DVB-T-based passive radar for silent surveillance of drones

Nowadays, Unmanned Aerial Vehicles (UAVs) and drones are used as threat’s vectors that create personal and public security issues. The unpredictable and complex motion along with the small Radar Cross Section (RCS) and low velocity makes the drone detection a challenging task for any radar system. In the context outlined earlier, the security level enabled by conventional active radar systems could be augmented by the cost-effective, non-intrusive and eco-friendly Passive Radar (PR) technology. As a PR system does not have its own transmitter, this allows reduced costs, intrinsic covert operation capability and the lack of additional electromagnetic pollution. To guarantee complete and continuous coverage, PR can effectively be integrated within conventional active radars not only to extend the surveillance coverage, acting as ‘gap-filler’, but also to reduce the probability of out of service of the surveillance system. Moreover, aiming at the monitoring of airport terminal areas or harbours, where the installation of additional sensors might be limited by regulations related to public safety and risk of interference with pre-existing systems, a network of PRs could easily be deployed to provide continuous and complete coverage. The stationary nature and the isotropic characteristic of many of the employable Illuminators of Opportunity (IoO) provide a persistent illumination of the targets of interest to generate Coherent Processing Intervals (CPIs) of long integration times (Tint) on receive to counteract the limited power density offered by the emitter. This certainly applies to many ground-based transmitters for analogue or digital radio/TV broadcasting. Among them, the emitters of the Digital Video Broadcasting-Terrestrial (DVB-T) are particularly attractive for counter-drone applications. Specifically, the high radiated power of these transmitters and the excellent coverage make them suitable for the detection of these small RCS and low altitude targets. In addition, the continuous emissions and the fine range resolution of about 20 m (equivalent monostatic range resolution yielded by a signal bandwidth of approximately 8 MHz) make them potentially able to continuously detect and discriminate closely spaced targets. Aiming at the detection of the low Signal-to-Noise Ratio (SNR) targets and at widening the DVB-T-based PR coverage area, very long integration times (up to few seconds) can be exploited if the migration effects are properly compensated. It is worth noticing that the use of long integration time allows also to improve the Doppler resolution as well as to discriminate between slowly moving targets and clutter contributions, which is of particular interest in a scenario with a high density of targets. By employing an Orthogonal Frequency-Division Multiplexing (OFDM) modulation, DVB-T signals are noise-like waveforms; thus, they provide ambiguity function with attractive properties that are nearly independent of the signal content and almost time-invariant. Eventually, since a DVB-T transmitter typically broadcasts multiple channels at different carrier frequencies, this provides the desired diversity of information that could be successfully exploited for both target detection and its localization. Recently, different authors have investigated the use of such sensor for counter-drone operations proving the capability of such technology to detect and localize small and medium drones up to a few kilometres from the PR receiver. Moreover, the capability of such sensor in simultaneous detection of drones flying near the airport area along with the conventional civil air traffic at farther ranges has been proved. This chapter reports the latest results of DVB-T-based PR for counter-drone operations obtained by the research groups of the University of Alcala´ and Sapienza University of Rome.