  The use of unmanned aerial vehicles (UAVs) for a variety of commercial,
civilian, and defense applications has increased many folds in recent years.
While UAVs are expected to transform future air operations, there are instances
where they can be used for malicious purposes. In this context, the detection,
classification, and tracking (DCT) of UAVs (DCT-U) for safety and surveillance
of national air space is a challenging task when compared to DCT of manned
aerial vehicles. In this survey, we discuss the threats and challenges from
malicious UAVs and we subsequently study three radio frequency (RF)-based
systems for DCT-U. These RF-based systems include radars, communication
systems, and RF analyzers. Radar systems are further divided into conventional
and modern radar systems, while communication systems can be used for joint
communications and sensing (JC&S) in active mode and act as a source of
illumination to passive radars for DCT-U. The limitations of the three RF-based
systems are also provided. The survey briefly discusses non-RF systems for
DCT-U and their limitations. Future directions based on the lessons learned are
provided at the end of the survey.
