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Abstract-This study addresses a novel application of global navigation satellite system-reflectometry (GNSS-R) delay-Doppler maps (DDMs), namely sea target detection. In contrast with other competing remote sensing technologies, such as synthetic aperture radar and optical systems, typically exploited in the field of sea target detection, GNSS-R systems could be employed as satellite constellations, so as to fulfill the temporal requirements for near real-time ships and sea ice sheets monitoring. In this study, the revisit time offered by GNSS-R systems is quantitatively evaluated by means of a simulation analysis, in which three different realistic GNSS-R missions are simulated and analyzed. Then, a sea target detection algorithm from spaceborne GNSS-R DDMs is described and assessed. The algorithm is based on a sea clutter compensation step and uses an adaptive threshold to take into account spatial variations in the sea background and/or noise statistics. Finally, the sea target detector algorithm is tested and validated for the first time ever using experimental GNSS-R data from the U.K. TechDemoSat-1 dataset. Performance is assessed by providing the receiver operating characteristic curves, and some preliminary experimental results are presented.Index Terms-Constant false alarm rate (CFAR), global navigation satellite system-reflectometry (GNSS-R), maritime surveillance, sea state, sea target detection.

In a recent paper, we have presented an efficient and simple approach to model electromagnetic propagation in urban street canyons, when both the transmitter and the receiver are below the rooftop level. The model is based on the modal expansion approach, and we have shown that it is computationally efficient and that it allows for deriving a straightforward expression of the average received signal strength. In that paper, the Line‐of‐Sight (LoS) propagation case was described and discussed in detail, and the complete derivation of results was provided. Conversely, for the Non‐Line‐of‐Sight (NLoS) around‐the‐corner case, only the final results were presented and briefly discussed. In the present paper, first of all we provide full details on the derivation of formulas for the NLoS case. In addition, we extend the discussion of results and properly analyze the obtained NLoS propagation loss expression. Last but not least, in contrast with our previous propagation model, we here propose a new formulation of the NLoS case that satisfies a key property in electromagnetics, that is, reciprocity, which was previously fulfilled only in the LoS case. The presented models are validated using empirical models, ray‐tracing algorithms, and experimental campaigns.

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