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

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.

Regards, Joerg

http://www.analogconsultants.com

In article <20050103184609.07944.00002635@mb-m07.aol.com>, 
bobgardner@aol.comma says...
> 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.

Mark Borgerson

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?

Joerg <notthisjoergsch@removethispacbell.net> wrote:

>Hi Joop,
>
>>It could very well have been an oldy. The light did seem a rectangular
>>red LED though, not a neon bulb. I do seem to recall some spinning
>>noises so I a rotating disk could have been involved.
>>That would make powering the device simpler (12V board power on
>>boats?). Perhaps I should have said ship. The whole vessel was quite
>>big, 3 masts with sails
>>(http://212.204.218.222/hanzestad/schepen/schip.php?id=42)
>>  
>>
>
>Wow, that is a big ship. Dance floor and all. But 160 passengers and 
>only eight restroom stalls? What if half the people on board got 
>seasick....?
>
LOL. The ship does not go on long trips anymore. It is primarily used
for daytrips with companies, organisations and such. It also does not
go onto rough seas, probably for the same reason. 
We did have a nice bit of wind without getting to storm level. Some
people did spend their time being really quiet in some corner inside
or below deck ;-) No continues run to the restrooms though...

>I'd think this ship deserves a much better echo system. You could use an 
>old scope as a display to make it look somewhat antique. Unless, of 
>course, you have to comply with the devices from a certain era. An LED 
>on a disk sound like half-hearted retro design though. But in the time 
>of three-masters they did not use echo. Well, it hadn't been invented 
>yet. They had a rope with regular knots that was let down once in a 
>while and, after the rope handling sailor had spit his chew tobacco into 
>the sea, the depth was hollered to the bridge in 'fathoms'.
>

You are maybe getting overly romantic now. I seem to recal from the
captain that the whole ship is from around 1900, so not super old. But
even a "retro design" of those days may be fine if it functions
similar as classic sail ships. And as far as I can judge it has been
done with a fair amount of good taste.

Over the years it has been equipped with plenty of modern stuff. Radar
screen, the mentioned depth meter and more. The radar screen also
showed the rain areas. Because of the wheater it had the captains
interest. More than the depth meter because this water is very
familiar to the crew. It only was switched on because I liked to play
with it ;-)

Joop

Hi Bob,

> It was 25 years ago (A long time ago, in a Galaxy far far away) that I 
> built that hand held sonar system. Today, with many years of 
> experience as an engineer, I understand about dynamic range. As a kid 
> fresh out of high school, it was a fair surprise. Then again, that's 
> where experience comes from! I am still a little surprised that with 
> the 1% duty cycle of the transmitter, the amplitude in the pool did 
> not decay measurably between pulses.


My foray into ultrasound started about 20 years ago and yes, at that 
time I also was a total rookie when it came to acoustics. Until a few 
days before the interview for my first job I wondered what the heck they 
would do with depth sounders on humans.

The duty cycle doesn't really matter much. It's the pulse amplitude and 
the path attenuation. Water is an almost ideal sound conductor and 
plaster walls are nearly ideal reflectors. At least until the pool gets 
old and chlorine or the occasional pH value drift takes its toll on the 
walls. Same for the bottom and the water surface on top. If the pool was 
tiled the reflection would be almost perfect. In a pool you get so many 
reflections back and forth that a short burst will create a plethora of 
echoes for a long time.

Regards, Joerg

http://www.analogconsultants.com

Joerg wrote:
> Hi Bob,
> 
>> Years ago I built a similar system for scuba diving, but it used a set 
>> of counters selecting LED's out of a matrix, with the detections, 
>> illuminating the selected LED. I started testing this thing in a pool 
>> and found that the pool surfaces were so reflective that the entire 
>> pool (olympic size) saturated with sound. I put a scope on the 
>> receiver and it showed continuous output. First I thought that the 
>> receiver was oscillating, so I took another transducer to the far end 
>> of the pool and the amplitude on the second transducer showed the same 
>> continuous signal. The amplitude change from the transmit pulses was 
>> not visible at all. The same piece of equipment worked fine in a local 
>> quarry.
> 
> 
> 
> Hard reflectors are the enemy of every sound system. In my field of work 
> it's situations like calcified blood vessels, bones or metal objects 
> such as an artificial heart valve that make things difficult. But we 
> always try our best to provide the largest possible dynamic range and 
> good gain control. That, and matched filtering, would also help in depth 
> sounding. On a ship this can become crucial when navigating a shallow 
> area where the sea bottom consists of hardened lava or rock.

