Navigation Sonar for the Ship Operator: Forward Looking Sonars and Multibeam Echosounders Explained Dr. Alexander Yakubovskiy Signal Processing Manager FarSounder, Inc. November, 2010

Abstract: A number of sonar technologies are offered in the marine market, each of which has a different capability and price point. To the ship operator, understanding the differences between these sonar technologies is an important aspect in understanding what type of sonar they need. This paper helps to explain the various sonar system characteristics, allowing captains, owners, and operators to better understand what is important when specifying their sonar requirements. This paper addresses the customers' perspective rather than the sonar engineers' perspective.

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Preface Forward Looking Sonars (FLS) are designed to detect obstacles in front of ships, such as sea bottoms and in-water obstacles, as well as to provide automatic navigation alerts. This is a relatively new type of sonar (at least for non-military applications), with a limited number of FLS models available. The importance of FLS has been recognized in last 10 years, and a number of companies now offer that kind of equipment. Here is a brief list: FarSounder FS-3DT and FS-3ER 3D Models, Interphase Twinscope, Tritech Eclipse, BlueView P900, Marine Electronics 6201 and SeaEcho, Reson SeaBat 7128, Echopilot (Gold, Platinum and future 3D), L-3 Communication Subeye. These products are each very different in technical specifications and system design style. For many decades bottom profiling has been achieved by using Single-Beam Echo Sounders (SBES). In the last 40 years Multi-Beam Echo Sounders (MBES) have also become a wellknown instrument for bottom profiling. So one good way to understand FLS technology is to compare it to the ideas of MBES. Many marine equipment consumers are now familiar with bottom mapping techniques, and maritime professionals also understand the quality of bottom profiling with standard echosounders. Many in these groups are less familiar with FLS and, when considering FLS, questions arise such as: - Can I see the bottom in front of the ship out to the same distance as I can see the bottom below the ship when using an SBES/MBES? - Can I see the bottom in front of the ship with the same quality (resolution) as the bottom below the ship when using an SBES/MBES ? - I have seen FLS systems listed for a minimum of $5,000 and other FLS systems for $80,000 - $250,000. What are the differences between these systems? Perhaps the more appropriate first question people should ask themselves is: HOW am I going to see the bottom in front of me in order to ensure navigation safety? To answer these questions one must approach this from both the customer point of view (what do I need?) as well as from the technical point of view (what may I expect?). This paper addresses the customers' perspective rather than the sonar engineers' perspective. Still, in order to answer the question "what may I expect?" some level of technical explanation is required. Despite the myriad of sonar and echosounder customers, there is still no popular book covering this technology. Available books tend to be limited to highly technical ones for the sonar professional or student. The following paragraphs are provided to explain the technical aspects in a popular way.

Sonar Technical Explanations Before, a thorough explanation can be given, we must first begin by defining the basic characteristics of a sonar and discuss the critical questions which arise from this terminology.

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Sonar (Echosounder) This device works similar to radar, but using an acoustic signal. It transmits a short signal ("ping") of some frequency and then listens to the echo from the targets.

Target A target is defined as any physical object that reflects the ping back to the sonar. If the object is relatively small and far away, one may consider it as a single point-like target. If the object is large, such as a piece of sea bottom (e.g. 500m x 500m), one may consider every small bottom patch as a separate target, and the whole object is built up of these patches (targets). This leads to the next question of: - What does it mean by "small" and "large" in regards to sonar? To answer this question, this takes us to the next definition:

Resolution Resolution is the ability to distinguish between two closely spaced things. A resolution cell is the minimum volume cell which can be seen by the sonar separately from the surrounding things. Users are often interested in whether they can see the real object as a single detected patch or if the object would be represented by multiple resolution cells ("imaged"). The smaller the cells are, the better the resolution and the closer the sonar image is to the real world. Technically, that "cell" size is defined by two things: 1) Beam and 2) Range Resolution.

Beam A beam is the spatial (angular) area where the acoustic energy is concentrated. A sonar transducer works like a flashlight, sending a more or less narrow beam in a given direction. As with a flashlight or projector, the beam is similar to a light cone. The angular size of the cone is usually referred to as the "beam width". Figure 1 illustrates the beam nature. When the acoustic beam hits the target piece of the sea floor), it is "ensonifying" (highlighting) the area located approximately across the beam axis (direction). This cross-section is often referred as "beam footprint". The size of this beam cross-section is the "crossrange resolution". Obviously, the greater the distance from the sonar, the greater is the footprint, or poorer is the cross range resolution.

