*4.1. Ocean Surface Currents*

This variable presents a gap in the required revisit time. The required revisit time is 3 h, and the required spatial resolution is 1–25 km, with a speed accuracy of 5 m/s and 10◦ in direction. A federation of three missions, Sentinel-3 (radar altimeter SRAL) and a specific combination with a constellation of Global Navigation Satellite Systems-Reflectometry (GNSS-R) instruments [12,13], microwave radiometer, altimeter radar [14] and SAR on small platforms, could improve the accuracy and the revisit time efficiently.
