Introduction to GNSS - GNSS Remote Sensing: Theory, Methods and Applications - page 11

In addition, there exist several Ground- and Space-Based Augmentation Systems
(GBASs and SBASs), such as the European Geostationary Navigation Overlay
Service (EGNOS), the GPS Aided Geo Augmented Navigation (GAGAN), the Joint
Precision Approach and Landing System (JPALS), the Local Area Augmentation
System (LAAS), the Wide Area Augmentation System (WAAS) and the Multi-
functional Satellite Augmentation System (MSAS). The working principle of these
systems is to make use of ground stations whose positions are precisely known. The
stations gather measurements from GNSS satellites and/or other sensors to create
a regional correction map that is sent back to orbiting satellites and then broadcast
to the users. This enhances the quality of the computed position by mitigating the
impact of certain errors (mostly atmospheric). Not less importantly, these systems
have also been designed to provide and enhance integrity services.
The gurus of navigation community predict that when GPS, GLONASS,
GALILEO, and Beidou/COMPASS will be fully operational and interoperable, four
times more satellites as compared to present may be available for navigation, posi-
tioning, and timing, providing more types of signals broadcasted on more frequen-
cies. This will surely result in enhancing the quality of the services and increasing
the number of potential users and applications. Furthermore, a number of space-
based augmentation systems and regional Navigation satellite systems will add
more satellites and signals to multiple systems of satellites and, as a result, improve
positioning performance in terms of accuracy, availability, reliability and integrity.
Global Positioning System (GPS) has become a clichéd terminology due to its
applications in navigators and cellular technology. The modern cell phones and
vehicles are equipped with GPS chipsets but the applications are far beyond these
two basic concepts and have wide applications in many terrestrial, air, marine and
space domains. At present, the civil applications have out marked the military
applications. The wide range of applications and the promises of the prospective
global navigation systems have paved the way for the development of strong
market for GNSS receivers and this market is growing at an enormous rate and
the prevailing market trends calls for enthusiastic contributors to participate in this
development phase and reap the fruits at the end of this endeavor.

1.2 GNSS Systems and Signals

1.2.1 GNSS Segments

GNSS is consisting of a space segment having a constellation of satellites in
Medium Earth Orbit (MEO) at the height of nearly 20,000 km and above, control
segment having monitoring and control stations to monitor/control/update the
constellation of satellites and the user segment ; consisting of receivers to give
position, velocity and time (PVT) of the static and mobile user all over the globe
at all time using at least four satellites of the constellation in view.