A beacon should work as I described in a separate response. A buoy could work too, but the buoy a) would need to be designed to float up from 4 km and still be functional at the surface b) the sub doesn’t necessarily know where it is below the ocean. GPS doesn’t work down there!
A beacon should work as I described in a separate response. A buoy could work too, but the buoy a) would need to be designed to float up from 4 km and still be functional at the surface b) the sub doesn’t necessarily know where it is below the ocean. GPS doesn’t work down there!