An onboard monitor that ensures the accuracy of data representing the calculated position of an airplane during final approach to a runway. This airplane position assurance monitor is a software function that uses dissimilar sources of airplane position and runway data to ensure the accuracy of the respective data from those dissimilar sources. ILS data and GPS or GPS/Baro data are the dissimilar sources of airplane position data used by this function. This function will calculate the airplane's angular deviations from the runway centerline and from the glide slope with onboard equipment and then compare those angular deviations to the ILS angular deviation information.