Noise Signal Modulation at the Ship Rolling and Pitching due to Fluctuating Interference of Beams

Amplitude modulation of a noise signal of the surface ship is formed both directly at noise emission, when the line of the shaft and the propeller is rotating, and in addition due to the ship rolling and pitching caused by the rough sea. At the same time additional low-frequency (no more than 0,2 Hz) amplitude modulation of the noise emission can be observed as a process of temporary variations of the received signal power. In this regard, it is interesting to find relation of the amplitude-phase modulation spectrum of the ship broadband signal at its rolling and pitching with the spectrum of rolling and pitching process. As a mechanism of formation of the noise signal modulation at the ship rolling and pitching the fluctuating interference of the beams, propagating from vertically dipping underwater point source near the water surface, is considered. The analytical ratios connecting the amplitude-phase modulation spectrum of the ship broadband signal at its rolling and pitching with the spectrum of rolling and pitching process are received.

As a result, the parameter describing cumulative conditions by which it is possible to determine spectral properties of modulation is determined. The type of the power variation spectrum can be substantially different depending on values of parameter K = 2kσ_H sinα 1 where k — the wave number, σ_H-root mean square value of the vertical component (source shift) due to rolling and pitching, α — the beam grazing angle at the surface. At value 2kσ_Hsinα< 1 results of observation of the signal modulation process can be identified with process of the ship rolling and pitching. At value 2kσ_Hsinα> 1 the signal modulation spectrum will not correspond to the spectrum of rolling and pitching process. With the increase of parameter K value the modulation spectrum extends, goes beyond the spectrum of rolling and pitching process. Significantly lower and high harmonics emerge. Results of computer modeling and natural experiments confirmed dependence of the type of the modulation spectrum on parameter K. Thus, presence of the additional mechanism of noise signal modulation formation at the ship rolling and pitching, caused by the phenomenon of fluctuating interference of the beams propagating from vertically dipping underwater point source near the water surface is found.

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