I. Introduction  
A. Existing Surveys and Reviews in the Literature  
B. Contributions and Organization of This Survey  

II. Radio Waves and Radar Basics  
A. Radio Waves Interaction with the Aerial Vehicles and Surroundings  
B. Radar Parameters  
C. Popular Radar Signal Processing Techniques  

III. Analysis of Aerial Threats and Challenges  
A. Capabilities and Features of Common UAVs  
B. Threats and Challenges from UAVs  
C. Limitations of Radar Systems  

V. DCT-U Using Communication Systems  
C. DCT-U Using JC&S  
D. DCT-U Using Communication Systems in Passive Mode  
E. Limitations of Communication Systems for DCT-U  

VI. DCT-U Using Passive Monitoring of RF Signals Radiated from UAVs  
A. RF Signals Radiated from UAV for DCT-U  
C. Limitations of Passively Capturing RF Signals Radiated from UAVs  

VII. Future Directions  
A. Communication Systems  
B. Single Long-range and High-resolution Radar  
C. Quantum Radars  
D. Hybrid Radars  
E. Sparse Signal Processing  
F. Open-Ended, Modular, and Reconfigurable Design  
G. Resilience to ECM  
H. Laser-Based Techniques  
I. Distributed Sensors and Sensor Fusion  
J. Regulating the UAV Air Traffic  
K. Airborne UAVs  
L. Space Assets  

VIII. Concluding Remarks