Returning to a SONAR Target

Electrical and electronic topics for small boats
jimh
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Returning to a SONAR Target

Returning to a SONAR Target

Let us say we are boating and looking for a particular target on a SONAR. The target is stationary and is on the sea bottom. The water depth is 100-feet. We are using a SONAR transducer with a circular pattern and a cone angle of 30-degrees of flat response. While boating we observe a target on SONAR. We immediately mark our position on our chart plotter as determined by our GNSS receiver. Sometime later we try to return to the position marked and can't find the target. What caused the errors?

FINDING THE TARGET

The first error in this process occurs in the SONAR. The target reflection is occurring from some point on the bottom that is in the cone of the SONAR signal. We assumed a circular pattern and a cone angle of 30-degrees. If the depth is 100-feet, then the radius of the circular area on the bottom that intersects our SONAR cone is defined by

circle radius = tan(coneAngle/2) x depth

--or--

circle radius = tan(15) x 100 = 26.8-feet

The actual target causing the reflection we see on the SONAR screen could be as far as 26.8-feet away from the SONAR transducer position directly overhead on the sea bottom.

The second error in the process occurs when the chart plotter marks the position. The position recorded is the position of the GNSS sensor, not the SONAR transducer. On a small boat it would be common that the SONAR transducer was mounted on the transom while the GNSS antenna was mounted at the helm. These two locations could be 10-feet apart. This is the GNSS antenna offset error

The third error occurs when the GNSS receiver deduces the position of its antenna. The deduced position is subject to many influences for accuracy. A nominal accuracy for an autonomous GPS without any assistance is plus or minus 5-meters in the horizontal plane. That is about 15-feet of error. This is the GNSS position error.

If we are particularly unlucky, all of these errors will lay on a straight line, and thus they will be additive. This gives us a maximum error for the waypoint position that is supposed to mark the SONAR target of

26.8-feet + 10-feet + 15-feet = 52-feet

of error in the waypoint position compared to the actual target location. The actual distance between the target and the recorded waypoint is 52-feet.

RETURNING TO TARGET

To get back over this target at some time in the future, we navigate to the recorded waypoint position. Our navigation is again subject to the error of the GPS position, or 15-feet. We can position the GPS sensor at the same position as the recorded waypoint, but we can only count on being within a circle of 15-foot radius. If we are again unlucky, the GPS position error will be in the opposite direction that it was when the waypoint was recorded. So not only is our waypoint 52-feet away from the target, when we return to the waypoint position there is a 15-feet error in the GPS range that, by chance, happens to be additive. Now the GPS sensor is 52+15 = 67-feet away from the target on the bottom.

But things are not done getting worse. If the orientation of the boat is also very unlucky for us, then the GPS tells us we are back to the waypoint, already 67-feet in error, the boat orientation will be aligned in the worst way, putting our SONAR transducer another 10-feet away from where it should be to find the target. The SONAR transducers is now 77-feet from the target.

Now we have to consider the SONAR cone coverage, but this time it actually helps us. Even though we might be out of position by 77-feet, we know that the SONAR cone is going to cover a circular area on the bottom of 27-foot radius. This puts the outer part of the SONAR code back to within 50-feet of the actual target. Unfortunately, we won't see that target on our SONAR.

CORRELATED v. UNCORRELATED ERRORS

In actual use, the errors do not tend to align for the worse outcome. The errors are likely to be uncorrelated. We repeat the analysis with the consideration that the errors will add as the root sum of squares. For the initial process of finding and marking the target, the uncorrelated errors will accumulate to

Waypoint Error = [ (SONARconeRadius)^2 + (GNSS antenna offset)^2 + (GNSS position error)^2 ]^0.5

Waypoint Error = (26.8^2 + 10^2 + 15^2)^0.5

Waypoint Error = 32-feet probable error between target location and waypoint location

Our stored waypoint is now only likely to be 32-feet in error from where the actual target is located on the sea bottom.

