How GPS works ?

GPS stands for Global Positioning System , GPS is constellation of 29 satellites placed in 6 planes at inclination of 55 degrees from earth’s equator . Each satellite is equipped with the highly precise clock i.e atomic clock , Atomic clock are extremely necessary for synchronization of each satellite . The satellites are placed such that minimum of 4 satellites are visible at any time from any place of the surface of earth . The satellites are placed at 20180 km away from the earth surface and the time taken by one satellite to complete one revolution of earth is 11 hrs and 58 minutes.

GPS operates at the frequency of 1575.42 Mhz.

GPS was developed by US army in 1970s for defence purpose only and later on it was made commercial, Nowadays it a common device used for the purpose of navigation,site surveys,path finding and many other purposes.

GPS that we carry in our hand is a GPS receiver , That shows our location,our speed and direction in which we are moving  by receiving signals from minimum of 4 satellites.

How GPS works ?  

My motive has always been is to explain my understandings in simple words. For understanding the working of GPS you have to be little bit imaginative.

You can take Paper and pencil in your hand for better understanding.

Let us consider that a person with GPS is standing at position A on the surface of earth , GPS receives signal from first satellite , the signals are electromagnetic waves which travels at the speed of light i.e 3*10­⁸  meter per second , let the distance be ‘p’ (Distance is calculated by the time taken by the signal to travel from satellite to GPS )  but it will not be known in which direction and where the GPS is , so probable  position for GPS receiver according to the satellite will be any position at the surface of imaginary sphere with radius ‘p’ and satellite at its centre .

Draw a circle (In 3 D it will be sphere ) as Earth , consider a position of GPS on earth and consider a satellite  A (mark it as point A ) on Space , Now draw a Circle ( In 3 D it will be sphere ) with satellite A as centre and distance between GPS and satellite A as radius .

Now GPS  receives the signal from another satellite , Let the distance be ‘q’ the , the position of satellite according to second satellite will be any position at surface of imaginary sphere with radius ‘q’ and satellite ‘B’ at the centre of sphere .

But with the combined information from satellite A and Satellite B the probable position for GPS is the surface where sphere x and sphere y intersects , the two imaginary spheres at an exact imaginary circle , let us call the circle as X . Now we have reached a closer approximation.

In the diagram drawn earlier consider an another satellite , satellite B at space , Draw another circle with satellite B as centre and distance between satellite B and GPS as radius , the two circles drawn with satellites as circle will intersect at 2 points on paper but in 3D it will intersect to form an another imaginary circle . so now keep in mind the only the intersecting imaginary circle of two imaginary spheres.

Now GPS receives the signal from third satellite , Let the distance be ‘r’  , the probable position of GPS according to third satellite will be a surface of sphere with radius ‘r’ and  satellite C at the surface centre of sphere . Now the imaginary sphere with radius ‘r’ will intersect the imaginary X at two points one at the surface of earth and another at space , let the intersection point at surface of earh be ‘N’ and Intersection point at space be ‘M’ , since GPS receiver will be place at the surface of the earth so position ‘M’ will be ignored and the only possibility left will be position ‘N’.

In the diagram drawn earlier consider an another satellite , satellite C at space , Draw another circle with satellite C as centre and distance between satellite C and GPS as radius , The new circle drawn will intersect the imaginary circle discussed above at two points , Consider only the position on earth for GPS location.

Combined Information from three satellite will provide the position on earth in 3 dimension , 4^th satellite will give the more precise information about the AMSL (Above Mean Sea Level )

With combined information from 4 satellites the Lat , Long and AMSL at the point where GPS is placed is known .

Now GPS receiver can show you in which direction you are moving and can show you the speed at which you are moving.

GPS calculates the direction by computing the lat-long at every point  and speed by computing change of lat-long with time at every point.

As satellites are equipped with the atomic clocks the GPS receiver are widely used at Wimax BTS’s working on TDD for synchronization of uplink and downlink stream of each base station to avoid interference.

Akela he chala tha janib manzil , Magar log saath aatey gae aur karwan banta gaya ..