Scanning Sonar Update

We have had 3 full cruising seasons and about 18000 nautical miles aboard Iron Lady now. We have seen a wide variety cruising situations from a circumnavigation of New Zealand to Fiji, Tonga, and French Polynesia. We initially thought our Furuno Scanning Sonar was a nicety but now find it an essential part of our arsenal in a wide variety of situations. Having said that, we can also say the learning curve is a very steep one and the way we are using it today is vastly different then it was three years ago. We thought it might be useful to review when and how we use the scanning sonar today, how we set it up in various situations and cover things we like, things we do not like and features that we wish we had (read marine electronics folks – are you listening?)

While the Furuno unit we have has been designed with many features for the commercial fishing industry, our uses are quite different. We primarily use the unit as an underwater obstacle collision avoidance device. It is therefore at its best when we are approaching an unfamiliar anchorage, insuring that we have a clear swing radius in all directions when at anchor, navigating in close quarter shallow water situations, transiting a narrow pass into a tropical lagoon, transiting a narrow passage, and navigating in areas where there are no depth soundings (and yes – there are more of them out there then you realize. Most of the lagoons in the Tuamotus in French Polynesia have few if any depth soundings.)

The common theme is that we are looking for obstructions that rise close to the surface that represent a collision hazard. When we first started using the Furuno, we typically tried to look out as far as we could. The problem with this strategy is one of geometry. Even at shallow beam angles, looking out more then 100 meters means we are looking down to depths which are of no concern to us.

It is worthwhile to look take a look at the geometry of the beam for a moment. The first is beam angle relative to the surface of the water. At beam angles of less then 5 degrees (with 0 degrees being level with the surface), the returns are distorted by return echoes from the surface. The more turbulent the surface, the worse the reflections. We therefore tend to operate at beam angles between 5 and 10 degrees with the final setting being determined by surface reflections. If you think about it, a shallower beam angle is more desirable as it allows one to reach our further while keeping the depth at the maximum range within limits that are important. It is also important to note that the vertical cone of the beam projected from the transducer is 8 degrees. Thus any tilt setting of less then 4 degrees results in the beam hitting the surface and unwanted surface reflections.

Without getting in to the trigonometry, using a 5 degree beam angle and looking out 100 meters, the depth we are looking down to is about 8.75 meters (not taking in to account cone angle which puts the lower edge of the cone around 16 meters). The point here is that we are really interested in objects that come to with 2 meters of the surface so 8.75 meters is well below our depth of concern. Looking out further only increases that depth so anything beyond 100 meters is really not providing any important information to us.

So early on we adjusted our strategy by setting the range we were looking out to 100 meters or less and using a 5 to 10 degree beam angle depending on surface conditions.

Taking a look first at situations where we are approaching an anchorage, a narrow pass or passage or are navigating in shallow waters with few or no soundings. In this situation, we normally set the unit to the horizontal scan mode so we are sweeping left to right of the bow. In addition to range and beam angle, the additional settings required on the Furuno are as follows – training direction (which direction relative to the bow the beam is pointing when centered), sweep (how many degrees left or right of the bow we are looking), gain (power setting), pulse length and power output. Normally training direction is set to look straight off the bow and sweep is 45 degrees either side of the bow. Pulse length and power output are normally set to short and min since we are not looking out a long way. To set gain, we adjust tilt angle to get a solid bottom return and then adjust the gain to get a clear return from the bottom without a lot of false echoes.

We also normally set a range bearing marker somewhere around 50 meters or at the point we are getting a bottom return. Interpretation of the display goes something like this. Uniform bottom shows as an arc at uniform distance from the center of the display. If the depth is greeter then the max depth at the given range, the display is blank. Objects projecting up from the bottom will result in a return that shows as closer to the boat then the arc of the surrounding bottom. This is important – if the return from the object is decreasing in intensity as we approach it, it means it will pass under the boat so long as it disappears entirely as we approach. If has not disappeared by the time it reaches the predetermined range bearing we have set up, we change course. If the object intensity is increasing as we approach it, it is time to alter course.

Again, turning to the geometry of the situation, the sonar is looking outward at an angle and not directly downward. As we approach an object that is not a hazard, it will gradually pass below the beam as we approach and this presents on the display as a weakening return. Just the opposite is try for an object that is a hazard as the it will remain in the beam and the return will get stronger as we approach.

We have actually refined our set up a bit more this year based on navigating in lagoons in the Tuamotus is French Polynesia. The lagoons are generally deep with shallow areas and coral heads. If we set the tilt angle to constantly display the bottom arc, small variations in the bottom contour are constantly showing up as we approach them. Using the sonar for long periods of time (meaning up to 4 hours) is very tiring as we are constantly trying to interpret the display. Our revised strategy is to tilt the beam angle up so that the bottom is not displayed at all. This is done after we have done all our initial set ups on the gain using the bottom to refine our gains settings. In this manner, the display is blank the vast majority of the time and therefore, requires nothing but a quick glance to see if there is a return at which time more intensive monitoring is done. Every once and again, we tilt the beam down to get a bottom return to insure that our gain settings are still on the mark.

Turning to the situation where we are approaching an anchorage, once we have achieved a position close to where we wish to anchor, we switch to a full 360 degree scan around the boat to insure that there are no objects of concern within our swing radius. (Makes for a much more peaceful night when there is a major wind shift – no worries that there is something our there waiting to grab us). During this procedure, we want to see a firm bottom return at the edge of our chosen range so tilt angle is adjusted downward as necessary and range is set to something a bit large then our swinging radius.

During this time, if we see an object that is of concern closer to the boat, the Furuno has a one push button feature which allows us to switch directly to a vertical scan in the direction the beams pointing at the time the button is pushed. The resultant vertical profile gives us a direction and depth of the object.

That about covers how we currently use the scanning sonar (in a very simplified manner). Time to spend a few moments on what like and what we do not like about the Furuno:

Likes:

1) Nice graphical presentation of where the beam is pointed relative to the bow, range, tilt angle, and depth including range and depth of the range/bearing marker;

2) Nice hot keys that allow switching between various modes on the fly;

3) It does things better then other current systems.

Dislikes:

1) Too much operating complexity;

2) Too steep a learning curve;

3) Too much reliance on operator interpretation of what is being seen;

4) Depth indicated by the display does not correspond to actual depth – typically actual depth is deeper – I suspect this is a result of transducer cone angle but whether this is true or not, the displayed depth should be accurate;

5) With a single transducer, too slow a scanning rate;

6) Mechanical complexity – two axis servo driven scanning;

7) Poor processing power – it simply takes a single transducer and displays what it sees as the transducer is mechanically manipulated in two axises;

8) Needs more hot keys to switch between modes with one touch of a button.

Want to haves as long as we are going for the gusto:

1) How about a multiple transducer, phased array scanning that captures surface to 90 degrees down and 90 degrees port to 90 degrees starboard in one go – no servos required?

2) In horizontal mode, a contoured bottom display colored to indicate depth and areas of concern;

3) In forward scanning mode, a “flyover” display that shows a contoured view of the bottom like Google Earth flyover;

4) Above requires more speed, better algorithms and a lot more processing horsepower.

We would appreciate hearing from other users of scanning sonar out there. Add your comments to the discussion and we will see if we can drum some interest from the marine electronics folks.

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