Advances in Military Technology

AiMT Vol. 14, No. 2 (2019), pp. 307-319

ISSN 1802-2308, eISSN 2533-4123
DOI 10.3849/aimt.01330

System of Systems Architecture for Generic Torpedo

Defence System for Surface Ships
G. Jomon*, J.V. Jojish and T. Santhanakrishnan
Naval Physical and Oceanographic Laboratory, Kochi, India

The manuscript was received on 25 April 2019 and was accepted

after revision for publication on 8 November 2019.

Abstract:
Protecting war ships from torpedo attack is considered as the most challenging task in
Anti-Submarine Warfare. Torpedo Defence System (TDS) is an essential fitment on mod-
ern high value platforms. Timely detection and identification of a torpedo attack is the
primary function of a TDS. Localisation and countermeasures against the weapon are
the next stage in torpedo defence. Detection of modern torpedoes with long endurance
can only be achieved using an underwater sensor suit with wide frequency coverage.
Tactical use of multiple counter measures is essential for evading an attacking torpedo
with intelligent homing mechanism. Individual sensors and counter measures are no
longer considered as a solution; they are rather considered as components of a large
network of systems to protect the platform from torpedo attack. A System of Systems
(SOS) architecture with multiple sensors, processing techniques, countermeasures and
tactics are presented in this paper for achieving fail-safe torpedo defence capability.

Keywords:
systems engineering, system of systems, requirement analysis, torpedo defence system

1. Introduction
‘Torpedo’ remains one of the oldest and deadliest weapons in naval warfare against
both surface and submarine targets. Depending on the size, torpedoes are classified as
heavy-weight and light-weight torpedoes. The older torpedoes use gyroscopic guid-
ance for either straight running or pattern running. However, the modern torpedoes
rely on acoustics for homing and attacking a target. Torpedo defence is a major area of
concern for both surface ships and submarines. The defence against any underwater
threat is mainly achieved through Anti-Submarine Warfare (ASW) operations, which
is a complex series of operations executed by a surface platform with the aim of de-

*
Corresponding author: Naval Physical and Oceanographic Laboratory, Thrikkakara P.O.,
Kochi-682021, Kerala, India. E-mail: jomong@npol.drdo.in
308 G. Jomon, J.V. Jojish and T. Santhanakrishnan

tecting and neutralizing the hostile submarines before they launch the weapons, mainly
torpedoes. In spite of the available advanced ASW systems, it should be taken into
consideration that nowadays, in shallow waters, the hostile submarines are likely to
succeed very often in launching their torpedoes before they are detected by the ASW
units [1]. Hence, present day maritime warfare requires systems that can directly en-
gage the attacking torpedo.
The two essential functionalities of a torpedo defence system are to detect the
torpedo and to help the platform to evade its attack. There are two approaches for the
latter task: one is hard-kill and the other is soft-kill. In hard-kill, the attacking torpedo
is destroyed by another torpedo, which is an anti-torpedo torpedo. In soft-kill, the
torpedo is lured away from the platform and its trajectory is controlled using decoys
till its battery-life gets exhausted. The soft-kill method is generally done in a layered
manner wherein the responsibility to defend the torpedo is passed on to different lay-
ers. Detection, tracking and classification of torpedo targets is the first step in torpedo
defence. Identification of the torpedo threat parameter is the next step. Executing es-
cape manoeuvre and tactical deployment of available countermeasures are the final
steps in torpedo defence.
Torpedoes work either in passive, active or mixed modes. In active homing, the
acquisition range is generally set by its ping rate. The general detection process is an
energy detection and modern torpedoes do have correlators and FFT processing [2].
Passive homing is more effective against surface ships because surface ships create
considerable noise due to propulsion. Passive homing torpedoes are more effective
against surface targets, however, they are not effective against submerged diesel elec-
tric submarines. The torpedo homing frequencies are generally in the high frequency
band because of the size limitations of the torpedo homing head. They use variable
pulse widths and multiple transmission waveforms.
Torpedoes usually follow helical pattern search. Torpedoes are limited by their
frequency of operation due to their small size as compared to the targets. Both active
and passive homing ranges are restricted by their operating frequency. To compensate
for these short ranges, wire guidance is used for extending the range. Modern torpe-
does have two-way wire communication with the torpedo fire control system.
Torpedo attack is a covert operation. Modern heavy-weight torpedoes attack
a ship either in autonomous or wire-guided mode. Torpedoes always run at higher
speeds compared to ships or submarines. Protecting war ships from torpedo attack is
considered as the most challenging task in Anti-Submarine Warfare. The main func-
tions of a Torpedo Defence System (TDS) are to detect, localize and to take evasive
and counter actions against an attacking torpedo [3]. Typical configuration of a torpe-
do defence system for surface ships is shown in Fig. 1.
System engineering practices are followed in developing defence systems. Sys-
tem Engineering is a robust approach to the design, creation, and operation of systems
[4, 5]. TDS is a complex system, which can be considered as a unique model of Sys-
tem of Systems (SOS). A system of systems is an arrangement of set of systems that
are heterogeneous and independently operable on their own but are networked together
for achieving unique capabilities. Both individual systems and SOS conform to the
accepted definition of a system in which each consists of parts, relationships, and
a whole that is greater than the sum of the parts [6]. SOS architecture is capable of
achieving operational capability beyond the level on which the systems can provide it
independently. This will demand more information sharing between systems and in-
 System of Systems Architecture for Generic Torpedo Defence
309
System for Surface Ships

