High Frequency Cylindrical Intercept Array a new high sensitivity - - PDF document

UDT 2020 UDT Extended Abstract Template Presentation/Panel

High Frequency Cylindrical Intercept Array – a new high sensitivity broadband intercept sensor for submarines

Nils Theuerkauf1, Sebastian Hess2

• 1Dr. rer. nat., Atlas Elektronik GmbH, Bremen, Germany
• 2M. Sc., Atlas Elektronik GmbH, Bremen, Germany

Abstract — The ATLAS High Frequency Cylindrical Intercept Array (HFCIA) is a small, passive acoustic sensor for submarines designed to detect and locate high frequency acoustic signals in the frequency band of 100 kHz to 600 kHz originating from other high frequency sonars. The HFCIA provides nearly constant beam characteristics and sensitivity over the entire frequency range which is achieved by a model and optimization based design approach.

1 Introduction

Rapidly emerging Autonomous Underwater Vehicle (AUV) technology is significantly increasing the threat to a submarine of detection by high frequency sonar. AUVs can be equipped with a variety of sonar systems such as side scan sonars, obstacle avoidance sonars, synthetic aperture sonars and other imaging sonars that are able to detect and classify submarines located within their sonar

• ranges. Other high frequency sonars such as diver

detection systems, sonar cameras and sea floor mapping sensors are also capable of submarine detection. Common to all these systems is that they are active sonars

• perating in a high frequency range.

The ATLAS HFCIA was designed to counter these modern threats. The compact sensor head contains sixteen conformal ceramic elements, twelve located around the circumference of the sensor head to ensure 360° horizontal sonar coverage, and four positioned on the top of the sensor head which enables the sensor to have vertical sonar coverage between elevation angles of approximately -15° and +90°. The ceramic elements allow for high accuracy bearing estimation. The electronics is located within the head and transfers the stave data via a fibre optical uplink to the inboard processing. The design is optimized in terms of acoustic performance and resilience to underwater explosive shock. HFCIA utilizes a new type of bi-conformal ceramics, which enables for nearly constant coverage and high sensitivity

• ver the entire frequency band ranging from 100 kHz to

600 kHz. The electronics is optimized for the sensor elements resulting in a low-noise system not limited by electrical self-noise. The compact sensor head can be easily installed atop of a rigid foundation fixed to the casing of a submarine. In this paper the design approach to fulfil the challenging specification and the system design are described. The key features of HFCIA are:

• Constant beam width over a wide frequency

range

• High sensitivity and detection performance
• Full azimuthal coverage
• No additional software beam former necessary
• Ease of maintenance due to exchangeable

receive modules The HFCIA development was finalized in 2019 and a first unit is already shipped to the customer and installed

• n a submarine. This development is part of an ongoing

project for Saab Kockums AB (SK) and Swedish Defence Materiel Administration with the Royal Swedish Navy as end customer.

2 System decomposition

The Sensor Head consists of a Ceramic Carrier, which provides pockets for the Receive Modules (RM) and integrates the pressure housing containing the electronic

• unit. The system is designed in a way that the individual

RM can be easily exchanged separately and independently from each other. The pressure housing containing the electronic unit is integrated into the Sensor Head. A cable connector is provided at the bottom of the housing. A shielded outboard cable connects the device to the Pressure Hull Penetrator mounted within the ship’s pressure hull. 2.1 Sensor Head The Sensor Head, shown in Figure 1, contains the 16 Receiver Modules placed on the Ceramic Carrier. Twelve RM are designed to be positioned 360° around the carrier to provide full azimuthal coverage. Four additional RM

UDT 2020 UDT Extended Abstract Template Presentation/Panel located on the top of the Sensor Head are mounted in a pyramid like arrangement to provide adequate coverage

• f the upper hemisphere (see Fig. 1).

Section 3 provides examples of the acoustic coverage

• btained with this arrangement of Receiver Modules.
• Fig. 1. HFCIA Sensor Head
• Fig. 2. Modular design of the HFCIA Sensor Head

2.2 Receiver Module The Receiver Module (see Figure 3) is the acoustic functional unit of the HFCIA. This functional unit converts the acoustic signal into an electrical signal. It consists of a composite ceramic which is embedded into a water-proof housing. The design allows for an easy exchange of Receive Modules and provides sufficient support to the ceramic element during explosive shock.

