. A part of the MARTA project was, therefore, devoted to comparing, even in the operational phase, different sensors on the market to select those that best meet the cited constraints. Apart from these basic requirements, this selection process also considered the possibility of configuring the hardware and software in an \"open\" way, i.e., flexible to different needs, as well as ensuring the data quality met the standards for use in an international scientific context.  
Based on a preliminary evaluation process, we selected a company that, in our assessment, manufactures and delivers products that meet our requirements [14]. In particular, we chose the sensors shown in Tables 1-5, along with their respective characteristics provided by the manufacturer (Atlas Scientific, Long Island City, NY, USA). We are still in the process of testing the characteristics and calibration of these sensors. Thus, although these results are intriguing, they are preliminary and require further investigation.  
\begin{table}
\begin{tabular}{l l} \hline Optimized Sensing range & 0–45 \({}^{\circ}\)C \\ \hline Accuracy & \(+/-\)(0.15 + (0.002 \(\times\) T)) \\ \hline Resolution & 0.001 \({}^{\circ}\)C \\ \hline Calibration & Single Point \\ \hline Data protocol & UART and I2C \\ \hline Operating voltage & 3.3–5.5 V \\ \hline Size (PCB/probe) & 13.97 mm \(\times\) 20.16 mm/6 mm \(\times\) 81 cm \\ \hline Weight (PCB) & 1.76 g/40 g \\ \hline \end{tabular}
\end{table}
Table 1: Temperature. PT-1000 Class-A high-purity platinum RTD probe connected to the EZO-RTD Circuit with UART interface. Specifications are provided by the manufacturer.  
\begin{table}
\begin{tabular}{l l} \hline Optimized Sensing range & 10–100,000 \(\mu\)S/cm \\ \hline Response Time & 90\% in 1 s \\ \hline Accuracy & \(+/-\) 2\% \\ \hline Resolution & 0.001 \\ \hline Temperature compensation & Yes \\ \hline Calibration & 2 or 3 point \\ \hline Data protocol & UART and I2C \\ \hline Operating voltage & 3.3–5.5 V \\ \hline Size (PCB/probe) & 13.97 mm \(\times\) 20.16 mm/12 mm \(\times\) 145.5 mm \\ \hline Weight (PCB/probe) & 1.77 g/43 g \\ \hline \end{tabular}
\end{table}
Table 2: Conductivity, salinity, and TDS (PPM). Platinum RTD probe connected to the EZO-RTD Circuit with UART interface. Specifications are provided by the manufacturer.  
\begin{table}
\begin{tabular}{l l} \hline Optimized Sensing range & 0.01–100 mg/L \\ \hline Response Time & 0.3 mg/L/ per s \\ \hline Accuracy & \(+/-\)0.05 mg/L \\ \hline Resolution & 0.001 \\ \hline Calibration & 2 points, \(-\)every year \\ \hline Maintenance & \(\sim\)18 months \\ \hline Data protocol & UART and I2C \\ \hline Operating voltage & 3.3–5.5 V \\ \hline Size (PCB/probe) & 13.97 mm \(\times\) 20.16 mm/16.5 mm \(\times\) 124 mm \\ \hline Weight (PCB/probe) & 1.7 g/52 g \\ \hline \end{tabular}
\end{table}
Table 3: Dissolved Oxygen. This is a quite delicate measure. We have selected a galvanic dissolved oxygen probe consisting of a PTFE membrane, an anode bathed in an electrolyte, and a cathode. Oxygen molecules diffuse through the probes membrane at a constant rate yielding a small voltage at the cathode. The probe is connected to the EZO-RTD Circuit with the UART interface. Specifications are provided by the manufacturer.