tern certain design modifications of systems. SOS architecture should include the fol-
lowing details:
• concept of operation: How the system is going to be operated by users during
an operational scenario,
• system details including data flow and functional information,
• end to end functionality with communications protocols.

Receiver Array
Hull mounted
array
Expendable
Decoy Towed Decoy

Fig. 1 Torpedo defence system configuration for surface ships

2. Generic Torpedo Defence System

TDS can be viewed as combination of complex systems working together cohesively
to achieve the common goal of defending torpedo attack on a surface ship. Detection,
classification, localisation and effective deployment of counter measures are the major
objectives of TDS, as described in Fig. 2. A generic contemporary torpedo defence
system comprises of five distinct elements:
• a set of acoustic sensors which detects an inbound torpedo threat,
• a highly automated target classifier to identify torpedo targets,
• target localization algorithm to localize torpedo targets,
• an automated information processor for threat evaluation, and tactical advice
for evasive manoeuvring and deployment of countermeasures,
• a range of countermeasure solutions such as towed decoys and expendable
decoys designed to lure the incoming torpedo away from its intended target
(soft-kill) or destruction of the torpedo (hard-kill).
The technical challenges encountered when deploying such systems cannot be
underplayed. Increasingly quiet propulsion systems mean that modern heavy-weight
torpedoes can now approach their targets in ‘near silent’ mode; bearing ambiguity
resolution is required to provide port/ starboard discrimination; a low false alarm rate
is necessary to ensure operator’s confidence and to conserve countermeasures ex-
penditure; and countermeasures themselves must be of sufficient power and fidelity to
ensure that they are effective against modern acoustic homing torpedoes.
At the moment, TDS systems are available for highly manoeuvrable small vessels
having limited Detection Classification and Localisation (DCL) capability with too
many false alarms and fairly effective soft-kill. Efforts are made to improve the DCL
significantly and the effectiveness of the soft-kill. Parallel to that, hard-kill systems are
under development, which are necessary for ‘close-in defence’ [7, 8]. One has to bear
in mind that torpedo defence systems can only protect satisfactorily if an underwater
310 G. Jomon, J.V. Jojish and T. Santhanakrishnan

sensor suite is used that has a full 360° coverage and has got sufficiently long detection
ranges under different sonic layer conditions. This means that hull-mounted sonar
combined with a variable depth towed array will be required to meet various operating
scenarios.

