A motion reference unit uses accelerometers and gyros to measure roll, pitch and heave many times a second. Survey vessels often carry a GNSS-aided inertial system that adds heading and position in the same package. Mounting matters: the unit should sit near the vessel's centre of motion, rigidly fixed, with accurately surveyed offsets to the transducer, because an attitude error of a tenth of a degree shows in the outer beams.
MRU took a jolt when the launch hit the dock, run a roll check before production.
Also heard as
- motion reference unit
- motion sensor
- IMU
- INS
Related terms
Roll
Rotation of a vessel about its fore-and-aft axis, measured to correct the angle of every sonar beam.
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.
Lever arms
The measured distances between sensors on a survey vessel, used to carry positions and motion to each sonar.
Patch test
A set of calibration lines over flat ground and slopes that measures sonar alignment biases and timing error.