Das Boot or the American Fleet Boat

[Bilge_Rat]

If you are looking at air v. sub, detecting an airplane on radar was easier and possible at greater range, even with WW2 technology, because the airplane sticks out all alone in the air with no competing false returns.

Detecting a sub on the surface from the air could only be done from a much shorter range, because the sub gets lost in the "surface clutter", namely the many false returns from radar waves which hit the surface of the sea and are reflected back to the radar set on the airplane. In WW2, the airplane has to be fairly close before the radar image of the sub would burn through the clutter and show up on the set.

So a radar equipped sub on the surface could have detected the airplane early enough to dive and survive rather than be surprised on the surface and killed. The U.S. Navy proved that in the Pacific.