# Precise Positioning of Autonomous Vehicles Combining UWB Ranging Estimations with On-Board Sensors
## 2 Related Work
### Precise Outdoor Positioning
After post-processing the data, a RMSE 3D positioning distance error of around 57 cm in the best case scenario was achieved. The results showed how the performance of the proposed sensor fusion improve the accuracy and availability of the system in denied GPS environments. Table 1 shows a summary of the related work. Although some UWB approaches have been found for outdoor applications, the literature shows how UWB technology is mainly used for positioning in indoor applications. Different publications that propose an UWB/INS positioning solution for IPS highlight that the performance of the UWB systems can be really affected by the environment. Depending on the environment different sources of error, such as multipath interferences, can affect the UWB system. Normally, in an indoor application, these kind of errors can be in somehow determined and mitigated beforehand, since the scenario is known and does not change over time. However, there are cases in which the testing scenario is not known and the mentioned errors cannot be determined previously. For overcoming these errors, [7] proposes an adaptive solution for tracking the dynamic changes in the performance of the UWB ranging estimates. In an outdoor application, it is not possible to characterise the environment, since it undergoes continuous changes. This supposes a great challenge when working with UWB systems, reflecting in a great variation of the uncertainty of the UWB ranging estimates. In the last years, several publications that evaluate the performance of an UWB/INS positioning solution for vehicles in an outdoor environment have reflected this issue. In general, the aim of these publications is to build an UWB/INS positioning solution that enhances the performance of GNSS based positioning systems, especially, in the areas in which GNSS signals are affected. Despite the fact of being applications based on vehicle's positioning, only in [4; 14] a real vehicle was used for evaluating the performance of the proposed solution. Furthermore, only in [4] an integrated UWB/IMU positioning solution is proposed, as [14] is more focused on integrating \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Article** & \begin{tabular}{c} **Outdoor** \\ **App.** \\ \end{tabular} & \begin{tabular}{c} **Motion** \\ **App.** \\ \end{tabular} & \begin{tabular}{c} **Sensor** \\ **Fusion** \\ \end{tabular} & \
\begin{tabular}{c} **Fusion** \\ **Alg.** \\ \end{tabular} &
\begin{tabular}{c} **Validation** \\ **Pos.** \\ \end{tabular} \\ \hline
[6] & No & Yes & \
\begin{tabular}{c} UWB/ \\ IMU \\ UWB/ \\ INS \\ \end{tabular} & 8 state-EKF &
\begin{tabular}{c} Real/ \\ Simulations \\ \end{tabular} & Yes \\
[7] & No & Yes & \
\begin{tabular}{c} UWB/ \\ INS \\ \end{tabular} & SHFAF &
\begin{tabular}{c} Real/ \\ Simulations \\ \end{tabular} & Yes \\
[8] & No & Yes &
\begin{tabular}{c} UWB/ \\ IMU \\ \end{tabular} & EKF/UKF & Real & Yes \\
[4] & Yes & Yes &
\begin{tabular}{c} UWB/ \\ IMU \\ \end{tabular} & EKF & Real & Yes \\
[5] & Yes & Yes & GPS/UWB/ &
\begin{tabular}{c} RKF \\ \end{tabular} & Real & No \\
[9] & No & Yes &
\begin{tabular}{c} INS/PDR/ \\ UWB \\ \end{tabular} & AKF & Real & Yes \\
[10] & No & Yes &
\begin{tabular}{c} INS/ \\ UWB \\ \end{tabular} & EKF & Real & Yes \\
[11] & No & Yes & \
\begin{tabular}{c} INS/ \\ UWB \\ \end{tabular} & PAKF &
\begin{tabular}{c} Real/ \\ Simulations \\ \end{tabular} & Yes \\
[12] & No & Yes &
\begin{tabular}{c} INS/Vision/ \\ UWB \\ \end{tabular} & EKF & Simulations & Yes \\
[14] & Yes & Yes &
\begin{tabular}{c} GPS/UWB/ \\ Bearing (V2V) \\ \end{tabular} & EKF & Real & Yes \\
[15] & Indoor/Outdoor & Yes &
\begin{tabular}{c} GPS/INS/ \\ UWB \\ \end{tabular} & KF & Real & Yes \\
[16] & Yes & Yes &
\begin{tabular}{c} GPS/ \\ UWB \\ \end{tabular} & KF & Real & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the main features of the related work that propose an UWB/INS positioning solution. GPS/UWB/Bearing for Vehicle-to-Vehicle (V2V) communications.