Underwater Communication

Hi all,

I have troubles with definition of the communication technique to use in my project.

I would just like to let communicate two arduino, one at sea level one at ground level with sea water as medium, as described in the attached scheme.
I am just trying to understand wich kind of technique I can use.

I made a link budget with RF Tx/Rx but it seems to be not applicable.

What about Ultrasound/Acoustic? Which one is the easier/cheaper to implement?

Consider that the only data that I have to transmit to the Arduino at the bottom, is just a 1/0 bit (ON/OFF) and no more.

Thank you in advance for your time!

Sound, either ultrasonic or audible, would work, but you need transducers intended for water (hydrophones) and amplifiers on each end.

RF energy does very poorly in water as you have found - do some research on "skin depth" - it is possible to use RF underwater (they have done it for years for submarines, but it requires a BIG antenna and very low frequency (seems to me it was something like 20khz). Ultrasonic works fairly well although you can run into issues with anything that creates a density change in the water (thermocline or salinity change (where a river etc runs fresh water into salt water) but as pointed out, you need transducers made for immersion. Again, lower frequency goes farther, but can carry less information per unit of time (often depth sounders for boats are dual frequency like 50khz and 200khz - the 50khz is for reading deep and the 200khz does not have the range, but does a better job of resolving detail like fish (well, actually the swim bladder in the fish filled with air creates the reflection)).

Regards the rf comms to subs - you forgot that a very large power is required too!

regards Allan

Search DTMF decoder/encoder for Arduino. Not very fast but should be ok to send steering commands and receive simple parameters like temperature, depth, battery charge, propeller rpm, compass heading .

There are hardware modules and stand alone libraries.

allanhurst: Regards the rf comms to subs - you forgot that a very large power is required too!

regards Allan

Very true - This is the one near me: 24.8khz, 1.2 megawatts and the antenna stretches between two hills :)

blimpyway: Search DTMF decoder/encoder for Arduino. Not very fast but should be ok to send steering commands and receive simple parameters like temperature, depth, battery charge, propeller rpm, compass heading .

There are hardware modules and stand alone libraries.

And the best part is he can just use his cell phone. ::)

https://www.google.com/search?q=underwater+acoustic+communication+arduino

The second link details an implementation using an Arduino and common parts.

gpsmikey is right about low RF frequencies; this is what you'll find that submarine modelers use but then they still only get a few feet of depth. Any 2.4GHz transmitter (as is common today) won't work; 2.4GHz is the same frequency that a microwave uses and I'm sure you understand how well water will absorb that frequency.

Thank you all for your seggestions!

I really appreciate them, I have just sent an email to http://www.dspcomm.com/ to understand what kind of devices I need to setup the communication (I guess an hydrophone and an underwater modem, am I right?)

blimpyway: Search DTMF decoder/encoder for Arduino. Not very fast but should be ok to send steering commands and receive simple parameters like temperature, depth, battery charge, propeller rpm, compass heading .

There are hardware modules and stand alone libraries.

blimpyway I did not understand why you talk about DTMF (consider that I am not an expert, I am just googling it :) )... Looking at these devices, it seems that they are useful for sending frequencies corresponing do binary digits (1/9*#), am I right? What did you mean? Thank you!

DTMF sends 4bits tones every 50ms or so. That totals to a ~80baud or 10bytes/sec data transmission rate.

You can also try any old-style data-over-audio communication used by modems over phone lines, like AFSK for example, should be ok for 1200 baud.

I think what blimpyway was saying that was implied, but not clearly stated was to consider using DTMF as a modulation scheme of an ultrasonic (or other) carrier although, I guess that you could use the DTMF frequencies direct if you had submersible transducers that could handle those lower frequencies (anybody with their head underwater where this was going on though would be sure someone was dialing someone somewhere!! )

Ok thank you very much gpsmikey/blimpyway!

I will try soon, thank you for your suggestions!