Re: [PSUBS-MAILIST] UW-DGPS

[Jon Hylands <hylands@ibm.net>]

On Sun, 19 Mar 2000 09:01:39 PST, "Paul Suds" <paul_suds@hotmail.com>
wrote:

> The GPS unit does all the triangulation and algorithm integrations of the 
> signals received from the satellites. This unit is on the surface. Placing 
> it underwater will attenuate the signals from the satellite, making it 
> non-functional. Say that you kept the unit above water, and "digitized" the 
> data and sent it to the sub. The sub would know exactly at all times where 
> the GPS unit was (were the surface craft was), but wouldn't know where the 
> sub was.

They way these systems work is to use 2 or 3 transponders on the surface,
who's locations are known (i.e., one on the bow of a ship, one on the
stern). The sub can do triangulation to calculate its position relative to
the surface ship, and thus get an absolute position.

You can do the same thing if you have 3 acoustic beacons at the bottom, if
you know their positions.

One other trick you can probably do with only a single surface transponder
would be to have three well-spaced microphones on the sub, and use a DSP to
do timing of incoming signals. You could then triangulate on the surface
beacon, and figure out where the sub is relative to the surface beacon, and
thus get an absolute position.

I'm going to do something along those lines for my autonomous sub
MicroSeeker as an intern measure until I can afford to dish out the $50K or
so for a real system.

For general interest, sound travels underwater about one foot every 500
microseconds (I think). If you have a micro-controller than can do
microsecond timing, it should be relatively simple to get input from three
different microphones spaced about a foot apart, and thus triangulate on a
surface beacon. For relatively shallow water (like under a couple hundred
feet), you could even do something like drop an anchor, and have the
accoustic transponder attached to it, and thus only need to do
2-dimensional triangulation using 2 microphones, since you can get absolute
depth from a pressure transducer directly.

Positioning is one of the big black holes for truly autonomous submarines.
Its pretty much the hardest problem to solve.

Later,
Jon

--------------------------------------------------------------
   Jon Hylands      Jon@huv.com      http://www.huv.com/jon

  Project: Micro Seeker (Micro Autonomous Underwater Vehicle)
           http://www.huv.com