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INTRODUCTION
Maritime traffic levels are related to economic growth: the international shipping industry is responsible for delivering about 90% of all trade worldwide by volume (with 7 to 9 billion tons loaded per year), and it is vital for bulk transport of raw materials, oil and gas. The linear regression between the economic growth of the nations in the Organisation for Economic Cooperation and Development (OECD) shows a 4% increase of imports and exports for a 1% increase in Gross Domestic Product (GDP). So, marine transportation is an integral, although sometimes less visible, part of the global economy. Other important marine activities include passenger transportation (ferries and cruise ships), national defence and fishing. If we also consider pleasure boats, even forgetting the millions of smaller leisure boats worldwide, the spatial density of marine traffic increases significantly, especially in areas close to harbours and coasts.
To improve the safety and the efficiency of marine traffic, as well as to protect the environment, Vessel Traffic Service (VTS) definitions and regulations were introduced by the International Maritime Organization (IMO) in 1985 and then updated in 1997 with the Resolution A.857(20) (IMO, 1997). VTS makes use, among other sensors, of powerful coastal radars to obtain an up-to-date marine traffic image. Vessels themselves can carry radars for navigation purposes. For decades these navigation radars have been designed as cheap pulsed magnetron devices emitting very short (e.g. less than 10-1[...]s) pulses with high peak power of the order of tens of kW. More recently, new solid-state technology has entered marine radar design (Zhang et al., 2013; Amato et al., 2010; Nelander and Tòth-Pᅢ l, 2009). With a solid-state power amplifier, marine radars are becoming more and more sophisticated and agile in frequency as well as in waveform in order to get better performance by pulse compression, all features that were not possible with the old magnetron transmitter. Since the solid-state amplifiers' power is of the order of tens of W, significantly less than magnetrons', they have to transmit longer pulses (e.g. up to 90-100 [...]s) in order to achieve the same energy on target, i.e. keep the detection probability unchanged with respect to magnetron radars. Their longer pulses...