During underwater missions LAUVs have no access to GPS and rely on dead reckoning for navigation. This navigation method consists of integrating the velocity and heading rate, provided by an IMU, to obtain the vehicle's position. We could compare it to being blindfolded and counting your steps while walking through a house. However if we miss a step or turn we might end up in the wrong place.
For this reason it’s important to make sure IMU data is reliable and the navigation system can use it correctly. To do so the vehicle is set on a square course at the surface, to receive GPS signal, before every mission. During straights IMU data is compared with the GPS to calibrate and ensure no errors are found.
However data is not everything! The system must use the information correctly in order to provide precise and reliable navigation. An assessment of reliability can be made by setting a figure eight square course, at surface, during approximately one hour. The pattern eliminates systematic errors, since the vehicle turns both ways, and a longer test time results in larger integration error, which helps to spot the problem. The GPS signal, received at surface, is recorded but removed from the filter to simulate underwater behavior. This way a reliable comparison can be made between the navigation solution and the vehicle's actual position.