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We present an analysis of the time of arrival (TOA) and time difference of arrival (TDOA) based positioning methods in Non-Line-of-Sight (NLOS) environments. Single path propagation is assumed. The positioning accuracy is investigated in terms of Dilution of precisions (DOPs) depending on different NLOS cases. We then show that the weighted DOP (WDOP) is equal to the Cramer-Rao Lower Bound (CRLB) when the number of Line-of-Sight (LOS) base stations (BSs) is sufficient (three or more). Some characteristics of the DOP in the Global positioning system (GPS) are introduced to investigate the performance of positioning accuracy. In addition, some new characteristics of the DOP and the relationship of the DOP between the TOA and TDOA based methods are derived. According to these characteristics, the system can improve positioning accuracy through proper choice of optimum BSs and location techniques.