Sea-Clutter
Sea-clutter are disturbing radar-echoes of sea wave crests. This clutter gets also a Doppler- speed by the wind. This means, the scenario „moves away”, i.e. changes with time, while for ground clutter it stays the same. Therefore, in practice, Sea-clutter is very difficult to control without some loss in detection.
Sea-Clutter can be seen here in the picture. The wind comes either from about 310° (NO) or from the opposite direction. (Unfortunately, whether the Doppler frequency is positive or negative cannot be recognized on the PPI-Scope.)
But this region, in which the radial speed of the waves is very small, is „cleaned” by the MTI system very clearly.