  Localization plays a key role for safe operation of UAVs enabling beyond
visual line of sight applications. Compared to GPS based localization, cellular
networks can reduce the positioning error and cost since cellular connectivity
is becoming a prominent solution as a communication system for UAVs. As a first
step towards localization, UAV needs to receive sufficient number of
localization signals each having a signal to interference plus noise ratio
(SINR) greater than a threshold, which is defined as localizability. On the
other hand, three-dimensional mobility of UAVs, altitude dependent channel
characteristics between base stations (BSs) and UAVs, its line of sight and
non-line of sight conditions, and resulting interference from the neighboring
BSs pose challenges to receive usable signals from the required number of BSs.
In this paper, we utilize a tractable approach to calculate localizability,
probability of successfully receiving usable signals from a certain number of
BSs, in a two-tier hexagonal cellular network. Localizability has an impact on
overall localization performance regardless of the localization technique to be
used. In our simulation study, we investigate the relation between the
probability of localizability with respect to number of participating BSs,
post-processing SINR requirement, air-to-ground channel characteristics, and
network coordination, which are shown to be the most important factors for the
localizability performance of UAVs.
