Measuring Round Trip Times to Determine the Distance Between WLAN Nodes

This technical report explores the degree of accuracy to which the propagation delay of WLAN packets can be measured using today’s commercial, inexpensive equipment. The aim is to determine the distance between two wireless nodes for location sensing applications. We conducted experiments and measured the time difference between sending a data packet and receiving the corresponding immediate acknowledgement. We found the propagation delays correlate closely with the distance, having only a measurement error of a few meters. Furthermore, they are more precise than the received signal strength indications. To overcome the low time resolution of the given hardware timers, various statistical methods are applied, developed and analyzed. For example, we take advantage of drifting clocks to determine propagation delays that are forty times smaller than the clocks’ quantization resolution. Our approach also determines the frequency offset between remote and local crystal clocks.