# Networked Unmanned Aerial Vehicles for Surveillance and Monitoring: A Survey
## 6 Conclusions  
In this paper, the applications of networked UAVs for surveillance and monitoring were investigated. Several typical problems across applications such as the navigation, deployment optimization, and coordination of UAVs were discussed. Specifically, we select some representative publications and classify them by targets of interest. In detail, we presented a review on the use of UAVs for general target surveillance, traffic monitoring, wildlife monitoring, and radio surveillance. The related research gap and future directions in the form of limitations of existing approaches and possible solutions are also discussed at the end of each main section. This paper mainly focused on some high-level topics of surveillance UAVs. Some other aspects that are related to surveillance UAVs, such as communication techniques between UAVs and low-level control techniques, are not covered.  
We notice that most of the existing publications on surveillance by UAVs have not taken into account the noise produced by UAVs. They can make strong noise especially when they are close to the targets of interest. While such noise may not impact monitoring performance in certain scenarios such as using UAVs to monitor road traffic, it does make a strong influence in sound-sensitive applications such as the surveillance of wildlife animals and criminal suspects. As these types of targets may not behave as normal more once, they realize that some unfamiliar objects (e.g., UAVs) are around. Thus, developing low-noise UAVs would be a quite useful direction for future study. Additionally, there is still room for the development of highly autonomous UAVs. State-of-the-art UAVs are mainly trained for specific and well-structured environments. They are very likely to fail to operate in unstructured or new environments. From this sense, developing UAVs that are capable of learning and on-the-fly adaptation to environmental novelty is necessary and challenging [118; 119]. Moreover, off-the-shelf UAV products, especially rotary-wing UAVs, are often restricted in operation time due to limitations in the payload. Therefore, the next generation of UAVs needs to be more energy-efficient than current systems and able to pursue complex goals in highly dynamic and even adversarial environments. Last but not least, as counter-UAV systems have been studied and developed [120; 121], the development of smarter UAVs that can avoid detection and tracking by such a system is meaningful, especially in military surveillance applications.  
Conceptualization, X.L.; funding acquisition, A.V.S.; methodology, X.L. and A.V.S.; investigation, X.L.; project administration, A.V.S.; supervision, A.V.S.; writing--original draft preparation, X.L.; writing--review and editing, A.V.S. All authors have read and agreed to the published version of the manuscript.  
Figure 11: Signal transmission of the proposed UTDOA by UAV-BS.  
Funding: This work was supported by the Australian Research Council. In addition, this work received funding from the Australian Government via grant AUSMURIB000001 associated with ONR MURI grant N00014-19-1-2571.