Global navigation satellite systems

Global positioning systems first became available for private use in 1995. Since the introduction of NAVSTAR-GPS (Navigation System with Time and Ranging-Global Positioning System) and GLONASS (Globaluaya Navigatsionnaya Sputnikovaya Sistema, Global Navigation Satellite System), such systems have quickly become indispensable in a wide range of applications. Above all, GPS is used today by a large user community. Artificial augmentations can be used to improve the performance of the systems in terms of accuracy, availability and integrity. In space-based augmentations, differential correction data, integrity information and additional ranging signals are transmitted from geostationary satellites. In ground-based augmentations, a reference station (based on a gee-referenced position) compares the position solution (calculated by the SatNav System) with the real coordinates. In that case, the correction data is transmitted by a telemetric system to roving receivers near the reference station. Both augmentation techniques are based on the generic satellite navigation systems GPS or GLONASS, and are dependent on the availability of the source system. To overcome the dependency of the civil European user community upon foreign military systems, an initiative to build up an autonomous European Satellite Navigation System with an own-space segment was initiated. Since 1999, the realisation of this project has been on the way, and the employment of the new system, named Galileo, is currently expected in 2008. (C) 2000 Elsevier Science B.V. All rights reserved.