A satellite navigation or sat nav system is a system of
satellites that provide autonomous geo-spatial positioning with
global coverage. It allows small electronic receivers to
determine their location (longitude, latitude, and altitude) to
within a few metres using time signals transmitted along a line-
of-sight by radio from satellites. Receivers calculate the
precise time as well as position, which can be used as a
reference for scientific experiments. A satellite navigation
system with global coverage may be termed a global navigation
satellite system or GNSS. As of April 2013, only the United
States NAVSTAR Global Positioning System (GPS) and the Russian
GLONASS are fully globally operational GNSSs. China is in the
process of expanding its regional Beidou navigation system into
the global Compass navigation system by 2020. The European
Union's Galileo positioning system is a GNSS in initial
deployment phase, scheduled to be fully operational by 2020 at
the earliest. India is also building the Indian Regio