GPS Underwater?

Hardware
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I have a fellow faculty member who would like to be able to track turtles underwater. Any idea how far underwater a GPS signal can propagate?

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None. But you might have a chance to get a GPS fix when the turtle surfaces for air.

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You would need to look at inertial guidance combined with GPS updates (hopefully enough time on surface to get lock - but you could incorporate a pressure sensor to detect when approaching the surface and “warm up” the GPS module… ) - this article is a good primer for IoT

https://www.analog.com/en/technical-articles/high-perf-inertial-sensors-propelling-internet-of-moving-things.html

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Thanks, Rftop. Does this depend on whether it’s fresh water or salt water, or the depth? Our current plan is to use a barometric pressure sensor to detect when the animal surfaces…

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I wouldn’t think salt/fresh water would matter, as the GPS antenna will need to be out of the water either way to get a fix. The same is true for a Cellular Antenna, if you want “real-time” updates.

Your System could “re-zero” it’s barometric readings to Sea Level after every GPS lock. That would help with drift from the pressure sensor and also barometric changes from weather patterns, but not tide.

I assume that you’d want to use replaceable batteries, so waterproofing the enclosure would probably be harder than the actual hardware design (IMHO). You would also want to keep the enclosure close to neutrally buoyant, I assume.

It’s an interesting project.

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Thanks, rftop. I found this article, which sounds like 3.27 cm is the maximum depth that a GPS signal can propagate. So you’re right, we will need the turtle to be entirely out of water. The turtles are going to be wandering around freely in a marsh, so I’m not sure about recharageables. Our plan is to get their location every hour when they are above ground, and transmit every 7 days. Hopefully the batteries will last that long, but I guess we could set up a low-battery SOS strategy.

I was going to use a Boron, which I hope, like the Photon, can go into a ridiculously low power mode: otherwise we may have to put tiny solar cells on the turtle :grinning:

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A Boron + GPS + Solar would be a great starting point.
Eventually, you may want to bypass the Boron’s Charging Circuit for a specialized Energy Harvesting IC.
That would allow for extended deployment times, as you can harvest energy from much lower voltage output from the Solar Panel, since the turtle might not keep the Solar Panel clean :sunglasses:.

But in the beginning, the Boron could be used to charge the Li-Po without the additional Energy Harvesting IC.

Eventually, you’d probably look at alternatives to the Li-Po storage since this device will be attached to an animal in the wild.

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Interesting, thanks for pointing this out, I had no idea these even existed. I found one inertial guidance system, but they said to call for a quote – never a good sign – and this product starts at $1899, a bit out of our price range.