Re: navigation was Re: [PSUBS-MAILIST] teathers

["Sean T. Stevenson" <cast55@telus.net>]

That sounds like a much less complicated solution than image comparison. 
Depending on how expensive these doppler units actually are, it may make more
sense to go that route.

I was just thinking about using a profiling sonar to do image comparison,
whereby a number of successive sweeps would produce a 3D surface at t=0, then at
t=1 a second image is generated, and then you do a curve-fit minimization to
adjust your six DOF deltas to make the two images match, using information from
the inertial system to provide your initial guesses.  This way, you could expect
reasonably fast convergence, but would probably still be limited by the accuracy
and speed with which you could generate the comparison image, and it would be
effectively useless on a featureless bottom.

-Sean


Quoting Antoine Delafargue <antoine.delafargue@gmail.com>:

> For navigation near the bottom, I think another (expensive) solution is a
> doppler sonar:
> 
> the change in the signal frequency after it bounces on the bottom is
> translated into a speed relative to it. 3 orthogonal beams give the full
> velocity vector.
> 
> I am not sure but I guess it is used by AUVs, ROVs, dynamically positioned
> boats, navy seals...
> A mixed task of mapping and positioning can be done too (cf Bill Stone
> experiments in Florida and with the AUV depthX)
> 
> Antoine
> 
> 
> On 1/15/08, Sean T. Stevenson <cast55@telus.net> wrote:
> >
> > Inertial navigation using (comparatively) low cost instrumentation is
> > possible, but the integration error over time precludes this from being
> > accurate enough for sustained duration navigation or navigation in
> > close-quarters.  I experimented with this using a 3DM-GX1 sensor from
> > Microstrain (combining orthogonal rate gyros, accelerometers and
> > magnetometers) - it worked okay, but to be useful for navigation an
> > inertial solution must provide a way to correct for gyro / integration
> > error using additional information, lest you have to surface often to to
> > obtain a fix and zero the error - it accumulates in a hurry.  One
> > possible method of doing this is terrain identification using sonar, but
> > I do not have experience with this sort of programming.  The idea would
> > be to fix position using GPS at the surface, maintaining your fix using
> > the inertial system in midwater, and then as soon as the bottom or
> > surrounding terrain could be accurately resolved by the sonar system, to
> > maintain the fix using bearings on resolvable landmark features.
> > Obviously, this requires some pretty advanced logic in the image
> > interpreter.
> >
> > Independent of surface support, the cheapest and most versatile acoustic
> > solution I would guess is a buoy system - where three or more buoys,
> > each equipped with a GPS and an acoustic transponder, may be placed
> > anywhere within the operating area.  Each transponder transmits
> > acoustically its position and time of fix, and the sub compares the time
> > of receipt with the time of transmittal to calculate its range from each
> > buoy, and then uses the multiplicity of buoy fixes to determine its own
> > position.  This is of course subject to errors as the buoys move on the
> > water surface, and is dependent on knowing the speed of propagation of
> > the acoustic signal through the water, which in turn depends on salinity
> > and temperature.  If all is constant and known, you have a better fix
> > than if you are operating under a thermocline, for example.
> >
> > -Sean





************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003.  Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.

If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
	removeme-personal_submersibles@psubs.org

Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.

PSUBS.ORG
PO Box 53
Weare, NH  03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************