## 4 Results
It can be appreciated that the SR is inversely proportional with respect to the length of the estimation window along the azimuth direction as expected from the theory. Moreover, ENVISAT data provide always better SR values with respect to ICEYE data. Conversely, the VR values retrieved from ICEYE benefits from the smaller radar wavelength with respect to the C-band one. This evidence encourages the use of such data for DCA-based marine applications. Finally, according to the DCA methodology in Section 3.2, we derive the radial surface velocity map for ENVISAT and ICEYE data. Figure 7 shows the result obtained by applying DCA methodology to ENVISAT data presented in Section 3.2. Unlike [41], which adopted a preprocessing step to join up to five frames to obtain a long strip over the area of interest, this work involves a single ENVISAT ASAR data frame acquired over the Ascending orbit already presented in [41]. Figure 6: (**A**) SR and (**B**) VR of ENVISAT and ICEYE data at the different azimuth window sizes on the ground. Figure 7 (on left) shows the SAR amplitude image relevant to the ASAR acquisition collected on 16 January 2009 over the Trieste gulf. This area has a distinctive and persistent wind, \"named Bora\", which originates from the mountains on the eastern side of the Adriatic Sea [41]. This specific wind phenomenon can result in significant surface currents as evidenced by the amplitude of the focused SAR images on the left of Figure 7. On the right side of Figure 7, the estimated radial surface velocity (\(v_{r}\)) is shown. The DCA is estimated on the SAR image over a sliding tile of 512 (azimuth) by 128 (range) pixels. This corresponds to a square patch on the sea surface of edge 2.5 km, a spectral resolution of 2.85 Hz, and a velocity resolution of almost 8 cm/s. The front of the surface current generated by the Bora wind is recognizable. The estimated radial component of the sea surface velocity is within the range of \(\pm\)1 m/s; the direction of the sea surface velocity is orthogonal to the flight direction. The red and blue colors (redshift and blueshift) of the estimated sea surface velocity component indicate a motion towards and away from the radar antenna, respectively [41,49]. Figure 8 shows the experimental results relevant to ICEYE data achieved by using sample data available on [54]. The DCA is estimated on the SAR image over a sliding tile of 400 (azimuth) by 400 (range) pixels. This corresponds to a square patch on the sea surface of edge 1.2 km and a spectral resolution of 6.1 Hz and a velocity resolution of almost 9.5 cm/s. Among the available ICEYE data, we select the data described in Section 2.2 because from a visual inspection of the Quick Look image, the sea surface backscattering shows an interesting pattern, which can be possibly related to a surface marine current. Furthermore, being that the area of interest is located in the Pacific Ocean, the scientific literature shows the capabilities of SAR to map the sea currents in ocean regions mainly characterized by large flows [16]. Therefore, since this work is a first demonstration of the application of DCA on ICEYE data, we select a case study that may potentially offer a promising starting point of DCA analysis. From a preliminary qualitative analysis of the amplitude SAR image, a variation of the backscattering in the marine surface region is well recognizable on the left of Figure 8.  
Since, as already stated in Section 2.2, for ICEYE only, Level-1A and Level-1B products are available, the DCA analysis starts from focused SAR data. Following the steps outlined in the block diagram of Figure 4, a map of surface radial marine velocity is generated according to the DCA methodology. It is shown (see the right of Figure 8) that \(v_{r}\) varies in the range of \(\pm\)1 m/s. Figure 7: **(Left) Amplitude SAR image acquired by ENVISAT sensor over Trieste area of interest on the 16 th January 2009.