Simultaneous Mobile Robot and Radio Node Localization in Wireless Sensor Networks

1 Summary Accurate and cost effective localization is a basic issue in the field of mobile robotics. This work shows how it can be solved by using wireless radio nodes as landmarks. Mechanical scanning is employed at the mobile robot to measure the angle of arrival of radio signals. A measurement integrity check is realized by means of normalized cross correlation. This enables the recognition of multi path propagation and radio interferers, thereby ensuring high robustness. A simultaneous localization and mapping approach is implemented to localize the robot and to build a map of static radio nodes simultaneously. This allows localizing the robot in unknown environments in an ad hoc and unobtrusive fashion. 2 Method to measure the angle of arrival The foundation of this work is a new method to measure the angle of arrival in wireless networks based on the received signal strength indicator (RSSI). It requires the antenna on the robot to be rotated about a vertical axis while recording the RSSI. This results in a radiation pattern in the azimuth plane. Its cross correlation with the known reference radiation pattern of the antenna allows to determine the relative angle. A detailed description of the method and a thorough error analysis are given in [1] and [2]. A schematic of the setup is shown in figure 1. To ensure a distinct radiation pattern, a monopole antenna is slightly modified by the attachment of a reflector plate. This increases the uniqueness of the angle given by the cross correlation. The advantages of this method are its robustness to antenna anisotropy of the static nodes and the possibility for measurement integrity by the consideration of the maximum correlation coefficient, which is a measure for the similarity between the recorded and the reference radiation patterns. Thereby the localization is robust in multipath environments. Further the presented method does not require any modification of the sensor network and thus allows the use of commodity radio hardware. It also does not assume any propagation models and therefore does not require any parameters to be known in advance enabling localization in completely unknown environments. Figure 19: Setup for the determination of the relative angle: a ϕ is the rotation angle of the antenna, r ϕ is the relative angle to the static node with position n x. 338 3 SLAM Based on this measurement method we apply the SLAM approach to simultaneously localize …