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Have you looked into adding navigation / mapping / and comms to your dive computer? My company has a side project building a dive watch for divers. It has a navigation system and communications aspect where divers can send messages to each other and track a dive buddy's position. The project is mostly for fun but there is a market for it.

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How do they track position? I know that (eg) GPS and Bluetooth don't work under water (or at least their range is so bad that they aren't useful). Seems like an interesting problem!

You are right! It is a very hard and interesting problem. Radio will not work underwater. Inertial/mag sensors can be small and cheap but the quality is poor. Therefore I have been using some 'intelligent' (hate to use that word these days) processing of the data. It is still a relative position estimate if you can minimize the drift you are good to go. To help, we have communications between divers using acoustics. Using some cleaver DSP techniques and information sharing between divers you can do a lot.

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