Torpedo
Localisation

Torpedo
Torpedo
Counter
Classification
Measures

Target
Detection

Fig. 2 Objectives of Torpedo Defence System

The following objectives are to be essentially met by the TDS:
• panoramic detection and tracking of all targets in the area of operation at max-
imum range,
• classification of torpedo targets at the earliest,
• localisation of torpedo targets using Target Motion Analysis (TMA) algorithms,
• threat analysis and recommendation of evasive manoeuvres and use of counter
measures,
• effective use of countermeasures to evade torpedo attack.

3. Torpedo Defence System: Concept of Operation

Submarines are lethal because of certain specific features. One of the most important
peculiarities of a submarine is stealth. The medium on which the submarine operates
makes it a very discreet and elusive target. They are slow and have long endurance on
periscope depth. However, they have limited endurance in submerged conditions due
to battery power. Since they operate in a quiet environment, the sonar sensors have an
additional advantage as compared to surface counterparts. They are vulnerable for
attack by surface units or air units and submarines efforts shall always be made to
avoid detection. They are lethal because they can very well fire a torpedo against
a detected target. They also carry missiles which are typically used against a receding
target.
To effectively detect a target, submarine uses a variety of sensors. The principal
sensor is sonar which shall work in multiple modes. The most common mode is listen-
 System of Systems Architecture for Generic Torpedo Defence
311
System for Surface Ships

ing to the radiated noise of the target by the passive sonar. The passive sonar has inter-
cept modes also to detect transmissions if any. Following that submarine can use its
active sonar, however, by operating it, the submarine compromises itself.
For obtaining the Fire Control Solution (FCS), the following sequence is carried
out by the submarine. The target is first detected, and then classified. The target mo-
tion parameters are obtained by carrying out certain manoeuvers which will give the
required parameters of the target, namely, bearing, range, course and speed. It will
then select a suitable weapon, most probably a torpedo, and it will attack the target as
per FCS recommendations. Thereafter, it has to get away to a safe location.
Torpedo defence systems typically follow the principle of layered defence which
utilizes detection, classification, localization, workout escape tactics and deployment
of countermeasures. The detection system is further divided into outer and inner layer
defence with respect to the ranges of detection. TDS should be capable of detecting all
the targets around the platform under all operating scenarios. Long-range detection
including left-right ambiguity resolution can use a towed array sonar, whereas the
hull-mounted sonar helps detect a torpedo at closer ranges and also from the head
sector [9, 10].
Unlike conventional ASW where target is persistent, in torpedo systems, the tar-
get appears suddenly and is alive for very short time, either achieving its mission or
failing. This demands that instead of operator detecting torpedo, the system has to
detect it automatically. Robust torpedo classification algorithms with very low false
alarm rate is necessary to ensure operator’s confidence and to conserve countermeas-
ures expenditure.
Torpedo detection is achieved typically using passive sonars. Passive sonars can
provide only the target bearing information. Complex TMA algorithms are essential
for localising the target from its bearing only information. Own ship manoeuvring is
essential for computing target motion parameters. But, there are restrictions in own
ship manoeuvring due to operational and safety reasons. Robust TMA algorithms are
essential for calculating target parameters with minimum own ship manoeuvre. The
TMA algorithms should accurately localise the attacking torpedo by computing its
course, range and speed.
Threat analysis is another important requirement of a TDS. Threat analysis sys-
tem assesses all possible threats in the vicinity and it assigns a threat priority level for
all the targets [11]. The system will recommend a course to steer, as well as a coun-
termeasure deployment to maximise escape probability. The system recommends
possible escape manoeuvres and tactics to deploy expendable and towed decoys. De-
ployment of hard-kill systems also will be recommended by the system.
Once detected, the inner layer of defence is achieved by acoustic counter
measures [12, 13]. Towed acoustic decoy and expendable decoys are having a unique
capability of seducing active and passive homing torpedoes. The towed acoustic decoy
is a programmable decoy enabling the ship to transmit any type of signature including
continuous wave, modulated waveforms, broad band noise, narrow band noise and any
kind of amplitude modulation enabling deception of modern light and heavy-weight
torpedoes. Also, the decoy carries intercept sensors, which can detect active transmis-
sions from torpedo. The decoy can function in multiple modes, namely the echo
repeater, broadband jammer and in autonomous mode. Hard-kill systems can also be
used for evading a torpedo attack. Sequence of operations of a generic TDS system is
described in Fig. 3.
312 G. Jomon, J.V. Jojish and T. Santhanakrishnan

