Time of flight based two way ranging for real time locating systems

Node positions in a wireless sensor network are required to track the nodes in real-time, but normally no prior information is given on deployment. A locating mechanism is required for such a purpose and in this paper a method is described to determine the point-to-point distance between two sensor nodes in an asynchronous location positioning system. A time-of-flight (TOF) based two-way ranging method, independent of network synchronization, is used. One critical factor that influences the round-trip time of a signal is the offsets between the crystal oscillators and their resolutions. These factors can educe temporal errors in the order of nanoseconds and eventually distance errors in the order of decimetres. The main idea of the proposed ranging method is to improve the accuracy and stability of estimated round-trip time for low frequency oscillators with inherent drift. It is demonstrated that a simple and effective iterative ranging algorithm reduces ranging error significantly. The analytical results of the algorithm are further corroborated by simulations.