How to transmit data underwater over 40m

You could buy a Fishfinder like this - all you want is the sensor (well two of them) so you might find that more cheaply.

Then you need to build a load of electronics to drive the sensor at one end and receive signals at the other .

This isn't mechanical engineering !

You MUST specify the data rate to get anywhere with this - it affects everything !

MK

Hi Alexandre,

If a low data rate is permissible you might find that you can get more distance with less power by using a lower frequency carrier. Infra-sound would carry greater distance as it is less easily absorbed in the medium.

John

Alternatives are, radiofrequency,  Light and sound.

RF has a pretty hard time penetrating salty water, and you require very low frequencies. Laser can get longer distances, but it has to be aimed and still won't go much farther than RF. Then ultrasound can get to very long distances, many kms if needed, but there is always the tradeoff of range and rate. And still ultrasound will always have limited latency, because of speed of sound in water.

I know you said no cable, Just curious why? would this be a corrosion issue? How about fiber optics?

1. How large can the communication system be?
2. How much power is available?
3. Is the water turbulent?
4. Is the water clear or murky?
5. Are there any obstacles in the water? (Is it a straight line-of-sight)?
6. Is there marine life that cannot be disturbed?
7. What data rate is adequate?
8. How deep is it?
9. Can an antenna stick out of the water?
10. Is there movement of one or both ends of the link?
11. Does the link need to be omni-directional?
12. Does the communication need to be real-time? (could a robot go back and forth?)
13. What is the cost target?
14. How much time is available for development?

There are a large number of answers to your question, including the technologies mentioned in other responses above and commercial communication systems and a couple of others I can think of, but if you want to narrow it down to a solution that works well for you, it would help to try to answer these questions.

Good luck in your endeavour.

Engineers and developers have been trying to do it for GPS for a number of years. Surface floating relay stations that receive signal from the underwater device is one option.

As the US military has discovered, transmitting from underwater is possible with very low frequency signals but the power, transmission rate and the technology, 2Km long antennas make it rather difficult. 

Hello Doug,

Thanks a lot for your questions, please find my answers below:

1. How large can the communication system be? 10x10x20 cm each
2. How much power is available? enough to transmit data during 48hrs -> see data rate
3. Is the water turbulent? There can be 3 knots/hr stream
4. Is the water clear or murky? Murky
5. Are there any obstacles in the water? (Is it a straight line-of-sight)? No obstacle
6. Is there marine life that cannot be disturbed? No
7. What data rate is adequate? 10 o/s
8. How deep is it? 25 meters max
9. Can an antenna stick out of the water? No
10. Is there movement of one or both ends of the link? yes on one end
11. Does the link need to be omni-directional? Yes
12. Does the communication need to be real-time? (could a robot go back and forth?) Real time
13. What is the cost target? from 30$ or 100$ max (minimum is the best)
14. How much time is available for development? 6 months to 12 months

So do you see any options?

Thanks a lot for your help,

Alexandre

Here is another discussion which shows the problems: RF communication through salt water . Basically without the resources of the US Navy, you would be hard pressed within the budget you have.

Clem

There are a couple of points in your responses to Doug's questions that need some more detail.

7 - Data rate - I'm not sure what units o/s are but let's assume it to be 10 x 64 bit words per second  - you can tell us if that won't do.

13 - Cost target - will you make just one or lots - if lots then how many - is the price target for both ends ?

30$ is close to impossible - certainly only achievable for quantities in 100k+

I haven't done business with Benthowave, they look like  a proper competent company - but not one that sells $5 sensors.

You might, just about, be able to make a cheapo echo sounder/fish finder kind of transducer for $5 - maybe you could find a Chinese source.

But then you need to package it up with the electronics, battery and however the data gets in an out - and all to a standard that will work under water.

The data rate is low so you can use a simple modulation scheme - where you just drive the transmitter at full power for short bursts. With a 40kHz transducer 640 bits per second is quite feasible.

You'll need a robust error detection/correction system so both transmitter and receiver will need a little micro processor.

Look at this:

https://www.amazon.co.uk/Lucky-Finders-Portable-Fishing-Sounder/dp/B06XFG59QW/ref=sr_1_1_sspa?keywords=echo+sounder&qid=…

The sensor would probably do but it wont be very omnidirectional. You could buy a couple of these fish finders, check what frequency they drive the transducer with and go from there.

Some of the things you've said suggest you want to buy a complete working system, these people do such stuff:

https://waterlinked.com/underwater-communication/

MK

I especially like the last link. Although pricey for my budget, the technology seems feasible.Thanks michaelkellett  for the share.

Clem