Re: Waterproof GPS/compass for long-distance swimmer

On Wed, 06 Jun 2007 22:45:19 -0700, Paul Hovnanian P.E. wrote:

Donald wrote:

_ wrote:

I coach a girl who does long-distance open-water swimming - like the
Channel, that sort of thing.

We think that some sort of GPS or at a minimum a compass would be a good
tool for her to wear, both while training and while actually doing the
swims.

One of the things we'd like would be some sort of feedback on direction.
There are a number of issues/characteristics with this:

Aural feedback may not work well. Bone conduction is a possibility, but
earphones probably won't stay put for the length of time she will need the
info.

An led indication of course could work; maybe a tri-colour attached to one
goggle (green ok, red veering left, yellow veering right, that sort of
thing).

There is a bit of body-motion to consider. She could wear a belt with the
unit in the small of the back, or on the head if it is not too large. The
back would have mainly roll; the head may have a bit of yaw as well, so
that might be trouble.

Initial setting of the course would have to be worked out - if the unit
were on the head, perhaps she could orient as desired, then press a button
to fix that course.

Course indications would need to be integrated over a short (~5 to 10 sec)
period. It might be helpful if this period could be randomly varied
within those limits, as some swims do not allow "pacing", and a repeating
signal could be used for this - unless it were irregular.

For training she often does multi-leg "loops", returning to a starting
point several times. For that some sort of GPS-type distance logging,
course & waypoint setting, etcetera would be a plus. This would also be a
BIG plus for some swims where current is a variable.

Waterproofing needs to be enough to withstand shallow immersion - say 30
feet.

Battery life might be a factor, some swims can be more than 10 hours.

Radio coms would also be a plus, but rather than get just a radio and have
a guide give info, a self-contained unit would be much better.

I can do small electronics, uc programming, and machining, but have very
little knowlege of what building blocks are out there and their qualities -
like how fine a resolution can you get from electronic compasses, what
tricks can you get GPS to do, will these blocks work if under 2 feet of
seawater, current draws etcetera...

GPS antenna will _not_ work in water. Spray is just as bad.

The swimmer I have seen in the news are towing a small raft behind them.

This is not a good solution for you ??

A further observation: Is it really wise to have a long distance swimmer
out in open water without an escort craft? That would be the logical
place (and pretty much a no-brainer) for the location of any navigation
equipment.

Of course not - but that's not even a consideration; don't worry. For swims
of any length there is a guide boat. What we are looking for is to reduce
the small course variations that can occur, and if possible to track
distance for training purposes.

Another poster has said that GPS units will not work in spray - is this a
guaranteed idea-killer? A unit mounted on the head would might be able to
see satellites for some proportion of the time.

A towed raft might not be oriented in the same direction as the swimmer,
and compass heading would then differ.
.