# Multi-Frequency Target Detection Techniques for DVB-T Based Passive Radar Sensors
## 1 Introduction
These parameters include the carrier frequency and, in turn, the radiating characteristics of the transmitter (i.e., the radiated power, the transmitting antenna gain and the corresponding pattern both on the horizontal and on the vertical planes, the adopted polarization, and the employed transmission mode among those specified by the DVB-T standard). As is well known, according to a conventional bistatic radar equation, these parameters directly affect the power level of the radar echo received from a point-like target. Moreover the parameters above further influence the receiver sensitivity as they concur to determine the backscattering characteristics for a complex target as well as the propagation channel electro-magnetic (EM) conditions, there including the dominant propagation mechanism and the possible presence of strong multipath and interfering transmissions. Due to the wide frequency band assigned to the DVB-T broadcast service (i.e., the upper portion of the very high frequency (VHF) band and the interval from 470 MHz to 862 MHz in the ultra high frequency, UHF, band), a large diversity could be experienced in terms of the above conditions and these might be time-varying during the radar operation. Therefore, in order to enhance the reliability of the considered PR system, proper strategies should be adopted aimed at improving the target detection performance by mitigating its instability tendencies. When exploiting broadcast transmitters, a tempting solution is offered by the joint exploitation of multiple signals emitted by the same illuminator of opportunity at different frequency channels. This possibility was investigated in [21; 22] with the aim of achieving finer range resolutions thanks to the widening of the resulting frequency bandwidth. To this purpose, coherent integration approaches were adopted together with advanced signal processing techniques to counteract the effects arising from the possible wide separation of channels in the frequency domain. In contrast, in [23] the concept of a non-coherent multi-frequency (MF) integration was introduced for a FM radio-based PR as a means to mitigate the time-varying characteristics of the exploited waveforms, which largely depend on the radio program content. Besides the expected improvement due to non-coherent integration of target echoes received on multiple frequency channels, the proposed MF operation was shown to yield a remarkable advantage with respect to a single frequency (SF) approach when operating in a severe interference scenario. Compared to a coherent integration, this advantage was obtained without requiring specific properties of the received signals nor setting demanding constraints on the receiver design, so that the proposed approach was demonstrated to be a practical solution for robust and effective PR based on FM radio signals. In this paper we aim at extending the range of applications of the MF operation presented in [23]. Specifically, its feasibility is investigated for a DVB-T-based PR sensor in order to increase the reliability of the resulting system. To this purpose different MF integration strategies are considered and proper modifications are proposed for these techniques to be effective in the newly considered application. A comparative analysis is first carried on by means of simulations, with reference to a set of specific case studies; the reported results give some insights into the properties of the proposed MF approaches. However, a full validation of the effectiveness and of the relative performance of the different multi-frequency techniques is provided through the application against live scenarios. To this purpose the conceived approaches are extensively validated against experimental data accounting for aerial and maritime surveillance applications. The reported experimental results are the fruit of a long-term collaboration between Leonardo S.p.A. (formerly Finmeccanica S.p.A., Italy) and the research group at Sapienza University of Rome.