Hi Joerg,
It was 25 years ago (A long time ago, in a Galaxy far far away) that I 
built that hand held sonar system. Today, with many years of experience 
as an engineer, I understand about dynamic range. As a kid fresh out of 
high school, it was a fair surprise. Then again, that's where experience 
comes from! I am still a little surprised that with the 1% duty cycle of 
the transmitter, the amplitude in the pool did not decay measurably 
between pulses.

Regards,
Bob

Hi Bob,

> Years ago I built a similar system for scuba diving, but it used a set 
> of counters selecting LED's out of a matrix, with the detections, 
> illuminating the selected LED. I started testing this thing in a pool 
> and found that the pool surfaces were so reflective that the entire 
> pool (olympic size) saturated with sound. I put a scope on the 
> receiver and it showed continuous output. First I thought that the 
> receiver was oscillating, so I took another transducer to the far end 
> of the pool and the amplitude on the second transducer showed the same 
> continuous signal. The amplitude change from the transmit pulses was 
> not visible at all. The same piece of equipment worked fine in a local 
> quarry.


Hard reflectors are the enemy of every sound system. In my field of work 
it's situations like calcified blood vessels, bones or metal objects 
such as an artificial heart valve that make things difficult. But we 
always try our best to provide the largest possible dynamic range and 
good gain control. That, and matched filtering, would also help in depth 
sounding. On a ship this can become crucial when navigating a shallow 
area where the sea bottom consists of hardened lava or rock.

Regards, Joerg

http://www.analogconsultants.com

Hi Joop,

>It could very well have been an oldy. The light did seem a rectangular
>red LED though, not a neon bulb. I do seem to recall some spinning
>noises so I a rotating disk could have been involved.
>That would make powering the device simpler (12V board power on
>boats?). Perhaps I should have said ship. The whole vessel was quite
>big, 3 masts with sails
>(http://212.204.218.222/hanzestad/schepen/schip.php?id=42)
>  
>

Wow, that is a big ship. Dance floor and all. But 160 passengers and 
only eight restroom stalls? What if half the people on board got 
seasick....?

I'd think this ship deserves a much better echo system. You could use an 
old scope as a display to make it look somewhat antique. Unless, of 
course, you have to comply with the devices from a certain era. An LED 
on a disk sound like half-hearted retro design though. But in the time 
of three-masters they did not use echo. Well, it hadn't been invented 
yet. They had a rope with regular knots that was let down once in a 
while and, after the rope handling sailor had spit his chew tobacco into 
the sea, the depth was hollered to the bridge in 'fathoms'.

Groetjes, Joerg

http://www.analogconsultants.com

Joerg <notthisjoergsch@removethispacbell.net> wrote:

>Hi Joop,
>
>>The last boat I was on had a depth meter with some rotating device
>>inside. Probably a rotating mirror. The round scale showed various
>>echos at different depths of which the 'real one' was usually of
>>maximum light output.
>>  
>>
>
>That was a real classic. I believe it is a little neon bulb on a 
>rotating disk. Echoes are amplified and the bulb hangs on the output of 
>the amp. I vaguely remember that Heathkit offered on of these. But I 
>didn't have a boat...
>
>Regards, Joerg
>
>http://www.analogconsultants.com

It could very well have been an oldy. The light did seem a rectangular
red LED though, not a neon bulb. I do seem to recall some spinning
noises so I a rotating disk could have been involved.
That would make powering the device simpler (12V board power on
boats?). Perhaps I should have said ship. The whole vessel was quite
big, 3 masts with sails
(http://212.204.218.222/hanzestad/schepen/schip.php?id=42)

If I would have forseen this discussion I would have paid more
attention ;-)