Recent advances in the field of Ultra-Wide Band (UWB) localization systems have brought a renewed interest to the problem of indoor localization systems, which have a wide range of applications. We propose in this paper to improve the localization performance of the Pozyx system based on UWB technology. The improvement in terms of localization accuracy is achieved using a multilateration algorithm combined with a strategy of anchors selection. This latter is proposed to overcome the problem of distance measurement errors caused by harsh environments. The conducted experiments and simulations show the effectiveness of the proposed algorithm.