###### Abstract  
In this work, we present the results of a comparative analysis between the first-generation Advanced Synthetic Aperture Radar (ASAR) sensor mounted on board the ENVISAT platform and the novel ICEYE micro-satellite synthetic aperture radar (SAR) sensor in measuring the radial velocity of ocean currents through the Doppler Centroid Anomaly (DCA) technique. First, the basic principles of DCA and the theoretical precision of the Doppler Centroid (DC) estimates are introduced. Subsequently, the role of the DC measurements in retrieving the sea surface current velocity is addressed. To achieve this goal, two sets of SAR data gathered by ASAR (C-band) and from the X-band ICEYE instruments, respectively, are exploited. The standard deviation of DCA measurements is derived and tested against what is expected by theory. The presented analysis results are beneficial to evaluate the pros and cons of the new-generation X-band to the first-generation ASAR/ENVISAT system, which has been extensively exploited for ocean currents monitoring applications. As an outcome, we find that with inherently selected methods for DC estimates, the performance offered by ICEYE is comparable to, or even better than (with specific parameters selection), the consolidated approaches based on the ASAR sensor. Nonetheless, new SAR constellations offer an undoubted advantage regarding improved spatial resolution and time repeatability. Doppler Centroid (DC); Doppler Centroid Anomaly (DCA); sea surface velocity; SAR micro-satellites; ICEYE +
Footnote †: journal: Marine Science and Engineering  
0000-0001-5000-0001-5000]C.M. for ocean, sea, and coastal applications mostly exploits the amplitude of the backscattered signal, for example, monitoring oil spills [8] and sea ice [9], ship detection [10], and high-resolution wind fields retrieval [11]. However SAR uses coherent radiation, and the complementary information carried in the phase of the received complex signal can be also exploited. By analyzing the complex backscattered (received) signal, it is possible to measure the Doppler properties of the scatterers. The latter are directly related to the motion of the scatterers along the radar line of sight (LOS) in both land and marine contexts. In ocean applications, these properties are currently used to retrieve near surface wind speed [12] and surface current [13]. The estimation of sea surface velocity [14; 15] from SAR images represents today a \"hot-point\". In this context, the process of hydrodynamic modulation of the sea surface roughness allows, for example, the estimation of the sea surface current signatures [14; 15; 16; 17]. Extracting quantitative parameters of currents based exclusively on SAR images, however, is a rather complex matter. Indeed, numerous variables, including the wind vector and bathymetry [18], must be taken into account for comprehensive analysis. In general, two SAR-based methods can be used to measure ocean surface current velocity: the along-track SAR interferometry (ATI) [19] and the Doppler Centroid Anomaly (DCA) [16] techniques. ATI, which was initially introduced in [19], needs the collection of data pairs from along-track displaced antennas. It measures the interferometric phase difference between the complex signals recovered from the two antennas, which is proportional to the Doppler shift of the backscattered radar echoes. ATI is particularly suited for fast-varying phenomena in marine observations, including sea current surface velocity and vehicle fluxes in aquatic environments [20; 21; 22; 23; 24; 25; 26]. This technique has undergone significant developments in recent years. However, ATI requires unique sensor characteristics, namely, the opportunity of having two antennas spaced along the azimuth direction, enabling the acquisition of images within a short time lag [27; 28; 29; 16; 20].