Interest in the use of Global Navigation Satellite Systems (GNSS) as a main source of navigation reference is increasing. The system employed for such a purpose should be capable of meeting the requirements of air navigation in terms of accuracy, availability, integrity, and reliability. At present, the accuracy requirements for all ?ight categories up to precision approach are summarized in Table 1. The accuracy requirement for Category I can be achieved most of the time using wide area differential systems such as the American “WAAS”, the European “EGNOS”, and the Japanese “MSAS”. The American Federal Aviation Authority (FAA) is developing a Local Area Augmentation System (LAAS) for categories II and III, involving the ?nal and precision approach phases of ?ight. The system includes at least four reference GPS receivers located at each airport, whereby GPS measurements are collected and processed in real time. The computed GPS differential corrections are sent to aircraft via a (VHF) radio link to calculate its location. LAAS preliminary test results have generally demonstrated accuracy of less than 1 meter. However, the percentage of system availability is still under evaluation. The cost of establishing LAAS for major airports is expected to be significant.