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Several sources of error disturb the range estimates utilized to infer the position of a mobile user (MU). This paper presents a method which includes two stages of error reduction for a round-trip time (RTT)-based indoor location system. First, systematic error due to non-line of sight (NLOS) propagation is mitigated by adapting the prior-NLOS- measurements-correction (PNMC) technique to the input of a particle filter (PF). Thereupon, the latter is used to filter out the random error in the range estimates which is not assumed to be Gaussian distributed, on the contrary to what is commonly used in the literature. Both techniques are described and their performance is assessed in a real indoor environment achieving a reduction of more than 50% in the total error.