Comparison of the Sea Surface Velocity Derived from Sentinel-1 and Tandem-X.

This paper presents a direct comparison of the sea surface radial velocity (RVL) derived from the two satellite SAR systems Sentinel-1 and TanDEM-X, operating at different frequencies and imaging modes. The RVL is derived from the Doppler centroid (Dc) provided in the Sentinel-1 OCN product and from the along-track interferometric phase of the TanDEM-X, respectively. The comparison is performed using an opportunistic collocated acquisition over the Pentland Firth strait, known for its strong tidal stream. This comparison shows that the two SAR systems observe similar sea surface circulation patterns with high spatial correlation coefficient (r ~10.8). It is also shown that, provided a common calibration reference is available, the two independently derived RVL are quantitatively in good agreement with a negligible bias and reasonable RMSE (~10.3 m/s). This encourages use of the synergy between different C- and X-band SAR systems, measuring sea surface velocity.