providing a plurality of models of GPS navigation, each model characterized by a model state vector which is updated periodically;

providing for each model a corresponding probabilistic data association filter (PDAF) for deriving, for each periodic update, a current value for the corresponding model state vector based on a current observation made on parameters affecting the corresponding state vector, wherein each PDAF determines whether the current observation is a correct observation or a false observation such that each PDAF updates the corresponding model state vector if the current observation is a correct observation and does not update the corresponding model state vector if the current observation is a false observation; and

applying an interacting multiple model (IMM) estimator to provide, for each period, a current value for a system state vector using the current values of the model state vectors for that period and their corresponding PDAFs.