A navigation system for a vehicle is described that includes an inertial measurement unit, a positioning unit, and a processing unit. The inertial measurement unit configured to provide a first set of data relating to operation of the vehicle and the positioning unit is configured to provide a second set of data relating to operation of the vehicle. The processing unit is configured to receive the data sets provided by the inertial measurement unit and the positioning unit and the processing unit comprises an error processing device. The error processing device is programmed with time updating error states and measurement updating error states relating to the data sets received by said processing unit and is configured to iterate both the time updating error states and the measurement updating error states based on a first criteria and iterate only the measurement updating error states based on a second criteria.