[jhgb]

Surely a GPS mast shouldn't be a problem? And an IMU may be more expensive than a GPS receiver, but compared to the cost of the whole vessel, it's probably negligible.

[siod]

Yeah a mast or tethered transmitter probably they way to go, but now you have the problem of operating both a submarine and a pseudo surface vessel.

Nothing insurmountable, but it's not as simple as running under everyone which is what the article implied (to me atleast).

[tgb]

Doppler velocity log is likely what they'd want.