## 4 Conclusions and Future Work  
In this paper, we analyzed existing localization techniques for mobile users and autonomous tractors collaborating in unstructured environments while performing complex agricultural tasks. Two well-known approaches for human operator locations, GNSS and DGNSS, and one approach for autonomous tractors, RTK-GNSS, were tested. Although both GNSS and DGNSS approaches yielded satisfactory localization accuracy in the test environment, we want to find an alternative localization system that can operate in GNSS inaccessible environments and thus complement existing techniques. Therefore, our focus was on determining the feasibility of the proposed hybrid localization approach combining RSSI information from wireless routers mounted on the autonomous tractor with RTK coordinates from the tractors. The hybrid solution was planned as a complement to the standard positioning techniques for mobile users. The complementary solution is activated in situations in which the GNSS signal is either poor or lost. Due to poor channel modeling, the RSSI based localization algorithm yielded a substantial positioning error. However, the localization system installed on autonomous tractor had a high accuracy of approximately 2.4 cm. Because the main assumption is that the mobile user will obtain the autonomous tractor coordinates when near the vehicle, a low position error on the vehicle could mitigate errors in the RSSI localization approach.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Row**} & \multirow{2}{*}{**Measurements**} & \multicolumn{2}{c}{**RTK-GNSS (cm)**} \\ \cline{3-4}  & & **RMS** & **S.D.** \\ \hline
1 & 122 & 2.71 & 2.10 \\
2 & 122 & 3.36 & 1.52 \\
3 & 122 & 1.36 & 0.87 \\
4 & 122 & 2.93 & 1.61 \\
5 & 122 & 1.65 & 1.05 \\ \hline All Rows & 610.00 & 2.40 & 1.43 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Statistics for the GNSS receiver using RTK correction signals on the autonomous tractor in motion.  
Thus, this approach may improve localization accuracy and may be used in harsh environments where the GNSS signal is severely attenuated due to dense foliage and obstructed LOS.  
Because of the implemented localization systems, the RTK on autonomous tractors and the RSSI on the UPD, future work will focus on utilizing the vehicle locations in the hybrid localization approach. Additionally, we will use the fourth wireless router at the base station to increase the number of anchors and will fine-tune the radio channel. After this is accomplished, we expect to achieve lower localization errors that will be acceptable for scenarios in which humans and robots work together.  
This paper is partially supported within the European Research Project RHEA. The Telecommunications Research Center Vienna (FTW) is supported by the Austrian government and the City of Vienna within the competence center program COMET. The authors thank to Soluciones Agricolas de Precision S.L., Spain for technical assistance.  
[PERSON] and [PERSON] developed the RSSI localization system running on a user portable device (UPD). [PERSON] and [PERSON] developed the RTK-GNSS localization system on autonomous tractor. Both groups participated with equal contribution in achieving the hybrid location solution and algorithms.  
The authors declare no conflict of interest.