In this paper, a 2.5D positioning system is analyzed, based on Received Signal Strength (RSS) measurements, collected from narrowband RF transmissions. The mobile node of the considered system uses multiple closely spaced receivers, collecting RSS measurements and combining them to mitigate multipath effects. Using simulations and optical approximation, the effect of antenna directivity on the positioning accuracy is analyzed. Moreover, a simple iterative method is proposed, that further improves the accuracy obtained using the multipath mitigation technique.