GPS
The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS) in the world. The GPS uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, that enable GPS receivers to determine their current location, the time, and their velocity (including direction).

GPS was developed by the United States Department of Defense. Its official name is NAVSTAR-GPS. The GPS satellite constellation is managed by the United States Air Force 50th Space Wing.

Similar satellite navigation systems include the Russian GLONASS (incomplete as of 2008), the upcoming European Galileo positioning system and the proposed COMPASS navigation system of China, and IRNSS of India.

Glonass
GLONASS (Russian: ГЛОНАСС) is a radio-based satellite navigation system, developed by the former Soviet Union and now operated for the Russian government by the Russian Space Forces. It is an alternative and complementary to the United States' Global Positioning System (GPS) and the planned Galileo positioning system of the European Union (EU).

Development on the GLONASS began in 1976, with a goal of global coverage by 1991. Beginning on 12 October 1982, numerous rocket launches added satellites to the system until the constellation was completed in 1995.

Following completion, the system rapidly fell into disrepair with the collapse of the Russian economy. Beginning in 2001, Russia committed to restoring the system, and in recent years has diversified, introducing the Indian government as a partner, and accelerated the program with a goal of restoring global coverage by 2009.

Galieleo
Galileo is a global navigation satellite system currently being built by the European Union (EU) and European Space Agency (ESA). The €3.4 billion project is an alternative and complementary to the U.S. Global Positioning System (GPS) and the Russian GLONASS. On November 30, 2007 the 27 EU transportation ministers involved reached an agreement that it should be operational by 2013.

Galileo is intended to provide more precise measurements than available through GPS or GLONASS (Galileo will be accurate down to the meter range), better positioning services at high latitudes and an independent positioning system upon which European nations can rely on even in times of war or political disagreement, since Russia or the USA could disable use by others (through encryption). Like GPS, use will also be free for everyone.

Compass
The Compass system (also known as Beidou-2) is a project by China to develop an independent global satellite navigation system. Compass is not an extension to the previously deployed Beidou-1, but a new GNSS system similar in principles to GPS and Galileo. The new system will be a constellation of 35 satellites, which include 5 geostationary orbit (GEO) satellites and 30 medium Earth orbit (MEO) satellites that will offer complete coverage of the globe.