Figure 1: Beam and footprint

Range Resolution The second thing that defines the resolution cell in 3D space is range resolution, i.e. resolution along the beam axis. Resolution in that dimension depends on: Ping Structure and Bandwidth.

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Ping Structure The ping structure includes the ping length (in seconds), the central frequency (in kHz) and the bandwidth (also in kHz), unless the ping is a single-frequency (pure tonal) signal. Along the beam axis the ping is ensonifying the area of physical length equal to the product of: Pulse_Length x Sound_Speed. Generally speaking, the shorter the ping, the smaller the range resolution cell is. However, the ping length cannot be shorter than at least a few periods of the central frequency. Otherwise this frequency would not be represented nicely. So one may expect better range resolution from high frequency sonars, due to the shorter ping.

Bandwidth The bandwidth of the ping is a more sophisticated feature. Ignoring the details for this paper, suffice it to say that engineers know how to process a wide-band ping (i.e. frequency modulated "chirp") in order to improve the resolution and make it better than the natural ping length. Interestingly, in nature there are two well-known bio-sonars; the sonar of a dolphin and the sonar of a bat. These two sonars are based on totally different approaches of how to improve the range resolution. The dolphin's ping is very short. The bat's ping is relatively long but extremely wide-band (highly sophisticated frequency modulated). Now back to the problem of how can one improve the cross-range resolution (i.e. make a narrower beam), and also how to get many beams (with different look directions) from a single sonar. That could be done by special kind of transducer design.

Single Transducer A single transducer (used in SBES) is basically just a piece of piezo-material. It can produce one beam only. The beam width depends on the physical size of the transducer and its frequency. The higher the frequency, the narrower the beam is for a given transducer size. If you want to look in different directions in order to see a larger picture area, your only way to do this with a single transducer is to mechanically turn it (or "scan"). That is the way marine radars and low price point imaging sonars work. You cannot scan too fast, due to the necessity of waiting for the return signal or echo. This is not such an issue for marine radar, as electromagnetic waves are propagating through air with a speed of 300,000 km/s, while sound speed propagation in water is only approximately 1,500 m/s. Therefore to get the echo from a given direction at a 150 m range one must wait for 0.2 sec. (0.1 sec. forward and 0.1 sec. return) The question is: - Is this considered a long time? This depends on the application. Imagine that you want to take a look in 100 different directions, scanning approximately with a beam width increment. It should take approximately 20 seconds for the 150 m max range example above. If your boat is running at 3 knots speed (1.5 m/s) and you are not trying to image any object of less than a couple of meters size on the fly, then a mechanical scanning solution may not be a problem. However, if your boat is running at 20 knots speed, your position would change by 200 m while you are doing the scan. So it is a tradeoff between scanning time (sonar output update rate), resolution and area coverage.

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Acoustic Array The way to solve this puzzle is with an "acoustic antenna array". An acoustic antenna array is a set of many transducers. Using the array, the sonar may produce many beams at a time without mechanical scanning. This is how MBES, multi-beam imaging sonars and 3D FLS work. As a comparison to radars, this is how long range military radars and radio intelligent systems work. Modern medical ultrasound is also based on the ideas of array antenna and multi-beam sonar. With an antenna array, one can either produce all the beams at once in parallel, or go beam by beam (which is known as electronic scanning). Electronic scanning takes additional time like mechanical scanning, but there are no mechanically rotating parts, which makes the transducer more reliable. There is a special type of electronic scanning which is almost as fast as parallel beam forming called "blazed array" technology. But this kind of scanning requires extremely wide frequency band transducers. This special case is implemented only in small, high frequency, very closerange imaging sonars such as BlueView1 and cannot be scaled effectively for long range navigation. Obviously, an array is more expensive than a single transducer and, in addition to the cost of the ceramics, one must also take into account the corresponding multi-channel electronics. But when considering to choose a multi-beam sonar with an antenna array, it is important to remember it is the most precise and fastest equipment. The question is now: - Do you really need it? This will be answered later on in this paper. Now let us suggest that you are a customer who has already researched this and made the decision on an acceptable transducer size, required resolution (which in fact means you also made the decision on operational frequency) and the type of sonar. Next question is: - How far might you expect it to operate to? It depends on what does "far" mean for you. - Does "far" mean "far down below" (such as with a down-looking echosounder)? - or "far ahead" (such as with an FLS)? The concept of “far” or “range” is related to sound propagation in the ocean. While the sound propagates in the ocean water, it experiences attenuation and multi-path propagation.