When we return to our stored waypoint, our GNSS position error and GNSS sensor offset also will be uncorrelated. The likely error will be

GPS Range and Location Offset Error = [ (GNSS antenna offset)^2 + (GNSS position error)^2 ]^0.5

GPS Range and Location Offset Error = (10^2 + 15^2)^0.5

GPS Range and Location Offset Error = 18-feet

This makes the chances of finding the original target better. On the surface of the water the SONAR transducer should be back to within 18-feet of the marked waypoint that recorded the target location. Unfortunately, we know that the marked waypoint position may likely have an error of 32-feet. But these two errors are also uncorrelated, so we assess the likely Total Error Distance between transducer and target to be

Total Return Location Error = [ (Initial Waypoint Error)^2 + (Returning GPS errors)^2)]^0.5

Total Return Location Error = [ (32)^2 + (18)^2) ]^0.5

Total Return Location Error = 36-feet, which is the likely distance between where the target is and where the transducer is upon returning

Now we look at the SONAR cone area on the bottom to see if the target will be within our cone of coverage. Unfortunately, in this example, the 27-feet radius of the SONAR cone may not detect the target that is 36-feet away.

IMPROVEMENTS

There are several areas for improvement in this example. At first, you might think that using a SONAR with a smaller cone angle would improve accuracy. It does, but only for the locating phase. If a transducer with a narrower cone angle were used to find the target, the initial error would decrease. For example, if we use a transducer with a 15-degree cone angle, the radius of bottom covered at 100-feet depth would decrease to 13-feet from 27-feet, reducing the error by half.

If the GPS sensor and SONAR sensor were co-located, the offset error could be reduced to zero. Let's assume we could get the two sensors within one-foot of each other, a tenfold reduction in that error.

If the GPS receiver utilizes augmentation, such as the Wide Area Augmentation System (WAAS), the ranging error could be reduced to about 7.5-feet from 15-feet, another reduction by half in the error.

The effect of these reduced errors would cause the initial waypoint error to decrease to

Waypoint Error = [ (SONARconeRadius)^2 + (GNSS antenna offset)^2 + (GNSS position error)^2 ]^0.5

Waypoint Error = (13^2 + 1^2+ 7.5^2)^0.5

Waypoint Error = 15-feet probable error between target location and waypoint location

Upon returning, we also experience an advantage. The GPS ranging error is again reduced, and the offset between GPS sensor and SONAR sensor is much more favorable. The new position error on return navigation is likely to be

GPS Range and Location Offset Error = [ (GNSS antenna offset)^2 + (GNSS position error)^2 ]^0.5

GPS Range and Location Offset Error = (1^2 + 7.5^2)^0.5

GPS Range and Location Offset Error = 7.6-feet

This makes the chances of finding the original target better. On the surface of the water the SONAR transducer should be back to within 7.6-feet of the marked waypoint that recorded the target location. Unfortunately, we know that the marked waypoint position may likely have an error of 15-feet. But these two errors are also uncorrelated, so we assess the likely Total Error Distance between transducer and target to be

Total Return Location Error = [ (Initial Waypoint Error)^2 + (Returning GPS errors)^2)]^0.5

Total Return Location Error = [ (15)^2 + (7.6)^2) ]^0.5

Total Return Location Error = 16.8-feet, or the likely distance between where the target is and where the transducer is upon returning.

Unfortunately, now the narrower cone angle works against us. The SONAR cone only covers 13-feet of the bottom. Again, we find that our SONAR transducer may not pick up the target. The solution perhaps is to use a narrow beam transducer to mark the target when initially finding it and storing a waypoint, and use a wider beam transducer to hunt for the target when trying to reacquire it upon return. Also on initial marking, better target accuracy may be obtained if the signal on the SONAR is peaked for strongest return signal before marking the waypoint.

