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Mike's Muses occasional articles...
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MikeB
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PostPosted: Sat Mar 10, 2007 12:12 am    Post subject: Mike's Muses occasional articles... Reply with quote

Diving with GPSFor some time now I have been getting some crazy ideas about how to implement GPS in areas that don't come immediately to mind. I had to go to London the other day so I had a few hours disconnected from the net. Shock, Horror what was I going to do??

Rather than daydream I took a portable PC and jotted down some thoughts. When I got back I formalised them into an article. I think this will become the first of a series that I will call Mike's Muses... We can also have Darren's Doodles, Roberts Raves (I was going to say Rants), then I dried up when I got to Lutz... Confused Answers on a postcard to...

Anyway, back on track, I was thinking about using GPS underwater. This may be a pure flight of fantasy or possibly a practical area for the use of GPS and short range communications.

Click here to read my thoughts...
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Tim Buxton
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PostPosted: Sat Mar 10, 2007 2:01 am    Post subject: Reply with quote

Learned Lutz's Lectures. Lucid Lutz's leanings. Lutz's lowdown on the hoe down.
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classy56
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PostPosted: Sat Mar 10, 2007 6:25 am    Post subject: Reply with quote

"Lutz cutz" or "that's your Lutz" or "Lutz in space"
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robertn
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PostPosted: Sat Mar 10, 2007 6:48 am    Post subject: Reply with quote

The main problem is the RF signals are too weak to get far under water, and get distorted traveling through water. I am sure the problems are solveable if someone puts their mind to it - GPS recievers are more senstive than they have ever been, but the R&D focus is on TTFF and accuracy rather than outright sensitivity. GPS is now driven by commercial not miltary motivators, and it is unlikely the any of the current crowd of multinationals is capable or prepared to take the commercial risk. Maybe some Military developments will migrate down to us, but a diver (or terrorist) with GPS postioning would be a formidible Military advantage, so it's unlikely to be announced publicly, let alone made widely availble.

I do know of a GPS device used by divers in special op's. It floats to the surface, gets a fix and then is reeled back in. Not exactly what you are wanting, but a good starting point none the less. Has the advantage all the technology needed is well proven and cheap (A reliable fishing reel and 100Meter water proof case.)
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mostdom
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PostPosted: Sat Mar 10, 2007 9:39 am    Post subject: Reply with quote

robertn wrote:
I do know of a GPS device used by divers in special op's. It floats to the surface, gets a fix and then is reeled back in. Not exactly what you are wanting, but a good starting point none the less.


I think the point of line of sight of clear sky is the biggest issue especially when you go deeper, however the floating gps idea has some milage.

Instead of carrying the bulk of a gps device, leave it on the boat or by your land based starting point and use radio to work out where you are in relation to the fixed gps unit. Kind of like a reverse fish finder where you are both the fish and the finder. A simple wrist device whould be all you would need to carry. Suppose it also solves the terrorist part as you need to drag a boat about with you?
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chris_w
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PostPosted: Sat Mar 10, 2007 4:06 pm    Post subject: Reply with quote

As a diver here in the UK and abroad who has navigated in visibilities down to 50cm I think this would be a very useful tool if used correctly.....

The first consideration is how do you get meaningful waypoints in there for your diveplan? it's not as easy as mapping on the surface and depth is a big player for decompression ( for example - this bearing at this depth)......... I think a more 3D view would need to be taken to make it work ..... perhaps integration with a dive computer would help........ especially when you consider a straight line may make the dive hazardous (sawtooth profiles etc.....)

Now for the signal issue......

I can think of a couple of solutions to this.........

Firstly it's not unusual to dive with an "SMB" - Surface marker buoy which you could attach the receiver part to (bearing in mind recreational depths generally stop at 30M for the majority the SMB wouldn't be an issue for this - commercial diving or technical diving would be a totally different matter)...... it's also possible to get a deployable "DSMB" Delayed Surface Marker Buoy which you could deploy from under the water if the situation warranted it..........

Assuming boat diving for a moment - you could also use a receiver on the boat then rebroadcast this signal from a transmitter under the boat - not sure what carrier would be best for this.....

To REALLY provide benefit you could include a location transmitter which sends the GPS signal back to the boat - not only could the skipper keep an eye on the divers whilst under the water and track them (if necessary) but lost divers on the surface would be a thing of the past!!

HTH
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chris_w
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PostPosted: Sat Mar 10, 2007 4:08 pm    Post subject: Reply with quote

Oh and if you get a prototype knocked up for next month I'll test it on the Zenobia for you :D
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RobBrady
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PostPosted: Sat Mar 10, 2007 9:29 pm    Post subject: Reply with quote

robertn wrote:
I do know of a GPS device used by divers in special op's. It floats to the surface, gets a fix and then is reeled back in. Not exactly what you are wanting, but a good starting point none the less. Has the advantage all the technology needed is well proven and cheap (A reliable fishing reel and 100Meter water proof case.)


