The electric field is the intrinsic characteristic of a ship. The measurement of the electric field of a ship is greatly
disturbed by noise in shallow sea. The measured signals can be processed using wavelet transform, adaptive filtering or
time-frequency analysis. Adaptive filtering can be used to remove a part of ambient noise. But the filtering effect is not
obvious because a certain type of industrial interference signal is also of an extremely low frequency, When wavelet
transform is used to remove noise, most noises from the electric fields of sea are removed. However, it also has nothing
to do with the periodical noises with an extremely low frequency. As far as time frequency method is concerned, the
original signals are first analyzed within the frequency band, and then they are compared with the measured ambient
signals. After the range of shaft-modulated signals are found, the signals of required frequency band are extracted for
further IFFT transform. The processing results show that the traffic characteristic of a ship is very clear, representing the
signals are extracted out.
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