## 5 Discussions, Conclusions, and Future Work  
The major contributions of this research are listed as follows: (1) Stability accuracy of UAV detection system is comprehensively modeled and simulated. The system's general kinematics, compensation principle, and major components including servo-motors and gyroscopes are all covered. (2) Assembly errors of axis perpendicularities, run-outs, and gyroscopes are defined, modeled, and introduced into the overall model. The effects caused by various assembly error sources can be comprehensively analyzed. (3) The effects caused by those assembly errors under different disturbance frequencies are studied. The simulation experiment results provide valuable data for real application scenarios. (4) Error sensitivity is proposed by carrying out standard deviation analysis. Based on the analysis, the most sensitive assembly errors are identified, and ranked in order of sensitivity as follows: x-axial component of pitch axis perpendicularity deviation, y-axial component of the azimuth gyroscope assembly deviation, and z-axial component of the pitch gyroscope assembly deviation. In real processes of manufacturing and assembly, the analysis results can be used to determine standards and priorities for possible improvements. The limits of this research are listed as follows: (1) So far, only assembly errors and UAV angular disturbances are studied. Other error sources and situations to further verify the proposed modeling and analysis method should be considered. (2) Simulation results provide only limited information, and further validations based on real experiments are needed. (3) The limitations, constraints used in this research, and the possibility of uncertainty will affect the results, such as the changes of error distributions, UAV disturbances, and target locations. (4) The effects of series versus parallel (or hybrid) component arrangement are not considered. (5) The effects of interface failures are not covered. As the next step, validations based on real experiments will be carried out. Other error sources and situations will be investigated, such as frictions, structural eccentricity, and inertia coupling. The linear disturbances of UAV and target movements may have great influence on the results. The effects of series versus parallel (or hybrid) component arrangement will be further studied. The interface performance of the system is very important. The reasons for interface failure include damage or wrong assembly of hardware, incompatibilities of data formats, and inferiority of control algorithms, and will be studied in detail. A similar control mechanism of a more complex system with more dimensions or new materials will be studied, such as the structures of two-axis-four-frame and three-axis-four-frame used on larger carriers. How to incorporate the proposed method to other working models of the system will be studied, such as target identification, tracking, positioning, etc. The following are available online at [[http://www.mdpi.com/2076-3417/10/7/2311/s1](http://www.mdpi.com/2076-3417/10/7/2311/s1)]([http://www.mdpi.com/2076-3417/10/7/2311/s1](http://www.mdpi.com/2076-3417/10/7/2311/s1)), MATLAB codes of the simulation works. Conceptualization, K.H. ; Data curation, K.H. and G.J. ; Formal analysis, K.H. ; Funding acquisition, K.H. and H.H. ; Investigation, K.H., H.H. and G.J. ; Methodology, K.H. and H.H. ; Project administration, K.H. and H.H. ; Resources, G.J. ; Software, H.G. ; Supervision, H.H. ; Validation, G.J. ; Visualization, G.J. ; Writing--original draft, K.H. ; Writing--review & editing, H.G. All authors have read and agreed to the published version of the manuscript. This research was funded by China Postdoctoral Science Foundation (Grant No.47662, the 66 th general grant batch of the military system).