[PSUBS-MAILIST] Surface controlled drones

[Sean T. Stevenson via Personal_Submersibles]

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 communication 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?
>
>Although 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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