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 Posted: Feb 07, 2012 - 05:35 PM |
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Joined: Dec 29, 2010
Posts: 466
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I would like to get your opinion on something.
I came across this great site on submersible ROVs at another site, and it occurred to me that these small remote controlled robots could be controlled wireless. Underwater communications is done through wires, it seems.
Does any one know what type of transmitter you would need for under water?
I doubt we could attain enough bandwidth for true color HD video, but I'm sure something can be attained at 320x240 maybe?
http://www.homebuiltrovs.com/seafox.html
http://www.rollette.com/rovrev2/index.html |
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Posted: Feb 07, 2012 - 05:43 PM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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Radio waves do not work, for all practical purposes, under water. Certainly not at the bandwidth needed for video.
Acoustic links are drastically short on bandwidth (by orders of magnitude).
Optical is limited by high absorption in water. If it is "murky", then a range of 1m might be VERY good. In clear water, a blue LED might be workable over some distance, but you have the problem of having to emit in all directions because the orientation of the ROV changes, making the received signal very weak.
Wire or optical fiber are far superior for video.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Feb 07, 2012 - 06:21 PM |
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Joined: Dec 11, 2007
Posts: 6848
Location: Cleveland, OH
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Of course a sub can always release a tethered buoy. The buoy has an antenna for an RF link to one's receiver. The cable to the buoy has a wired link with it.
For shallow deapths, and slow speeds the buoy is just dragged along on the surface behind the sub, with an automatic cable tensioner.
If the sub needs to go to greater deapths, or be silent, or move at high speeds, it just reels in the buoy.
JC |
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Posted: Feb 07, 2012 - 06:34 PM |
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Joined: Aug 09, 2009
Posts: 255
Location: UK
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JC just beat me to it. I was just about to suggest exactly the same thing.
On the other hand I think it is usual for this type of ROV to be supplied power via the tether so unless you plan to run on batteries there is little to gain from this IMO.
Anyway to expand a little on what Jim rightly says, read this Communication with submarines |
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Posted: Feb 07, 2012 - 08:55 PM |
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Joined: Sep 04, 2002
Posts: 21257
Location: Orlando Florida
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| How about a bell 202 modem using 1200bps fsk into a hydrophone? |
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Imagecraft compiler user
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Posted: Feb 07, 2012 - 09:31 PM |
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Joined: Nov 06, 2005
Posts: 592
Location: Vilnius, Lithuania, Continental EU
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An useful range of roughly from 30 to 300 kHz can be considered for RF or ultrasonic underwater use.
Bandwidth, nevertheless, shall be a bottleneck for video according to Nyquist's formula.
Perhaps SSTV or alike would be feasible. |
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Posted: Feb 07, 2012 - 10:32 PM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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In order to get any sort of video over an underwater channel that does not use wire or optical fiber, it would have to be slow-scan. That would probably end up being seconds per frame.
Ultrasound and its brethren all have a big problem underwater with multi-path. There will be multiple signals arriving at the receiver, just because of temperature irregularities in the water. If it is ocean, you also have to deal with the thermocline and salinity layering, all of which can give you multiple transmission paths, each with a different transit time. Not totally unlike a "house of mirrors". It is, at the very least, really, really, tough.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Feb 08, 2012 - 12:07 AM |
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Joined: Aug 09, 2009
Posts: 255
Location: UK
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Assuming the ROV is to be self-powered and entirely remote then DocJC's suggestion has the added advantage of being a means to locate the thing if it happens to get stuck and/or the batteries die.
Good luck with the project. Living on the coast myself, building an ROV is something that has been on my to-do list for a while.  |
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Posted: Feb 08, 2012 - 12:12 AM |
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Joined: Dec 29, 2010
Posts: 466
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I'm very surprised at the challenges involved with handling video underwater.
I assumed something like wifi would be possible but apparently not.
Also, if i remember correctly, vlf is indeed currently used by the military for intra sea communications, but, vlf does seem to pose a threat to marine life. It seems like whales, dolphins and other mammals occasionally beach as a result of heavy vlf transmissions. |
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Posted: Feb 08, 2012 - 12:15 AM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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Whales and dolphins are thought to be affected by very strong sonar.
Subs do not transmit VLF as far as I know. It is transmitted from shore to subs that are near but not at the surface. Given where the transmitters are, their impact on marine life should be very small. Sonar, with very high sound levels, are a very different matter.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Feb 08, 2012 - 12:25 AM |
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Joined: Aug 09, 2009
Posts: 255
Location: UK
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ka7ehk wrote:
Subs do not transmit VLF as far as I know.
