Example of a 30-min measurement covering a vessel passage on 21 May 2022. (a) Spectrogram of power spectral density. (b) Azigram showing only the bearings for the data points in (a) that are >70 dB. The 70 dB is chosen for visualization purposes. The white color denotes the non-visualized bearings. (c) 2.5 min track of the vessel Admiralen (MMSI: 258352500). The vessel headed northwest with a speed over ground of about 31 knots. In (a) and (b), the x-axis is local time (HHMMSS) and calculation parameters are shown in Table 1.