• Fig. 3.. Image of HFCIA Receive Module

2.3 Sensor Electronic The Sensor Electronic is located in the pressure resistant housing which is a part of the Sensor Head. The Sensor Electronic is designed to process the analogue signals from the 16 receive ceramics. Therefore, it contains a Low Noise Amplifier (LNA), fully integrated Voltage Controlled Amplifier (VCA) with gain and filter functions, Analogue Digital Converter (ADC) and a Field Programmable Gate Array (FPGA) for control and transfer via a fibre optical uplink. The electronic unit features signal conditioning like a/d- conversion, various filters and a down sampling for a high data output. To improve the SNR of the received signals, an automatic gain control amplifies the dynamic range of the ADC to attenuate high level signals and amplify low level signals. This prevents the HFCIA from clipping when active sonars ping in close distance.

3 Design approach

The design of the HFCIA presented numerous technical challenges to be solved. At the beginning of the design phase a selection of possible concepts was identified and these concepts were subsequently assessed in terms of achieving the design goals, whilst minimizing the technological risk. This resulted in a receive module with a bi-conformal surface. Main acoustic requirements for the development were:

• Reception frequency band: 100 to 600 kHz
• Vertical detection sector θ-3dBV: ≥ 7.5° in band
• Full coverage of the azimuth
• No limitation by own noise
• Good horizontal bearing estimation better than 10°

A further design goal was that the HFCIA shall enable a horizontal bearing estimation by using a 12 stave cylindrical array. Therefore, the sensitivity of the individual staves should decrease continuously with the azimuth angle to facilitate the bearing estimation algorithm. For a high detection range the ceramic in combination with the electronics shall not be own noise limited. Numerical optimization was used to minimize the impact

• f electrical self-noise. The model incorporates electro-

acoustical properties of the sensor and the properties of the electronics. This facilitates constant coverage and low self-noise over the wide frequency range. This modelling approach reduced the number of test samples to be produced during the development and helped to speed up the development process. The initial validation and the following automatization of the model (rapid modelling approach) was an important step to reach the final design. The identified performance benchmarks met by the design are:

UDT 2020 UDT Extended Abstract Template Presentation/Panel

• A high sensitivity and low electrical self-noise in

the desired frequency band.

• No limitation by own noise.
• An intrinsic constant beam characteristic over a

wide frequency range; the HFCIA was intended to

• perate from 100 to 600 kHz.
• Vertical detection sector θ-3dB: ≥ 7.5° in band
• A full azimuthal coverage and better bearing

estimation accuracy than required.

• No multi-channel processing is required for this

approach, due to the shape of the ceramic.

4 Measurements

• Fig. 4. Image of HFCIA in test environment

The HFCIA Receive Module is designed to provide a smooth slope in the horizontal beam pattern. The smooth slope is desired in order to support the bearing estimation

• algorithm. A smooth decrease of sensitivity with

increasing beam angle is required, because the difference in signal energy has to be reliably detected by the bearing estimation algorithm. A smooth slope over the entire frequency band is desired. To verify the performance of the Receiver design measurements of RM beam patterns were performed. Figure 4 shows a HFCIA sensor head mounted on a turn table in a water test tank facility. The arrangement allows for measurements of the horizontal and vertical characteristics.

• Fig. 5 shows the normalized horizontal characteristics of

the HFCIA element for three frequencies (200 kHz, 500 kHz, and 600 kHz). These characteristics provide a similar slope for all shown frequencies. The undulations

• f the characteristic are an intrinsic property of the design

and cannot be avoided completely.

• Fig. 5. Beam characteristic horizontal
• Fig. 6. Beam characteristic vertical

The desired vertical characteristic differs from the desired horizontal characteristic. While for the horizontal characteristic a constant slope is of advantage, for the vertical characteristic a constant coverage of approx. 15° is desired. As for the horizontal direction, the challenge is to provide this shape over the entire frequency band. Figure 4 shows the measured vertical stave characteristics for the same three frequencies. The beam width is nearly constant and does not vary much with frequency. These measured patterns were fed into simulations to validate the overall performance of the sensor in respect to the bearing estimation accuracy. The simulations showed that the design goal with respect to desired coverage and bearing accuracy was achieved.

Acknowledgements Author/Speaker Biographies