Discussing this article with MikeB the other night, I came up with what I thought to be my first ever original thought Surprised We even spoke about patents. Yep, some clever b*gger (and probably everybody else as well) has already come up with it... see above Crying or Very sad
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robertn
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PostPosted: Sun Mar 11, 2007 2:05 am    Post subject: Reply with quote

Now I think we are onto something here. We don't need the diver to have GPS. He wants to know where he is (in 3D) in relation to that boat.

So why not use well proven depth sounder/fish finder - ultrasonic sound, technology.
Combine this with Aircraft Air traffic control Radar Technology - Secondary radar to be precise. A transponder in the aircraft, on "hearing" the radar ping, responds by transmitting a code. The radar then recieves the code, and reports the 3D position and the code to the Air traffic controllers. Here we do this to the divers. a transponder replys to the depth sounder ping's. The dive boat then works out where the divers are. It can then transmit as part of the ping, the range/bearing/depth for the divers watch to display. The depth is not that important, as the diver knows this anyway.

One catch - a secondary radar installation costs millions, it would need to be done cheaper.......

Is there anyone in the marine electrinics game kicking around looking for the next cool product. Maybe this is something to run (swim) with.....
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7andy
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PostPosted: Sun Mar 11, 2007 12:49 pm    Post subject: Reply with quote

...don't use radar Smile use sound - we know it travels well underwater. The transmission rate might have to be a bit low (~300 baud??!!) but there's not a lot of data to transmit.

7&Y
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mikealder
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PostPosted: Sun Mar 11, 2007 3:10 pm    Post subject: Reply with quote

robertn wrote:
So why not use well proven depth sounder/fish finder - ultrasonic sound, technology.
Combine this with Aircraft Air traffic control Radar Technology - Secondary radar to be precise. A transponder in the aircraft, on "hearing" the radar ping, responds by transmitting a code....
This on the right direction, but as 7andy has pointed out Radar and water simply don't mix.
However if the diver were equipped with an active device that could respond when "pinged" to transmit a unique ID this might work (this is similar to the radar ATC system in principle), but would have to be either radio or acoustic transmission, the airborne radar system is IFF - Identification Friend or Foe.

The accuracy of the position of a diver (or number of divers) would depend on the number of acoustic sensors on the surface or submerged to a known depth, the more the better, the actual depth of a diver would be better provided local via water pressure sensing, then transmitting the depth back to the surface.

There is more information on this subject Here towards the end is a section on D-TAG, which is similar in some respects to the original "thoughts"

The cost for such a system (even a basic one) is going to be considerable, hydrophone equipment is not cheap, neither would the computer equipment needed to process the audio signals, about the only affordable portion of such a system is the GPS part to pin point the position of the boat! - Mike
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nej
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PostPosted: Mon Mar 12, 2007 3:00 pm    Post subject: Reply with quote

As others have said, radio signals slow dramatically in water, so even if you could get a signal down to your GPS, it would be way off, as timing is everything in GPS.

The solution as I see it, and as others have indicated, is to have a few fixed-point GPS on a boat or bouys, and attach to these (underwater) a radio transceiver. By the divers having their own radio transmitters, the underwater distance could be measured between the boat and the divers' radios (I assume underwater transmission is a fixed speed, as in air) , and this distance used to extrapolate the fixed GPS position from the boat. This traingulation system would be quite similar to how mobile-phone tracking works.
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philpugh
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PostPosted: Mon Mar 12, 2007 4:03 pm    Post subject: Reply with quote

nej wrote:
As others have said, radio signals slow dramatically in water, so even if you could get a signal down to your GPS, it would be way off, as timing is everything in GPS.

The solution as I see it, and as others have indicated, is to have a few fixed-point GPS on a boat or bouys, and attach to these (underwater) a radio transceiver. By the divers having their own radio transmitters, the underwater distance could be measured between the boat and the divers' radios (I assume underwater transmission is a fixed speed, as in air) , and this distance used to extrapolate the fixed GPS position from the boat. This traingulation system would be quite similar to how mobile-phone tracking works.


Transmission speed of radio-waves in water will be a constant speed but will be different in water with different salinity (the dielectric constant will vary). Also, like radio waves in air, the path-length will depend upon refraction effects and will vary with water-pressure and temperature. Normal GPS units have to allow for these effects (sats low in the sky are more prone to refraction effects). These effects and the potential errors they produce give rise to some of the wandering-whilst-stationary effects that are often remarked on in GPS forums.

For short distances (like are being discussed here) it may be that these will not make much difference.

For my money something using ultrasonics would probably be as accurate over these distances and should be significantly less expensive to implement. Three tethered bouys with US transducers and divers with IFF style responders, should give a reasonable 3D location and if one of the bouys has a GPS unit attached then a positional fix should be possible.

Now I wouldn't mind betting that the military have something like this (i.e. positional fix for divers) in service already.
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alncc1701
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PostPosted: Thu Mar 15, 2007 11:50 pm    Post subject: Reply with quote

Don't know much about the technicals but these guys claim they have underwater GPS

http://www.underwater-gps.com


Al
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p800
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PostPosted: Sun Mar 18, 2007 1:21 pm    Post subject: Reply with quote

This might be a solution.

http://yro.slashdot.org/article.pl?sid=07/03/12/1834257&from=rss
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