I see a few things wrong with this.

1. You are assuming that the boat does not move from it's original position.
In my experience this would be the exception. (YMMV)
2. I would deploy a delayed surface marker buoy only when needed. (Drift
diving, or to give me something to hang on to if I need to do a
decompression stop etc.) Such buoys have long lengths of lightweight line.
>50 metres. I would routinely dive to 35 metres. Others I know routinely
dive to below 40 metres. Now imagine that this length of line also has to be
used in conjunction with a GPS antenna cable.

a) the weight of the line/cable combo becomes significant. (and you should
not try to use the cable only for obvious reasons)
b) What effect does this long length of cable have on the GPS signal at
about 1.45Ghz?
c) When diving in any kind of current, the buoy will not be directly
overhead, as it will be dragged by the current. Depending on local
conditions, the buoy position could be a significant distance away from the
diver's X,Y position. This error will be greater the deeper the diver gets.
The position reported by the GPS will be the buoy position only.

This sort of rig would also take time to deploy. Winding it back down is a
non starter. Normally if anything goes wrong with the deployment of a
delayed SMB, such as the reel jamming, I would let it go, rather than try to
recover it underwater. (my preference only, do not try this at home)

You mention 'clear water thats too busy to safely surface?' If that is the
case then
i)You should not be diving there. There are any number of reasons why you
might have to surface at any time.
ii)Your boat should be flying the 'A' flag or your local equivalent. This
warns other vessels that there are divers in the area.
iii)I would not want to deploy a buoy in such waters for fear of having it
caught in a surface vessel.

Basically the simplest way I know of being sure of getting back to your boat
is to deploy ann SMB on your way up and let the boat pick you up. (again my
preference only, do not try this at home) Placing too much trust in
technology where there are such serious consequences arising from failure of
such technology is asking for trouble. The only thing I agree with you on is
that whatever method is used it should be simple.

Should we not take this to rec.scuba now........

Joe



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dave King
Sent: 12 May 2003 20:37
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [OT]: Underwater navigation


Instead of something complicated what about something simple.

Take the principle of dipping sonar where they dip the head into the water
from a helicopter to ping.

Invert it

So now you take a gps antenna with ground plane and put it onto a float.
I'm assuming this is clear water thats too busy to safely surface?
A hundred feet of cable put the float on top for almost all sport divers
A few seconds on the right gps will give you a course and distance
back to the origin.

Wind the cable and float back down or let it stream but it should allow you
to find the way back to the safe area. If you had a full face mask you
could even
rig in that radio...

My 2 cents

Dave

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