1. A navigation system comprising: an inertial measurement unit configured to measure inertial motion of a vehicle; an aiding source configured to provide observational measurements of motion for the vehicle; and a navigation computer coupled to the inertial measurement unit and the aiding source, wherein the navigation computer is configured to calculate a predicted state and an error covariance data based on the measured inertial motion received from the inertial measurement unit and the observational measurements from the aiding source; calculate variance increments based on attitude uncertainty in the predicted state; add the variance increments into a process noise covariance matrix for the predicted state; and calculate an update for the motion of the vehicle based on the predicted state, the error covariance data, and the observational measurements.