Accurate and real-time measurement of network characteristic is essential to optimize performance of wireless mesh networks. We present architecture for online measurement of wireless mesh networks, and address the problem of location selection of measurement nodes. In order to obtain the best location of measurement nodes, an ellipse secant algorithm is developed, which combine the signal detection capability
with the transmission capability. The optimized location of measurement nodes is selected inside a set of ellipses with the measured node and data centers respectively as the focus. Simulation results show that the online measurement architecture can achieve best tradeoff between signal detection capability and transmission capacity, and guarantee the quality of online measurement. The ellipse secant algorithm has computational
complexity lower than the random selection algorithm.