It is also interesting to look at water depth. It really does not affect the outcome because the SONAR coverage area on the bottom is a function of water depth and cone angle. Water depth is already included in the calculation by its influence on the bottom coverage area. If the water depth were less, the area covered on the bottom would decrease, but it would similarly decrease when hunting for the target upon return.

CONCLUSIONS

A further consideration is the behavior of the SONAR. The signal response of the transducer does not just immediately stop at its rated cone angle, nor is it completely uniform within the cone area; the sensitivity just decreases beyond the cone radius, and it is also variable within the cone radius. If care is taken to mark the target initially by peaking the target signal strength, then the target is more likely to be well aligned with the transducer location when the waypoint it taken. On return, some response, albeit weaker, from targets outside the cone radius will be helpful in finding the target.

jimh
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Re: Returning to a SONAR Target

I can contribute an example of the problem of returning to a SONAR target that was marked with a waypoint taken from a GPS position solution. I had some reliable data about the position of a wreck in Lake Huron. We approached the waypoint position carefully until our GPS receiver showed we were at that location. Unfortunately, our SONAR had no sign of any bottom echoes that would indicate the wreck of a very large ship on the bottom. We then began a search pattern, working out from the waypoint. In a short time we began to see bottom echoes from the wreck. Our distance from the waypoint was probably about 100-feet. In this instance the GPS position used to locate the waypoint was taken by another person, on another boat, on another day, with another GPS receiver. The target location was well known as they had divers who investigated, and their boat was probably tied onto the wreck or very close to it. We returned to hunt for the target more than a year later, with a different boat, different GPS receiver, different SONAR. We did not find the wreck by echo sounding at the same waypoint, but quite close to it.

porthole
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Re: Returning to a SONAR Target

Two waypoints, the second taken on a reciprocal course will greatly increase your chances of relocating a target. Learning to optimize your sonar settings and due diligence on your search patterns will increase your odds as well.

Wreck hunting (or any sonar target) has come a long way from the days of LORAN A and flashers or paper recorders.

If you are wreck hunting, knowledge of the wreck is a big help. If you know the wreck has a cleared height of 30' for instance, then saving the waypoint when you first start to see a return is not going to help much. Save it when you see 30' of relief.

For those who have a bit of difficulty getting their bearings while circling around, a bleach bottle, sash weight and the appropriate amount of string is a big help.
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Duane
1999 Outrage 21
1999 Yamaha SW Series II 200

jimh
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Re: Returning to a SONAR Target

porthole wrote:Two waypoints, the second taken on a reciprocal course will greatly increase your chances of relocating a target.

I don't quite understand the mechanism that causes this to occur. Please explain in more detail why you think this outcome occurs.

Also, if you record two waypoints, which one do you return to?

I hope you don't say that you first go to one of the two positions and then head for the other position. That is not returning to a waypoint. That is searching for something along a line between two positions. You're not returning to one position, but to many positions along a line. And, of course, the more places you look for the original target, the more chance you will have of seeing it again.

fno
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Re: Returning to a SONAR Target

More than one waypoint will get you on the wreck faster. As divers and bottom fishers, finding a wreck with two or three waypoints increases the reliability and timeliness of finding a wreck or reef. For the larger ones, usually the bow and stern are saved as waypoints and any path in between those two will show the wreck on SONAR--as long as it is set up properly. The roll and yaw (correct my terms if necessary) of a boat also should have some influence on target acquisition, shouldn't it?

Putting an anchor down 600 feet away and ending up over the wreck is another story that is part science and part magic.

jimh
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Re: Returning to a SONAR Target

I agree, you can "return to a SONAR target" by that method, but you really are not returning to the target, just searching anew for it in a pre-defined area. I also accounted for this approach when I suggested that when returning to the wreck you use a wider cone angle for the SONAR.

