Wide Area Augmentation System
American Institute of Aeronautics and Astronautics eBooks · 1996
I N time, the Global Positioning System (GPS) will be used for a wide variety of aircraft operations. However, aircraft use of any satellite-based navigation system raises significant concern with respect to integrity (hazardous but undetected faults), reliability (continuity of service), time availability, and accuracy. After all, a single satellite malfunction would affect users over a huge geographic area. A navigation system with integrity warns its users if position errors may be greater than a prespecified alarm limit. Clearly, radionavigation systems used by aviators must have integrity, and the integrity requirement depends on whether the system is the primary navigation aid or supplements another system. The wide area augmentation system (WAAS) is a safety-critical system consisting of a signal-in-space and a ground network to support enroute through precision approach air navigation. It is designed to augment GPS so that it can be used as the primary navigation sensor. The WAAS augments GPS with the following three services: a ranging function, which improves availability and reliability; differential GPS corrections, which improve accuracy; and integrity monitoring, which improves safety. The WAAS is shown in Fig. 1. As shown, the WAAS broadcasts GPS integrity and correction data to GPS users and also provides a ranging signal that augments GPS. In the near future, the WAAS signal will be broadcast to users from geostationary satellites. Inmarsat-3 satellites will be the first to carry the WAAS navigation payload, and they are scheduled to be launched in 1996.