  In this letter, the localization of terrestrial nodes when unmanned aerial
vehicles (UAVs) are used as base stations is investigated. Particularly, a
novel localization scenario based on received signal strength (RSS) from
terrestrial nodes is introduced. In contrast to the existing literature, our
analysis includes height-dependent path loss exponent and shadowing which
results in an optimum UAV altitude for minimum localization error. Furthermore,
the Cramer-Rao lower bound is derived for the estimated distance which
emphasizes, analytically, the existence of an optimal UAV altitude. Our
simulation results show that the localization error is decreased from over 300m
when using ground-based anchors to 80m when using UAVs flying at the optimal
altitude in an urban scenario.
