# Efficient Multipath Clutter Cancellation for UAV Monitoring Using DAB Satellite-Based PBR
## 1 Introduction  
With the progress of technology and cost reduction, unmanned aerial vehicles (UAVs) have been widely used in the military, industrial, and civil fields, for which the increasing accessibility poses unprecedented threats to infrastructure and public safety. Therefore, more and more attention has been paid to the corresponding monitoring technologies as detection is required before proper disposal. Compared with other surveillance means such as laser and photoelectricity, radar-based systems have an insensitivity to environmental changes and strong all-day, all-weather applicability and, thus, are used in many applications [1; 2; 3; 4] in which the feasibility for UAV detection has been shown in [5; 6; 7]. Unfortunately, the high radiation power from active radar aggravates the complexity of the surrounding electromagnetic environment, and its use might be limited to avoid other severe safety accidents in some sensitive areas, such as oil epochs and granaries. For such scenarios, passive bistatic radar (PBR) based on an illuminator of opportunity becomes an attractive solution because of the feasibility of non-cooperative surveillance while staying radio-silent. Unlike conventional monostatic systems, passive radar utilizes the scattering of existing radio signals from targets to perform surveillance, which is accumulated according to the reference signal obtained with an additional receiving channel pointing towards the illuminator. Thus, target detection can be completed with less power consumption as long as a suitable radio emitter covers the target area, and the system's impact on the surrounding electromagnetic environment can be ignored. PBR usually uses a reference signal with stable radiation power, such as broadcast and cellular communication, to realize habitus monitoring, the capability of which has been shown to have many different terrestrial illuminators [8; 9; 10; 11; 12; 13; 14]. In practical usage, due to similar reasons as for active radar, the power density of electromagnetic radiation is limitedin many cases [15; 16], and thus, the system capability might be affected by the availability of appropriate illuminators. Spaceborne illuminators can be applied to more scenarios because of the broader coverage area provided by their giant footprints and evenly distributed power density. In recent works, [17; 18] used global navigation satellite system (GNSS) signals to realize UAV detection. Due to the low ground power density, a substantial receiving antenna is needed to intensify the echo from weak targets, dramatically increasing system costs. As an alternative, digital satellite TV systems (DVB-S) located in synchronous orbit are used in [19; 20; 21] to enlarge the detection range and to analyze the micro-Doppler signature of targets. Although with a higher power density and negligible emitter motion, the Ku band signals, more precisely from 10.7 GHz to 12.75 GHz, exacerbate the environmental attenuation and radar cross-sectional (RCS) fluctuation, limiting the gain from the coherent processing interval (CPI) increment. Therefore, the feasibility of UAV monitoring using spaceborne illuminators with lower radiation frequency is worthwhile to explore further. Usually, target echoes are much weaker than direct wave and multipath interference, which inevitably leads to a massive mask effect in the two-dimensional cross-correlation function (2D-CCF). The influence on the detection performance needs to be prevented by the cancellation of multipath clutter, and many suppression algorithms have been proposed in previous works [22; 23; 24; 25; 26], some of which, such as [27; 28; 29; 30], are designed in the sub-carrier domain for PBR based on orthogonal frequency division multiplex (OFDM) broadcasting and thus are not applicable to emitters such as DAB satellite.