Detection and tracking of targets using

acoustic sensors

Analysis of target parameters and

identification of torpedo targets

NO
Is it a torpedo?

YES

Threat analysis and Recommendation

of counter measures

Tactical use of counter measures

Fig. 3 TDS Sequence of operation

4. Requirement Analysis
Translating user requirements to system specifications is an essential stage during
system design phase [14, 15]. Many rounds of interactions and iterations are essential
for capturing the user requirements. The mapping of high-level user requirements to
system specifications forms the foundation for building the system capability. Systems
engineering describes the system requirements as operational, functional and physical
requirements [16].

4.1. Operational Requirements

The operational requirements of a TDS for surface ships shall specify the detailed
operational specific requirements, such as basic mission requirements, performance
needs, operational boundaries etc., as given in Tab. 1.

4.2. Functional Requirements

Detection, tracking, classification, localisation and decoying of an attacking torpedo
are the major functions to be carried out by a TDS. These functions can be realised by
selecting proper system configuration and by using signal processing algorithms.
Functional requirements of TDS are given below:
• detection of targets using wideband acoustic sensors,
 System of Systems Architecture for Generic Torpedo Defence
313
System for Surface Ships

• use of variable depth sensors to enhance detection probability,

• use of adaptive and advanced signal processing techniques for target detec-
tion [17],
• automatic target detection and tracking,
• robust torpedo classification algorithm using artificial intelligence and neural
networks,
• multi-sensor data fusion and use of Kalman filter for target localization [18],
• simple and effective Human Machine Interface,
• use of multimode intelligent acoustic decoys for torpedo counter measures,
• destroying of attacking torpedo using anti–torpedo weapons.
Tab. 1 Operational requirements of TDS
No. Category Requirements
1 Mission Requirements • to detect, track, classify and locate active, passive
and wake homing torpedo attacks
• to decoy acoustic and non-acoustic homing torpe-
does
• to destroy an attacking torpedo using hard-kill
rockets
• locating and initiating counter attack on torpedo
firing platform
2 Performance • long-range torpedo detection at firing range
Requirements • detection and localisation of torpedoes using pas-
sive, active or intercept sonars
• detection of targets at shallow water and deep water
and at different acoustic layers
• protecting ship from torpedo attack using decoys or
using hard-kill options
3 Operational Needs • automatic operation with minimum human inter-
vention
• quick deployment and retrieval of the system
• high probability of detection and minimum false
alarm rate
• fail-safe system with adequate redundancy
• system should be able to fit on all major classes of
surface ships
4 Operational • target detection and tracking at own ship speed
Boundaries upto 22 knots
• system should withstand escape speed of the plat-
form
• basic operations up to sea state 4 and system should
withstand all sea states

4.3. Physical Requirements

Physical configuration of TDS consists of wet end systems, launch systems, onboard
electronics systems and counter measures and their launching systems as shown in Fig.
4. Wet end systems comprise of acoustic and non-acoustic sensors which are in direct
314 G. Jomon, J.V. Jojish and T. Santhanakrishnan

contact with sea water. Launch systems are winch and handling systems for launch and
retrieval of wet end systems. Onboard electronics houses the processing and display
subsystems for processing sensor data and displaying the results. Acoustic decoys and
anti-torpedo rockets are launched from pneumatically controlled launchers for defend-
ing torpedo attack.

