Side-to-side rotation, measured by the motion sensor and applied to every beam so depths land in the right place. Because outer beams are long, a small roll error makes a large vertical error: a 0.1 degree bias moves a beam at 60 degrees in 20 m of water about 6 cm up or down. Roll bias between motion sensor and transducer is found in the patch test from reciprocal lines over flat seabed.
Roll's off by 0.08, the swath tilts opposite ways on the reciprocal lines over the flat.
Also heard as
- roll angle
Related terms
Pitch
Bow-up or bow-down rotation of a vessel, which shifts sonar footprints forward or aft along the track.
Heave
The vertical up-and-down motion of a vessel caused by waves, measured and removed from sonar depths.
Patch test
A set of calibration lines over flat ground and slopes that measures sonar alignment biases and timing error.
MRU
Motion reference unit: the sensor that measures a vessel's roll, pitch and heave to correct sonar data.
Wobble
A rocking or rippling artefact in multibeam data that follows the vessel's motion, caused by attitude or timing errors.