This chapter presents an overview of radar systems and their main applications that have pushed radar toward new paradigms and system concepts. In particular, the chapter discusses the basic theory and concepts behind a classical radar system. The basic radar nomenclature and architecture are summarized in brief, and a summary of the main radar functionalities and applications is given. This is followed by the new challenges that modern radar systems are facing, with special emphasis on a high degree of adaptability to the environment and the capacity to cooperate in a network. Such new features are meant to enhance the radar performance in terms of detection, tracking, and automatic target recognition in a dynamic environment and to enable a radar system to be able to handle several tasks by managing its resources autonomously and intelligently.

In this chapter, we introduce the principal concepts and functionalities that photonics can bring in the microwave systems. The discussion will focus in particular on the application of photonics to radars and to electronic warfare (EW) systems. The analysis will compare the performance of several standard microwave subsystems with the latest results obtained by using photonics, either coming from the research labs or, where available, from the pioneering microwave photonics industry. The potentials of photonics in improving the performance with respect to the standard approaches are stated. Moreover, several novel peculiar possibilities that photonics can enable are highlighted. In order to be fruitful to the largest technical audience, the analysis will avoid discussing the photonic solutions in deep details, which would require the experience of a photonic specialist to be fully understood. Nevertheless, the descriptions will let the reader getting the main concepts, and the reported numbers will allow a direct comparison with the standard microwave system counterparts. For those who will be willing to reach a deeper insight, several references are provided. After an introductory paragraph on the general benefits brought about by photonics, this chapter takes into account the basic functionalities of microwave systems: the generation and detection of the microwave signal, its transmission and distribution, the signal conditioning for the beamforming function in phased array antennas (PAAs), and the signal filtering. The potentials coming from the integration of photonics are also discussed in a dedicated paragraph. Finally, the main points raised in this analysis are summarized.

The aim of this chapter is, thus, not only to underline the advantages of radar networks, but also to describe the major technical issues for their correct operation. First, a brief introduction to the concept of radar networks will be provided, together with an overview of the main benefits and the most relevant applications. Secondly, the problem of radar network categorization has been examined. A first categorization is based on the different transmitting and receiving options of the network sensors, which can be monostatic, bistatic (i.e., the transmitting and receiving antennas are widely separated) or a combination of mono/bistatic. A second categorization is based on the distinction between centralized and decentralized processing approaches instead. In particular, the advantages and disadvantages of centralized and decentralized approaches will be considered, with particular attention to the final system performance and additional complexity trade-offs. Third, the chapter will extensively discuss the network synchronization and data fusion issues, which are vital to ensure excellent operation of the radar network. Synchronization directly impacts on radar network performance. For this reason, the main causes that could lead to the mis-synchronization of the radar network elements will be presented. Later, the data fusion problem in the context of the radar networks will be investigated with particular emphasis on the multitarget tracking (MTT) issue. Finally, the chapter concludes with a recent scientific experimentation conducted by the NATO Science and Technology Organization Centre for Maritime Research and Experimentation, in which a network composed by two highfrequency surface wave (HFSW) radar systems was deployed in the Ligurian and northern Tyrrhenian Seas for maritime surveillance. The description of the experimental example is aimed to prove the effectiveness and the scientific relevance of radar networks and their decisive impact for homeland security and human progress.

in this chapter discusses the requirements for electronic warfare (EW) systems, the current solutions based on standard electronic technologies, and their limits. Here, we analyze what photonics can implement in this field. In fact, this chapter takes into account the several photonics-based solutions proposed so far for the electronic warfare. In particular, these solutions focus on the following applications: microwave photonic links (MPLs), instantaneous frequency measurement (IFM) systems, channelized receivers, and scanning receivers. In the case of the scanning receiver, we also report on a field trial run in a naval scenario. It is worth underlining here that most of the photonics-based solutions proposed so far are still at the level of research results. In fact, in order for photonics to be a serious contender in radio-frequency (RF) systems, it needs to demonstrate either a quantum leap in performance or completely new capabilities in comparison with digital electronics. As can be read in the examples below, several photonicsbased solutions are coming close to this point. The reader can therefore find in the following the seeds of what we expect to see soon in the real application fields.

In this book chapter the authors present concluding remarks on the application of photonics for radar networks and electronic warfare systems. They state that the main features that photonics can bring to the microwave field are the frequency flexibility (allowing the operation from few gigahertz up to several tens of gigahertz with the very same devices) and the precision (in terms of phase stability, also in conjunction with the fiber distribution). These can be exploited in microwave systems to reach new performance and new functionalities.