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1.
INTRODUCTION - GNSS
Global Satellite Navigation Systems (GNSS) is the generic term for satellite navigation systems that provide autonomous geo-positioning with global coverage (Figure 1). GNSS-1 is the name given to existing systems (i.e. Global Positioning System [GPS] and GLObal'nayaNAvigatsiomayaSputnikovaSistema [GLONASS]). When GPS is used with corrections from Ground Based Augmentation Systems (GBAS) or Space Based Augmentation Systems (SBAS), it is known as Differential GPS (DGPS). GNSS-2 will include additional second-generation systems (e.g. Galileo, Compass [CNSS], Quasi-Zenith Satellite System [QZSS] and the Indian Regional Navigational Satellite System [IRNSS] etc.).
Figure 1.
GNSS Relationships and Terminology.
1.1.
Rapid Growth in Civilian Use
Only twenty years ago, civilian use of GNSS was mainly limited to merchant ships' crews and surveyors; today there are millions of receivers being used in all forms of transport and commerce as well as a wide variety of recreation activities. The major breakthrough came with the introduction of Navstar, more commonly known as GPS; development started in 1973 and the system was declared operational at the end of 1993. 'Cold War' rivalry between the Soviet Union and the United States of America (USA) led to the 'Space Race' and the almost parallel development of the Soviet system GLONASS. After initial success, GLONASS fell behind in the 1980s and 1990s when too few satellites were launched to sustain a working constellation.
1.2.
Augmentation Systems
Increasing civilian use of GPS resulted in the introduction of Selective Availability (SA), which was designed to deny the full accuracy of the system to civilian users and possible adversaries. Unfortunately for the US military authorities, this was easily defeated by means of DGPS, and early commercial services were quickly followed by government funded GBAS (e.g. the US Coast Guard DGPS Network) and by SBAS (e.g. the US Wide Area Augmentation Service [WAAS], the European Geostationary Navigation Overlay Service [EGNOS] and the Japanese Multi-functional Satellite Augmentation System [MSAS]). SBAS all involve the use of Geo-Stationary Earth Orbit (GEO) satellites which are placed 36,000 km above the Equator and rotate in synchronism with Earth rotation.
1.3.
More Recent Developments
The ease of use and inherent accuracy of GPS made it very popular with millions of yachtsmen, private pilots and hikers as well as commercial...