Attenuation The attenuation is the acoustic energy loss in the water. The longer the propagation path, the more energy is lost. Also, the higher the frequency, the more the loss. So if you need high resolution (which requires a narrow beam), and a long range at the same time, you need a low frequency to reach that distance, as well as a large transducer to produce a narrow beam at that frequency. An example of one such system is the Kongsberg Marine EM 120 MBES. It can map the bottom all the way down to the deepest ocean, 11,000 m at the Marianas Islands. It 1 www.blueview.com

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has a frequency of 12 kHz and an antenna array size of 7 m.

Multi-Path Propagation In shallow water the sound gets reflected by both the ocean bottom and surface. At a horizontal range much greater than water depth the sound hits boundaries ("multi-bounce"), many times while propagating. At that long range the sound reflected by the same object propagates in many different paths (referred to as "acoustic rays"), with different kinds of bounces. At the sonar receive array point all these rays get mixed up, and it is hard to tell the target depth by analyzing this signal. (This is somewhat analogous to trying to see through a long narrow gap using a flashlight). Sonar engineers often state that 10 to 12 water depths is the maximum operational horizontal range limit. This is not an exact maximum limit, as it depends on how hard (reflective) the bottom is, how rough the surface is and how advanced the signal processing is. But it is definitely difficult to profile the bottom beyond 10 water depths; at that range you can usually only detect that "there is something reflecting there in this direction". At that range, the depth of the target cannot be nicely determined.

Playing with Multi-Beam Echosounders Down-Looking 2D Multi-Beam Echosounder While doing a bathymetry survey (bottom profiling), an MBES produces a fan of beams as shown in Figure 2. The beam set is 2D, while the beams look down and across the ship's course. With a single ping the sonar can tell depths for a bottom cross-section covered by the beam's footprints (pink). The total width of that cross-section is usually about 2 x mean water depth.

Figure 2: Down-looking 2D MBES

While moving straight ahead, depth measurements are ping-to-ping combined ("mosaiced") to build the strip of bottom chart. That strip is usually called the "swath". Precise ship and sonar positioning in reference to Earth is required for mosaicing. Therefore, complimentary to the sonar, one needs GPS and roll/pitch/heave/heading sensors. Next, the survey ship travels along the legs (zigzagging), in order to cover the survey area with swaths. The wider the swath, the more the acceptable spacing between survey legs is, and the less is survey time for a given bottom area. For the customer this means that the greater the bottom coverage is with a single ping, the less expensive the survey is. Note that the final

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bottom chart is 3D, but it is built from 2D slices. You can find an informative video explanation of MBES at the Kongsberg Maritime website2.

Forward-Looking Vertical 2D Now let us turn the above described beam set forward and make it a vertical slice. See Figure 3. The beam set is still 2D, with the beams looking forward and along the ship's course. The beam set is tilted towards the bottom. The upper beam is almost sliding along the ocean surface (horizontal) and the other beams looking more down towards the seafloor. If all the beams are of the same width, then the greater the distance, the larger the footprints.

Figure 3: Forward-looking vertical 2D

Figure 4 explains what we will see at the sonar output: the acoustic echo intensity distribution versus range and vertical angle, if we compensate for the attenuation loss.

Figure 4: Normalized echo intensity vs range and vertical angle

Dark blue means a low intensity echo, red means a loud echo. The bottom does not look like a solid line, as some bottom patches are good reflectors, while others are poor. At distances less than 10 to 12 water depths, the bottom depth can be measured. At the longer ranges it is hard to tell. It is only possible to say "A bright blip far away means there are some strong reflectors, which might be a navigation obstacle or might be a big rock not too high above the surrounding bottom".

2 http://www.km.kongsberg.com/KS/WEB/NOKBG0240.nsf/AllWeb/620F423FA7B503A7C1256BCD0023C0E5 ?OpenDocument

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The question is now: - Is this a result which can ensure safe navigation? If you are ready to slow down or even stop the ship after finding an obstacle in this picture, then the answer is "yes". A good example of vertical 2D is FLS Platinum from EchoPilot Marine Electronics3. If you want to figure out what is at the left and right of this single vertical slice, and find the way for safe obstacle avoidance maneuvers, then the answer is "no". You need a 3D picture.

