GLOBAL POSITIONING SYSTEM

The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides reliable location and time information in all weather and at all times. It was established in 1973. The Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services. This system consists of three segments: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments.
Space SegmentThe space segment consists of a nominal constellation of 24 operating satellites that transmit one-way signals that give the current GPS satellite position and time.  
Control SegmentThe control segment consists of worldwide monitor and control stations that maintain the satellites in their proper orbits through occasional command manoeuvre, and adjust the satellite clocks. It tracks the GPS satellites, uploads updated navigational data, and maintains health and status of the satellite constellation.
User SegmentThe user segment consists of the GPS receiver equipment, which receives the signals from the GPS satellites and uses the transmitted information to calculate the user’s three-dimensional position and time.
Other satellite based navigation systems are:
a) GLONASSThe Global Navigation Satellite System (GLONASS) is based on a constellation of active satellites which continuously transmit coded signals in two frequency bands, which can be received by users anywhere on the Earth's surface to identify their position and velocity in real time based on ranging measurements. The system is a counterpart to the United States Global Positioning System (GPS) and both systems share the same principles in the data transmission and positioning methods. GLONASS is managed for the Russian Federation Government by the Russian Space Forces.
b) GALILEO Galileo is a global navigation satellite system (GNSS) currently being built by the European Union (EU) and European Space Agency (ESA). Its aim is to provide a high-accuracy positioning system upon which European nations can rely independent of the Russian GLONASS and US GPS systems which can be disabled for commercial users in times of war or conflict.
When in operation, it will use the two ground operations centres, one near Munich, Germany, and another in Fucino, Italy and will consist of 30 satellites (27 operational + 3 active spares). This will become fully operational by the year 2014.
c) BEIDOU NAVIGATION SYSTEMThe BeiDou Navigation System (COMPASS) Navigation Satellite System is a project by China to develop an independent satellite navigation system. It may refer to either one or both generations of the Chinese navigation system. The first BEIDOU system, officially called BEIDOU Satellite Navigation Experimental System, or known as BeiDou-1, consists of 3 satellites and has limited coverage and applications. It has been offering navigation services, mainly for customers in China and from neighboring regions, since 2000. The second generation of the system known as Compass or BEIDOU-2 will be a global satellite navigation system consisting of 35 satellites, is still under construction. It is planned to offer services to customers in Asia-Pacific region by 2012 and the global system will be started by 2020.
d) GAGANGPS Aided Geo Augmented Navigation ‘‘GAGAN’’ is an augmentation system to enhance the accuracy and integrity of GPS signals to meet precision approach requirements in Civil Aviation and is being implemented jointly by AAI and ISRO. The goal is to provide navigation system for all phases of flight over the Indian airspace and in the adjoining areas. GAGAN will increase safety by using a three-dimensional approach which will provide course guidance to the runways to help to reduce the risk of controlled flight into terrain i.e., an accident whereby an airworthy aircraft, under pilot control, inadvertently flies into terrain, an obstacle, or water.