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 , 4th 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.
