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The Devil's Kettle is a pothole in the rock where a small river disappears underground in the Judge C.R. Magney State Park in Minnesota. Would it be possible to trace the path of the drainage conduit by throwing in a small waterproof GPS tracker?

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    $\begingroup$ I think you're going to have to explain what the "devil's cattle" are, and what kind of GPS tracker you have in mind. A GPS receiver on its own doesn't let you track anything: it also needs a way to communicate its location to the rest of the world. In some areas this is practical to do via a mobile phone network; in more remote places you might need to communicate with a satellite (which probably isn't feasible in a "micro"-anything). $\endgroup$ – Pont Mar 1 '18 at 21:42
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    $\begingroup$ There's a big difference between Cattle & Kettle $\endgroup$ – Fred Mar 2 '18 at 2:22
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    $\begingroup$ This question is not about earth science it's about the applicability of surface use satellite communication technology when it is placed underground. $\endgroup$ – Fred Mar 2 '18 at 2:27
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GPS units require a clear view of the sky to fix their position. Because unlike radio signals used for communications, once the GPS signal have bounced in a wall becomes useless for computing the receiver's position. Therefore, the GPS would loss fix shortly after entering the pothole.

The studies made in the area that determined the point at which the water reenters the river, claim that the flow of water keep objects at the bottom of the pothole, explaining why they never resurface. So there is a high chance that you wouldn't see your GPS ever again.

However, if your GPS were to resurface, it would fix its position again soon after regaining a clear view of the sky. So, if you recover it (or if it had the capability to transmit its position in real time), you won't see a record of the track followed by the device, you would see only the entry and exit point.

For such problems, similar to a car going trough a tunnel, the GPS is usually augmented with inertial navigation systems that can estimate the displacements and compute the position by dead reckoning from the last GPS fix. Despite that the turbulence within the water flow in the pothole would make very challenging to keep track of the trajectory of the device underground, a sophisticated enough device should be able to do it.

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    $\begingroup$ It's not that the signals become useless after reflecting from a wall, it's that they simply don't penetrate rock. $\endgroup$ – jamesqf Mar 2 '18 at 4:04
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    $\begingroup$ @jamesqf Yes, they don't. But in this case is more like a tunnel, as it is not completely surrounded by rock. I don't know how is the penetration of the signal in fresh water, in snow is pretty good, in sea water is null, but in fresh water I don't know. Anyway, a GPS don't work in a tunnel or indoors not because the signal don't penetrate concrete or rock, but because in such places the indirect signal is way stronger than the direct signal, therefore rendering useless for positioning. Otherwise, making the receiver antenna bigger and/or the satellite power stronger would solve the problem. $\endgroup$ – Camilo Rada Mar 2 '18 at 5:32

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