The chapter shows examples of current and emerging applications for passive radar on moving platforms. The application largely depends on the platform the radar is installed on, so different platforms are first considered mainly sea- and air-borne. The applications described include target detection and passive imaging of terrain or moving objects. Sample results obtained from real signals during field test campaigns are presented. Then, selected technologies applied in passive radar on mobile platforms for multichannel signal acquisition, processing, and for system synchronization are discussed.

Passive operation is an important operational concept for future airborne radar platforms. Successful target detection requires an understanding of the underlying clutter statistics in order to set the detection threshold accurately. In addition, clutter statistics are important for modelling of passive radar performance and simulation of passive bistatic radar to aid in the development of new target detection algorithms. In this chapter, the clutter statistics of data from an airborne platform are studied over a range of bistatic geometries and receive polarisations. The illuminator is a digital video broadcast-terrestrial (DVB-T) station and the collected data spans both ground and maritime regions. The key contributions in this chapter include a study of amplitude statistics using a number of common distributions, understanding the impact of the reference signal quality and an analysis showing the distribution accuracy as the integration time and bistatic range varies.

This chapter provided an overview of the fundamental elements of a DVB-T SAR system. The power budget, sensitivity and resolution have been calculated. These calculations indicate potential for long distance operation and a modest spatial resolution when using a single DVB-T channel for imaging.

An image formation processor has been presented. The algorithm is based on BPA, so while it is computationally expensive it is able to form images for arbitrary bistatic data acquisition geometries. For airborne receivers, where physical space could be limited, it could be sufficient to use a single receiving channel to collect both the direct and reflected signals for imaging. DVB-T pilot signals seem to have little effect on imaging. An analysis on effects of DVB-T pilot signals on imaging shows their effects do not seem to be pronounced and that is due to the BPA operation as a spatial, rather than temporal filter. Algorithms to compensate for direct signal artefacts and for uncompensated aircraft motion errors have been derived.

An experimental system using mostly off-the-shelf components has been described. Software-defined radios are relatively straightforward for this purpose. An experimental campaign designed to verify system components and to begin further exploration of DVB-T SAR has been presented and experimental results have been discussed, in quasi-monostatic and multiple bistatic measurements.

Overall, through independent efforts from numerous scientific groups, the feasibility of DVB-T SAR has been solidified. The natural next step is to probe this technology further to fully understand and extract its potential, and the following are the authors' own thoughts.

One possible route to explore is DVB-T SAR phenomenology. A passive system like this is both bistatic and low-frequency, while at long stand-offs the transmitter is likely to be at near-grazing angle. As such, it is important to understand possible indirect propagation effects. On the one hand, this can set limits on the operational distance due to the local landscape, but on the other hand, it can investigate potential for beyond-the-hill vision, where higher frequency systems face restrictions. A glimpse of such capability has been shown in [19], where a receiver was on a ground moving vehicle and the resulting imagery produced echoes behind a hill blocking visual LOS. Additionally, UHF bands are known to have an adequate degree of penetration through foliage, while adding bistatics has the potential to increase signal-to-clutter ratio (SCR) compared to monostatic, low-frequency measurements (e.g. [46]). Demonstration of capabilities such as those could add operational value for DVB-T SAR.

Another strand could be to consider advanced SAR signal processing techniques, and the possibilities here are many. To name a few, one example is to investigate methods of spatial resolution improvement and that could be done in a number of ways. DVB-T stations broadcast on multiple RF channels (Table 6.1). Ways of combining their bandwidths, as has been shown for DVB-T inverse SAR [47], could be a way of improving range resolution. Considering multistatic approaches to resolution improvement, through coherent [8] or non-coherent [7,9] methods could be another way of enhancing image information space.

This chapter is concerned with the study of signal processing techniques and operational strategies for passive radar systems mounted onboard moving platforms and aimed at ground moving target indication (GMTI) applications. Specifically, the attention is focused on space-time adaptive processing (STAP) methodologies for systems equipped with multiple channels on receive. The spatial degrees of freedom are adaptively exploited for space-time clutter cancellation and slow-moving target detection and direction of arrival estimation. Specific solutions are devised and integrated into the typical signal processing architecture of passive radar to address the main limitations deriving from the passive bistatic framework.

This chapter provides a brief outlook into the future trends in passive radar. Particularly, it is mentioned how passive radars constitute a key component in system-of-systems architectures. Afterwards, two sections are devoted to preliminary research activities aiming at investigating the potentials of new illuminators of opportunity for passive radar.