## 2 Materials and Methods  
Several illuminators explored for passive bistatic radar (only one example of a published paper is cited for each technology) such as the global navigation satellite system (GNSS) [11], Long-Term Evolution (LTE) signal [12], WIFI [13], WIFI (DVB-T) [14], Digital Audio Broadcast (DAB) [15], FM broadcast [16], drone RF signal [17], video surveillance [18]. These illuminators became a boosting tool for passive radar, thereby becoming an important technology for both military and civil applications. Table 1 highlights some of the issues for the abovementioned potential signals. Many passive FSR approaches, other than DVB-S-based, addressed issues on airborne targets such as RCS verification of different aircraft in [19], detection and classification of helicopters in [20], a feasibility for PFSR implementation in [21; 22; 23]. Signal characterization and optimization was also presented in [24; 25] to improve the detectability of airborne targets. A GNSS signal was further used in [26] to present a signal model and experimental validation of a SISAR image of detected aircraft. Signal from DVB-S was also explored as an illuminator of opportunity for both ground and airborne target detection. For example, [PERSON] et al. [27] and [PERSON] et al. [28] performed an analysis of the DVB-S signal waveform for radar application. [PERSON] et al. [28] used a flat plate reflector within a close range to establish some detection using the system. Another feasibility study involving a simulated signal model and some experimental results was presented in [29; 30]. Nevertheless, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Potential Passive Signal** & **Pros** & **Cons** \\ \hline Global navigation satellite system (GNSS) & Global coverage (even in open sea) and in the availability of multiple sources (different satellites and constellations). & No continuous signal from a single satellite on a specific location to be used for continues monitoring. \\ Frequency modulation (FM) broadcasting & High power levels provided and wide coverage. & Low resolution as FM transmission use lower frequency as carrier signal; thus, the low-RCS target such as Drone is difficult to detect. \\ Long-Term Evolution (LTE)/Mobile telecommunication & Excellent ambiguity function (Range and Doppler resolution). Multiple drone detection. & Limited to only low-altitude target because depending on height of the tower and antenna tilting, which normally pointing to the ground. \\ Ground-based Radar & Constant and controlled coverage. Drone tracking. Multiple drone detection. & All materials do not reflect a conventional radar signal, especially low-RCS-like Drone. Besides the conventional ground-based radar system cannot see behind obstacles. \\ WIFI/RF signal & Low cost, possible for drone and pilot localization, as well as drone make and model identification. Multiple drone detection. & Extensive traffic on 24.5 and 5.8 GHz bands which could implicate difficulties in finding drones in the background noise. \\  & & Directional antennas on commercial drones make it virtually undetectable from the wrong direction. \\  & & Poor performance in poor weather conditions, slow detection in a concept as used for forensic evidence of drone intrusions. & It takes many pixels to cover a wide angle. \\ Optical sensors (camera) & Advanced infrared cameras can ”see” in the dark. & High maintenance for lens likely must stay clean to maintain functionality. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Potential signal as passive radar and its fundamental suitability features. some unresolved challenges identified envisaged the derailing factor of DVB-S implementation for high-altitude targets, and the difficulty experienced while maintaining the effective detection range for small RCS targets is an issue of concern [31; 32]. Consequently, it derails the use of DVB-S-based systems for airborne target detection including the drone. Some works integrating FSR into a satellite-based system could be found in [33; 34].