# Cerulean Inertial Measurement Unit (CIMU) The CIMU 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. Note that having a good IMU calibration is one of the dominating factors for good system performance. The CIMUs requires 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 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.ceruleansonar.com/c/ethernet-rov-locator/cerulean-inertial-measurement-unit-cimu.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.