## 1 Introduction
Specifically, it can be obtained via the estimation of the residual Doppler shifts of the received radar echoes, the Doppler Centroid Anomaly (DCA) approach [4], or by means of interferometric acquisitions with two antennas displaced in the along-track direction, Along-Track Interferometry (ATI) approach [13; 18]. Both approaches have been attempted to get the radial ocean current, especially over regions of well known strong currents, as in the Gulf Stream [12; 19] offshore of the USA and in the Agulhas Greater Current offshore of South Africa [20]. In [4], other experiments have been presented in coastal areas of the Gulf of Normandy, which is characterized by intense tidal currents. Based on many successful experiments, steps aimed at providing operational services for coastal ocean monitoring have been proposed in recent years [21] and in some cases, implemented. However, regarding currents, the existing examples of applications are mainly limited to the above-cited sites characterized by significant water circulation phenomena. This work focuses on the DCA, which, similarly to the phase offset for ATI, is induced by the movement of the scattering centers, thus providing a direct measurement of the radial component of the sea surface currents [4]. In particular, we applied the technique of retrieving the radial surface sea current from the DCA in the Mediterranean Sea, a semi-enclosed sea where the sea currents are weak and the geophysical phenomena in coastal areas are strong, due to the interactions between the airflows and the orography. In this respect, this work acts as a feasibility study. At present, the topic of deriving sea surface currents from SAR involves many issues, both from the engineering and geophysical points of view. The former because of the typically large non-geophysical velocities associated with to the satellite attitude and the SAR instrument imaging characteristics; the latter because of the difficulty to split the different phenomena contributing to the DCA, such as the wind, the waves and the currents [22]. For the investigation of the above open issues, we considered the DCA method and we focused on a case study involving the Naples bay and the adjacent coastal region. Among the Italian coastal zones, the Gulf of Naples and the surrounding coastal regions are particularly interesting as first of all they are areas influenced by numerous environmental, socio-economic, and interacting cultural factors (strong anthropogenic impact, intense maritime traffic, the presence of polluted rivers, tourist and economic activities and the presence of four protected areas) [23]. The area is also interesting from an oceanographic standpoint as both coastal wind-driven and open-sea currents influence the local hydrodynamics [24; 25; 26]. In particular, several factors make this an area of general oceanographic interest, among them: (i) the presence of the Gulf of Naples, characterized by dimensions comparable to the internal Rossby deformation radius; (ii) the presence of extended shallow water areas at the edge of the continental slope; (iii) a coastal circulation shaped by both the interaction between the larger scale flows with the coastal topography and by the local wind forcing [27]. The peculiar dynamics of water circulation in this area qualify this region as a natural challenging laboratory both for investigating oceanographic phenomenology [27] and for testing the skill of ocean and atmospheric observational platforms as well as numerical models [23]. Furthermore, the choice of the zone is driven by the availability of a large historical data archive regularly acquired over the years. As also better specified in the following sections, we focus in particular on the use of the ENVISAT SAR archive due to the available larger azimuth bandwidth compared to the most recent data gathered by the Sentinel-1 sensor.