[orangepurple]

I think periscope detection is only possible since the 1970s

  The opportunity to detect periscopes was exploited in early radar experiments that prompted the development of the AN/APS-116 radar manufactured by Texas Instruments in the 1970s. The AN/APS-116 is an Xband, high-resolution, fast scanning system developed specifically to provide a periscope detection capability on the carrier-based S-3 aircraft. The AN/APS-137 is an upgrade of this radar used primarily on the S-3; a limited number are also used on the land-based P-3 aircraft.

https://www.jhuapl.edu/Content/techdigest/pdf/V18-N01/18-01-...

[petre]

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