This paper is a first introduction to the concept of using Global Navigation Satellite Systems (GNSS) as illuminators of opportunity in a passive bistatic real-time radar system for maritime target indication applications. An overview of the system concept and the signal processing algorithms for Moving Target Indication (MTI) is provided. To verify the feasibility of the system implementation as well as test the developed signal processing algorithms, an experimental test bed was developed and the appropriate experimental campaign with the new Galileo satellites and a ferry as the target was carried out. The results confirm the system concept and its potential for multi-static operation, with the ferry being detected simultaneously by two satellites.

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In this paper, we present a new technique to exploit the data acquired simultaneously by multiple radar sensors carried by multiple air platforms to increase the cross-range resolution of inverse synthetic aperture radar (ISAR) images of rotating targets. This distributed ISAR technique is devised for two different cases: 1) multiple-input-multiple-output (MIMO) case with each platform carrying an active radar that transmits and receives RF waveforms and 2) multistatic case with a single platform carrying an active radar (transmitting and receiving) and the remaining platforms equipped with passive sensors (namely, receiving only). The processing chain proposed for the distributed ISAR is shown, together with the results obtained against simulated ISAR data for both the MIMO and the multistatic cases. The performance analysis shows that the proposed technique is able to provide an increase of the cross-range resolution up to the number of platforms in the multistatic case and even higher in the MIMO case, if the platforms are properly located. This is of great benefit in applications where the target rotation angle is insufficient to guarantee the desired resolution. A typical case is the imaging of ship targets with rotation induced by the sea swell structure under low sea state conditions. To make the results appealing for practical application, the performance degradation is also analyzed arising from errors in the knowledge of both the target rotation motion and the acquisition geometry. Experimental data collected by a ground-based radar operating together with a rotating platform are processed by following the presented distributed ISAR technique to validate the proposed approach

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The exploitation of the Global Navigation Satellite Systems (GNSS) as transmitters of opportunity in passive radar systems for maritime surveillance is particularly attractive because of the main advantages consisting in a global coverage (even in open sea) and in the availability of multiple sources (different satellites and constellations). The main drawback stays in the restricted power budget provided by navigation satellites. This makes necessary to conceive, define and develop innovative moving target detection techniques specifically tailored for the system under consideration, in order to make this technology a powerful tool for persistent surveillance of sea areas of interest. To this aim, a long integration time Maritime Moving Target Indication technique is proposed in this work, and its effectiveness is proved against experimental data involving a small maritime target, not detectable by conventional MTI techniques. Obtained results prove the feasibility of a maritime MTI mode for GNSS based passive systems.