Global Positing System

GLOBAL POSITIONING SYSTEM
Presented by
PRANAY MONDAL
M.Tech. (Communication System Engg.)
ROLL - 1351015

CONTENTS :-
 Introduction
 What is GPS?
 Basic principal of GPS
 Information in a GPS signal
 GPS segments
 Errors in GPS
 DGPS
 GPS application
 Conclusion

INTRODUCTION :-
 Radio-based navigation system developed by U.S. Department of
Defense (DOD)
 Initial operation in 1993
 Fully operational Capability in 1995
 System is called NAVSTAR
 NAVigation with Satellite Timing And Ranging
 Referred to as GPS
 Provides specially coded radio signals that can be processed in a
GPS receiver.
– position
– velocity
– time

 What is GPS? :-
• Based on a constellation of about 24
satellites.
• Transmit coded radio signals that can be
processed in a GPS receiver by position,
velocity and time.
 The system measures the time delay
between the signal transmission and
signal reception of the GPS signal.
 The signals carry information about
the satellite’s location.
 Determines the position of, and
distance to, at least three satellites, to
reduce error.
 The receiver computes position using
trilateration.

BASIC PRINCIPAL OF GPS :-
Trilateration :

Trilateration (cont.) :-
2D & 3D Positioning by Satelites

INFORMATION IN A GPS SIGNAL :-
The GPS signal contains 2 types of data.
 Ephemeris data –
 status of the satellite
 current date
 time
 Almanac data -
 orbital information for that satellite and for every other
satellite

GPS SEGMENTS :-
 Space segment
 Control Segment
 User Segment

SPACE SEGMENT :-
 24 satellites that orbit the
earth at about 11,000
nautical miles, once every
12 hours.
 Each planes has about 55°
tilt relative to Earth's
equator in order to cover
the polar regions.
 Each satellite is identified
by number and broadcasts
a unique signal.

SPACE SEGMENT (CONT.) :-
 Satellite Signals-Code-Phasing Ranging
 Each satellite has a unique no & unique signal.
 It continuously broadcasts its signal and also sends out a
time signal every time it starts.
 The receiver has a copy of each satellite signal and
determines the distance by recording the time between when
the satellite says it starts its signal and when the signal
reaches the receiver.

SPACE SEGMENT (CONT.) :-
 Satellite Signals-Code-Phasing Ranging (cont.)
 Distance is calculated using the velocity equation.
Velocity = Distance/time
 Rearranging the equation for distance
Distance= velocity * time
 If the system knows the velocity of a signal and the
time it takes for the signal to travel from the sender to
the receiver, the distance between the sender and the
receiver can be determined.

CONTROL SEGMENT :-
The CS consists of 3 entities:
i) Master Control Station , ii) Monitor Stations ,
iii) Ground Antennas

CONTROL SEGMENT (CONT.) :-
 Master Control Station :
 responsible for overall management of the remote monitoring and
transmission sites.
 Monitor Stations :
 Each of the monitor stations checks the exact altitude, position,
speed, and overall health of the orbiting satellites.
 uses measurements collected by the monitor stations to predict
the behavior of each satellite's orbit and clock.
 The prediction data is up-linked, or transmitted, to the satellites
for transmission back to the users.
 ensures that the GPS satellite orbits and clocks remain within
acceptable limits.
 Ground Antennas :
 Ground antennas monitor and track the satellites from horizon
to horizon.
 They also transmit correction information to individual
satellites.

USER SEGMENT :-
 GPS receivers are generally
composed of an antenna, tuned to
the frequencies transmitted by the
satellites, receiver-processors, and
a highly-stable clock.
 They can also include a display
for showing location and speed
information to the user.

USER SEGMENT (CONT.) :-
 GPS receiver has to know two things:
 The location of atleast three satellites above one’s.
 The distance between us and each of those satellites
 A standard GPS receiver can give you several
information-
 How far you've travelled (odometer)
 How long you've been travelling
 Our current speed (speedometer)
 Our average speed
 A "bread crumb" trail showing us
exactly where we have travelled on
the map
 The estimated time of arrival at
our destination if you maintain
our current speed.

ERRORS IN GPS :-
 Selective Availability or SA –
hamper very precise positioning accuracy
 Ionosphere and troposphere delays-
Signal delayed upon going through the ionosphere and
troposphere.
 Receiver clock errors –
the receiver timing references will have some small error.
 Orbital errors -
Also known as ‘ephemeris errors’, these are inaccuracies in the
satellite’s reported position.

ERRORS IN GPS (CONT.) :-
 Multipath error – can produce large location errors.
 Signal attenuation -
Non-restricted GPS signals (1.575 GHz) are blocked by
steel and concrete structures .

DGPS :-
 Technique used to correct some of these errors.
 Referred to as “differential GPS” or DGPS.
 In DGPS, two GPS receivers are used.
 One receiver is located at an accurately surveyed point
referred to as the “base station”.
 A correction is calculated by comparing the known
location to the location determined by the GPS satellites.
 The correction is then applied to the other receiver’s
(known as the “rover”) calculated position.

DGPS METHODS :-
 Post-processing
 Corrections performed after the data is collected.
 Special software required .
 Real-time
 Corrections are performed while the data is being
collected.
 Need special equipment to receive the DGPS signal.

GPS APPLICATIONS :-
 GPS in the air
 GPS on land
 GPS in sea
 Military Uses for GPS
 GPS in scientific research
 Applications for your business
 Monitor Nuclear Explosions
 Every Day Life
 Set Your Watch!

CONCLUSION :-
 GPS will find more civilian uses
 The applications using GPS are constantly growing.
The cost of the receivers is dropping while at the same
time the accuracy of the system is improving.
 DOD has promised to eliminate Selective Availability

Global Positing System

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