**5. Conclusions and Future Work**

In this paper, we proposed a new coverage flight path planning algorithm based on footprints' sweeps fitting and a sparse waypoint graph for unmanned aerial vehicles' (UAVs) navigation in three-dimensional (3D) urban environments with fixed convex obstacles. The main goals of the proposed algorithm are to reduce the computational time, number of turns, and path overlapping while finding a minimum length path that passes over all reachable points of an area or volume of interest for UAVs flying at low-altitudes in 3D urban environments. We devised a novel footprints' sweeps fitting method by considering the UAV sensor footprint as a coverage unit that guarantees complete coverage of the AOI with fewer and longer sensor footprints' sweeps. Furthermore, we generate a sparse waypoint graph by connecting footprints' sweeps endpoints by considering footprints' sweeps visiting sequence, obstacles' effect, and maneuverability constraints to obtain a complete coverage flight path. Simulation results have shown that the proposed algorithm can achieve better performance compared to the closely related global coverage flight path planning algorithms. It finds a feasible path from the urban environments inhabiting substantial number of obstacles without sacrificing the guarantees on computing time, number of turns, perfect coverage, path overlapping, and path length. The proposed CPP algorithm is practical, effective, feasible, and it has important significance in UAV practical application in urban environments. In addition, the proposed algorithm is directly inspired by the real-life applications of UAVs, and it is applicable for various coverage related applications in 3D urban environments with fixed obstacles. In future work, we are planning to enhance the proposed CPP algorithm for spatially distributed regions coverage.

**Author Contributions:** Conceptualization, S.L.; Data curation, A.M.; Investigation, A.M. and S.L.; Methodology, A.M.; Project administration, S.L.; Software, A.M.; Supervision, S.L.; Validation, A.M. and S.L.; Visualization, A.M.; Writing—original draft, A.M.; Writing—review & editing, S.L.

**Funding:** This work was supported by Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korean Government (MSIT) (No. 2015-0-00893, Technology Development of DMM-based Obstacle Avoidance and Vehicle Control System for a Small UAV).

**Conflicts of Interest:** The authors declare no conflict of interest regarding the publication of this manuscript.
