[PSUBS-MAILIST] Underwater Com

[via Personal_Submersibles]

Sean,
That's the best summary on the subject I've ever heard in a short amount of 
 space.
Thanks,
Jim T.
 
 
In a message dated 6/2/2014 6:39:24 P.M. Central Daylight Time,  
personal_submersibles at psubs.org writes:

Most through-water communication devices employ acoustic  transmission 
instead of electromagnetic, with the exception of some extremely  low-frequency 
systems. The reason is that seawater, being conductive, acts  like a massive 
Faraday cage surrounding the transmitter. Electromagnetic  attenuation 
through seawater is significant, and gets worse with increased  frequency. 
Microwaves are wholly unsuitable. Extremely low frequency radio  waves have been 
used with some success, but due to the low frequency, are  limited to low 
data rates insufficient for encoding real-time voice comms.  Human hearing 
covers from about 20 Hz to 20 kHz, so to encode the audible  spectrum you need 
a bandwidth of twice that. This is why CDs are sampled at  44.1 kHz. Now, 
intelligible voice comms don't require the whole spectrum - you  can get away 
with just a few kHz to encode the typical frequencies in a human  voice, but 
even that is too much data for real-time communicat! ion at  frequencies 
which will not be attenuated so much by the seawater as to make  them useless. 
Think text messages that are so slow that you can see each  character 
arriving individually - that's about what you can hope for with  underwater radio 
that still has some useful range.  Of course, this is  dependent on your 
requirements. More available transmission power, or less  required range, and 
you may be okay, but it is hardly efficient. 
In contrast to radio waves, acoustic signals are not as easily  attenuated 
in water, and as such are frequently used for communication,  navigation and 
imaging. (I.e. why we use sonar underwater instead of  radar).  Voice comms 
can be frequency shifted to frequencies outside of  human hearing, 
transmitted as an ultrasonic sound wave through the water, and  frequency shifted 
again on the receive side to make real-time voice comms  feasible. What's 
more, with acoustic signals, increased salinity actually  helps rather than 
hinders, as the greater the density of the water, the less  the attenuation with 
distance. 
Sean


On June 2, 2014 3:27:20 PM MDT, "Nathan.tuttle via  Personal_Submersibles" 
<personal_submersibles at psubs.org> wrote:  
Hey I am a submersible enthusiast and an expert engineer.

One thing that is bugging me is why communication with devices at great 
depths (the very bottom of the ocean) seems to be so hard.

The thing I am working on is miniature drones fully equipped and deployed 
en masse to scan and collect data from the bottom of the ocean.

Primarily, I want to find Amelia Earharts wreckage ;)

My question is, would it be difficult to create an underwater device that 
can communicate via microwave to surface?

Microwaves on the electromagnetic spectrum can pass through things in a 
line of sight manner if there is nothing obstructing them.

But I am weak on my physics and maybe the several billion tons of water 
that it has to pass through would squelch the signal.

Is there a means of telecommunication with high enough bandwidth to 
transfer signals from that distance and that depth?

Althoug!

 h our

earth is covered 70% of water. I think we have seen technology come to the 
point where a mass deployment of small controllable drones equipped with 
detection devices could search the sea floor.

I would great appreciate your input.

Sent from my iPad
 
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