Underwater Protection System - IEEE Xplore

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Underwater Protection System Arne L0vik*, Arnt Rune Bakken*, Johnny Dybedal* Tor Knudsen**, Jon Kj0ll**

*Kongsberg Defence &Aerospace AS, Kirkeg'ardsv. 45 3600 Kongsberg, Norway "

Norwegian Defence Research Establishment, FFI, Horten, Norway

With the emerging asymmetrical threat to harbours, vessels and areas of special interest the need for both fixed and portable protection systems are imminent. This paper describes a system approach to the problem of protecting vital assets from divers and swimmers all the way from the initial detection through to the choice of reaction including the use of UUV and non-lethal acoustic deterrent units. A vital part of the system described is a novel new sonar system using wideband technology for increased sonar performance in shallow and highly reverberant environments. Further the frequency range has been lowered compared to the standard diver detection sonar of to-day, to give a potential for improved detection ranges and thereby provide more time for carrying out an appropriate reaction. The wideband technology may also lead to a new set of classification tools. The paper outlines the system and specifically discusses tests done in harbours and enclosed environments with the sonar system against divers and UUVs. Detection and classification issues are discussed and examples of obtained system performances are given.

1. INTRODUCTION The increased focus on security for fixed installations and own ships in out-of-area situations has lead to the development and delivery of both above and underwater protection systems. This paper will concentrate on the latter bearing in mind that the information from the above water situation may prove useful in the process of false alarm reduction. The work to be presented here are threefold: one part is sonar systems for confined water, aiming to detect small and difficult targets in difficult environments, secondly to have underwater vehicles for inspection, survey and issuing countermeasures and finally the sensor integration, data fusion and overall sonar operation. The intruder protection capability is composed of these elements, the ability to detect and classify and the ability to react. The integration of these capabilities in a command and control unit is essential for the total system capability.

2. SYSTEM DESIGN CONSIDERATIONS Coastal protection underwater, being in a harbour or in a confined area, will be complicated by the reverberant conditions and the normally busy traffic in the area. As with most systems of this kind that operates 24/7 it is of prime importance to ease the burden of the operator. Thus the philosophy in the design has been to let the system only give a warning when something is unusual or abnormal and requires the attention of the operator. This again puts severe requirement to the signal processing and the ability to reliably reduce the number of "false alarms" i.e. events not requiring the attention of the operator. An important factor in the design of a system is to provide sufficient the time to react. That is the time from detection through classification, tracking, deciding which procedure to follow and finally implement adequate reaction in time to prevent any damage. Thus the time needed from detection to reaction may be substantial and thus require a large detection distance of the sonar system. If we consider perceived underwater threats to be divers, assisted divers, UUV and mammals delivering weapons, the speed of advance will differ considerably from some 0.5m/s to 6 m/s. Time to reach target is illustrated as a function of distance in the figure below.

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Figure 1 Typical travel times as afunction ofdistancefor diferent possible threats

This time has to be compared to the sonar detection range and the sum of the times required to track and classify and the time to react. In Figure 1 the time needed to detect, classify and initiate reaction is assumed to be 8 minutes. Thereafter the countermeasure, or reaction force, is assumed to travel to the target with a speed of 3 m/s. The total time needed to position the reaction force or the countermeasure near the target is therefore shown as "time to acquire" in the figure. The times here are illustrative but indicate that a free swimming diver may be stopped if detected before some 400 meters while a diver with an underwater delivery vehicle would need to be detected at around 1500 meters. The use of trained mammals offers a difficult if not impossible task both due to the speed and the low target strength. Thus it is clear that a very important requirement to the sonar system is a sufficiently large detection distance for small targets under difficult and reverberant environmental conditions. The good thing however is that the geometry for the system is constant, while the environment varies with tide, seasonal and traffic in the area. The targets will have low Doppler shift with the exception of mammals, which in turn may be the least likely ones to appear. Long range detection requires low frequency systems. This is easily seen by examining Figure 2 which shows the signal attenuation as a function of range for three different frequencies. The echo at two km range from a 20 kHz sonar suffers about 97 dB less absorption loss than that from a 100 kHz sonar.

