The document summarizes an integrated underwater surveillance system consisting of new active and passive sonar systems. The LASAR 40 active sonar system can detect divers out to 1100 meters, longer than standard diver detection sonars. It operates between 30-45 kHz for optimal performance in ports. The system takes a layered approach to surveillance across multiple zones and can detect and track both divers and marine mammals like dolphins.

1. Integrated underwater surveillance system.
Arne Løvik, Arnt Rune Bakken, Endre Marken and Martin August Brinkmann
Kongsberg Defence & Aerospace as
Kirkegårdsveien 3, 3600 Kongsberg, Norway
Abstract — The C-Scope integrated surveillance is a well
established system in the vessel traffic management area II. ABOVE WATER SURVEILLANCE
and has over the past years been complemented with a Protection against illicit underwater activities in
novel underwater surveillance and protection part with a coastal areas, ports, harbors or in confined areas is
layered system approach. The new development of both complicated by the reverberant conditions, and the
active sonar systems for longer range and passive systems
normally high levels of surface traffic.
for large area detection and classification has been
completed. The paper introduces the new wide-band To ease the burden on the operator, the system
sonar units and gives some insight to the implemented should only give a warning when something is unusual
classification concept. The active sonars cover the or abnormal and requires attention. This puts severe
frequency range from 3-8 kHz and 30-40 kHz, while the requirements on the signal processing and the ability to
passive system is from 10-5000 Hz. The paper will reliably reduce the number of 'false alarms' (i.e. events
concentrate on the active part of the systems and gives not requiring the attention of the operator.)
examples with results from some installations.
It is therefore very important to combine above-
Keywords-component; surveillance system, sonar water information with sonar information in order to
improve on classification and thus reduce false alarms.
I. INTRODUCTION There are many VTS (Vessel Traffic Service) and Port
Management Systems installed in important ports
New changes to port security address not only the around the world. These systems already handle all
normal or friendly traffic, but also systems that secure important above-water sensors. Therefore, it was
the normal port operation, and deter and prevent any decided to integrate the sonar information in an existing
hostile operation against the harbor. platform, the C-Scope Vessel Traffic Management
The mere cost of an incident that halts normal System, in order to provide a common operational
operation of a major port will incur significant picture on track level, for targets above and below the
economical and operational consequences to a country surface.
or region. In this paper we will discuss threats from The system operates 24/7 with a minimum number
intruders, such as surface vessels, underwater vehicles, of operators. It was therefore vital that all information
divers and swimmers, all being targets that will hide from above- and underwater sensors are collected,
their real identity and intent. sorted, tracked and fused.
Diver detection sonar systems with detection ranges Fused tracks are compared with ship data in
from some hundred meters to 600-800 meters on a good databases, and alarms of underwater activities are given
day have been available for some time. But before only for sonar tracks that pass alarm zones and do not
countermeasures or reaction procedures can be set into correlate with expected targets on the surface. Based on
operation, the security system will have to perform all the results form the trials the following features are
the tasks from detection, classification, alerting the
recommended:
operator to confirmation of the threat being real by the
operator before the operator chooses the best reaction • Open system architecture.
procedure for the incident.
• GIS (Geographic information system) that
Studies have shown that the time to perform these shows tracks and detections in geo-referenced
tasks will be very limited, and thus work was standard sea charts and land maps in both 2D
undertaken to develop sonar and systems with larger and 3D.
detection ranges and integrate them into a system
• Map layers that can be used by the operator to
allowing the timely performance of port security.
display available information for the area.
The project discussed in this paper began in 2007,
• Advanced sensor fusion covering all above- and
aiming to develop and test new long-range awareness
sonar systems in a real environment at Haakonsvern underwater sensors.
Naval base, which could form the basis for a permanent Fig. 1 shows an example of fused tracks, giving the
installation at the base. operator a coherent view of all large surface vessels in
The sonar was to be integrated into a system the area.
including other above- and below-water sensors and
reactors.

2. Figure 1. C-Scope Tracking service
Figure 2. Layered approach with three zones.
It is also important to be able to study an incident in
detail, for example to investigate the cause of an The threats perceived in the different zones are
automatic alarm. The C-Scope therefore has Recording normally
and Replay service taps that feed system data back Inner zone: divers, UUV, scooters, RIB
exactly as it appeared, with complete detail of the
following: Middle zone: DDV, UUV, Midgets, RIB
Radar, sonar video Outer zone: submarines. UUV, Midgets, other
Radar, sonar tracks,AIS tracks and fused tracks The targets to be detected are generally smaller and
slower as we move towards the final assets in the inner
Operator inputs zone.
Images from the CCTV system In addition to the zone structure, two other
Audio from the maritime communications system approaches are often found, one is the total volume
coverage and the second being control of choke points.
Bearing lines from the direction finders.
For the early warning and preparedness, the more
layers the more awareness is obtained.
III. UNDERWATER SURVEILLANCE AND PROTECTION
(USP) The choice of the volume or choke approach is often
An important factor in underwater surveillance and one of cost and severeness of the threat to the area in
protection is providing sufficient time for the operator question.
to react. For a system to be complete different sensors will be
To achieve maximum time for reaction a layered required for the different zones.
approach is often considered. An example is shown in The other zone is more like ASW type sonar, while
the Fig. 2. the inner zone by many has been compared to MCM
operations.
Further different reaction measures must be
included to move from a surveillance system to a
protection system. In this paper the reaction side of the
system will not discussed, however the KONGSBERG
C-Scope system may be fitted with reaction units from
the soft warning to the hard kill type.
Going back to the sensor side the range of
application from ASW to MCM creates a set of
different sonars dedicated for operation in the near
coastal environment and with the requirement of having
extremely low false alarm rate and with a high degree of
automation in the decision or classification process.
For the coastal and harbor surveillance
KONGSBERG has developed in cooperation with FFI
and the RNoN a set of new wide-band sonar systems.