Onboard Counter
Wet end systems Launch systems electronics measures

Fire Control Decoy

Winch System Launchers
Towed Array
system

Intercept Display Launchers for

Array Processor weapons

Signal
Hull Mounted Processor
Array

Power
Amplifier
Flank Array
Ship
navigational Recorders
sensors
Non Acoustic
Sensors

Fig. 4 Physical configuration of TDS

5. SOS Architecture for TDS

Defending torpedo attack is the most challenging task in underwater warfare. Inde-
pendent systems working in isolation are not sufficient to defence torpedo threat.
A consortium of sensors and countermeasure systems working with synergy under
common command and guidance system is essential for achieving this most difficult
task. A system of systems architecture for TDS is presented in Fig. 5. Major building
blocks of TDS and their functional details are explained below.

5.1. Acoustic and Non-acoustic Sensors for Torpedo Detection, Tracking and Clas-
sification
TDS essentially requires different types of sensors for effective torpedo detection
under various operating conditions. The major considerations while selecting the sen-
sors are its operating band, depth of operation and possibility of positioning it away
from self-noise. The following five types of sensors are essentially used in TDS:
• hull-mounted array,
 System of Systems Architecture for Generic Torpedo Defence
315
System for Surface Ships

• towed array,
• intercept array,
• flank array,
• non-acoustic sensors.

Threat perception and

Information Processor
Tactics
1. Target parameter
1. Threat Evaluation
extraction
2. Threat Prioritisation
2. Data Association
3. Evasive Tactics
3. Threat identification
4. Tactics for use of
4. Localisation
counter measures

Command and Control

1. Sensor coordination
2. Managing
information processor
3. Evasive maneuver
4. Deployment of
counter measures
5. Engaging counter
attack

Counter Measures
1. Acoustic Towed
Sensors
Decoy
1. Hull mounted Sonar
2. Acoustic Expendable
2. Towed Array Sonar
Decoys
3. Intercept Sonar
3. Decoys against wake
4. Flank Array sonar
homers
5. Non Acoustic
4. Mobile Decoys
Sensors
5. Hardkill rockets
6. Counter attack on
enemy platform

Fig. 5 SOS architecture for TDS

Identification of torpedo targets is an essential requirement of TDS. All counter-
measure actions are initiated based on this classification. Target parameters extracted
using different sensors are used for torpedo classifications. Bearings, frequencies,
modulation frequencies are some of the features used for torpedo classification. Tor-
pedo specific characteristics are different for different stages of torpedo run. Torpedo
Confidence Level (TCL) is determined after analysing available information. Torpedo
alarm is generated when TCL crosses set threshold.
316 G. Jomon, J.V. Jojish and T. Santhanakrishnan

5.2. Information Processor for Target Localization

The TMA algorithm running on the Fire Control System (FCS) accurately computes
the bearing, range and course of the torpedo at any instant using the target bearing
derived from passive sonar sensors. Kalman filter based TMA algorithms is commonly
used. Target parameters can also be derived using data association technique. The
target observed bearings observed by different sensors located at different geometrical
locations are used for computing target parameters.

5.3. Tactics for Effective Torpedo Defence

In a multi torpedo attacking scenario, threat evaluation and threat prioritization are the
first step for torpedo defence. Recommendations for evasive manoeuvre and tactics for
counter measure deployment are finalised based on the threat scenario. Evasive ma-
noeuvre is finalised as per the torpedo counter attack tactics. Sequence of operation of
counter measure devices and their modes of operation are finalised as part of torpedo
defence tactics.

5.4. Command and Control Unit

Command and control system is the most important system in TDS. This unit coordi-
nates activities of various systems in the SOS architecture. Command and Control Unit
analyses the situation based on data available from various systems and take tactical
decisions to maximise escape probability in a given scenario.

