## 3 Results
### Tests  
The first batch of tests took place in small lakes due to their moderate environmental conditions. First, we checked and improved the performance of the primary drifter by fine-tuning the system parameters. In particular, it was necessary to do the following:  
* verify the water tightness and buoyancy of the vehicles, together with the correct functioning of the two thrusters.
* fine-tune the parameters for the motion estimator, the motion controller, and the guidance law modules responsible for the autonomous following of generic trajectories.
* prescribe a batch of trajectories and verify the ability of the vehicle to follow them. In particular, the path-following error, defined as the error between the geodetic position of the vehicle and the desired geodetic position on the path, had to decrease over time.  
After succeeding in this first phase of testing (see Figure 6), a second set of lake trials was performed with the MARTA Smart Drifters Cluster. In this second scenario, we checked the communication protocol between the vehicles and the capability of primary drifters to track the position and velocity of each secondary drifter, and input information to the MPC module. Moreover, we verified the quality of the measurements collected with the sensors mounted on the drifters and finally the satellite communication.  
Figure 5: The primary drifter is following the central position of two secondary drifters (orange circles), following a trajectory (red line) computed interpolating predictions of the centroids (circles on the trajectory).  
A final test was held in the sea location of Tellaro, (La Spezia-IT) (Figure 7). On this occasion, we verified that a primary drifter was able to autonomously follow a very small cluster (one and two elements in two different trials) of passive drifters for an extended period. We also verified that each drifter was able to communicate its sensors' measurements (Figure 8), thus we collected satellite data communications, representing them in real time on a dedicated graphic user interface (Figure 9).  
Figure 6: Path following error–norm of error between the geodetic position of the vehicle and the desired geodetic position on the path.  
Figure 7: Captured picture of a motorized drifter (on the right) following a single passive drifter (on the left) during the Tellaro sea trial.  
Power consumption and energy harvesting are crucial points in the project. Two lightweight photovoltaic panels, laminated with plastic material (19 cm \(\times\) 14.5 cm in size,  
Figure 8: T and pH sensor measurements collected and communicated by a single secondary drifter during the Tellaro sea trial.  
Figure 9: Example of transmitted data, represented on the dedicated GUI. A single primary drifter is following a single secondary drifter (ID 1) with temperature, pH and DO sensors mounted on it.  
with peak power of 4.5 Wp and MPP voltage of 2.65 V) and made with proven technology for installation in an intensive marine environment, were used. The storage consists of four Li-ion 26650 5000 mAh cells. With this configuration, the drifters turned out to have virtually unlimited autonomy, even in winter and in cloudy conditions. Figure 10 displays an example of the energy balance on board the primary element during one of the sea tests.  
In the exceptional case of severe meteorological events extending over time (more than a week), each drifter element is programmed to put itself into a stand-by state as soon as it reaches a power shortage condition. When the energy level of the batteries allows it again, the recorded data is then transmitted on a delayed basis.