Inaccurate range estimates often restrict indoor position-ing systems, resulting in a more remarkable drawback when using an already-deployed IEEE 802.11 network. This is the case of the time de-lay based location system that this paper deals with. The main causes of these inaccuracies are multipath and non-line-of-sight (NLOS) ef-fects. These effects can be solved to a large degree by characterizing arrival times and range estimation errors. For this reason, this pa-per analyzes multipath and NLOS effects involved in the round-trip time (RTT) discrete measuring process, which is conducted before each range estimate. RTT observations obtained in this process for differ-ent real indoor environments provide useful statistical information that allows to make the work extendable to other similar scenarios. More-over, from this statistical information, the nodes in the network can estimate several parameters of the range estimates distribution while performing the location process. These are used to reduce the error caused by multipath components and to predict and correct the NLOS biases produced. In this way, the NLOS error is dynamically estimated and corrected, achieving better results than classical approaches based on static parameters. Corresponding author: J. Prieto (jprieto@cedetel.es).