For proper use in the INS, each individual sensor may require calibration. Magnetic sensors require a thorough field calibration before they can be used to allow them to account for local magnetic disturbances. Many sensors, such as transducer-based sensors, require regular factory calibration to adjust for transducer variations over time. Additionally, magnetic sensors require a declination to account for their position on the Earth's surface.
The INS may also perform a start-up calibration to establish a bias for each sensor. The Greensea INS typically performs a start-up bias procedure that lasts between 20 and 60 seconds. During the calibration process, the INS checks that sensor noise levels are acceptable and, if the unit is determined to be stable, establishes a bias for the accelerometers and gyros. The INS also implements a Zero Velocity Correction algorithm to update the calculated biases of the accelerometers and gyros during operation.
The INS also needs to know the placement of sensors with respect to the coordinate frame center of the vehicle. Greensea's INS provides a configuration file to specify the transform between each sensor's instrument frame and the vehicle's frame. Appendix A provides an example sensor transform configuration file.
Additionally, Greensea's INS can transform the navigation solution to a designated point, such as the center of a pipe tracking coil that's in the front of the vehicle. This transform is specified just as a sensor transform in the sensor configuration file as the "ref_point".
Greensea provides utilities in our graphical workspace to configure this configuration file.