By expanding the search by travelling along a line, the beamwidth of the SONAR on the bottom is effectively increased in massive proportion. For example, if you travel along a line between two waypoints that are 200-feet apart, then the SONAR beam (assuming 30-degree cone and 100-feet of water) sweeps the bottom over an area of 54-feet x 200-feet or 10,800-feet^2.

In my initial presentation the SONAR beam only sees a circle of 2 π r or 168-feet^2. By travelling along the line for 200 feet you increase the size of the SONAR beam a factor of 64-times.

porthole
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Re: Returning to a SONAR Target

jimh wrote:
porthole wrote:Two waypoints, the second taken on a reciprocal course will greatly increase your chances of relocating a target.

I don't quite understand the mechanism that causes this to occur. Please explain in more detail why you think this outcome occurs.

Also, if you record two waypoints, which one do you return to?

OK, I'm confused, aside from your scientific account on how the SONAR works, the title of the thread is "Returning to a SONAR Target"
You changed the description in your reply above.

I'm no engineer or scientist, but I know how to find a SONAR target, e.g. wrecks or structure. And with trying to keep it simple, my comment above is a simple way to return to a SONAR target.

It can be explained probably in a lot more detail, but sometimes simpler is easier to understand.

I'll try.
For example:
If Waypoint 101a gets you a target return that you are satisfied with, go past the target, turn 180 degrees and go back towards the target. When you get a satisfactory return, mark it as 101b. In practice, especially on smaller targets, Splitting the waypoints 101a and 101b should get you closer to an optimum target return and can be saved as 101c

This does take more practice then just saying it, but in 25 plus years of wreck diving, I have found a target or two, and have repeatedly gone back to many of them, many times.

Modern GPS devices and state of the art SONAR devices sure make it a bit easier then years ago.

Last edited by porthole on Wed Jan 04, 2017 4:34 pm, edited 1 time in total.
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Duane
1999 Outrage 21
1999 Yamaha SW Series II 200

porthole
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Re: Returning to a SONAR Target

fno wrote:Putting an anchor down 600 feet away and ending up over the wreck is another story that is part science and part magic.

Just need to spend some time practicing it. On smaller boats (typically a six pack) we usually would just anchor into a wreck.
On some of the larger boats we either anchored directly, followed by chaining in or using a sand anchor up wind and drifting back, then dropping a grapple and tie in.

So, along the lines of the subject title for those that are curious.

This example is using modern equipment, but can be done with old tech as well.

Find a suitable SONAR target and mark it (saved waypoint, bleach bottle etc, electronic marking is better).

Once you are satisfied with location that you wish to dive or fish, get close to the location and drift over your target. The idea here is a slow pass.
Blue arrow

Once over your now well marked SONAR return experience comes into play. Based on wind and tide, drift for several hundred feet depending on depth from your mark.
Some chart plotters allow you to move the curser and leave a telltale line on the screen. Move that curser and line directly over the SONAR target and past it by several hundred feet.
Black arrow

Here you drop your sand anchor and allow the wind and tide to have you drift back over your SONAR target. Go a bit past the target, set your anchor then pull or motor yourself up over the the wreck/structure.
Red arrow

That's the simple way, 2 sand anchors allow fine tuning, but that is a different subject.
Last edited by porthole on Wed Jan 04, 2017 4:35 pm, edited 1 time in total.
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Duane
1999 Outrage 21
1999 Yamaha SW Series II 200

porthole
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Re: Returning to a SONAR Target

jimh wrote:Also, if you record two waypoints, which one do you return to?
I hope you don't say that you first go to one of the two positions and then head for the other position. That is not returning to a waypoint.

Well I respectively disagree here.

If record 1, 2 or more waypoints around a sonar target, and I return to any of the saved waypoints, I have in fact returned to the waypoint. Whether there is a SONAR target under that waypoint is another story.
Last edited by porthole on Sat Jun 24, 2017 12:31 pm, edited 1 time in total.
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Duane
1999 Outrage 21
1999 Yamaha SW Series II 200

jimh
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Re: Returning to a SONAR Target

It's not much of a problem to navigate a boat to the latitude and longitude of a waypoint. The goal is to get back to the stored position AND see the target on SONAR at that location. If there's no target on the SONAR, you are not, as the TOPIC line requires, "returning to a SONAR target."

