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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