5.5. Acoustic and Non-Acoustic Counter Measures

List of countermeasure devices used as part of TDS are listed below:
• acoustic towed decoy,
• acoustic expendable decoys,
• mobile decoys,
• decoys against wake homers,
• hard-kill devices,
• counter-attack on enemy platform.
Acoustic homing torpedoes can be seduced and misguided using false acoustic
sources called decoys [19, 20]. Towed, expendable and mobile decoys are commonly
used against both active and passive homing torpedoes. Wake-disturbing decoys are
against wake-homing torpedoes. Anti-torpedo devices are used as explosives to de-
stroy attacking torpedoes. The last stage in torpedo defence is attacking enemy
launching platform through a counter attack to prevent further attacks.

6. Data Flow and Communication

SOS architecture of TDS is an intensive communication system. It follows distributed
communication network architecture with a central command and control unit. Differ-
ent systems in the SOS are working independently to achieve a common goal of
torpedo defence. Command and Control Unit will coordinate between systems to
achieve best results. Systems are communicating through Ethernet links using TCP/IP
or UDP protocols. System to system interface protocol is clearly defined. Dataflow
and communication diagram between systems are shown in Fig. 6.
Target bearing information received from sensors is used for torpedo identifica-
tion and for computing torpedo motion parameters like course, range and speed. This
 System of Systems Architecture for Generic Torpedo Defence
317
System for Surface Ships

information will be used by tactics processor for evaluating the threat and to workout
escape tactics. Based on the recommendations of the tactics processor, Command and
Control Unit will deploy suitable counter-measure to evade torpedo attack. Command
and Control Unit is getting inputs directly from all the systems. If the normal data flow
is affected due to malfunctioning of any of the participating systems, Command and
Control Unit will get available data through alternate route. Standard communication
protocols are proposed here to ensure future system growth by adding more systems
and for smooth future upgradations.

Sensors

Target Parameters

Information Processor

Torpedo motion
parameters

Tactics Processor

Recommendations on Deployment of
tactics counter measures

Counter
Counter measures
Measures

Fig. 6 Dataflow and communication diagram

7. TDS with SOS Architecture vs. Contemporary Systems

Defending torpedo attack on a surface ship is a complex task. Wide variety of sensors,
countermeasures, information processing and decision making are involved in achiev-
ing this difficult task. Contemporary torpedo defence systems try to defend torpedo
attack by deploying available set of sensors and counter measures. These systems are
generally deployed for defending specific classes of torpedoes. These independent
systems without proper centralised command and control system may not be sufficient
to defend modern intelligent torpedoes. TDS with SOS architecture is a collection of
task-oriented and dedicated systems that pool their resources and capabilities together
to create a complex system which offers more functionalities and higher performance
than simply a sum of the constituent systems. TDS with SOS architecture is having the
capability to defend wide spectrum of torpedo threat. This system provides holistic
view of the problem and provides solutions as per threat perception analysis. Central-
ised command and control system with distributed communication network is the
backbone of SOS architecture.
318 G. Jomon, J.V. Jojish and T. Santhanakrishnan

8. Conclusion
An SOS architecture for TDS system is presented in this paper. TDS system based on
this architecture is capable of defending any potential torpedo attack on a surface plat-
form. This system is capable of detecting and tracking of all types of torpedoes and
decoying acoustic and wake homing torpedoes. TDS is a mission critical system. Ro-
bustness of the system is paramount. SOS architecture ensures redundancy for all
critical functions. The uniquely distributed communication network with a central
Command and Control Unit makes the system more robust against failures. A consor-
tium of sensors and countermeasure systems working with synergy under common
command and guidance system is the main advantage of SOS architecture. Coordina-
tion of geographically distributed and functionally independent systems for achieving
the most difficult task with perfection is achievable though this architecture. TDS with
SOS architecture is an essential fitment onboard all high-value platforms. Independent
systems without centralised command and control system are not appropriate to defend
surface ships against the threat from modern intelligent torpedoes.

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