According to the wiki article I linked ELF/VLF communications are one-way only due to the infeasibly long wavelengths involved. From what I understand they use the earth itself as an antenna. |
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Posted: Feb 08, 2012 - 12:50 AM |
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Joined: Sep 04, 2002
Posts: 21257
Location: Orlando Florida
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| Not to mention the bit rate is in hertz. I used to think my 300 baud Hayes was slow. At least you could read the chars as they filled the screen. |
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Imagecraft compiler user
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Posted: Feb 08, 2012 - 01:01 AM |
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Joined: Nov 22, 2002
Posts: 12045
Location: Tangent, OR, USA
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The U.S. Navy has a transmitter for the Pacific Ocean at Jim Creek Washington. The antenna is hung between two mountains east of Seattle; it is a multi-element beam pointed west. Transmitter power is 1.2 megawatts. The references don't say, but I believe the frequency is in the low 10's of KHz. Data rate of a similar station in Maine is 200 baud, FSK, encrypted.
In these cases, the antenna is definitely in the air.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Feb 08, 2012 - 01:08 AM |
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Joined: Mar 07, 2010
Posts: 221
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| what about throwing a digital camera on it and just recording? you would of course have to encase in in something waterproof, but that might be the simplest way. by the way, thanks for posting this! i never imagined such a thing could be built by amateurs! this is going on my todo list as well |
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Posted: Feb 08, 2012 - 05:14 AM |
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Joined: Dec 29, 2010
Posts: 466
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testdrone wrote:
what about throwing a digital camera on it and just recording? you would of course have to encase in in something waterproof, but that might be the simplest way. by the way, thanks for posting this! i never imagined such a thing could be built by amateurs! this is going on my todo list as well
Your welcome.
This is a great project worthy of its own forum. The challenges that must be overcome are very particular and interesting. The different solutions can get very creative.
Getting back to our topic. If we are transmitting in the hertz range, I do believe we may attain the long distances. however, at these speeds, we will not be able to transmit much data. Live video footage will not be possible.
Also, to attain, lets say a continuous 40 decibels needed to cover the long distances will exhaust our battery quickly.
After reading some material, not much progress has been made in the underwater wireless department. |
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Posted: Feb 08, 2012 - 05:21 AM |
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Joined: Dec 29, 2010
Posts: 466
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ka7ehk wrote:
The U.S. Navy has a transmitter for the Pacific Ocean at Jim Creek Washington. The antenna is hung between two mountains east of Seattle; it is a multi-element beam pointed west. Transmitter power is 1.2 megawatts. The references don't say, but I believe the frequency is in the low 10's of KHz. Data rate of a similar station in Maine is 200 baud, FSK, encrypted.
In these cases, the antenna is definitely in the air.
Jim
I read something similar about the submarines not being long enough to support the length of the antennas required for transmission.
Even in full length submarines, communication appears to be one way -which came to me as a surprise. |
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Posted: Feb 08, 2012 - 06:42 AM |
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Joined: Jan 09, 2007
Posts: 1868
Location: Arlington, Texas, U.S.A.
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Posted: Mar 18, 2012 - 04:34 PM |
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Joined: Dec 29, 2010
Posts: 466
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I thought I would share this article with you, since it has a lot to do with the discussion: its about neutrinos used to send deep sea messages and it can be found here: http://m.popsci.com/science/article/201 ... gh-bedrock
All we need now is to reduce the size of particle generators and we're in business! |
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Posted: Mar 18, 2012 - 06:26 PM |
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Joined: Jan 03, 2006
Posts: 4414
Location: Hemel Hemsptead, UK
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| And the size of the receivers, I guess - neutrinos are notoriously unreactive with normal matter. I believe 700 tons of Argon is one way to detect individual neutrinos... |
_________________
Neil Barnes
www.nailed-barnacle.co.uk
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Posted: Mar 18, 2012 - 10:07 PM |
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Joined: Dec 29, 2010
Posts: 466
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barnacle wrote:
And the size of the receivers, I guess - neutrinos are notoriously unreactive with normal matter. I believe 700 tons of Argon is one way to detect individual neutrinos...
While we're in the topic of Neutrinos, too bad they found them to not be faster than the speed of light.
I personally hold Albert Einstein accountable for discouraging others from building a time machine. I'm sure some bright genius would have built one by now had had it not been for his theories. :wink: |
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Posted: Mar 18, 2012 - 11:02 PM |
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Joined: Nov 11, 2003
Posts: 3881
Location: Chicago Illinois USA
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Quote:
That would probably end up being seconds per frame.
Rather like watching netflix. |
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Discursive design,
Torby
Some days, it's just not worth chewing through the restraints.
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Posted: May 18, 2012 - 01:28 AM |
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Joined: May 18, 2012
Posts: 1
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Hi all,
On a related matter. I have recently got the designs for an ROV which uses CAT5 cable (tether) to send power to lighting etc. The plans claim that the cat5 cable acts as an antenna and allows an RC transmitter to control the ROV thrusters.
My question is, do you think this would still be the case in saltwater, and would the range be reduced? Or, is the case(as I read it), that the cable effectively takes the signal right to the sub and RC receiver, and hence could work at any (100ft?) depth?
Note, I'm not trying to send video along the cable.The cable is strictly for power to the lights.
Any help much appreciated.
Cheers
Marcus |
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