This paper addresses the problem of indoor tracking of tagged objects with Ultra High Frequency (UHF) Radio Frequency Identification (RFID) systems. A new and more realistic observation model of the system is proposed, where the probability of detecting a tag by a reader is described by a Beta distribution. We model the probability of detection as a function of both the distance and the angle between the tag and the reader. The considered model also accounts for the possibility of a tag being in a dead-zone, that is, in a space where the tag cannot be detected even if it is well within the range of a reader. For tracking, we propose the use of the particle filtering methodology that takes into account the asynchronous nature of the measurements. The needed parameters for modeling the system are obtained from laboratory experiments and the performance of the algorithm is shown by extensive computer simulations.