How far aft of the tow point the towfish actually sits. It is usually calculated from cable out, tow point height and fish depth, assuming a straight cable or a simple catenary, and applied along the vessel's track to position the sonar data. Wrong layback shifts every contact along track and makes one object appear in two places on reciprocal lines.
Contacts are 20 metres apart on the reciprocal lines, layback must be off by about ten.
A contact that jumps along track between opposite-direction lines points to layback; one that jumps sideways points to heading or offsets.
Also heard as
- cable layback
- fish layback
Related terms
Towfish
A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.
Flying the fish
Controlling a towfish's height above the seabed by adjusting cable length and boat speed.
Contact
A target picked out of side-scan or multibeam data that may be an object worth investigating.
Side-scan sonar
A sonar that images the seabed to either side of its track, revealing objects and textures by their echoes and shadows.