  The use of unmanned aerial vehicles (UAVs) for different applications has
increased many folds in recent years. The UAVs are expected to change the
future air operations. However, there are instances where the UAVs can be used
for malicious purposes. The detection, tracking, and classification of UAVs is
challenging compared to manned aerial vehicles (MAVs) mainly due to small size,
complex shapes, and ability to fly close to the terrain and in autonomous
flight patterns in swarms. In this survey, we will discuss current and future
aerial threats, and provide an overview of radar systems to counter such
threats. We also study the performance parameters of radar systems for the
detection, tracking, and classification of UAVs compared to MAVs. In addition
to dedicated radar systems, we review the use of joint communication-radar
(JCR) systems, as well as passive monitoring of changes in the common
communication signals, e.g., FM, LTE, and any transmissions that may radiate
from a UAV, for the detection, tracking, and classification of UAVs are
provided. Finally, limitations of radar systems and comparison with other
techniques that do not rely on radars for detection, tracking, and
classification of aerial threats are provided.
