This work is focused on integrating a monopulse system into a passive RFID (Radio Frequency Identification) reader operating in the ISM (Industrial, Scientific and Medical) band centered at 915 MHz to identify the sense of motion of RFID tags installed on vehicles. The channel between the reader and the tag is modeled using the basic Friis equation for free space with measured values of antenna gains. The statistical behavior of both readers and tags is derived from measurements using actual components and it is introduced in the system model using the Monte Carlo Method to obtain more realistic performance figures at the system level.