## 1 Introduction
Additionally, the metallic ship hull leads to a polarization inversion of the impinging signal, that, after a further reflection upon the sea surface, is mostly in RHCP, as the transmitting one. All such aspects strongly affect target visibility in conventional GNSS-R data and compromise reliable maritime surveillance using GNSS-R systems. In order to overcome these severe limitations of conventional GNSS-R, ad-hoc GNSS-R instruments specifically designed for ship detection applications were envisaged in [20]. Such receivers are conceived to work in a backscattering configuration rather than the conventional forward-scattering one and equipped with an RHCP channel instead of the LHCP one. Here, these RHCP back-scattering GNSS-R instruments are referred to as _unconventional_ GNSS-R. A graphical comparison of conventional and unconventional GNSS-R systems is depicted in Figure 1.  
The benefits of the backscattering geometry were highlighted first in [16] and then proved theoretically in [19] by means of a link budget analysis using the analytical model for the ship radar cross section (RCS) presented in [23], while expected performance of unconventional GNSS-R have been assessed on both a theoretical and a simulation basis in [20; 21], respectively. In particular, in [21] it was demonstrated that best detection performance is achieved with intermediate viewing angles, low wind speed, and ship perfectly facing the transmitter, i.e., ship hull normal to the mean incidence plane. In such configurations, signal-to-noise-clutter ratio for the ship target return larger than 7 dB were obtained from airborne altitudes. However, performance analyses carried out in such previous works are based on the fundamental assumption that sea clutter can be reliably described by means of the Geometrical Optics (GO) model, which represents the sea surface scattering model commonly adopted within the GNSS-R community [9]. Indeed, it is well known that GO can accurately predict sea surface scattering at microwaves in far-from-grazing incidence and only in a rather narrow angular region including the forward-scattering direction, whereas it significantly underestimates diffuse scattering outside this region [24; 25]. Additionally, GOcannot catch cross- and de-polarization effects arising in the incidence plane as it is derived under the tangent plane approximation. Accordingly, an accurate investigation, design and simulation of unconventional GNSS-R systems for maritime surveillance applications might call for a more appropriate modeling of sea surface scattering in L-band, where GNSSs operate, and in a wide set of observation directions. In this paper, we aim at providing useful guidelines for a fast and accurate simulation of the sea-reflected GNSS signal at any imaging geometry, polarization channel, and sea state. Such guidelines could be fruitfully exploited in future research activities for a more accurate analysis of the expected detection performance using GNSS-R in unconventional configurations and could then support the design of ship-detection-oriented GNSS-R systems. To this end, we carry out an analysis of the signal-to-noise ratio (SNR) for the sea-reflected GNSS signal in arbitrary acquisition geometries. This is accomplished by providing an estimate of the strength of the sea-reflected signal, and, then, of the related SNR by accounting for the thermal noise power. Sea surface scattering is here described via the recent Bistatic Anisotropic Polarimetric Two-Scale Model (BA-PTSM), which provides an accuracy comparable to other advanced scattering models, e.g., the second-order Small Slope Approximation (SSA2), but at a much reduced computational complexity [26]. Accordingly, BA-PTSM is a good candidate for fast and accurate simulation of Earth-reflected GNSS signals over wide regions.