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Comment by eps | Hacker News Reader

Small GPS receiver card (chip?), GSM module with prepaid SIM, a bit of flash storage to record GPS data and a battery - all sealed in a rugged ball. This shouldn't be that costly, no? Or will it be hard to make the battery last beyond few days?

At the very least this should give the first surface point for that undergraund stream.

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Water is essentially non permeable by radio waves at the frequencies you would use for GSM and GPS, which is why communicating with submarines is extremely tricky. I've always assumed (but never been able to find information on) convicts with GPS tracking anklets taking baths and unknowingly getting in trouble for breaking the recorded path.

Both the GPS and the phone module is going to be useless while underground, so perhaps it will be better to turn them off altogether and use a separate timer to reconnect them after a few days. A disconnected battery can last quite a long time.

Even an energized battery, on a low duty cycle, (Transmitting once a day at low power), can last upwards of 20 years.

You'd probably need to make it very slightly more dense than water, or it may get trapped at the surface in an underground cave. This means it won't surface by itself once it leaves the cave system, and as water blocks RF pretty effectively, you'd not get a signal. To make it work, you'd need to add some additional mechanism to allow it to become more buoyant over time - perhaps a slow chemical reaction creating gas?

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