Underwater GPS

[got4boyz]

Thanks archman for that thread. I did do a search on GPS and like you said, it didn't work of course. I also tried different searches without finding any info.

Thanks Steve for the * tip. I'll use it to turn all my 3 letter words into 4 letter words! LOL

Thanks for the SONAR thread link Don. That's cool, but I want more. I want to navigate to places not just back to where I started!

The other underwater GPS thread is discussing the possibility of doing what the Scuba-Nav already does. I'm wanting to know what others think of this device not a discussion of whether it will work or not. It's out there to buy, so it must work. They also have a pro model that has a 100 ft cable.

Now many of you say you don't want to pull a float around but I know many of you do. I'd like to hear from you about what you think about this.

Here's a scenario:

You are shore diving and want to go to a couple nearby sites, let's say a boat wreck and a plane wreck. You have the GPS coordinates for those sites so you make a waypoint for them before the dive into your GPS unit. You then just follow the path to the boat wreck first and to the plane wreck next.

On the way to the plane wreck you see a cool fish and want to follow it for awhile, so you do. By looking at your GPS unit you can now continue on to the plane wreck from where you quit chasing the fish. On the way back to shore a current has drifted you off course many feet. With a compass you'd have to guess how far of mark you had drifted and try and get back to your starting place using your guesstimation of the direction you needed to go on your compass. But if you had a GPS unit with you it wouldn't be a problem. You can see exactly what direction you need to go to get back to your shore entry. Wouldn't this make for a more enjoyable dive?

 

[Don Burke]

Now many of you say you don't want to pull a float around but I know many of you do. I'd like to hear from you about what you think about this.

Here's a scenario:

You are shore diving and want to go to a couple nearby sites, let's say a boat wreck and a plane wreck. You have the GPS coordinates for those sites so you make a waypoint for them before the dive into your GPS unit. You then just follow the path to the boat wreck first and to the plane wreck next.

On the way to the plane wreck you see a cool fish and want to follow it for awhile, so you do. By looking at your GPS unit you can now continue on to the plane wreck from where you quit chasing the fish. On the way back to shore a current has drifted you off course many feet. With a compass you'd have to guess how far of mark you had drifted and try and get back to your starting place using your guesstimation of the direction you needed to go on your compass. But if you had a GPS unit with you it wouldn't be a problem. You can see exactly what direction you need to go to get back to your shore entry. Wouldn't this make for a more enjoyable dive?

You can do this with a SONAR system as well. Prior to the dive, figure out range and bearing from each of your points of interest to the point where the transmitter unit is to be placed. It's a technique similar to pilots using VOR/DME or TACAN to go from point to point by doing the trig in their heads. In your case, you can have your computer or GPS unit do the trig ahead of time and you'll have a list of ranges and bearings on a wetnote..

I've tried chasing fish while pulling a float. It was a lot of work and my gas consumption went to hell.

My GPS often is thirty-five or so feet off. That's not a big deal when I'm trying to find a wreck with a few hundred horsepower pushing me. It would suck big time with fin power in low viz.

 

[ArcticDiver]

Sounds like you have already made up your mind about this toy.

Now that more folks have posted I think it wouldn't be accurate enough for my use. Plus, dragging 50' or 100' of cable around would be a royal pain. Plus, it would only be good for shallow diving.

But, putting an eTrex on the float; taking the float to the precise coordinates; mooring the float; and descending from there would be cool. Plus, it would be cheaper; always a good thing for me.

If you get a few hours of experience with it be sure to post your impression then. I can always be persuaded to change my mind.

 

[archman]

this version of "false GPS" would have utility in underwater archaeology and shallow surveys... I wouldn't be suprised if jury-rigged setups weren't already in use by these folks.

Divers that...
A) stay shallow
B) don't move much

...would be fine for this technology, provided they don't get annoyed with a strung surface cable. The application would be best used to mark points of interest, to better position a boat on subsequent dives. It would not be effective as a navigational tool due to it's...
A) annoying and potentially dangerous tether
B) high power drain (GPS units suck batteries pretty hard)
C) awkwardness underwater (most portable GPS units have small screens and are not diver-ergonomic)

B & C could be rectified with improved GPS models. You're still stuck with the dang tether, that would require near-constant attention. Personally I'd "put up with" false GPS on dedicated survey trips, and then use the data to create line-of-bearing maps for later dives using the good 'ol compass and slate. I don't see a realistic market for recreational diving, though. Maybe as a rental...

**
This thread should be bumped to the general equipment forum, and maybe folded into the existing thread(s) on this topic.

 

[got4boyz]

Sounds like you have already made up your mind about this toy.

Not really. I love my GPS and what it can do, so I'm very intrigued by this idea, but I have my reservations. That's why I'm asking questions. I don't want my excitement about something that seems cool to cause me to waste lots of money if it's not really worth it.

I've done very little shore diving and have never pulled a float, so I don't know what that is like, but I know people who dive all of the time pulling floats. I was hoping to hear more from them too. Those of you who don't like pulling floats are not going to be as receptive to the idea as to someone who is used to pulling a float around behind them. And since I've never done it, I don't have any idea what it's like.

Guess I need to go buy me a float and try that out first.

And you're right archman, this probably should have been in the general equipment forum. Didn't think about that!

Thanks y'all for your comments!

 

[divebunnie]

OOh this sounds interesting.

