Submarine detection is a major research field of all militaries throughout the world.
The primary advantage and first layer of defence of a submarine is to remain hidden. Once discovered, submarines have very little in the way of defence and are easily destroyed.
Therefore it has been recognised as early as the first world war (yes there were many submarines used in WW1) that detecting them is very important, and this has sparked a 'cat and mouse' arms race between detecting a submarine, and research into techniques for a submarine to avoid detection.
Aircraft are seen as a major element in submarine detection as they can cover much area when searching. Techniques developed since WW1 to contemporary times are:
- Visual observation of exhaust cowls, periscopes or disturbance in water surface by aircraft
- dropping of sonar buoys to send regular 'pings' to detect any objects underwater. Contrary to popular belief, active sonar is not activated unless detected in modern submarines as your position would be revealed. Sonar buoys do not have this problem.
- passive sonar 'nets' or buoys deployed to listen for submarines so they can be detected undetected.
- magnetic sensors in the aircraft to detect unusual magnetic fields caused by metal in submarines
- radar was a major detection method in WW2 and was the main cause of U-boat detections. 10cm wavelength radar was used throughout this period.
- listening to radio traffic from submarines - this can give away its position instantly
- Leigh light - an incredibly strong searchlight beam to detect submarines in WW2.
- more contemporary technologies include:
- magnetic anomaly detection, detection of metal from an aircraft
- hydrodynamic wave detection (searching for disturbance in surface water from a potential 'wake')
- laser detection - beaming lasers into the ocean and measuring reflections to search for submerged objects
- infra-red detection if any part is above the surface
- forward infra-red detection of warmer water rising to the surface from underwater nuclear submarines