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Figure 2 Two way attenuation

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of range for the frequencies 20 kHz, 40 kHz and 100 kHz. (Francois and Garrison JASA 72 sept 82, JASA 72 dec 82)

The preferred sonar for diver detection is therefore a wideband low-to-medium frequency sonar with a large receiver aperture. This will allow the exploitation of the frequency dependant characteristics of the target for classification, long range detection and high resolution in both range and bearing. The prime sonar will be active since the noise from free-swimming divers is low. However, in the passive mode, the sonar is used for alert and possible classification clues.

3. SYSTEM LAYOUT The general system configuration of the Underwater Protection System, UPS, is shown in figure 3 below

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Figure 3 General system configuration

The two main branches of a complete system are the above and the underwater side. Seen from the underwater surveillance viewpoint the above water sensors are used primarily for false alarm reduction by track association and removal of surface tracks. In the following a brief summary of the underwater part is given.

3.1 Operator Display, Command and Control The Operator Display processor is used to collect, present and filter position dependent information, providing decision support to numerous planning and analysis activities related to domain awareness. The Operator Display is the primary user interface giving the operator access to all the information available in the system together with the ability to control all system functions. The Operator Display handles vector and raster sea charts and land maps seamlessly. S-57 and DTED (Digital Terrain Elevation Data) map formats are used as standard.

Figure 4 Operations Centre.

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The Command and Control Module is used to collect, analyze, present and filter all-source surveillance and intelligence data into a common recognized operational picture, to assess the threat situation relative to vital assets and to assign and monitor joint reaction missions. The Command and Control Module can operate in a network of operations centres cooperating according to a defined hierarchy. Data security and data integrity is handled through multi-level security and role-based operator access.

3.2 Multi Sensor Integration System The Multi Sensor Integration (MSI) System integrates information from the underwater sensors and provides the underwater operational picture. MSI takes advantage of underwater sensor overlapping to combine information provided by the individual underwater sensors. This process also takes into account geographical information including bottom topography, known underwater objects and input from above-water sensors like radars, electro-optical sensors if available The MSI ensures track numbering consistency when tracks flow between different sonars, and includes advanced functionality for false alarm reduction. Included in the MSI is the Sonar Configuration and Optimization (SCONOP) Module that provides sonar performance prediction capabilities for active and passive sonars taking into account actual bathymetric and oceanographic parameters. The acoustic model is LYBIN which is developed by the Royal Norwegian Navy and has been in operational use onboard Norwegian and other Navies submarines and surface vessels for more than 20 years. Operator interactions are performed via the Operator Display where specific windows for parameter setting and data presentation are provided.

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Figure 5 Illustration of sound propagation

3.3 Sensors, Inspection and Identification Systems The following sections give a brief description of the various sonar and sensor types and also the inspections and identification systems available in the UPS configuration. Other subsystems can be introduced to the network if required. Typical sensors are: * Long range awareness sonar (LASAR) * Passive awareness sonar (PASAR) * Diver detection sonar (DDS) * Underwater inspection vehicles (C'Inspector)

3.3.1 Long Range Awareness Sonar The Long Range Awareness Sonar (LASAR) is a wideband sonar that provides active mode capabilities for long range underwater surveillance. High performance in demanding harbour environments is achieved through the use of wide bandwidths and long linear receiver arrays. Separate transmit and receive arrays provide the flexibility to adapt the sonar configuration to different harbour constraints. The Long Range Awareness Sonar can detect small targets (i.e. diver bottles) at more than 2500 meters in good conditions. Operator interactions are performed via the Operator Display. The LASAR comes in two versions LASAR 10 and 40. The first may operates in the frequency band from 2-25 kHz and the latter from 35 to 50 kHz. The LASAR systems also have passive modes of operation.

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Figure 6 Long Range Awareness Sonar

3.3.2 Passive Awareness Sonar The Passive Awareness Sonar (PASAR) includes a hydrophone array with integrated data acquisition and signal conditioning. The Passive Awareness Sonar includes facilities for automatic calibration, and the hydrophone array is deployed in a known shape to support left/right ambiguity resolution. Operator interactions are performed via the Operator Display.

Figure 7 Passive Awareness Sonar

3.3.3 Diver Detection Sonar The Diver Detection Sonar (LASAR 40 or SM 2000/DDS9000 a 90 kHz Imaging Sonar) provides active mode capabilities for diver detection primarily used for point protection purposes. The systems can be deployed over-the-side, mounted on the quayside or installations in the water column or on the seabed. The Diver Detection Sonar can detect divers and delivery vehicle at more than 500 meters range in good conditions. Operator interactions are performed via the Operator Display.

