# Localization of Radio Sources Using High Altitude Platform Station (HAPS)
## 5 Conclusions  
This study evaluates the feasibility of radio source localization using a high-altitude-platform station (HAPS) at an altitude of 20 km, addressing the limitations of conventional ground-based receiving antennas in urban environments. Through numerical analysis, the effectiveness of HAPS-based localization is assessed for both single-source and multi-source scenarios under varying environmental conditions.  
For single-source localization, results indicate that accuracy deteriorates with increasing horizontal distance. However, radio sources within 60 km could still be accurately localized, even in urban areas. Additionally, tilting antenna elements downward improved accuracy for low-power sources or those located below the HAPS. The study also found that edge diffraction from buildings had minimal impact on accuracy, while the overall detection percentage improved. These findings confirm that HAPS-based localization of a single source is feasible and reliable.  
For multi-source localization, accuracy declined as the number of sources increased, particularly when sources were closely spaced, leading to reduced angle resolution. However, increasing element spacing and incorporating antenna directivity into the MUSIC method significantly enhanced localization accuracy. In scenarios involving mobile terminals with closely spaced, low-power sources, the system achieved sufficient estimation accuracy when cell side lengths were between 3 km and 10 km. However, at 1 km cell spacing, localization errors increased, though these were mitigated by further increasing the element spacing.  
Overall, this study demonstrates the effectiveness of HAPS-based localization but highlights the need for higher angle resolution when sources are closely spaced, compared to ground-based systems. Additionally, antenna calibration and HAPS attitude adjustments were idealized in this study, and optimization of computation time and power consumption was not addressed. An initial study on these issues was conducted in [23], but further investigation will be carried out in our future work. Future research will also focus on real-world performance evaluations, practical calibration techniques, and computational efficiency enhancements to advance HAPS-based localization toward real-world deployment.  
Furthermore, since our HAPS-based localization system achieves an estimation accuracy within several hundred meters, the possible location of the radio emitter can be significantly narrowed down. This suggests that conventional techniques, such as DEURAS or UAV-based systems, can then be further utilized to precisely pinpoint the source. Integrating these methods into a hybrid system will be explored in our future work.  
Conceptualization, G.K.T.; Methodology, G.K.T.; Software, Y.F.; Validation, Y.F.; Investigation, Y.F.; Writing--original draft, Y.F.; Writing--review & editing, G.K.T.; Supervision, G.K.T. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflicts of interest.