Abstract: A system and method is described for providing pilots with navigational guidance when landing in low visibility conditions and/or at airports that do not contain a ground infrastructure in place to support precision approach. The navigational guidance is obtained by fusing one or more navigational algorithms and/or error models to improve navigation performance to near Instrument Landing System (ILS) performance levels. The navigational algorithms and error models include a gravity error model, a Global Positioning System (GPS) receiver measurement model, a Wide Area Augmentation System (WAAS) troposphere model, encrypted corrections, and a dynamic range error model.