DIDSON Resolution
In high-frequency mode, the image is formed from transmitted pulses sent out in 96 beams. Each beam is 0.3° wide and 14° high, spaced at 0.3° intervals. When the sonar is tilted down, these slices of sound each ensonify a radial line on the bottom. The 96 lines side-by-side span the sonar display. In low-frequency mode the same sector is spanned by 48 beams each 0.4° wide, spaced at 0.6° intervals.
Resolution is affected by the size of the acoustic pixels. The width of the pixel increases with increasing range. The height of the pixel increases with increased window length. A simple way to calculate the acoustic pixel width is to divide the cross-range of the display at desired range R by the number of beams. In the High-Frequency mode there are 96 beams covering this cross-range. In the Low-Frequency mode there are 48 beams covering that distance. The cross range of the display at range R is R/2. Here are two examples.
Assume we are interested in the acoustic pixel size at the range of 30 meters when the window-length is 40 meters. At this range the sonar is in Low-frequency mode with 48 beams covering the cross-range. At 30 meters, the cross-range is 15 meters and the cross-range resolution is 1500 cm/48 = 31 cm (that is a bit over one foot!) A display window has 512 pixels from the maximum range to the minimum range. The acoustic pixel height is simply the window length divided by 512. In this example it is 40 m/512 or 4000 cm/512 = 8 cm (~3 inches). Those are large acoustic pixels and one would not expect much detail.
In the second example, the object of interest is only 2 meters from the sonar. The window-length is set to 1.25 meters. We have an acoustic pixel width of 100 cm/96 = 1 cm and an acoustic pixel height of 125 cm/512 = 0.24 cm. This small acoustic pixel will show detail.