## 4 Experiment
### Experiment Results  
After four months of sparse activity, logs gathered on the base station were compiled, sanitized and analyzed. Since the two equipped tractors were rarely operating simultaneously, we focused on the logs of days providing the most extensive data. A summary of the activity during these days is presented in Table 1. Day 1 was the experiment setup; we checked that communication was well established between the three nodes, and moved RASP01 and RASP02 to make them enter and exit communication range with each other and with sensors. On Day 3, the tractor carrying RASP01 was spreading fertilizer, and on Day 4 it sowed the fields. Day 5 featured the longer trip from RASP02. Table 2 compiles global observations made throughout the experiment. When tractors were active, we reported an average speed of 16 km/h, which was not an hindrance to message delivery. Opportunistic sensing and the data mule approach is therefore a viable solution in the agricultural context where vehicles speed is moderate. Although cellular network coverage was quite poor on Montoldre site, opportunistic networking via ad hoc Wi-Fi proved to be effective to convey small messages such as environmental data. Under the right circumstances, the communication range exceeded our expectations with a recorded maximum range of 468 m. This result is further supported by Figure 8, which shows the number of actual contacts at a given time between two nodes versus an estimate derived from the actual node positions and a fixed range of 200 m. More often than not, actual contacts were established, whereas the estimate did not consider the nodes to be within radio range. Contact durations (i.e., the time periods during which the nodes are in contact and can communicate) are presented in Figure 9. On Day 5, half of the contacts lasted less than 3 min while only a tenth of the contacts exceeded 10 min. The few contacts exceeding 1 h were due to RASP02 instance remaining active in the warehouse, within range of the base station. The relatively moderate speed of the tractors combined with the long range Wi-Fi interfaces resulted in long enough contacts allowing for wide communication time periods. Since the size of exchanged messages, whether they were gossiping relating to the opportunistic communication protocol or sensor data, was quite small (a few bytes), such a time frame is sufficient to perform opportunistic sensing (i.e., collect sensor data and opportunistically forward these data). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Average Speed**} & \multicolumn{4}{c}{**Communication Range**} \\ \cline{2-6}  & **Maximum Range** & **Average Range** & **Median Range** \\ \hline
15.9 km/h & 468 m & 237 m & 185 m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tractors average speed and radio-contact range. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Trip Duration**} & \multicolumn{4}{c}{**Number of Messages Exchanged**} & \multicolumn{2}{c}{**Number of GPS Positions**} \\ \cline{2-6}  & **RASP00/RASP01** & **RASP00/RASP02** & **RASP01/RASP02** & **RASP01** & **RASP02** \\ \hline Day 1 & 5h09 & 46 & 110 & 48 & 5 & 147 \\ \hline Day 2 & 4h50 & 28 & 47 & 3 & 0 & 28 \\ \hline Day 3 & 6h21 & 56 & 0 & 0 & 266 & 0 \\ \hline Day 4 & 7h31 & 334 & 0 & 0 & 1005 & 0 \\ \hline Day 5 & 7h44 & 219 & 0 & 51 & 0 & 2063 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data of active days. Nevertheless, it is worth noting that the experiment did not go entirely smoothly. Sometimes the RaspberryPi was slow to receive the GPS signal, which not only led to missing positions, but also to clock desynchronization and erroneous timestamps since the system clock was set to GPS time. However, logs could be resynchronized using contacts and exchanged messages with the base station. Due to farming vehicle vibrations, sometimes the device got its GPS antenna or its power cord  
Figure 8: Real contacts vs. estimated contacts of RASP00, RASP01 and RASP02 on Day 1.