This paper presents a positioning algorithm to improve the performance of location systems based on two-way TOA techniques. This algorithm, which we call passive-TDOA, consists of computing the time-difference-of-arrival (TDOA) measurements from the TOA messages that some terminals generate in the positioning process. Passive-TDOA offers accurate results under optimum geometry. This work goes a step further and evaluates the accuracy of the algorithm in scenarios with adverse geometry. Accordingly, scenarios with access points in line and non-line of sight are studied through simulation and compared with the accuracy achieved by means of regular TOA.