Far Below & Long Away
Not in a galaxy but in a sea close to you are things that are just as nasty as Mr Darth Vader. Today charts are no longer legended: “Here be Monsters”, but uncharted objects are just as harmful as any monsters that are still out and under there. They may be animate and inanimate, temporary, permanent and even growing. Even twin transducer depth sounders only tell you what depth is there right now. Granted that allows a savvy, experienced helmsman to read trends and predict, but sonar – especially forward looking 3D – offers much more, to the benefit of mariner and Cetacean alike. Ian Bowles of FarSounder explains.
A VESSEL TRAVELLING AT 15 KNOTS WILL TAKE TWO minutes to go half a mile. If a target or obstacle is detected directlyahead of a vessel, is this enough time to evaluate the situation and take [manoeuvring] action to avoid it? The reader’s immediate reaction might be no, but the questions will also be asked as to how exactly this target is detected. If visually, then the human brain can, and likely will, make a decisive judgment. If by radar,then time is needed to perform even the most basic of plotting to recognise what is happening, leaving very little time for evasive action. In these cases, the target is above the surface.
For a target below the surface, it is likely to be stationary or very slow moving, and so it may be thought that this is enough time to take evasive action with only a minimum course adjustment; a little like weaving through fishing buoys or vessels.
The argument here is that sonar is a practical aid to navigation and the greater the range, the more beneficial it can be. There are different types of sonar and there are practicalities and limitations for all. From the two-dimensional fish finding type through military grade ones to three dimensional navigation sonars. Vessels currently using sonars for navigation are generally using them when navigating at manoeuvring speeds, in poorly charted areas that may contain floating or submerged hazards to navigation; or simply in waters that are unfamiliar to the navigators. Increasing the detection range gives the navigators more time to assess the situation and allows for other practical applications.
In this changing environmental world of global warming, energy exploration and eco-tourism, unexplored and uncharted waters are being cruised on a regular basis. Nations are also taking a greater responsibility in animal resource management and, in our industry, marine mammals in particular. NOAA, for example, recently introduced a ruling that temporarily restricted vessel access to certain geographical areas, and implemented speed restrictions in others, where certain whales have migratory routes or calving grounds.
In “post disaster” environments, such as after a major meteorological event, it is often the case that the underwater situation has changed. After Hurricane Katrina in 2005, the bottom topography in the Gulf of Mexico was significantly altered, with underwater obstructions lying uncharted throughout busy navigation areas. Until survey vessels document and report these changes, sonar is important for first responders, working vessels and helps ports to get back in operation as soon as possible.
For fish finding, range is more the priority and so a two-dimensional display is quite suitable. For navigation, two dimensions are not sufficient, because in addition to range and bearing to in water or underwater targets, the navigator also needs depth information. Giving all three dimensions offers the navigator an excellent alternative view of the waters ahead of the vessel; something relatively new but longed for. Using sonar for navigation can help vessel owners and operators avoid costly, dangerous and environmentally damaging collisions and groundings.
Current Navigation Technologies
Current navigation technologies, such as GPS, RADAR and electronic charts, are widely accepted and standard fit on most classes of vessels. Until now, state-of-the-art navigation has relied on historical surveyed charts, GPS systems and depth sounders to determine position and water depth. Chart data is often inexact as coastlines and shipping channels can shift. Transient objects such as sandbars, lost shipping containers, ship wrecks, whales, floating logs and other debris are not shown. Additionally, many charts use 70-year-old (or more) survey data that often predates GPS. This means that even ‘charted’ obstacles are not necessarily where the chart places them.
The introduction of and use of navigation grade sonar is not meant to replace anything, rather to augment the mariner’s box of navigation tools by offering a real-time picture of the waters ahead.
An echo sounder will tell the ship how deep the water is, but it can’t warn of the dangers ahead. The introduction of and use of navigation grade sonar is not meant to replace anything, rather to augment the mariner’s box of navigation tools by offering a real-time picture of the waters ahead. This further enables the mariner to make critical navigation decisions with a more complete understanding of the real-time situation.
Current Sonar Technologies
Until the advent of 3D systems, vessel operators were limited to one- or two-dimensional views, with limited distance capabilities, limited performance in shallow waters, and a narrow field of view. Usually these products are from the recreational, fishing specific or hydrographic markets and therefore not suited to commercial applications or navigation; yet they may still be marketed and sold for this purpose. At the other end of the spectrum there are military grade sonar systems that, again, are designed for a specific market capability. They also tend not to be commercially viable navigation options for a commercial vessel operator.
