An experimental study in forming a long synthetic aperture in the sea
Successful experimental testing of synthetic aperture techniques for sonar systems have been very limited and only few results, including very stable CW signals, have been published. These experimental results and theoretical investigations indicated that the spatial coherence and the cross-correlation properties of the received signal's segments over time are serious limitations on the effective size of the synthetic aperture. The aim of the present investigation is to extend the testing of a synthetic aperture technique, the ETAM algorithm and to experimentally examine its performance for CW, pseudo-random signals and broadband ship noise. The results reported here show the limitations of this technique and they are of special interest for operational systems development. In the CW experiments, the transmitted signal was generated with high temporal coherence and loss of the spatial and temporal coherence of the received signal was introducedonly by the medium and the stability of the towed array. In the experiments that included the pseudo-random signal and the ship noise, the temporal coherence of the transmitted signals was deliberately chosen to be poor in order to study the effects and the performance of the algorithm with broadband signals.The related experimental results show that for received signals, whichhave their segments over the synthesizing period highly cross-correlated, a synthetic aperture array gain was achieved which corresponds to the length of an equivalent fully populated array.