It has been possible since WWII, only not from tens of miles: "During the early months of the Battle of the Atlantic in World War II, British ships using the r

It has been possible since WWII, only not from tens of miles: "During the early months of the Battle of the Atlantic in World War II, British ships using the radar set Model 271 were able to detect the periscope of a submerged submarine at a distance of 800 m (0.50 mi) during tests in 1940." https://en.m.wikipedia.org/wiki/Submarine_snorkel https://en.m.wikipedia.org/wiki/Submarine_snorkel