Use when you must fuse INS and GNSS in a loosely coupled error-state integration filter: assemble the 5-state psi-angle error model of the horizontal INS drift from the specific forces, discretize it into the state transition matrix, predict the position, velocity and heading error states and their covariance between GNSS fixes, and apply the GNSS position measurement update that drives the estimated error state toward the innovation through the Kalman gain.
