  The adoption of Unmanned Aerial Vehicles (UAVs) for public safety
applications has skyrocketed in the last years. Leveraging on Physical Random
Access Channel (PRACH) preambles, in this paper we pioneer a novel localization
technique for UAVs equipped with cellular base stations used in emergency
scenarios. We exploit the new concept of Orthogonal Time Frequency Space (OTFS)
modulation (tolerant to channel Doppler spread caused by UAVs motion) to build
a fully standards-compliant OTFS-modulated PRACH transmission and reception
scheme able to perform time-of-arrival (ToA) measurements. First, we analyze
such novel ToA ranging technique, both analytically and numerically, to
accurately and iteratively derive the distance between localized users and the
points traversed by the UAV along its trajectory. Then, we determine the
optimal UAV speed as a trade-off between the accuracy of the ranging technique
and the power needed by the UAV to reach and keep its speed during emergency
operations. Finally, we demonstrate that our solution outperforms standard
PRACH-based localization techniques in terms of Root Mean Square Error (RMSE)
by about 20% in quasi-static conditions and up to 80% in high-mobility
conditions.
