Satellite navigation systems provide a number of significant benefits over traditional ground-based navigation aids. These benefits include global coverage, improved accuracy, and the ability to support area navigation (the ability to navigate along any arbitrary course as opposed to only along defined routes). The U.S. Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS) are used today for a number of safety-critical applications, including navigation of aircraft and ships. Other satellite navigation systems are emerging and are expected to be in use for such applications within the next decade.