Investigating the Effect of Ocean Waves on GNSS-R Microwave Remote Sensing Measurements

Abstract

Global Navigation Satellite System-Reflectometry (GNSS-R) is an innovative technique for ocean remote sensing. It exploits signals of opportunity from navigation constellations, to look primarily at the ocean surface roughness. This dissertation investigates the capabilities of GNSS-Reflectometry to convey information about sea state, and the response of GPS reflected signals to different wind and wave conditions. This is done through the use of real GPS-R data, as well as through simulations of the scattering of GPS signals from realistic ocean surfaces. A retrieval of ocean roughness parameters is carried out on four GNSS-R datasets, collected onboard the UK-DMC Satellite. Measured Delay-Doppler Maps (DDMs) from GPS-R data are least-square fitted to DDMs simulated using a theoretical (Zavorotny-Voronovich, or Z-V) model. The retrieved parameters are compared and validated against measurements from co-located NDBC buoys, and theoretical calculations, and a reasonable agreement is observed.

A GPS scattering simulator is then presented, that uses explicit 3D ocean surface representations, and an innovative facet-based polarimetric scattering model, called the Facet Approach (FA). The results of the GPS scattering simulator are first analysed in the spatial domain, as 2D maps of normalized radar cross section and polarization ratio. These maps exhibit clear features related to the explicit waves of the underlying sea surface.
A detailed analysis of noise-free idealized DDMs of both scattered GPS power and polarization ratio is then carried out for a variety of different ocean surfaces, both linear
and non-linear. This analysis stresses in particular the importance of wave directionality as a crucial parameter that influences the DDM sensitivity to sea surface roughness
ii and wave direction, and the polarization effects on the scattered signal. It is revealed that polarization is another important parameter, as it can convey information on wave direction and directionality, and potentially be used to identify nonlinearities on the sea surface. Finally, an investigation of subsets of noise-free idealized DDMs, computed at a high DD resolution, is presented, and its potentials as a tool for detecting the explicit waves on the sea surface are highlighted.

The research and analyses of this PhD dissertation represent novel contributions to the field of GPR-Reflectometry. In particular, the analysis of satellite GPS-R data is the first one that makes use of the whole DDM, and of data collected onboard a satellite. The results from the GPS scattering simulator provide a comprehensive description of how and to what extent different parameters of the ocean surface, linked to wind and waves, influence the scattering of GPS signals. Furthermore, they identify polarization as a new crucial parameter for future GNSS-R missions, since it provides additional information about sea-state, and might be used as a potential indicator of sea surface nonlinearities.

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