Re: [PSUBS-MAILIST] SONAR

["Sean T. Stevenson" <ststev@uniserve.com>]

When I was originally designing the PLC control schematic for my non-existent sub, I had the idea of using the signal from a simle downward firing sonar to implement an "auto-altitude" setting, 
which would keep the vehicle at a specified setpoint above the bottom as you move along.  Subsequent discussions with Jim MacFarlane of ISE led me to the conclusion that this feature doesn't 
work very well, since any extraneous echoes (such as fish, debris, stirred up bottom, etc.) would result in a huge error, and the only way to combat this (and not send your sub lurching up to 
compensate) was to average the signal over a time interval, resulting in the response being too slow when traversing.

The compromise is just to use the auto-depth, which is easy since the depth transducer is very consistent, and just jog the setpoint manually as you move along.  The fishfinder still can provide 
altitude information, but it is just passive.  A 360 degree navigational sonar is what you really need for obstacle avoidance and location.

-Sean


On Fri, 10 Aug 2001 13:25:55 -0700, Ed Greany wrote:

>I suppose I should address this topic since it deals with Sonar. I was a
>Sonar Supv. on board when I was on the boats.
>
>The typical fish finder works just like a fathometer or as Capt. Nemo
>called it, a "sonar altimeter" which is a pretty good term except an
>altimeter measures the height above a horizontal plane such as a sea
>level. In water, it is a little different in that it measures the depth
>below the keel. A depth gauge meter measures the distance you are below
>sea level - or actually the depth below the surface of the water you are
>presently at in case your lake is above sea level to start with.
>
>All (active) sonar works on the same principal you described with a
>"ping" and measures the time differencial for the return bounce off an
>object. Of course many physics of sound principles apply such as
>salinity, temperature, seaweed and other obstacles, temperature layers,
>etc. But in the perfect setting, the ping goes down from the keel in a
>particular pattern (cone, globe) and bounces back to a receiver. The
>fish, target, or other object would bounce back as a faster rate than
>say, the bottom, and this is displayed as a "hole" in your sound pattern
>and shows on the screen as "fish".
>
>I thing two meters would be very valuable in a P-Sub. First, and most
>important, is a depth gauge. You HAVE to know your depth. Second, a
>fathometer (or fish finder) would be very helpful for locating
>underwater "invisible" objects such as big rocks, pier pilings, sunken
>hulls, or other items you don't want to connect with - just view.
>
>Ed, STS-2(SS)
>
>Captain Nemo wrote:
>> 
>> Adam,
>> 
>> The finder I use in my sub came with a diagram describing the shape of
>> the sonar "cone" projecting below the boat.  When I see a fish image
>> displayed, what I'm seeing is the return bouncing off something
>> situated somewhere in that cone.  Without checking the manual (and
>> remember, I bought it 12 years ago) I think my transducer projects a
>> 20 degree cone.  Basically, unless I'm getting a false return, it's
>> only telling me there's something down there in that general area
>> below the boat, but it's close enough to work with in most practical
>> circumstances not requiring extreme precision.
>> 
>> I use mine as a "sonar altimeter" to give me a readout of the distance
>> between my keel and the bottom; and it also provides some visual
>> representation of what's below me.  Not high tech, perhaps; but much
>> better than nothing at all.  In use, it's been very helpful,
>> especially since my sub's design does not enable me to look straight
>> down visually.  I'm presently rebuilding the control panel, and the
>> fishfinder is staying aboard.
>> 
>> VBR,
>> 
>> Pat
>> 
>> 
>> 
>>      ----- Original Message -----
>>      From: Adam
>>      To: personal_submersibles@psubs.org
>>      Sent: Friday, August 10, 2001 5:41 PM
>>      Subject: [PSUBS-MAILIST] SONAR
>> 
>>      Okay,
>> 
>>      I have a question about "civilian" SONAR.  I have seen fish
>>      finders in action but I never really understood what I was
>>      looking at.  I saw a little picture of a boat and then the
>>      water under us.  And a couple fish :)     I'm wasn't really
>>      intrested in the fish but the technology when I saw this at
>>      the tender age of...???  um??? like 12? I think?
>> 
>>      Anyway my question is.... when OI saw it it a apeared as a
>>      2d section...  a plane straight down from the boat....  Now
>>      this image may have deviated a little from what I actually
>>      saw in my memory, but if that is what I saw then that
>>      doesn't seem very useful for our purposes.
>> 
>>      Basiclly, what i'm asking is: Can someone explain to me how
>>      fish finders work.  Not the fundamental PING and Return time
>>      with the calculation involving time, distance, and the speed
>>      of sound in water, but in what type of plane does the ping
>>      get read.
>> 
>>      In otherwords, it ping in 3D, but is it read in 3D and if
>>      so, how is that displayed using a 2D display.....  I hope I
>>      have explained my queston sufficiently.
>> 
>>      Adam