# Precise Positioning of Autonomous Vehicles Combining UWB Ranging Estimations with On-Board Sensors
## 1 Introduction  
Currently, the vehicle industry is focusing on developing autonomous vehicles in order to improve driving safety and efficiency. For optimal operation, an autonomous vehicle requires to know with a high accuracy its global position. Traditionally, this issue has been covered by Global Navigation Satellite Systems (GNSS) in combination with odometry and inertial data of the vehicle. The result of this data fusion under good GNSS conditions is a significantly reliable and accurate positioning system [1]. However, there are certain environments where GNSS signal reception is limited or impossible, compromising the positioning and navigation of autonomous vehicles. Every satellite system needs, among other requirements, an unobstructed Line-of-Sight (LOS) to four or more satellites in order to provide an enough accurate positioning solution. In some environments, such as urban canyons or tunnels, this can be a great challenge, since the satellite signals need to overcome numerous obstacles before reaching the vehicle's receiver. When GNSS signals are not available, the positioning is only based on the vehicle's inertial and odometry data [2]. Positioning estimates based on odometry and inertial estimates have a tendency to drift with time, increasing the positioning error. Therefore, if GNSS signals are blocked during a long period of time, the vehicle's position will be extremely inaccurate. This unreliable performance of satellite-based positioning systems in some particular environments generates a need for a complementary positioning system that can guarantee the continuity of accurate positioning. Ultra-Wideband (UWB) [3] is one of the most promising technologies to be used as a complementary positioning system providing accurate ranging estimates to be fused with on-board sensors like odometers or inertial sensors. Thus, some previous work have been found with this focus [4; 5]. However, most existing UWB systems are designed for indoors positioning [6; 7; 8; 9; 10; 11; 12], and less attention has been given to apply this technology for autonomous vehicles. Furthermore, in the case of automated manoeuvres, as required in PRoPART project, highly accurate solution is needed working not only outdoors but also at high speed over large distances. Although the UWB range is limited, in the case of PRoPART several UWB nodes can be deployed on the gantry poles required for other purposes and installed every 200 m. It can be said that other equipment such as Road Side Units (RSUs) are required on these gantry poles to be able to do an automated manoeuvre. These RSUs will send the required corrections to the Real Time Kinematic (RTK) system on-board through Vehicle-to-Everything (V2X) communications and will use radar technology to detect if the lane is free or not before the lane change. Thus, the developed system will permit to know if another equipped or non-equipped vehicle is occupying the lane or otherwise if it is safe to do the manoeuvre. Hence, this use case allows us to deploy the UWB nodes easily and without additional infrastructure to cover large distances. Another important issue is to perform tests in a real environment because the performance of the solution is deeply affected by the environmental conditions and surrounded objects. Hence, ad-hoc calibration methods are required to comply with the accuracy and range requirements for an specific scenario. To tackle the above challenges, this paper makes the following contributions:  
* Equip the Road Side Units (RSUs) located aside the roads with UWB fixed nodes for redundancy and robustness in areas where the coverage of GNSS is poor. These ranging units will allow the automated vehicle to calculate its distance to known points on the road. * Development of an accurate and robust positioning solution based on the data fusion of UWB ranging estimates together with inertial and odometry data. This solution enhances the vehicle's positioning in those environments where the GNSS signal reception is limited or impossible.