Cold Waters Nuclear Submarine game/sim website by PMC.
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In general terms if you are sneaking around listening to enemy boats/ships, you only get sierra contacts in the tactical map, however when you take peek at the surface using ESM mast, radar mast or periscope, you have those sierras turn into Master contact types.
Sierra contact types are only held on sonar, Master contact types are from two different sources for example sonar and periscope.