The three-dimensional offsets from a common reference point to the GNSS antennas, motion sensor, transducers and waterline. Software uses them to move a position measured at the antenna down to the sonar head and to compute induced heave as the vessel rolls and pitches. They are measured with a total station or laser scanner when equipment is installed, and a few centimetres of error appears in every depth.
Lever arms for the new pole are in: 1.82 starboard, 0.35 aft, 2.10 down from the reference point.
Most offset blunders are sign errors, because each software package picks its own positive directions for starboard, forward and down.
Also heard as
- offsets
- vessel offsets
- sensor offsets
Related terms
MRU
Motion reference unit: the sensor that measures a vessel's roll, pitch and heave to correct sonar data.
Transducer
The device that turns electrical pulses into sound in the water and converts returning echoes back into signals.
Pole mount
A sonar transducer fixed on a rigid pole over the side or bow of a boat instead of built into the hull.
Static draft
The vertical distance from the waterline down to the transducer face while the vessel is at rest.
Patch test
A set of calibration lines over flat ground and slopes that measures sonar alignment biases and timing error.