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A small tethered float with accelerometers that ascends along the path of the surfacing submarine.

Submarines typically conduct powered or semi-powered ascents. No forward motion = no steerage.

Add some steering and if needed power - if you’re lucky you only need to steer to the path with the energy of the vertical ascent.

I think the point was that you have enough forward movement for steerage, your tethered float will always be behind you.

That's why I mentioned powering the float if needed ... I suppose it's more of a tethered drone at that point.