A novel RF time-of-flight estimation method for asynchronous wireless sensor network nodes

Recently, ranging based on RF TOF has been expected to give a good performance for distance measurements in wireless sensor networks (WSNs). However, short distance and thus, extremely short TOF measurements make precise TOF estimation for asynchronous nodes in WSNs big problems to challenge. For precise RF TOF estimation, previous research have developed some heterogeneous clock systems (HCSs) generating a virtual resolution which is much more precise than the operating clocks in ranging nodes. However, while the virtual resolution then makes it possible for precise TOF estimation, present HCSs need a precise wireless synchronization which is often difficult to implement. Therefore, as an indispensable solution, we propose a new asynchronous TOF estimation method for the present HCSs. The basic idea of the proposed method is to calculate TOF from the round trip time and reply time measured in two ranging nodes. However, because of relative timing changings for every time and every node pair of measurements in WSNs, we have done a carefully theoretical analysis to find out a unique TOF estimation formula for all the cases. Also, we have developed a new HCS operation way to apply the method.