Introduction to GNSS - GNSS Remote Sensing: Theory, Methods and Applications - page 8

geometry can be observed every SI (International System of Units) day with about
4 min displacement. The GPS ground-segment is composed by a primary Master
Control Station at Schriever Air Force Base (Colorado, US) and ten dedicated
ground antennas and monitor stations around the globe.
The GPS has been widely used for positioning, navigation and timing (PNT)
globally to all users at all times since 1994. For example, the southern California
Permanent GPS Geodetic Array (PGGA) was the first established in early 1990s
across the Pacific-North America plate boundary to continuously monitor crustal
deformation, providing the first temporal-spatial details of crustal movements
before, during and after a major earthquake (Bock et al. 1997 ). Since GPS
is highly precise, continuous, all-weather and real-time with signals that travel
through the Earth's atmosphere, additional scientific applications of GPS were
being explored by scientists and engineers. Notably, Ware ( 1992 ) suggested limb
sounding the Earth's atmosphere and ionosphere using GPS atmospheric refracted
delay signals. Following, the GPS/Meteorology Mission (GPS/MET) using the GPS
radio occultation technique was launched and successfully provided accurate, all
weather, global refractive index, pressure, temperature and the ionospheric total
electron content (TEC) measurements and electron density profiles, which have
been widely applied in the atmosphere and ionosphere (Rocken 1997 ; Hajj and
Romans 1998 ).
Over the last decade, the U.S. has implemented several improvements to the
GPS service, including new technology and new signals for civil use and increased
accuracy and integrity for all users, all while maintaining compatibility with existing
GPS equipment. GPS modernization has now become an ongoing initiative to
upgrade the Global Positioning System with new capabilities to meet growing
military, civil, and commercial needs. The program is being implemented through
a series of satellite acquisitions, including GPS Block III and the Next Generation
Operational Control System (OCX). The U.S. Government continues to improve the
GPS space and ground segments to increase performance and accuracy. With the
development of the new global navigation satellite systems (GNSS) constellations,
including the U.S.'s GPS, Russia's GLONASS, and the coming European Union's
GALILEO system and China's Beidou/COMPASS system as well as a number
of Space Based Augmentation Systems, such as Japan's Quasi-Zenith Satellite
System (QZSS) and India's Regional Navigation Satellite Systems (IRNSS), more
applications and opportunities will be exploited and realized for multi-frequency
and multi-system GNSS.

1.1.2 GLONASS

The Global Navigation Satellite System (GLONASS) by former Soviet Union was
fully operational since 1995 but suspended and partially operated after collapse
of USSR and now has got the latest up-gradation and is fully operational since
2010 by Russia. The GLONASS (abbreviation of GLObal'naya NAvigatsionnaya