The purpose of the article is to analyze why you can't quite count on a SONAR target always being there, and to explain the errors that cause this. As I said before, there are only two things you can do to directly reduce the errors related to marking the location of a SONAR target:

--locate the GPS sensor and SONAR sensor as close to each other as possible to minimize the distance between sensors;

--get the best possible GPS position solution to minimize the error in the actual position that will be recorded and stored as the waypoint.

Without a physical mark on the water surface, like a buoy, indicating the location of a SONAR target, you can be close to a SONAR target, not see any indication of it on a SONAR display, and have no clue exactly where the target lies from your present position. You have to search for it all over again.

I recalled this story in an earlier posting, but let me retell it. In trying to find the location of some big wrecks with very prominent SONAR targets, I was getting frustrated by discovering that many of them WERE marked with buoys placed there by divers. All you needed to do was get within a few hundred feet of the wreck and you'd see the buoy. The last time we were out wreck hunting, I intentionally masked my view of the water and looked only at the chart plotter, trying to put the boat right on the waypoint location I had for the wreck. Once I did, there was no SONAR target echo to be seen. I then began a slow search pattern, still not looking up from the chart plotter and SONAR displays. In a few moments I found the wreck on screen. Then I looked up and saw a small buoy tied onto the wreck, and now very close to the boat.

The recreational diver community or the charter boats that serve it have done a good job marking many of these wrecks. The marker buoys are usually small and a typical boater might pass them rather closeby and not even notice them. But if you have a clue what is below, you'd know what the buoy was there to mark.

ASIDE: I just bought a side scan SONAR and I hope to get some fun from it, again, looking for wrecks. Also, last summer on one really calm late morning, I was out kayaking along the shore. The water clarity and sun angle were both just perfect, and I could see the bottom at a depth of 25-feet or more as I paddled along. I was quite amazed to find an old wooden shipwreck less than a mile from my house. Unfortunately, I had no way to mark its location or record my position via GPS. I hope to find that wreck again, this time from my boat, and get a good GPS location for it. Maybe by next August, when the water gets warm enough for swimming, I can snorkel over it. It looked quite interesting.

jimh
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Re: Returning to a SONAR Target

The problem of returning to a SONAR target and the solution offered by PORT' is similar to a general problem in navigation when position fixing is not precisely accurate. Two examples of this old problem can be found in two well-known aviation navigation problems from long ago.

Francis Chichester used the method of intentional off-course navigation to safely locate and fly to small islands in the Pacific in his airplane c.1929, navigating only by rather crude methods of lines of positions from observations of the sun. The intentional off-course method involves intentionally aiming to the left or right of the target destination, until you reach a predetermined line of position, then turning left or right as appropriate, and flying down that line of position toward the target. By intentionally aiming to be far left or right, you can be certain which way to turn when on the final line-of-position to the target. (See more about Chichester at <https://en.wikipedia.org/wiki/Francis_Chichester#Air_pilot> )

The famous 1937 flight of Amelia Earhart was a similar attempt to navigate a long distance over water to a small island. It ended in tragedy. Once in the vicinity of the island, there was not a clear indication which way to turn toward the island on the pre-computed line of position (based on the sun's position).

