# Multi-Frequency Target Detection Techniques for DVB-T Based Passive Radar Sensors
## 1 Introduction  
Passive radar (PR) sensors exploit existing transmitters as illuminators of opportunity to perform target detection, localization, and possibly imaging [1; 2]. A receiving system is appropriately designed to collect the echoes reflected from targets and sited to provide coverage of specific areas. Since there is no need to build and to operate dedicated transmitter equipment, PR sensors offer a number of advantages such as contained costs of realization and maintenance, possible covert operation, and low vulnerability to electronic countermeasures. Moreover, they do not generate additional electromagnetic pollution and have a limited impact on the landscape thanks to their reduced size, therefore they are usually referred to as \"green systems\" and can be employed in areas where conventional radars could not be deployed (e.g., in close proximity to residential areas). Different transmitters for telecommunications, radio navigation, and remote sensing applications can be potentially used in this opportunistic manner, however broadcast transmitters are usually preferred solutions for surveillance purposes owing to their wider coverage. Nowadays the transmissions for Digital Video Broadcasting-Terrestrial (DVB-T) represent one of the most attractive choices as they offer a quite stable range resolution of about 20 m (equivalent monostatic value) thanks to the orthogonal frequency division multiplexing (OFDM) modulation based on persistent sub-carriers that span a total bandwidth of up to 8 MHz. Moreover, by employing an OFDM modulation, the DVB-T signals are noise-like waveforms, thus, they provide more attractive ambiguity function properties that are nearly independent of the signal content and almost time-invariant [3; 4; 5; 6; 7; 8; 9]. The potential of DVB-T-based PR sensors has been recently investigated for both air traffic control (ATC) and coastal/maritime surveillance applications. With reference to the first application, the performance of such systems suffers from the vertical-plane radiation patterns of typical DVB-T transmitters that are tailored to avoid wasting too much power above the horizontal plane. In factDVB-T-based PR has been shown to allow a reliable detection capability, mainly against targets flying at low altitude, at distances up to tens of kilometers [8; 9; 10; 11; 12; 13; 14; 15]. Therefore, such sensors can be fruitfully integrated, with the role of gap-fillers, within conventional ATC solutions to provide continuous coverage. The exploitation of DVB-T-based PR in maritime surveillance applications has a number of positive implications and has recently attracted significant interest from the passive radar community [16; 17; 18; 19; 20]. For instance, compared to the ATC application, the expected radar coverage benefits from the longer integration times allowed by vessels and from the vertical pattern of the transmitting antennas. This implies the possibility of employing DVB-T-based PR sensors for monitoring typical maritime traffic as well as for detecting small boats close to the coast [16; 17; 18]. In contrast, the wide horizontal beam and high power level of the broadcast transmitters allows the availability of such signals even at large distances from the coast and this enables their exploitation for short range surveillance by PR receivers on moving maritime platforms [20]. Furthermore, the low carrier frequencies of the considered waveforms, especially when channels in the VHF or low UHF bands are exploited, might allow the detection of targets beyond the normal line-of-sight horizon thanks to the anomalous propagation phenomena typical of sea paths where super-refractive conditions are prevalent [19]. Obviously, target detection performance depends highly on the selected transmitter of opportunity and, in particular, it significantly varies with the employed DVB-T channel. In fact the selection of a given DVB-T channel among those emitted by a specific illuminator of opportunity implies a number of radar parameters must be inherently fixed.