What are the different ways to detect submarines?

The ones I know of are:

• Passive sonar (via ASW ships and helicopters)
  
  
• Active sonar
  
  
• Tracking the wake (via MPA)

Are there any other methods of detecting a submarine?https://snaptube.cam/ 9apps Additionally, are there unique ways for a submarine to contact friendly forces on the surface without giving away their position (besides frequency hopping)?

 

Thermal

 

Satellites had not been invented yet.

 

magnetic anomaly

 

During WW2, helicopters did not have dipping sonar, and could only detect visually. They also carried very little payload to attack with.

Others.
1. Visual - depending on opaqueness of the sea, a submarine could be seen down to 120-150 feet depth. by blimps or aircraft.

2. Magnetic Anamoly Detector(MAD) - Saw it's first use in WW2. Primarily aboard aircraft.

3. Sonobouy(Allied only) - passive sonar of limited range and duration.


Contacting of friendly forces ashore...
To use the radio submarines had to be surfaced or very near the surface. To use frequency hopping, the forces ashore had to have a radio capable of it(unlikely for partisans or guerrillas). Thus, methods were used to minimize detection(hooded lamps, light through periscope).

The Germans were working on a burst transmission system(Kurier) but it did not enter service before the war ended. Although, partisans or guerrillas would not have the proper equipment to read it either.

 

One of the most significant antisubmarine "weapons" of WWII was radar. WWII submarines spent most of their time on the surface, cruising or attacking. German and American subs in particular had their greatest success when able to attack on the surface at night. The proliferation of radar on escort ships and aircraft, especially long-range maritime patrol aircraft, was a key factor in turning the tide of the Battle of the Atlantic. Conversely the Americans had the radar edge in the Pacific.

 

Did they manage to crack the Kriegsmarine enigma code finally which helped to know where to look for the u-boats?