Global Positioning System :-
Global Positioning System GPS is a radio navigation system that allow people on land in sea or in the air to determine their exact position, 24 hours a day anywhere in the world in every weather condition.
In the modern era of communication satellites are widely used.A satellite is a relay station which is placed in to orbit by human.They are sometimes called artificial satellites to distinguish then from the natural satellites such as moon.satellites are used for various purposes such as militaryand civilian earth observation satellites, communication satellites, navigation satellite, weather satellites, and research satellites.The orbit used by these satellites may be defined as geostationary earth orbit, medium earth orbit or low earth orbit.
GPS Global Positioning System are used for navigation on aeroplanes, ships, ground vehicles, and by individuals.Also the relative positioning and time data is used to study movement of tectonic plates to understand the earthquicks, astromical observations, telecommunications.
Satellite system can be classified based upon their orbit in to following three types.
• Geostationary Earth Orbit
• Medium Earth Orbit
• Low Earth Orbit
Geostationary Earth Orbit:-
A Geostationary Earth Orbit GEO is an orbit directly above the earth equator at 0 angle altitude with a period equal to the earth rotation period.An object in Geostationary Earth Orbit appeared to be fix in the sky if it observed from the earth.Communicaton satellites and weather satellites are generally placed in the Geostationary Earth Orbit, so that the satellite earth antennas paced on the earth that communicate with them do not have to track them, but can placed permanently at the fix position where they stay.
Medium Earth Orbit:-
A Medium Earth Orbit MEO is located above the earth at altitude of 2,000 kilo meters and below geostationary earth orbit altitude of 35,786 kilo meters.The satellites placed in this orbit are generally used for navigation such as Global positioning system GPS at an altitude of 20,200 KM.For example GLONASS a Russian satellite based navigation system is at an altitude of 23,222 Km from the earth.
Low Earth Orbit:-
A Low Earth Orbit LEO is also known as low orbit, is generally defined for the earth surface up to an altitude of 2,000 km.But the commonly used definition of LEO Low Earth Orbit is from 160 km to 2,000 km above the earth surface.Less energy is required to place a satellite in to Low Earth Orbit and it require low power amplifiers for communication.Therefore this orbit is used for many communication applications.Most of the satellites, like International Space Station, The Space Shuttle, and the Hubble Space Telescope are all exits in Low Earth Orbit.