Transcript GLOBAL POSITIONING SYSTEM NAME : S.J.VIJAI CLASS : I – M.sc (C.S) ROLL NO : APU – 15

GLOBAL POSITIONING
SYSTEM
NAME
: S.J.VIJAI
CLASS
: I – M.sc (C.S)
ROLL NO : APU – 15
Why do we need GPS?
• Trying to figure out where you are
is probable man’s oldest pastime.
• Finally US Dept of Defense
decided to form a worldwide
positioning system.
• Also known as NAVSTAR (
Navigation Satellite Timing and
Ranging Global positioning
system) provides instantaneous
position, velocity and time
information.
Components of the GPS
GPS
Space Segment
Control Segment
User Segment
Space Segment:
• 24 GPS space
vehicles(SVs).
• Satellites orbit the
earth in 12 hrs.
• 6 orbital planes
inclined at 55 degrees
with the equator.
• This constellation
provides 5 to 8 SVs
from any point on the
earth.
Control Segment:
• The control segment comprises of 5 stations.
• They measure the distances of the overhead
satellites every 1.5 seconds and send the corrected
data to Master control.
• Here the satellite orbit, clock performance and
health of the satellite are determined and determines
whether repositioning is required.
• This information is sent to the three uplink stations
User Segment:
• Receivers decode
signals from satellites.
• The receiver tasks:
– Selecting one or more
satellites
– Acquiring GPS signals
– Measuring and
tracking
– Recovering navigation
data
User Segment:
• The Precise Position Service
– PPS is the highest level of dynamic
positioning based on the dual freq P-code
– The P-code is a very long pseudo-random bi
phase modulation on the GPS carrier which
does not repeat for 267 days
– Only authorized users, this consists of SPS
signal plus the P code on L1 and L2 and
carrier phase measurement on L2
GPS Satellite Signal:
• L1 freq. (1575.42 Mhz) carries the SPS code and
the navigation message.
• L2 freq. (1227.60 Mhz) used to measure
ionosphere delays by PPS receivers
• 3 binary code shift L1 and/or L2 carrier phase
– The C/A code
– The P code
– The Navigation message which is a 50 Hz signal
consisting of GPs satellite orbits . Clock correction
and other system parameters
How does the GPS work?
• Requirements
• Triangulation from satellite
• Distance measurement through travel time
of radio signals
• Very accurate timing required
• To measure distance the location of the
satellite should also be known
• Finally delays have to be corrected
Triangulation
• Position is calculated
from distance
measurement
• Mathematically we
need four satellites
but three are sufficient
by rejecting the
ridiculous answer
Measuring Distance
• Distance to a satellite is determined by
measuring how long a radio signal takes to
reach us from the satellite
• Assuming the satellite and receiver clocks
are sync. The delay of the code in the
receiver multiplied by the speed of light
gives us the distance
Getting Perfect timing
• If the clocks are perfect sync the satellite
range will intersect at a single point.
• But if imperfect the four satellite will not
intersect at the same point.
• The receiver looks for a common
correction that will make all the satellite
intersect at the same point
Error Sources
• 95% due to hardware ,environment and
atmosphere
• Intentional signal degradation
– Selective availability
– Anti spoofing
Conclusion
• GPS is considered a dual-use technology,
• GPS has become a widely deployed and
useful tool for commerce, scientific uses,
tracking, and surveillance.
• GPS's accurate time facilitates everyday
activities such as banking, mobile phone
operations, and even the control of power
grids by allowing well synchronized handoff switching.
Thank you