8051 and sonar (depth finder)

In article <LWBCd.7133$yV1.3847@newssvr14.news.prodigy.com>, 
notthisjoergsch@removethispacbell.net says...
> Hi Mark,
> 
> >>Whats a 'radar mile' in water? 12usec per nautical mile out and back is the
> >>radar rule of thumb. Is speed of sound in water 15000 fps? So is there a 'blind
> >>spot' at multiples of the ping prf like in radar?
> >>
> >>    
> >>
> >IIRC, the speed of sound in seawater is about 1500m/second.  It varies 
> >slightly with salinity, depth and temperature.
> >
> >Depth finders usually use fairly low PRFs---perhaps on the order of 1 to 
> >5Hz.   At 1Hz,  that would allow depths of up to 750m without a 'blind 
> >spot.  5Hz allows up to about 150m without problems.  Most simple depth
> >finders can't put out enough power or receive enough signal beyond about 
> >400 feet in any case.
> >
> >For shallow waters,  up to a 100 feet,  you can use much faster PRFs.
> >  
> >
> 
> You could also use PRF staggering and a piece of nifty software to sort 
> out alias echoes from the real one. Same in Radar. Its just not done in 
> some systems, but it could.
> 

It's probably simpler and more useful to use a variable PRF.  Start with
a slow PRF and look for an echo.  If  you get a solid echo  at 400 feet,
stick with the slow PRF---as the user is not too likely to worry about
the difference between 395 feet and 400 feet anyway.  In any case
beam spreading and geometry are likely to broaden the echo.

If you get a strong echo (or multiple echoes)  at a shallower depth,
adjust the PRF accordingly.  That will also give you faster response
when the water is getting thin---and attractive feature when feeling
your way up a narrow channel or looking for an anchorage.

At the same time that you adjust PRF, you can adjust the pulse width,
transmitter power, and receiver gain.

Except for high-end fish-locating depth finders, there is usually no
great need to track both distant and near targets as is the case
with radars.

Mark Borgerson