SIMULATION OF GNSS RETURNS FOR DELAY–DOPPLER ANALYSIS OF THE OCEAN SURFACE Maria Paola Clarizia 1,2 , Maurizio di Bisceglie 1 , Carmela Galdi 1 , Christine Gommenginger 2 , Meric Srokosz 2 1 Universit` a degli Studi del Sannio, Piazza Roma 21, I-82100, Benevento, Italy 2 National Oceanography Centre, Southampton, UK 1. ABSTRACT GNSS-Reflectometry is a revolutionary approach to Earth Observation and is recently undergoing rapid advances, especially because the signals of opportunity scattered worldwide from the ocean surface can be used to investigate many important geo- physical properties in a near-real time. Global sampling and high temporal resolution of GNSS-R signals encourage application in both ocean surface scatterometry (sea roughness, wind speed and direction) and ocean altimetry (sea surface height and mean sea level). Recent studies on scatterometry using GNSS-R have shown the possibility to retrieve the sea surface roughness, with airborne [1] and spaceborne [2] GPS-R receivers using a 2-D representation of the scattered signal power in the delay and Doppler domain, known as delay-Doppler map (DDM). The typical approach is to perform a least square fitting of measured DDMs with simulated DDMs using the theoretical Zavorotny-Voronovich (Z-V) model [3], to retrieve the optimal directional Mean Square Slopes (MSS), representative of the surface roughness. The Z-V model gives an expression of the average scattered power, in the Geometric Optics (GO) limit, through a bistatic radar equation, adapted to the GNSS case. Some new analysis and results presented in [2] show how GNSS-R can be used to retrieve wave properties from satellite data, but at the same time highlight some important differences between DDMs derived from GPS-R data and simulated DDMs from the theoretical model. These differences are probably due to some scattering mechanisms that are not properly considered in the GO-based model. As opposed to [1] and [2], we use here a different approach, in that we simulate the whole end-to-end microwave scattering of GNSS signals from the sea surface. Our approach stems from the idea that accurate retrievals of sea surface height and sea roughness require a better description of the microwave/ocean interaction and scattering mechanisms, as well as a more realistic representation of the ocean surface. The GNSS-R simulator performs the generation of the transmitted GPS signal, the scattering by a realistic ocean surface, and the signal reception and processing in the delay-Doppler domain to produce a DDM as the final output. The fundamental steps to be implemented are: a) The simulation of a realistic sea surface, with specific statistical and spectral properties; b) the selection of a realistic scattering model; c) the implementation of GNSS-R processing to produce the final scattered power in the delay-Doppler domain (DDM). To generate the sea surface, we filter a white Gaussian process with the sea surface spectrum described by Elfouhaily et al. [4]. This ensures the twofold advantage of preserving the Gaussian statistics of the sea surface heights (a realistic assumption) and imposing the desired spectral properties to the sea surface. The output of the filter is a snapshot of a sea surface, which can be modelled as an ensemble of facets, tilted and oriented in arbitrary directions, and whose size is chosen according to the spectral requirements and criteria induced by the scattering parameters. The scattering model that we adopt to effectively represent the GPS scattering from the sea surface is a semi-deterministic two-scale model. The two-scale model assumes that the sea surface shape is made by a larger (gravity) roughness scale, and a smaller (capillary) scale, and therefore accounts for the two complementary scattering mechanisms. The scattering from the large-scale roughness is usually described using the Kirchhoff Approximation (KA) or Physical Optics (PO) [5]. The KA is based on the assumption that the radius of curvature of the waves is much larger than the electromagnetic wavelength (19 cm in the case of GPS), such that the surface can be locally approximated by its tangent plane. According to the KA, the scattered power from each facet, in the quasi-specular regime, is concentrated within a narrow cone around the specular direction. The KA in the high frequency limit reduces to the Geometric Optics (GO), where the scattering occurs in the specular direction only.