Low frequency active sonar - reverberation and performance in the Baltic

Active sonar is an important tool for detection of submarines. For the oceans, where high-frequency propagation of sound suffers from attenuation caused by the salinity, LFAS (low-frequency active sonar) has attracted much interest in recent years. Sonar frequencies below 1 kHz have been tested with good results. One of the purposes of the BAROC (Baltic Acoustics on Rocky Outcrops) experiment carried out in May 2002 jointly by FOI and FWG (in Germany) was to assess the potential of LFAS in the Baltic. The detection ranges in shallow water are typically limited by bottom reverberation. Making some simplifying assumptions, it follows that the detection range will increase with the target strength, decrease with the scattering strength (for the bottom), decrease with the horizontal beam width and increase with the bandwidth. A small horizontal beam width and a large bandwidth can be achieved at a high (centre) frequency. Nevertheless, the BAROC experiment provided the following results: 1) A detection range of 8 km against a 10 dB target with a 1,5 kHz pulse of bandwidth 1 kHz, and a detection range of 4 km against a 10 dB target with a 4.0 kHz pulse of bandwidth 2 kHz. 2) SS increase with frequency for the frequencies studied (1-6 kHz), and the increase can apparently be significant enough to compensate for the decrease of the horizontal beam width and the increase of the bandwidth. 3) SS is very high in the area, because of irregular bottom topography and rocky outcrops. Comparison to detection ranges for traditional sonar system remain to be made. Additional topics for future research include the frequency dependence of the target strength TS, and 3D modelling capability will be required for an improved understanding of how SS depends on grazing angle and frequency.