Mechanical Rotating Is it possible to scan the beam set by utilizing mechanical rotation and then build a 3D bottom? Yes, it is. But that means: •

Every slice takes a separate ping and time for listening for the echo all way down to the maximum range.

Therefore, for example, 30 slices with a 450 m max range each means at least 20 sec to build the whole 3D picture. •

Vertical slices are not aligned, because the ship is moving.

Therefore, additional time for image post-processing is required. Taking into account that in 20 seconds the ship should travel 100 m with a 10 knots speed, this solution is hardly acceptable for large or even medium ships as that style of navigation is hardly practical and not cost-efficient. It may be acceptable for recreational boats. Note that we are now talking about the connection between obstacle avoidance sonar and ship piloting. In fact, that is the most practical approach to the whole problem of navigation safety. We shall return to that point of view later on.

Forward-Looking Horizontal 2D Now let us turn the beam set into a horizontal position and tilt it in the vertical. First, let us say the beams are narrow enough and the beams are tilted towards the bottom in order to ensure a small enough footprint. The result is shown in figure 5 . It is similar to down-looking MBES. Depth can be measured for the bottom strip covered by footprints.

Figure 5: Forward-looking horizontal 2D 3 www.echopilot.com

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The last ping's and previous pings' results can be mosaiced into a 3D bottom (ship and sonar positioning sensors required) if the ship is running straight ahead. If there is a sharp turn, the whole bottom ahead is a new bottom, never ensonified and therefore hard to mosaic. One more option in this approach is to let the beams be wide in the vertical dimension, and tilt for the angle shallow enough. A wide strip of bottom in front of the ship would be ensonified with a single ping.

Figure 6: Forward-looking Horizontal 2D, wide beams

Figure 7 depicts intensity distribution vs. horizontal angle and range, compensated against the propagation loss, for that type of sonar.

Figure 7: Normalized echo intensity vs range and horizontal angle

It is not possible to measure the bottom depth for every bottom point at long range, as the footprints are too large, but that kind of view ("radar style”) is useful as a potential obstacle alarm. Even without depth estimation, the user can understand that the large bright blip is a big reflector and possibly an obstacle. A third possible approach is to scan above said 2D horizontal beam set of narrow beams vertically and so build the 3D bottom. Again, scanning (either mechanically or electronically)

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takes additional time.

Combining Single Beam Vertically and Horizontal Scanning One example of such approach is the Interphase Twinscope4. This FLS has two transducer arrays: one is scanning a narrow beam vertically to build a vertical bottom slice directly ahead of boat while another is scanning another narrow beam horizontally to build the bottom image ahead of the boat. As both scans take time, the customers often use it as follows: "stick it in vertical mode, set the [depth] alarm, and forget about it until a reef, whale, or thick school of fish gets in the way. You can then switch over to horizontal mode to figure out your escape options"5.

Forward-Looking 3D Now imagine the sonar which produces a 3D beam set as shown in Figure 8. Such a sonar can build a 3D bottom image ahead of the ship with a single ping – without scanning (true 3D).

Figure 8: Forward-looking 3D

This is the fastest way to look forward, but it is also a lot more complex. See the FarSounder FS-3DT FLS as an example of true 3D6. The screen shot in Figure 9 shows the bottom image

Figure 9: Depth (as surface) and echo intensity (as color from green to red), 3D FLS 4 See www.interphase-tech.com 5 See www.cruisersforum.com 6 See www.farsounder.com

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(produced from single ping), of the "Black Bart" shipwreck near Panama City, Florida. The global bottom depth is 22 m. The detected shipwreck is marked with spheres (an upright bridge of 10m height above the bottom is clearly visible) with the color denoting echo intensity. The shipwreck is a large object at 165 m range. The small yellow object at 83 m is a small rock. The white "wire mesh" shows a [horizontal angle, horizontal range, depth] grid, depth down to 50 m. As before, it is possible to mosaic the images. But now seamless mosaicing can be done even if the ship is turning. Overall, here is:

What the customer may expect from the forward-looking sonar: •

•

•

Horizontal range where detection of typical navigation obstacle is available (no depth estimation) This could be quite long, the transducer installation size is a limit. If a ½ m transducer size is installable, ranges can reach out to more than 1000 m. Horizontal range where bottom depth estimation is available This is usually limited to about 10 times the global water depth. Depth measurement accuracy and depth resolution This could be quite high (close to a survey echosounder), at small horizontal ranges up to 2 water depths, as the beam footprints are small enough here. But the practical value of that high accuracy for navigation is not as relevant, as that close range is not the critical area from a navigation safety point of view – as it would be too late to avoid the obstacle at such a short range. The accuracy lessens at longer horizontal ranges.

