## 5 Conclusions  
In this paper, we presented an analysis of the SNR for the sea-reflected GNSS signal in arbitrary acquisition geometries, including the backscattering direction, which is of key relevance in maritime surveillance applications using GNSS-R. Sea surface scattering has been described by means of the recent BA-PTSM, that extends the original TSM to bistatic geometries and anisotropic random rough surfaces, such as sea surface, and provides analytical expressions for the polarimetric covariance matrix. BA-PTSM has the advantages of wider validity limits, especially with respect to GO, and accuracy comparable to other advanced scattering models, e.g., SSA2. For power budget analysis, standard GNSS-R processing chain, comprising coherent and incoherent integration steps, has been assumed and the sea surface signal strength has been evaluated at the output of the typical GNSS-R processing chain. Typical spaceborne and airborne GNSS-R receivers, namely SGR-ReSi and GOLD-RTR, respectively, have been considered. The sea scattering area has been evaluated by means of a numerical tool accounting for the receive antenna beamwidth and the actual configuration of iso-delay and iso-Doppler lines.  
Numerical results comparing BA-PTSM with classical GO model have been presented for both spaceborne and airborne GNSS-R receivers and for both LHCP and RHCP receiving channels. It has been shown that for spaceborne GNSS-R, as long as standard GNSS-R processing chain is considered, GO can be safely adopted as the scattering model regardless of the polarization channel, acquisition geometry, and sea state. Conversely, for airborne GNSS-R simulation of sea surface, GO is accurate enough only in near-to-specular scattering directions, where SNR exhibits the largest values. In far-from-specular acquisition geometries more accurate scattering models, such as BA-PTSM, are required for a proper modeling and simulation of the sea surface return. Nevertheless, it should be noted that BA-PTSM reduces to GO at and around the specular reflection direction and, therefore, it could be used as the scattering model for fast and accurate simulation of GNSS-R signals reflected off sea surface regardless of the acquisition geometry, polarization, sea state, and receiver altitude.  
Highlighted guidelines might be exploited for a fast and accurate simulation of sea-reflected GNSS signals in arbitrary acquisition geometries, which is an important preliminary task for a proper assessment of ship detection performance achievable with GNSS-R, also including unconventional imaging configurations. Further research works might focus on the evaluation and simulation of GNSS-R processing chains more suited to ship detection applications, such as bistatic SAR, as well as accurate link budget analysis for the ship target echo using advanced sea scattering models, such as BA-PTSM, for describing ship-sea EM interaction.  
**Author Contributions:** Conceptualization, G.D.M., A.D.S., A.I. and D.R.; methodology, G.D.M., A.D.S., A.I. and D.R.; investigation, G.D.M., A.D.S., A.I. and D.R.; validation, G.D.M., A.D.S., A.I. and D.R.; writing--original draft preparation, A.D.S.; writing--review and editing, G.D.M., A.D.S., A.I. and D.R. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research has been partly funded by the Italian Ministry for Education, University and Research within the PON \"Ricerca e Competitivita\" 2007--2013 program under contract SCN_00393_54E \"S4E--Sistem di sicurezza e protezione per l'Ambiente Mare\".  
**Conflicts of Interest:** The authors declare no conflict of interest.