The constant job of holding a towed sonar at the right altitude, commonly about 10 percent of the range scale and within 8 to 20 percent in many specifications, so shadows are long enough to measure without losing range. The winch operator pays out cable as the bottom drops away and hauls in as it shoals, while the coxswain holds a steady speed. Fly too high and small contacts throw shadows too short to see; fly too low and the fish risks hitting bottom.
Flying the fish at 12 metres on the 100 metre range, give me another 20 of cable before the drop-off.
Also heard as
- fish height
- towfish altitude
Related terms
Towfish
A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.
Layback
How far astern of its tow point a towed sensor actually rides, measured horizontally.
Acoustic shadow
The dark area behind an object in a side-scan image where the object blocked sound from reaching the seabed.
Side-scan sonar
A sonar that images the seabed to either side of its track, revealing objects and textures by their echoes and shadows.