•

Update rate This is the fastest for the true 3D FLS, but cannot be less than the 2-way propagation time, which is 1.7 s for a 1000 m horizontal range. For a scanning FLS – the update rate is at least a few times greater (for the same range).

•

Graphic user interface (GUI) These are important features, as the GUI, as well as the alarm, is what motivates the user to start an obstacle avoidance maneuver. Typically, the graphic windows selection include: a) Long range (beyond 10 water depths) sector view, radar style, with automatic target (potential obstacles), detection. b) 3D bottom view up to 10-12 water depths, with automatic depth alert.

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What the customer needs The following table explains what is critical for the FLS customer.

Alert type

Range to obstacle general case

Range to obstacle Suggested response example for: 800 m detection range 10 m global water depth

Seconds before collision example for scenario: 10 knots speed

Potential obstacle max detection ahead: range depth unknown

800 m

Prepare to maneuver (decide on escape route)

160 sec

Obstacle ahead: max depth critical depth estimation range

100 m

Emergency maneuver

20 sec

These results clearly show that time is critical. It is also important to take into account that time for safe maneuvering is at least 3 x ship_length / speed7. A recent case to consider is the bulk carrier Shen Neng 1 which grounded on the Australian Great Barrier Reef on April 03, 2010. The 225 m long Shen Neng traveled at 12 knots speed, so the time required for a safe maneuver was about 110 sec. In comparison, for a 6 m length recreational boat running with the same 12 knots, the minimum time for safe a maneuver is 4 sec. Now let's take a close look at the Royal Majesty cruise ship grounding in 1995 at Rose and Crown Shoal near Nantucket Island, Massachusetts. In this example, the ship was off course due to a GPS issue. The last 5 nautical miles of Royal Majesty's path shown in figure 10. If the true ship's position was available, then while looking at the map the user would understand that the ship was heading into dangerous shallow area. But with standard nautical chart coloring and contours it is not quite clear how dangerous the situation is. To clarify the matter, NOAA bathymetry data inside the red XY area shown in 3D and "jet" colormap (from blue to red) in figure 11. Figure 10: Royal Majesty path to grounding 7

See "Ship's Guidance and Control" by Kohei Ohtsu, www.soi.wide.ad.jp/class/20050026/slides/04/

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Figure 11: Royal Majesty grounding area, NOAA bathymetry

From that image we can see that the bottom rises up dramatically. Water depth remains deep (more than 14m) up to the very last moment, and then it becomes very shallow in just the last 600m of ship's way. The Royal Majesty's speed was 14 knots, so it took her only 85 seconds to pass these 600m. Now let's guess the ship equipped with the forward-looking sonar with 800m detection range and a 60º horizontal angle field-of-view. That sonar field of view is shown in figure 12, overlayed with NOAA bathymetry top view.

Figure 12: Royal Majesty grounding area: NOAA bathymetry top view and sonar field of view

The Royal Majesty's overall length is 173 m, so at the ship's position shown in figure 12 she still had enough space and time for the escape maneuver, if the shoal edge had been detected by FLS.

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The following picture shows the example: how FarSounder forward-looking sonar can see that kind of underwater scene.

Figure 13: Possible FLS view of Royal Majesty grounding site, 3D FLS with 800m detection range

The sharp shoal edge is clearly visible at sonar display, and the automatic alarm will sound. So FLS is a valuable instrument to prevent groundings and collisions.

Conclusions The best way to make a decision on "what type of FLS is good for me?" is to think about the interaction between the sonar and the ship's guidance and control. •

Are you an owner/captain of a large ship?

•

Do you prefer to be able to make obstacle avoidance maneuvers right after obstacle detection rather then stop or slow down and spend time investigating what is your best escape route?

•

Do you need to travel with a constant cruising speed to save time and fuel?

If you answered "yes" to all, then a true 3D long range FLS is your best option. •

Are you an owner of relatively small recreational boat?

•

Are you ready to stop or slow down when an obstacle is detected ahead and you are not sure about an escape route?

•

Is travel time not so important for you?

If you answered "yes" to all, than the less expensive scanning 2D FLS may be good enough and more cost effective for you. In any case, consider FLS to ensure your navigation - and have a safe journey!

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