The method of finding a known SONAR target by making a waypoint that is intentionally off-target by a distance far greater than the navigation error, then running down a line that is known to cross over the target, is precisely the same intentional off-course navigation method used by Chichester.

porthole
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Re: Returning to a SONAR Target

jimh wrote:ASIDE: I just bought a side scan SONAR and I hope to get some fun from it, again, looking for wrecks.
Did you buy a real side scan like a Klein? Or one of the many consumer MFD models we now have available?
jimh wrote:Unfortunately, I had no way to mark its location or record my position via GPS.
Land ranges
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Duane
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jimh
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Re: Returning to a SONAR Target

I bought a used LSS-1, first-generation Lowrance side scan in a separate module which will do the processing and not tax my old HDS-8 too much. It was about ten-percent of the cost of getting that new HDS-9 CARBON. About 0.01-percent of the cost of something professional like you're referring to.

On the wreck, yes, I did take note of a few landmarks and ranges. I hope I wrote them down in my notes up North when I got back to shore. I don't remember them now. The lake bottom was very strange this year. A lot of new sand was deposited on our "beach" which is normally not very sandy. Last winter the bay never froze over, and the high winds in winter storms must have really moved around a lot of sand. I suspect that perhaps that action removed some sand off this wreck. Based on the size of this wooden ship, it must have been built about 100-years ago.

porthole
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Re: Returning to a SONAR Target

Who knows, maybe you found something historic, or just somebody's derelict.

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Duane
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jimh
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Re: Returning to a SONAR Target

I didn't discover it; it seems the local divers are well aware of it.

porthole
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Re: Returning to a SONAR Target

Yes, I am aware your find is in Michigan, was just posting a 'story'

The Lake George boat, although claimed by the person in the article, was known to divers, just not made public. Us divers don't want our "secret" sites to get out you know.
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Duane
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jimh
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Re: Returning to a SONAR Target

PORT'--yes, I have sensed that among the recreational diver community there is a practice that knowledge about the location of certain wrecks is to be kept in limited distribution. However, as I have mentioned in some narratives of my wreck hunting, I have found that a surprising number of wrecks have unofficial marker buoys tied-on to them, so if you knew the general location of the wreck those marker buoys would put you right on it.

ASIDE: I am too old to take up SCUBA diving, so I will have to be content with just snorkeling above a few wrecks in shallow water and just looking at SONAR echoes from wrecks in deeper water.

jimh
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Re: Returning to a SONAR Target

Follow-up 2017 on the wreck in shallow water south of Northport marina:

We had the boat in the water in early June. Lake Michigan water is extremely clear in the spring due to the cold temperatures; there is no organic material growing in 50-degree water. And the lake had been very calm for several days. We went looking for the wreck I had located in the kayak last fall. The water was so clear we found the wreck very easily. I think the winter storms had scoured off more sand, as more of hull was exposed. We got a good location mark on the chart plotter, and when the water warms up--maybe in August--we'll be back to swim over the wreck. It looks quite interesting.

The wreck may also be a good target to practice with my side scan SONAR. I bought a used side scan SONAR over the winter, and I hope to get some enjoyment from it looking for bottom targets of interest.

jimh
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Re: Returning to a SONAR Target

More follow-up on the wreck in shallow water near Northport: the water (surface temperature) finally warmed up a few weeks ago to more than 70-degrees, so we set off one afternoon to swim on the wreck. It turns out the wreck is located in the middle of a sailboat race course used by the local Northport Youth Sailing School, and there was a flotilla of small cat-rigged prams piloted by youngsters out on the course. We had to postpone investigation of the wreck until after 4 p.m., the end of the sailing school activity.

Eventually we were able to anchor over the wreck, and I had a chance to swim on it with mask and snorkel. The wreck is in about 10 to 12-feet of water. I discovered that a surface water temperature of 70-degrees does not extend to the lake bottom at 12-feet depth, so I declined to swim down very far onto the wreck. There is a very prominent steel boiler exposed. The wreck is about 60-feet long and is now exposed from the lake bottom by about two feet of hull, with the boiler about four feet off the bottom.

The next adventure is going to be to install the LSS-1 side scan SONAR and see what sort of echoes we can get off this wreck. This will give me a chance to learn how to use the LSS-1 and see if the transducer installation is going to work.