navigation was Re: [PSUBS-MAILIST] teathers

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

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




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