(inspired by a question on reddit, I’ll post a reply too)
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There has to be a solution that would have allowed them to send a buoy up to transmit a mayday and coordinates like some life boats do. This would help the rescue teams to narrow the search grid. I wouldn’t have gone down in that thing, but if I WAS, I would at least feel better with something like that.
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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!