I have just got back from working on a project where at one point we were creating a GPS map of a particular reef, noting the depth and life forms at certain points whilst someone on the surface registered the actual way points. This system would have been great, as we could have guaranteed far more accuracy, wouldn't have needed to synchronise with the surface (not always so easy at 18M). We had to tow a float along with us anyway so that the person at the surface could see where we were, so that would not have been a problem either.
Any ideas of cost? I am guessing it will be out of the current budget of a conservation team, but who knows??
By the way, yes… towing a float is a bit of a pain, for those of you who haven't done it before, but I guess sometimes it is necessary. We were quite isolated, so it was just one more safety method.

 

[Falcon99]

I'm a big GPS fan, but dragging a bouy around behind me all day long is not something that I do for fun. This system is not any good for deep diving since the bouy umbilical is only 55' long. Also remember that if the wind changes the location of the boat can change during the dive. Trying to preload the the location of the anchor point on the seafloor is a non-trivial mathematical exercise. Any diver that can use this this device to reliably find the boat anchor is probably already very good at underwater navigation.

Other than that, it sounds like a great gadget!

Chris

Chris, the big problem here is obviously getting around the water's inability to pass L-band RF - not to mention the host of decoding issues that would come with the water-air boundry

I've seen another system that converts this over to what looks like a ~50 Khz sonic data medium that allows the information to be displayed on an independant diver unit. This still has some problems mandated by the laws of Physics. You still have to calculate a 3-D position from the boat to the diver. This cannot be accomplished with a single-point device such as a unit on the boat talking to the remote. Even if a two-way communication protocol is implimented, you could only get the distance from the unit to the boat. I can't do much with that. Water is a hard nut to crack when it comes to this.

just my $.02 worth

James

 

[got4boyz]

divebunnie, The ScubaNav is $400 and comes with only 55' cable. The professional model is more heavy duty and comes with 100' cable. It is very pricey at $1200! Then you still have to provide a GPS unit. The site if you are interested is www.longbeachdive.com.

 

[archman]

I sincerely hope that these long cables have some sort of angle leveler built into the base. Otherwise you're going to have to guesstimate the horizontal difference between the float and the GPS. At 100 feet what appears to be a vertical line could be off by a significant margin.
If you:
A. maintain a constant depth
B. know how much float line you've released
C. know the oblique angle between true vertical and the float line

... then it's possible to determine the "drift error" from the float line. Sounds like a lot of work, right? Now factor in:
A. not maintaining a constant depth
B. changes in the oblique angle due to shifting current flows
C. variations in current regimes along compass bearings (i.e. which way do I compensate for drift now?)

Now this technology becomes a friggin' nightmare regarding both accuracy AND precision. I would not have any faith on it deeper than 50 feet, even with a full dive team doing the mapping. I can predict massive variation from repetitive or even concurrent surveys. The scientist and reef ecologist in me is screaming out "bad data!"

Now at clear shallow depths lacking current, you could probably do much better with the thing. You could eyeball the float and visually mark it directly overhead... this solves all of your error problems (except natural GPS error) right off the bat. You would still have to follow a very strict protocol for marking points. Both you and your dive buddy would be doing nothing else on the dive, needless to say. The California website advertising these units is being extremely conservative in their error reporting, and bordering on absurd for California waters. You'll note in the FAQ that they "cop out" of a full explanation.

7. How accurate is the Global Positioning System ?
The short answer is: depending on the type of civilian GPS receiver used, down to 3 meters of accuracy. The long answer is: understanding and determining the accuracy of civilian GPS depends on many variables, and factors, and is too long a discussion for a FAQ page.

 

[DoUDive2]

Chris, the big problem here is obviously getting around the water's inability to pass L-band RF - not to mention the host of decoding issues that would come with the water-air boundry

I've seen another system that converts this over to what looks like a ~50 Khz sonic data medium that allows the information to be displayed on an independant diver unit. This still has some problems mandated by the laws of Physics. You still have to calculate a 3-D position from the boat to the diver. This cannot be accomplished with a single-point device such as a unit on the boat talking to the remote. Even if a two-way communication protocol is implimented, you could only get the distance from the unit to the boat. I can't do much with that. Water is a hard nut to crack when it comes to this.

just my $.02 worth

James

Wow! I've spent the last 30 minutes trying to figure out how to move this thread to the General Equipment forum. I never did figure it out, but somehow it seems to have magically moved by itself. It must be an act of :god: !

Back on topic:
I agree that GPS signals cannot penetrate water to any significant depth. That is why the receiving antenna must be on the surface float and the signals are passed to the diver's receiver via the 55' umbilical line. I have heard that military diver's have a similar system in which the antenna is deployed to the surface only when a fix is needed. The system gives the surface location of the antenna float, not the actual location and depth of the diver. WAAS is not available in all dive locations. So the accuracy of the system is probably about 50 ft 95% of the time.

I'm waiting for the differential version in which the boat acts as a GPS base station. The float is not attached but uses its own propulsion system and keeps itself stationed above the diver using a sonar transponder based guidance system. The location of the diver is displayed on the diver's gage console and is decoded from the sonar signals sent from the float. How's that for dreaming!

3-Dimentional positioning of the diver underwater using sonar trilateration seems like a great idea, but it is way too expensive for me. However, I am interested in sonar devices such as Neverlost, Eye Sea, and XIOS which provide bearings back to the boat. They are still expensive, but I have read some interesting threads that describe their operation. see: http://www.scubaboard.com/t26321.html If you keep the batteries charged and dry, they beat dead reckoning.

Chris