Challenges for RF two-way time-of-flight ranging in Wireless Sensor Networks

In applications where a priori determination of location is infeasible, node localization schemes are desirable, which allow the node to estimate its location during network operation. The majority of these schemes are based on ranging between node pairs, which should ideally be performed without adding cost or size to the sensor node. Two-way time-of-flight schemes can fulfill this desire, by utilizing the measurement of the time-of-flight of electromagnetic waves to determine the distance between two sensor nodes. In this paper, we present the implementation and analysis of such a ranging scheme. Because a small error in time measurement can result in a large distance estimation error, the focus of this work lies on the determination and analysis of influencing factors, which limit the accuracy of round-trip-time measurements. We analyze two main contributing factors to the accuracy of the ranging scheme, namely the radio transceiver clock quantization and the link quality during round-trip-time measurement. These effects and their impact on the overall ranging error have been investigated by means of simulation and experimentation. Initial ranging errors as large as 24 m RMS were observed, which could be reduced to errors between 5 and 8 m RMS by utilizing compensation techniques.