Broadband LFM signal source for a module-based diver detection sonar

BROADBAND LFM SIGNAL SOURCE FOR A MODULE-BASED DIVER DETECTION SONAR KRZYSZTOF ZACHARIASZ*, WALDEMAR LIS*, ANDRZEJ ELMINOWICZ** * Gdansk University of Technology, Faculty of Electronics, Telecommunications and Informatics, Department of Marine Electronic Systems Narutowicza 11/12, 80-233 GdaĔsk Poland , **OBR Centrum Techniki Morskiej S.A. Dickmana 62, 81-109 Gdynia, Poland A new approach to design of the diver detection sonar’s sounding pulse source is presented. It is based on modules comprising 8 or 16 elements of the transducer grouped on the transmitting channel. The basic advantage of this solution is that it reduces significantly the number of group transmitters. The required output power and topology of the transmitters were determined through theoretical estimation and measurements in a four terminal network. Measurements were taken of the frequency characteristics in the transducer modules including the compensation system used. It is demonstrated that the source level is most irregular in the transducer under examination when the transducer’s voltage has a constant amplitude, i.e. with parallel compensation. It is also demonstrated that by appropriately selecting elements for a series compensation of the transducer, the frequency characteristics becomes more regular and source level is increased at the extreme ends of the working bandwidth. The result is a compensated characteristics on the receiving side. INTRODUCTION In recent years the threat of terrorism has led to the launch of a number of initiatives designed to improve the security of marine sites, in particular sea ports, anchorages and offshore facilities. Underwater security relies on active and passive sonars for the detection of divers, small underwater vehicles and submerged diver delivery vehicles. The sonars are manufactured by a number of companies worldwide. 269 Installed at the Polish Navy Port, the Active Stationary Sonar [1] has been in operation for several years with very good results. It was built by OBR CTM in a project called KRYL. The company is currently working on a new generation modular sonar [2] for the detection of underwater objects in ports and anchorages. Its special feature is that it can be adapted to the geometry of the site it is surveilling (the shape of the pool, configuration of the quays and breakwaters and the type of bottom). The sonar is easy to install, portable or stationary, can be installed on quays, ships and in open bodies of water. It is protected from disturbances and changing conditions of sound propagation. One of the research problems the project is addressing is the development of methods for integrating sonars and modules of acoustic barriers. The objective is to build a cost effective and efficient system of underwater security which can successfully operate in a variety of large-scale sites and ensure that mutual disturbances and reverberation levels are kept to a minimum. Because sonar parameters must be changed in a wide range (especially the observation sector), a module-based design is required. Modules are primarily used in the transducer, transmitter, receiver and signal processing. The number and type of modules in the sonar depend solely on the parameters of the site and its conditions. Special software is used to connect the modules. 1. SONAR CHARACTERISTICS The diver detection sonar (DDS) uses a broadband piezzocomposite transducer [3] a product of a US-based company MSI. It has 128 transmit-receive sections placed on the side of a cylindrical casing. Vertical beam pattern has a constant width at 12° because it operates in shallow waters. The horizontal width of the sounding beam and its orientation, however, must be adapted flexibly to the geometry of the area under surveillance. To build a modular structure of the sonar’s source of transmitting pulses, the elementary sections of the transducer are grouped (on the transmitting side) into modules operated by individual group transmitters. With this structure it is easy to configure that part of the array which is excited. The operation of the individual transmitters within the group can be easily controlled by low voltage settings made in the main level system. The dimensions of the transducer module determine the minimal angle by which the transmit sector width and position can be changed. The sector’s highest level of precision is achieved with single section modules. This, however, would require a 128 channel transmitter. While the stroke angle for 8 or 16 element modules is wider, it is still acceptable (22.5° or 45°). This structure is matched by lower numbers of group transmitters (16 or 8) but with a higher output power. The ease of observation sector changes is not the only factor speaking in favour of the modular design of the sonar’s transmitting channel. It was also important to consider whether the following can be corrected depending on the size of the module: • dispersion of the transmit patterns for the elementary sections of the transducer; • irregularity of the frequency characteristics in the working bandwidth. The need for correcting the characteristics became apparent from the results of measurements taken by the manufacturer of the transducer. They showed (Fig.1) significant fluctuation of source level SL in the sections with some as high as 8 dB in the middle of the working bandwidth. 270 148 146 SL[dB] 144 142 140 138 136 0 8 16 24 32 40 elements number 48 56 64 Fig.1. Spread of source level SL [dB ref. 1µP/V/1m] for 64 array elements The dispersion of source level can be corrected separately for each section using single element transmit modules. But this is not necessary because the beam patterns of the transducer’s elementary sections are wide and for the practical widths of the scan sector many adjacent sections have to be excited (40 for a 90° sector). This leads to averaging on the acoustic side. As a result, even if non-identical sections are connected in parallel – just like in a multiple elements module - the beam pattern is slightly smoothened out. Its shape and level of ripple resemble that of the theoretical pattern which was obtained for identical elements. This is confirmed by the results of measurements taken by MSI, as seen in Fig.2. 170 SL [dB] 160 150 140 130 120 -100 -75 -50 -25 0 25 50 75 100 Angle[deg] Fig.2. Horizontal transmit beam patterns for a 16 element module of the cylindrical transducer Figure 3 shows the irregularity of the frequency characteristics. As you can see the drop in SL at the edges of the working bandwidth (60÷80kHz) reaches 6 dB. A similar irregularity is seen in the receiving sensitivity which adds up to the drop of 12 dB for the transmit-receive frequency characteristics. 271 170 b 160 S L [dB ] c 150 a 140 130 50 60 70 80 90 100 f[kH z ] Fig.3. Normalized source level SL in the frequency function for a single element (a) and modules with 8 (b) and 16 (c) elements The decr (...truncated)