Global Positioning System
Space-based satellite navigation system used to locate the
place
Project was developed in 1973 by U.S. Department of Defense
It became fully operational in 1995
There are different GPS systems
NAVSTAR - US
GLONASS - Russia
Galileo - EU
BeiDou - China.
QZSS - Japan
IRNSS – India
(Indian Regional Navigation Satellite System
IRNSS-1A,1B,…… )
SPACE SEGMENT-
GPS satellites - 24 satellites
GPS orbits at an altitude of 20,200 km and
Period of revolution -12 hours
Powered by solar cells & nickel cadmium batteries
The satellites continuously orient themselves to point their
solar panels toward the sun and their antenna towards the
earth. Orbital planes are inclined at 55˚ with respect to equator.
Orbits are separated by 60˚
At least 5 to 8 satellites are always within line of sight from any
location on the planet
Life time of satellite- 7.5 years and replacements are constantly
being built and launched into orbit
CONTROL SEGMENT
Master Control System Monitor Stations and Ground Antennas
• MASTER CONTROL STATION located at Falcon Air Force Base
in Colorado Springs, The control segment comprises a
master control station and five monitor stations outfitted
with atomic clocks that are spread around the globe.
• Checks the exact altitude, position, speed, and overall health
of the orbiting satellites. The control segment ensures that
the GPS satellite orbits and clocks remain within acceptable
limits.
• A station can track up to 11 satellites at a time
• They measure the distances of the overhead satellites every
1.5 seconds and send the corrected data to Master control.
• The five monitor stations monitor the GPS satellites and then
send information to the master control station where
abnormalities are revised and sent back to the GPS satellites
through ground antennas.
• Schriever Air Force Base in Colorado
• Cape Canaveral
• Florida
• Hawaii
• Ascension Island in the Atlantic Ocean
• Diego Garcia Atoll in the Indian Ocean
• Kwajalein Island in the South Pacific Ocean.
Atomic clock
• An atomic clock principle: When electromagnetic radiation
interacts with the excited states of certain atoms, there is a
hyperfine transition in the microwave region, or an
electron transition in the optical or ultraviolet region of the
emission spectrum of an atom is used as a frequency
standard
55Cs - 137 9192631770 Hz
37Rb - 87
User Segment
GPS receiver, receives the signals from the GPS
satellites and determines
(1) how far away it is from each satellite and
(2) Latitude
(3) Longitude
(4) Altitude
(5) Velocity
• GPS receivers are generally composed of
• 1. an antenna tuned to the frequencies transmitted
by the satellites
• 2. receiver-processors
• 3. highly-stable clock (commonly a crystal
oscillator). They can also include a display for
showing location and speed information to the user.
A receiver is often described by its number of
channels (this signifies how many satellites it can
monitor simultaneously). As of recent, receivers
usually have between twelve and twenty channels.
• GPS receivers calculate distances to satellites as a function of
the amount of time it takes for satellites' signals to reach the
ground. A GPS Receiver is a L-band radio processor capable of
solving the navigation equations in order to determine the
user position, velocity and precise time (PVT), by processing
the signal broadcasted by GPS satellites
• The Global Positioning System carriers are in
the L band, centered at 1176.45 MHz (L5),
1227.60 MHz (L2), 1381.05 MHz (L3), and
1575.42 MHz (L1) frequencies. L band waves
are used for GPS units because they are able
to penetrate clouds, fog, rain, storms, and
vegetation
• C/A code or Coarse Acquisition Code(Each code consists of
1,023 chips and is sent at a rate of 1.023 megabits per
second. The code sequence repeats every millisecond).
• P-Code or Precise Code
OTHER components of receiver
• Receiving Antenna:
• LNA (Low Noise Amplifier):
• IF Amplifier:
• Analog to digital converter
• Microprocessor :
A microprocessor is a computer processor that incorporates the functions
of a central processing unit
• How GPS Determines a Position
The working of the Global positioning system is based on the
‘trilateration’ means measuring distance
Triangulation means measuring angle
The position is determined from the distance measurements to
satellites. Four satellites are used to determine the position of
the receiver on the earth. The target location is confirmed by
the 4th satellite. And three satellites are used to trace the
location place.
How Accurate GPS ?
Physical Obstructions
The measurements of arrival time can be skewed through
large masses such as buildings, mountains, trees, etc.
Atmospheric Effects
GPS devices mainly affected by solar storms, Ionospheric
delays
A major source of error in GPS arises from the fact that the
speed of the radio signals is constant only in a vacuum which
means that distance measurements may vary as the values of
the speed of signal vary in the atmosphere
GPS applications
• GPS receivers are used by soldiers, in aircrafts, F-16 fighters,
Navy ships, minesweeping
• GPS has become important for all military operations and
weapons systems.
• GPS is also used by car racers, hunters, bikers, and cross-
country skiers
• GPS is used by police, fire, emergency medical service
enabling the quickest possible response in death or life
situations.
