GPS (Global Positioning System)

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.