## 1 Introduction  
The exploitation of Global Navigation Satellite System (GNSS) signals scattered off Earth' surface for remote sensing purposes was first demonstrated in the late 1980s by [PERSON] [1] and, later, by [PERSON] for ocean altimetry [2]. Since then, GNSS-Reflectometry (GNSS-R) concepts and systems have been successfully applied in numerous Earth observation applications over ocean [3, 4], land [5, 6] and cryosphere [7, 8]. For a deep review of GNSS-R principles, missions and applications, the interested reader is referred to [9]. A traditional application of GNSS-R data over oceans is the analysis of the sea state, including the estimation of the wind speed and root mean square (RMS) sea surface slope. To boost sea state retrieval performance, GNSS-R receivers are typically designed to acquire and process the signal coming from a portion of sea surface surrounding the specular reflection point, i.e., the so-called _glistening zone_, where most electromagnetic (EM) energy of the sea echo is reflected. Additionally, cross- and de-polarization effects due to sea surface make the right-hand circular polarization (RHCP)-transmitted signal mostly scattered in left-hand circular polarization (LHCP). Accordingly, past and current GNSS-R receivers are typically equipped with an LHCP receiving channel, while the presence of also an RHCP channel is left so far to biomass and soil moisture retrieval using polarimetric GNSS-R measurements [10]. Here, we refer to such forward-scattering GNSS-R receivers collecting the LHCP component of the scattered signal as _conventional_ GNSS-R. Recent applications of conventional GNSS-R, including the ongoing NASA CYGNSS mission, focus on both ocean and land remote sensing, see e.g., [11, 12, 13, 14]. Additionally, in the last decade, GNSS-R has been investigated for ship detection applications [15; 16; 17], where it might offer important and intriguing advantages compared to other remote sensing technologies, including synthetic aperture radar (SAR) and optical/multi-spectral sensors. As a matter of fact, GNSS-R allows for global and seamless monitoring of the Earth' surface using low-cost, light-weight receivers that can count on the free availability of >100 GNSS signal sources, including GPS, GLONASS, Galileo, and BeiDou-2. Revisit time offered by GNSS-R systems can be as low as few hours when launched in constellation formation and can be further improved by tracking multiple GNSSs [17]. Such key factors make GNSS-R an intriguing, despite not yet mature, technology for near real-time maritime surveillance [18]. Indeed, feasibility of ship detection using GNSS-R data is still an open issue and requires further investigations. Ship detection using GNSS-R has been investigated in numerous works, ranging from theoretical [16; 19; 20], simulation [21] and experimental [15; 17; 22] analyses. The general consensus of all such works is that conventional GNSS-R is scarcely suited to the ship detection problem, especially from spaceborne altitudes, due to the extremely poor GNSS power density-that can be as low as \(3\times 10^{-14}\) W/m\({}^{2}\) at the Earth' surface-, and GNSS-R spatial resolution, that can be as large as tens of kilometers over oceans. Besides this intrinsic limitation of the GNSS power budget, additional issues are related to the acquisition geometry and polarization of conventional GNSS-R. Indeed, in the presence of a ship within the observation area, the different mechanisms arising in the EM interaction between sea surface and ship, namely multiple reflections, make only a negligible quota of the ship echo be scattered in LHCP and in the specular direction [21; 23], and, then, collected by conventional GNSS-R receivers. Conversely, double-bounce ship-sea and sea-ship scattering contributions, which are predominant in most acquisition configurations, are stronger in far-from-specular directions due to the mirror-like behavior of the ship hull, the direction of the ship peak radiation depending on the orientation of the ship against the transmitter [23].