3. The active sonar is called LASAR, Long Range new and challenging problems for the design of the
Awareness sonar and the passive counterpart PASAR. beamformer and a novel and flexible approach has been
developed [3]. The adopted projection method with
The PASAR operates in the frequency range 10- suitable tapering works extremely well with these
5000 Hz while LASAR has different frequency bands,
nonlinear array.
3-8kHz, 10-20kHz and 30-45kHz.
Longer arrays may be used in ports with very high
The active sonar LASAR can operate in active and
reverberation, and the bandwidth can be chosen
passive mode simultaneously, or separately. For the depending on the environment, giving a range
LASAR 40, active band is 30-45kHz and the passive
resolution from a few cm and up. The sonar is therefore
band covers 1-45kHz. In combined mode the passive
very adaptive to different environmental conditions.
part is mainly used for detection of fast going smaller
surface crafts and for classification on its own or in The frequency bands covered are:
combinations with the active retrieved information.
Active: 30 - 45 kHz
From this discussion it should be clear that the
LASAR 5, 3-8kHz sonar is aimed for the outer zones Passive: 1- 30 kHz
giving long range detection of larger targets such as The transducer unit may be mounted on the seabed,
submarines, UUV and midgets. The PASAR is often over the side of a vessel or as a pier mount. The
used in combination with the LASAR 5. The transducer processing unit is made in one compact portable unit
array of the LASAR 5 is a linear array giving angular suited to meet the operational requirements. The first
resolution of 0,7 degrees and less. two units were installed at Haakonsvern Naval base and
In the following a discussion of the LASAR system the next section presents some of obtained test results.
is given exemplified by the LASAR 40.
V. CLASSIFICATION
IV. LONG RANGE AWARENESS SONAR: LASAR 40 The over all classification scheme consists of two
separate branches:
The LASAR 40 was developed in the project as a • one related to attributes associated to the target
part of the test installation at Haakonsvern Naval base as such
in Bergen, Norway. The main goal was to detect small
targets such as divers or unmanned underwater vehicles • the other to the behaviour of the target specially
(UUVs) consistently, at a range of 1000 meters or more in relation to the perceived threat.
The sonar is designed to be installed at fixed points The target related attributes are typically among the
in a port, or alternatively as a moveable containerized following
system for port security, or as a portable device for fleet • echo strength
protection. To help in classifying targets, the system
includes a passive chain in parallel to the active part. • echo variation with aspect
Internal studies [2] and others [1] show that the • echo variation with frequency
'optimal' frequencies for this type of application are
• target speed
more in the range of 30 to 40 kHz than around 80 to 100
kHz, which is today commonly used for diver detection • target acceleration
sonar.
• target turn rate
Another variable in a busy port is the wake from
• emitted noise
passing vessels.
Many studies have been performed on the effects of • noise spectrum or characteristics
bubbles in the water and the caused attenuation. Some To define the attributes above from the returned
of the effects are related to the bubbles generated by the echoes is in itself an interesting task, the target strength
wave motion, i.e. related to the sea state. The general for instance could be related to the peak value in and an
conclusion is clear: the higher the frequency, the more area of returned energy or to the average of some sort.
the effect on sound propagation. This follows from the Further a number of additional parameter could be
fact that the smaller bubbles live longer and are more defined related to the echo mass; its extent, major axis,
abundant than the larger ones. perimeter or others.
To preserve the portability and adaptability to The various attributes will have at least two values,
different geometries, a modular system was conceived a static on and a temporal. The static giving the value
with a single separate transmitter and a number of linear for the attributes per ping and the temporal giving some
receiving arrays. The receiver may be configured to measure of it temporal variation or statistical behavior.
provide a 60 to 360 degree field of view, without any
loss or difference in the performance. The flexibility in For the classification process the most important
the number of receiving arrays makes it possible to factor is to select those attributes that will give the
optimize the sonar to environmental conditions in “best” classification result for the group of targets
different ports. involved. Best classification will normally mean a
classification that will yield high rate of correct
However the configuration with linear array forming classification and low rate of false classification.
the receiving array for the different geometries imposes

4. Figure 3. Scatter plots with density estimation, (a) for a static set and (b) for a temporal set for object classes diver and marine life.

