Overview
The International Civil Aviation Organization (ICAO) uses the term 'GNSS' to represent the collective global satellite navigation capability for civil aviation applications. GNSS is a world wide system and includes one or more satellite constellations, aircraft receiving equipment and any augmentation systems required to deliver the service.
The core satellite constellation is the Global Positioning System (GPS). The Russian Federation is building its GLObal NAvigation Satellite System (GLONASS), and plans to offer full service coverage by 2010.
These systems provide independent capabilities and can be used in combination with future core constellations such as the European 'Galileo' system.
Airservices and Honeywell, in conjunction with the US Federal Aviation Administration, are developing and certifying an ICAO compliant GBAS (Ground Based Augmentation System), a precision landing system to meet Category I requirements, replacing ILS (Instrument Landing System).
The links on the left will give you more detailed information about the systems.