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Figure 8 Diver Detection Sonar.

3.3.4 Underwater Inspection Vehicle, C'Inspector The C'Inspector Underwater Inspection Vehicle is used for close range inspection of underwater objects based on the onboard sensor suite which includes side scan sonar, mechanical scanning sonar with selectable scanning sector and video camera. C'Inspector is operated from a portable operator console which means that the vehicle can be operated from either a shore based operations centre or a patrol vessel depending on the situation. The Underwater Inspection Vehicle is equipped with the WAP (Wideband Acoustic Positioning) system for underwater localisation. This enables the vehicle to continuously report its position to the Multi Sensor Integration System ensuring the continuous visibility of the vehicle on the Operator Display.

Figure 9 C'Inspector

3.4 Counter Measures The counter measures range from simple warning systems to weapon system for protection of the facility. The counter measures are fully integrated into the networked system and may be operated locally or from the central operator station. The following sections give a brief description of the various counter measures available in the UPS configuration. Other subsystems can be introduced to the network if required.

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3.4.1 Underwater Speaker The underwater speaker or one way communication system is integrated into the LASAR 10 transmitter and may transmit audio messages and act as a warning system transmitting continuous warning messages or other acoustic signals.

3.4.2 C'Guard Underwater Sound Deterrent Unit The unit allows the operator to transmit infra and audible sound in the water at levels which are at least unpleasant to an intruding diver. The sound level is controlled by the operator. The technology is based on a modified air gun technology to allow higher firing rates than used in seismic exploration applications. The same airgun is used in the acoustic influence sweep system AGATE which is in operation with the Royal Norwegian Navy. The system may be operated from a small RIB or as a fixed installation.

3.4.3 C'Inspector C'Inspector may be used as a counter measure to inspect the target and make a diver aware that his presence is known.

3.4.4 Minesniper The reaction vehicle, Minesniper, is similar to the C'Inspector vehicle. The main difference is that the Minesniper includes a directed charge. The Minesniper is operated from a portable operator console, the same or similar to the C'Inspector, which means that the vehicle can be operated from either a shore based operations centre or a patrol vessel depending on the situation. The Minesniper is equipped with the WAP (Wideband Acoustic Positioning) system for underwater localisation. This enables the vehicle to continuously report its position to the MSI System giving a continuous track of the vehicle on the Operator Display.

4. SOME RESULTS This part will give some examples of results from the sonar and the false alarm reduction scheme. The general signal processing chain is illustrated in the following figure

Figure 10 Signalprocessing chain

The chain is the normal one up to the background removal, which includes a filter taking away the stationary objects in the sonar image and emphasising targets with some movement. The tracker is an adapted multi hypothesis tracker with classes of dynamic behaviour models allowing tracking and classification to be linked. The UPS Tracker is designed using the JDL (Joint Directors of Laboratories) Data Fusion Modelling. Figure 11 shows a block diagram of the process.

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The following steps are included in the Data Fusion process 1. Common Referencing for aligning all data to common time and coordinate reference 2. Data Association for association of detections and tracks, using relevant information from digital maps, known fixed objects, and high-reverberation areas for reduction of false alarm rate: * Hypothesis Generation (gating) * Hypothesis Evaluation (scoring) * Hypothesis Selection 3. State Estimation for calculation of full target state including bearing, range, course, speed, and corresponding uncertainties.

The UPS Tracker automatically detects and tracks a number of different targets simultaneously such as divers, swimmers and swimmer delivery vehicles (SDV). The UPS Tracker provides: 0 Improved detection and reduced false alarm rate 0 Reduced ambiguity 0 Timely and accurate situation awareness The final track information is provided to the operator on top of a map or pictorial background. An example of the final display format showing the actual track of a scuba diver is given below:

Figure 12 Screen dump of sonar image overlaid aerial photo with diver track in red.

5. CONCLUSIONS This paper has outlined a general Underwater Protection System (UPS) with all the elements from detection to reaction. It has been emphasised that the need for a seamless integration is required in order to meet the false alarm requirement and need for quick reaction when alerted. From the work performed so far, we have made some general conclusions: * There has to be an area specific system configuration * Sensor and data fusion is an essential property in order to keep the false alarm rate low and still provide a high probability of detection. Long range awareness sonar is crucial in order to reduce cost and complexity of the total system.

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