Achieving a transmission data underwater at 100 mts depth

I need to develop a device that should transmit its position and some telemetry data underwater at a maximum depth of 100 m. Considering that RF waves don't propagate well underwater, I guess I'll have to use acoustics as solution. However, I have no idea what kind of material (Tx/Rx) I should use to achieve a transmission data using sound at that depth. Is there any microcontroller (ST or Arduino) I could use in order to achieve that? Any suggestion will be apreciated.

Welcome to ETO!
I think that will be a major challenge.
How will the device determine its position?
Do you need to transmit to the water surface?
What data transmission rate is acceptable?
Is two-way transmission required?

Hi alec_t. Thanks for replying! To answer your questions:
1) For localisation, I have an idea but I'm not sure if it's feasible: The submerged device would have a sort of ultrasonic speaker for sending data. The signal would be "heard" by a Rx at the surface (a buoy). Then, power signal received by the buoy could be coded for determine distance between the device and the buoy.
2) Yes, data have to be transmitted at the surface and eventually to a boat and/or onshore
3) I believe a low rate (40-80kbps) should be work.
4) One-way (device to buoy) would be suitable.

alec_t said:

How will the device determine its position?

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That's a major issue, seeing as you don't have GPS. One thing you can do is to collect GPS data just before the device is deployed
and use a crude form of inertial navigation (Using an accelerometer such as is in your cell phone.)

alec_t said:

Do you need to transmit to the water surface?
What data transmission rate is acceptable?
Is two-way transmission required?

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You could talk to these guys.
https://www.benthowave.com/products/

JPolo said:

Considering that RF waves don't propagate well underwater

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there are techniques for communications using VLF (3 khz to 30 khz) radio that does propagate through water. it could also be used with 3 receivers at the surface to triangulate the transmitter position and depth. the data rate won't be very high, but unless you are transmitting video or sonar data, it doesn't have to be. spelunkers have been using such techniques for decades to transmit data and triangulate position through rock, which is much more dense than water. you can find more info (and maybe other ways of doing it) [here]

Is this salt water?

JPolo said:

3) I believe a low rate (40-80kbps) should be work.

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unclejed613 said:

there are techniques for communications using VLF (3 khz to 30 khz) radio that does propagate through water.

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Those two are incompatible.

I think there are certain "colors" of light that pass through water well and some that do not. Maybe a laser pointed up. 410nm light?
light vs water
I found LEDs at 405 and 445nm. LED Lasers at 401 through 410.

Personally I would look into military sonar for ideas. A transponder may help with position location. The frequency of the transmitted carrier signal will likely be the thing that determines data rate. There are such things as thermal layers in water which would complicate things so the conditions in your planned will need looking into. I am not sure about these but submarines can use them to confuse sonar, and I suspect there would be interference to light too (diffraction?).

Tracy

Vlf is one of my interests, receiving rather than transmitting.
Antennas for Vlf tend to be very large, unless you went for something like a magnetic loop, efficiencies tend to be very low.
Subs use Vlf though I'm not sure what depth they can communicate at.
Sonar might be a little easier to get working.
Have a look to see if you can find any existing products and see what they use.

JPolo said:

The submerged device would have a sort of ultrasonic speaker for sending data.

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JPolo said:

I believe a low rate (40-80kbps) should be work.

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he's working with the same conflict between data rate and spectrum width here too... he would need an acoustic device with 40-80khz bandwidth. with an OFDM modulation scheme, he might be able to cram 40kb/s of data into a signal 30khz wide. that's something an arduino doesn't have the horsepower for though.

i did find this paper. it's about fresh water radio propagation, i'm keeping my eyes open for anything similar about salt water. from what i see in the fresh water paper, is that between 10khz and 100khz is quite usable. i'll have to dig deeper into the text to see what their test conditions were. i would think that within a few hundred meters though, RF between 10 and 100khz would be a good choice... you're not trying to talk to a submarine at 500 meter depth a half a world away. it might even be possible at such close range as what the OP described, that bands in the low HF (up to 10Mhz) part of the spectrum could work.

one thing that just came to mind, is that the speed of light in water is different than it is in air, and so antenna lengths would need to be recalculated for this difference. the speed of light in salt water is approximately 2/3 of what it is in air, so antennas need to be about 2/3 their normal length.

JPolo said:

I need to develop a device that should transmit its position and some telemetry data underwater at a maximum depth of 100 m. Considering that RF waves don't propagate well underwater, I guess I'll have to use acoustics as solution. However, I have no idea what kind of material (Tx/Rx) I should use to achieve a transmission data using sound at that depth. Is there any microcontroller (ST or Arduino) I could use in order to achieve that? Any suggestion will be apreciated.

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Why not just use wire? You'll need to power it, why not send the data over an undersea twisted pair data cable to the surface buoy?

gophert said:

Why not just use wire? You'll need to power it, why not send the data over an undersea twisted pair data cable to the surface buoy?

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Data and power over the same twisted pair wire.

ronsimpson said:

Data and power over the same twisted pair wire.

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Not what I intended to say but, is that possible? Like "X10" I guess?

Had I to attempt something like this I would focus on having the transducer problem solved first.

Don't model train systems use modulated AC over two wires (tracks) to power and control trains? The same thing could be done over two wires but only in one direction - down.

Mike.

Power & data on the same wire. I put an inductor on each end of the cable so the impedance is high at 100khz but remains low at dc. The transmitter reaches arround the inductor and shakes the cable at 100khz. The receiver input is capacitor coupled around the inductor.

Interesting concept Ron.
One time I put togther a system thad did this I modulated the voltage from 8v to 12v using a tranny and a 5v reg, bitrate was only a few 100 but it works well still and that was a few years back. Decoupling the line with a choke would work at high data rates.
'Reaches around the inductor and shakes the cable' I like it.

dr pepper said:

Reaches around the inductor and shakes the cable

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One of the reasons X-10 has problems is now days there are many computers and switch mode power supplies on the line. Each supply has real good capacitors across the power line. The point of the filters in a switching power supply is to short the power line out at 100khz. (switching frequency) Back when most loads are resistive or inductive X-10 worked well. (ok, better) I opened up many of my power strips and added inductors to each power wire to make then high impedance at 100khz.

Maybe a real shot in the dark, but if there are thoughts of running a twisted pair, what are the practicalities of running a supported fibre optic cable and having whatever data rate you want?

Another thought is that, assuming failrly low data rate, if saltwater only one insulated conductive lead may be required, the other being the water itself?

Tracy

BusyBee said:

Maybe a real shot in the dark, but if there are thoughts of running a twisted pair, what are the practicalities of running a supported fibre optic cable and having whatever data rate you want?

Another thought is that, assuming failrly low data rate, if saltwater only one insulated conductive lead may be required, the other being the water itself?

Tracy

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Or a battery for power and fiber optic for data.