5. Two example of a set of such attributes are shown in temporal set.
the Fig. 3. The Fig. 3 (a) for a static set and (b) for a
The values of the attributes related to the target can All negative values of K is not threatening behavior,
be used in a probability based classifier such as Naïve likewise zero is no problem. When the K value
Bayes classifier. Bayesian classifiers assign the most approaches 1 the behavior is clearly hostile. Fig. 4
probable class to a given example described by its shows experimental values of the K for unknown
attribute vector. Learning such classifiers can be greatly targets and for diver threats. The figures show that K for
simplified by assuming that the attributes are diver threats has a narrow distribution around 1, while
independent variables for the given class. K has a much wider distribution for unknown targets.
Let X = (x1, ..., xn) be a vector of observed random
variables, called attributes. The Naïve Bayes classifier VI. SONAR AND SYSTEM TESTS
will then give the most probable class Ci. The The installation and test at the Norwegian
probability of having class Ci given the attributes X, is Haakonvern Naval base has now been underway since
by Bayes’ theorem; early 2007, with different test systems and sonar heads.
The latest is the prototype LASAR 40 of which two
heads are installed along the main pier. The transducers
. are bottom mounted.
Tests have been devised to look at detection range
The conditional probability P(X|Ci) is the against various targets, their aspect and the seasonal
predictions that the model makes about the data X when variations, among others. The achieved detection ranges
its parameters have a particular value x. The “prior” have been compared with predictions using the sonar
distribution P(Ci) states what values the model’s performance model LYBIN, supplied by the Royal
parameters might plausibly take. The normalising factor Norwegian Navy [4].
P(X) is only based on the values the attributes take on. In general, there is a good correspondence between
Bayesian Classifiers are known to be the optimal the simulations and the obtained results. There is a
classifiers, since they minimize the risk of consistent detection to 1100 meters or more against
misclassification. However, they require defining divers with closed or semi-closed systems with the
P(X|Ci), i.e. the joint probability of the attributes given prototype of the LASAR 40.
the class. Estimating this probability distribution from a The production model is now under delivery to the
training dataset is a difficult task, since it may require a YUNUS project in Turkey for the protection of their
very large dataset even for a moderate number of two largest naval bases. This integrated underwater
features in order to significantly explore all the possible surveillance system is based on a volume coverage
combinations. approach and counts many sonars to cover the area in
In the framework of the Naïve Bayes Classifier, the mono and bi-static mode of operation.
attributes are assumed to be independent from each Fig. 5 shows an illustration from another installation
other given class. This allow us to write the P(X|Ci) as: where both diver and mammals are detected and tracked
correctly.
.
The Naïve Bayes Classifier is therefore fully defined
by the conditional probabilities of each attribute given
the class.
When it comes to the behavior pattern it will
normally be related to the threat the behavior imposes.
Typical patterns for a diver with a dedication to reach a
goal, an erratic behavior of a school of fish, the tidal
Figure 4. K factor for unknown targets and for diver threats.
variation of clutter and many more.
A first simple but efficient measure of the behavior
is the travel distance compared to the distance travelled
towards the area of protection, high value asset. This
measure K can be defined by
,
where v is the speed of the target and vn is the
component of the velocity vector towards the high value
asset, positive towards the asset, negative away.
5

6. Figure 5. Operator display with diver (red) and dolphins (green).
VII. SUMMARY
This paper has presented a set of new sonar systems,
active and passive for coastal and port surveillance. The
LASAR 40 sonar is lower in frequency than standard
diver detection sonar and gives longer detection ranges
than those normally achieved. The major benefit of
longer range come in the volume coverage approach to
surveillance as opposed to a choke point approach
where range necessarily is a prime factor.
The low frequency LASAR 5 gives large volume
coverage in coastal areas for detection of larges targets,
submarines to midgets.
To reduce the burden on the operator a sophisticated
classifier has been introduced. The classifier uses
attributes associated with the target itself and with the
behavior of the target.
Tests of the systems in different waters and
environmental conditions show very robust and good
detection and classification performance for the
automatic sonar processing system.
REFERENCES
[1] M. G. E. D. Colin, S. P. Berens and M. A. Ainsie, (TNO),
“Optimal frequency for diver detection sonar,”.
[2] A. Løvik, H. Aagedal, E. Mathisen and E. Marken (Kongsberg
Defence & Aerospace). ”Examples of the Combined Use of
Active and Passive Sonar for Underwater Harbour
Surveillance,” UDT UK, 2008.
[3] R. Otnes (FFI) and Nanna Skjei (Kongsberg Defence &
Aerospace), “Projection-based tapering for conventional
beamforming on nonlinear sonar arrays,” Oceans USA, 2010.
[4] K. T. Hjelmervik (FFI), E. M. Dombestein (FFI), T. S. Såstad
(FFI), J. Wegge (FFI) and S. Mjølsnes (NDLO), “The Acoustic
Raytrace Model Lybin – Description and Applications,” UDT
UK, 2008.
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