This book is divided into two parts. The first one deals with models and techniques, thus focusing on the SAR system viewpoint. This part provides the necessary background for a complete understanding of the second part, which is devoted to maritime surveillance applications. In the following, a brief description of book chapters is provided.

The analytical description of the scattering of electromagnetic fields from the sea surface is an old but still open problem. Initially devoted to removing the sea clutter from radar acquisitions, sea scattering models gained growing interest as instruments to investigate the sea physical characteristics and today new challenges call scientists to enrich their models with the presence of ships on the sea surface. The opportunity of remotely retrieving information about geometrical and electrical properties of the sea surfaces, the possibility of using radar satellites to monitor ships in oceans and coastal waters as well as the efficient analysis of microwave links in marine environment depend on the capability of modeling the interaction of the electromagnetic field with the sea surfaces. Such an analysis has to account for the involved dependence of the scattered field on incident wavelength, surface roughness and dielectric properties, polarization, look angle, and so on. In addition, time variance and hydrodynamic wave -wave interactions affect the power distribution in sea scattering phenomena.

The synthetic aperture radar (SAR) is a microwave remote sensing imaging system that consists of a pulsed radar mounted on a moving platform. Usually, the latter is an aeroplane (airborne SAR) or a satellite (spaceborne SAR) moving along an approximately straight-line trajectory (line of flight, or track) with an approximately constant velocity. Transmitted pulses are usually linearly frequency modulated (LFM), that is, they are chirp pulses, as they are called in radar jargon. The SAR imaging system resolution along the range (or cross-track) direction is obtained by matched-filtering the received radar echoes, as in standard pulsed radars; therefore, range resolution depends on chirp bandwidth: it improves as the chirp bandwidth increases.

This chapter has a twofold objective: on one side, it provides the basics of synthetic aperture radar (SAR) polarimetry; on the other side, SAR polarimetry is specialized with respect to the sea surface observation for marine and maritime applications.

Synthetic aperture radar (SAR) provides a particularly convenient way of observing the ocean. Indeed, spaceborne SAR sensors were used from their early years to estimate physical parameters (e.g., currents and winds) and to detect the presence of ships (which usually appear as bright spots over the sea dark background) with allweather day-and-night capabilities. In view of all these applications, it is important to obtain data with the highest possible radiometric accuracy. In this context, special attention must be paid to SAR ambiguity problems. Due to the potential appearance of displaced and attenuated replicas of strong signals ("ghosts") related to bright structures present on the coast or to ships over the sea surface, this issue is especially significant for ship-detection applications, since it may dictate a relevant increase in the number of false detected targets.

In this chapter, the problem of ship detection from SAR images is introduced. This topic is of great significance in maritime surveillance, since it can help in the identification of noncollaborative ships. The techniques used for ship detection are mainly based on a two-step procedure: a first prescreening detection phase, followed by a discrimination phase, necessary to lower the false alarm rate due to ship-alikes (e.g., ambiguities, see Chapter 5). In the chapter, the pre-screening phase for ship detection is focused.

Intertidal zones are coastal areas that fall dry once during each tidal cycle. This chapter demonstrates how high -resolution synthetic aperture radar (SAR) imagery of coastal zones can be used to study morphodynamical changes in these zones, since exposed intertidal flats show up as dark or bright patches on SAR imagery. In addition, submerged sandbanks can cause SAR image signatures through variations of the surface current fi eld. Multifrequency SAR data can be used for the derivation of surface roughness parameters, and the polarimetric decomposition of dual-copolarization SAR data helps in identifying coastal habitats. And fmally, we demonstrate how high resolution SAR imagery can be used for archaeological surveys in areas that are difficult to access.

The major topic of this chapter is space-borne synthetic aperture radar (SAR) surveillance of ocean regions that are permanently or seasonally covered with ice. Surveys of sea ice conditions are required for production of ice charts, and sequences of SAR images are used for monitoring movement and deformation of the ice. Ice charts show the regional distribution of open water areas and different ice types, often supplemented by additional information such as areal fractions of ice types and floe sizes. Ice drift is displayed in maps of vector fields, which may be complemented with data quantifying their errors or reliability and the degree of deformation. A less extensive but nevertheless important topic of this chapter is the surveillance of icebergs. Ice charts and information on ice movements and iceberg occurrences are essential for the safety of ship traffic and offshore operations, and for several scientific questions. Issues addressed here concern methods for retrieving information about ice conditions, using different SAR systems and technologies.

This chapter addresses the physical mechanisms behind SAR imaging of oil spills in the open ocean and proceeds with a discussion on the emerging SAR information retrieval techniques for detecting and characterizing these slicks. Section 9.1 covers oil spill information items possibly derived from SAR and limitations of existing methods. Challenges and trade-offs faced by operational service providers in retrieving these items are discussed in Section 9.2. Section 9.3 is about SAR image interpretation: the main contrast drivers for oil spills are explained and surface scattering models aiding interpretation are reviewed. In Section 9.4, the main techniques for oil spill detection and characterization are illustrated and current research questions are discussed. Concluding remarks and notes on further readings are given in Section 9.5.

In this framework, monitoring techniques aim at fusing multiple sources from both satellite and terrestrial monitoring data for a better understanding of the marine picture for near real time applications. SAR sensors are a valid monitoring technique, often considered a complementary means to the traditional coastal-based ones. SARs are able to acquire images independently from daylight, meteorological conditions, and national borders; in addition, modern sensors also offer a wide spatial coverage and usually operate in constellation thus reducing the revisit time and allowing, consequently, the control in open sea areas. The main issue concerned with SAR is the impossibility to identify ships, although high-resolution SAR (HR-SAR) with its submeter spatial resolution can recognize at least the ship type. In addition, these sensors are usually characterized by high mission costs compared with optical sensors.

Monitoring of sea state and sea surface wind is certainly of fundamental importance for studies on climatology, meteorology, weather forecast, and ocean circulation. However, it has a great relevance also in the framework of maritime surveillance, particularly for navigation safety. In fact, of course navigation safety strongly depends on weather forecasts; in addition, knowledge of sea surface winds and sea currents allows predicting paths followed by fl oating sea ice and icebergs, which constitute a serious danger for navigation. Also, knowledge of wind speed and surface currents allows predicting the spreading and movements of oil spills over the sea surface. Microwave remote sensing systems, such as synthetic aperture radar (SAR) systems, have been recognized since their fi rst operational uses as the ideal sensors for the monitoring of sea state and sea surface winds.