Comparison of measured and modelled reverberation and echo level in littoral waters

Abstract

Sonar performance models are used both when planning and carrying out anti-submarine warfare operations. The models allow the sonar operator to get an estimate of what ranges the sonar is able to detect an enemy submarine. For active sonar, the transmission loss and reverberation levels are important input to the sonar equations which are used to model the sonar performance. Here we run two different sonar performance models, Lybin and MSTPA, for an environment in the Norwegian Trench on the Western coast of Norway. A present pipeline is used as a target of opportunity. The reverberation and echo levels are modelled. The output of the two models are compared to each other, but also to measurements made in the same area. These measurements were collected during a NAT III experiment in 2002. NAT III was a joint research program that included TNO, Thales Underwater System, FFI, and the Dutch, French and Norwegian navies. The modelled reverberation compares fairly well to measurements, particularly in the flat region in the Trench. The complex terrain close to the coast is an exception, which is expected since only a single set of bottom parameters were used in the modelling. For improved modelling of this complex environment a detailed map of the bottom parameters would be needed. The two models differ strongly on the echo level, and both models fail to estimate the echo level. This may be due to the presence of a strong sound channel at the sonar depth. The oceanography in the area of the experiment is known for its strong variability, and the presence of such sound channel are therefire usually local. By removing the sound speed minimum in the profile, the estimated echo levels of both models improve.