Principle of Operation
The 3D sonar comprises a phased array transducer that will usually be mounted in the bow or stem of the vessel, facing forward. This in turn is connected by a special cable to a junction box and fromthere to the processor on the bridge.
The forward-looking horizontal field of view is a practical 60° to 90° with a practical range of up to ½ nm. Vertical field of view is approximately 10° up to the surface and 50° down. For a useful navigation tool a real time presentation is necessary and the whole volume would be pinged approximately every second with an overall refresh of around one to two seconds to achieve this. Shallow-water operation is another important factor when considering navigational usefulness. Bottom mapping, that is the amount of sea floor that is detectable ahead of the vessel, is a function of depth. For example, with a sonar that has a bottom mapping capability of 8 x the depth of water, if the water depth is 10m, it will map the sea floor out to 80m. In practice, a bottom mapping capability of 10– 12x the depth of water should be expected. However, “in water” targets can still be detected out to the full range of the system, but their depth will not be known.
It is important for any sonar system to have an easy-to-use man machine interface allowing for easy interpretation of essential data. This is fairly new technology and too much time should not be taken away from the other key navigation aids.
In a comparison of the visual picture vs sonar, the example shows a breakwater as seen through the bridge windows and the same view on the 3D Forward Looking Sonar. It is clear on the sonar display that there are other obstacles in front of the breakwater but below the surface.
Shipstrike is the largest killer of the endangered Right and Great Whales. At certain times of the year during migration, certain areas are restricted for passage or have significantly reduced speeds. It is commonly reported that sonar is harmful to marine mammal life, and it needs to be clarified that this is often in relation to low frequency and high power systems used predominantly for military applications. It is important not to confuse navigation sonar systems with military low frequency ones. Generally a sonar in the 30kHz to 70kHz is in the same frequency range as other accepted marine electronics (echo sounders and fish finders for example) and usually at a lower power.
3D sonar data’s inclusion in today’s integrated bridge management systems (IBS) was the next logical step. Today’s IBS can now have sonar overlay as an option to the radar overlay on the electronic chart. With the advent of various data recording devices, more and more capability is often sought. Sonar data is no different and can also be archived, either directly or in conjunction with a vessel’s VDR.
Limitations and Expectations
Targets such as containers, whales, rocks, reefs, ice/icebergs, other vessels, buoys, pilings, etc. (to an 8 dB target), are the benchmarks for the types of targets that can be expected to be detected. Limitations for commercial vessel operators of all classes are usually related to speed and range. Larger vessels and high-speed vessels need sufficient time to evaluate potential dangers and act accordingly, although for vesselsat manoeuvring speeds, the range requirement is significantly less.
Current vessel speed for a sonar in the ½ nm range is in the region of 20 to 25 knots. Future research and development over the next one to two years anticipates increasing the range to approximately 1nm or more, and with a speed up to 35 knots. As speeds and ranges increase, to keep the full real-time picture, fast refresh rates become of even more importance, although there will be some latency at longer ranges.
In regards to the development of a long-range/high-speed navigation sonar as discussed above, there are specific technical factors that must be dealt with. These are of minimal effect on current shorter-range systems. For instance, there is a trade-off in choosing an appropriate frequency which will still offer enough signal to noise ratio (SNR) to counter the effects of long-range attenuation of the system. At higher speeds, a challenge to overcome may be hull specific: high-speed fl ow noise issues. Therefore, the form factor of the Transducer Module and how it is mounted must be optimised for different high speed hull types.
One new concept that has been introduced is a Bistatic system. Where a traditional transducer is Monostatic, with transmitter and receiver in the same module, the Bistatic version will house those same components in separate, but smaller form factor, modules. This allows for installation on a greater variety and size of hull types, from sailing vessels and fine line motoryachts, to catamarans. For the latter one module is in each hull. At short ranges it may be appropriate to regard the Sound Speed Profile as a constant (for navigation grade sonars, not necessarily for security sonar systems). At longer ranges, a vertically varying sound speed profile must be compensated for.
The resolution of Long-Range Targets is also a challenge and requires an added level of Bathymetric Testing and Ground Truthing as well as compensation in the Fixed Frame of Reference.
The need for a navigational solution to groundings and collisions has been recognised for hundreds, if not thousands of years. It is expected that navigation grade sonar will become increasingly attractive to operators of all superyachts. 3D sonar represents an extraordinary advance in the technology and a revolutionary change in the way vessels navigate.
Opening image: istock.com