Measurement covering 71 h. (a) Accumulated energy distribution of different frequency versus bearing. (b) Bearing distributions of selected frequencies. B1 and B2 denote two bearing ranges. (c) GPS tracks of the ferries. The azigram calculation parameters are shown in Table 1. In (a), the frequency and bearing resolutions are 1 Hz and 1°. The ferries are Lysoey (MMSI: 257056190), Flatoy (MMSI: 257055990), Faeroes (MMSI: 257055910) and Huftaroy (MMSI: 257055930), and they were built in 2019 according to the registration.