Measurements of correlation loss and time-spreading in linear FM sweeps and pseudo-random noise signals transmitted over long ranges in shallow water
The results of some transmission measurements made in shallow water are described. The ranges were about 15 km, 20 km, and 34 km, and the sound source was either suspended about 10 m above the bottom or was on the bottom in about 60 m of water depth. The signal was received on a verticalstring of hydrophones at 10, 20, 40, 80 and 160 m depths. Both linear FM sweeps and pseudorandom noise signals with different time/bandwidth products were transmitted. The centre frequencywas 1087.5 Hz. The sound-speed profile was of a very downward refracting type, forcing greatinteraction with the sea bottom. In the calm weather prevailing, the sound experienced nearly specular reflection from the sea surface. The two shortest ranges were over a hard sandy bottom, resulting in severe multipath propagation that caused acoustic travel times to fluctuate by about 130 ms around their mean. The longest range was over a soft lossy bottom and very little fluctuation intravel time was observed. The loss in processing gain compared with the theoretical gain was generally more severe for the shallow receivers than for the deep receivers, which were situated below a steep gradient in the sound-speed profile. The multipath spread was generally also more severe for the shallow receivers. The pseudo-random noise signals experienced a great increase inprocessing loss with increasing pulse length, while processing loss for the FM-swept signals showedvirtually no increase for pulse lengths of up to 16 s. The transmission loss over the 20 km range was modelled using the parabolic equation. The modelled results agree well with the measurements.