Figure 1
From: Range-Based Localization for UWB Sensor Networks in Realistic Environments
Example location scenario. The network consists of four static reference nodes R1–R4 (indicated by receiving antenna in green/black) and one target node T1 (indicated by a robot with transmitting antenna). The blockage by the wall between Node R1 and T1 creates an NLOS situation, but for the others reference nodes, T1 is in an LOS position. The 
are the range estimates between the reference nodes and the target node.