GPS (Global Positioning System) is the only system today able to show you where your exactly position on the earth at anytime and any weather condition. 24 satellites are all orbit around the earth at 11,000 nautical miles or approximately 20,200 kms. above the earth. The satellites are placed into six different orbital planes and 55 degree inclination. They are continuously monitored by ground stations located worldwide.

GPS ELEMENTS We can divide GPS system into three segments.

SPACE SEGMENT

USER SEGMENT

CONTROL SEGMENT

SPACE SEGMENT The space segment comprises a network of satellites . The complete GPS space system includes 24 satellites, 11,000 nautical miles above the earth, take 12 hours each to go around the earth once or one orbit. They are orbit in six different planes and 55 degrees inclination. These positions of satellites, we can receive signals from six of them nearly of the time at any point on earth. Satellites are equipped with very precise clocks that keep accurate time to within three nanoseconds ( 0.000000003 of a second or 3e-9)


This precision timing is important because the receiver must determine exactly how long it takes for signals to travel from each GPS satellite to receiver.
Each satellite contains a supply of fuel and small servo engines so that it can be moved in orbit to correct for positioning errors.
Each satellite contains four atomic clocks. These clocks are accurate to a nanosecond .
Each satellite emits two seperate signals , one for military purposes and one for civilian use.
     SOME SPECIFICATION OF SATELLITE

Weight     930 kg.(in orbit)

Size     5.1 m.

Travel Velocity      4 km/sec

Transmit Signals      1575.42 MHz and 1227.60 MHz

Receive at      1783.74 MHz

Clocks      2 Cesium and 2 Rubidium

Design life      7.5 year (later model BlockIIR 10 years)

USER SEGMENT As the pilot fly , the GPS receiver continuously caculates the current position and display the correct position / heading.The GPS unit listen to the satellite's signal and measure the time between the satellites transmission and receipt of the signal. By the process of triangulation among the several satellites being received, the unit computes the location of the GPS receiver. GPS receiver has to see at least four satellites to compute a three dimensional position (it can compute position with only three satellites if know altitude). Not only latitude and Longitude , but altitude as well. There are numerous forms of display among the various manufacturer. No frequency tuning is required , as the frequency of the satellite transmissions are already known by the receiver.

CONTROL SEGMENT The control Segment of GPS consist of:

Master Control Station ( one station ): The master control station is responsible for overall managment of the remote monitoring and transmission sites. As the center for support operations , It calculates any position or clock errors for each individual satellite from monitor stations and then order the appropriate corrective information back to that satellite.

Monitor Stations ( four stations ): Each of monitor stations checks the exact altitude , position , speed , and overall of the orbiting of satellites. A station can track up to 11 satellites at a time. This check-up is performed twice a day by each station as the satellites go around the earth.

OPERATION The principle of GPS is the measurement of distance between the receiver and the satellites. The satellites also tell us exactly where they are in their orbit above the earth . The receiver knows our exact distance from satellite , knows the distance between satellites. GPS receivers have mathematical method by computer to compute exactly where the GPS receiver could be located.