1. 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

2. Introduction space-based global navigation satellite system (GNSS)
Reliable location and time information in all weather and at all times
Three major segments - The space segment (SS), a control segment (CS), and a user segment (US).
GPS time is accurate to about 14ns

3. 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.

4. Components of the GPS

5. 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.
There are more than 24operational at a time as more satellite s are launched to replace the older ones

6. 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

7. User Segment: Receivers decode signals from satellites.
The receiver tasks:
Selecting one or more satellites
Acquiring GPS signals
Measuring and tracking
Recovering navigation data

8. 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

9. GPS Satellite Signal:
L1 freq. ( Mhz) carries the SPS code and the navigation message.
L2 freq. ( 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

10. 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

11. Triangulation Position is calculated from distance measurement
Mathematically we need four satellites but three are sufficient by rejecting the ridiculous answer

12. 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

13. 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
Very costly

14. Error Sources 95% due to hardware ,environment and atmosphere
Intentional signal degradation
Selective availability
Anti spoofing

15. 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 hand-off switching.

16. Thank you