Inertial Measurement Unit (IMU)

The IMU in your Receiver or Transceiver device calculates the attitude of the device with respect to both gravity and magnetic north. This allows the device to convert its relative estimate of the distance and direction of the ROV to an absolute estimate of the ROV location.

The Mk II Receiver and Mk III Transceiver share the same physical sensors (gyros, accelerometers, and magnetometers) but use different algorithms to fuse the sensor data. For convenience, we call the Mk II algorithm the "Mk II IMU" and we call the Mk III algorithm the "CIMU" (short for "Cerulean IMU").

Note that a good IMU calibration is one of the dominating factors for good system performance.

Mk III Transceivers with serial numbers of 5535 and lower use the Mk II IMU. For units in warranty, Cerulean Sonar will update Mk III Transceivers with serial numbers of 5535 and lower to use the CIMU. Contact Cerulean Sonar for details if you are interested.

The Mk II is now by default shipped in the CIMU mode.

Both IMUs require periodic calibration.

• The accelerometers are subject to slight aging effects, and to temperature effects.

• The gyros are subject to slight aging effects, and to temperature effects.

• The magnetometers are subject to variations in the Earth's magnetic field strength and to magnetic disturbances due to electrical currents and and nearby soft iron or other ferrous objects. The closer a disturbance source, the smaller it can be and still affect the north-finding.

The Mk II IMU will force a recalibration every time it is powered on. The CIMU is recalibrated at your option.

The Earth's magnetic field varies greatly over geographically small areas. This is why we recommend a CIMU magnetometer recalibration every time the system is moved to a different area (rough rule of thumb, recalibrate if you move 50 km or more).

A significant error source for the IMU is often nearby electrical currents. IF the Receiver or Transmitter are mounted on the ROV (and this is NOT typical), a big error source can be the power systems and ferrous materials on the ROV. This is why we recommend a CIMU recalibration every time the device is mounted in a different way on the ROV.